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Yun et al. Soft Sci 2023;3:12 https://dx.doi.org/10.20517/ss.2023.04 Page 13 of 23

was also tested. Figure 4I shows that the temperature of the case shell grew more slowly with the coated heat
sink than with the original heat sink. After 15 min of stress testing, a maximum temperature drop of 7 °C
was achieved.

PHASE CHANGE MATERIALS FOR THERMAL PROTECTING
PCMs are suitable for thermal management solutions in a variety of situations because they can absorb or
release heat from the target through the phase transition mechanism. Although PCMs have the potential to
enhance the performance of thermal management, the impact of their concentration on heat transfer
characteristics, such as latent heat, remains uncertain. In addition, improved numerical models and
analytical techniques are necessary to evaluate the cooling performance of PCMs in various situations .
[126]
Nevertheless, their shape adaptability and flexibility make them highly effective for equipment thermal
control because they can maintain a consistent temperature throughout the phase transition process.

In recent years, PCM substrates have been studied extensively as viable solutions for the heat management
of flexible electronic devices. Researchers have reported that these materials possess adequate flexibility and
resilience to adapt to changes in the shape of the body [127,128] .

In wearable devices, these flexible PCMs can be used as TPSs. Nie et al. created a polymer-based TPS that
can be used in wearable devices . As illustrated in Figure 5A, the wearable device uses a functional heating
[129]
component that generates heat energy, which is employed through its phase shift of n-eicosane to avoid
overheating of the skin. To achieve this, an n-eicosane heat-absorbing microsphere with a melamine-
formaldehyde resin coating is combined with PDMS to provide a thermal insulation function [Figure 5B].
Through optimization, it was demonstrated that the thermally protective substrate could withstand
complicated deformations of more than 150% and minimize the peak skin temperature rise by 82% or more.
Furthermore, studies conducted on the skin of a mouse have shown the thermal protection potential of the
device for wearable thermal management.

Shi et al. presented the concept of a PDMS substrate comprising an embedded PCM with a thin copper film
on top . A functional soft composite that serves as a TPS for wearable electronic devices was proposed,
[130]
comprising two key components: a PCM and a thin metal film. As illustrated in Figure 5C, the paraffin
(transition temperature: 18-71 °C), located beneath the metal film, can absorb excessive heat above its
transition temperature to reduce heat dissipation to the skin. Because of its high thermal conductivity, the
copper film implanted in the PDMS efficiently modifies the heat flow directions to reduce heat transfer to
the skin further. The proposed thermal protecting substrate has been demonstrated to reduce the peak
temperature rise by more than 85% compared with a conventional substrate. In addition, Figure 5D shows
finite-element analysis FEA simulations and temperature tests on pig skin and a comparison with
conventional substrates. The functional soft composite substrate was found through FEA to lower the
maximum temperature increase significantly (by 65%), and no skin burns were observed when the device
was applied to pig skin in the experimental trial. Furthermore, research has been conducted on utilizing
composite PCM materials to enhance the phase transition characteristics and conformal contact on the
skin.

Jung et al. fabricated an advanced thermal skin (ATS) using a hybrid-based approach that exhibits
outstanding thermal diffusivity and thermal storage properties . The ATS is composed of a silver flake/
[131]
PDMS serpentine structure (SPS) and a sodium-acetate-based hydrogel matrix (SAHM). By incorporating
thermally conductive metallic fillers, the SPS made efficient heat dispersion possible throughout the ATS
network. The intrinsic softness of the SPS also allowed the ATS to conform to the highly malleable surface