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

mode and materials, elucidating comprehensive guidelines for practical cooling solutions in wearable
devices and defining suitable heat dissipation modes across diverse wearable platforms. In Section
“Thermal management structures for wearable devices”, the essential mechanisms for managing the
thermal conditions of the user, both internally and externally, are introduced, along with the effective
structures for thermal management in wearable devices. In five major sections, thermal management
structures, multifunctional applications for flexible substrates and wearable devices, and active thermal
management techniques for dynamically changing materials and radiative cooling structures are explored.
The review concludes with a roadmap for the practical application and development of fully integrated
multifunctional thermal management systems for wearable devices and highlights future directions and
opportunities in thermal management for advanced wearable devices.

THERMAL MANAGEMENT STRUCTURES FOR WEARABLE DEVICES
Wearable devices are generally composed of sensors, electrodes, and flexible substrates, which are combined
with integrated circuits and other parts to enable real-time monitoring of biosignals in daily life. The devices
typically include a variety of sensors. Sensors for electrocardiograms (ECGs) [1,51-56] , EMG [57-60] , and
photoplethysmography (PPG) [61-65] are used to monitor heart rate, body motions, and pulse oximeters,
respectively. Temperature and humidity are measured by resistance sensors [66-73] , while Galvanic skin
response sensors have become popular tools for stress monitoring because of their ability to utilize
sweat [74-77] . Sweat sensors detect glucose levels through an electrochemical process in sweat transported
through microfluidic channels [78-84] .

Thermal management through heat transfer mode
Figure 1 shows an overview of thermal control considerations for wearable devices. Generally, different
pathways of heat dissipation contribute to wearable device thermal management: conduction, radiation,
convection/evaporation, heat absorption/release, and thermoelectric (TE) cooling. The transfer of heat
resulting from the temperature difference between two physically contacting objects is known as thermal
[85]
conduction . According to Fourier’s law, the rate of heat transmission across a medium increases as the
[86]
temperature decreases .

where q denotes the heat flux, λ is the thermal conductivity coefficient and ∂T/∂x is the temperature
gradient. Heat transmission in the steady state is primarily defined by effective thermal conductivity. Most
heat created by wearable devices is transmitted to the skin or, when fitted with a heat sink, to the ambient
[87]
air . Efficient heat dissipation is critical to prevent heat transfer to the skin, which can be achieved by
either attaching a flexible heat sink to the device or introducing nanofillers with high thermal conductivity
embedded in the substrate material .
[88]

[89]
Radiation is an electromagnetic wave emitted by an object with a temperature above absolute zero . The
energy dissipation rate depends on the temperature differential between the object and its immediate
surroundings, the surface emissivity, and the effective surface area. Thus, surface absolute temperature is the
main factor used to determine total radiant heat power through the Stefan-Boltzmann law .
[90]