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Yun et al. Soft Sci 2023;3:12 https://dx.doi.org/10.20517/ss.2023.04 Page 11 of 23

cooler keeps flat and stiff under sunlight exposure, while a bare sample without a radiative cooler becomes
soft. In the corresponding IR images, the surface temperatures of the samples with and without the radiative
cooler are 24 °C and 38 °C, respectively. Therefore, the transformative platform based on gallium (T =
melt
29.76 °C) maintains its rigid mode owing to the significant cooling performance of the radiative cooler.

EVAPORATIVE COOLING MATERIALS AND DEVICES
One cooling solution in the human body is evaporative cooling, particularly sweat evaporation, which can
transfer a significant amount of heat. However, wearable devices lack the capacity to regulate moisture
because of their hydrophobic properties, which cause discomfort and ineffective cooling during perspiration
resulting from heating. Excessive residual perspiration on the skin absorbs heat because water is denser and
[119]
more heat-resistant than air, which can be detrimental to thermal comfort and cooling effectiveness . An
evaporative cooler moves sweat and maximizes breathability, making it easy for sweat or moisture to
evaporate. Cooling power caused by evaporation can dissipate heat at a higher level than other passive
coolers . Furthermore, an evaporative cooler is comfortable for the user because it enables the skin to
[120]
breathe and sweat to evaporate naturally. The effectiveness of evaporative cooling depends highly on
[121]
environmental conditions such as temperature, humidity, and wind speed . In addition, an evaporative
cooler requires direct contact with the skin for sweat transport. Therefore, careful consideration of the
specific requirements of the wearable device is necessary before choosing evaporative cooling technology.

Recently, Peng et al. reported integrated cooling (i-Cool) textiles for enhancing sweat transportation .
[120]
These textiles provide enhanced sweat evaporation capacity and high cooling effectiveness for sweat
evaporation, in addition to a liquid sweat draining function, by efficiently integrating water transport routes
and heat conducting pathways. In contrast to traditional fabrics, i-Cool textiles efficiently remove a
significant amount of heat from the skin by wicking sweat as well as creating heat conduction channels for
quick evaporation, as illustrated in Figure 4A. Figure 4B shows optical and scanning electron microscopy
(SEM) images of i-Cool (Cu) textiles. Nylon 6 nanofibers not only coat the Cu surface but also fill the pores.
Compared with those found in the pores of the Cu matrix, the nanofibers on the skeleton of the Cu matrix
are denser and have less space between them. The difference in capillarity caused by the variance in shape
makes unidirectional water passage possible from the inner to the outer surface.

Chen et al. developed aramid nanofiber-MnO nanowire (ANFMN) hybrid membranes with heat
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dispersion and sweat transport . Using a hydrothermal process, vacuum filtration, and steam
[122]
modification, a bilayer ANFMN hybrid membrane was constructed with hydrophilic aramid nanofiber-
MnO nanowire (HAFMW) and hydrophobic MnO -nanowires@MnO -nanosheet (HMWMN), as
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illustrated in Figure 4C. The ANFMN hybrid membranes have strong radiation heat dissipation capabilities,
as shown by the greater IR emissivity of the HMWMN layer and the 3.8 °C temperature differential between
clothes and membranes in the IR image. Remarkable sweat transportation capabilities were demonstrated
through simulation, with sweat moving from the HMWMN layer into the HAFMW layer in only 13.4 s.
When sweat contacts the HMWMN layer, it penetrates below and progressively moves up to the AFMW
layer as time passes. SEM images of HAFMW and HMWMN are shown in Figure 4D. MnO nanowires
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with a high aspect ratio are entangled with one another to form a network structure in the HAFMW layer,
and smaller-diameter aramid nanofibers are entangled between the MnO nanowires. The HMWMN layer
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morphology changed little from before to after the hydrophobic alteration.

Zhang et al. developed a metafabric that integrates nanofiber membranes to combine evaporative cooling
with radial cooling . A hierarchical metafabric that can selectively emit IR radiation and reflect sunlight is
[123]
made up of layers of cellulose acetate (CA), aluminum oxide (Al O ), and polyamide 6 (PA6). CA/Al O was
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