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Page 4 of 23 Yun et al. Soft Sci 2023;3:12 https://dx.doi.org/10.20517/ss.2023.04

Figure 1. Overview of thermal management for flexible and wearable devices. Flexible and wearable devices that can monitor various
biosignals, such as heart rate, body motion, pulse oxygen, temperature, psychological stress, and sweat analysis. Reproduced with
[53] [118]
permission . Copyright 2019, American Association for the Advancement of Science . Copyright 2019, National Academy of
Science [63] . Copyright 2017, Wiley-VCH GmbH, Weinheim [141] . Copyright 2019, Wiley-VCH GmbH, Weinheim [77] . Copyright 2020,
Wiley-VCH GmbH, Weinheim [84] . Copyright 2019, American Association for the Advancement of Science. Novel cooling structures
based on heat dissipation mechanisms for flexible/wearable devices: (1) high-thermal-conductivity materials; (2) passive radiative
cooler; (3) evaporative textile; (4) phase change material; and (5) TE device. Reproduced with permission [115] . Copyright 2020, Springer
Nature [43] . Copyright 2022, Wiley-VCH GmbH, Weinheim [120] . Copyright 2021, Springer Nature [132] . Copyright 2021, Springer
Nature [135] . Copyright 2019, Springer Nature.

where |T| represents the absolute temperature, ε indicates the emissivity of the surface, and σ is the Stefan-
Boltzmann constant. Modifying the emissivity, transmittance, and reflectance of the material showed that
radiative thermal control has promising cooling potential [91,92] . The radiative cooler not only facilitates the
release of internal heat generation but also impedes the absorption of solar energy. The design of the
radiative cooler leverages the spectral properties of solar and atmospheric radiation. In particular, strong
emissivity in the atmospheric window (ATW; 8-13-μm wavelength range) optimizes thermal energy
emission to the outside space and limits solar energy absorption in the solar spectrum (0.3-2.5-μm
wavelength range) [93,94] . The overall radiative cooling power P (T) consists of four contributions to the
cool
power terms and is defined as [95]

where P (T sample ) is the power radiated by the structure per unit area, P (T ambient ) is the absorbed power per
rad
atm
unit area from the atmosphere, P is the incoming solar power absorbed by the structure per unit area, and
Sun
P cond+conv is the conductive and convective heat exchange powers.
Heat transfer through thermal convection occurs through the movement of liquid or air. The air/vapor
permeability plays a crucial role in heat transfer from the body to the environment through convection and
[96]
evaporation . Increasing permeability improves heat dissipation and can also be used to lower
temperatures by modifying convective and evaporative heat transfer processes. The temperature differential
between an object and its surroundings determines the rate of convection heat loss, as defined by Newton’s
law .
[97]