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Page 2 of 23 Yun et al. Soft Sci 2023;3:12 https://dx.doi.org/10.20517/ss.2023.04

INTRODUCTION
Wearable technology has advanced significantly in the past decade, with a range of body-worn electronic
and electro-optical devices, such as smart watches, bands, glasses, and goggles, becoming available to
consumers . The development of flexible and stretchable electronic materials is driving the creation of
[1-3]
new and improved wearable devices to meet the needs of various industries, including the consumer, health,
biomedical, and industrial sectors. These devices, such as smart clothes, wearable displays, computing
devices, and health technologies, have the ability to monitor real-time physiological and biomechanical
signals or provide physiological stimuli [4-11] . The design and development of advanced wearable devices still
pose challenges because of the need to integrate electronic, electrochemical, electro-optical, or multiple
types of functionality on a platform that is soft, compact, lightweight, flexible, and stretchable [12,13] . To do so,
various manufacturing technologies, such as laser processing [14-17] , transfer printing [18-21] , and inkjet
printing [22-26] , have been used to fabricate a flexible/stretchable device platform. Ensuring long-term
reliability and biocompatibility during human body motion, especially in outdoor activities involving
external heat exposure and metabolic heat generation, adds to the challenges in material/structure
development and device design. Although quantifying heat generation is difficult because of the diversity of
wearable device structures and platforms, for devices in contact with the skin, it is necessary to maintain a
[27]
temperature lower than skin temperature (31.1 °C to 35.4 °C) during heat generation .

Effective thermal management and control are crucial for the reliable performance of advanced wearable
devices, which feature miniaturization, integration, and ultrathin designs [28-38] . The integration of thermal
regulators into these devices is challenging because various requirements must be met, including thermal,
mechanical, ergonomic, and application-specific needs. The use of rigid materials with high thermal
conductivity is problematic in wearable devices because they are unsuitable for flexible or stretchable
structures. To address this issue, nanofillers can be incorporated into flexible composites to improve the
overall thermal conductivity [39-41] . However, this approach presents challenges because the thermal contact
resistance between nanofillers increases upon tensile deformation, resulting in mechanical contact loss.
Moreover, wearable devices that contain highly thermally conductive materials, such as electrodes and
components, can become rapidly heated under external heat or direct sunlight, causing adverse problems
for the user and the device [42,43] . For example, rapid heating of wearable devices, which can occur in a matter
of hours or minutes depending on the thermal environment, can result in skin burns of varying
severity [44,45] . Surface temperatures of optoelectronic devices lacking proper heat sinks can rapidly increase
upon power application, potentially causing skin burns from the light-emitting diode (LED). Low power
levels of ≤ 5 mW can raise surface temperatures above normal skin temperature, elevating the risk of burns.
Exposure to higher power levels of ≤ 10 mW can cause first-degree burns in just 2 min and third-degree
burns after 10 min .
[46]
Sweat expelled from heated skin can generate artifacts on electro-based sensors, such as those used for
electromyography (EMG) or electroencephalography (EEG), and can mechanically hinder the adhesion of
[47]
wearable patches . Accumulated heat can degrade the performance of batteries and wireless
communication devices [48,49] . Furthermore, all-weather or all-day devices that must maintain a stable
temperature level in various thermal environments require active thermal management that can heat or cool
the external device surface or device-skin interface [28,50] .

This review covers the latest technology for thermal management in wearable devices, including the use of
various materials and structures. Previous reviews have primarily focused on presenting theories and
concepts related to thermal management or organizing only thermal management methods. In this article,
thermal management methodologies are systematically classified based on the underlying heat transfer