polymers
Review
Advanced Functional Materials for Intelligent
Thermoregulation in Personal Protective Equipment
Alireza Saidi
1,2,3,
* , Chantal Gauvin
3
, Safa Ladhari
2
and Phuong Nguyen-Tri
1,2,
*
Citation: Saidi, A.; Gauvin, C.;
Ladhari, S.; Nguyen-Tri, P. Advanced
Functional Materials for Intelligent
Thermoregulation in Personal
Protective Equipment. Polymers 2021,
13, 3711. https://doi.org/10.3390/
polym13213711
Academic Editor: Teofil Jesionowski
Received: 1 October 2021
Accepted: 21 October 2021
Published: 27 October 2021
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4.0/).
1
Department of Chemistry, Biochemistry and Physics, Université du Québec à Trois-Rivières (UQTR),
3351 Boulevard des Forges, Trois-Rivières, QC G8Z 4M3, Canada
2
Laboratory of Advanced Materials for Energy and Environment, Université du Québec à
Trois-Rivières (UQTR), 3351 Boulevard des Forges, Trois-Rivières, QC G8Z 4M3, Canada; safa.ladhari@uqtr.ca
3
Institut de Recherche Robert-Sauvé en Santé et en Sécurité du Travail (IRSST), 505 Boulevard de Maisonneuve
Ouest, Montréal, QC H3A 3C2, Canada; [email protected]
* Correspondence: [email protected] (A.S.); Phuong.nguyen-tri@uqtr.ca (P.N.-T.)
Abstract:
The exposure to extreme temperatures in workplaces involves physical hazards for workers.
A poorly acclimated worker may have lower performance and vigilance and therefore may be more
exposed to accidents and injuries. Due to the incompatibility of the existing standards implemented
in some workplaces and the lack of thermoregulation in many types of protective equipment that
are commonly fabricated using various types of polymeric materials, thermal stress remains one of
the most frequent physical hazards in many work sectors. However, many of these problems can
be overcome with the use of smart textile technologies that enable intelligent thermoregulation in
personal protective equipment. Being based on conductive and functional polymeric materials, smart
textiles can detect many external stimuli and react to them. Interconnected sensors and actuators
that interact and react to existing risks can provide the wearer with increased safety, protection, and
comfort. Thus, the skills of smart protective equipment can contribute to the reduction of errors and
the number and severity of accidents in the workplace and thus promote improved performance,
efficiency, and productivity. This review provides an overview and opinions of authors on the current
state of knowledge on these types of technologies by reviewing and discussing the state of the art of
commercially available systems and the advances made in previous research works.
Keywords:
thermoregulation; personal protective equipment; smart textiles; performance;
productivity
1. Introduction
1.1. Thermal Stress in the Workplace
Thermal stress is among the most common physical hazards in various work sectors. In
fact, any worker exposed to a high heat load through a combination of his or her metabolic
heat during work, environmental factors (air temperature, humidity, air movement, heat
transfer by radiation), and the clothing requirements of his or her job can suffer health
problems [
1
]. In addition, exposure to extreme temperatures in workplaces involves
physical hazards for workers. Workers in firefighting, construction, mining, smelting
and primary metal processing, metal product manufacturing, forestry, agricultural, food
manufacturing, and police services are among the most exposed sectors to heat-related
hazards. Workers in construction, agriculture, fishing, logging, forestry, and other outdoor
activities are at risk of cold stress [2].
Indeed, the exposure to extreme temperatures can lead the worker to a state of heat
stress, which occurs when the body is unable to maintain its temperature between 36 and
37
◦
C [
3
]. Heat syncope, heat exhaustion, heat stroke, dehydration, heat cramps, miliary
eruptions, hyponatremia, and rhabdomyolysis are among the diseases or health disorders
due to heat exposure. Hypothermia, immersion feet, and frostbite are the most significant
Polymers 2021, 13, 3711. https://doi.org/10.3390/polym13213711 https://www.mdpi.com/journal/polymers
Polymers 2021, 13, 3711 2 of 80
injuries and illnesses caused by exposure to extreme cold [
2
]. Therefore, the prevention of
thermal stress risks should be a priority in order to avoid any negative effects on workers’
health and safety [
4
]. Adequate prevention of heat stress risks not only provides a sense of
comfort for the worker toward his work environment, but it can also have a positive impact
on the productivity rate and result in a decrease in the employer’s number of injuries [5].
In addition to being a direct cause of serious injuries in the workplace, thermal stress
can indirectly lead to accidents and other types of injuries. A poorly acclimated worker may
have reduced performance and alertness and therefore may be at greater risk of accidents
and injuries [
6
–
8
]. One of the main risks indirectly related to working in extreme cold is the
decreased manual function, which can quickly impair task performance and increase the
risk of accidents or intensify a hazardous situation [
9
]. Research has shown that manual
dexterity is impaired during work in cold storage warehouses [
10
]. Cold can also reduce
alertness and impair cognitive performance, increasing the risk of inappropriate mental
actions leading to accidents. Indeed, one study was able to demonstrate that reaction time
and signal detection decreased in workers exposed to a temperature of -20
◦
C for more
than 45 min [11].
Exposure to extreme temperatures can also temporarily reduce work capacity and af-
fect productivity [
2
]. As a result, thermal stress can directly alter operational capacity, both
by decreasing work tolerance and by requiring changes in work schedules, such as longer
rest and recovery breaks [
2
]. For some professions, such as firefighters, the interaction be-
tween high physical exertion and heat is the main cause of death [
12
]. According to studies
conducted in the United States, thermal and physiological stress during interventions is
associated with an increased risk of cardiovascular accidents, which are the most common
cause of death among firefighters [
13
]. In addition to the impact of heat on cardiovascular
behavior, the thermoregulatory mechanisms of the human body under thermal stress and
the physiological changes they imply can alter the functions of several organs related to
the absorption and chemical metabolism. Heat exposure has been shown to be associated
with increased pulmonary and dermal absorption of xenobiotics [14].
The protection of workers against thermal risks becomes even more important since,
according to experts, the current climate change context will contribute to emphasizing
the impact of thermal constraints in the workplace [
15
–
17
]. Over the past few decades,
many research studies related to thermal management have been witnessed, as shown in
Figure 1.
Figure 1.
Published data for personal thermal management from 2000 to 20 June 2020. (
a
) Research articles published
during the last two decades. (
b
) Review articles and book chapters published during the last two decades. Reproduced
with permission [3]. Copyright 2020, Elsevier.
Polymers 2021, 13, 3711 3 of 80
As a result of the importance of preventing the risks of thermal stress, recommenda-
tions and measures have been planned by the authorities. These regulations recommend
redesigning the workstation, reducing the workload, and wearing appropriate personal
protective equipment (PPE) to ensure that thermal stress thresholds are not exceeded.
However, some studies have shown that despite compliance with these regulations, some
workers may be subject to thermophysiological constraints depending on their age, sex,
physical fitness, or state of health [
12
]. Moreover, these types of measures against thermal
stress are sometimes far from being applicable in certain environments such as agricul-
ture [
18
,
19
]. Regulations are sometimes very cautious and sometimes overestimate the level
of thermal stress, while for heavy work in indoor workplaces, they may underestimate
exposure [
20
]. Prevention measures remain unclear and sometimes unrealistic in the face
of reality [21].
1.2. Personal Protective Equipment Design Challenges
In addition to several gaps in the established regulations to counter the risks of thermal
stress in the workplace, PPE can accentuate the impact of thermal stress, as many of these
items of equipment lack comfort [
22
]. PPE is designed primarily to protect workers against
external hazards such as chemical, biological, thermal, and mechanical. Various polymeric
materials are commonly used for the fabrication of PPE [
23
]. For instance, protective
gloves can be made with polymers (nitrile, latex, neoprene, poly(vinylacetate), polyvinyl
chloride (PVC)), with woven or knitted textiles materials (aramid fibers (Kevlar
®
), high-
performance polyethylene (HPPE)), coated or not with polymers, in single or multiple
layers [
24
–
27
]. Depending on the protection required, different synthetic materials can
be used also in the fabrication of protective clothing, such as meta-aramide (Nomex
®
),
para-aramide (Kevlar
®
), polybenzimidazole (PBI), melamine (Basofil
®
), polyphenylene
benzobisoxazole (Zylon
®
), and polyimide for heat and flame hazard, polyurethane (PU),
chlorinated polyethylene (CPE), polytetrafluoroethylene (PTFE), PVC, and polyvinylidene
chloride (PVDC) as impermeable layers and moisture barriers [
26
,
28
], or activated carbon
impregnated foam, fluoro-polymer coatings, polyurethane nonporous membrane, or elas-
tomers for chemical protection [
23
,
29
–
31
]. Being a multidisciplinary field calling for several
technological knowledge, the materials used in the design of protective equipment has been
the subject of several technical reviews. While some of these studies have been devoted to a
global state of the art of materials used and the evolution of associated needs [
23
,
26
,
32
,
33
],
others analyzed specific developments and needs to counter a particular type of risk, for
example, reviews specifically dedicated to advances and applications of materials for chem-
ical protective clothing [
25
,
30
]. Some contemporary research studies have even evoked a
potential application of nanofiber materials in protective clothing. These materials obtained
from nanoparticles mixed with polymer solutions can offer greater breathability, a selective
filtration potential along with an improved liquid chemical and aerosol particle retention
capability compared with current commercially available membranes [25,34,35].
However, the materials used in the design of several types of PPE tend to avoid
the adequate dissipation of body heat [
36
]. Thus, workers such as firefighters or metal
fabricators may be exposed to more thermal and physiological stresses due to their type of
protective equipment [
37
]. As reported by occupational health and safety experts, workers
often find protective equipment uncomfortable, too hot, or too bulky, which does not
encourage them to wear it regularly, thus accentuating potential risk situations [38].
Given the existing shortcomings in the prevention of thermal stress in workplaces
due to conventional conception in the design of protective equipment and the inefficiency
of the established standards and recommendations, it is essential to develop new tools
and equipment to ensure thermal risk management adapted to the individual situation of
the worker and his or her work environment. In such a context, smart textile technologies
integrated into personal protective equipment have a very great potential to respond to
many issues related to thermal risks. Thus, using them in the development of PPE presents
great potential for the field of occupational health and safety [22,39–41].
Polymers 2021, 13, 3711 4 of 80
Being based on textronic (e-textiles), conductive textiles, functional textiles, and flexi-
ble and extensible electronics, smart textiles can contribute to the development of thermal
regulation systems [
42
,
43
] to better protect workers against the risks of thermal stress
while offering them greater comfort. They can also be used in the development of tools for
measuring external and internal garment temperatures, as well as body temperature [
39
,
40
].
In addition to being the basic textile material, polymeric materials are also widely used
in the production of smart textiles whether in the design of sensors or actuators, their
methods of integration into textiles, conductive yarns fabrication, conductive polymers
coating, functional coating, or embedding conductive fillers [44–46].
Recent technical reviews often report on knowledge in the area of smart textiles
[44,45,47]
,
including a number of studies that mention their potential use in PPE design [
40
,
48
,
49
].
Although some other studies have made reviews of the heat stress state in conventional
PPE [
50
,
51
], to our knowledge, no reviews are specifically related to the analysis of smart
textile technologies for the prevention of thermal stress risks while wearing PPE. In fact,
despite the studies that have separately reviewed heating, cooling, or thermal sensor
technologies integrated into clothing [
44
,
45
,
48
], no study exists on a complete analysis of
all the technologies that facilitate intelligent thermal management in PPE. Furthermore,
the continuous evolution of smart textile technologies in an increasingly connected world,
both at the societal and industrial levels, requires an update of knowledge to better support
the adaptation of such technologies to occupational health and safety applications.
In spite of the recent technological progress, a preliminary analysis has shown that
most of the current commercial solutions are dedicated to the fields of sport and leisure,
and very few are related to occupational protective equipment [
52
,
53
]. Indeed, heating
systems integrated into different types of clothing and accessories have emerged in recent
years [
54
]. However, these systems suffer from a lack of comfort and are difficult to use in
a work context.
While some integrated systems have presented risks of overheating [
55
], others suffer
from a lack of temperature control [
56
]. Integrated cooling systems are usually based on
passive devices composed of multilayer structures or functional coatings, which limits their
reactivity to temperature variations [
57
]. Moreover, active integrated cooling systems re-
main cumbersome and energy consuming [
43
] and sometimes not very efficient in extreme
climatic conditions [
58
]. The development of self-regulating temperature systems using
functional materials of phase change materials types [
59
,
60
] has attracted the attention of
many research groups [
59
]. However, these materials in their current state remain limited
by their overall enthalpy of phase change or thermal window. They are active during
their phase change period but cease to function when the phase change is completed [
61
].
Despite the emergence of commercial products incorporating smart textile technologies,
garments with integrated sensors capable of detecting thermal stressors in order to mitigate
the risk of contact and prolonged exposure to extreme temperatures in workplaces are also
rare. Although isolated cases have been developed for some trades in a few countries [
62
],
most work remains limited to research [63].
Using the potential of advanced materials both in the design of conductive textiles
and in the development of thermal sensors and actuators to be integrated in protective
equipment can provide a reliable solution to fill current lacks in the design of intelligent
thermal management tools in the context of occupational health and safety. Therefore,
the present study aims to present a review of current knowledge of these technologies
facilitating smart thermoregulation in personal protective equipment.
2. Temperature Sensor
This part of the study focuses on systems that provide data on the body temperature
of an active person. It also discusses the sensors that can be integrated into PPE in order
to facilitate the acquisition of the temperature of the microclimate under the clothing or
the outside temperature with the objective of warning the worker in case of prolonged
exposure to extreme temperatures.
Polymers 2021, 13, 3711 5 of 80
Real-time monitoring of body temperature is very important in order to prevent in time
the occurrence of disorder in many organs during exposure to high thermal stress [
64
]. The
calculation of body temperature is commonly based on the measurements of the core body
temperature (T
c
) and the skin temperature (T
s
). While T
c
is adjusted by thermoregulatory
mechanisms of the body, T
s
is affected by blood circulation and is related to heart rate (HR)
and metabolic rate [
64
]. Therefore, temperature sensors used for body temperatures (T
s
and T
c
) must operate efficiently over a temperature range of 35 to 40
◦
C and ideally offer a
measurement accuracy of 0.1
◦
C [65].
2.1. Methods to Measure Body Temperature
Various types of analog electrical sensors have been deployed in recent years to
measure body temperature (T
s
and T
c
). These sensors are generally based on thermis-
tors, resistance temperature detectors (RTDs) [
66
] (Figure 2a–e), or thermocouples [
64
]
(Figure 2f,g).
Figure 2.
Temperature sensors: (
a
) Concept of the flexible temperature sensor embedded within the fibers of a textile yarn;
(
b
) Bending of the uncovered flexible resistance temperature detectors RTD; (
c
) RTD Close-up sensing area. (
d
) Resistance
temperature detectors embedded within a braided polyester yarn; (
e
) Cross-section of the braided temperature-sensing yarn
((
a
–
e
) [
7
]); (
f
) Lightweight and flexible conductor materials in a thermocouple array with copper-coated cellulose textiles [
8
];
(
g
) A cross-sectional schematic of encapsulation for a thermistor within a yarn. The standard encapsulation is composed of
three layers: a polymer resin, packing fibers, and a knitted sheath [9].
Rectal thermometry is the most accurate method for measuring body temperature,
and its value is recognized as the most representative of core body temperature [
64
]. It
has been widely used as the standard measurement in many heat stress studies, including
work on the development of heat stress indices [
67
–
70
]. However, rectal thermometry is an
intrusive method that requires private arrangements and is therefore unsuitable for the
continuous monitoring of workers with high physical activity [
65
]. Although heart rate
can be used for indirect inference of T
c
[
71
,
72
], some other studies have also proposed an
estimation of T
c
from T
s
[73,74].
Thus, in order to contribute to the protection of individuals against thermal aggressors,
the scientific community has been interested in the development of temperature sensors
that can be integrated into personal protective equipment [
75
]. These sensors could measure
Polymers 2021, 13, 3711 6 of 80
T
s
and monitor the microclimate temperature between the body and the clothing or the
outside temperature during exposure to thermal aggressors. While much work has been
dedicated to the development of temperature sensors based on smart textile technologies
and flexible electronics, a very limited number of studies have been devoted to the systems
integrated into clothing.
In fact, the main motivation for the development of textile or flexible sensors has
been to overcome the obstacles that hinder portable temperature detection despite the
progress made [
76
]. Most thermistors or thermocouples used in wearable technologies [
77
]
are sensitive to deformation, which can impair temperature sensing with bending or
twisting of the sensor [
76
]. To counter the strain dependence of this type of sensor, some
researchers have proposed a hybrid approach based on the integration of a small rigid
thermistor embedded in a flexible and extensible matrix [
78
]. In one of these selected works,
an NTC-type thermistor (having a negative temperature coefficient) in association with
conductive textile threads was integrated in a bamboo belt to monitor the body temperature
of newborns. Despite an encouraging detection accuracy of 0.1
◦
C of the prototype tested
in a hospital setting, the concept lacked mechanical strength due to the use of knots to
ensure the connection between the sensor and the signal-transmitting conductive textile
threads [
79
]. In more recent work, the aspect of mechanical strength could be improved by
encapsulating a standard thermistor in a polymer resin microcapsule, then embedding it in
the fibers of a yarn, and then incorporating it into a textile structure [
78
,
80
–
83
]. As part of
this work, ongoing optimizations have been made, including encapsulating the commercial
thermistor in a microcapsule of thermally conductive resin to improve the sensitivity of the
sensor [
82
] or connecting the sensor leads to a microcontroller and a Bluetooth module for
wireless transmission of the collected data [
78
,
80
]. However, the proposed concepts still
require further optimization, particularly in terms of detection accuracy, as differences of
0.5 to 1
◦
C were observed between the reading and the actual temperature of the sample
surfaces [80,82].
Temperature sensors can also be manufactured from textile materials composed of
conductive fibers or yarns using conventional textile manufacturing technologies such
as weaving, knitting, or embroidery [
65
]. Depending on their operating principles, these
types of sensors can be classified as thermocouples or RTD-type detectors [84].
Textile thermocouples
: They exploit the Seebeck effect, which is based on the devel-
opment of a corresponding potential difference between the junctions of two different
metal structures due to the temperature difference between the junctions [
65
]. Struc-
tures with textile electrode pairs consisting of graphite fiber/antistatic fibers, non-woven
graphite/silver-coated yarns, or hybrid knitted steel/alloy constantan wire composition
have been used to design textile thermocouples [
85
,
86
]. However, these thermocouples
exhibit a non-linear relationship between potential change and temperature and are charac-
terized by low accuracy and sensitivity compared to conventional wire thermocouples [
65
].
In addition, they are also sensitive to changes in environmental relative humidity [86].
Textile RTDs
: They use the temperature dependence of materials with electrical
resistivity to determine temperature. These sensors can be developed by incorporating
wires or conductors with a high temperature resistance coefficient into the fabric [65].
Therefore, fibrous sensors of RTD types could be developed by inserting metal wires
(copper, nickel, and tungsten) in a knitted structure [87], by integrating metallic filaments
in the middle of a double-knitted structure with different densities of metallic wire incorpo-
ration [88], by using cotton yarns coated with a PEDOT-PSS conductive polymer solution
and a polystyrene encapsulation layer embeddable in a textile structure by weaving or
stitching [
89
], by embroidering chromium–nickel austenitic stainless steel threads on a
textile substrate [
90
], or by embroidering a hybrid thread composed of polyester fibers
and a stainless steel micro thread on a fabric [
91
], which could be inserted in the outer
layer of firefighters’ clothing [
92
]. This last work was able to demonstrate that textile RTDs
offer increased accuracy and sensitivity, shorter response time, and better linearity with
temperature compared to thermocouples [
65
]. However, these sensors could not provide
Polymers 2021, 13, 3711 7 of 80
localized temperature measurements, as the measurement is instead performed over the
entire area of the textile [78,92].
Some studies, on the other hand, have reported an optical sensing approach for mea-
suring body temperature by integrating optical fibers into the textile structure [
93
]. As a
result, a distributed Bragg reflector with the ability to reflect light of specific wavelengths
and transmit it to other wavelengths has been used [
94
]. The Bragg reflector was encap-
sulated with a polymeric substance and then woven into the fabric structure [
95
]. The
authors have also analyzed mathematically the transmission of heat from the skin to the
environment via the Bragg reflector and used a weighted coefficient model to estimate body
temperature considering the wavelength shift as a function of temperature. They have
also reported a high accuracy of
±
0.18
◦
C in a range of 33 to 42
◦
C [
95
]. A new method of
integrating optical fibers constituting a Bragg reflector into a hollow double-walled fabric
structure has also been proposed in a recent study [
96
]. Despite the high accuracy provided
by Bragg reflectors, the concept is far from being applicable to the design of a wearable
device, as it requires connection to at least one amplified broad-spectrum light source and
an optical spectrum analyzer [
96
]. The design of a textile heat flux sensor has also been
proposed by investigating a method of inserting a constantan yarn into three different
textile structures (polyamide-based knitted fabric, non-woven aramid, and aramid-based
woven fabric), which is followed by several treatment and post-treatment steps including
the electrochemical deposition of copper on the constantan yarn to obtain a thermoelectric
yarn [
97
]. Figure 3 shows some examples of integrated flexible sensors in textiles and yarns.
Figure 3.
Thermal detection of smart textiles. (
a
) Illustration of spatiotemporal sensor mapping of the body with temperature
and accelerometer (heart beat and respiration); (
b
) Wearable textile with embedding stretchable–flexible electronic strips;
(
c
) Exploded view of a sensor island. Reproduced with permission [
98
]. Copyright
©
2021, Wicaksono et al. (
d
) Health
monitoring textile with temperature-sensing yarns; (e) A schematic of the textile thermograph (d,e) [78].
Polymers 2021, 13, 3711 8 of 80
2.2. Flexible Temperature Sensors
Although these studies are still at a very preliminary stage, some research groups have
attempted to develop shape memory textile sensors. The concept is based on the use of
shape memory polymers sensitive to external stimuli such as light or temperature. Recently,
the innovation of sol gels, conductive polymers, and copolymers as biomaterials enabled
the miniaturization of biological analyses in an integrated chip with new generation sensors
using a Si light source with a wide visible wavelength range as an optical biosensor [99].
Temperature sensing functionality can be obtained by spinning shape memory poly-
mer fibers, such as polyurethane fibers, with other types of fibers to make textile fabrics, or
by coating shape memory polymer emulsions on a woven or knitted fabric [
100
]. Other con-
figurations of shape memory materials applicable to fabrics include silicon [
101
], nanofibers,
and shape memory foams. In order to facilitate the characterization of the thermal sensitiv-
ity of textile shape memory sensors, a shape memory coefficient based on the change of
deformation angle with temperature variation was suggested [102].
Many researchers have also worked on the development of flexible temperature
sensors with the deposition of materials that facilitate temperature detection on flexible
polymeric substrates using printing, coating, and lamination techniques [65] (Figure 4).
Figure 4.
Schematic illustration of flexible sensors materials. Clockwise from the right top: polyimide
(PI) [
103
], polyurethane (PU) [
104
], pectin [
105
], silk [
106
], cellulose [
107
], paper [
108
], ecoflex [
109
],
polydimethylsiloxane (PDMS) [110].
If they maintain their mechanical strength, these types of sensors can then be at-
tached to fabrics or integrated into textile structures [
100
]. In this context, several studies
investigated the development of flexible temperature sensors based on graphene as a
highly conductive material from an electrical and thermal point of view [
111
,
112
]. There-
fore, electrical resistance temperature-sensing layers have been developed by printing
a graphene oxide formulation on polyimide and polyethylene terephthalate substrates,
which is followed by infrared firing to obtain a material with a negative temperature
coefficient [
113
]. A layer with an RTD property having a positive temperature coefficient
(PTC) was also developed by deploying the plasma-assisted chemical vapor deposition
method of graphene nanosheets on a polydimethylsiloxane (PDMS) substrate [
114
]. In
addition, a stretchable thermistor was designed by integrating a graphene-based dispersion
in a PDMS-based matrix as a detection channel, which was associated with electrodes
formed from silver nanofilaments in polycarbonate membranes [
111
]. Thanks to the use
of graphene, temperature sensitivities very close to those of metal oxide materials used
Polymers 2021, 13, 3711 9 of 80
in classical sensors have been obtained in a flexible structure [
113
]. However, the stretch-
able structure based on graphene has shown strong variations in its thermal behavior as
a function of mechanical deformation [
114
], which may constitute a limitation for their
integration in textile structures.
Printing techniques were also used to design flexible temperature sensors [
115
]. The
most notable works include the screen printing of a carbon-based ink on a polyimide sheet
to obtain a PTC thermistor-type structure [
43
], the screen printing of various resistive
inks on polyethylene naphthalene being protected by a passivation layer of dielectric
ink and plasma post-treatment to improve the temperature resistance coefficient of the
printed layer [
116
], the ink-jet printing of a dispersion based on nanoparticles of nickel
oxide in the space between two silver-printed electrodes using a polyimide substrate to
develop an NTC thermistor [
117
], a 100
×
100 pixel array all-CMOS (Complementary
metal–oxide–semiconductor) monolithic microdisplay system has proven possible to create
a high-optical power efficiency all-CMOS microdisplay [
118
], and the ink-jet printing of a
silver complex dispersion on a polyimide substrate to obtain a layer with PTC thermistor
behavior [
119
]. Overall, the printed thermosensitive structures were able to offer high
temperature sensitivity, while having very low hysteresis during heating and cooling
cycles [
116
,
117
,
119
]. Screen printing of PEDOT-PSS conductive polymer and carbon nan-
otubes dispersion on polyimide substrates and the use of silver-based printed electrodes
has also allowed the development of RTD layers. Then, the printed RTD layers were
combined with radio signal transmittances to design a label [
120
] or bandage [
121
] to
be placed on an individual’s skin to communicate with an external reader device [
120
].
Printed temperature sensors have also been developed on paper substrates [
122
,
123
]. In
their current state, these types of development are rather intended for the packaging field
and require work to reformulate the inks used to make them compatible with non-porous
polymeric substrates with surface properties different from those of paper [64].
The formation of composite layers on flexible substrates has also been another method
for the design of flexible temperature sensors. In this register, a composite film with RTD
properties could be obtained by coating a mixture of poly o-methylaniline and manganese
oxide (Mn
3
O
4
) on a solid substrate [
124
]. In addition, a composite film based on tellurium
nanofilaments in a poly-3-hexylthiophene matrix deposited on a flexible substrate was
used to obtain RTD behavior [
125
]. The deposition of graphite particles dispersed in a
PDMS matrix on inter-digitalized copper electrodes prefabricated on a polyimide substrate
was also deployed to obtain a composite film demonstrating RTD properties [
126
]. The
dispersion of multiwall carbon nanotubes in a toluene solution of polystyrene–ethylene–
butylene–styrene (SEBS) deposited on gold electrodes fabricated on a polyimide substrate
resulted in a composite film showing NTC-type thermoelectric characteristic of a sensi-
tivity comparable to the highest values for metals [
127
]. In a similar study, a mixture
of multiwall carbon nanotubes and a polyvinyl benzyl derivative with trimethylamine
coated on a pair of gold electrodes fabricated on a polyimide film led to the formation of a
composite film with RTD behavior and a sensitivity comparable to that of metals [
128
]. The
combination of a binary composite film of polyethylene and polyethylene oxide loaded
with nickel microparticles with a passive RFID antenna has led to the design of a portable
RTD temperature sensor. Despite the portability of this prototype sensor, it had three
times the sensitivity of similar commercial sensors and a significant measurement error of
±
2.7
◦
C [
85
]. In this framework, an array of 16 RTD-type temperature sensors was also
fabricated with narrow serpentine gold traces using a microlithography technique on thin
layers of polyimide to design an electronic skin to be fixed to the skin by the action of Van
der Waals forces [129].
2.3. Radio-Frequency Identification (RFID)
As part of the development of flexible temperature sensors, other work has opted
for radio-frequency identification (RFID) tags to be placed on the skin to measure T
s
.
For example, these studies have contributed to the development of a passive ultra-high
Polymers 2021, 13, 3711 10 of 80
frequency (UHF) RFID tag, which is based on the temperature dependence of the ring
oscillator frequency and allows data to be sent to a reader at 868 MHz with a range of
2 m [
130
]. Similar work has developed a flexible RFID tag comprising a commercial
microchip providing direct thermal reading and an antenna designed with copper adhesive
transferred onto a polycaprolactone membrane to be attached to the individual’s arm or
abdomen with hypoallergenic cosmetic glue. The label allowed the data collected to be
sent in a band of 780–950 MHz and a range of 30–80 cm to a nearby reading device [
131
].
According to the analyses of this study, the label placed on the skin requires that the label
itself does not alter the locally measured T
s
and must allow the natural perspiration of
the skin to be preserved [
131
]. In a similar work, a modular patch with two detachable
components, including a reusable inner part housing electronic element (the antenna,
the integrated circuit, and the battery) and a disposable cover encapsulating the sensor
associated with a medical-grade adhesive ensuring adhesion to the skin surface, made
it possible to develop a real-time epidermal temperature sensor using UHF-type RFID
communication [
132
]. In addition to a deviation of 0.6
◦
C from reference measurement
methods, the influence of human variability and environmental conditions on the sensitivity
of this sensor remains to be clarified [132].
Advanced materials have also been applied to the optimization of certain types of
portable devices such as portable in-ear devices, which is a new technological trend in
recent years to measure body temperature and other physiological parameters through
sensors that hold. A dispersion based on graphene, as a highly conductive material known
for its strong optical absorption in the infrared range, has been coated on the silicon
substrate of the lens of IR thermopiles used in portable in-ear devices with the aim of
increasing the accuracy of measurements in such a thermopile [133].
2.4. Textile Prototypes with Flexible Temperature Sensors
The overall analysis of the research on temperature sensors integrated in textile
structures, textile sensors, and flexible temperature sensors has shown that the vast majority
of these studies remain at the level of proof-of-concept of components that are still to be
integrated in clothing, although some work is dedicated to temperature sensors integrated
in work clothing. In one of these studies, the ambient temperature and heat flux through
the garment could be measured by a modified PTC grade sensor network integrated in
the outer and inner side of the firefighters’ protective clothing with the transmission of
the collected data to an external reader device using the Zigbee communication protocol.
The prototype, tested on a thermal manikin in the laboratory, had yet to be validated in
an operational environment [
134
]. A work jacket for oil workers operating in extreme
cold was also developed using an embedded IR temperature sensor and two combined
humidity/temperature sensors. The jacket consisted of one humidity/temperature sensor
on the outside of the jacket, a second pair of sensors placed on the opposite side of the jacket
on the inner side, and the IR sensor, which was integrated on the inside of the sleeve for
non-contact measurements of T
s
at the wrist [
135
]. This jacket equipped with temperature
sensors could be optimized by, among other things, placing a layer of heat-reflecting film
in the lining of the jacket on the inside to reduce the influence of the person’s heat on the
outside temperature measurements and adding a layer of elastomeric material around the
outside sensor to reduce the heat flow through the jacket in the vicinity of the sensor [
136
].
A smart glove and an armband each comprising two electrodes made of conductive
textiles to measure the galvanic skin response and a sensor from a commercial digital
thermometer detecting T
s
were developed to assess the conditions of soldiers in real time.
Both were tested on about 40 subjects, but the assembly remained cumbersome, and the
main signal transmission lines were fabricated with electrical wires that could be damaged
during use or maintenance [
137
]. A thermistor microencapsulated in a wire [
78
,
82
] has
been integrated into a cuff, glove, and sock for measuring T
s
[
138
]. The cuff contained four
wires each with a thermistor, while the glove and sock were based on a set of five wires
each containing a thermistor. The contact pressure on the hands was found to influence the
Polymers 2021, 13, 3711 11 of 80
measurements due to the deformation of the sensor wire structure in the glove. In addition,
the fit of the sock can also affect the measurements, as can the wearing of a shoe or walking,
which appear to strongly influence the temperature measurements. These measurement
errors seem to show that monitoring the foot skin temperature by sensors integrated in the
textiles could be challenging for applications where accurate measurements are required.
According to this study, fabrics containing sensor yarns should be manufactured according
to the contact pressure exerted at the temperature measurement emplacement [138].
2.5. Commercial Textile with Temperature Sensors
Due to the need to monitor patient health or athlete performance, more and more
portable products with temperature sensors have appeared on the market in recent years.
Some integrate temperature sensors into their structure and others are based on the deploy-
ment of advanced materials. Among the commercial devices for biometric sign detection
in the form of portable accessories in recent years, Biofusion (by Biopeak, Ottawa, Canada)
and QardioCore (by Qardio, San Francisco, US) offer integrated systems that use contact
RTD-type temperature sensors to measure T
s
from the chest.
Based on printable electronics techniques, flexible temperature sensors have also been
produced and have entered the market to serve areas such as transportation, logistics, food
supply chain, and home appliances. Thanks to their flexibility, their integration into textile
structures seems conceivable. However, their adaptation to textile structures still requires
a certain number of technical challenges to be taken up, especially in terms of durability
in wear or maintenance, especially in washing [
139
]. These types of flexible sensors such
as those proposed by the company PST sensors (Cape Town, South Africa) are mainly
printed thermistors associated with an electronic chip. The conductive ink used in these
types of development is based on a composition that, once printed, demonstrates RTD
properties [113,120].
Then, circuits containing these types of printed thermistors can be combined in a
hybrid system with wireless data transmission protocols [
140
]. According to the manufac-
turers of these types of flexible thermistors, the sensors developed provide measurement
accuracy ranging from
±
0.1 to
±
0.25
◦
C. While providing a very low response time of 100
to 250 ms, these flexible temperature sensors have the advantage of operating with low
working powers in the nano or micro watt range. Graphene conductive layers with RTD
characteristics, demonstrating a very high sensitivity to temperature changes [
111
], have
recently been successfully used in the design of a connected insole based on an integrated
thermistor to continuously monitor temperature changes in patients’ feet and detect early
signals of foot ulcers in diabetics (Smart Insole by Flextrapower, New York City, NY, USA).
These types of products for the medical field may be of interest for knowledge transfer
toward an occupational health and safety application.
Regarding products marketed in the form of temperature sensors integrated into
clothing, a very limited number of products exist on the market. These products were
mainly developed to help protect firefighters [
39
]. In this context, the companies Ohmatex
(Aarhus, Denmark) and Viking (Esbjerg, Denmark) jointly presented a firefighter suit
containing thermal sensors integrated inside and outside the firefighter’s clothing to
monitor environmental and near-body heat, respectively. The sensors are connected to LED
displays on the sleeve and shoulder of the jacket. Above a certain temperature threshold
detected on the outside or inside the jacket, the flashing of the display alerts the user.
Despite the presence of an integrated electronic device, this garment had the advantage of
withstanding at least 25 wash cycles.
The Balsan fire jacket (by TeckniSolar Seni, Saint-Malo, France) was also equipped
with temperature and humidity sensors. A temperature sensor on the outside of the jacket
measures the environmental temperature and a pair of temperature/humidity sensors
on the inside of the jacket measures microclimatic conditions close to the body. When
parameters exceed a certain level, an audible and visual alarm alerts the firefighter [
62
,
141
].
Polymers 2021, 13, 3711 12 of 80
2.6. Apparels Measuring Thermal Stress
The review of research literature for measuring body temperature tools and sensors
that can be integrated into protective equipment to assess the microclimate under the
clothing or the environmental temperature in order to develop warnings in case of very
high thermal stress are presented in Table 1.
Table 1. Temperature sensors to be integrated into textile apparels.
Technology Used Integration Method Operating Temperature Range Reference
Temperature-sensing yarns
incorporated in a knitted
fabric
An off-the-shelf thermistor encapsulated
into a polymer resin Multi-Cure
®
9-20801
(Dymax Inc.) micro-pod embedded
within the fibers of a polyester yarn
Physiologically relevant
temperature range of 25–38
◦
C
[78]
Electronic temperature
sensing yarn
Knitted polyester-based armband
demonstrator using a polyester yarn with
embedded thermistor encapsulated into a
polymer resin Multi-Cure
®
9-20801
(Dymax Inc.) and connected to an
Arduino Pro Min Hardware
Tested to measure the temperature
of a hot object of 65
◦
C
[83]
Yarn with embedded
thermistor
NTC thermistor soldered to copper
interconnects and encapsulated with a
cylindrical micro-pod made of
conductive resin (Multi-Cure
®
9-20801 by
Dymax Inc.), then embedded in a
polyester yarn
Tested in a range of 0 to 40
◦
C [82]
Yarn with embedded
thermistor
A commercial temperature-sensing
element within a polymeric resin
micro-pod embedded in the fibers of a
polyester yarn
Tested in a range of heating-cooling
cycle of 25–38
◦
C
[81]
Yarn with embedded
thermistor
Commercially available NTC thermistor
encapsulated in a polymer micro-pod
made of UV curable resin (Multi-Cure
®
9001-E-V-3.5 by Dymax Inc.) embedded
into the fibers of a thermoplastic
monofilament yarn spun from liquid
crystal polymer (Vectran
TM
)
NTC sensitive to 25–38
◦
C [80]
Thermistor integrated into
textiles
Embedded NTC thermistor and
conductive textile yarns (Shieldex
®
silver
plated polyamide) in a belt made of soft
bamboo yarns
25 to 43
◦
C [79]
Embroidered hybrid resistive
thread (RTD)
(1) Hybrid thread composed of three
strands. Each strand contains 33
polyester fibers; only one includes one
resistive stainless steel microwire, (2) The
surface of the hybrid thread is covered by
a silicone lubricant, (3) The sensor is
embroidered in a helical meander-shaped
structure into the carrier fabric made of
KERMEL
®
, Lenzing
TM
FR, Technora, and
antistatic fibers
Temperature calibration (40 to
120
◦
C); rapid temperature cycling
(−40 to 125
◦
C)
[92]
Embroidered resistance
temperature detector (RTD)
Conductive silver R.STAT
®
yarn as
humidity and chromium–nickel
austenitic stainless steel yarn as thermal
sensors embroidered on a cotton
substrate
Validated for 20
◦
C to 100
◦
C and 50
to 98% of RH
[90]
Polymers 2021, 13, 3711 13 of 80
Table 1. Cont.
Technology Used Integration Method Operating Temperature Range Reference
Temperature-sensing knitted
resistance temperature
detector (RTD)
Metal wire inlaid in the middle of a rib
knitted structure of polyester fabric
Validated at 20–50
◦
C [87]
Dip dyed yarn by
PEDOT-PSS as RTD
RTD yarns fabricated by: (1) Dip dyeing
cotton yarns in PEDOT-PSS solution, (2)
Applying a silver paste applied at the
two ends of the dyed threads to form
electrical pads, (3) Creating
encapsulation layer by dip dyeing the
yarns in polystyrene to better protect
against dust and moisture
Validated for −50 to 80
◦
C [89]
Metal wires incorporated in a
knitted fabric (RTD)
Knitted temperature-sensing fabric
developed with two different wire inlay
densities and a fine metallic filament
embedded within the courses of a
double-layer knitted structure made of
poly acrylic/wool yarns
Validated at 20–60
◦
C [88]
Flexible platinum-based
resistance temperature
detector (RTD) integrated into
textile
Sensors manufactured by electron beam
evaporation followed by
photolithography on Kapton
®
polyimide
foils, then cutting the foil into stripes
each containing an individual sensor and
connecting lines, which are then inserted
into a fabric during the weaving process
Validated for 25 to 90
◦
C [142]
Optical fiber Bragg grating
(FBG) based sensor integrated
into textile
Encapsulating the optical fiber with
polymeric (copolymerization of
unsaturated methyl ethyl ketone
peroxide (MEKP) and cobalt
naphthenate) filled strips, then
embedding it into the fabric by
combining large and small pipes together
in fabrication
Validated for body temperature
ranging from 33 to 42
◦
C
[95]
Optical fiber Bragg grating
(FBG)-based sensor integrated
into textile
A textile structure of hollow double-wall
fabric was adopted as a base, and
quasi-distributed FBG sensors were
embedded by the methods of cross-walls
and between-walls for smart fabric
sensor development
Validated in a T
env
range of 20 to
130
◦
C with 10
◦
C steps and then
decrease back to 20
◦
C with the
same procedure
[96]
Textile thermocouple
Four different textile thermocouples:
(1) Flat textile composed of pairs of textile
electrodes: graphite non-woven—woven
fabric with nirtil static fibers, (2) Linear
textiles composed of pairs of textile
electrodes: thread of Nitinol—static
fibers—thread of steel fibers, (3) Flat
linear thermocouple manufactured from
pairs of electrodes: graphite
nonwoven—silver-covered polyamide
yarn, (4) Hybrid thermocouple composed
of pairs of electrodes: steel knitted
fabric—constantan wire
Validated for temperatures up to
70
◦
C and 90
◦
C
[86]
Polymers 2021, 13, 3711 14 of 80
Table 1. Cont.
Technology Used Integration Method Operating Temperature Range Reference
Thermocouple
(1) T
s
measured by a thermocouple
placed at the armpit with an elastic belt
made of spandex, (2) T
env
and the heat
flux through the garment measured by
modified platinum sensor array
integrated into the outer garment of
firefighters, (3) Sensors associated to a
planar textile-based antenna made of
conductive yarns
Heat flux sensor is able to operate
in the range of −70 to +500
◦
C
[62]
Textile heat flow sensor
Insertion of a constantan wire within
three different textile structures
(polyamide-based knit, aramid
non-woven, woven aramid-based),
followed by a local treatment with
polymeric resin to allow the partial
copper deposition, then an
electrochemical deposition of copper on
the constantan wire to obtain a
thermo-electrical wire and finally a
post-treatment for polymer removal
Tested in a range of 30 and 80
◦
C
and 0 to 150% moisture content
[97]
Sensorized glove/upper-arm
strap
(1) A glove with two textile electrodes
integrated inside in the proximal phalanx
of the index and middle fingers on the
inside of the glove and a temperature
sensor placed in the tip of the ring finger
of the glove, (2) Upper arm strap
confectioned with two integrated textile
electrodes and a temperature sensor
placed in the inner lining of the strap
Validated for T
s
measurements
averaging 34
◦
C
[137]
Platinum sensor integrated
into a jacket
Modified platinum sensor array (welded
on Kapton
®
polyimide foil) integrated
into the outer firefighting garment
(composed of external impermeable,
thermal insulation Gore-Tex
®
PTFE
membrane, and internal comfort layers)
to measure T
env
and the heat flux
through the jacket
Able to operate in the range of −70
to +500
◦
C
[134]
Working jacket with
integrated sensors
Sensors and wireless communication
integrated into a commercialized
Wenaas
®
working jacket, while packing
sensors on the textile by vacuum molding
using biocompatible silicon, and wiring
external sensors to the main sensor
module by conductive yarns also coated
with silicon after vacuum molding
Verified in a climatic chamber −20
to 25
◦
C with RH 0% to 50%
[136]
Working jacket with
integrated sensors
Infrared temperature sensor and two
combined humidity–temperature sensors
integrated into the jacket in three
different areas, using two different
packages: (1) sensor enclosed into a
pouch made from Gore-Tex Paclite
®
PTFE membrane, and (2) only the
opening of the sensor covered with
membrane made form Gore-Tex Paclite
®
Validated at 22
◦
C and −5
◦
C [135]
Polymers 2021, 13, 3711 15 of 80
Table 1. Cont.
Technology Used Integration Method Operating Temperature Range Reference
Firefighting clothing with
integrated sensors
A firefighting garment with three main
integrated components: physiological
sensors (including the body temperature),
fire-related sensors (including field
temperature), and the computing node
N/A [63]
Sailing garment with
integrated sensors
The electronic system is consisted of a
master system and a slave system placed
inside a waterproof pocket above the cuff
of a waterproof sailing top garment made
of coated and laminated woven fabrics
N/A [143]
Thermosensing armband,
glove, and sock based on yarn
with embedded thermistor
Temperature-sensing garments (armband
and glove made of polyamide/spandex,
sock made of cotton) containing
thermistor soldered to copper
interconnects and encapsulated with a
cylindrical micro-pod made of
conductive resin (Multi-Cure
®
9-20801 by
Dymax Inc.)
Tested at 23
◦
C and validated for T
s
ranging from 28 to 33
◦
C
[138]
Printed polymeric PTC and
NTC thermistors
Carbon-based paste screen printed on
Kapton
®
polyimide foil
Validated at a range of 30 to 42
◦
C [43]
Printed polymeric PTC and
NTC thermistors
Resistive inks screen printed on
polyethylene naphthalate and protected
by a dielectric ink (CYTOP-like
fluro-polymer) as a passivation layer,
followed by a plasma post-treatment
Validated at a range of 20 to 90
◦
C [116]
Printed polymeric NiO based
NTC thermistor
Stable NiO ink (suspended in ethylene
glycol aqueous solution) inkjet-printed in
between two silver conductive electrodes
on a polyimide substrate, then thermally
cured at 200
◦
C for an hour
Validated at a range of 25 to 250
◦
C [117]
Printed resistance
temperature detector (RTD)
Silver complex ink inkjet printed on
Kapton
®
polyimide foil
Validated at a range of 20 to 60
◦
C [119]
Printed smart bandage
Temperature sensor fabricated by
PEDOT-PSS/CNT paste screen-printed
on a nm-thick-SiO 2-coated Kapton
®
polyimide, then cured at 100
◦
C for
10 min
Validated for 22 to 48
◦
C (normal T
s
≈ 29 to 31
◦
C)
[121]
Printed wearable resistance
temperature detector (RTD)
Shadow mask printing of
PEDOT-PSS/CNT suspension on
SiO
2
-coated Kapton
®
polyimide
substrate and silver electrodes by screen
printing
Validated at a range of 22 to 50
◦
C [120]
Printed paper-based thermal
sensor
(1) Ionic liquid, 1-ethyl-3-methyl
imidazolium bis (trifluoromethylsulfonyl)
imide ([EMIm][Tf2N]), inkjet printed on
a regular paper, (2) Two gold electrodes
deposited on the paper substrate through
magnetic sputtering evaporation setup
Thermal responses validated at 25
and 45
◦
C
[123]
Printed resistance
temperature detector (RTD)
on paper
Silver nanoparticle ink inkjet printed on
specific coated paper substrate
Validated at a range of
−
20 to 60
◦
C
[103]
Polymers 2021, 13, 3711 16 of 80
Table 1. Cont.
Technology Used Integration Method Operating Temperature Range Reference
Stretchable graphene-based
resistance temperature
detector (RTD)
(1) Silver nanowire first filtered as
electrodes using polycarbonate filter
membranes, (2) Graphene/nanocellulose
dispersion then filtered as the detection
channel to connect electrodes, (3) PDMS
base and curer poured on top of the
filtered films, then degassed and cured,
(4) Solidified PDMS with embedded
silver electrodes and graphene detection
channels peeled off from the
polycarbonate membrane to obtain a
stretchable device
Validated at a range of 30–100
◦
C [111]
Graphene-based wearable
resistance temperature
detector (RTD)
Graphene nanowalls deposited on a
polydimethylsiloxane substrate with
plasma-enhanced chemical vapor
deposition technique and
polymer-assisted transfer method,
associated to silver paste electrodes
Validated at 35 to 45
◦
C [114]
Flexible graphene-based
resistance temperature
detector (RTD)
Graphene oxide-based formulation
printed on Kapton
®
polyimide and
polyethylene terephthalate substrates
reduced by infrared heat lamp and then
annealed at 200
◦
C
Validated in a range of 30 to 180
◦
C [113]
Flexible composite-based
resistance temperature
detector (RTD)
Ni microparticle-filled binary polymer of
polyethylene and polyethylene oxide
composites with copper tape strips-based
RFID antenna
Validated at a range of 35 to 42
◦
C [85]
Flexible composite-based
resistance temperature
detector (RTD)
HCl doped poly-o-methyl
aniline/Mn
3
O
4
nanocomposite spin
coated on glass substrate
RT characteristics in the
temperature range of 35–185
◦
C
with repeatability in the range of
75–185
◦
C
[124]
Flexible composite-based
resistance temperature
detector (RTD)
Dispersions of multiwall CNT drop
casted onto gold electrodes fabricated on
a polyimide substrate
Validated in a range of 20 to 60
◦
C [127]
Flexible composite-based
resistance temperature
detector (RTD)
Graphite/PDMS composite dispensed on
flexible polyimide films, associated to
copper electrodes
Validated at 30 to 110
◦
C [126]
Flexible CNT-based composite
Multiwall CNT/polyvinyl benzyl
chloride derivative with trimethylamine
(PVBC_Et3N) dispersions drop casted
onto a gold electrode pair supported on a
polyimide film
Validated for 20–40
◦
C [128]
Flexible composite-based
thermoelectric nanogenerator
A composite of the tellurium
nanowires/poly (3-hexylthiophene)
(P3HT) dropped onto a Kapton
®
polyimide flexible substrate associated to
two silver electrodes
A heat source of 24.8
◦
C [125]
Polymers 2021, 13, 3711 17 of 80
Table 1. Cont.
Technology Used Integration Method Operating Temperature Range Reference
E-patch
A modular patch with electronics
elements: (1) The thermometer
prototyped by attaching a flexible
adhesive-backed copper foil on a
polyethylene terephthalate substrate, (2)
The loop enclosed between two layers of
a medical-grade adhesive dressings to
attach the tag over the skin
Validated for T
s
ranging from 32.7
to 34.7
◦
C
[132]
E-skin sensor
Two main technologies compared: (1)
Arrays of 16 temperature sensors relying
on thin serpentine traces of gold,
fabricated using microlithographic
techniques with thin layers of polyimide,
(2) Multiplexed arrays of 64 sensors
based on PIN diodes formed by
patterned doping of nanoscale
membranes of silicon
T ranging from 27 to 31
◦
C and 30.7
to 32
◦
C (during mental and
physical stimulus tests)
[129]
Dual-heat-flux associated with
two double-sensors
Two double-sensors with dual-heat-flux
embedded in the neck pillow, while using
rubber sheets to simulate the
subcutaneous tissue layer of the neck
during experiments
Tested at 32–38
◦
C [144]
Heater-less deep body
temperature probe
Dual-heat-flux method wired sensors
placed on the skin, each probe containing
the two insulators on a rubber sheet
Validated at 36.5–37.5
◦
C [145]
Double-sensor thermometer
The sensor consists of two temperature
probes on each side of a standardized
insulator placed in a plastic shell
Validated at 36–37.8
◦
C [146]
Double-sensor thermometer
Combined heat and skin sensors specially
sealed in a polycaprolactone-based
enclosing cover
Validated at 10, 25 and 40
◦
C [147]
Double-sensor thermometer
Combined skin and heat flux sensors
specially sealed in a
polycaprolactone-based enclosing cover
Validated for a body temperature of
36–38
◦
C
[148]
Wearable thermistor
T
s
measured by a textile strip wristband
containing a NTC thermistor
16–42
◦
C [149]
Wearable thermometer
Array of 4 × 4 Silicon Kelvin precise
sensor thermometers
integrated into a textile-based affixation
aid to the arm, associated with a signal
processing chain
25–41
◦
C [150]
Wireless connected
temperature sensor
T
s
of the hand measured by a connected
temperature sensor
0–100
◦
C [151]
Wireless connected
temperature sensor
The system consists of a transceiver, a
microcontroller, and a digital
temperature sensor enclosed in a
polycarbonate covering to be placed
under the subject’s arm
Validated for T
s
(36.7 to 37.3
◦
C) in
an ambient environment
[152]
Polymers 2021, 13, 3711 18 of 80
Table 1. Cont.
Technology Used Integration Method Operating Temperature Range Reference
Long-range RFID tag
RFID rigid tag based on temperature
dependence of the frequency of the ring
oscillator integrated in a ceramic package
and assembled to a matched impedance
dipole antenna designed on
high-dielectric constant ceramic
substrates
35 to 45
◦
C [130]
Epidermal RFID-UHF tag
Tag and antenna layout with adhesive
copper transferred on a polycaprolactone
membrane attached on a skin with a
hypoallergenic cosmetic glue
Validated at 30 to 42.5
◦
C [131]
Remote HR and body
temperature monitoring
A temperature sensor integrated into a
polyurethane flexible substrate wearied
on the left thumb, while being connected
to a programmed microcontroller
Validated for body temperature
range of 36.6 to 37.2
◦
C
[153]
Remote HR and body
temperature monitoring
A portable temperature sensor connected
to an analogue microcontroller
measuring the body temperature, with
the final product being packaged in a
small lightweight polymeric package
Validated for body temperature
range of 36.6 to 39.4
◦
C
[154]
Wireless humidity and
temperature sensor
A semiconductor temperature and RH
sensor affixed to the internal surface of an
N95 filtering face-piece respirator made
of highly hydrophobic nature of
polypropylene
Validated for 30–36
◦
C and 60–89%
RH
[155]
Wearable in-ear thermometer
(1) Thermal sensors integrated into a
textile based earbag in order to measure
the tympanic temperature inside the ear,
T
s
, and T
env
, (2) The earbag added to a
resizable headset shielding the outer ear
Validated for the body temperature
range of 34.5 and 37
◦
C
[156]
Graphene-coated lens of IR
thermopile sensors for an
ear-based device
(1) Graphene/isopropyl solution
drop casted over the silicon substrate of
the lens of commercial IR thermopile
being associated to a microcontroller
collecting the temperature measured, (2)
The ear hook-type enclosure 3D printed
using Accura Xtreme polymeric resin,
while covering the thermopile with a
silicone cushion
Validated at T env of 21
◦
C and a
body temperature range of 36.5 to
37.5
◦
C
[133]
2.7. Temperature Sensors Challenges
Concerning the studies dedicated to temperature sensors that can be integrated in
textiles, the present state of the art has found that a lot of work is dedicated to the design
of temperature sensors based on smart textiles and flexible electronics [
53
,
93
,
157
], and a
very limited number of studies on sensors integrated in clothing has been identified. A
hybrid approach has been proposed to integrate rigid thermistors in a flexible matrix in the
textile structure. Despite several works related to integrated thermistors, some prototypes
lack mechanical strength, while others require optimizations regarding detection accuracy.
Another method has been to design fibrous sensors such as RTDs or thermocouples.
According to the studies analyzed, fibrous thermocouples require significant optimization
effort, because in addition to low sensitivity and low measurement accuracy, they have
proven to be sensitive to environmental humidity. Although the textile RTDs developed in
analyzed studies have provided better accuracy, higher sensitivity, and shorter response
time compared to textile thermocouples, these sensors were not able to provide localized
Polymers 2021, 13, 3711 19 of 80
temperature measurements. Therefore, the use of textile RTDs to measure temperatures in
micro or macro environments remains to be validated. The integration of Bragg reflector-
type optical fibers to measure body temperature, which has provided high accuracy, is far
from being applicable to a portable device, as such concepts require connection to fixed
optical systems. The same observation is valid for concepts that have integrated heat flow
sensors in textile structures. Being intended to be eventually integrated in clothing, textile
temperature sensors need to be validated for mechanical or wash resistance in future work.
In addition, experts in flexible electronics have shown great interest in the develop-
ment of temperature sensors on flexible polymeric substrates. Graphene layers deposited
on flexible substrates have demonstrated RTD properties of very high temperature sensitiv-
ity. However, in an extensible configuration, the RTD graphene layers have shown thermal
properties sensitive to mechanical deformations. Layers with RTD properties have also
been developed on flexible substrates by depositing different types of dispersion (based
on carbon, nickel oxide, silver complex, and mixing PEDOT-PSS with carbon nanotubes)
using printing techniques. These heat-sensitive printed layers were able to ensure high
temperature sensitivity while demonstrating low hysteresis in the heat–cooling cycles.
The formation of composite layers on flexible substrates also allowed the fabrication of
flexible temperature sensors. Among the various developments, composite layers based on
carbon nanotubes have made it possible to obtain thermal sensitivities comparable to those
of metals. However, in many studies on composite layers, electrodes based on precious
metals such as gold have been used. Despite the advantages of some concepts for flexible
temperature sensors, significant efforts are required to integrate them into clothing. From
a general point of view, work on textile-integrated temperature sensors, textile sensors,
and flexible temperature sensors seems to remain at the level of proof of concept with very
few connected device demonstrators and even fewer prototypes of garments equipped
with temperature sensors. In addition, the influence of various environmental parameters
on the performance of these types of sensors remains unknown. Among the few studies
on the design of garments with integrated temperature sensors, very few were dedicated
to protective equipment, and almost all the work was carried out in the laboratory with
tests on very few subjects. The effectiveness of these concepts has yet to be validated in
operational environments. In addition, in most of these studies, conventional electrical
wires were used for electrical connections or to ensure the transmission of collected signals.
These types of structures containing electronics can be vulnerable to mechanical constraints
during use and maintenance. The use of structures based on conductive textiles is to be
expected in order to ensure a better mechanical resistance in use. Clothing equipped with
temperature sensors that incorporate rigid thermistors embedded in textile fibers also
require optimization efforts in order to reduce the impact of mechanical stresses on the
quality of the sensor reading. The literature also mentions the influence of the fibrous struc-
ture surrounding the sensor on the reading [
78
]. Not only have few studies been carried
out in this area, but an in-depth knowledge of the influence of the multilayer structures of
various types of protective equipment on the performance of integrated sensors remains to
be developed.
Among the commercially available products, flexible temperature sensors seem to be
able to ensure high measurement accuracy and very short response times. Being mainly
based on a very thin printed structure, this type of sensor requires relatively low power
supplies of the order of microwatts. These products, which are currently manufactured
on flexible polymeric substrates, are mainly dedicated to the fields of warehousing and
logistics. In order to extend their application to clothing, research is still needed to ensure
their reliability and durability in use. Very few products including garments with integrated
temperature sensors currently exist on the market. These products are mainly dedicated
to the protection of firefighting workers. These types of protective equipment, which
include temperature sensors incorporated into their structure, can warn the firefighter
when predefined temperature thresholds inside or outside the garment are exceeded.
Polymers 2021, 13, 3711 20 of 80
3. Heated Actuator
In recent years, the textile industry has proposed multiple solutions to offer better
protection against the cold during outdoor winter activities. The use of various types of
textile materials has made it possible to reduce heat loss from the body while ensuring the
transmission of moisture from sweat through a garment that must remain water and wind
resistant [158].
Despite technological advances in textile materials engineering, these types of gar-
ments still need to be improved. Indeed, most garments against extreme cold remain
bulky by being based on multilayer fibrous structures taking advantage of the thermal
resistance of textile materials, which depends mainly on their content of air trapped inside.
In addition, highly insulating garments can sometimes limit body and arm movement and
reduce manual dexterity, thus affecting individual performance. A feeling of discomfort
may be particularly accentuated when clothing against the cold is worn in combination
with other clothing [
39
,
159
]. In addition, it has always been difficult to correctly estimate
the optimal clothing or number of layers to wear for sustained physical activity under
varying environmental conditions [55].
3.1. Heating Garment Technologies
During intense activities in the cold, excessive perspiration, and consequently the
humidification of the inner layers of the garment, can lead to a considerable decrease
in thermal insulation, thus increasing the risk of cold-related injuries [
160
,
161
]. To offer
a better level of comfort and higher endurance during activities in extreme cold, warm
personal clothing has been proposed by actors of the textile industry. These types of
garments also aim to offer more personalized solutions to individuals, incorporating
additional technologies to their basic textile structure. The development of personal
warming garments is of particular interest in a work context in order to protect workers
against injuries directly or indirectly related to prolonged exposure to extreme cold [
161
].
These garments can be presented under four main categories according to their principle
of operation: (1) Electric heating garment; (2) Fluid-flow-based heating garment; (3) Phase
change material heating garment; and (4) Chemical heating garment [55].
3.1.1. Electric Heating Garment
Among the different categories of personal heat garments, this study has mainly fo-
cused on electric heat garments, as they can provide heat in a sustained and durable manner
throughout the performance of tasks in extreme cold, depending on the endurance of their
portable power source. In addition, their structure incorporating a heating element could
provide heat distribution in a space-saving, thinner cold protective garment
[162,163]
. The
integration of electronic modules in combination with electric heating elements facilitates
the creation of garments with adjustable heating levels that can even be adjusted to the
individual’s personal situation [164].
3.1.2. Fluid-Flow and Airflow Based on a Tubing System
In contrast to electric heating garments, fluid-flow-based heating garments are very
bulky. Almost every example of this type of heating garment, based on a flexible tubing
system for circulating liquid or hot air, requires an external energy source and fluid supply.
In addition, the tubing system integrated into the garment makes it rigid, which may limit
its usability during activities [55]. Nevertheless, due to their thermodynamic efficiency in
heating the human body and the heat exchange capacity of specific areas, airflow-based
heating garments have been successfully applied for medical surgery [165].
3.1.3. Phase Change Material Heating Garment
Heating garments based on phase change material (PCM) also have important limita-
tions despite a very interesting potential and many dedicated efforts. The most important
limitation of this technology is its temporary heating effect. Although it is active during
Polymers 2021, 13, 3711 21 of 80
its phase change period, the release of heat ceases when the PCM, initially in a liquid
state, solidifies with exposure to cold. Thus, in order to recover its heat source based
on a phase change mechanism, it is necessary for the PCM to move away from the cold
environment to reach its liquid state again [
59
]. It has also been reported in the literature
that the integration of microencapsulated PCM into garments by coating and fiber-spinning
techniques shows a low heating effect due to their low mass. In addition, their effect may
gradually disappear when clothes are washed several times [60].
Since the thermal regulation capacity of textiles incorporating PCM is highly depen-
dent on the amount of material deployed, the incorporation of PCM pockets in clothing
generally leads to heavy clothing and may only be suitable for people for whom, depending
on the activities, the extra weight is not a problem [
60
]. To address these problems, a great
deal of research is underway. However, significant efforts still seem necessary to optimize
the global enthalpy of phase change and the thermal window of the PCMs to ensure a
sustained heat release effect [
59
,
61
] to meet the requirements of continuous hours of activity
in cold weather.
3.1.4. Chemical Heating Garment
Chemical heating garments are mainly based on chemical energy converted into
thermal energy by oxidation during the reaction of chemical substances and are mostly
used in diving suits to protect divers in cold water. The integration method remains
primitive, because the reactive material placed in cushion-like packages is glued by an
adhesive to the inner surface of the garment. The heat-generating chemicals are kept in
separate compartments inside the cushion. When the user presses the pad, the barrier
between the substance’s breaks, and the reagent mixes is generating heat. Although this
system can use a mass of selected reagents to provide a highly exothermic chemical reaction
free of gaseous by-products, the released temperature is difficult to control and of limited
duration [166,167].
3.1.5. Power Source
Despite durable heating throughout the duration of cold work, the low capacity of the
batteries to ensure the proper functioning of the integrated heating system during long
exposures to cold remains one of the major drawbacks of textile structures incorporating
electric heating elements [
38
]. The rapid development of telephones and laptops has led to
the availability of powerful and durable batteries that can also be used for auxiliary heating.
However, these batteries may have disadvantages in terms of weight, space requirements
in the garment structure, and the danger of overheating for some types of lithium-ion
batteries [
38
]. The problem of efficient power supply for electrical functions is a major
challenge in the design of intelligent textiles. Therefore, a lot of work has been under-
taken to develop new methods for integrating energy sources [
168
] and textile structure
batteries [
169
], while searching for new regenerated energy sources such as solar energy,
sound wave power, human movement, or even friction energy from clothing
[170,171]
.
Since this is a topic of important scientific interest that affects the entire field of smart
textiles, the analysis of advances in flexible and portable energy storage for different types
of electronic textiles requires a comprehensive study separate from the present one. Thus,
the review of the literature concerning heating actuators has mainly focused on techniques
for the development of electric heating elements that would offer a more efficient energy
consumption with current portable energy sources, as well as a better heat input while
ensuring flexible structures in order to better withstand mechanical stresses during the use
and maintenance of personal protective equipment.
3.2. Conductive Heated Actuator
The functioning of electric heating garments is based on the Joule heating principle
also known as ohmic heating [
172
], according to which the passage of an electric current
through a conductor generates heat by affecting the integrity of the conductive body.
Polymers 2021, 13, 3711 22 of 80
According to Joule’s first law, the heating power of this principle is proportional to the
product of the resistance of the conductive body and the square of the electric current
flowing [
173
]. In early versions of electric heating garments, the heating element was based
on an integrated electric heating wire or a 3D heating pad composed of electric wires or
graphite elements [
174
]. Despite their advantages in terms of increased comfort in the
cold, some users have pointed out disadvantages such as clutter, restriction of movement,
overheating, and problems with the durability of the electrical wiring system during use
and maintenance [
55
]. In addition, electric wire heating had technical limitations, as by
restricting heating to the path of the wire, it failed to produce uniform heat over a selected
area [163].
In order to circumvent these drawbacks, the design of heating elements based on
conductive textile fibers or the deposition of conductive layers on the surface of textiles
has been proposed by the scientific community [
166
]. Based on the technology used, these
types of heating elements can be divided into five categories: (i) textile substrates coated
with compositions based on silver particles; (ii) textile substrates coated with conductive
polymers; (iii) heating elements based on carbon fiber or carbon-based compositions; (iv)
heating textiles based on yarns of metallic compositions, and (v) hybrid heating textiles
using simultaneously passive heating actuators and electric heating elements.
3.2.1. Silver Coated Yarns
With the goal of solving the problems associated with the use of electrical wires as
an integrated heating element in clothing, several works have attempted to apply metallic
textile wires or wires made from metallic compositions [
175
]. In part of this work, heating
elements were developed by sewing seams of metallic textile threads on the surface of
various types of fabrics to simulate the embroidery process. In an analytical study, con-
ductive yarns based on silver-coated Vectran
TM
fibers (a type of aromatic polyester) [
176
]
were sewn in serpentine shape on three stretchable knit fabric composed of cotton–elastane,
polyester–elastane, and nylon–elastane in different variations. It was found that different
levels of heat can be generated depending on the number of yarn passes, the spacing of the
coil curves, and the type of knit, which also dictate the level of electrical power required [
54
].
Based on the knowledge developed on the spacing required between the coil curves and
the number of yarn layers superimposed to obtain the best thermal response in terms of
electrical power versus temperature [
54
], a prototype wrist heater providing a temperature
range from 33 to 40
◦
C was developed using the same type of conductive textile yarn [
177
].
According to these studies, the creation of heating elements from embroidered conductive
textile yarns could allow the generation of a much higher heat range than heating elements
based on electric heating wires by applying the same power supply. According to the
authors of this study, the influence of substrate fiber content, stitch configuration, and
increased heating zone still requires further work [
54
]. Using the same technique, a heating
element based on a silver wire was designed to provide heat close to body temperature
with a power of 5 W supported by a portable 10 V battery with a capacity of 6000 mAh for
8 to 10 h of supply [
178
]. In addition, the power supply and saturation time for a given
temperature were analyzed for a heating element designed by sewing a silver-coated nylon
thread onto a polyester-based fabric to elucidate the power level required to achieve heat
levels in the range of 27 to 43
◦
C [
166
]. All these results can contribute to the optimization
of heating element design with embroidery techniques on an industrial scale.
As knitted fabrics offer flexible and stretchable structures, the creation of knitted
heating elements has attracted particular attention from the scientific community in recent
years [
179
–
182
]. In this context, the heat production of two silver-coated textile yarns
with different electrical resistance, embedded in a traditional wool knitted fabric, has
been studied by applying various levels of electrical tension for more than one hour. The
results of this study showed that the total electrical resistance of the conductive knit fabric
decreases significantly when the fabric is heated, as the linear resistance of the conductive
yarns as well as the resistance of the contact points between the superimposed conductive
Polymers 2021, 13, 3711 23 of 80
yarns in the knit structure decreases with increasing temperature [
182
]. Studying the
behavior of a silver-coated polyester yarn embedded in three different knit structures
showed that the maximum equilibrium surface temperature of heated knit fabrics is
strongly correlated with the energy consumption density. Furthermore, the maximum
equilibrium surface temperature can be influenced by the knitting method, as the electrical
resistance of some structures seems to remain more stable than others during the heating
process [183].
Analysis of the design of weft knitted heating pads using three different types of
conductive textile yarns embedded in two knitted fabrics of similar structure, but with dif-
ferent main yarns (acrylic and polyester respectively), showed that the electrical resistance
of the conductive yarn and the composition of the knitted textile fibers surrounding the
conductive textile yarn greatly influence the heat generated at a fixed supply voltage. The
authors concluded that the acrylic yarn of the knitted fabric would have better heating and
heat retention properties compared to polyester when using the same type of conductive
yarn [
184
]. The influence of the design and the method of integrating the conductive yarn
on the heat generated was also studied by integrating a silver-coated textile yarn into a
fully knitted structure to compare it to stitches on the surface of a shoe insole. Depending
on the design and the type of textile threads surrounding the conductive thread in the
fabric, temperatures higher than the body temperature could be obtained with electrical
powers as low as 1.7 Watt provided by portable low-voltage batteries [185].
3.2.2. Metallic Textile Heating Elements
In addition to silver or silver-coated conductive textile yarns, other types of conductive
textile yarns with a metallic composition were also considered for the design of heating
elements. The study of the behavior of steel wire-based heating panels using single and
multilayer steel wire integrated in clothes showed that the thermal effect obtained, and
the time required to reach an equilibrium temperature at a fixed voltage, depended on the
number of wire folds in the cloths [
186
]. In addition, the criteria for selecting conductive
yarns for knitting an electric heater was explored using two types of steel yarn, two
types of silver-coated polyamide yarn, and one polyester/steel blend yarn, each of which
was knit in two patterns: (1) wool/polyamide knit with a 1 m long conductive yarn in
three rows of loops; (2) a conductive area in a multiply knit fabric [
187
]. While finding
that the maximum equilibrium temperature of the heating elements was influenced by
the method of integration of conductive yarns, the authors concluded that an optimal
heating element should contain conductive yarns with low electrical resistance and minor
variations in electrical resistance to elongation, providing good temperature uniformity
during the heating process while being mechanically suitable for knit structure. In this
work, silver-coated polyamide yarns in a three-ply configuration were able to provide
the most uniform heating zones while being technically suitable for a knit structure [
187
].
Analysis of the method of manufacturing flexible heating fabrics by integrating a copper
coil filament between two pieces of flexible interlining fabric using the thermal adhesion
process has demonstrated that reducing the copper wire spacing and the applied tension,
while improving the thermal conductivity of the textile structure of the fabric, not only
increases the temperature and heating rate but also helps to maintain the fabric at a uniform
temperature [188].
In this context, a fabric with variable insulation properties was developed with a
structure consisting of three fleece layers and two interlayers comprising copper filament
spirals and Nitinol as a temperature-sensitive shape memory element. The inner layers,
being heated by the passage of an electric stream, made it possible to increase the thickness
of this part of the fabric during the heating process, thus ensuring the increase in the
insulation of the fabric due to the increase in air present in the transverse direction of the
fabric. The heat-induced physical change in the conductive spirals could be electrically
adjusted, providing a means to control the overall insulation level of the fabric [
159
].
Finally, a heated knitted fabric was developed using a conductive elastic yarn of composite
Polymers 2021, 13, 3711 24 of 80
structure that included an elastane filament as a core and a steel filament combined with
rayon fibers as a sheath wrapped around the core. Composite yarns of varying degrees of
tension were embroidered on the surface of commercial knitted fabrics to obtain heating
fabrics. According to the analysis of the thermomechanical behavior of heating fabrics
based on conductive elastic yarn, despite reasonable cyclic stability in tensile tests, the
temperatures obtained seemed to decrease with increasing tensile stress but still reached a
stable thermal equilibrium after the application of the deformation [189].
3.2.3. Mathematical Models for Metallic Heating Textiles
To facilitate the design of electric heating elements based on metallic textile wires,
some research work has proposed mathematical models to better anticipate the behavior of
the heating textile to be developed. In one of these studies, the thermomechanical proper-
ties of knitted structures based on silver-coated textile yarn were mathematically modeled
as a function of the influence of the contact pressure at the structural bonding points on the
heating level. Thus, considering the relationship of the electrothermal property of the mate-
rial and the structural parameters of the knitted fabric, the resulting temperature and loop
resistance of a knitted fabric of uniform width can be predicted. Practical validation of the
model with a heated knitted fabric based on silver-coated polymeric yarn showed that the
maximum temperature obtained at a fixed supply voltage would depend on the structure
of the knitted fabric in plain, ribbed, and interlock stitches [
190
]. The same research group
proposed a second model to predict the electrothermal behavior of a steel wire knitted
structure, whose predictive accuracy was subsequently evaluated with experimental trials
of integrating conductive steel wires into double-ply knitted fabrics of interlock and solid
structures [
191
]. The results of this study showed again that the maximum temperature
obtained and the reaching of a heating temperature equilibrium state at a given voltage
would depend on the structure of the knitted fabric. Based on the analyses performed,
steel wire-based heating elements can generate a greater amount of heat at very low power
supply voltage, and therefore, its use would be recommended over silver-coated yarns
when a high level of heat is required. This study also recommends an interlock structure
for the design of heated knitwear due to better stability and higher temperature supplied
compared to solid knitwear at the same electrical supply voltage [192].
Another theoretical model has been proposed to control the temperature of conductive
knitwear of various courses and stitch yarns based on the quantitative relationship between
the electrical resistance of a conductive knitwear and the temperature provided. According
to this model, by knowing the initial resistance and thermal diffusivity [
193
] of the knitted
fabric, as well as the applied voltage, it would be possible to predict the temperature
provided by the knitted fabric. Experimental validation of the model with silver-coated
yarns in the design of five woolen knitwear, with the same loop density but different
loop arrangements, has demonstrated the dependence of the maximum temperature ob-
tained on the type of loop arrangement [
193
]. Another model predicting the electrothermal
properties of conductive knitwear was proposed by taking into consideration the thermal
capacity of conductive and non-conductive yarns, the electrical resistance, and the thermal
capacity of the heated knitwear. Experimental validation of the model, which also consid-
ered the coefficient of thermal conductivity, the mass, and the initial temperature of the
fabric, showed that the coefficients of thermal conductivity and the thermal capacities of
electrothermal fabrics depend on the type of conventional fiber used and the density of
the loops of the knitted fabric. Experimental validation of the model using the integration
of silver-coated yarns in three types of wool, acrylic, and cotton knitted fabrics with three
different densities for each type of knitted fabric showed that the maximum temperature
and time required to reach a stable heating temperature depend on the types of expanded
textile fibers and the loop density of the knitted fabric [194].
These types of patterns have also been proposed to predict the design of heated
woven fabrics. In order to express the relationship between various parameters of a heated
woven fabric, an equation was proposed based on the resistance of the fabric, the heat
Polymers 2021, 13, 3711 25 of 80
output power, the DC voltage, the number of parallel conducting wires, the length of the
single conducting wire, the resistivity of the conducting wire, and the cross-sectional area
of the conducting wire. Validation experiments using the integration of silver filaments
and silver-coated yarns in identical cotton fabrics concluded that the conductive yarns
or filaments must be uniformly distributed in order to avoid overheating on parts of
the heating fabric [
195
]. It was observed that silver-coated yarns would not be suitable
for the design of heating fabric due to their poor thermal stability. In addition, silver
filaments would be a better choice compared to steel wires in such structures to avoid wire
breakage [195].
According to some of the models discussed, knowing the electrical resistance of a
conductive tissue can greatly contribute to predicting its electrothermal behavior [
193
,
195
].
Therefore, theoretical models suggested by some experts to predict the overall resistance
of a conductive knitted fabric can be taken into account. Studies such as the modeling of
the resistance of conductive knitwear from the length-related resistance and the contact
point resistance associated with the analysis of the electromechanical behavior of such
knitwear [
196
], the modeling of the resistive network for conductive knitwear stitches [
197
],
and the estimation of the resistance of conductive knitwear from a macroscopic view
by considering the surface resistance of the conductive yarns [
198
], can be considered
in such an approach to the design of a heated knitwear. In the same context, a derived
simulation model has been developed to calculate the electrical resistance of a conductive
woven fabric by considering its structure as well as the density and arrangement of the
integrated conductive yarns. Once the radius of the warp yarn and the resistance of a unit
of conductive yarn were known, the electrical resistance of the conductive woven fabric
could be calculated. By validating the model using the integration of a silver-coated nylon
wire in three woven structures with different weft density and constant warp density, the
study demonstrated that for the same fabric size, the electrical resistance can be adjusted
by controlling the fabric structure and the arrangement of conductive wires [199].
In order to facilitate the design of a heating element in a textile with a versatile design,
and to overcome the technical challenges related to the integration of a conductive wire
in a textile structure, coating techniques have been deployed to form conductive and
heating zones on the surface of textile substrates. The deposition of a silver particle-based
conductive ink on the surface of one polyester/cotton fabric resulted in a heating element
that provided a maximum temperature of 33
◦
C with power supplies as low as 1.4 Watt and
a time of about 10 s to reach the equilibrium heating temperature [
200
]. In a similar work,
the deposition of a dispersion containing silver nanofilaments on a cotton woven fabric
created a heating zone that could provide 50
◦
C heat at an applied power density as low as
0.05 W/cm
2
. Despite such performance, due to the relatively low environmental stability
of silver nanofilaments, the developed heating fabrics lost their performance after two
months of storage under ambient conditions. In addition, the created conductive layer was
damaged during washing, and its thermal performance was significantly reduced [
201
].
In order to take advantage of the benefits of using silver nanofilaments in the design of
a heating element, techniques such as the one proposed for the fabrication of heating
membranes based on nanosilicon carbide and thermoplastic polyurethane covering the
silver filaments [
202
] should be considered. Although these types of membranes may
offer good thermal stability and better mechanical properties, their integration into textile
structures remains to be explored.
3.2.4. Textile Substrates Coated with Conductive Polymers
The formation of polymeric conductive layers on textile substrates has also been
explored for the design of electrical heating textiles. The in situ polymerization of poly
(3,4-ethylene dioxythiophene) p-toluene sulfonic acid (PEDOT-PTSA) on a polyester web
by coating has allowed the development of a very flexible and lightweight heating textile
with a durable and high heating potential that still required high supply voltages [
203
].
The deposition of a polypyrrole coating on a nylon-based knitted fabric was also used to
Polymers 2021, 13, 3711 26 of 80
create a textile heating element. However, voltages as high as 18 volts were required to
generate temperatures in excess of 45
◦
C. In addition, the provided temperature appears to
be altered during the elongation of the fabric [
191
]. Vapor-phase polymerization of poly
(3,4-ethylene dioxythiophene) on a cotton fabric has made it possible to develop a heating
element that can reach 28 and 45
◦
C with voltages of 4.5 and 6 volts, respectively. By means
of a vapor-phase post-treatment for the deposition of a protective layer against moisture, it
was possible to achieve better protection of the polymeric heating element against abrasion
and mechanical deformation. According to the analyses performed, cutting, sewing, and
partial weaving would not appear to alter the electrical conductivity and electrothermal
responses of the heating layer [
204
]. Although these types of developments are very
interesting, due to the technical challenges and high cost of scaling up vapor deposition
techniques to meet the high-volume production requirements of the textile industry, it
is difficult to envisage soon the use of vapor deposition processes to create textile-based
electronic components [205].
3.2.5. Heating Elements Based on Carbon Fiber or Carbon-Based Compositions
Carbon fibers are also very interesting candidates in the design of electric heating
textiles because of their good thermal efficiency and ability to generate uniform heat
quickly [
206
,
207
]. Allowing a very high rate of electricity conversion, carbon fibers can
promote the design of heating elements with versatile surface temperatures depending on
the desired design while providing an average lifespan of up to 100,000 h [
55
]. Examples of
work in this context are the development of a heating element in the form of a composite
layer based on recycled carbon fiber in a polyurethane resin producing heat ranging from
26 to 96
◦
C [
208
], the development of an anti-icing/de-icing device with the integration of a
carbon fiber composite laminate in a multilayer structure requiring electrical currents of 2 to
4 amps to provide the desired electrical power density [
209
], and the evaluation of a carbon
fiber-based electric blanket to warm patients during abdominal surgery, demonstrating a
performance equivalent to that of forced hot air heating technologies and superior to that
of hot water circulation mattresses in tests conducted in the hospital environment [210].
A few studies have also been devoted to the use of carbon fiber-based heating elements
in the design of electric heating garments. The evaluation of an electric heating vest with a
carbon fiber-based heating element on a thermal manikin in a cold climate chamber has
shown that the application of too high temperatures can lead to a reduction in heating
efficiency due to a significant loss of heat to the environment, thus demonstrating that
the heating power should be adjusted according to the external temperature [
211
]. The
influence of ambient air velocity and the influence of the suit of clothing worn on heating
efficiency was also studied by testing an electric heating vest, equipped with six carbon
fiber-based heating elements, on a thermal manikin. The combination of the vest with
knitted underwear and a military uniform in different orders demonstrated that the order
of the clothing combination can significantly influence the heating efficiency. Indeed, the
best heating efficiency was obtained when the heating vest was worn as a middle layer
in the middle of the other clothing. It has also been found that the heating efficiency of
the heating vest decreases with increasing cold air velocity [
212
]. The efficiency of an
electric heating garment containing seven carbon yarn-based heating pads was compared
to that of a heating garment containing 14 PCM pockets during tests conducted under
identical conditions using a thermal manikin operating in the thermoregulatory model
control mode. According to the analyses performed, the electric heating garment can show
a more efficient heating power and a significantly higher total thermal insulation compared
to the PCM at low airflow velocities, whereas no significant difference was observed at high
airflow velocities [
162
]. In addition, the analysis of different methods of applying carbon
fiber in the design of an electric heating garment has shown that the use of carbon fiber
can lead to a rapid temperature increase as well as a high recovery rate when disconnected
from the power supply, so that such a heating element has the necessary characteristics for
precise temperature control. Based on the results obtained, it was also recommended to
Polymers 2021, 13, 3711 27 of 80
take into consideration the human body heat dissipation principles and that of the garment
surface in the design of the garment as well as a sandwich-type heating element design to
promote better heat input [
213
]. Despite the advantages of a carbon fiber-based heating
element, its integration into clothing still requires further work to optimize its resistance to
washing [213] and energy consumption [208,209,211,213].
3.2.6. Efficiency of Heating Clothing Based on Yarns of Metallic Compositions
In parallel with the numerous studies dedicated to the development of textile electric
heating elements, some work has also been devoted to the evaluation of the efficiency of
electric heating garments. The evaluation of a heated sleeping bag incorporating heating
fabrics in the foot area on a thermal manikin, and subsequently on eight human subjects in
the controlled conditions of a climatic chamber, has demonstrated the capacity of such a
concept to keep feet and toes warm throughout the test period [
214
]. The optimal operating
conditions for a heated glove with heating elements attached to the back of the layer
adjacent to the fingers were determined by testing under controlled laboratory conditions.
To this end, the study attempted to identify the heating power required to maintain the
finger temperature above 15.6
◦
C, which is known as the minimum ergonomic design
standard for space suits [
215
]. In a similar work, the evaluation of the performance of an
electric heating glove on a thermal hand model identified the electrical power required
to maintain thermo-neutral skin temperature of the hand during exposure to extreme
cold. According to the observations, three additional watts was required to maintain
the thermal comfort of a hand in moderate wind compared to a calm air circulation at
−
10
◦
C. This study also concluded that finger dexterity may also depend on the structure
of the heating element and its flexibility as well as the glove configuration and fingertip
design [
161
]. Another study evaluated the ability of an electric heating vest in warming
up and improving the performance of elite sprint swimmers. Skin thermal imaging and
measurements of tympanic temperature, heart rate, thermal comfort, and thermal sensation
of male participants wearing a heated vest followed at a swim session showed a real
beneficial warm-up effect compared to a group of unheated participants. However, no
significant effect was observed for the female swimmers tested, suggesting a sex difference
with possible links to gender differences in perceived discomfort [216].
In order to offer more comfort and ease in the execution of tasks during activities
in the cold, clothing allowing control or self-regulation of the temperature has also been
studied. In this context, a vest with temperature control capability was developed by
combining steel wire-based heating panels, in several configurations from one to four
layers, with a digital temperature sensor and a microcontroller. These components, being
worn on a carrier, were subsequently attached by means of Velcro strips under a multilayer
cotton/polyester/polyamide garment. A user interface on an external handheld device
was also used to control and display the temperature. In a self-regulating temperature
mode, the heating circuit was activated by the microcontroller if the value measured by
the temperature sensor fell below a preset value [
217
]. Evaluation of this garment with
a copper thermal manikin in a cold climate chamber showed that the maximum heating
temperature would depend on the number of folds in the panels. According to these
analyses, the single-layer heating elements could operate longer, while the power supply
period became shorter for the high number of panels due to the lack of power supply. By
comparing different types of batteries of identical capacity, the authors also concluded
that nickel-metal hydride batteries would be more appropriate for cold environments with
an instantaneous heating effect, while for circumstances requiring continuity, lithium-ion
batteries providing stable heating would be more advantageous [
217
]. The effectiveness of
a glove comprising an electric heating element and a temperature controller measuring
the T
s
of the fingers was examined by recording the thermal sensation of human subjects
wearing the gloves in a climatic chamber. The results showed that such a glove would
maintain the temperature of the back of the hand and fingers within a comfort zone. The
tests showed that in addition to improved thermal sensation and comfort in the fingers,
Polymers 2021, 13, 3711 28 of 80
the thermal sensation and whole-body comfort sensation increased slightly with the use of
electrically heated gloves in cold weather.
By applying a power switching method based on the self-monitoring of the heating
element temperature, a heating textile with the ability to quickly reach various temperature
levels, having a uniform temperature distribution band and ensuring the maintenance
of the defined temperature, was developed. To realize such a concept, copper-coated
polyurethane filaments were embroidered on a cotton fabric to design the heating ele-
ment and an RTD-type temperature sensor. To ensure temperature self-regulation, an
on–off control system referencing the temperature in real time was used to maintain the
target temperature in the embroidered circuit, independent of the internal microclimate
and external climatic conditions, as well as the battery voltage level [
56
]. In addition, an
analytical study carried out an experimental characterization of the design parameters
of a self-regulating heating garment [
164
]. For this purpose, a heating actuator based on
serpentine stitching of silver-coated filaments was integrated into a three-layer garment
comprising the heating element formed on the knitted base layer, a layer of aluminum foil
in the center to improve heat retention, and a textile cover layer on the outside. In order
to study the temperature control system, the garment was developed in three versions:
(1) no control circuit; (2) the self-regulating garment with closed-loop T
s
feedback using
thermistors placed at various locations on the skin and a control system based on a mi-
crocontroller; and (3) the self-regulating but user-controllable garment with control of the
thermistor feedback to maintain the internal temperature of the garment at a desired level
and the use of an additional potentiometer to allow the user to control the set value of each
actuator. According to the analyses of this study, total temperature self-regulation may be
inadequate in complex thermal environments, indicating the need to consider ambient and
body thermal effects in the thermal management of the temperature self-regulating system.
By placing control of the system in the hands of the wearer, the self-regulating garment
could overcome some of the challenges associated with complex environments by relying
on the thermal sensation of the wearer [164].
3.2.7. Hybrid Heating Textiles
Some studies have also looked at the combination of electric heating elements and
functional heating materials to ensure better energy efficiency. In one of these studies, the
influence of the use of phase change materials on the energy consumption of electrically
heated garments was investigated [
218
]. For this purpose, several configurations of the
same garment were developed by associating, or not, an electric heating element with
a PCM-coated layer. Tests carried out on the different versions of the garment using a
bionic skin model at 33
◦
C in a climatic chamber at
−
15
◦
C showed that the association
of an electric heating element with a layer containing PCM can considerably optimize the
distribution of heat in the garment, thus improving the thermal protection performance of
the garment. In addition, the PCM coating with a melting point of 27
◦
C allowed the im-
plementation of a self-regulating temperature mechanism whereby when the temperature
produced by this layer fell below 27
◦
C, the conductive fabric was automatically energized,
and conversely, when the temperature exceeded 29
◦
C, the conductive fabric was switched
off. Such a hybrid configuration also resulted in energy savings of about 30% with the
temperature control process [218].
By using textile fibers, such as cotton, polyester, or acrylic, containing metals of ceramic
compounds (e.g., platinum, alumina, or silica derivatives), fabrics with the ability to absorb,
reflect, and emit far-infrared waves have been developed. Using such potential, heating
elements have been proposed for protective clothing against cold in recent years [
160
].
Some commercial products claim that their technology can capture thermal radiation
emitted from body heat and then, by reacting as a reactive mirror, use thermal far-infrared
rays to reflect energy back to the body [
219
]. The analysis of the integration of far-infrared
wave reactive heating panels has shown an effect of local heat, but it is not enough to
increase the temperature of fingers and toes during physical activities in the cold. However,
Polymers 2021, 13, 3711 29 of 80
their association with electric heating elements could still contribute to an optimization of
energy consumption in electrically heated clothing [
220
]. In addition, a very recent study
has proposed a dynamic exploitation of infrared radiation in textile structures in order to
create thermal effects that are adaptive to the environment. Thus, a textile with dynamically
adaptive optical properties, allowing the regulation of thermal radiation, has been designed
with a structure composed of elliptically shaped dimorphic fibers of triacetate and cellulose.
The fibers fused side by side were knitted and subsequently coated with multiwalled
carbon nanotubes [
221
]. By arranging the electromagnetic spectrum and wave propagation
of thermal radiation by controlling the distance-dependent electromagnetic interactions
between the conductive elements of scales less than or equal to the desired wavelength,
it was possible to create an adaptive aperture of IR radiation in the textile depending
on the thermal response of the body against cold or in warmth with an inverse physical
effect [
221
]. According to the authors, further research is needed to optimize the observed
triggering effect and to address cost and human testing concerns.
3.3. Commercial Warming Clothing
The study of commercial products for heating actuators was mainly oriented toward
electric heating products for sustained heating. On the other hand, few or no products
were identified in the other three categories of heating garments, i.e., those based on fluid
flow, phase change material, or chemical heating garments.
Indeed, commercially available electric garments use different technologies. Five types
of technologies were defined in this study to classify companies and/or products, based on
the review of scientific literature and information found on the websites of these heating
product companies. The five types of technologies are conductive heating elements, electric
heating wires, carbon fiber-based heating, graphene layer-based heating, and Positive
Temperature Coefficient (PTC) conductive layer technology. Some types of technology
such as conductive heating elements have been deliberately defined as quite generic, as
it is often very difficult to know exactly what the technology of many products on the
market consists of, as the information available on websites is often not very detailed,
sometimes insufficient, or confusing. A sixth technology has been added but contains only
one product that is distinct from the others. It is a face mask that warms and humidifies
the air inhaled, which was first developed for people with asthma or respiratory disease
(ColdAvenger). Table 2 shows the number of companies listed for the different heating
technologies and the types of products they offer.
Table 2. Number of companies listed for electric heating actuators for use in intelligent thermal management.
Type of Product
Technology
Warm Clothing (Jacket, Vest, Shirts, Pants, Gloves,
Scarf, Beanie, Socks)
Heated
Insoles (and
Socks)
E-Textile Mask Total
Fabricant
1
Brand Sold on
Online Platform
2
Smart
Apparel
3
Conductive heating
elements
8 3 3 2 3 19
Electric heating wires 8 1 1 3 13
Heating based on
carbon fibers
20 9 29
Graphene technology 2 1 3
Technology PTC 1 2 3
Inspired air heating 1 1
Total
38 14 4
3 8
1
68
56
1
Manufacturing company (with a website);
2
Brand sold exclusively on online platforms (ex.: Amazon) and whose manufacturer does not
have a website;
3
Intelligent heating clothes offering self-regulation of the temperature.
Polymers 2021, 13, 3711 30 of 80
Nineteen of the companies identified were classified under the generic category
of conductive heating elements because they provide very little detail on the heating
technology used in their products on websites or data sheets. However, images, videos,
and promotional interviews of these companies suggest that, for example, the heating
elements used by some companies are based on conductive textiles (Makita, Zanier, Soleno
Textile), conductive elastomers laminated to textiles (New Textile Technologies—NTT,
Loomia), printed heating elements (Digitsole, which offers insole heaters, Conductive
Transfers), or heating elements knitted into clothing (Odlo, Myant & Helly Hansen).
Among the 13 companies analyzed that use electric heating wires, Interactive-wear
produces heating textiles made with embroidered, single-layer Litz yarns that meet auto-
motive quality standards to minimize the risk of hot spots. The Volt Smart Yarns company
manufactures garments and heating textiles using different types of yarns (stainless steel,
copper, nickel, etc.). The other companies in this category produce heated clothing, but
it is often difficult to have the details of the heating wire technology. For example, Gyde
Wearable Technology announces that these garments contain micro-wire heating zones,
but they do not provide more detail. Gerbing sells a jacket with a heat output of 77 watts,
making it the warmest product Gerbing has to offer. The jacket contains more than 30
m of MicrowirePRO
®
heating wire in seven different heating zones (front, back, collar,
sleeves) for complete body heating. It also has three outlets that can power heating gloves
(at different temperatures than the rest of the jacket), pants, and socks.
Twenty-nine of the companies classified use a carbon fiber-based heating system to
provide warm clothing such as jackets, vests, and shirts, as well as beanies, socks, or
gloves. Duran, a Chinese company, claims to be the first company to have developed
and commercialized carbon fiber heating yarns, and it is the only one capable of precisely
controlling fiber strength during production to
±
5% (per meter). According to their website,
Duran holds 14 international patents and 18 national patents for electric heating products.
A heating element made from carbon fibers can quickly reach the desired temperature
in just a few seconds. It can even have a long lifetime, up to more than 1000 working
hours, as for the Arris company’s heating vest with a constant temperature of 40 to 80
◦
C.
For example, the information provided by the manufacturers’ website shows that Verseo
uses very thin, stretchy carbon fibers, that Heated Gear and EGEVogue use a silver-coated
polyester thermal lining to reflect heat in addition to the carbon fiber heating elements
and benefit from a hybrid system, Colcham offers a safe heating system by providing
short-circuit protection, and that Octocool claims to use more carbon fiber (60 to 80% more)
in their heating jacket than other competing brands. Vinmori, a Chinese company, states on
its website that it uses Toray carbon fiber from Japan to improve the emission wavelength
of the heating panel to reach values of 3 at 14
µ
m, with most heating wavelengths between
2 and 10
µ
m and can cause greater heat dissipation. This company also uses a temperature
control system that ensures that the heating panel can quickly reach its highest heating
temperature in 3 min. In addition, a built-in NTC-type thermistor temperature sensor can
automatically detect the panel temperature every 0.3 s. Thus, the heating panel can operate
at the specified temperature, with the accuracy of 0.3
◦
C regardless of the external ambient
temperature, and avoid excessive temperature that may expose the body to the risk of
burns. In order to ensure a firmer and safer circuit, carbon fibers wrapped in a polyester
film were considered. In addition, to ensure the electrical connections in its products, the
company has favored the use of conductive wires with a thermoplastic elastomer resistant
to low temperatures in order to maintain the mechanical strength of the wire and avoid its
breaking even at −40
◦
C.
Three of the companies listed use one heating technology based on graphene layers
(Firefox Heated Coats, AGPTek, Vulpes). According to the available information, this
technology allows products that are light, resistant (to traction, bending, friction, cold,
washing), durable, and offering good thermal performance. Graphene elements, in addition
to allowing an equal and efficient distribution of heat, can be used safely in various
Polymers 2021, 13, 3711 31 of 80
conditions of temperature, humidity, or exposure to water and under high mechanical
stress.
Two of the listed companies, Nuova Heat and Nissha GSI Technologies, manufacture
electronic textiles based on PTC technology for applications in the medical and industrial
fields, such as aerospace, automotive, military, consumer goods, etc. The thermoregulatory
PTC technology is based on a high temperature expanding resin layer that is loaded
with conductive particles (often carbon). Such a film can control the temperature itself
by regulating the heating power using its electrical resistance response to temperature,
which varies with the expansion of the resin causing the distance between the conductive
particles to increase. At low temperatures, its resistance is lower, so its heating power is
greater, resulting in a rapid increase in temperature. As the temperature rises, its resistance
increases, and therefore, its heating power decreases, thus controlling the temperature
(Okutani, Yokota, Matsukawa and Someya, 2020). Once deposited on a textile structure,
the PTC layer heats evenly over the entire surface of the textile and self-regulates to a
specific temperature, thus reducing the possibility of overheating the garment. According to
manufacturers, products using PTC technology have the potential to be safer and even more
efficient compared to those using more traditional yarn or carbon fiber technologies. PTC
heating elements from Nuova Heat, a U.S.-based company, are manufactured by depositing
a conductive ink printed on a nylon fabric containing traces of silver as electrodes that
can reach 55
◦
C in a few seconds with the passage of a 9 V direct current. Only one
company identified uses a technology based on a conductive carbon-based PTC layer. This
is Kinesix Sports, whose product, which allows self-regulation of the heating temperature,
is described in detail below. This company uses flexible, lightweight heating pads made
from PTC-type carbon ink encapsulated between two extra-thin layers of polyester.
In general, about 50 companies offer clothing and accessories that include heating
technologies, mostly integrated in jackets and vests (sleeveless), but also in pants, body
suits, gloves, socks, scarves, and beanies. Although 14 of these companies were only found
on online sales platforms such as Amazon, most of them have a website where they present
their products and features and sometimes explain the technology used. For jackets and
vests specifically, two-thirds of the products listed have three heating zones, two of which
are located on the chest and one on the upper back. In addition to these three zones, many
products also offer heating zones on the collar to warm the neck, on the pockets to warm
the hands, on the lower back and, for only a few products, on the sides of the body or on
the arms. Most heated garments such as jackets and vests use a lithium-ion battery (4 V,
5 V, 7.4 V and 12 V), which allows the heating elements to provide heat higher than body
temperature. In addition, many of the commercially available jackets and vests have USB
ports that allow the battery to be used to charge mobile devices. Two-thirds of jackets and
vests allow three temperature settings, for example, 25
◦
C/35
◦
C/45
◦
C for some products
or 45
◦
C/55
◦
C/65
◦
C for others. These settings provide continuous heat for periods of
time that can be, for example, around 15 h, 7 h, and 5 h for some products, or 4.5 h, 3 h, and
2 h for other products, depending on the temperature supplied and the power available.
Usually, an LED control switch is integrated into the chest of the garment to allow the user
to adjust and interpret the heat settings at different levels. Most commercially available
products can be washed according to the manufacturer’s instructions. They are mainly
aimed at the sports, leisure, or generic markets. A few garments and vests stand out
because of their particular features or performance. For example, some companies offer
heated shirts, jackets, or vests with plugs that allow the same energy source to be used
to connect heating gloves (Warm & Safe Heated Gear, California Heat, Gerbing), heated
pants, or heated socks (California Heat, Gerbing). Other companies offer independent
heating zones to separately adjust the temperature of certain areas, such as the front, back,
and hands (via garment pockets) (Arris), front and back (Vinmori), or body and hands
(Ptahdus). Some companies offer continuous adjustments of the heating temperature via a
variable switch (Warm & Safe Heated Gear) or with the help of a smart phone application
(Odlo, Clim8, Vulpés).
Polymers 2021, 13, 3711 32 of 80
In addition, this study identified three companies that have implemented systems
that allow self-regulation of the heating temperature thanks to integrated thermal sensors
that measure the temperature inside the garment or that of the skin. Clim8 proposes an
intelligent thermal system integrated in a textile panel, in the form of a sweater adjusted
to the body. This sweater is equipped with thermal sensors integrated in the fibers and
controlled by a smart phone application. Once the temperature is set by the user, the sensors
measure in real time the temperature of the microclimate, and the system activates when
the temperature detected by the sensors is below the reference threshold and deactivates
above this temperature. The mobile application of this system still allows manual activation
and control of the garment heating. The company announces that the heating elements
are positioned on the vital parts of the body. However, the available images and videos
show that the technology seems to be present at least on the front and back of the sweater.
Other companies such as Odlo and K2 also use Clim8 technology. Odlo has developed,
with Clim8 and Twinery, the I-Thermic system integrated into a knitted sweater that can
be worn alone or under a jacket. Although few details are provided on Odlo’s website, it
seems that the heating elements are knitted in the shape of a coil. The company says that
with this option for total control of the personal microenvironment, it is not necessary to
wear an extra layer under the winter sports jacket. Equipped with a battery offering 4 h of
autonomy, Odlo’s I-Thermic sweater seems safe, since the heating elements and software
are set not to exceed 37
◦
C and stop immediately in case of higher temperatures, avoiding
overheating.
In association with Helly Hansen, the Canadian company Myant has announced a
line of active thermal workwear that provides thermal regulation for low-temperature
environments. These garments feature an electronic textile layer and include a base
layer top, leggings, socks, balaclava, and gloves. Equipped with textile heating elements
and integrated temperature sensors, the system detects the skin temperature and the
temperature of the microclimate close to the body to trigger a reaction by actively supplying
heat through the textile to regulate the temperature. Being designed using advanced
knitting technology, these workwears have a tailored design to better keep the sensors
and actuators in contact with the body. Note that the company Myant, according to the
information available on its website, seems to have the will to contribute to the future of
work through smart textiles, artificial intelligence, and the Internet of Objects. In addition
to a platform to measure the physiological parameters of workers with smart textiles, they
want to be able to measure the environmental conditions (temperature, humidity, CO
2
and
methane levels, noise level, etc.) of a workplace.
Another Canadian company, Kinesix Sports, is working on the development of an
intelligent heating jacket equipped with five thermal sensors capable of monitoring the
temperature inside and outside the jacket in real time, and it includes 12 heating pads
made from PTC carbon ink encapsulated between two layers of polyester. The ink used
for the pads is specially designed to stop heating when the maximum temperature of
40
◦
C is reached, thus avoiding overheating. The system, based on a technology called
ThermoAdapt, exploits artificial intelligence, more precisely automatic machine learning,
to adapt to and anticipate temperature variations as the jacket is used. The heating pads,
powered by an external battery, are in four independent zones of the jacket. The system
constantly and independently adjusts and regulates each zone according to the temperature
selected by the user. In addition, a thermal sensor located on the outside of the jacket can
detect sudden temperature changes in the outside environment in order to instantly stop
or activate the system. The four thermal sensors positioned inside the mantle, near each
heating zone, help the system understand whether it is necessary to heat the entire body or
only a specific part of the body. However, the system also allows the heating system to be
activated manually if necessary. The heating pads are removable so that they can be easily
replaced in the case of a malfunction.
Regarding an occupational health and safety application, this study also identified
a few companies that offer products targeting workers in various industries, including
Polymers 2021, 13, 3711 33 of 80
construction, heavy industry, or all types of cold outdoor work (post office, airport runways,
etc.). Some offer clothing that can be worn under a uniform or work clothing (Mobile
Warming, Warm Fitness, Volt Smart Yarns, Techniche). Others offer high-visibility heated
jackets, vests, or hoodies (Mobile Warming, Dewalt, Makita). Finally, five companies offer
products dedicated to workers: Milwaukee, Dewalt, Bosch, Makita, and Myant-Helly
Hansen (including an intelligent garment offering self-regulation of body temperature that
was described above). Among the range of products for use in the workplace, Makita’s
jacket provides 28 h of warmth with an 18 V battery.
3.4. Heated Actuator Challenges
Among the different categories of heated clothing designed to provide better comfort
during activities in extreme cold, this study focused on electric heated clothing providing
continuous heat within the limits of their energy sources, while offering the possibility of
developing space-saving structures with a reduced thickness (Table 3).
Despite a very good potential, at the current state of technological advancement, PCM-
based garments do not have the capacity to provide sustained and durable heat throughout
a working day in a cold environment due to the temporary heating effect of PCM-based
heating elements, the low thermal effect and durability problems of microencapsulated
PCM coated on the textile, and the high weight and reduced sweat evacuation in PCM
pocket-based garments. Therefore, significant work is still required to achieve a sustained
heat effect from PCM garments.
Table 3. Heated actuator.
Technology Used Integration Method Operating Temperature Range References
Silver ink-based printed
heater
Heat-curable ink (Fabinks-C4001 silver
ink) direct dispenser printed on
UV-curable ink (Electra EFV4/4965
dielectric) as printing interface and
untreated woven polyester/cotton fabric
Heating up to 33
◦
C [200]
Ag nanowire-coated heating
fabric
Heating fabric made of pre-cleaned bare
cotton fabric dipped in ethanolic
solutions of silver nanowires for 5 min,
then dried at 80
◦
C for 10 min
Heated up to 50
◦
C under an
applied power density (30–150
◦
C
can be obtained according to the
applied voltage)
[201]
Silver filament-based heating
membrane
Flexible and waterproof heating
nano-silicon carbide (SiC)/thermoplastic
polyurethane (TPU) hybrid membranes
(prepared by pouring modified
nano-SiC/TPU solution into a mold with
silver filaments)
Depending on the applied voltage
(1.4–5.14 V), a maximum
temperature of 20–160
◦
C
[202]
PEDOT coated-based heating
fabric
In situ polymerization of poly
(3,4-ethylene dioxythiophene)
p-toluenesulfonic acid (PEDOT-PTSA) on
a textile polyester fleece
With a surface resistance down to
10 Ω/sq can even reach 170
◦
C by
applying 24 V
[203]
PEDOT coated-based heating
fabric
Vapor phase polymerization of PEDOT
coatings on the textiles (pineapple and
cotton fiber-based fabrics)
Cotton-coated fabric generated
28
◦
C when connected to a 4.5 V
battery and 45
◦
C when connected
to a 6 V battery
[204]
Poly pyrrole-coated textiles
Polyamide knitted fabric impregnated
soaked with pyrrole and then dipped into
polymerization solution of the dopant
(p-toluenesulfonic acid) and the oxidizing
agent (Iron (III) chloride hexahydrate)
45 to 105
◦
C produced depending
on the heated surface area
[191]
Polymers 2021, 13, 3711 34 of 80
Table 3. Cont.
Technology Used Integration Method Operating Temperature Range References
Carbon fiber-based composite
as a heating element
Polyacrylonitrile-based (T-800s) recycled
carbon fiber sheet with polyurethane
binders (three types were used: Primal
ECO-16, Resin HF-05A, and Emuldur DS
2361 PU)
Heating up to 96
◦
C (20 to 96
◦
C
range)
[208]
Carbon fiber-based
electroconductive heating
textile
Carbon-based electro-conductive textile
(from Gorix Inc.) integrated in a carbon
fiber composite laminate and woven
glass fiber plies
Tested at 0
◦
C, −10
◦
C, and −20
◦
C
in an environmental chamber
[209]
Carbon fiber-based heating
elements
A commercialized carbon fiber-based
resistive-heating blankets (SmartCare by
Geratherm Medical AG) compared with
air or water-warming systems
Providing 42
◦
C during
120–150 min
[210]
Carbon fiber-based heating
elements
A heating garment based on a carbon
fiber fabric with carbon content that can
be divided into surface and linear heating
N/A [213]
Vest based on a Carbon
polymer heating element
Electrically heated vest (six strips of
carbon polymer heating elements made
from
the ultrathin, biothermal carbon fiber
inserted into six front and back sacks
inside a polyester woven vest) worn with
cotton knit underwear and a military
uniform (polyamide/cotton) in different
sequences
Heating up to 24 to 26.5
◦
C
depending on the placement of the
elements
[212]
Vest based on a carbon
polymer-based heating
element
Electrically heated vest (carbon polymer
fabric-heating element in a polyester vest
with polyamide batting) of four-layer
structure with protection layer,
heat-insulating layer, heat-generating
layer, and base layer
Providing 34
◦
C around torso skin
and 38
◦
C on the outside surface of
the electrically heated vest, tested at
0
◦
C and −10
◦
C; 30% RH; 0.4 m/s
of air velocity
[211]
Electrically heated garment
based on carbon heating
wire-based garment versus
chemically heating garment
Two heating technologies compared: (1)
Two types of heated ensembles by
embedding seven heating elements into
the vest (each heating pad was
manufactured by sandwiching carbon
heating wire between two layers of
high-density woven polyester fabrics), (2)
polyester-based ensembles with 14
chemical body warmers
Validated at 2.0 ± 0.5
◦
C and 85 ±
5%; 44
◦
C by the electrically heated
garment and 46
◦
C by the PCM
garment
[162]
Stitched heating actuator
A single-trace serpentine pattern of
silver-coated Liberator40
®
conductive
fiber (by Sysco Advanced Materials, Inc.)
that has a polyester Vectran
TM
core
(Kuraray Co. Ltd.) stitched on an
elastomeric knit fabric
Heating up to 33–40
◦
C [177]
Stitched heating actuator
Electrical heating system using Liberator
40
®
conductive fiber with a polyester
Vectran
TM
core stitched on stretch knit
fabrics (cotton/spandex,
polyester/spandex, nylon/spandex)
20–140
◦
C heat generated
depending on the number of thread
layers, the thread spacing, and the
knit fabric type and fabric covering
[54]
Polymers 2021, 13, 3711 35 of 80
Table 3. Cont.
Technology Used Integration Method Operating Temperature Range References
Sewn silver-based yarn
Heating element based on conductive
yarns made from stainless steel fibers or
polymer yarns that have been coated
with silver or copper
A maximum temperature of
37–39
◦
C
[178]
Stitched silver-coated heating
actuator
Heating actuator made of stitching
silver-coated polyamide yarn over
polyester plain woven fabric
Heat generated in a range of 27 to
43
◦
C
[166]
Silver-coated yarn vs.
Stainless steel
Two types of stainless steel and two types
of silver-coated polyamide with different
linear density and yarn structures
A maximum temperature of
38–55
◦
C depending on the knit
structure
[187]
Silver-coated yarn-based
woven fabric
A simulation model derived to compute
the resistance of conductive woven fabric,
validated with two silver-coated
conductive polyamide 6 and polyamide
6.6-based yarns blended with cotton in
three woven structures
N/A [199]
Silver-coated polymeric
yarn-based heating element
Thermo-mechanical properties of knitted
structures mathematically modeled and
validated on an elastomeric and silver
yarn knitted structure
27.4
◦
C, 30.1
◦
C, and 31.6
◦
C
depending on the plain, rib, and
interlock structures while applying
3 V
[190]
Silver plating yarn-based
heating knit
Silver plating compound yarns fabricated
by twisting three silver filaments and
polyester staple fiber spun yarns utilized
in three types of knit (plain stitch, ribbed
stitch, and interlock knit)
25–70
◦
C can be produced
depending on the applied voltage
and the knit structure
[183]
Ag nanowire-coated heating
fabric
Two conductive yarns (silver-coated yarn
with polyamide 6 and polyamide 6.6
inner fibers) embedded into normal
knitted woolen fabrics
25–55
◦
C produced depending on
the applied voltage
[182]
Woven silver filaments or
coated silver yarns-based
heating element
Relation of function of parameters of the
heating fabric expressed by an equation
for a design prediction, validated on
woven fabrics made of cotton/Tencel™
lyocell blend using different conductive
components such as silver filament,
silver-coated yarn, and coated silver
knitted fabric
Three different fabrics with set up
resistance of 10 Ω, 14 Ω, and 18 Ω,
providing different levels of
temperature
[195]
Steel-based fiber panels
Panels construction made of continuous
stainless steel filament yarns based on
metal fibers with polyester yarns
30–50
◦
C depending on the amount
of the ply of the pad
[186]
Fine copper wire and fusible
interlining fabrics
Non-woven and woven interlining as
substrates, bonded fabrics of nylon and
cotton, copper wires all bonded by
thermal fusing
21–95
◦
C produced depending on
the applied voltage
[188]
Heat-insulated shape-memory
element-based EHG
The fabric made of three layers of
non-wovens from the blends of flax and
steel fibers and the two interlayers
included spirals, made from Nitinol
(NiTi) or copper (Cu) wire
34–40
◦
C produced depending on
the applied voltage
[159]
Polymers 2021, 13, 3711 36 of 80
Table 3. Cont.
Technology Used Integration Method Operating Temperature Range References
Conductive-coated
yarn-based knitted or sewn
fabrics
Knitted structures by using different
conductive yarns made of stainless-steel
fibers covered by polyester fibers
(DA5393, DA5340; Bekitex 50/1)
35–60
◦
C produced depending on
the design and the fiber type
[185]
Weft knitted heating pads
Acrylic, polyester as main yarns and
three different conductive yarns
(Copernic non-insulated (9 Ω),
Thermaram hybrid (5.8 Ω),
Thermotech-N non-insulated (9.6 Ω))
Copernic
(35.2–48.8
◦
C)/Thermaram
(33.4–60.28
◦
C)/Thermotech-N
(35.4–48.4
◦
C) depending on the
main yarn composition
[184]
Conductive knitted fabric
based on elastic-conductive
composite yarn
A spandex filament as the core and a
stainless-steel filament combined with
rayon fibers as a helically wound sheath
around the spandex core, embroidered
on fabric knit with spandex content
Tested at 20
◦
C, 65% RH and heat
generated in a range of 30 to 90
◦
C
depending on the applied voltage
[189]
Conductive knitted fabric
based on stainless steel yarn
A physical model in order to predict the
electrothermal behavior of stainless-steel
knitted structure, validated by a
stainless-steel heating fabric associated to
bus-bars of highly conductive
silver-coated polymeric yarn
Produced heat depends on the knit
structure: 1.5 V applied: 35.6
◦
C
(plain) 42
◦
C (interlock); 3 V
applied
◦
C (plain) 84
◦
C (interlock)
99
◦
C
[192]
Conductive knitted fabric
based on silver-coated yarns
A theoretical model proposed to control
the temperature of conductive knitted
fabrics, validated by conductive knits
made of two types of conductive yarns (a
monofilament of 68.6 Ω/cm and a
silver-coated yarn of 1 Ω/cm embedded
into different knitted wool fabrics)
25 to 60
◦
C depending on the
applied voltage and the loop
arrangement
[193]
Conductive knitted fabric
based on silver-coated yarns
An electrothermal model considering
thermal conductivity coefficient, specific
heat capacitance, fabric mass, and initial
temperature, validated by the average
temperature of the knitted fabric of wool,
cotton, and acrylic blended with
silver-coated conductive yarns
45 to 70
◦
C, depending on the blend
type and the loop density
[194]
Conductive knitted fabric
based on silver-coated yarn
The resistance of conductive knitted
fabrics modeled by contact resistance and
the superposition of the length-related
resistance and contact resistance,
validated on two overlapped conduct
yarns and conductive knitting stitches
(composed of silver coating yarn and a
cotton yarn) under unidirectional
extension
Initial resistance of two overlapped
yarns varying from 2 to 6 Ω
[196]
Conductive knitting stitches
Equivalent resistance of a knitted stitch
with different courses and different wales
modeled, validated on knitting materials
that included one non-conductive yarn
made of wool and three
Statex-conductive silver-coated yarns,
designed in two types of knitting stitches
(jersey and intarsia)
The global resistance depends on
the course/wale’s configuration
[197]
Polymers 2021, 13, 3711 37 of 80
Table 3. Cont.
Technology Used Integration Method Operating Temperature Range References
Conductive knitted fabric
based on silver yarn
A sheet resistance method to compute the
resistance of conductive fabrics from a
macroscopic view, validated on a knitted
fabric (wool associated with two
conductive silver-coated yarns resistance
of 1 Ω/cm and 4.7 Ω/cm)
An equivalent lump resistor of the
conductive fabric paths is modeled
[198]
Electrical heated sleeping bags
Heating sleeping bag was developed by
incorporating heating fabrics into the feet
region of the bag (no precision on the
heating element composition)
Tested at 5.5
◦
C and −0.5
◦
C, 80%
RH; 0.4 m/s wind speed, with a
heating capability from 22 to 34
◦
C
[214]
Electrical heated glove
Heating plates fixed in the back side of
the limiting layer of the fingers in glove
(no precision on the heating element
composition)
(a) Tested in an environmental
temperature of −130
◦
C; (b) the
gloves are supplied active heating
to keep the finger temperature
higher than 15.6
◦
C
[215]
Electrical heated garment
A jacket with integrated heated elements
(Powerlet rapidFIRe Proform Heated
Jacket Liner by Warren)
Produced heat of 50
◦
C tested on
subjects after swimming in the pool
water temperature of 27.6
◦
C (Air
temperature 23.4
◦
C, 56% RH)
[216]
Controlling the heating
temperature of the vest based
on a steel-based fiber panel
Heating vest composed of a heating
system based on pads using stainless
steel yarns with single-, double-, three-,
and four-ply configuration. The heated
panels were mounted onto the carrier
using Velcro tapes worn under a garment
made of cotton as outer layer, polyester
as the lining, and polyamide as the
net-like fabric
Depends on the amount of the ply
pads and the power source
[217]
Temperature-regulated
clothing
A newly developed metal composite
embroidery yarn made of
polyurethane-coated copper filaments for
both temperature sensing and heating
textile
Operating temperature set to 20 to
40
◦
C
[56]
The self-regulating garment
Heating garment composed of: (1) The
actuator based on silver-coated polyester
Vectran™ multifilament yarn stitched in
a serpentine pattern, (2) The garment
designed in a three-layer assembly: the
heating element on the outside of the
polyester/spandex knit base layer; an
aluminized biaxially-oriented
polyethylene terephthalate film layer
above to improve heat retention; and a
textile cover layer on the outside, (3) The
self-regulated garment device with
integrated closed-loop T
s
feedback using
NTC thermistors placed immediately
underneath each zone and a
microcontroller-based control system; (4)
The user-controllable self-regulated
garment with the thermistor feedback
Generated heats from 20 to 80
◦
C
depending on the applied power
[164]
Polymers 2021, 13, 3711 38 of 80
Table 3. Cont.
Technology Used Integration Method Operating Temperature Range References
PCM associated with heating
textile
Clothing system consisting of four layers:
(1) Cotton fabric, (2) Non-woven
polyester fabric treated with/without
PCM enclosed in small polymer
micrometric spheres with or without
conductive heating fabric, (3) Non-woven
polyester fabric, (4) Waterproof
breathable fabric as the outermost layer
25–33
◦
C depending on the
structure
[218]
CNT-coated triacetate
cellulose-based fibers
Metatextile with dynamically adaptive
infrared optical properties to directly
regulate thermal radiation. Each fiber is
elliptically shaped, with triacetate and
cellulose components fused side by side,
knitted, and then coated by few-walled
CNTs in a process similar to solution
dyeing
N/A [221]
Water-perfused trousers
Water-perfused trousers with an adjusted
water temperature of 43
◦
C
Tested in an ambient environmental
temperature
[222]
In order to overcome the disadvantages of the conventional use of electric heating
wires, heating elements based on conductive textile fibers have been developed in recent
years. Within this context, several methods have been proposed to design heating elements
based on metallic textile wires (fibers coated with a composition containing metallic par-
ticles) or based on metallic compositions (i.e., based on copper, steel, silver fibers, etc.).
However, the analysis of the research work has shown that obtaining such textile heating
elements requires the control of many parameters. Concerning the heating elements de-
signed with the embroidery of metallic (textile) threads, the number of thread passages,
the spacing between the threads, and the composition of the base fabric have an impact on
the heating temperature and the level of electrical power required. Despite the advantages
of heating elements embroidered with metallic threads in terms of energy consumption,
research is still needed to better control the influence of the fiber content of the base fabric
and the enlargement of the size of these heating elements. Despite the advantages of a
flexible and stretchable structure of the knitted heating elements, their design is also a
technological challenge. It has been shown that the thermal effect achieved in heated
knitted fabrics depends on the type of conductive yarn, its mechanical properties, the
structure of the knitted fabric, the knitting method, the composition of the textile fibers
surrounding the conductive yarn in the knitted fabric, and the number of plies in a possible
multilayer structure. It has also been shown that with the right design and conductive
yarns with appropriate electrical resistance, knitted heating elements working with low
power supplies could be developed. In a possible approach to integrating knitted heating
elements in protective equipment, special attention must be paid to such parameters, in
particular the structure and composition of the layers constituting the workwear.
In order to facilitate the design of electric heating elements based on metallic textile
yarns, some mathematical models have been developed to predict the thermoelectric
behavior of heating fabrics or knitted fabrics [
223
]. With these models, the maximum
equilibrium heating temperature and the time required to reach it can be calculated from
the thermal and structural properties of the fabric and the electrical characteristics of
the conductive yarns. However, as these models have been applied to specific types
of conductive yarns or fabrics, their applicability in the design of protective equipment
with particular compositions and structures remains to be validated. From a general
point of view, very little work has been done on the durability and characterization of
the electromechanical behavior of electrical heating elements based on metallic textile
Polymers 2021, 13, 3711 39 of 80
wires. However, such technical information is necessary for the integration of these heating
elements in protective equipment.
Heating elements with versatile designs can be formed on the surface of flexible
substrates using coating techniques. Silver particle-based coatings ensure low energy con-
sumption and very short times to reach the maximum equilibrium temperature. However,
their low washout durability can be a very important shortcoming. In addition, coatings
based on silver nanofilaments have poor stability in ambient air. Therefore, encapsulation
techniques would be necessary to protect them in a possible integration process in protec-
tive equipment. Despite the flexibility and lightness offered by the coated layers based on
conductive polymers and their ability to provide stable heat at high temperatures, they
require a fairly high energy consumption and present certain failures from a mechanical
resistance point of view. On the other hand, carbon fibers have been the subject of research
work as well as numerous industrial developments in recent years. Indeed, due to their
good thermal efficiency, rapid attainment of uniform heat, rapid recovery of the initial
temperature when the power supply is switched off, and a very high electricity conversion
rate, carbon fiber-based heating elements are ideal candidates for the implementation of
precise temperature control. However, further research is needed to optimize the wash
resistance and energy consumption of carbon fiber heating elements.
Despite the large number of studies dedicated to the development of new types of
heating elements, little work has been done on the design or efficiency of heating garments.
Furthermore, few studies have been devoted to the use of heating garments in a work
context or to the development of protective equipment with heating elements. Indeed,
most studies have been carried out in the laboratory with few human subjects. As some
studies have highlighted a difference in the sensation of comfort expressed between male
and female subjects when using heated clothing, more investigation is also needed in order
to define the optimal heating conditions. Based on the results of previous studies, the
impact of factors such as the combination of the heated garment with other clothing or
environmental conditions on the performance of the heated garment in a work environment
should be studied in order to obtain the best possible thermal performance. Extending
the research on conventional (electric) heated gloves, the influence of the structure of the
heating element as well as the design of a protective glove with heating elements on the
dexterity of the fingers remains to be studied.
In addition, the association of electric heating elements with far-infrared wave reactive
heating panels or PCM-based heating elements to ensure a better energy consumption
efficiency proposed in the literature is one of the concepts that remains to be explored
in the structure of protective equipment and an active work context. The association of
temperature-sensitive shape memory materials with electric heating elements allowing the
placement of textiles with insulation properties that vary with the level of heating, used
as a means to control the overall degree of insulation of the fabric, is another concept that
could be applied to protective equipment to provide better protection to the worker.
As with the literature review, research on products containing heating actuators has
focused mainly on electrical heating garments. As this is a dominant technological trend
and there is strong industrial competition between the various players in this sector, several
companies did not provide any information regarding the technology used in the design
of the electric heating elements of their products. Despite all the known limitations of
electric wire-based heating elements, this technology still seems to attract the attention
of a significant number of manufacturers because of the simplicity of its implementation.
However, because of the advantages of using carbon fibers in the design of heating elements,
this technology seems to be the new trend among manufacturers. Positive temperature
coefficient (PTC) heating elements are also a growing category of technology because of
their ability to self-regulate the heating temperature to a specific level. Due to numerous
advantages such as quick response to temperature change commands, good thermal
efficiency, uniform heating capacity, etc., carbon fiber or PTC-based heating elements can
Polymers 2021, 13, 3711 40 of 80
form the basis for future work on the integration of heating elements in personal protective
equipment.
The analysis of commercial products has also shown that more and more warming
garments allow several areas of the body to be heated, while enabling the temperature to
be varied using an integrated control switch or wireless temperature control. Although a
number of these types of electric heat garments are also intended for workers in different
industries, the heating zones are fixed, and temperature settings are often limited to
three levels and restricted temperature ranges. Not only are these products unable to
provide a customized solution, but such structures can also present serious overheating
problems when used during intense work activities. Therefore, the few products offering
independent heating zones and allowing interruption or adjustment of the temperature
of each zone separately, as well as garments offering temperature control using a variable
switch may be of interest for adaptation to use in work environments. In future work, it
may be important to study the impact of independently controlling the heating temperature
of different parts of the body based on the heat loss of different parts of the body, which can
vary considerably depending on physical activities performed and the type of equipment
worn (helmet, harness, etc.).
Thanks to advances in portable technologies, a limited number of products that allow
self-regulation of the heating temperature using integrated thermal sensors that measure
the microclimatic temperature inside the garment or the skin have been launched on the
market over the last two years. As this is a very recent technology, the effectiveness of
such systems, as well as their impact on the physiological aspects of people performing
cold work tasks, remains to be studied. In addition, the integration of self-regulating
temperature actuators into personal protective equipment structures requires significant
research efforts.
4. Cooling Actuator
Among the various means of intelligent thermal management, cooling actuators are
the technological solutions most dedicated to the occupational health and safety applica-
tion. Since the evaporation of sweat is the most efficient way for the body to cool down,
it is practically impossible to do so when wearing fully enclosed protective equipment
such as protective clothing against chemical, biological, radiological, or nuclear CBRN
hazards [
224
]. In addition, the weight, stiffness, and multilayer design of many protective
equipment such as those used by firefighters can increase the energy cost associated with
wearing them during work [
225
]. Increased metabolic heat production and decreased
body heat dissipation under the protective layers of such equipment can lead to decreased
physical performance and increased risk of heat stress [
226
]. In some workplaces, it
is not economically viable or practically impossible to make environmental changes to
reduce ambient temperatures. Such cases include hot open environments and large work-
places such as deserts, steel mills, smelters, mines, and metallurgical plants [
58
]. Due
to the requirements for the design of protective equipment, small variations in thermal
properties introduced in their design have had little or no effect on heat exchange with
the environment [
227
]. As a result, personal cooling garments have been proposed to
provide an effective method for cooling the body under protective equipment or in hot
environments [
58
]. Based on microclimatic cooling focused on the regulation of body
surface temperature, personal cooling garments have been deployed to promote the body’s
heat exchange with the environment through the heat transfer by conduction, convection,
radiation, and evaporation [228].
4.1. Cooling Garments Categories
Personal cooling garments can be divided into two main categories according to their
passive or active cooling system. Passive cooling garments include conductive, phase
change (PCM) cooling, and evaporative cooling elements. Active cooling garments include
thermoelectric, air ventilation, and circulating fluid coolers [
57
,
229
]. While the performance
Polymers 2021, 13, 3711 41 of 80
of passive cooling garments is likely to be greatly affected by environmental conditions,
user activity, and the resulting generation of body heat, the effect of active cooling garments
is relatively stable and less likely to be affected by environmental conditions [57].
4.2. Phase Change Material Integration in Cooling Garments
The present study was particularly interested in the analysis of active cooling garments
that could provide sustained cooling, depending on their power sources. For passive
cooling garments, the detailed analysis focused instead on conductive and evaporative
cooling elements. As the integration of PCM cooling elements in garments has been studied
in various studies, their state of the art has been widely documented [
59
,
60
], demonstrating
that their application for persistent cooling requires significant research efforts.
Indeed, PCM cooling garments use the energy of latent heat to maintain the microcli-
mate temperature close to the skin temperature. The cooling mechanism is based on the
melting of a substance going from a solid state to a liquid state that allows the absorption
of body heat transported to the skin surface. This type of cooling is effective when PCMs
change from their solid to liquid phase. Therefore, the cooling effect is only effective
within a narrow temperature range of the microclimate that triggers a phase change of
the material [
57
,
58
]. Being a relatively simple system to deploy, the PCM-containing layer
requires direct contact with the skin for a superior efficacy [
57
]. Since the efficiency of the
thermal effects and their duration depend mainly on the latent heat storage capacity of
the PCM itself, the quantity of PCM used is the main factor affecting thermal efficiency
and the amount of energy absorbed or released at the time of phase change [
230
]. In order
to achieve good thermal productivity, cooling elements in the form of pockets containing
PCM were integrated into the cooling garment design. However, these types of pockets
have some disadvantages such as obstruction to sweat evacuation or the stiffness and
weight of the pockets, reducing the mobility of the user [
58
]. Indeed, the conventional
duration of the cooling effect of PCM embedded in the textile is 15 min and can rise to a
maximum of 2 h depending on the number of layers, the mass, and the area covered by the
material, but at the cost of a significant increase in the weight of the garment, which will
increase the energy expenditure of the individual [230].
To overcome these problems, experts proposed the coating of microencapsulated
PCM on fibers or fabrics. However, as the amount of microencapsulated PCM inserted
into textiles to ensure thermal productivity increases, the permeability (to air, vapor, and
moisture) of the fabric decreases. In addition, as the stiffness of the fabric increases, its
softness and flexural strength decreases. Furthermore, despite efforts to improve the
resistance of PCM microcapsules to washing, abrasion, and high temperature, it has been
reported that the material can lose up to 60% of its heat storage capacity after a few
washes [
230
]. With respect to their integration into personal protective equipment, the
flammable structure of some PCMs would not be suitable for work environments in direct
contact with fire [
58
,
230
]. In addition, the thick and sometimes multilayered structure of
personal protective equipment can negatively influence the effectiveness of the PCM-based
element by delaying the release of latent heat [
59
]. Although several research groups have
attempted to overcome some of the limitations of PCM-based cooling elements by chemical,
physical, and mechanical means such as improving their stability during phase change, the
cooling capacity of this technology remains relatively low [59].
4.3. Active Cooling Actuator
Personal cooling garments were initially developed to reduce the effect of thermal
stress in hostile aerospace and industrial environments. Even if the first developments date
back 50 years, research on the optimization and effective integration of these devices into
clothing continues [58].
According to a first observation, a large part of the work on cooling garments is
dedicated to fluid cooling garments (FCG). These garments employ a conduction cooling
system that circulates cooled fluid inside a garment close to the skin surface. The cooled
Polymers 2021, 13, 3711 42 of 80
fluid can be a liquid such as water or compressed or ambient air. A network of pipes
attached to the inside of the garment conducts the cold fluid through the garment and
returns it to a cooling device after conduction heat exchange with the body. The cooling
system typically contains a pump, a reservoir, and a control valve [
231
]. To date, the main
application areas for these garments have been in space suits during extra-vehicular activi-
ties, sunlit aircraft cockpits, military operations, mining, and the warm-up or cooldown
phases of elite athletes. They may also be advantageous for workers working in vehicles,
as it is convenient to attach the refrigeration unit or compressed air system to them [
58
].
As this technology has been in use since the 1960s, a significant part of the last ten years of
research on FCG has been dedicated to the study of the physiological response of the body
under cooling conditions.
Since the conduction mechanism requires direct and continuous contact between the
tubular network of FCG and the skin, the contact pressure and the uniformity of tube
distribution could have a major impact on the heat exchange between FCG and the body.
In order to promote this heat exchange, the inner textile layer of FCG to be worn close to
the skin should have good thermal conductivity and provide good moisture management,
while ensuring a good fit to the body and good tactile properties. In addition, the material of
the tubes, their thermal conductivity, overall length, internal diameter, and wall thickness,
as well as the flow rate and temperature of the circulating fluid are other parameters that
influence the effectiveness of FCG. The distribution of the tubes is another important factor
affecting the efficiency of FCG in cooling different areas of the body or intermittent and
regional cooling. In addition, liquid and air-cooled garments are limited by their required
power and total system size [58,231].
4.3.1. Fluid Cooling Garment Design
As a result, several studies have been devoted to the optimization of FCG design
in recent years. In this context, the comparison of two water FCGs of identical tubular
networks but different textile structures on a thermal manikin have shown that the type
of knitted fabric used to contain the tubes greatly affects the heat transfer in the garment.
For example, double Jersey fabrics with naturally curved structures that accommodate the
tube would provide a better cooling effect than single Jersey fabrics that require additional
material, such as foam interlining, to accommodate the tube, leading to a lower heat
transfer coefficient [
232
]. As an interruption of liquid flow can occur with the compression
of the tubes integrated in the FCG garment, an optimization of the integration of the tubes
into the textile was proposed by inserting them directly into the modified structure of a
specific knit fabric that included a spacer containing channels produced during the knitting
process. This development, which aimed at a better ergonomic contribution, remained to
be validated on human subjects or thermal manikins [
233
]. Based on a series of sequential
tests evaluating the physiological and psychological sensations of the individual, the
arrangement and fixation of the tubes, the textile materials, and the assembly of the piece
were progressively improved in order to propose a process for the design and conception
of an FCG garment hood. Despite the proposed methodology, the study remains limited
due to the testing of only one male subject [234].
As part of the development of FCG for a space suit and in order to determine whether
the capacity of the mechanical pump was appropriate for this system, the heat removal
capacity of the system was determined by applying a thermodynamic heat exchange model.
The equation was subsequently validated by comparing the theoretical values with the
values obtained by thermocouples recording the entry and exit temperature of the FCG
suit [
235
]. In another theoretical study, a model considering the metabolic heat, convective
heat flux, and radiation heat flux of the environment was set up to analyze the effects of
different factors in the performance of FCG in a warm environment and to identify the main
limitations preventing optimal performance. Model validation tests on a thermal manikin
and the thermal resistance analysis demonstrated that the flow rate of the liquid circulation
had a greater effect on the thermal resistance between water and the environment than
Polymers 2021, 13, 3711 43 of 80
between water and the skin. According to the same analyses, the coolant flow rate and the
ambient temperature would greatly affect the duration of action of the FCG garment [
236
].
Some experts have proposed the presence of a cooling control system in FCG garments
to adjust the temperature and flow rate of the coolant circulation according to the microcli-
matic temperature changes close to the skin. Thus, with the decrease in metabolic activity,
the wearer of the garment would not experience undesirable body heat loss and thermal
discomfort due to excessive cooling [
229
]. Indeed, some work has focused on the develop-
ment of devices to control the flow rate of fluid circulation, since earlier studies on human
subjects had shown that intermittent cooling could reduce the effect of thermal stress in
a manner equivalent to continuous cooling by FCG while allowing moderate peripheral
cutaneous vasodilatation to be maintained compared with the cutaneous vasoconstriction
of over-cooled skin [
237
]. Such methods of intermittent cooling, involving a 2-min cycle
of operation and 2 min of shutdown, have also been compared to continuous cooling or
alternate cooling based on a change in the direction of flow every 2 min in a water-based
FCG through tests performed on a thermal manikin [238]. According to the results of this
study, the risk of overcooling is very low with alternate cooling, which would also have
increased system efficiency by more than 50% compared to continuous cooling. However,
intermittent cooling was not considered to be very advantageous, as some of the potential
efficiency gains from this mode could be lost due to off-cycle losses [
238
]. The controlled
cooling mode of a water-based FCG, activated at a T
s
of 34.5
◦
C and deactivated at a T
s
of
33.5
◦
C, demonstrated longer periods of heat stress management compared to continuous
and intermittent cooling modes [
239
], and it was included in an analytical study examining
T
s
feedback to activate an FCG when T
s
was in the range of 33 to 35
◦
C [
240
]. Thus, it was
demonstrated that in addition to reducing energy requirements, control of an FCG by the
T
s
of the individual could reduce thermal stress in the same way as constant cooling [
240
].
As in humid environments, water circulation in the space between the skin and the
dense layers of personal protective equipment can lead to the appearance of steam and
cause skin burns; thus, researchers have proposed water-based FCG garments with a
self-transpiration capacity induced by oozing water from 20 pores in the tubular network
for cooling with heat loss by evaporation [
241
]. The self-permeable FCG garment was
designed with a tube attached to the outer surface of the garment to improve moisture
absorption and was subsequently tested by a few male subjects to demonstrate that such a
garment could effectively lower T
s
without increasing the moisture content of the garment.
However, the cooling effect was delayed until a sufficient dose of water was released
and evaporated [
241
]. The same concept was taken up in a second study that proposed
the presence of only 10 pores in the tubular network for evaporative cooling combined
with control of water vaporization by the individual as an additional evaporative cooling
function. Tests conducted in a climate chamber on male subjects controlling the evaporation
process in the garment with a control button demonstrated the ability of a controllable
perspiration FCG in reducing T
s
without causing an increase in garment moisture from the
start of cooling [
242
]. Despite the great potential of FCG function control systems, all the
research work analyzed was limited to validation tests in a laboratory environment.
More recently, nanofluids have drawn the attention of scientists due to their high rates
of heat transfer, which allows them to be used in various industrial uses. A new class of
nanofluids, “hybrid nanofluids”, has recently been used to further improve the rate of heat
transfer. The current phenomenon particularly concerns the analysis of the flow and heat
transfer of SWCNT (single-wall CNT) MWCNT (multiwall CNT)/water hybrid nanofluid
with activation energy through a moving wedge [
243
]. However, this technology has not
yet been used in the development of PPE.
4.3.2. PCM-Based Suspensions as Cooling Actuator
In order to overcome some limitations on the use of cold water in an FCG garment
with respect to the weight of the cooling tank or the influence of ambient heat on the
water temperature, some research has proposed the use of other liquids to be circulated
Polymers 2021, 13, 3711 44 of 80
in the tubular network of FCG [
244
,
245
]. Evaluation of the use of microencapsulated
PCM-based suspensions as a coolant in an FCG worn on a thermal manikin has shown
that the inlet temperature, the flow rate, and the concentration of the microcapsules were
the most influential parameters on the heat dissipation by such a system. With proper
adjustment of these parameters, significantly better heat dissipation could be achieved
with the application of a suspension of PCM instead of water. In addition, the use of a PCM
suspension could improve the performance of the cooling garment without an apparent
increase in pump power [
246
]. A laboratory-scale study of a liquid carbon dioxide cooling
garment worn by male subjects showed that these types of FCGs were effective in relieving
thermal stress by lowering the T
s
and Trec values of individuals, thereby enhancing worker
productivity in a hot, humid environment with a relatively lighter portable cooling system
compared to similarly sized FCGs operating with cold water [247].
4.3.3. Air and Gas Circulation as Cooling Actuator
FCG garments using air circulation in an integrated tubular network have also been the
subject of recent studies. Examination of an air FCG garment with a stationary compressor
generating dehumidified air blown through a tubular network covering certain body
regions under a chemical protective suit has shown that such a device would significantly
reduce the effect of thermal stress. Tests carried out on human subjects have also shown
that with this type of clothing, working hours could be considerably extended [248].
The gas expansion cooling garment is another type of personal cooling garment based
on an integrated tube network distribution. Its operating principle is the endothermic
vaporization of liquefied carbon dioxide (CO
2
), based on the distribution of CO
2
at high
pressure through a pressure relief valve in which the gas pressure drops to ambient pressure
(Figure 5). During this thermodynamic evolution, the liquid CO
2
is transformed into vapor
and absorbs energy equal to the heat of vaporization of the gas and allows cooling of its
immediate environment [229].
Figure 5.
The air treatment system of the cooling garment with front view and back view. The
prototype is composed of three parts: the layers forming the garment, the air treatment system, and
the distribution channels. Reproduced with permission [249]. Copyright 2019, Springer Nature.
Despite its relatively lower total weight compared to water or air FCG garments and
its high cooling capacity, the gas expansion cooling garment has a relatively short service
life. In addition, the escape of CO
2
from a closed environment can lead to hazardous gas
concentrations if the device is used simultaneously by several workers in proximity [229].
In order to address some of the limitations of this type of cooling garment, a portable
system using atmospheric discharge of CO
2
at high pressure has been proposed to im-
prove working conditions in hot and humid environments [
229
]. Thus, a prototype was
developed. It consisted of a three-layer textile structure, an air treatment system using
an atmospheric discharge of highly pressurized liquid CO
2
to cool and dehumidify the
airstream taken from the environment, two identical cylinders of saturated two-phase CO
2
connected to a mixing chamber located inside a mixing box equipped with a heat sink,
Polymers 2021, 13, 3711 45 of 80
and distribution channels made of PVC tubing placed between the moisture-absorbing
mesh layers of the garment, distributed at the back and front of the body. In this approach,
the treated air was directed over the body to create a cool microclimate under the gar-
ment that cooled the body through convective heat transfer and assisted the evaporation
of condensed sweat [
229
]. The evaluation of the performance of this prototype through
tests carried out on male subjects in a hot and humid climate chamber demonstrated the
capacity of such a concept to improve the thermal comfort of people by reducing thermal
stress such as T
c
and HR and the sensation of humidity. However, the conclusions of this
study remain to be confirmed under real operating conditions and with other populations
regarding the sex of participants, average age, and body weight. Some modifications
should also be considered in the design of this prototype for use under personal protective
equipment [229].
4.3.4. Air Blast Cooling
Air blast cooling is another principle used. These types of clothing blow air onto the
body and extract heat from it, improving the evaporation of sweat produced on the surface
of the skin, while at the same time promoting heat exchange by convection using the speed
of air passage over the body surface [
58
]. Most of these garments consist of two layers:
an outer layer of waterproof fabric that prevents air leakage to the environment and an
inner layer of air-permeable material that is directed between two layers toward the skin
surface [
250
]. Since large air movements promote the evaporation of sweat, in some cases,
the use of a compressor attached to the garment has been considered in order to project
forced air. In addition, the use of a cooling device to cool the projected air could result in
a greater temperature difference between the skin and its environment, thus promoting
convective heat loss [227].
4.3.5. Fan-Assisted Garment
This literature review has shown that from a portability perspective, most studies
over the last ten years have focused on cooling by ventilation. These types of garments
contain built-in fans to blow ambient air onto the skin surface to facilitate the evaporation
of sweat. With the use of integrated mini fans a few centimeters in diameter, the cooling
garment can remain light [
251
]. Although their cooling performance may be impacted
by ambient air temperature or humidity, their great advantage is that they rely on the
human body’s thermoregulatory mechanism to dissipate heat, thus eliminating the risk of
overcooling [226,229].
In this context, tests conducted on male subjects in a climatic chamber have demon-
strated the effectiveness of a ventilator-cooled garment in increasing heat loss while main-
taining a constant T
s
value during exercise in a hot and dry environment [
252
]. Calculation
of the physiological strain index (PSI) with data collected during tests conducted in a
climate chamber on male subjects wearing a cooling garment under a military suit showed
that the projection of air onto the torso of individuals was more effective in a hot and dry
environment compared to a hot and humid environment. However, the results showed an
identical reduction in perspiration rates in both climatic conditions [253].
Some research groups have also made performance comparisons with passive cooling
garments. Comparison of a jacket equipped with two ventilators on both sides of the
abdomen and a vest with 21 pockets of PCM cooling under identical conditions showed
no significant difference in the performance of the two garments in terms of torso T
s
and
HR of the female test subjects. However, the PCM garment provided a greater decrease in
the microclimate temperature close to the skin and a better thermal sensation, while the
fan-assisted garment further decreased the microclimate humidity [254]. The comparison
of a cooling vest with frosted pockets and a fan-cool garment allowed the study of the
subjective perceptions of workers in the horticultural and cleaning sectors when using such
equipment during their workday. The data collected showed that male workers’ choice
was more influenced by thermal comfort, while female workers paid more attention to
Polymers 2021, 13, 3711 46 of 80
tactile comfort and the feel of the fabric. This suggests that gender differences need to be
considered in the design of this type of cooling clothing [255].
Studies have also focused on optimizing the design of fan-assisted garments. The
integration of two fans at five different locations in the upper back, lower back, middle
back, upper front, and lower front of a cooling vest being examined on a breathable
thermal manikin showed no significant difference in total torso cooling or total dynamic
evaporation resistance of the garments (Figure 6). However, the local area corresponding
to each ventilator was better cooled [
250
]. The effectiveness of a fan garment in providing
greater comfort to workers working in offices with a warm environment was examined
by wearing a short-sleeved shirt containing two ventilators on the abdomen associated
with two side openings in the chest area and a third in the upper back. Tests conducted
on female subjects with low physical activity in a warm laboratory environment showed
that ventilation reduces T
s
at the location of the ventilators, as well as the average T
s
of the
torso. However, a variation on the mean whole body T
s
and T
rec
was not observed [251].
Figure 6.
Small fans and openings on ventilated jacket located at different torso sites. The both fans are placed at (
a
) the
upper back; (
b
) the lower back; (
c
) the mid back; (
d
) the chest (upper front); (
e
) the belly (lower front). Reproduced with
permission [250]. copyright 2013 Elsevier.
Using numerical simulation of a series of two-dimensional models of convective and
evaporative heat transfer to the skin surface, the efficiency of a fan-cooling garment was
examined by considering different configurations in terms of the number and diameter
of fans as well as different airflow speeds. Simulations showed that convective and
evaporative heat transfer could be improved by the formation of vortex currents produced
when the inlet air flows are high or when the space between the skin and the garment is
wide enough [
256
]. Comparison of a continuous cooling mode with intermittent cooling
on a 2-min operating and 2-min off cycles in a fan-cooled garment showed that constant
ventilation could reduce heat stress to a greater extent during recovery phases. However,
tests conducted on subjects wearing the garment cooling under a bullet-proof vest showed
better perceptual benefits with intermittent ventilation during work and better perceptual
benefits with constant ventilation at rest [
257
]. The use of ventilators has also been extended
to the design of full-face respirators. A comparative examination of a conventional mask
with a modified mask providing air under the mask near the forehead and a second
modified mask providing air from the forehead to the eyes and into the breathing zone
found that air projection through the integral ventilation reduced the T
s
of the face and
minimized the increase in T
c
while improving the subjective assessment of comfort and
thermal sensation in the test subjects [258].
4.3.6. Thermoelectric Cooling
Thermoelectric devices using thermoelectric cooling based on the Peltier effect [
259
]
have also been used in the design of personal cooling garments [
260
], as shown in Figure 7.
Polymers 2021, 13, 3711 47 of 80
Figure 7.
Illustrations of (
a
) the powers harvested by the human body [
261
]: (
b
) Several applications of wearable electronics.
(
c
) A typical flexible thermoelectric generator (F-TEG) on a sphere. (
d
) The unit of the fiber-based F-TEG. Reproduced
with permission [
262
]. Copyright 2017 WILEY. (
e
) A wearable thermoelectric power generator with a fiber-based flexible
substrate. Reproduced with permission [
263
]. (
f
) The reported maximum ZT (ZTmax) for the fiber-based thermoelectric
materials in recent years [264–276]. Reproduced with permission [277]. Copyright 2020, Elsevier.
A temperature-controlled glove was developed by combining thermoelectric modules
with heat sinks in the form of mini-fans and a thermistor placed close to the skin. Using a
feedback microcontroller of the integrated thermistor, the applied voltage could be used
to cool or heat the modules. Despite the validation of the demonstrator developed on
human subjects at the laboratory level, the optimization of the glove size and the area
of thermoregulation remain to be investigated [
278
]. A cooling helmet based on thermo-
electric refrigeration was proposed by implementing two air-cooled and water-cooled
refrigeration modules that each included a thermoelectric element. Tests conducted on a
thermal manikin revealed that the flow rate of the water circulation had a greater impact
on the cooling capacity of the helmet and the coefficient of performance of the system [
259
].
A thermoregulatory garment was also proposed using the connection of a portable thermo-
electric module to a network of air distribution tubes knitted into the garment. By changing
the direction of the electric power supplied to the thermoelectric module, the modes of
operation could be switched between cooling and heating. By examining the relationship
between weights and thermal resistance of commercially available heat sinks, the study
proposed a method to find the minimum weight of heat sinks for a portable thermoelectric
system [279].
A flexible thermoelectric system has also been developed using elastomer layers,
sandwiching rigid thermoelectric modules between two extensible sheets separated by
an air gap to achieve low module thermal conductance and improved flexibility. Then, a
demonstration vest was put in place covering the back, chest, and abdomen with more
Polymers 2021, 13, 3711 48 of 80
than 140 flexible thermoelectric modules [
280
]. Despite the small size of the thermoelectric
modules allowing for portable solutions, it appears that these systems have relatively high
electrical energy consumption and require the use of appropriately sized batteries [229].
4.3.7. Active Evaporative Cooling Garments
The optimization of evaporation, being considered as the most efficient physiological
means for heat dissipation, has also been the subject of studies on the development of
cooling garments [
226
]. Conventional evaporative cooling garments take advantage of
the high latent heat of water evaporation and provide a cooling effect by facilitating
evaporation through a highly absorbent fabric structure [
241
,
281
–
283
], as shown in
Figure 8
.
Then, the cooling effect lasts until all the moisture in the cooling garment evaporates. In
this mechanism, the evaporation of water from a wet media or surface is typically used to
cool the skin [53].
Figure 8.
(
a
) Schematic of an evaporative cooling vest. (
b
) Corresponding cross-sectional schematic
and thermal resistance network presenting different heat and mass transfer processes involved in
evaporative cooling of the wearer. (
c
) A plot of body cooling, convective loss, and evaporative heat
fluxes. (
d
,
e
) Schematic of evaporative vests with the (
d
) louver and (
e
) slitted shading structures.
Reproduced with permission [282]. Copyright 2020 Elsevier.
Polymers 2021, 13, 3711 49 of 80
However, an evaporative cooling garment has the disadvantage of not being func-
tional when worn under dense protective clothing. In addition, its effectiveness is greatly
reduced with high ambient humidity [
226
]. One of the approaches proposed to improve
the performance of evaporative cooling clothing in a humid environment has been the
combination of a ventilation mechanism to wick moisture away more efficiently [
284
]. To
circumvent some of the problems associated with evaporative cooling garments, portable
and motorized evaporative cooling systems have also been explored. In this framework,
a motorized vapor compression device assembled in a backpack configuration has been
proposed to be combined with a cooling garment containing refrigerant lines [
285
,
286
].
Despite very satisfactory cooling rates using a motorized approach, the concept remains
very cumbersome and impractical [286].
4.4. Comparison of Cooling Strategies
In view of the multitude of methods available for the design of personal cooling
garments, some studies have focused on making comparisons between different techniques
in order to propose the best cooling strategies for different conditions. Comparison of
a garment containing two pockets of cooling PCM with a vest containing two fans on
the front and back and a cold water FCG on human subjects under identical laboratory
conditions found that for short cooling periods, active cooling techniques provided rapid
initial reductions in T
c
, whereas a PCM-based device was more influential on T
c
[
287
].
Evaluation of five cooling conditions for people wearing firefighter suits in a hot, humid
environment showed that maximum T
c
could be further reduced when a water-based
FCG garment was combined with air ventilation from protective equipment ducts [
288
].
A study on a thermal manikin in combination with human testing, which compared the
performance of a fan-assisted garment with two cooling PCM garments and a water-based
FCG for military use, found that the fan-assisted garment also improved physiological
responses in subjects to a lesser extent compared to other methods [289].
Cooling capacity, ability to keep the skin dry, operating time, and portability are
characteristics that make it easier to choose the right cooling technique according to en-
vironmental conditions and activity. To this end, a comparative table has been proposed
by experts [
226
]. Data collected from various studies in the literature show that FCG
and vacuum desiccant garments provide the greatest cooling capacity. However, such
comparisons are highly subjective, as depending on the climate, the number of cooling
elements, and areas covered, some of the characteristics presented in Table 4 may vary.
Table 4. Characteristics comparisons of various types of cooling apparel [58,226].
Personal Cooling Garment Cooling Capacity (Watt) Average Weight (Kg) Average Operating Time
By liquid circulation 50–600 3–5 3 to 6 h
By air circulation 270–320 4–5 2 to 6 h
By ventilation 75–350 0.5–1 2 to 8 h
By evaporation 50–70 1–3 1 to 2 h
By vacuum desiccator 320–370 3–4 2 to 3 h
For PCM materials 50–140 4–5 20 to 40 min
From the perspective of the use of personal cooling garments in workplaces, universal
methods have been proposed to facilitate the evaluation and selection of the most appro-
priate system according to the climate and the nature of the activity. Within this context,
a cooling garment performance scale was proposed in order to present the potential suc-
cess of an integrated system to provide thermal comfort under different environmental
conditions. For this, a factor in the form of a dimensionless number between 0 and 1
was proposed, whereby the smallest value corresponds to the system’s lesser capacity to
achieve thermal comfort [
58
]. In a related study, a method for calculating the effectiveness
of a personal cooling garment in meeting the requirements of different types of work tasks
has been suggested. This method considers the cooling capacity, weight, and operating
Polymers 2021, 13, 3711 50 of 80
time of the integrated cooling system, on the one hand, and the work rate, type of terrain,
slopes, or work sites to be covered by the worker on the other hand [
290
]. However, in
order to accurately predict the time required to complete a task, additional methods that
include additional information on body heat loss with or without cooling clothing and the
effect of cooling on the body and its physiology are needed [290].
4.5. Hybrid Cooling Garments
Due to the shortcomings of the cooling methods used in the design of personal cool-
ing garments and the complexity of selecting the best strategy for different activities and
environments, some experts have opted to implement hybrid cooling technologies [
291
].
Although they appear to be more efficient than those using a single technology, hybrid cool-
ing garments can become more cumbersome than systems with a single technology [
229
].
The combination of frozen pads with integrated fans was one approach explored. In this
context, a garment containing three frozen gel pockets and two fans mounted on the lower
back was tested in a warm and humid climate chamber. The results of the tests carried out
on male subjects confirmed the effectiveness of such a hybrid cooling garment in reducing
physiological stress during exercise. However, the concept remains to be validated for
other types of activities and with subjects of other fitness characteristics [292].
The effectiveness of garments equipped with frozen pads and integrated fans was
also validated in a study of 130 Hong Kong workers in the construction, horticulture, and
outdoor cleaning, catering, and airport parking sectors, who generally expressed higher
levels of perceptual comfort when wearing the cooling garment [
293
]. A concept combining
PCM pockets with cold water circulation was also studied.
For this purpose, PCM pockets integrated into a jacket to cool the torso were associ-
ated with a water pipe concealed through the PCM pads to circulate cold water from a
microcooler to refreeze the PCM and extend its duration of action. Simulation work was
used to optimize the parameters related to the type of PCM and the coolant circulation
and to adjust the jacket’s tightness. Subsequently, tests conducted on human subjects
with a prototype developed from the simulated optimizations showed that hybrid cooling
would remain effective for at least two hours of work indoors without sacrificing thermal
comfort [294].
Over the last five years, several studies have been dedicated to exploring hybrid
cooling garments combining PCM cooling elements and integrated fans (PCM/fans) to
ensure better performance in hot and humid climates [295].
To evaluate the performance of PCM/fan hybrid cooling garments, a prototype con-
taining four fans and 24 pockets of PCM [
296
] and a garment with two fans and 24 pockets
of PCM [
297
] were tested on thermal manikins. The presence of fans greatly improved
evaporative heat loss compared to the situation where the fans were turned off. Although
PCM actuators offer limited cooling time, a hybrid garment would provide a certain level
of cooling throughout the test period due to the presence of fans in both hot/dry and
hot/humid environments [
296
,
297
]. In addition, the study of a jacket with eight PCM
pockets and two fans on the lower back by a sweaty thermal manikin in a hot and humid
climate also showed that a higher cooling power would be achieved by hybrid cooling
compared to PCM-only or fan-only cooling configurations [
298
]. A suit containing 24 PCM
pockets and four fans distributed across the lower back of the jacket and the side pelvis
of the pants was also tested on a thermal manikin in hot/dry and hot/humid climates.
The results revealed that in dry conditions, the cooling speed in the initial phases was
higher with the use of PCM without turning on the fans. On the contrary, in wet conditions,
the cooling speed was lower without the fans. In addition, hybrid cooling provided a
significant continuous cooling effect for the duration of the tests. According to activity
simulation tests conducted on the thermal manikin, although the PCM alone or the fan
alone can provide some degree of cooling for light work, it is indeed the hybrid cooling
that leads to an optimized performance for heavy work conditions [299].
Polymers 2021, 13, 3711 51 of 80
A study conducted on human subjects concluded that PCM/fan-cooling garments
could effectively reduce heat stress during exercise in a warm, moderately humid envi-
ronment. Indeed, the use of a suit containing 18 PCM pockets in the upper body and six
thigh pockets in combination with two ventilators on the lower back of the jacket and two
ventilators on the lateral pelvis of the pants reduced subjects’ T
c
, mean T
s
, HR, and PSI,
while improving subjective perceptions during exercise and recovery phases [300].
A similar combination of 24 PCM pockets and four fans was also validated for a warm
indoor environment simulated by a climatic chamber by demonstrating a reduction in
the mean T
s
and total sweat production of subjects, who also expressed good thermal
sensations, skin moisture, and comfort compared to tests without cooling [
295
]. The
effectiveness of a cooling jacket equipped with two fans on the lower back area and eight
PCM pockets distributed on the front and back of the body was also evaluated through a
series of 14 field studies conducted during the summer with 140 Hong Kong construction
workers. Wearing the vest during break phases led to a significant reduction in thermal
sensation, RPE (rating of perceived exertion) scale, HR, and PeSI (perceptual strain index)
in the subjects compared to breaks without cooling. This PCM/fan vest also showed a good
ability to attenuate workers’ perceptual heat strain index during breaks of limited duration
but with much less effect for extended breaks. However, a thorough study of optimal
work–rest duration with cooling by a hybrid cooling garment remains to be done [301].
4.5.1. Design Optimization of Hybrid Cooling Garments
A few studies have also been devoted to optimizing the design of PCM/fan garments
to ensure better management of cooling energy using an additional layer of insulation in
the garment structure [
302
,
303
]. A vest with a structure of two layers of firm fabric and
equipped with a pair of fans installed on the lower back and eight PCM pockets evenly
distributed on the front and back of the body was tested during the rest phases of the
male subjects in activities in a hot and humid climatic chamber. The results of these tests
highlighted the ability of the hybrid jacket to decrease participants’ T
c
and HR and improve
their subjective perceptions [
303
]. For the optimization of a wetsuit with two fans in the
lower back of the jacket, two fans in the side pelvis region of the pants, 18 PCM pockets
placed on the front and back of the jacket, and six pockets in the thigh area of the pants, a
polyethylene insulation sheet has been inserted between the PCM pockets and the outer
layer of the garment. The results of tests performed on active human subjects in a hot
and humid climatic chamber demonstrated that such a design could provide a relatively
cool microclimate around the wearer’s body while minimizing the rise in the average
T
s
. The study suggests the use of such a design for moderate physical activities in a hot
environment thanks to an extended duration of the cooling of the PCM ensured with the
presence of an additional insulation layer [302].
A similar study taking into consideration the same arrangement of 24 PCM pockets
and four fans placed in the jacket and pants of a suit proposed the integration of an
insulating layer composed of polyethylene foam on the outer surface of the PCM pockets
to reduce the heat absorption of the hot environment and extend the operating life of
the PCM used. Subsequently, Tanabe’s thermoregulation model coupled with a model
of heat and moisture transfer through the garment was used to numerically study the
performance of this new hybrid cooling garment. According to parametric digital analyses,
the environmental heat absorbed by the PCM can decrease thanks to the increase in
thermal resistance provided by an additional insulating layer. The validation of the model
by tests carried out on males in a hot and humid climatic chamber also demonstrated that
the presence of an insulating layer in the structure of a garment with PCM/fans could
considerably reduce the environmental heat absorbed by the PCM. Thus, the total PCM
melting time and the effective cooling time could increase [291].
Polymers 2021, 13, 3711 52 of 80
4.5.2. Numerical Analysis of Hybrid Cooling Garments
Some work has also been devoted to the numerical analysis of the performance of
PCM/fan garments under different conditions [
256
,
304
,
305
]. In one of these studies, a
mathematical model was proposed to calculate the transient transfer of heat and humidity
through layers of clothing incorporating PCM pouches and fans. Once validated by exper-
iments performed by a prototype placed on a hotplate, the model was integrated into a
bioheating model in order to simulate an individual working in hot and dry conditions
at different metabolic rates. Numerical simulation results showed that running the fans
during the transient period of sweat absorption by layers of interior fabric could cause
unwanted heating effects and increase the melt fraction of the integrated PCM. However,
these unwanted effects were eliminated by running the fans after the end of this transitional
period to achieve increased heat loss in the torso region, which therefore improved the
comfort and feel levels at tested metabolic rates [
306
]. Another digital model has been
proposed [
305
] to analyze heat and humidity transfer through a PCM/fan combination
having 24 PCM pockets and four fans with the same arrangement described in the work
of [
295
,
300
]. For this purpose, a clothing heat and moisture transfer model coupled with a
multimode human thermoregulation model was developed to determine thermophysio-
logical responses under dynamic environmental conditions. In addition, the parts covered
and not covered by the PCM pockets were considered, and a method for calculating the
apparent heat capacity was used to address the behavior of the PCM. The moisture barrier
effect of the PCM pockets as well as evaporation and condensation on the surface of the
PCM pockets were also considered in the model. Model validation of the data from [
300
]
of PCM/fan combination showed that heat absorption from the external environment by
the PCMs and condensation of moisture on the surfaces of the PCM pockets proved to be
the two major problems in hybrid cooling garments. However, proper ventilation could
play an important role in removing a large amount of moisture and latent heat from this
clothing system [
305
]. The performance evaluation of a suit equipped with 24 PCM pockets
and four ventilators distributed in the jacket and pants was also the subject of numerical
analyses including the simulation of different types of warm environments. According to
numerical analyses of T
c
and T
s
values, high ambient temperature and RH
≥
70% would
weaken the performance of such a suit. However, for better cooling efficiency in conditions
of very high environmental temperatures or RH, the properties of the PCM used and their
level of insulation should be optimized [304].
4.6. Advanced Material Based Passive Cooling Strategies
With the development of advanced materials and the progress made in the elaboration
of conductive textiles, these types of concepts have also been exploited for the implemen-
tation of passive cooling strategies in textile structures [
57
,
307
,
308
]. Examples of recent
work in this field are the creation of artificial leather with very high thermal conductivity
by mixing silver-coated nylon yarns with polyester yarns in a laminated structure using a
polyurethane and methyl cellulose resin [
309
], the design of thermally conductive fabric
with hybrid conductive yarns made of polyester yarns combined with copper filaments
in two different alignments [
310
], the development of thermoregulatory textiles based on
thermally conductive composite fibers of highly aligned boron nitride/polyvinyl alcohol
having been synthesized by 3D printing to take advantage of the in-plane thermal per-
formance of boron nitride [
311
], and the numerical simulation using the finite element
method of heat transfer concepts through an aligned carbon nanotube layer to be integrated
between two layers of textiles to ensure partial heat redirection to a cold reservoir in the
design of a firefighting garment [312].
It has also been reported that mixing phase change materials with active cooling
components such as metals and/or highly conductive ceramics and encapsulated soluble
alcohols such as xylitol that cool in contact with water vapor could allow a PCM to
repeatedly lose heat and thus create an effect similar to a recharging of the cooling effect
of the PCM during exposure to heat. A study has shown that depositing a mixture of
Polymers 2021, 13, 3711 53 of 80
PCM/highly conductive metals on the surface of a sweater could allow the development
of a textile layer creating a multistage cooling effect [
313
]. Nafion
®
, being a selectively and
highly water-permeable, sulfonated tetra fluoro ethylene-based fluoro-polymer copolymer,
has been the subject of recent work to develop a reversible moisture sensitive garment
to support personal thermoregulation in warm environments. For this purpose, smart
textile structures based on Nafion
®
, activated by moisture change, have been developed
with the ability to rapidly and reversibly change their porosities or thermal insulation
levels in response to the individual’s level of perspiration [
314
]. Indeed, a perspiration
pore mimicking structure comprising a network of flaps on a sheet of Nafion
®
could
respond to a moisture gradient by automatically opening or closing to regulate the flow of
air through the pores, thus providing humidity and temperature control. Nafion
®
tapes
inserted between two layers of variable thickness have also demonstrated the ability to
adjust the air gap and change the thermal insulation between two layers of fabric [
314
].
Figure 9 shows some examples of smart textiles with thermal effects.
Figure 9.
Wearable thermal textile: (
a
) Schematic illustration of the thermal regulation textile. The thermoregulation is
established by conductive composite fibers. Adapted with permission [
2
]. Copyright 2017, American Chemical Society.
(
b
) Mimic of thermo-adaptive functionality of human skin on one single Nafion flap. Reproduced with permission [
4
].
Copyright 2017. (
c
) Schematic of the ZnO nanoparticle-embedded textile. The spectrum was designed to be transparent to
thermal radiation and reflective for sunlight for human body. Adapted with permission [
5
]. Copyright 2018 WILEY-VCH.
(
d
) Thermal radiation management illustration of smart textiles with patterned silver strips on a PET substrate and combined
VO
2
nanoparticles. The thermal textile reversibly reflected heat at high temperature and was transparent to IR light at low
temperature. Adapted with permission [6]. Copyright 2019 WILEY-VCH.
Polymers 2021, 13, 3711 54 of 80
Shape memory polymers have also shown great promise in the development of
thermoregulating textiles. These materials sensitive to external stimuli have the capacity
to memorize a permanent macroscopic shape, be manipulated and fixed to a temporary
shape under specific conditions of stress, and then later return to their original state by
no longer being subjected to thermal, electrical, or environmental stress [
102
,
315
,
316
], as
shown in Figure 10.
Figure 10.
Shape memory polymers. (
a
) Schematic representation of sample deformation during shape memory testing cycle.
Reproduced with permission [
317
]. Copyright 2015, Elsevier. (
b
) Shape memory cycle of two hot stages (red background)
and two cold stages (blue background). The shape changes occur during the hot phase. Reproduced with permission [
318
].
Copyright 2018 Elsevier. (
c
) Main stages of thermally induced shape memory polymers [
319
]. (
d
) Classification of shape-
changing polymers. Reproduced with permission [320]. Copyright 2015 Elsevier.
Polymers 2021, 13, 3711 55 of 80
Concerning temperature-sensitive shape memory polymers, large changes in thermo-
mechanical properties occur across the glass transition temperature of the melting point
temperature of the crystals of their soft segment. In addition to these changes, it has also
been shown that this type of material may exhibit changes in moisture permeability above
and below this point [160]. For textile structures, this behavior can be very useful, as they
can provide thermal insulation at cold temperatures and permeability at high ambient
temperatures [
102
]. These materials are particularly interesting for creating cooling effects.
Indeed, when a textile containing a shape memory polymer reaches the glass transition
temperature, it transforms into a fabric permeable to water vapor and heat, allowing the
release of body heat after intense activity or a rise in environmental temperature.
The material may return to a less permeable structure when the temperature drops [
321
].
Despite this potential, their cooling capacity is by no means comparable with that of the
techniques presented in Table 4 [
58
]. The recent use of textiles containing shape memory
polymers in commercial products [
321
] could suggest their association with techniques
used in the design of personal cooling garments (Table 4), but the current literature review
could not find such studies, or they must be rare.
Multilayer garments are another area for improving the performance of conventional
evaporative cooling garments. This approach involves the integration of hygroscopic
materials, of the desiccant or super-absorbent type to promote the absorption of the vapor
produced by perspiration or by the liquid included in an internal reservoir [
322
]. Based
on studies that have demonstrated the increased evaporation rate of water through the
addition of desiccant materials, the desiccant cooling method has been combined with
the vacuum cooling technique to achieve better performance. To promote the integration
of desiccant elements into the garment structure, membrane technologies have also been
proposed to separate the water contained in the cooling core from the desiccant material
present in the absorption core. Polymeric membranes of the polyurethane or polyester
type, being waterproof but permeable to water vapor, were chosen to allow water vapor to
pass through while retaining the condensed water. Once a vacuum is created by a pump,
the operation of these types of garments relies on the absorption of the vapor or adsorption
by the desiccant in order to maintain the driving force for water evaporation [
226
]. Despite
the high cooling capacity of vacuum desiccant cooling garments, very few studies have
been devoted to them [
229
]. Examples of recent studies in this field are the integration
of vacuum desiccant pads into a garment [
125
], the evaluation of an evaporative cool-
ing garment to absorb heat and water vapor under an astronaut’s suit to be combined
with a lithium chloride-based absorbent radiator to reject heat into space [
323
], and the
development of a membrane desiccant fiber for vacuum desiccant cooling in view of the
development of a vacuum desiccant garment [
226
]. It should be noted that the performance
of desiccant systems based on evaporative cooling is much better in dry climates than in
wet climates [57,322].
4.7. Commercial Cooling Garments
Commercially available cooling garments use different technologies. Table 5 shows
the number of companies listed for each of the seven types of commercially available
cooling technologies: active cooling systems such as circulating coolant (liquid and air)
devices, gas expansion devices, air ventilation devices, thermoelectric devices, and passive
cooling systems such as phase change materials (PCM) and evaporative cooling, as well as
hybrid systems using two technologies.
Polymers 2021, 13, 3711 56 of 80
Table 5. Number of companies listed regarding cooling actuators used in thermal management.
Type of Product Technology
Vest
1
Jacket
Leggings
(Chaps)
Other
Clothing
2
Ballistic
Vest
Gloves Helmet Total
By liquid circulation 3 2 5
By air circulation 1 1 1 3
By air ventilation 2 1 1 1 5
By gas expansion 1 1
Thermoelectric 1 1
By PCM
3
11 1 12
By evaporation 2 1 3
Hybrid system 2 1 3
Total 22 1 2 5 1 1 1 33
1
Some products (3/20) are sold exclusively by distributors.
2
Clothing can be vests, shirts, short or long pants, leggings, short or
long-sleeved jackets.
3
Phase change material.
Although the current study focuses primarily on active cooling systems, several
commercially available products use PCM, a passive system, as the cooling technology
(12 products). Most of the products are sleeveless vests, most often available in one size
(or two sizes), with adjustment straps around the torso to ensure as close contact with the
body as possible. The vests are equipped with PCM pockets on the front and back. Most
companies do not provide details on the phase change materials used, claiming that their
material is lighter and more effective than water or frozen gels and safer than ice water for
the skin, which can cause frostbite. However, FlexiFreeze uses frozen water as a cooling
principle but with specific packaging, claiming that for the same weight, water is a more
efficient means of cooling than frozen gels. The company offers a vest with 96 ice cubes
distributed on the front and back, weighing 1.4 kg. The FlexiFreeze product, similar to
the AlphaCool Ice Vest, using a water-based product, has packaging that slows down the
melting of the ice. Techniche uses pockets containing a non-toxic, non-flammable, and non-
combustible carbon-based liquid. Overall, the majority of identified cooling vests appear
to have similar properties: a temperature around 15
◦
C, a cooling period of approximately
2 to 3 h, a cooling capacity reactivation time of approximately 35-45 min, and a weight
varying between 1.0 and 2.3 kg. There are a few exceptions. For example, AllTuff USA PCM
vests are available in three charging temperatures, 5
◦
C, 15
◦
C, or 25
◦
C. The Ergodyne vest
allows a reactivation time of 5 to 15 min. ClimaTech Safety’s CM2000 vest can provide
an extension of the cooling period from 2 h (standard) to 4 h with the addition of another
cooling layer attached by Velcro over the first layer.
Of the commercially available products using a passive water evaporative cooling
system, three are presented herein. The first product is a thin, light, and flexible combination
of a sleeveless shirt and shorts, which was developed by UNICO Swiss Tex GmbH. This
close-fitting suit can be worn underneath clothing. It is made of a three-layer laminate
consisting of two waterproof but breathable polyester membranes that cover a hydrophilic
fabric [
324
]. The fabric acts as a container that can be filled with 30–60 mL of water using
a syringe. This system lowers the skin temperature by 4
◦
C, and the cooling effect can
last 40 min depending on the activity. The second product is the HyperKewl
™
PLUS vest
from Techniche. This vest is made of specialized fabric and fibers that allows for rapid
absorption, stable water storage, and good evaporation. It is activated by soaking it in
water and then removing the excess. This fabric is machine washable and can run for
150 wet/dry cycles. The third product is Ergodyne’s nylon-based Chill-Its 6687 vest that
acts as a reservoir that can be filled with 400–450 mL of water. With patented technology,
the vest gradually releases water by evaporation from the inside out, keeping the user cool
and dry.
As a hybrid cooling system, one of the most interesting is that of the SurgeCool
company, which has developed a vest using two technologies: a liquid circulation cooling
system (active) combined with a frozen gel cooling system (passive). Instead of being
Polymers 2021, 13, 3711 57 of 80
equipped with a large ice tank (stationary or portable) and an injection pump, as most
liquid circulation cooling systems are, SurgeCool replaced these elements with a gelling
polymer pack. The liquid circulating through the vest tubes is cooled to a temperature
of approximately 18–22
◦
C by the cooling pack, which will gradually melt and lose its
cooling effect after 2 h. This assembly of the two technologies allows a more global cooling
effect of the body, spreading the cold from the cooling pack over the whole vest in a certain
way and for a longer period. The vest, with a single cooling pack, can be worn with the
refrigerant pack on the front or on the back depending on the user’s work preferences. The
vest weighs less than 1 kg.
UNICO Swiss Tex GmbH has developed a ballistic cooling vest together with the Empa
Research Institute (Switzerland) that also uses two technologies: a passive evaporation
system and an active system with fans. A panel, Coolpad, is filled with water, which
evaporates through a membrane, cooling the panel. According to the company, the existing
Coolpads were unsatisfactory: being subjected to high mechanical stress in the vest, they
often leaked. Fortunately, a new laser diode welding technique has made it possible to
produce thin, flexible, and reliable panels that do not leak, despite the mechanical stresses
to which they are subjected. Two fans blow air through a spacer knit behind the Coolpad
and provide additional cooling. The compression-stable, flexible spacer knit with low
resistance to airflow was developed in cooperation with the Eschler company. A water
refill is required for approximately three hours of use. The two fans, which can be recharged
from a socket or a car cigarette lighter, can last three to four hours. This ballistic vest has
been tested with the police officers of the Zurich City Police, who appreciated it. Finally,
Techniche also offers a hybrid cooling product, simply combining two passive systems in
the same vest: their PCM technology, CoolPaxTM, with their water evaporation technology,
HyperKewlTM PLUS.
4.8. Cooling Actuator Challenges
Cooling actuators are the most studied technological solutions in an occupational
health and safety context among the various means of intelligent thermal management
(Table 6).
Table 6. Cooling actuator.
Technology Used Integration Method Operating Temperature Range References
High thermal conductive
artificial leather
Silver-plated polyamide yarn blended
with polyester yarn (base layer)/dry or
wet laminated resin (PU resin, solvent,
methyl cellulose)
N/A [309]
CNT-based fabric
The concept of heat transfer through a
layer of aligned CNT stacked between
two textile layers (insulation material)
Simulation conditions: (1) Hot
environment (40
◦
C) and light work
(332 W); (2) Hot
environment/strenuous work (889
W); (3) Firefighting environment
(58
◦
C) and light work; (4)
Firefighting environment and
strenuous work
[312]
Thermally conductive copper
filament
Hybrid conductive yarns made of
polyester yarn pooled with copper
filaments of different diameters using
cover yarn technique
N/A [310]
Thermally conductive
composite fibers
Thermally conductive and highly aligned
boron nitride/polyvinyl alcohol
composite fibers synthesized by 3D
printing
Simulation conditions: T
s
(37
◦
C);
T
env
(25
◦
C)
[311]
Polymers 2021, 13, 3711 58 of 80
Table 6. Cont.
Technology Used Integration Method Operating Temperature Range References
Nafion-based interlayer for
adaptive insulation
Nafion
®
N117 polymer from Dupont
(polymeric chains including both
hydrophobic polytetrafluorethylene
backbone and hydrophilic perfluoroether
sulfonic acid side chains) dried and
annealed at 130
◦
C before using
Tested at 32
◦
C, 90% RH [314]
Blend of PCM/highly
conductive metals
UnderArmour
®
polyester/spandex shirt
with PCM/ACC (ACC, i.e., active cooling
component blend: highly conductive
metals and or/ceramics, encapsulated
dissolvable alcohols such as xylitol)
micro-printed inside the shirt
Tested in a climate chamber:
35 ± 1
◦
C; 55 ± 6% RH
[313]
PCM
Cooling vest made of polyester and
separate pockets containing
21 PCM packs
Tested at T = 55
◦
C, RH = 30% [325]
Peltier effect created by
conductive fabrics
Direct current applied across two
dissimilar polypyrrole-coated fabrics
Temp drops from 40 to 22
◦
C during
30 min while thermoelectricity
decreases from 0.16 to 0.1mV
[326]
Temperature-controlled glove
A modified polyester glove with
integrated thermistor placed closed to the
skin and thermoelectric coolers attached
to the textile with thermally
conductive epoxy
Tested at 21
◦
C, 9
◦
C, −9
◦
C [278]
Flexible thermoelectric device
(cooling and heating)
Double elastomer layer design,
sandwiching thermoelectric pillars
between two stretchable sheets separated
by an air gap
(1) From heating temperature
change of 10
◦
C to the cooling
temperature change of −8
◦
C
depending on the applied current;
(2) T
s
kept at 32
◦
C in a T
env
varying from 22 to 36
◦
C
[280]
Potable thermoelectric device
(cooling and heating)
The thermoelectric unit conversion unit
supplying cool or warm air through a
tree-like rubber tube network knitted into
an undergarment
T
s
of the manikin fixed at 34
◦
C,
tests performed at 21
◦
C
[279]
Thermoelectric cooling helmet
Helmet based on both air-cooled and
liquid-cooled thermoelectric refrigeration
using polyvinyl tubing network
Tested at 30, 32, 34, 36, 38, and
40
◦
C, while maintaining the
average temperature of the thermal
manikin at 32 to 34
◦
C
[259]
Air-cooling garment
(ventilation)
A ventilated vest blowing ambient air
using flexible vented polymeric ducts
woven into the vest across the front and
back of the garment
Tested in hot (45
◦
C), dry (10% RH),
ambient
[252]
Air-cooling garment
(ventilation)
Air-cooling garment composed of textile
materials and flexible polymeric tubing,
and environmental air ventilation along
the torso
Tested in 40
◦
C–30% RH; 30
◦
C–70%
RH
[253]
Air-cooling full-face piece
respirator (ventilation)
Silicone-based modified full face piece
respirator supplying air into the mask
using an axial fan, flexible PVC tubing,
and customized ports
Tested at 32
◦
C dry bulb (TAIR) and
50–60% RH
[258]
Air-cooling garment
(ventilation)
Short sleeve jacket made of
cotton/polyester with two integrated
ventilation units
Approved at T = 34
◦
C, RH = 60%,
air velocity = 0.4 m/s.
[250]
Polymers 2021, 13, 3711 59 of 80
Table 6. Cont.
Technology Used Integration Method Operating Temperature Range References
Air-cooling garment
(ventilation)
A short-sleeve shirt with two integrated
ventilation units
Climate chamber (38
◦
C, 45% RH, 3
kPa water vapor pressure, 0.4 m/s
air velocity)
[251]
Air-cooling garment
(ventilation) versus frozen
pads
Two cooling vests are compared: vest A
(flame-resistant fabric containing four
pieces of frozen gel pads) and vest B
(inflaming retarding fabric with two
small fans and three pieces of frozen
gel pads)
N/A [255]
Air-cooling garment
(ventilation)
A polyester-based jacket with two
integrated small fans compared with a
polyester-based vest incorporated with 21
PCM packs
Tested at 32
◦
C, RH = 50% [254]
Forced-air ventilation
A forced air ventilation built into a textile
body armor
Tested at 40
◦
C, 20% RH [257]
Numerical modeling of ACG
(ventilation)
Series of micro-fans, placed in a textile
ribbon and attached to a woven textile
garment
Simulation performed at 27–30
◦
C.
40% RH
[256]
Vacuum desiccant cooling
garment
Garment with 12 vacuum desiccant
cooling pads based on a semi-permeable
and a microporous hydrophobic PTFE
membrane, polypropylene honeycomb
spacer, and multilayered
polyamide/polyethylene bag
Validated at 40
◦
C and 50% relative
humidity
[322]
Wearable engine-driven
evaporative cooling system
The cooling system consists of an
engine-driven vapor-compression system
coupled with a cooling garment
including refrigerant lines
Tested at 37.7–47.5
◦
C [285]
Wearable engine-driven
evaporative cooling system
Engine-driven vapor compression system
assembled with a cooling garment
consisted of the insulation, the heat
transfer surface, and the refrigerant
tube layer
Performs over a range of ambient
temperatures (37.7–47.5
◦
C),
evaporator refrigerant temperatures
(22.2–26.1
◦
C), and engine speeds
(10,500–13,300 RPM)
[286]
Evaporative cooling garment
The cooling generated by evaporation of
water from a porous, hydrophilic pad
sandwiched between a Nafion pocket
and a hydrophobic expanded PTFE
laminate
Tested on a simulated skin at a
temperature of 33.2
◦
C
[323]
Evaporative cooling vest
A quilted polyamide outer layer, a
water-repellant polyamide liner, and an
elastic trim of cotton/polyester
Tested at 36
◦
C/33% RH,
36
◦
C/67% RH, 40
◦
C/27% RH,
40
◦
C/54% RH
[284]
Liquid cooling clothing
A vest with a network of fine PVC tubes
sandwiched between two-layer polyester
mesh, a backpack storing a pump,
batteries, and an ice pack cooling
reservoir
Tested at 36
◦
C/33% RH,
36
◦
C/67% RH, 40
◦
C/27% RH,
40
◦
C/54% RH
Liquid cooling garment
Two cooling garments compared: (1) A
light-weight vest (polyester mesh inside
and PU laminated polyester fabric in
pocket area) filled with superabsorbent
acrylic resin pads, (2) a PVC tubed vest
connected to a cold liquid reservoir
placed in a sealed bag
Tested at 30
◦
C, 50% RH [225]
Polymers 2021, 13, 3711 60 of 80
Table 6. Cont.
Technology Used Integration Method Operating Temperature Range References
Liquid cooling garment
Two types of polyethylene
spandex-based garments with different
PVC tubing length for the cooling liquid
circulation
Tested at 35
◦
C and 50% RH [327]
Liquid cooling garment
Two types of polyethylene
spandex-based cooling garments with
different PVC tubing length
Tested at 35
◦
C and 50% RH [328]
Liquid cooling garment
A long-sleeved T-shirt (Coolmax
®
polyester knitted fabric) and a vest
(Coolmax
®
polyester knitted fabric)
constituting the insulation layer of the
coolant PVC-based tubing system
Tested in climatic chamber
26
◦
C–30% RH and 35
◦
C–30% RH
[329]
Liquid cooling garment
Long-sleeve underwear made of a
specially developed two-layer knitted
fabric (polyester/elastomer as the inner
layer and cotton/elastomer as the outer
layer) with a spacer module for
PVC-based tubing
Climatic chamber at 30
◦
C, of 40%
RH, and 0.4 m/s of air velocity
[330]
Liquid cooling garment
Tube-lined (PVC-based) perfusion vest
(polyester based) using field-portable
cooler
Tested at 33
◦
C, 60% RH [331]
Liquid cooling garment
(water-perfused suit)
A commercially available
water-perfusion vest made of polyester
and laminated around silicone tubing
connected to a backpack made of silicon,
polyamide, and polyester
Tested at 33
◦
C, 60% RH [231]
Liquid cooling garment
Three cooling vests compared: an
ice-based cooling vest, PCM cooling vest,
and water-perfused suit
Tested at 35.2
◦
C; 49.2% RH; <1 m/s
[224]
Liquid cooling garment
(water-perfused suit)
Water-perfused suit compared to PCM
and ice vest
Tested at 35
◦
C and 50% RH [224]
Liquid cooling garment
(1) Cotton shell liquid cooling vest with
flexible tubing routed throughout vest,
(2) cotton vest shell with four PCM packs,
(3) polyester vest with 22 PCM packs, (4)
cotton shell vest with five gel ice packs
Tested at 32
◦
C and 92% RH [332]
Numerical simulation of a
liquid cooling garment
Numerical simulation using a finite
element method. The model validated
thermal manikin, chiller, and liquid
cooling
Simulated at body temperature of
40
◦
C and an external temperature
of 23
◦
C
[333]
Fittable liquid cooling
clothing
A cooling garment composed of
polyvinyl tubing attached with silicone
rubber tubing on the trunk area and
adjustable with Velcro straps
Tested at 35.89 ± 1.25
◦
C, 35% RH [334]
Liquid cooling garment
A vest covering the chest and composed
of heat exchanger polyvinyl silicon tube
line, an ice-water backpack reservoir, and
a small battery-operated motor pump
39.4
◦
C dry bulb temperature; 41.2%
RH; 32.7
◦
C wet bulb globe
temperature
[335]
Polymers 2021, 13, 3711 61 of 80
Table 6. Cont.
Technology Used Integration Method Operating Temperature Range References
Liquid cooling garment
Two different liquid cooling garments
(outer layer single jersey wool knitted
fabric with plain weave and fusible
interlining versus 10
×
3 rib wool knitted
fabric; with any interlining) but the same
tubing lengths and the inner layers
Tested on manikin temperature of
40 ± 1
◦
C and a test cabin
temperature of 23 ± 1
◦
C
[232]
Liquid cooling garment
A knitted fabric used for the front and
back of the cooling (two-layer piece for
the sides of the garment made of
polyester Coolmax
®
/spandex, spacer
piece for tubing made of polyester
Coolmax
®
/Spandex, pieces for the top
and bottom of the garment made of
cotton/spandex, channel for tube
implementation made of polyester
Coolmax
®
/spandex
Tested at 20
◦
C and 65% RH [233]
Liquid cooling hood
Flexible PVC tubing distributed based on
the thermal sensitivity of different body
areas in a garment made of cotton or
polyester/spandex
Tested at 24
◦
C with RH of 24 + 2% [234]
Liquid cooling garment for
NDX-1 space suit
Polyester spandex-based garment with a
tubing network of flexible PVC tubes
Tested when T
s
between 30 and
37
◦
C
[235]
Heat transfer model of liquid
cooling garment
A spandex/cotton garment including
flexible PVC cooling tubing system, the
check valve, the switch, the micro-pump,
the portable power supply, the ice pack,
and the liquid reservoir
Tested on manikin surface
temperature of 35
◦
C
[236]
Liquid cooling garment with
PCM suspensions
Microencapsulated PCM (particles
wrapped by a thin polymer shell,
Microtek
®
) suspensions used as the
cooling fluid compared to a water liquid
cooling garment made of cotton
Tested at an inlet temperature of the
cooling garment of 11, 13, 15
◦
C;
and the T
c
of the thermal manikin
37
◦
C
[246]
A thermoregulatory model
implanted for the liquid
cooling garment
Fiala’s thermoregulatory model
implemented in a liquid cooling garment
environment
Validated at a 700 W metabolic rate [336]
Liquid cooling garment
Spandex clothing without any cooling
device compared with: (1) a liquid
cooling and ventilation garment
integrating a vinyl tube knitted to
spandex underwear, (2) a liquid cooling
made of elastic spandex with
self-perspiration induced by water
permeation from pores created on the
vinyl-based tubes
Tested at 27
◦
C and 47% RH [241]
Liquid cooling garment
controlled by T
s
Liquid cooling garment made of cotton
or Nomex
®
aramid fabric woven or
laminated around small-diameter Tygon
®
flexible polymeric tubing
Tested at 30
◦
C and 30% RH [239]
Liquid cooling garment
controlled by a T
s
feedback
Modeling several studies using a
water-perfused liquid cooling garment
(T
s
controlled, constant and pulse cooling
methods)
Tested at 30
◦
C, 30% RH [240]
Polymers 2021, 13, 3711 62 of 80
Table 6. Cont.
Technology Used Integration Method Operating Temperature Range References
Liquid cooling garment
Liquid cooling garment made of cotton
or Nomex
®
aramid fabric woven or
laminated around small Tygon
®
flexible
polymeric tubing (intermittent and
continuous cooling methods)
Tested at 30
◦
C and 30% RH [237]
Liquid cooling garment
controlled by different
algorithms
A mobile liquid cooling garment made of
spandex fabric and vinyl tubing tested at
continuous, alternating, and pulsed
cooling
T
s
of the manikin is varying from 27
to 35
◦
C depending on the cooling
control strategy
[238]
Liquid CO
2
-based liquid
cooling garment
A cooling garment based on the
endothermic vaporization of liquefied
CO
2
(Porticool, Inc) with vaporized cool
and dry CO
2
vented over a thin cotton
layer
30
◦
C WBGT [247]
Air-diffusing garment
(tubing)
A dry air ventilation provided with an
air-diffusing garment made of 3D space
knitted fabric and stellate tubing worn
between an underwear and impermeable
chemical protective clothing
Tested at 25
◦
C, 50% RH, 0.2 m/s
wind
[248]
CO
2
-based air cooling
garment (gas expansion
garment)
The air treatment system using an
atmospheric discharge of highly
pressurized liquid CO
2
to cool and
dehumidify the constant stream of air in
a cooling garment made of polyester
outer layer, moisture-wicking fabric
middle layer, polyester mesh inner layer,
and PVC tubes
Tested at 35.7
◦
C dry bulb and 86%
RH
[337]
CO
2
-based air cooling
garment (gas expansion
garment)
Tested at T env = 22
◦
C and 40% RH
and climate chamber with a
dry-bulb temperature of 30 ± 1
◦
C
and 60% RH
[249]
CO
2
-based air cooling
garment (gas expansion
garment)
Air-cooling systems analyzed by
calculating the cooling capacity of the
gaseous CO
2
-free jet expansion by three
different approaches in a cooling garment
made of polyester outer layer,
moisture-wicking fabric middle layer,
polyester mesh inner layer, and PVC
tubes
CO
2
used to cool a constant hot and
humid airflow set at 37 ± 0.5
◦
C
(dry bulb) and 69 ± 1% RH
[338]
Wearer-controlled
vaporization garment
Two cooling systems compared: (1) a
simulated liquid cooling and ventilation
garment integrating a vinyl tube knitted
to a spandex underwear, (2) a liquid
cooling made of elastic spandex with
self-perspiration induced by water
permeation from pores created on the
vinyl-based tubes
Tested at 27
◦
C and 47%, RH [242]
Hybrid cooling garment
(liquid cooling/air cooling)
Fiberglass-based helmet containing
solution-associated air cooling and water
cooling
The cooling capacity validated for
the temperature changing in the
helmet (25–40
◦
C) and (25–35
◦
C)
for the temperature changing of
LED driving modules
[339]
Hybrid cooling garment
(liquid cooling/air cooling)
(1) Liquid cooling and ventilation
garment made of vinyl tubing and
spandex fabric, (2) liquid cooling garment
made of elastic spandex and polyester
Validated in a typical laboratory
environment
[340]
Polymers 2021, 13, 3711 63 of 80
Table 6. Cont.
Technology Used Integration Method Operating Temperature Range References
Hybrid cooling garment
(PCM–liquid cooling)
Combining PCM with water pipes buried
in the PCM in a cooling garment made of
cotton lining, porous polyester support
fabric, floss insulation vest, and PVC
tubes
N/A [294]
Hybrid cooling garment (gel
pads–air cooling)
A hybrid cooling vest with light taffeta as
the shell fabric integrating two fans and
three gel packs
Tested at (1) 25
±
1
◦
C/60
±
3% RH
(standardize the initial body
condition); (2) outdoor WBGT
(26.31 to 35.60
◦
C)
[293]
Hybrid cooling garment
(frozen pack–air cooling)
A commercially available hybrid cooling
vest (airproof outer fabric and meshed
inner fabric) integrating three frozen gel
packs made of water-based gel and
fire-retardant fabric and two small
detachable electronic fans
33
◦
C and 75% RH with partial
water vapor pressure of 3750 Pa
[292]
Hybrid cooling garment
(PCM-Air cooling)
24 PCM packs and four fans embedded
in a cooling garment made of polyester
Approved at 34.0
◦
C, RH = 75%,
and 28%
[297]
Hybrid cooling garment
(PCM–air cooling)
Cooling uniform (cotton/polyester)
containing two ventilation units and 24
PCM packs inserted into separate pockets
and vertical ventilation pathways
Tested at air temperature of 22
◦
C,
50% RH; and evaporative resistance
tests performed at 40% RH
[297]
Hybrid cooling garment
(PCM–air cooling)
A jacket with polyamide taffeta based
outer layer and mesh spacer inner fabric
containing eight PCM packs and two fans
inserted at the lower back of the vest
Tested at 34.0
◦
C, 60% RH, and
V = 0.4 m/s
[298]
Hybrid cooling garment
(PCM–air cooling)
Four ventilation fans and 24 PCM packs
integrated into a cotton/polyester-based
cooling uniform
Tested at (1) 30
◦
C, 47% RH (three
different air velocities of 0.4 m/s;
0.15 m/s; 1 m/s)
[299]
Hybrid cooling garment
(PCM–air cooling)
Long-sleeved jacket including a
polyamide outer and a mesh liner layers
with 24 PCM packs and four integrated
ventilation fans
Tested at 36 ± 0.5
◦
C and RH = 59
± 5%
[300]
Hybrid cooling garment
(PCM–air cooling)
Long-sleeve cotton/polyester jacket and
pants containing 24 PCM packs and two
ventilation fans installed at the lateral
pelvis area
Tested at T = 34.0
±
0.5
◦
C, RH = 65
± 5% and V = 0.15 ± 0.05 m/s
[295]
Hybrid cooling garment
(PCM–air cooling)
Two fans and eight PCM packs inserted
inside a jacket with a polyamide outer
layer and mesh spacer inner fabric
Tested in environmental
temperature ranging from 29.2 to
31.3
◦
C
[301]
Hybrid cooling garment
(PCM–air cooling)
Vest with polyester inner and polyamide
taffeta outer layers, containing two
ventilation fans and eight PCM packs
Tested at 37
◦
C, 60% RH, and
V = 0.3 m/s; 450 W/m
2
solar
radiation
[301]
Hybrid cooling garment
(PCM–air cooling–insulation)
Cooling uniform with polyester mesh
fabric lining and cotton outer layer
containing four fans, 24 PCM packs, and
one expanded polyethylene insulation
layer between PCM packs and the outer
clothing layer
Approved at 36
◦
C, RH = 59% [303]
Polymers 2021, 13, 3711 64 of 80
Table 6. Cont.
Technology Used Integration Method Operating Temperature Range References
Hybrid cooling garment
(PCM–air cooling)
Cooling uniform with polyester mesh
inner fabric and cotton outer layer
containing four fans, 24 PCM packs, and
one insulation layer made up of
expanded polyethylene foam placed onto
the outer surface of PCM packs
Simulation and experimental
validation: 36
◦
C and 59% RH (and
air velocity in experiments of 0.10
±
0.05 m/s)
[302]
Hybrid cooling garment
(PCM–air cooling)
A mathematical model developed for
transient heat and moisture transfer
through clothing layers incorporating
PCM packs and ventilation fans for a
cooling garment made of polyester inner
and cotton outer layers
The simulation cases of the planned
parametric study: 25
◦
C and 50%
RH (ambient); 40
◦
C and 35% RH
(hot, dry)
[291]
Hybrid cooling garment
(PCM–air cooling)
A numerical model developed to analyze
heat and moisture transfer through the
hybrid personal cooling garment made of
polyester inner fabric and a cotton outer
layer containing four fans and 24 PCM
packs
Validated with data collected at T
env = 36.0 ± 0.5
◦
C, 59% RH
[306]
Hybrid cooling garment
(PCM–air cooling)
Cooling garment made of polyester inner
and cotton outer layers containing 24
PCM packs and four ventilation fans
Conditions used in the numerical
parametric study: (1) RH = 50% and
T = 32, 34, 36, 38, and 40
◦
C; (2)
T = 36 and 40
◦
C and 30, 50, 70, and
90% RH
[305]
Air cooling garment
compared to a PCM garment
and a liquid cooling garment
Three different cooling garments
compared: PCM garment; air cooling
garment; and liquid cooling garment
Tested at 31.20 (0.20)
◦
C and 70
(1.90)% RH
[304]
Air cooling garment
(ventilation) versus PCM
versus liquid cooling garment
Four different commercial cooling
garments compared: Ventilation Vest
(Entrak), PCM Cool Under Vest (Steele),
PCM PCVZ-KM Vest (Polar), and liquid
cooling garment Hummingbird II (CTS)
Thermal manikin (35
◦
C, 40% RH);
Human subjects (42
◦
C, 20% RH)
[287]
Air cooling garment
(ventilation) versus PCM
versus vapor compression
Four commercial cooling garments
compared: Ventilation Vest (Entrak),
PCM Cool Under Vest (Steele), PCM
PCVZ-KM Vest (Polar), and a
direct-expansion vapor-compression
refrigeration garment Hummingbird II
(CTS)
Air (dry bulb)
temperature = 42.2
◦
C; 20% RH;
Mean radiant temperature = 54.4
◦
C
[289]
An important part of the research work on personal cooling garments has been de-
voted to fluid circulation cooling systems. Although their effectiveness has been approved
by several studies, these garments are heavy and cumbersome and seem practical only for
occupations in which workers do not travel frequently, such as workers working in vehicles
with the refrigeration unit or compressed air system at a standstill. In addition, cooling
units consisting of an ice cube tank, proposed for better portability of the system, remain
limited due to the operating time and require frequent recharging of the tank. Furthermore,
recent studies on the optimization of parameters such as the textile layer design of the fluid
circulation cooling garments, their tubular network, their assembly, the capacity of their
fluid injection pump, circulation flow rate, etc., seem to be limited to validation tests in the
laboratory and on very few human subjects.
The automatic control of the appearance of steam around the tubular network to
reduce the risk of skin burns in humid environments and the use of PCM suspensions
to improve heat dissipation without an apparent increase in pump power are examples
of other concepts proposed for optimizing the performance of cooling garments through
Polymers 2021, 13, 3711 65 of 80
fluid circulation, which generally operates with cold water. However, tests in operational
environments are still needed to validate these concepts. Although many studies are
devoted to fluid cooling garments, this analysis identified only two studies that were
conducted in an operational environment.
Fluid circulation cooling garments with integrated temperature and flow rate control
systems using intermittent or alternate circulation and T
s
feedback activation would
reduce system energy consumption and the risk of overcooling while improving efficiency.
However, all work on these systems has been limited to tests on thermal manikins and few
tests on individuals in the laboratory.
In order to take advantage of the relatively low weight and high cooling capacity of
gas expansion cooling garments, while circumventing their limitations in terms of low
operating time and exhaust gas, very recent studies have proposed some optimizations
for use in hot and humid environments. These results have yet to be confirmed under
real operating conditions. In addition, design modifications are still necessary to facilitate
their use under protective equipment. The literature review also revealed that despite
the greater efficiency of air blast cooling garments using an air compressor, most of the
work had favored cooling by fan ventilation for better portability of the system. Although
fans allow a good decrease in the humidity of the microclimate close to the skin, their
performance seems to be influenced by the temperature or humidity of the ambient air and,
according to some studies, their beneficial effect lies at the level of the local Ts without a
remarkable influence at the level of the total T
s
or T
c
. Despite optimization work on fan
placement, additional openings in the clothing, or intermittent cooling modes, no particular
benefit could be observed in the studies analyzed. On the other hand, the few studies
devoted to the thermoelectric cooling garment seem to be limited to proofs of concept
and show a relatively high electrical energy consumption in their current state. However,
the small dimensions of thermoelectric modules, combined with recent work on flexible
module design, suggest that portable thermoelectric solutions with heating and cooling
capacity can be implemented. In order to overcome the low efficiency of evaporative
cooling garments worn under dense protective equipment and in humid environments,
the introduction of a ventilation mechanism to evacuate moisture, or motorized vapor
compression devices associated with cooling lines, have been proposed. In their current
state, these concepts are cumbersome and impractical to carry to work.
Vacuum desiccant cooling garments have a high cooling capacity, as they involve
hygroscopic materials to improve the performance of evaporative cooling garments [341].
However, very few studies have been devoted to them. Research continues to propose new
structures or new types of desiccant materials [
226
]. As they are passive systems, their
operating time remains limited, and their performance is reduced in humid climates.
Despite comparative studies and the establishment of generic tables comparing the
performance of various cooling systems, the selection of the most appropriate system is
sometimes a complex task. One of the reasons for this difficulty is the shortcomings of
the test methods used to evaluate the various types of personal cooling garments. Many
studies have used thermal manikins. Although useful for a first draft, manikins cannot
adequately simulate the spatial and transient thermal behavior of humans or realistic
thermophysiological responses, such as changes in T
c
and T
s
. They are also limited by the
lack of a vasoconstrictor response initiated in human skin when cooled [58].
In addition, testing on human subjects is mostly limited to a restricted number and
gender. In addition, differences in methodologies (i.e., exercise duration and intensity),
subject characteristics (i.e., gender, fitness level, acclimatization, and hydration level),
and cooling system properties (i.e., cooling duration, number of cooling elements, and
their location) sometimes lead to confusion about the results presented in the literature.
Indeed, the diversity of experiments and methods has sometimes led to different results
that are not necessarily confirmed and are sometimes contradictory. For example, the
impact of technology on physiological parameters may vary from one study to another.
In addition, the application of laboratory results in the field may be compromised by
Polymers 2021, 13, 3711 66 of 80
ergonomic problems in a real work environment due to the varied and complex forms of
movement in comparison with the simulated treadmill running-type tests adopted in many
studies. In order to facilitate the choice of the appropriate system, some universal methods
have been proposed for theoretical calculation of the performance of the cooling garment
according to the climate and the nature of the activity. However, these methods do not
consider the loss of body heat and the impact of cooling on the physiological aspects of the
body, which can vary from person to person.
Faced with the complexity of selecting the best strategy for different environments
and activities, and to circumvent the limitations of different systems, some experts have
proposed hybrid cooling technologies. The combination of frozen gel pads with fans for a
greater reduction of physiological constraints in the activity phases, and the circulation of
cold water through PCM pockets to increase the operating time, have been the concepts
explored through a limited number of studies. However, the review of the literature
revealed several studies concerning hybrid cooling garments combining PCM actuators
and fans (PCM/fans) in order to promote heat loss by evaporation using fans and to
create a synergistic effect to obtain better performance in hot and humid climates while
ensuring portability. The integration of insulating layers to reduce the environmental heat
absorbed by the PCM in optimized versions of PCM/fan garments has increased the total
melting time of the PCM and the effective cooling time. Despite great potential, moisture
condensation on the surfaces of the PCM pockets, reducing the efficiency of the system,
and the weight of the PCM pockets have proven to be two limitations of this type of hybrid
cooling garment. Furthermore, a hybrid PCM/fan system has not yet been explored in a
protective equipment structure.
In addition, the design of high thermal conductivity layers based on advanced ma-
terials or conductive textiles to improve the heat exchange of the garment, the mixing of
PCM with conductive materials to create an effect of repeated heat loss, the use of materials
reversibly sensitive to humidity or temperature have not yet been explored in association
with active cooling systems in the design of hybrid cooling garments. These types of mate-
rials should also be studied in the optimization of the thermal performance of personal
protective equipment. In view of the progress made and the use of new technologies in the
design of personal cooling garments, the decision on the effectiveness or ineffectiveness of
these new systems in reducing the body’s thermal stress requires more studies, modeling
or simulations, in order to judge their performance under particular conditions.
Concerning research on products containing cooling actuators, analyses have shown
that almost half of the products identified use passive cooling principles such as PCM or
evaporative cooling. As in the scientific literature, most cooling garments based on liquid
circulation or compressed air are stationary and less intended for workers who must move
frequently. Despite the relatively simple design of fan cooling garments, no protective
equipment containing integrated fans could be identified in the analyses. Gas expansion
and thermoelectric systems appear to be still under development and not widely available
on the market. Despite a great deal of research work dedicated to hybrid systems, there
are few products combining two types of technologies. Moreover, none of these products
combines an active fan system with a passive PCM system, even though they have been
praised in the scientific literature.
5. Conclusions
Despite the standards governing working conditions and the advances in the de-
velopment of more efficient protective equipment, thermal constraints remain a major
occupational health problem. In such a context, thermoregulation systems, which make
textiles capable of detecting, reacting, and adapting to thermal stimuli, offer great poten-
tial for improving the performance of personal protective equipment during exposure to
extreme temperatures.
Therefore, the present study was conducted in order to better document the current
state of knowledge on the technologies facilitating intelligent thermal management by
Polymers 2021, 13, 3711 67 of 80
reviewing the existing technologies currently available on the market and the developments
carried out in the framework of previous research work. Particular attention was paid
to the collection of scientific and technical information on systems that can potentially
be integrated into personal protective equipment for intelligent and sustained thermal
management throughout the execution of tasks. Based on the knowledge gathered and
discussions on the current gaps in studies and marketed products, the efforts still to be
done and the development or adaptation strategies to be deployed in personal protective
equipment were discussed.
Indeed, the potential of smart textiles and advanced functional materials can be
greatly exploited in the development of integrated temperature sensors, heating or cooling
actuators, and wearable devices or in the optimization of their performance. The use of
advanced functional materials in combination with active cooling or heating technologies
to establish hybrid systems providing improved performance are among the most viable
solutions to implement in the short term. Moreover, combining two cooling or heating
technologies to create a synergistic effect to optimize the performances is one of the most
interesting strategies to consider. Finally, the results of numerous laboratory studies and
some products recently developed in the industry remain to be deployed in workplaces
through field studies.
Author Contributions:
Methodology, A.S. and C.G.; validation, A.S., C.G. and P.N.-T.; writing—
original draft preparation, A.S., S.L., C.G. and P.N.-T.; writing—review and editing, A.S., C.G., S.L.
and P.N.-T.; supervision, A.S. and P.N.-T.; project administration, A.S.; funding acquisition, A.S. and
C.G. All authors have read and agreed to the published version of the manuscript.
Funding:
This research was funded by the Institut de recherche Robert-Sauvé en santé et en sécurité
du travail (IRSST), grant number 2019-0036. This is an invited review article and the authors received
a full fee waiver for the publication.
Acknowledgments:
This work was supported by occupational health and safety funds granted
by the Institut de recherche Robert-Sauvé en santé et en sécurité du travail (IRSST) [grant number
2019-0036, 2019].
Conflicts of Interest: The authors declare no conflict of interest.
Abbreviations
CNT Carbon nanotube
CPC Chemical protective clothing
CPE Chlorinated polyethylene
LED Light-emitting diode
HR Heart rate (number of heartbeats per unit of time)
HPPE High-performance polyethylene
FCG Fluid cooling garment (cooling clothing by circulation of fluid)
NTC Negative temperature coefficient
PB Poly benzimidazole
PEDOT-PSS Sodium poly (3,4-ethylenedioxythiophene)-poly (styrene sulfonate) polymer complex
PCM Phase change material
PDMS Polydimethylsiloxane
PeSI Perceptual strain index
PPE Personal protective equipment
PSI Physiological strain index
PTC Positive temperature coefficient
PU Polyurethane
PTFE Polytetrafluoroethylene
PVC Polyvinyl chloride
PVDC Polyvinylidene chloride
RFID Radio-frequency identification
RH Relative humidity
RPE Rating of perceived exertion (scale of perception of effort)
RTD Resistance temperature detector (electric resistance temperature detector)
T
c
Core (internal) body temperature
Polymers 2021, 13, 3711 68 of 80
T
s
Skin temperature
T
rec
Rectal temperature
T
env
Environmental temperature
UHF Ultra-high frequency
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