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Yun et al. Soft Sci 2023;3:12 https://dx.doi.org/10.20517/ss.2023.04 Page 9 of 23

Because of the outstanding cooling performance, zero power requirement, and versatile platforms, several
researchers have attempted to integrate stretchable/flexible wearable devices with a cooling solution, as
summarized in Table 2. For example, Lee et al. fabricated a colored passive radiative cooler (CPRC)
[117]
composed of a metal-insulator-metal (MIM) structure . Figure 3A is a diagram of the CPRC, which
consists of a SiO (650 nm) and Si N (910 nm) bilayer as an emitter and Ag film (100 nm) as a metal
4
3
2
reflector. The origin of each color was controlled by varying the thickness of the insulator layer (i.e., the
SiO capacity) in the MIM, which was determined by interference in the 1D stacked layers. It was shown
2
experimentally that, in the daytime, the CPRC was 3.9 °C cooler than the ambient air throughout the day.
They also demonstrated cooling performance integrated with wearable devices [Figure 3B]. Al foil was used
as a substrate considering compatibility with the wearable device, and a MIM and emitter layer were
deposited on Al foil and encapsulated using PDMS for protection and outstanding emissivity in the thermal
infrared (IR) region. The flexible and stretchable mechanical properties make integration of the radiative
cooler and wearable devices possible. In addition, the radiative cooler should consist of nonmetallic
materials to avoid interference with wireless communications.

Xu et al. designed highly versatile on-skin electronics with a radiative cooler substrate considering user
comfort . Using a phase inversion method with polystyrene-block-poly(ethylene-ran-butylene)-block-
[118]
polystyrene (SEBS), isopropyl alcohol, and chloroform, a radiative cooler substrate was fabricated. The
multiscale porous SEBS structure is depicted in Figure 3C, where the micropores exhibit high solar
reflectivity while maintaining low long-wave IR reflectivity (4-13 μm). Bioelectronic devices, including
various biosignals (ECG, temperature, humidity, and body motion), were fabricated by spray printing silver
nanowires onto a multiscale porous SEBS substrate, as shown in Figure 3D.

Kang et al. developed a thermally stable near-field communication (NFC)-based patch-type tissue oximeter
(PTO) with a radiative cooler . The nanovoid/microvoid polymer (NMVP) used a SEBS and poly(methyl
[42]
methacrylate) (PMMA) bilayer for radiative cooling [Figure 3E]. By obtaining Mie scattering enhancement
and antireflection effects through the porous PMMA layer, the NMVP could achieve high solar reflection
and heat emission. The PTO could measure muscle oxygenation during activity with compact dimensions
(20 × 17 × 2 mm). Figure 3F displays a photograph of the NMVP-integrated PTO, with all layers flexible and
conformal contact to the skin. Figure 3G exhibits the difference in temperature between black elastomer
(BE), white elastomer (WE), and NMVP on the skin when exposed to sunlight. The BE and WE reached
40 °C and 35 °C, respectively, 8 min later, while the NMVP stayed at the usual skin temperature. Stability
tests of the PTO with and without NMVP were also performed. The PTO with NMVP exhibited a tissue
oxygen saturation (StO ) value of approximately 80%, which was an accurate measurement compared to the
2
normal state, i.e., approximately 80%). However, the PTO without NMVP showed an unstable
measurement (StO value of ~67%) because of heating by solar absorption.
2

Furthermore, Byun et al. reported gallium-based transformative electronics integrated with a radiative
[43]
cooler for outdoor use . The transformative electronics system (TES) was used to switch the shape and
stiffness between rigid handheld and soft wearable configurations. The TES used gallium (T = 29.76 °C,
melt
where T represents the melting point of gallium) as a core material to implement stiffness tuning by
melt
changing the phase between solid and liquid. However, the TES has difficulty maintaining a rigid form in
outdoor applications because of excessive heat accumulated from itself or environmental sources, such as
the sun. Figure 3H shows a conceptual illustration of the TES integrated with a multi-layered, flexible, and
stretchable radiative cooler (m-FSRC) that can be used in the rigid mode under sunlight. The integrated
radiative cooler can reflect solar energy and radiate thermal energy. Thus, the transformative platform
minimizes the temperature increase during continuous operation under sunlight exposure. The m-FSRC is