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Yun et al. Soft Sci 2023;3:12 https://dx.doi.org/10.20517/ss.2023.04 Page 15 of 23

THERMOELECTRIC COOLING MATERIALS AND DEVICES
TE cooling, which exploits the Peltier effect to transfer energy directly from electricity to heat, is attracting
significant interest owing to its many benefits. These include being quiet, motionless, miniaturized,
lightweight, capable of cooling and heating, and eco-friendly because TE does not require the use of
refrigerants. However, the high cost and low power efficiency of TE materials remain to be addressed.
Furthermore, TE cooling systems generate heat as a by-product, so it is important to design a structure with
heat dispersion. Because it is one of the few miniaturized active coolers that can be applied to wearable
devices, not only conventional rigid TE coolers but also TE coolers with various flexible structures have
been reported [133,134] . The development and application of wearable technology with TE coolers is based on
the principle of TE cooling. As shown in Figure 6A, Kishore et al. investigated the impact of the thermal
resistances of the heat source/sink and the TE material parameters on the performance of a TE cooler .
[135]
They lowered the skin temperature by 8.2 °C in ambient air, representing a 170% improvement over the
cooling performance of commercial TE modules.

Conventional TE coolers have bulky, inflexible components, which are a significant obstacle to their
application in flexible systems. To solve this problem, several studies on the construction and materials of
flexible TE coolers have been conducted. Hong et al. fabricated a wearable TE device that had high elasticity
[136]
owing to the combination of elastomer and rigid TE pillars . The TE device is easily embeddable into
clothing and is suitably thin, flexible, and lightweight. A double elastomer layer structure, consisting of an
air gap insulating layer sandwiched between two stretchable sheets and high-ZT inorganic TE pillars with
an optimal aspect ratio and spatial density, is used to provide mechanical flexibility and excellent cooling
performance [Figure 6B]. The top stretchable sheet stretches and the bottom sheet contracts when the
double-layer TE device is bent. To achieve low thermal conductivity and high flexibility in the TE device,
TE pillars with a high aspect ratio are implanted with several elastomer layers and air gap thermal
insulation. This eliminates the need for the massive heat sinks commonly used in previous devices. Without
using a heat sink, the TE device lowers the temperature by 10 °C.

[137]
Zhang et al. proposed inorganic TE wire using a thermal drawing process . Figure 6C shows optical and
SEM images of the fiber. The core material is composed of p-type Bi Sb Te and n-type Bi Se and
0.5
3
1.5
3,
2
borosilicate glass is used as the cladding material. At a ΔT of 140 K, the TE power generator, composed of
inorganic TE fibers, exhibits a high output power of 3 W. The fibers are woven into the fabric to create a
flexible TE device, as depicted in Figure 6D, which shows the experimental temperature profile captured by
an IR camera. Using two sets of p-n core fibers and applying a 2 mA current, a simulation of a temperature
profile is produced to assess the TE cooling capability. The analysis of the TE cooling performance
demonstrates that the devices can reduce the temperature by 5 °C. Fiber-woven fabrics can serve not only as
TE generators, which use heat from the body to produce electricity, and TE coolers, which cool the body
through the application of low currents, but also as thermal sensors. Moreover, because of their thin
composition, these fabrics retain air permeability in the covered area, a crucial factor for extended comfort
on curved human skin. Combining the cooling/heating modes of TE with a wearable sensor makes active
temperature regulation possible.

Lee et al. assembled a skin-like, stretchable, and dual-mode thermo-haptic device for a virtual reality
experience . The conceptual process of the skin-like thermo-haptic (STH) device is shown in Figure 6E.
[138]
The device was designed to make it possible to sense cold and heat in virtual reality. The cooling and
heating are achieved by TE with system feedback. The p- and n-type telluride TE pellets, which are capable
of rapid temperature variation with the application of voltage, are connected by Cu electrodes coated with
polyimide on one side and encapsulated in a thin layer of thermally conductive elastomers. The Cu