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Yun et al. Soft Sci 2023;3:12 https://dx.doi.org/10.20517/ss.2023.04 Page 17 of 23

while the PLCL zone was cooled through heat release. This resulted in a temperature differential between
the TE generator regions, leading to the generation of power (as shown in Figure 6H). The proposed system
showed outstanding performance compared with relying solely on radiative cooling. The system can
maintain performance during stretching and generate energy throughout the day to ensure the viability of
eco-friendly energy harvesting when coupled with a TE generator system.

CONCLUSION AND OUTLOOK
In this review, recent advances in thermodynamic technology that make it possible to manage thermal
conditions efficiently for external/interfacial wearable devices on the skin of users are summarized, ranging
from microstructures/nanostructures to dynamically changing materials. With the advances in wearable
devices, the progress in this field has been rapidly evolving over the past decade, resulting in many
promising achievements, along with new ideas and strategies. Thermal management schemes offer a variety
of potential pathways toward efficient thermal management of wearable devices through the modification
and optimization of structures or properties to improve performance based on each thermal mechanism.
Highly beneficial applications have been demonstrated both conceptually and experimentally, including
structured thermal conductors, passive radiative coolers, evaporative textiles, PCM-based coolers, and TE
devices. Nonetheless, thermal management technologies for wearable devices are in their infancy, limited to
laboratory settings, and must mature to meet the demands of practical and industrial applications. This
entails addressing material and manufacturing issues for thermoregulators and wearable devices that have
not yet received adequate attention.

Wearable device applications require careful consideration of a variety of biomedical needs, including
nontoxic/nonirritant qualities, breathability, regulatory compliance, and aesthetics. Because past studies
have been few, interdisciplinary collaboration and rigorous clinical testing are necessary to assess these
requirements. The growth of wearable applications requires low-cost mass production and ongoing basic
research. Population diversity and operating conditions must be considered in comprehensive studies,
especially studies of heat transfer into and out of human skin . Other than conventional thermal
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conductors, such as heat sinks, there is still a lack of well-tested and proven thermal management structures
in practical, field-tested devices. During long-term, repetitive device operation in various thermal
environments, thermoregulator-integrated wearable devices should be tested to ensure that they can provide
users with practical monitoring and information. Most thermal management schemes previously
introduced have been conceptually demonstrated in limited settings, depending only on one of their
thermal management mechanisms. In reality, thermal structures in outdoor/indoor thermal environments
cannot operate solely as independent thermal mechanisms but are affected by the interaction of multiple
thermal mechanisms. Convergence studies in materials and structures covering complex thermal
mechanisms are necessary to manage the thermal conditions of wearable devices realistically. Active
thermal management is a promising field for fundamental research. The potential for thermal diodes,
thermal transistors, and other innovative thermal control devices to provide exciting opportunities is
evident; however, there is currently a lack of conceptual designs for practical and versatile solid-state
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thermal control devices . The exploration of novel concepts and physical mechanisms that extend beyond
established TE processes is a rich area for investigation.

In conclusion, with the recent rapid development of thermodynamics and manufacturing technology,
wearable electronics based on thermoregulators have become an advanced platform for practical/future
applications but still have limitations. Further technological advances are necessary to meet industry
demands, such as high-efficiency thermal management, fast thermal modulation, cost-effective
manufacturing, active thermal management, and fully integrated multifunctional devices.