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Integrated cooling (i-Cool) textile of heat conduction and sweat transportation for personal perspiration management

October 2021

Authors:

Wei Li

Changchun Institute of Optics, Fine Mechanics and Physics

Bofei Liu

Stanford University

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Perspiration evaporation plays an indispensable role in human body heat dissipation. However , conventional textiles tend to focus on sweat removal and pay little attention to the basic thermoregulation function of sweat, showing limited evaporation ability and cooling efficiency in moderate/profuse perspiration scenarios. Here, we propose an integrated cooling (i-Cool) textile with unique functional structure design for personal perspiration management. By integrating heat conductive pathways and water transport channels decently, i-Cool exhibits enhanced evaporation ability and high sweat evaporative cooling efficiency, not merely liquid sweat wicking function. In the steady-state evaporation test, compared to cotton, up to over 100% reduction in water mass gain ratio, and 3 times higher skin power density increment for every unit of sweat evaporation are demonstrated. Besides, i-Cool shows about 3°C cooling effect with greatly reduced sweat consumption than cotton in the artificial sweating skin test. The practical application feasibility of i-Cool design principles is well validated based on commercial fabrics. Owing to its exceptional personal perspiration management performance , we expect the i-Cool concept can provide promising design guidelines for next-generation perspiration management textiles.

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ARTICLE

Integrated cooling (i-Cool) textile of heat

conduction and sweat transportation for personal

perspiration management

Yucan Peng 1,9, Wei Li 2,3,9, Bofei Liu1, Weiliang Jin 2, Joseph Schaadt4,5, Jing Tang1, Guangmin Zhou1,

Guanyang Wang6, Jiawei Zhou 1, Chi Zhang 7, Yangying Zhu1, Wenxiao Huang1, Tong Wu1,

Kenneth E. Goodson7, Chris Dames4,5, Ravi Prasher 4,5, Shanhui Fan 2& Yi Cui 1,8 ✉

Perspiration evaporation plays an indispensable role in human body heat dissipation. How-

ever, conventional textiles tend to focus on sweat removal and pay little attention to the basic

thermoregulation function of sweat, showing limited evaporation ability and cooling efﬁciency

in moderate/profuse perspiration scenarios. Here, we propose an integrated cooling (i-Cool)

textile with unique functional structure design for personal perspiration management. By

integrating heat conductive pathways and water transport channels decently, i-Cool exhibits

enhanced evaporation ability and high sweat evaporative cooling efﬁciency, not merely liquid

sweat wicking function. In the steady-state evaporation test, compared to cotton, up to over

100% reduction in water mass gain ratio, and 3 times higher skin power density increment for

every unit of sweat evaporation are demonstrated. Besides, i-Cool shows about 3 °C cooling

effect with greatly reduced sweat consumption than cotton in the artiﬁcial sweating skin test.

The practical application feasibility of i-Cool design principles is well validated based on

commercial fabrics. Owing to its exceptional personal perspiration management perfor-

mance, we expect the i-Cool concept can provide promising design guidelines for next-

generation perspiration management textiles.

https://doi.org/10.1038/s41467-021-26384-8 OPEN

1Department of Materials Science and Engineering, Stanford University, Stanford, CA, USA. 2E. L. Ginzton Laboratory, Department of Electrical Engineering,

Stanford University, Stanford, CA, USA. 3GPL Photonics Lab, State Key Laboratory of Applied Optics, Changchun Institute of Optics, Fine Mechanics and

Physics, Chinese Academy of Sciences, Changchun, China. 4Department of Mechanical Engineering, University of California, Berkeley, CA, USA. 5Energy

Technologies Area, Lawrence Berkeley National Laboratory, Berkeley, CA, USA. 6Department of Mathematics, Stanford University, Stanford, CA, USA.

7Department of Mechanical Engineering, Stanford University, Stanford, CA, USA. 8Stanford Institute for Materials and Energy Sciences, SLAC National

Accelerator Laboratory, Menlo Park, CA, USA.

These authors contributed equally: Yucan Peng, Wei Li. ✉email: yicui@stanford.edu

NATURE COMMUNICATIONS | (2021) 12:6122 |https://doi .org/10.1038/s41467-021-26384-8 | www.nature.com/naturecommunications 1

1234567890():,;

Satisfaction with the thermal environment for human body is

signiﬁcant, not merely due to the demand for comfort, but

more importantly because thermal conditions are crucial for

human body health1. Heat-resulted physiological and psycholo-

gical problems not only can be threatening for human health2,

but also negatively inﬂuence labor productivity and society

economy3. Personal thermal management focusing on thermal

conditions of human body and its local environment is emerging

as an energy-efﬁcient and cost-effective solution4,5. Without

consuming excess energy on managing the temperature of the

entire environment6,7, innovative textiles have been designed for

controlling human body heat dissipation routes8,9. In general,

human body dissipates heat via four different pathways: radiation,

convection, conduction and evaporation10. Recently, textiles with

engineered radiative properties11–16, convective and conductive

properties17–19 have been demonstrated as promising approaches

for personal thermal management especially for mild scenarios.

However, for intense scenarios, textiles for ideal personal per-

spiration or evaporation management are still lacking.

For the delicate human body system with a narrow tempera-

ture range (36–38 °C core temperature at rest and up to 41 °C for

heavy exercise)20, evaporation plays an indispensable role in

human body thermoregulation. Even at a mild state, about 20% of

heat dissipation of the dry human body relies on the water vapor

loss via insensible perspiration10,21. With further increase of heat

load, liquid sweat evaporation contributes to more and more heat

loss and becomes the major route for human body heat dissipa-

tion in intense scenarios such as heavy exercise and hot/humid

environments, where excess heat cannot be dissipated efﬁciently

by other pathways22,23. State-of-the-art textiles for daily use are

usually sufﬁciently good at water vapor transmission to ensure

comfort at the mild state (See Supplementary Note 1 and Sup-

plementary Figs. 1–2 for more discussion)24. Nevertheless, the

cooling performance of conventional textiles is to be improved

when human body is in more intense scenarios, such as moder-

ate/profuse perspiration situations in which liquid sweat is

inevitably present.

In order to avoid increased wettedness on the skin which

causes less comfort in such cases25,26, state-of-the-art textiles,

including moisture management fabrics, tend to focus on sweat

removal. Textiles made of natural ﬁbers, such as cotton, show

strong water absorption capacity, which can help alleviate sense

of wettedness quickly27. In spite of diminished absorbing ability,

synthetic ﬁbers (with proﬁled cross-section), such as polyester,

are developed to possess enhanced moisture transportation than

natural ﬁbers to deliver water to the textile surface for faster

evaporation28,29. Microﬁbres are also explored for improved

wicking30. Besides, strategies including surface hydrophilicity/

hydrophobicity modiﬁcation31–33, multiple-layer design with

differential wettability34,35 and hierarchical design of multiscale

interconnected pores with capillarity gradient36,37 are reported to

realize better controlled directional water transportation. These

textiles serve as a buffer absorbing water to provide dry sense for

people and can potentially offer a comparatively larger surface

area for evaporation.

However, how to efﬁciently unlock the cooling power of sweat

evaporation for human body thermoregulation and design textiles

based on laws of human body perspiration process have not been

taken into account. In the aspect of thermoregulation, sweat is

secreted to be evaporated and take away the excess heat. Never-

theless, although sweat evaporation does happen on the con-

ventional textiles, human skin underneath is not effectively cooled

since heat for vaporization is not efﬁciently drawn from the skin

because of the limited heat transfer38–40. One extreme case is that

only the textile surface rather than human skin can be cooled. In

other words, the sweat absorbed in the conventional textiles

shows decreased evaporative cooling efﬁciency in cooling the

human body, which means sweat is less efﬁciently utilized. Also,

even regarding evaporation rate of conventional textiles, it is

relatively restrained because skin heat cannot be efﬁciently

delivered to the evaporation interface to accelerate evaporation.

The inefﬁcient cooling effect will lead to further perspiration, and

meanwhile the slow sweat evaporation, will result in the accu-

mulation of sweat in the textile. This process may undermine the

buffer effect of the textiles once the absorption limit of the fabric

is reached, at which point the human body will get wet and sticky

again. The excessive perspiration can also cause potential risk of

dehydration, electrolyte disorder, physical and mental deteriora-

tion or even death41. Moreover, when people are in highly active

scenarios, the maximum cooling power of sweat evaporation that

can be achieved actually limits the maximum activity level of

human body42. Accordingly, in addition to decent wicking

property, an optimal textile for perspiration scenario should show

high evaporation ability and more importantly high sweat eva-

porative cooling efﬁciency to utilize sweat in a highly efﬁcient

manner, to provide adequate cooling effect using minimized

amount of sweat.

In this work, we propose a novel concept of integrated cooling

(i-Cool) textile of heat conduction and sweat transportation to

achieve the as-mentioned goals based on human body perspira-

tion process, as illustrated in Fig. 1a. We introduce heat con-

ductive components into the textile and divide the functionalities

of heat conduction and sweat transport into two operational

components. The heat conductive matrix and sweat transporta-

tion channels are integrated together in the i-Cool textile. The

synergistic effect of the two components results in excellent

performance at sweat wicking, fast evaporation, efﬁcient eva-

porative cooling for human body and reducing human body

dehydration. As shown in Fig. 1b, the sweat transport channels

can pull liquid water up from skin and spread it out in the sweat

transport channels for evaporation. On the other hand, the heat

conductive matrix can efﬁciently transfer skin heat to the eva-

poration spots that are integrated on the heat conductive

matrix43,44. Therefore, combined with large evaporation area and

efﬁcient heat conduction from skin, sweat absorbed in the water

transportation channels can be evaporated quickly into air, taking

away a huge amount of heat from the skin. The efﬁcient heat

removal from the skin provides improved evaporative cooling

effect and decrease skin temperature effectively, which will con-

sequently reduce human body dehydration. As illustrated in

Fig. 1c, compared to the conventional textiles, the i-Cool textile

functions not only to wick sweat but also provide heat conduction

paths for the accelerated evaporation and efﬁciently take away a

great amount of heat from the skin. Furthermore, the enhanced

evaporation ability and high sweat evaporative cooling efﬁciency

can prevent the i-Cool textile from ﬂooding to a much greater

extent and avoid excessive perspiration. The improved evapora-

tive cooling effect does not mean more sweat needs to be gen-

erated or even evaporated. Therefore, the i-Cool textile can help

human body achieve enhanced cooling effect with greatly reduced

sweat secretion by using the sweat in a highly efﬁcient manner.

Results and discussion

On the basis of the i-Cool functional structure design principles

as outlined above, we selected copper (Cu) and nylon 6 nano-

ﬁbres for proof of concept. It is worthwhile to mention that Cu

and nylon 6 nanoﬁbres are not the only choices. Other materials

satisfying the design principles can be applied as well. Here, Cu is

well-known for its extraordinary thermal conductivity (~400

W·m−1·K−1), and nylon 6 nanoﬁbres are capable of water

wicking. As illustrated in Supplementary Fig. 3, electrospinning

ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-021-26384-8

2NATURE COMMUNICATIONS | (2021) 12:6122 | https://doi.org/10.1038/s41467-021-26384-8 | www.nature.com/naturecommunications

was utilized to generate nylon 6 nanoﬁbres, which were trans-

ferred to the heat conductive Cu matrix prepared by laser cutting.

With press lamination, the i-Cool (Cu) textile with desired

functional structure design was fabricated. The photograph of as-

fabricated i-Cool (Cu) textile is displayed in Fig. 2a. Nylon 6

nanoﬁbres not only cover the Cu top surface, but also ﬁll inside

the pores, as shown in the magniﬁed photograph of the bottom

side of the i-Cool (Cu) textile in the inset of Fig. 2b. Nanoﬁbres

on the skeleton of Cu matrix are denser with smaller void space

among the nanoﬁbres than the ones in the pores of Cu matrix,

which can be clearly observed in the scanning electron micro-

scope (SEM) images in Fig. 2b and Supplementary Fig. 4. The

capillarity difference resulted from the morphology difference

beneﬁts one-way directional water transportation from inner

surface to outer surface. To evaluate the performance of the

i-Cool (Cu) textile, we selected cotton textile as the main control

textile since it is arguably the most widely used and accepted

textile in human history. We have also chosen other well-known

activewear fabrics for comparison purposes.

Liquid water transport characterization. Textiles designed for

perspiration scenarios must be able to wick sweat from the skin

(in contact with textile bottom) and spread it out. Correspond-

ingly, we tested in parallel the i-Cool (Cu) textile and commercial

textiles including cotton, Dri-FIT, CoolMax and Coolswitch via

mimicking the sweat transport process from the human body skin

to the outer surface of the textile. Textile samples covered a

certain amount of liquid water on the platform respectively, and

the wicking rate was calculated via dividing wicking area by

wicking time for every sample (Supplementary Fig. 5). It turned

out that the interconnected nylon 6 nanoﬁbres in the i-Cool (Cu)

textile was able to quickly transport liquid water from bottom to

top and spread it out, which exhibited comparable or higher

wicking rate in comparison with conventional textiles (Fig. 2c).

Besides, due to the unique structure design and the nanoﬁbre

morphology variation from i-Cool (Cu) bottom to the outer

surface, i-Cool (Cu) exhibits good one-way water transport

property. As displayed in Supplementary Fig. 6a, the water dro-

plet added onto the inner side of i-Cool (Cu) can be transported

to the outer surface and spread out very quickly while little water

remained on the inner side. In reverse, water transportation was

limited when the water droplet added to the outer side. As a

comparison, for cotton, the testing time on the outer side and

inner side was almost the same no matter which side the water

droplet was added onto (Supplementary Fig. 6b), which means

the conventional cotton fabric shows no one-way transport cap-

ability. Also, in the scenario of adding water onto inner side, the

water spreading rate on the inner surface and outer surface (Sinner

and Souter) and one-way transport index (µ) were deﬁned (See

Methods for more details) and plotted in Supplementary Fig. 745.

The i-Cool (Cu) shows obviously different Sinner and Souter, and

very large µ, while Sinner and Souter are very similar for cotton and

its µis very close to 1, which demonstrates the apparent one-way

sweat transport advantage of i-Cool (Cu) again. This property can

also help faster evaporation, because sweat can spread on the

outer surface quickly and liquid water transport to the nanoﬁbres

right on the heat conductive Cu matrix is preferential37.

Water vapour

Heat

conductive

matrix

Sweat transportation

channels

Sweat

Skin

Sweat

wicking and

spreading

Heat

conduction

Sweat

evaporation

Cool skin

i-Cool textile

Conventional textile Skin T

Low

High

Skin

Heat conduction

Sweat wicking Sweat evaporation

Sweat

Fig. 1 Schematic of the functional structure design of integrated cooling (i-Cool) textile of heat conduction and sweat transportation for personal

perspiration management and its working mechanism. a, Schematic of the i-Cool textile. The synergistic effect of the heat conductive matrix and sweat

transport channels provides a solution to textile in personal perspiration management. b, Schematic of the working mechanism of the i-Cool textile. When

human body perspires, the water transport channels can wick sweat from the skin surface and spread sweat onto the large-area top surface made of ﬁbers

quickly. The heat conductive matrix transfers human body heat efﬁciently to where the evaporation happens, to assist fast evaporation. Meanwhile, it can

deliver the evaporative cooling effect to human body skin efﬁciently. c, Comparison between conventional textiles and the i-Cool textile. Conventional

textiles usually offer comfort via buffer effect of absorbing sweat, which is helpful to relieve discomfort of wet and sticky sense. However, its limited

evaporation rate and evaporative cooling efﬁciency cannot provide effective cooling effect for skin and may undermine the buffer effect soon. Different

from normal textiles, the i-Cool textile functions not only to transport sweat but also provide an excellent heat conduction path for the accelerated

evaporation and taking away a great amount of heat from the skin, which can prevent the i-Cool textile from ﬂooding to a much greater extent and avoid

excessive perspiration. Therefore, the i-Cool textile can help human body achieve enhanced cooling effect with greatly reduced sweat, by using the sweat in

a highly efﬁcient manner. The weight contrast in red arrows drawing illustrates the heat transport ability difference. The dot size and density contrast in the

sweat evaporation drawing shows the different evaporation ability. The drop size contrast in the sweat drawing illustrates that i-Cool textile can help

reduce sweat consumption.

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NATURE COMMUNICATIONS | (2021) 12:6122 | https://doi.org/10.1038/s41467-021-26384-8 | www.nature.com/naturecommunications 3

Thermal resistance measurement. To quantify the enhancement

of heat transport capability of the i-Cool (Cu) textile, we per-

formed the measurement of thermal resistance using cut bar

method, as illustrated in Supplementary Fig. 8. Using this

method, we measured the dry thermal resistance of the i-Cool

(Cu) textile and other commercial textile samples all under an

additional contact pressure of ~15 psi (103 kPa). As exhibited in

Fig. 2d, the i-Cool (Cu) textile shows about 14–20 times lower

thermal resistance compared to the conventional textiles (See

Supplementary Note 2 and Supplementary Fig. 8 for more details

and discussion). A thermal resistor model was built up to inter-

pret the measured thermal resistance. It was found out the nylon

6 nanoﬁbre layer contributes to the major thermal resistance, and

increasing the thickness of heat conductive matrix (Cu) will only

cause minor increase of thermal resistance (Supplementary

Fig. 9). It provides support for the possibility of extending the

i-Cool concept into fabrics of various thickness.

Transient droplet evaporation test. We further used a transient

droplet evaporation test to compare the evaporation performance

of the i-Cool (Cu) textile and the conventional textiles. Figure 2e

illustrates the experimental setup: A heater placed on an insu-

lating foam was used to simulate human skin with a thermo-

couple attached to the heater surface; We added liquid water at

37 °C to mimic sweat onto the artiﬁcial skin, then textile samples

covered on the wet artiﬁcial skin immediately; The power density

of the artiﬁcial skin was maintained constant during the mea-

surement. During the whole evaporation process, skin tempera-

ture was always monitored and recorded. For example, a group of

typical curves of skin temperature versus time are shown in

Supplementary Fig. 10. Generally, the curves can be divided into

three stages for every tested textile sample. Initially, when water

was just added onto the artiﬁcial skin, skin temperature dropped

sharply. Then, skin temperature was relatively stable only ﬂuc-

tuating in a small range in the evaporation stage. Eventually, skin

temperature rose again quickly once water was completely

evaporated.

Two pieces of important information can be obtained through

comparing the curves of i-Cool (Cu) and the conventional

textiles. Firstly, the evaporation time with i-Cool (Cu) was much

shorter, which indicates that i-Cool (Cu) exhibits higher

evaporation rate. This conclusion can also be veriﬁed by

measuring the mass loss of liquid water over time during the

evaporation test (Supplementary Fig. 11). Secondly, skin

temperature with i-Cool (Cu) textile was lower than the

conventional textiles during evaporation, demonstrating human

body can evaporate sweat faster with even lower skin temperature

when a person wears i-Cool textile. The summarized comparison

of average skin temperature and average evaporation rate between

the i-Cool (Cu) textile and the conventional textiles is displayed

in Fig. 2f (0.1 mL initial water, 422.5 W/m2power density,

ambient temperature: ~22 °C). The i-Cool (Cu) shows 2.3–4.5 °C

lower average skin temperature and about twice faster average

evaporation rate compared to the conventional textiles.

Furthermore, measurements under assorted skin power density

and initial liquid water amount for i-Cool (Cu) and cotton were

performed. With different experimental parameters, the average

evaporation rate was calculated and plotted versus the average

skin temperature during evaporation in Supplementary Fig. 12a

and Supplementary Fig. 12b. In our measurement range, a linear

0.0 0.1 0.2 0.3 0.4 0.5 0.6

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

i-Cool(Cu),T

skin

=35

CCotton,T

skin

=35

i-Cool(Cu),T

skin

=40

C Cotton,T

skin

=40

)

h/Lm

(

Initial water amount (mL)

i-Cool(Cu),T

skin

=30

CCotton,T

skin

=30

31.16

34.64

33.83

35.68

33.51

0.8381

0.42726

0.45755

0.44273 0.46414

i-Cool(Cu) Cotton Dri-FIT CoolMax Coolswitch

tare

ksega

Skin temperature

0.4

0.6

0.8

1.0

Evaporation rate

Average evaporation rate (mL/h)

****

1.895E-4

0.00315

0.00392 0.00381

0.00271

0.0000

0.0005

0.0010

0.0015

0.0020

0.0025

0.0030

0.0035

0.0040

0.0045

nat

iserl

mrehT(K*m

/W)

69.5

48.43

72.33

6.67 9.37

100

arg

W(m

/s)

wicking

Textile

sample

Insulating

foam

Heater

Thermocouple

Water

Cover

Sweat

Fig. 2 Wicking performance, thermal resistance and transient droplet evaporation test of the i-Cool (Cu) textile. a, Photograph of as-prepared i-Cool

(Cu) textile. Scale bar, 1 cm. b, SEM image of nylon 6 nanoﬁbres in the pores of heat conductive matrix (blue dash box) and on the top of heat conductive

matrix skeleton (red dash box). Sweat tends to be transported to the nanoﬁbres on the heat conductive matrix skeleton due to the morphology difference.

Scale bar, 1 µm. Inset is the magniﬁed photograph of the bottom side of i-Cool (Cu) textile showing its integrated heat conduction channels and water

transport channels. The holes are 2 mm in diameter and 3 mm pitch. Scale bar, 4 mm. c, Wicking rate of the i-Cool (Cu), cotton and other commercial

textiles. It shows how fast water underneath the textile can be pulled up and spread on the top surface. Double asterisks, Statistical signiﬁcance between

the i-Cool (Cu) and labeled sample, Welch’st-test p< 0.1; Asterisk, Statistical signiﬁcance between the i-Cool (Cu) and labeled sample, Welch’st-test p<

0.001. d, Thermal resistance of the i-Cool (Cu), cotton and other commercial textiles measured by cut-bar method (See more discussion in Supplementary

Note 2). Asterisk, Statistical signiﬁcance between the i-Cool (Cu) and labeled sample, Welch’st-test p< 0.001. e, Schematic illustration of the transient

droplet evaporation test. f, Average skin temperature and average evaporation rate of the i-Cool (Cu) textile and the conventional textiles (initial water

amount: 0.1 mL, skin heater power density: 422.5 W/m2). Asterisk, Statistical signiﬁcance of average skin temperature between the i-Cool (Cu) textile and

other textile samples, Welch’st-test p< 0.001. Statistical signiﬁcance of average evaporation rate between the i- Cool (Cu) textile and other textile

samples, Welch’s test p< 0.001.g, Fitted average evaporation rate of i-Cool (Cu) and cotton versus initial water amount at different skin temperature. All

the error bars represent standard deviation of measured data.

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relationship between the average evaporation rate and the average

skin temperature was observed with a certain amount of initial

water. Employed the linear ﬁtting relationship and replotted from

Supplementary Fig. 12, Fig. 2g shows the ﬁtted relationship

between the average evaporation rate and the initial water

amount at different skin temperatures for the i-Cool (Cu) and

cotton. Generally, the average evaporation rate increases as the

initial water amount increases and it shows an approaching

saturation trend as the initial water amount reaches a certain

level. This is perhaps consistent with the change trend of average

evaporation area during the drying process when the initial water

amount is changed. It is obvious that the i-Cool (Cu) exhibits

overall higher evaporation rate than cotton. Besides, i-Cool (Cu)

can achieve this with lower initial water amount and lower skin

temperature, indicating the superiority in sweat evaporation of

the i-Cool functional structure design.

Steady-state evaporation test. In order to further characterize the

evaporation features of i-Cool (Cu) and analyze its advantages over

conventional textiles, we performed a steady-state evaporation test.

Compared to the transient droplet evaporation test above, the

steady-state evaporation test can help derive additional useful

indexes to differentiate the evaporation property of textiles during

human body perspiration. The measurement apparatus is illu-

strated in Fig. 3a. Similarly, a heater placed on an insulating foam

was used to simulate human skin. Thermocouples and a water inlet

which were sealed in a thin acrylic board were attached to the

artiﬁcial skin surface. Not adding a certain initial amount of water,

water heated to 37 °C was pumped onto the skin surface at a

speciﬁc rate continuously, and textiles on it wicked the intake

water. Power density of the skin was adjusted to maintain skin

temperature stable at 35°C. The system with textile samples ﬁnally

reached a steady-state. By changing steady-state evaporation rate (i.

e. water pumping rate), the corresponding stable water mass gain

and power density can be measured for different textiles.

Figure 3b exhibits the measured water mass gain ratio (i. e.

water mass gain/textile sample dry mass*100%, denoted as W)of

i-Cool (Cu), cotton and Dri-FIT versus increasing evaporation

rate (denoted as v). Firstly, it was observed that the water mass

gain ratio of i-Cool (Cu) was always lower than cotton and Dri-

FIT at the same evaporation rate, indicating that less sweat is

required to “activate”i-Cool (Cu) to reach the same evaporation

rate compared to the conventional ones. For example, when the

steady-state evaporation rate was 1.1 mL/h, i-Cool (Cu) only

showed about 20 percent of water mass gain ratio, while Wof

cotton was approximately 130 percent. This phenomenon was

also in accordance with the transient droplet evaporation test

results. Furthermore, we ﬁtted the curves in Fig. 3b and calculated

water mass gain ratio gradient (dW/dv), as shown in Fig. 3c. dW/

dvof i-Cool (Cu) is apparently smaller than the conventional

textiles, even if all of them displayed water mass gain increase as

the growth of evaporation rate. Besides, dW/dvof cotton and Dri-

FIT rises rapidly with the increase of evaporation rate, especially

cotton. It means that it becomes even more and more difﬁcult to

achieve higher evaporation rate. Nevertheless, this index for

i-Cool (Cu) stays almost unchanged in the measurement range.

During real human body perspiration, these features of i-Cool

(Cu) enables it to fast evaporate sweat before sweat accumulates a

lot and to retain a relatively dry state even during very profuse

perspiration that requires high evaporation rate.

The measured power density (denoted as q) of artiﬁcial skin in

this test is shown in Fig. 3d. Overall, the skin power density with

i-Cool (Cu) was higher than the conventional textiles when they

were at the same evaporation rate, demonstrating the cooling

ability of i-Cool (Cu) during perspiration is stronger. It is

worthwhile to mention that i-Cool (Cu) is easier to reach higher

evaporation rate, thus the cooling power difference between

i-Cool (Cu) and conventional textiles in practical use can

be further enlarged. Besides, the curves in Fig. 3d were ﬁtted

and power density gradient (dq/dv) could be derived, as displayed

in Fig. 3e. This index (dq/dv) exhibits the cooling power

increment rate when evaporation rate increases. Obviously, dq/

dvof i-Cool (Cu) is much higher than cotton and Dri-FIT, which

means i-Cool (Cu) can provide much higher cooling power when

every unit of sweat evaporates. To be speciﬁc, dq/dvof i-Cool

(Cu) is about 3 times higher than that of cotton and Dri-FIT.

Furthermore, to some extent, dq/dvcan be converted into sweat

evaporative cooling efﬁciency (denoted as η) (See Supplementary

Note 3 for more discussion). Based on our estimation, the

evaporative cooling efﬁciency of i-Cool (Cu) is 0.8~1, while ηof

cotton and Dri-FIT is only 0.2~0.4 (Supplementary Fig. 13).

Therefore, we demonstrated i-Cool (Cu) shows evident advan-

tages in both evaporation ability and sweat evaporative cooling

efﬁciency, which makes it to be promising in next-generation

textiles for personal perspiration management.

Artiﬁcial sweating skin platform with feedback control loop.

Human body is capable of adjusting itself to maintain home-

ostasis in the means of feedback control loops46. Taking per-

spiration as an example, when the human body temperature

exceeds a threshold, the sympathetic nervous system stimulates

the eccrine sweat glands to secrete water to the skin surface. In

reverse, water evaporation on the skin surface accelerates heat

loss and thus body temperature decreases, which will reduce or

suspend the perspiration of human body (Fig. 4a)47,48.

To mimic human body perspiration situation and show the

performance difference between the i-Cool (Cu) textile and the

conventional textiles, we designed an artiﬁcial sweating skin

platform with feedback control loop, as illustrated in Fig. 4b. In

this system, an artiﬁcial sweating skin that can generate sweat

uniformly from every fabricated perspiration spot was built up

and served as the test platform. Power was supplied to the

artiﬁcial sweating skin platform to generate heat ﬂux simulating

human body metabolic heat. A syringe pump and a temperature

controller were utilized to provide continuous liquid water supply

at a constant temperature (37 °C) for the artiﬁcial sweating skin.

A thermocouple was attached to the artiﬁcial sweating skin

platform surface, monitoring skin temperature with a thermo-

couple meter that transmitted skin temperature data to the

computer in real time. Subsequently, the internal set program

could instantly alternate the pumping rate of the syringe pump

that corresponds to the sweating rate of artiﬁcial sweating skin,

which realized the feedback control loop imitating human body’s

feedback control mechanism.

To achieve uniform water outﬂow through each artiﬁcial sweat

pore mimicking human body skin sweating, we designed the

artiﬁcial sweating skin platform as illustrated in Fig. 4c. In the

bottom, an enclosed small cuboid cavity connecting to water inlet

acted as a water reservoir. When water was pumped in, water in

the reservoir was forced out upwards through the channels on the

reservoir cap. On the top of it, a perforated hydrophilic heater

was attached to generate heat, in the meantime through which

water can ﬂow out. The uniform “sweating”from each artiﬁcial

sweat pore was realized by the fabricated Janus-type wicking layer

with limited water outlets that was placed above the perforated

heater (See Supplementary Note 4–5 and Supplementary

Figs. 14–16 for more details and discussion).

We believe that the measurement results obtained with the as-

built artiﬁcial sweating skin platform can provide reasonable

parallel thermal comparison among the textile samples, even

though this set-up cannot fully represent the human body due to

the lack of some other feedback control mechanisms such as

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blood ﬂow feedback control and the differences in size, shape,

thermal capacity, etc. With the realization of scale-up, we expect

to conduct the human physiological wear experiment42 in the

near future.

Artiﬁcial sweating skin test. On the artiﬁcial sweating skin

platform, we ﬁrst performed a demonstrative experiment to

intuitively show the sweat evaporative cooling efﬁciency differ-

ence. In this experiment, the same power density was used for the

i-Cool (Cu) textile and cotton textile while the sweating rate was

varied for different ones to realize the same skin temperature

(34.5 °C), then we observed the condition of the artiﬁcial skin

device and the textile samples after stabilization of 30 minutes. As

shown in Supplementary Fig. 17, bare skin remained almost dry.

The skin with the i-Cool (Cu) textile also remained dry while

there was a little water absorbed in the sample. Nevertheless,

there was a much larger amount of water remaining on both the

skin platform and the cotton textile. These results intuitively

demonstrated the i-Cool (Cu) can cool down the skin more

efﬁciently consuming much less sweat.

Then, we performed measurements with constant skin power

density for i-Cool (Cu) and other commercial textile samples, to

mimic an exercise scenario of human body (See Supplementary

0.0 0.2 0.4 0.6 0.8 1.0 1.2

225

300

375

450

525

600

675

sne

dre

oP (W/m

)

Evaporation rate (mL/h)

i-Cool(Cu)

Cotton

Dri-FIT

◊

0.2 0.4 0.6 0.8 1.0 1.2

100

150

200

250

300

350

400

dq/dv (W/m

*h*mL

-1

)

Evaporation rate (mL/h)

i-Cool(Cu)

Cotton

Dri-FIT

0.0 0.2 0.4 0.6 0.8 1.0 1.2

100

120

140

)%(

itar

ags

Evaporation rate (mL/h)

i-Cool(Cu)

Cotton

Dri-FIT

◊

0.2 0.4 0.6 0.8 1.0 1.2

100

150

200

250

300

dW/dv(%*mL

-1

*h)

Evaporation rate (mL/h)

i-Cool(Cu)

Cotton

Dri-FIT

Textile sample

Insulating foam

Heater

Thermocouple

Water flow

Liquid Water

Water evaporation

Steady state:

Water flow rate (in)

= Evaporation rate (out)

Fig. 3 Steady-state evaporation test of the i-Cool (Cu) textile, cotton and Dri-FIT. a, Schematic illustration of the measurement apparatus and method.

b, Measured water mass gain ratio (W) at different evaporation rate (v). Triangle, Statistical signiﬁcance between the i-Cool (Cu) and cotton, Welch’s

t-test p< 0.1 at 0.3 mL/h, p< 0.001 at 0.7 mL/h, p< 0.01 for others. Diamond, Statistical signiﬁcance between the i-Cool (Cu) and Dri-FIT, Welch’s test

p< 0.05 at 0.3 mL/h, no statistical signiﬁcance at 0.5 mL/h, p< 0.01 for others. c,dW/dvobtained by ﬁtting data in (b). i-Cool (Cu) can achieve a certain

evaporation rate with much lower water gain. The required water gain increase for larger evaporation rate is also reduced. d, Measured power density (q)

at different evaporation rate (v). Triangle, Statistical signiﬁcance between the i-Cool (Cu) and cotton, Welch’st-test p< 0.05 at 0.3 mL/h, p< 0.001

at 0.7 mL/h, 0.9 mL/h, p< 0.01 for others. Diamond, Statistical signiﬁcance between the i-Cool (Cu) and Dri-FIT, Welch’s test shows no statistical

signiﬁcance at 0.3 mL/h, p< 0.05 at 0.5 mL/h, p< 0.01 at 0.7 mL/h, 0.9 mL/h, p< 0.001 for others. e,dq/dvobtained by ﬁtting data in (d). The i-Cool

(Cu) can show enhanced cooling effect with higher sweat evaporative cooling efﬁciency. All the error bars represent standard deviation of measured data.

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Note 6 and Supplementary Fig. 18 for more discussion for this

measurement). All the measurements were performed from the

same initial state. The skin temperature and sweating rate (i.e.

water pumping rate) after stabilization were measured. Figure 4d

shows the experimental results when skin power density was

~750 W/m2and ambient temperature was 22 °C. The cooling

performance of i-Cool (Cu) is very similar to the bare skin, which

is recognized as the most efﬁcient cooling approach since sweat

evaporation can directly take away heat from the skin. Compared

to the conventional textiles, i-Cool (Cu) exhibited evidently lower

skin temperature (~2.8 °C lower than cotton, ~2 °C temperature

difference with Dri-FIT and Coolswitch, ~3.4 °C temperature

difference with CoolMax). The sweating rate provided for the

conventional textiles was over 2–3 times as much as i-Cool (Cu).

It proves that conventional textiles cannot achieve better cooling

effect even with much more available sweat. On the other hand,

i-Cool (Cu) is able to unlock the cooling power of sweat more

efﬁciently, which can deliver improved cooling effect with

reduced sweating dehydration. As a result, conventional textiles

would become highly wet after perspiration, whereas i-Cool (Cu)

could retain a much drier state (insets of Fig. 4d), which is a

comprehensive effect of evaporation ability and sweat evaporative

cooling efﬁciency.

We tested the Cu heat conductive matrix and nylon 6

nanoﬁbre ﬁlm separately. The departure of the heat conduction

component and water transport component makes both of them

less efﬁcient in evaporative cooling, as exhibited in Supplemen-

tary Fig. 19. These tests illustrate the key factor to achieve an

effective cooling effect is the integrated functional design of heat

conduction and sweat transportation. Different cotton samples

with various area mass density were also tested (See Supplemen-

tary Note 7 and Supplementary Fig. 20 for more details). In our

experiments, the thinnest cotton sample (26.5 g/m2) that is too

transparent to be practically used still exhibited around 1.5 °C

higher skin temperature than the i-Cool (Cu) textile. These results

further validate the superiority of the i-Cool structure that is an

integrated one with both heat conduction and sweat

transportation.

The artiﬁcial sweating tests under different skin power

densities to simulate changed human body metabolic heat

production were also conducted. As displayed in Fig. 4e, the

enhanced cooling performance showing lower skin temperature

and reduced sweating rate in comparison to conventional textiles

was still true when different skin power densities were applied. It

veriﬁes the advantages of i-Cool in a wide range of heat

production.

Besides, the evaluation of performance under diverse ambient

environment conditions was performed, especially in high

temperature circumstances and high relative humidity surround-

ings in which perspiration is more likely to happen. At the

ambient temperature of 40 °C, the evaporative cooling perfor-

mance of i-Cool (Cu) textile and the conventional textiles is

shown in Fig. 4f. The cooling performance distinction between

the i-Cool (Cu) and the conventional textiles was still very

apparent. To take a step further, we decreased skin power density

of the artiﬁcial sweating skin to make skin temperature lower

38.58 39

42.03 41.66

42.56

41.55

1.31 1.44

2.41 2.29

2.58

2.26

35.0

37.5

40.0

42.5

45.0

erutarepmetnikS(

)h/Lm

targ

tae

RH ≈80%

*****

0.4 0.8 1.2 1.6

Bare skin

i-Cool(Cu)

Cotton

Dri-FIT

CoolMax

Coolswitch

erutarepmetnikS(

Sweating rate (mL/h)

Conventional

textiles

i-Cool(Cu)

Cotton

35.47 35.95

38.82

38.07

39.38

37.95

37.41 38.26

40.82 40.57

42.68

40.35

39.95

40.85

43.18 42.84

44.43

42.59

0.31 0.46

1.38

1.14

1.56

1.1

0.93 1.2

2.02 1.94

2.62

1.87

1.74

2.03

2.78 2.67

3.18

2.59

35.0

37.5

40.0

42.5

45.0

erutarepmetnikS(

oC)

750 W/m

880 W/m

1035 W/m

)h/Lm(etargnitaewS

Tambient = 22 °C

Body temperature

abc

Artificial sweating

skin

Thermocouple

Data reading

Feedback control

gde

40.01

40.99

43.82

43.08

44.69

43.09

1.76

2.08

2.98

2.75

3.26

2.85

35.0

37.5

40.0

42.5

45.0

metnik

)h/Lm(etargnitaewS

Tambient = 40 °C

*****

water

Capillary force

water

Wicking layer

Hydrophobic “baffle”

Perforated heater

Water reservoir

*****

Syringe

pump

Water

temperature

controller

Power

supply

Thermocouple

meter

Fig. 4 Artiﬁcial sweating skin platform with feedback control loop and measurements on it. a, Schematic of human body temperature self-regulation

mechanism. When body temperature increases, human body perspires to cool down its own temperature, which leads to reduction or suspension of

perspiration in reverse. b, Schematic of the artiﬁcial sweating skin platform with feedback control loop simulating human body temperature self-regulation

mechanism. c, Schematic of the detailed structure of the artiﬁcial sweating skin. The schematic in the red dash box shows the working mechanism of the

modiﬁed Janus-type wicking layer which realizes uniform sweating mimicking human skin sweating scenario. d, Measurement results of skin temperature

and sweating rate for bare skin, i-Cool (Cu) and commercial textiles (skin power density: 750 W/m2, ambient temperature: 22 °C). Insets show the

photographs of i-Cool (Cu) and cotton after one-hour stabilization during the tests. Asterisk, Statistical signiﬁcance of skin temperature and sweating rate

between the i-Cool (Cu) and other textiles, Welch’st-test p< 0.001. e, Measurement results of skin temperature and sweating rate for bare skin, i-Cool

(Cu) and other conventional textiles under different skin power densities. Asterisk, Statistical signiﬁcance of skin temperature and sweating rate between

the i-Cool (Cu) and other textiles at 750 W/m2, 880 W/m2and 1035 W/m2, Welch’st-test p< 0.001. f, Measured skin temperature and sweating rate at

high ambient temperature (40 °C). 750 W/m2power density was applied. Asterisk, Statistical signiﬁcance of skin temperature and sweating rate between

the i-Cool (Cu) and other textiles, Welch’st-test p< 0.001. g, Measured skin temperature and sweating rate in high relative humidity ambient (~80%).

Asterisk, Statistical signiﬁcance of skin temperature and sweating rate between the i-Cool (Cu) and other textiles, Welch’st-test p< 0.001. All the error

bars represent standard deviation of measured data.

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than ambient temperature to compare bare skin, i-Cool (Cu) and

cotton, to see if the high thermal conductivity design in the i-Cool

(Cu) will cause adverse effect for skin temperature. Consequently,

skin temperature with the i-Cool (Cu) was almost the same as

bare skin and showed better performance than cotton, as shown

in Supplementary Fig. 21, indicating its evaporative cooling effect

surpassed the opposing heat conduction from the ambient. In

addition to high ambient temperature, we also investigated the

performance of i-Cool (Cu) and other conventional textiles in a

high relative humidity (RH) environment (Fig. 4g). As the relative

humidity was raised, skin temperature with all the textile

swatches rose correspondingly. Nevertheless, the skin tempera-

ture of the i-Cool (Cu) was still much lower than the conventional

textiles.

Moreover, we performed measurements to see how the

parameters in the functional structure design of i-Cool (Cu)

inﬂuence its performance (See Supplementary Note 8 and

Supplementary Fig. 22 for more details). The results provide

additional guidelines for personal perspiration management

textile design.

i-Cool practical application demonstration. To further study the

cooling effect of the i-Cool textile on human body, we developed

a thermal simulation considering the coupled heat transfer,

moisture vapor and liquid water transfer processes based on the

actual human body with complex structure and dynamic phy-

siological responses (See Supplementary Note 9, Supplementary

Dataset 1 and Supplementary Fig. 23 for more details)49–51. The

simulation results show that the i-Cool textile with improved

evaporation ability and sweat evaporative cooling efﬁciency can

achieve temperature reduction in both the skin temperature and

core temperature of the human body compared to that with

conventional textiles (Supplementary Fig. 23), which further

validates the potential of the i-Cool structure design in efﬁcient

evaporative cooling for the human body.

To bridge the gap between i-Cool (Cu) concept demonstration

to practical use, we demonstrated the feasibility via fabricating the

i-Cool textile based on commercial fabrics. Firstly, we veriﬁed the

replacement of Cu matrix by polymer materials with heat

conductive coatings. As shown in Supplementary Fig. 24, the

i-Cool textiles using silver (Ag) coated polyester (PET) and

nanoporous polyethylene (NanoPE) matrices exhibit almost the

same performance as i-Cool (Cu) in the artiﬁcial sweating skin

test (experimental parameters: same as Fig. 3d). Furthermore, we

fabricated i-Cool textiles based on commercial knitted fabrics

made of PET ﬁbers. Here, we chose Dri-FIT and CoolMax which

were already tested as control samples as the substrates. Figure 5a

illustrates the fabrication process: holes were cut by laser cutting

on the original fabric, after which it went through a facile

electroless plating process. The Ag coating was deposited onto

every ﬁber’s surface of the fabric. Next, cellulose ﬁbers were ﬁlled

into the holes of the fabric, and prepared nylon 6 nanoﬁbre ﬁlm

was transferred onto the fabric via press lamination to realize the

i-Cool (Ag) textile which possessed the desired i-Cool structure. It

is worthwhile to point out the fabrics we selected and the

electroless plating method are not the only choices. Other textile

material and other methods offering heat conductive coatings can

be utilized. Alternatively, heat conductive ﬁbers can be applied as

well for the heat transport matrix. Figure 5b shows the

photograph of the i-Cool (Ag) textile sample swatch (Dri-FIT

as substrate). The photograph viewing from the i-Cool (Ag)

bottom is exhibited in the inset of Fig. 5c, and the SEM images of

the Ag coated PET ﬁbers (Fig. 5c, Supplementary Fig. 25) show

the Ag coating is conformal and uniform. The branched structure

formed in the electroless plating process can potentially enlarge

evaporation area as well. The photograph and SEM images of

i-Cool textile with CoolMax substrate are shown in Supplemen-

tary Figs. 26 and 27.

Successively, we performed the same steady-state evaporation

test and artiﬁcial sweating skin test for the i-Cool (Ag) textile. In

the steady-state evaporation test, the curves of i-Cool (Ag) plotted

with curves of i-Cool (Cu), cotton and Dri-FIT (Fig. 5d, e)

exhibited that i-Cool (Ag) exhibited very similar performance to

the i-Cool (Cu) textile. Compared to the original Dri-FIT textile

acting as the substrate, i-Cool (Ag) owns signiﬁcantly improved

evaporation performance and evaporative cooling efﬁciency,

which is owing to the i-Cool functional structure. Also, in the

artiﬁcial sweating skin test, i-Cool (Ag) and i-Cool (Cu) presented

comparable cooling performance for personal perspiration

management, which was signiﬁcantly improved in contrast to

cotton and Dri-FIT. This is also true for the i-Cool textile

prepared with CoolMax substrate (Supplementary Fig. 28). With

only sweat transportation channels, the modiﬁed Dri-FIT and

CoolMax showed weaker cooling performance (Supplementary

Fig. 28), which veriﬁes the i-Cool structure combining heat

conduction with water transportation provides superior strategy

in personal perspiration management. These results demonstrate

the feasibility of readily applying the i-Cool concept to

practical usage.

In summary, we report a novel concept of i-Cool textile with

unique functional structure design for personal perspiration

management. The innovative employment of integrated water

transport and heat conductive functional components together

ensures not only its wicking ability, but also the fast evaporation

rate, enhanced evaporative cooling effect and reduction of human

body dehydration for human body via utilizing sweat in a highly

efﬁcient manner, which was demonstrated by the transient and

steady-state evaporation test. An artiﬁcial sweating skin platform

with feedback control loop simulating human body perspiration

situation was realized, on which the i-Cool (Cu) textile shows

comparable performance to the bare skin and apparent cooling

effect with less provided sweat compared to the conventional

textiles. Also, the structure advantage maintains under various

conditions of exercise and ambient environment. Besides, the

practical application feasibility of the i-Cool design principles was

demonstrated, exhibiting decent performance. Therefore, we

expect the i-Cool textile will open a new door and provide new

insights for the textiles for personal perspiration management.

Methods

Textile preparation. The Cu matrix used in the i-Cool (Cu) textile sample (main

text) was prepared with Cu foil (~25 µm thickness, Pred Materials) laser cut via

DPSS UV laser cutter. A pore array (2 mm diameter, 3 mm pitch) on the Cu foil

was created to realize the Cu matrix. Nylon 6 nanoﬁbre ﬁlm was prepared by

electrospinning. The nylon 6 solution system used in this work is 20 wt% nylon-6

(Sigma-Aldrich) in formic acid (Alfa Aesar). The polymer solution was loaded in a

5 mL syringe with a 22-gauge needle tip, which is connected to a voltage supply

(ES30P-5W, Gamma High Voltage Research). The solution was pumped out of the

needle tip using a syringe pump (Aladdin). The nanoﬁbres were collected by a

grounded copper foil (Pred Materials). The applied potential was 15 kV. The

pumping rate was 0.1 mL/h. The distance between the needle tip and the collector

is 20 cm. After collecting nylon 6 nanoﬁbres of desired mass, the nylon 6 nanoﬁbre

ﬁlm (~4.5 g/m2, ~25 µm thickness) was transferred and laminated on the Cu

matrix. A hydraulic press (MTI) was used to press nylon 6 nanoﬁbres both into the

holes and on the top of the Cu matrix. The fabricated i-Cool (Cu) was ~45 µm

thick and 107.7 g/m2. The varied parameters of the i-Cool (Cu) textile are shown in

Supplementary Fig. 22. To fabricate the i-Cool (Ag) textile sample, same pore array

as above was cut by laser cutter (Epilog Fusion M2 laser cutter) for the Dri-FIT or

CoolMax textiles. Then, the fabric was cleaned and modiﬁed with polydopamine

(PDA) coating for 2 h in an aqueous solution that consists of 2 g/L dopamine

hydrochloride (Sigma Aldrich) and 10 mM Tris-buffer solution (pH 8.5,

Teknova)52. For electroless plating of silver (Ag), the PDA-coated fabrics were then

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dipped into a 25 g/L AgNO

solution (99.9%, Alfa Aesar) for 30 min to form the

Ag seed layer. After rinsing with deionized (DI) water, the fabric was immersed

into the plating bath solution containing 4.2 g L−1Ag(NH

)

+(made by adding

28% NH

·H

O dropwise into 5 g L−1AgNO

until the solution became clear again)

and 5 g L−1glucose (anhydrous, EMD Millipore Chemicals)53 for 2 h. Next, the

fabric was turned over and placed into a new plating bath for another 2 h. After

drying, cellulose ﬁbers were ﬁlled into the cut pores by extraction ﬁltration of paper

pulp. Then, nylon 6 nanoﬁbre ﬁlm (~2-2.5 g/m2) was added onto it by the same

process described above. The as-prepared i-Cool (Ag) (based on Dri-FIT) is ~175

g/m2. The one based on CoolMax is ~199 g/m2. For i-Cool samples based on Ag-

coated PET and NanoPE ﬁlm matrix, the PET matrix (~50 µm thickness) and

NanoPE matrix (~25 µm thickness) were prepared by laser cutting in the same way,

and went through the same Ag coating process and nylon 6 nanoﬁbre ﬁlm

lamination. The cotton textile sample was from a common short-sleeve T-shirt

(100% cotton, single jersey knit, 135 g/m2, ~400 µm thickness, Dockers). The Dri-

FIT textile sample was from a regular Dri-FIT T-shirt (100% PET, single jersey

knit, 143 g/m2, ~400 µm thickness, Nike). The CoolMax textile sample was from a

T-shirt made of 100% CoolMax Extreme polyester ﬁbers (100% PET, single jersey

knit, 166 g/m2, ~445 µm thickness, purchased from Galls.com). The Coolswitch

textile sample was from a Coolswitch T-shirt (91%PET/9% Elastane, French terry

knit, 140 g/m2, ~350 µm thickness, Under Armour).

Material characterization. The optical microscope images were taken with an

Olympus optical microscope. The SEM images were taken by a FEI XL30 Sirion

SEM (5 kV) and a FEI Nova NanoSEM 450 (5 kV).

0.0 0.2 0.4 0.6 0.8 1.0 1.2

225

300

375

450

525

600

675

ewo

P(W/m

)

Evaporation rate (mL/h)

i-Cool(Ag)

i-Cool(Cu)

Cotton

Dri-FIT

35.47

35.95 36.29

38.82

38.07

0.31

0.46 0.57

1.38

1.14

erutarepmetnikS(

0.0

0.4

0.8

1.2

1.6

)h/Lm(etargnitaewsartxE

Tambient = 22 °C

****

0.0 0.2 0.4 0.6 0.8 1.0 1.2

100

120

140

)

(

itarni

ags

taW

Evaporation rate (mL/h)

i-Cool(Ag)

i-Cool(Cu)

Cotton

Dri-FIT

Laser

cutting

Electroless

plating

i-Cool (Ag) textile

nanofibre film

Fibre

filling

Ag coated

fabric

Original fabric

Press

lamination

)

Electrospun

Fig. 5 Practical application feasibility demonstration of the i-Cool functional structure via i-Cool (Ag) textile. a, Illustration of the fabrication process of

i-Cool (Ag) textile based on a commercially available fabric. b, Photograph of as-fabricated i-Cool (Ag) textile based on Dri-FIT as the substrate. Scale bar,

1 cm. c, SEM image showing the uniform and conformal Ag coating on the PET ﬁbers of the fabric substrate. Scale bar, 50 µm. The inset shows the

photograph of i-Cool (Ag) viewing from its bottom. Scale bar, 4 mm. d, Measured water mass gain ratio of i-Cool (Ag) and other textiles at different

evaporation rate in the steady-state evaporation test. Omega symbol, Statistical signiﬁcance between the i-Cool (Ag) and i-Cool (Cu), Welch’st-test p<

0.1 at 0.3 mL/h and 0.5 mL/h, no statistical signiﬁcance for others. Phi symbol, Statistical signiﬁcance between the i-Cool (Ag) and cotton, Welch’s test p<

0.05 at 0.3 mL/h, p< 0.001 for others. Sigma symbol, Statistical signiﬁcance between the i-Cool (Ag) and Dri-FIT, Welch’s test shows no statistical

signiﬁcance at 0.5 mL/h, p< 0.05 at 0.7 mL/h, 1.1 mL/h, p< 0.01 for others. e, Measured power density of i-Cool (Ag) and other textiles at different

evaporation rate in the steady-state evaporation test. Omega symbol, Statistical signiﬁcance between the i-Cool (Ag) and i-Cool (Cu), Welch’st-test p<

0.01 at 0 mL/h, 1.1 mL/h, p< 0.05 at 0.7 mL/h, no statistical signiﬁcance for others. Phi symbol, Statistical signiﬁcance between the i-Cool (Ag) and

cotton, Welch’s test p< 0.001 at 0.7 mL/h and 0.9 mL/h, p< 0.01 for others. Sigma symbol, Statistical signiﬁcance between the i-Cool (Ag) and Dri-FIT,

Welch’s test p< 0.05 at 0.5 mL/h, p< 0.01 at 0.3 mL/h and 0.7 mL/h, p< 0.001 for others. f, Measured skin temperature and sweating rate of the i-Cool

(Ag) textile on the artiﬁcial sweating skin platform with feedback control loop. Asterisk, Statistical signiﬁcance of skin temperature and sweating rate

between the i-Cool (Ag) and other textiles, Welch’st-test p< 0.001. All the error bars represent standard deviation of measured data.

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Wicking rate measurement. The wicking rate measurement method was based on

AATCC 198 with modiﬁcation. 5 × 5 cm textile samples were prepared ahead.

0.1 mL of distilled water was placed on the simulated skin platform by pipette.

Then textile samples were covered on the water, and the time of water reaching the

circle of 1.5 cm in radius on the top surface of textile was recorded. Wicking rate

was calculated using wicking area divided by wicking time.

One-way water transport characterization. A 5 × 5 cm textile sample was ﬁxed

onto an acrylic frame that had a 4 × 4 cm square hole. Camera was placed right

above the frame or underneath the frame to shoot videos. In total 20 µL of deio-

nized water was added onto one side of textile sample and the water transport

process was ﬁlmed. The testing time was calculated by an image processing soft-

ware (SketchAndCalc Area Caculator). We calculated the Sinner ,Souter and µat the

testing time of 15 s. S

inner

or S

outer

=water spreading area/testing time. µis a one-

way transport index, which is deﬁned as S

outer

inner

Thermal resistance measurement. The cut bar method adapted from ASTM

5470 was used to measure thermal resistance. In this setup, eight thermocouples are

inserted into the center of two 1 inch × 1 inch copper reference bars to measure the

temperature proﬁles along the top and bottom bar. A resistance heater generates a

heat ﬂux which ﬂows through the top bar followed by the sample and then the

bottom bar after which the heat is dissipated into a large heat sink. The entire

apparatus (top bar, sample, bottom bar) is wrapped in thermal insulation. A

modest pressure of approximately 15 psi was applied at the top bar to reduce

contact resistance, and no thermal grease was used due to the material porosity.

The temperature proﬁles of the top and bottom copper bars are then used to

determine both the heat ﬂux and the temperature drop across the sample stack,

which can derive the total thermal resistance (RTOT ). Plotting the RTOT versus the

number of sample layers, the sample thermal resistance with contact thermal

resistance between samples can be obtained from the slope of the line.

Water vapor transmission property tests. The upright cup testing procedure

was based on ASTM E96 with modiﬁcation. Medium bottles (100 mL; Fisher

Scientiﬁc) were ﬁlled with 80 ml of distilled water, and sealed with the textile

samples using open-top caps and silicone gaskets (Corning). The exposed area of

the textile was 3 cm in diameter. The sealed bottles were placed into an environ-

mental chamber in which the temperature was held at 35 °C and relative humidity

was 30 ± 5%. The mass of the bottles and the samples was measured periodically.

By dividing the reduced mass of the water by the exposed area of the bottle (3 cm in

diameter), the water vapor transmission was calculated. The evaporative resistance

measurement was based on ISO 11092/ASTM F1868 with modiﬁcation. A heater

was used to generate stable heat ﬂux mimicking the skin. A metal foam soaked with

water was placed on the heater. A waterproof but vapor permeable ﬁlm was cov-

ered on the top of the metal foam to protect the textile sample from contact with

water. The whole device was thermally guarded. For different textile samples, we

adjusted the heat ﬂux to maintain the same skin temperature (35 °C) for all

measurements. The ambient temperature was controlled by the water recirculation

system at 35 °C, and the relative humidity was within 24 ± 4%. The evaporative

resistance was calculated by Ref ¼ðPsPaÞA

HRebp, where Psis the water vapor

pressure at the plate surface, which can be assumed as the saturation at the tem-

perature of the surface, Pais the water vapor pressure in the air, Ais the area of the

plate test section, His the power input, and Rebp is the value measured without any

textile samples.

Water vapor thermal measurement. The artiﬁcial sweating skin platform was

utilized in this measurement. A steady power density (580 W/m2) and water ﬂow

rate (0.25 mL/h) were adopted. An acrylic frame (thickness: 1.5 mm) with a

crossing was laser cut and placed on the platform to support the textile samples

avoiding the liquid water contact. Stable skin temperature was read. The ambient

was 22 °C ± 0.2 °C, 40 ± 5% relative humidity.

Transient droplet evaporation test. The skin was simulated by a polyimide

insulated ﬂexible heater (McMaster-Carr, 25 cm2) which was connected to a power

supply (Keithley 2400). A ribbon type hot junction thermocouple (~0.1 mm in

diameter, K-type, Omega) was in contact with the top surface of the simulated skin

to measure the skin temperature. The heater was set on a 10 cm-thick foam for heat

insulation. During the tests, water (37 °C) was added onto the simulated skin and

textile samples were covered on the simulated skin immediately. The skin tem-

peratures with wet textile samples during water evaporation were measured with an

assorted combination of initial water amount and generated area power density of

simulated skin. The average evaporation rate was calculated by dividing the initial

water amount by evaporation time. The end point of the evaporation was deﬁned

as the inﬂection point between the relatively stable range and the rapid increase

stage of temperature. The average skin temperature referred to the average tem-

perature reading spanned the evaporation stage in which skin temperature was

relatively stable. The mass of wet textile samples was measured by a digital balance

(U. S. Solid, 0.001g accuracy) to track the water mass loss during the evaporation.

The tests were all performed in an environment of 22 ± 0.2 °C, 40 ± 5% relative

humidity.

Steady-state evaporation test. The skin was simulated by a polyimide insulated

ﬂexible heater (McMaster-Carr, 25 cm2) which was connected to a power supply

(Keithley 2400). It was covered by a 1.5 mm-thick acrylic board with grooves made

by laser cutting (Epilog Fusion M2 laser cutter) on its top surface. A ribbon type

hot junction thermocouple (~0.1 mm in diameter, K-type, Omega) was sealed in a

groove by PDMS to measure the skin temperature. A needle connected to a tube

and a syringe pump (Harvard, PHD 2000) was also sealed in one groove of the

acrylic board, but with head exposed for water outage. The heater was set on a

10 cm-thick foam for heat insulation. During the tests, water in the tube was heated

by a proportional–integral–derivative (PID) temperature controller (Omega

Engineering) at 37 °C before ﬂowing onto the artiﬁcial skin. Textile samples were

placed on the artiﬁcial skin surface. The applied power density was adjusted to let

measured skin temperature ﬂuctuate around 35 °C. After stabilization for a period

of time, the mass of wet textile samples was measured by a digital balance (U. S.

Solid, 0.001g accuracy), and power density was recorded. The tests were all per-

formed in an environment of 19.5 ± 0.3 °C, 35 ± 5% relative humidity.

Fabrication of Janus-type wicking layer with limited water outlets.Aﬁlter

paper (Qualitative, Whatman) was used as the wicking layer. An acrylic board was

laser cut into a mask with Epilog Fusion M2 Laser and placed on the top of the

ﬁlter paper. Polydimethylsiloxane (PDMS) base and curing agent (Sylgard 184,

Dow Corning) with mass ratio of 10:1 were dispersed into hexane (Fisher Scien-

tiﬁc) with volume ratio 1:10. The PDMS solution was sprayed onto the masked

ﬁlter paper that was on a heating plate, which helped with faster volatilization of

hexane. After drying and curing, the PDMS formed hydrophobic coating layer only

on the uncovered place of the top surface of the ﬁlter paper, which could absorb

and transport water from the bottom surface but provide limited water outlets on

the top surface.

Artiﬁcial sweating skin test with feedback control loop. The water reservoir

(5 cm × 5 cm × 2.5 mm) with water inlet (whole part size: 8 cm × 8 cm × 3.5 mm)

was made by 3D printing (FlashForge Creator Pro). A cover with a 9 × 9 hole

(diameter: 3 mm) array (hole array area: 5 cm × 5 cm, whole part size: 8 cm ×

8 cm× 1.5 mm) was also 3D printed and bound with the water reservoir part. The

water reservoir was connected to a syringe pump (Harvard, PHD 2000). The

pumped water was heated at 37 °C by a heater (Omega Engineering) and a

proportional–integral–derivative (PID) temperature controller (Omega Engineer-

ing). A polyimide insulated ﬂexible heater (McMaster-Carr, 25 cm2) with laser cut

water outlets was adhered to the holey cover. The heater was connected to a power

supply (Keithley 2400). Then, the fabricated Janus-type wicking layer with limited

water outlets was attached to the heater layer to serve as the skin surface. A ribbon

type hot junction thermocouple (~0.1 mm in diameter, K-type, Omega) connected

to a thermocouple meter (Omega Engineering) was in contact with the top surface

of the Janus-type wicking layer to measure the skin temperature. The thermocouple

meter, syringe pump and power supply were all controlled by a LabView program,

which can alter the pumping rate (extra sweating rate) according to the thermo-

meter reading (skin temperature) in real time. Before the test, the artiﬁcial sweating

skin platform was ﬁlled with water in advance. The perspiration threshold skin

temperature was set to be 34.5 °C, over which the sweating rate was linearly

dependent on skin temperature47,48. The relationship between pumping rate and

skin temperature was set as pumping rate (mL/h) =0.32*skin temperature (°C)

−11.04, which was decided according to previous research and reasonable human

body perspiration rate range. The whole set-up was in a space without forced

convection. No chamber with cover for the set-up was used to avoid water vapor

accumulation except the high-humidity test. In the high-humidity test, a humidiﬁer

was placed next to the testing platform and they are enclosed together to change

the humidity. The initial air temperature in the chamber was 22 °C but about

1–2 °C reading variation of the ambient temperature thermometer was observed,

perhaps due to the water vapor condensation, but no obvious inﬂuence on the skin

temperature was observed. In other cases, if no ambient temperature and relative

humidity are speciﬁed, the ambient temperature was 22 ± 0.2 °C and ambient

relative humidity was 40 ± 5%.

Data availability

The data that support the ﬁndings of this study are available from the corresponding

author upon reasonable request.

Code availability

The code for thermal simulation of actual human body is available from the

corresponding author upon reasonable request.

Received: 16 May 2021; Accepted: 10 September 2021;

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Acknowledgements

We acknowledge the great help of P. Zhu, C. Lau, G. Gerboni, Z. Yu, and Y. Zheng. Part

of this work was performed at the Stanford Nano Shared Facilities and the Stanford

Nanofabrication Facility. J.S., C.D., and R.P. acknowledge the support of the Laboratory

Directed Research and Development Program (LDRD) at Lawrence Berkeley National

Laboratory under contract # DE-AC02-05CH11231.

Author contributions

Y.C. and Y.P. conceived the idea. Y.P. designed and conducted the experiments. Y.P.,

W.L., and B.L. performed the feedback control loop construction and programming.

W.L. and W.J. conducted the simulation. B.L. drew the schematics. J.T. and G.Z. helped

with sample preparation. J.S. and J.Z. performed the thermal resistance measurement.

G.W. helped with statistical analysis. Y.Z. and C.Z. helped with laser cutting process.

W.H. and T.W. provided helpful discussion. Y.C. and R.P., C.D., S.F., K.G. supervised the

project. All the authors provided helpful discussion on this project and contributed to

manuscript writing.

Competing interests

The authors declare no competing interests.

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Additional information

Supplementary information The online version contains supplementary material

available at https://doi.org/10.1038/s41467-021-26384-8.

Correspondence and requests for materials should be addressed to Yi Cui.

Peer review information Nature Communications thanks Yi Li for their contribution to

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Thermal management is of great significance for human activities. Attaining thermal comfort not only requires thermal control of building's macroenvironment, but also additionally personal thermal regulation. Radiative cooling technologies are anticipated to effectively lower building energy utilization and provide optimal thermal comfort for individuals in hot weather. Nevertheless, these static and single‐purpose characteristics lack the ability to adjust to rapidly changing weather conditions, often leading to excessive cooling. To overcome this challenge, the emergence of dual‐mode smart flipping technologies has provided a pathway for dynamically adjusting the heating or cooling of objects in response to fluctuations in ambient temperature. First, the underlying principles of dual‐mode smart flipping are shown. Then the evolving materials and approaches of smart flipping are given an introduction to adapt to different environments under external stimuli, such as mechanical flipping, temperature, humidity, and so forth. Afterward, we present the recent applications of dual‐mode smart flipping materials and devices in personal thermal management, thermoelectric generation, energy‐saving buildings, and smart windows. Finally, the challenges and outlook of dual‐mode smart flipping are presented and future directions are identified.

Superabsorbent fabric: in situ polymerisation of macroporous starch–sodium alginate–polyacrylate hydrogel on fibre surface

Article

Full-text available

Jun 2023
CELLULOSE

A comfortable microenvironment can be obtained by applying superabsorbent fabrics in personal protective equipment. We prepared a superabsorbent and high-water-retention fabric via in situ polymerisation of starch–sodium alginate–polyacrylate (St–SA–PAA) hydrogel on the surface of a polyester fabric. The pore-size distribution of the gel on the fabric was 30–91 μm. Utilising the capillarity of the polyester fibre, the water-absorption rate of St–SA–PAA gel could be increased from 2.8 to 10 g/min when deposited on the fabric. The water absorption capacity of the superabsorbent fabric reached 343 g/g, which exceeded that of the reported superabsorbent fibres. In addition, the supporting effect of the polyester fibre resulted in improved mechanical properties of the gel after water absorption. Interestingly, the superabsorbent fabric exhibited isotropic characteristics, and its water absorption capacity and softness was adjustable by controlling the amount of deposited gel. Further, its swelling behaviour conformed to second-order kinetics. This study provides a new approach for the development of comfortable damp-heat-management fabrics. Graphical abstract

Thermoregulating textiles and fibrous materials for passive radiative cooling functionality

Article

Full-text available

May 2023
Mater Des

Developing thermoregulating textiles based on passive radiative cooling strategies has recently attracted considerable attention as an emerging means of providing localised thermoregulation for wearers. This cooling mechanism not only has been used to enhance the thermal comfort in both indoor and outdoor spaces, but it has also been regarded as one of the promising methods of reducing dependency on energy-intensive air conditioning systems. This article reviews recent advances in developing fibrous structures with passive radiative cooling functionality including mid-infrared (MIR) transparent textiles, and textiles with passive daytime radiative cooling (PDRC) effect. The fundamental working mechanisms, fabrication methods, and reported performances of these functional textiles are reviewed and compared. Specific attention is paid to recent technological advances in obtaining the radiative cooling functionality via conventional manufacturing methods including coating, fibre spinning, nanofibres electrospinning, weaving/knitting of fabrics, and nonwovens. Passive radiative cooling mechanisms of several natural species are also discussed, to guide the development of novel thermoregulating textiles. This article provides a comprehensive review of the status of passive radiative cooling technology in textiles and delineates some possible pathways for future improvements with potential applications in sportswear, protective clothing, and other wearable textiles.

Recent progress in thermal management for flexible/wearable devices

Article

Full-text available

Apr 2023

Thermal management for wearable devices is evolving to make ubiquitous applications possible based on advanced devices featuring miniaturization, integration, and ultrathin designs. Thermal management and control integrated with wearable devices are highly desirable for various applications for human body monitoring, including external heat exposure and metabolic heat generation, in various activities. Recently, dynamic change materials have been integrated with micro/nano thermal management platforms to address the potential for active thermal management. In this article, recent advances in the architecture of effective thermal management in wearable devices are reviewed, along with the essential mechanisms for managing thermal conditions for users in external/internal thermal environments. Appropriate thermal management approaches are proposed for the design and integration of materials/structures tailored to specific targets in wearable devices. In particular, this review is devoted to materials/structures based on five thermal management strategies: conduction, radiation, evaporation/convection, heat absorption/release, and thermoelectric (TE). Finally, the challenges and prospects for practical applications of thermal management in wearable devices are discussed.

“Skin-like” fabric for personal moisture management

Article

Full-text available

Apr 2020

Personal moisture management fabrics that facilitate sweat transport away from the skin are highly desirable for wearer’s comfort and performance. Here, we demonstrate a “skin-like” directional liquid transport fabric, which enables continuous one-way liquid flow through spatially distributed channels acting like “sweating glands” yet repels external liquid contaminants. The water transmission rate can be 15 times greater than that of best commercial breathable fabrics. This exceptional property is achieved by creating gradient wettability channels across a predominantly superhydrophobic substrate. The flow directionality is explained by the Gibbs pinning criterion. The permeability, mechanical property, and abrasion resistance (up to 10,000 cycles) of the fabric were not affected by the treatment. In addition to functional clothing, this concept can be extended for developing materials for oil-water separation, wound dressing, geotechnical engineering, flexible microfluidics, and fuel cell membranes.

Reversible Water Transportation Diode: Temperature-Adaptive Smart Janus Textile for Moisture/Thermal Management

Article

Full-text available

Dec 2019
ADV FUNCT MATER

“Air‐conditioning” textiles with thermal‐ or moisture‐managing functions are of high interest for not only improving human comfort but also reducing energy consumption. However, making the textile sensitive to the surrounding environment and exhibit adaptive thermal/moisture management still remains a great challenge. Herein, a double‐sided synergetic Janus textile is developed, featuring reversible diode‐like water transportation and adjustable thermal convection upon temperature change. The incorporated responsive polymer networks with inverse transitions on the opposite sides provide synergistic surface energy gradients and capillary gradients that generate drying and cooling effects (with 50% faster water evaporation and 1.2–2.3 °C cooler than with cotton fabric) in hot weather while offering thermal preservation (120 s longer needed to be cooled down and maximumly 3.3 °C warmer than with cotton fabric) in a cold environment. This method could provide ideas for the development of more adaptive textiles and clothing to address maximum personal comfort in demanding situations. A double‐sided synergetic Janus textile that features moisture/thermal management has been developed. Its temperature‐adaptive reversible wettability gradient and pore size change give rise to smart regulation of the microclimate near the skin: dissipating moisture/heat in hot weather while weakening undesired heat loss in cold environments. This study may open the gate for designing smart textiles addressing self‐adaptive personal comfort.

Bioinspired Janus Textile with Conical Micropores for Human Body Moisture and Thermal Management

Article

Full-text available

Aug 2019
ADV MATER

Excessive sweat secreted from the skin often causes undesired adhesion from wetted textiles and cold sensations. Traditional hydrophilic textiles such as cotton can absorb sweat but retain it. A hydrophobic/superhydrophilic Janus polyester/nitrocellulose textile embedded with a conical micropore array with a hydrophilic inner surface that can achieve directional liquid transport (with an ultrahigh directional water transport capability of 1246%) and maintain human body temperature (2–3 °C higher than with cotton textiles) is demonstrated. When the hydrophobic polyester layer with large opening of hydrophilic conical micropores contacts the liquid, the Janus polyester/nitrocellulose textile can pump it to the superhydrophilic nitrocellulose layer through the hydrophilic conical micropores driven by capillary force. The Janus polyester/nitrocellulose textile can weaken undesired wet adhesion and heat loss due to the removal of liquid. The water wicking and air permeability of the Janus polyester/nitrocellulose textile is comparable to those of traditional cloths. This study is valuable for designing of functional textiles with directional water transport properties for personal drying and warming applications. A hydrophobic/superhydrophilic Janus polyester/nitrocellulose (PE/NC) textile with asymmetric hydrophilic conical micropores can be constructed by combining a simple laser perforating method and plasma modification. This textile can unidirectionally pump the excessive sweat from the hydrophobic layer to the superhydrophilic layer through asymmetric hydrophilic conical micropores from large to small openings, thereby avoiding undesired sticky and excessive cold sensations from sweat.

Photonic thermal management of coloured objects

Article

Full-text available

Oct 2018

The colours of outdoor structures, such as automobiles, buildings and clothing, are typically chosen for functional or aesthetic reasons. With a chosen colour, however, one must control the radiative thermal load for heating or cooling purposes. Here we provide a comprehensive calculation of the tunable range of radiative thermal load for all colours. The range exceeds 680 Wm⁻² for all colours, and can be as high as 866 Wm⁻², resulting from effects of metamerism, infrared solar absorption and radiative cooling. We experimentally demonstrate that two photonic structures with the same pink colour can have their temperatures differ by 47.6 °C under sunlight. These structures are over 20 °C either cooler or hotter than a commercial paint with a comparable colour. Furthermore, the hotter pink structure is 10 °C hotter than a commercial black paint. These results elucidate the fundamental potentials of photonic thermal management for coloured objects.

Advanced Textiles for Personal Thermal Management and Energy

Article

Apr 2020

To realize improved human body thermal comfort and reduce energy consumption on building heating and cooling, personal thermal management emphasizing energy management of human body and its local environment is emerging as a promising solution. Advanced textiles are being invented and developed to effectively regulate heat exchange between human body and its surroundings. Here, recent progress on advanced textiles for personal thermal management and its significance in energy efficiency are reviewed. We will mainly discuss textiles with engineered properties targeting at passively controlling human body heat dissipation routes, the active warming and/or cooling textiles, and the responsive textiles that offer adaptive personal thermal management ability according to the external stimuli. An outlook discussing important challenges and opportunities in this field is also presented.

High Heat Flux Evaporation of Low Surface Tension Liquids from Nanoporous Membranes

Article

Jan 2020

Water is often considered as the highest performance working fluid for liquid-vapor phase change due to its high thermal conductivity and large enthalpy of vaporization. However, a wide range of industrial systems requires using low surface tension liquids where heat transfer enhancement has proved challenging for boiling and evaporation. Here, we enable a new paradigm of phase change heat transfer, which favors high volatility, low surface tension liquids rather than water. We utilized a nanoporous membrane of ≈600 nm thickness and <140 nm pore diameters supported on efficient liquid supply architectures, decoupling capillary pumping from viscous loss. Proof-of-concept devices were microfabricated and tested in a custom-built environmental chamber. We used R245fa, pentane, methanol, isopropyl alcohol, and water as working fluids with devices of total membrane area varying from 0.017 cm² to 0.424 cm². We realized device-level pure evaporation heat flux of 144±6 W/cm² for water and the highest evaporation heat flux was obtained with pentane at 550±90 W/cm². We developed a three-level model to understand vapor dynamics near the interface and thermal conduction within the device, which showed good agreement with experiments. We then compared pore-level heat transfer of different fluids, where R245fa showed approximately 10 times the performance of water under the same working conditions. Finally, we illustrate the usefulness of a figure of merit extracted from kinetic theory for evaporation. The current work provides fundamental insights into evaporation of low surface tension liquids, which can impact various applications such as refrigeration and air conditioning, petroleum and solvent distillation, and on-chip electronics cooling.

Temperature Regulation in Colored Infrared-Transparent Polyethylene Textiles

Article

Mar 2019

Effectively regulating heat flow between the human body and its environment not only increases thermal comfort but also presents a novel and potentially cost-effective approach to reducing building energy consumption. Infrared property-engineered textiles have been shown to passively regulate radiative heat dissipation for effective cooling and warming of the human body. However, a lack of dyes that can tune the textile color without compromising the infrared properties remains a major impediment to textile commercialization. Here, we report a new strategy utilizing inorganic nanoparticles as a coloring component for scalable brightly colored, infrared-transparent textiles. The as-fabricated composite textiles not only show a high infrared transparency of ∼80% and a passive cooling effect of ∼1.6°C–1.8°C but also exhibit intense visible colors with good stability against washing. This facile coloration approach will promote the commercialization of radiative cooling textiles in temperature-regulating wearable applications for effective energy savings.

Apparent evaporative cooling efficiency in clothing with continuous perspiration: A sweating manikin study

Article

Mar 2019
INT J THERM SCI

Apparent evaporative cooling efficiency has typically been determined by applying a pre-wetted fabric “skin” on a dummy (“manikin”) simulating human thermal physiology, to understand the effective cooling components of body perspiration in clothing systems. This procedure is only a very rough approximation of real life, as the pre-wetted fabric does not have the capacity to continuously push extra moisture into the clothing layers, which would happen with continuous sweating of the body. In this study, a sweating torso was used to mimic different sweating situations (100, 175 and 250 g/h) with twelve single-layer (SI) and multilayer (MU) clothing systems to understand the effect of continuous sweating and its interaction with clothing materials on cooling efficiency. Our experiments revealed that evaporative cooling efficiency is affected differently by continuous sweating when compared to pre-wetted fabric “skin” approaches. With continuous sweating, up to 15% (24 W m⁻²) cooling power came from the so-called “heat pipe effect” and/or wet conduction and 24% (44 W m⁻²) evaporative latent heat was gained from the environment. It was found that the increase of perspired moisture can affect evaporative cooling efficiency for hydrophilic materials in dual ways. For each 75 g/h sweat rate increase, the in-plane moisture transfer can raise the evaporative cooling efficiency of SIs at least 3–12% and the transplanar moisture transfer may reduce the evaporative cooling efficiency by at least 2–7% and 2–9% for SIs and MUs, respectively. For hydrophobic materials, the evaporative cooling efficiency was less affected by different levels of perspiration due to low wicking. Results also showed the negative correlation of evaporative cooling efficiency of hydrophilic materials with fabric evaporative resistance and thickness. Our study contributes to the understanding of the effective sweat cooling power and evaporative latent heat from environment for the clothed human body with continuous sweating. It also provides insight into the interaction between liquid and water vapor transport, and material design for optimizing the evaporative cooling.

Dynamic gating of infrared radiation in a textile

Article

Feb 2019

A cloth that adapts to the heat Textiles trap infrared radiation, which helps keep us warm in cold weather. Of course, in hot weather, this is less desirable. Zhang et al. constructed an infrared-adaptive textile composed of polymer fibers coated with carbon nanotubes. The yarn itself expanded and collapsed based on heat and humidity, which changed the spacing of the fibers. Wider fiber spacing allowed the textile to breathe but also altered the infrared emissivity of the textile. This allowed for better heat exchange under hot and wet conditions. The self-adjusting emissivity of the textile could help toward wearable thermal-management attire. Science , this issue p. 619

Biomimetic Fibrous Murray Membranes with Ultrafast Water Transport and Evaporation for Smart Moisture Wicking Fabrics

Article

Dec 2018

Both antigravity directional water transport and ultrafast evaporation are critical to achieving a high-performance moisture wicking fabric. The transpiration in vascular plants possess both of these features, which is due to their optimized hierarchical structure composed of multi-branching porous networks following Murray’s law. However, it remains a great challenge to simultaneously realize the ultrafast water transport and evaporation by mimicking nature’s Murray networks in the synthetic materials. Here, we report a synergistic assembly strategy to create a biomimetic micro/nanofibrous membrane with antigravity directional water transport and quick dry performance by combing multi-branching porous structure and surface energy gradient, overcoming previous limitations. The resulting fiber-based porous Murray membranes exhibit an ultrahigh one-way transport capability (R) of 1245%, a desired overall moisture management capability (OMMC) of 0.94, and an outstanding water evaporation rate of 0.67 g h⁻¹ (5.8 and 2.1 times higher than the cotton fabric and Coolmax fabric, respectively). Overall, the successful synthesis of these biomimetic porous Murray membranes should serve as a source of inspiration for the development of moisture wicking technologies, providing personal comfort in hot or humid environment.

Integrated cooling (i-Cool) textile of heat conduction and sweat transportation for personal perspiration management

Abstract

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