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Tang et al. Soft Sci. 2025, 5, 11 https://dx.doi.org/10.20517/ss.2024.62 Page 3 of 21

conditions. This is where materials such as Ag Se come into play, offering a tougher alternative to n-type
2
Bi Te TE material. Ag Se demonstrates not only high TE efficiency but also enhanced mechanical
2
2
3
robustness, which is vital for wearable devices subjected to continuous mechanical stress [28-32] . Carbon-based
materials, particularly carbon nanotube (CNT)-based and graphene-based, have also emerged as promising
candidates for flexible TE sensors, maintaining flexibility and cost-effectiveness. CNTs can be tailored for
both n- and p-type properties through different strategies, and their inclusion in polymer matrices has been
shown to enhance both electrical conductivity and Seebeck coefficient, leading to improved TE
performance . Incorporating graphene into various matrices can also enhance the TE performance of
[33]
composite materials . However, their TE properties still need further optimization for commercial use.
[34]
Additionally, the advancement of hydrogels and nanomaterials provides a promising material basis for
wearable, flexible sensors, which can be used for designing disposable in-situ health monitoring sensory
systems. Nanomaterials are widely used in soft electronics, including sensors, due to their excellent
mechanical flexibility, and their certain properties for human utilization such as ultralight weight and high
breathability. Hence, such materials can be utilized as the substrates for TE-based sensors, thus ensuring
their flexibility and practicality. However, due to the intrinsic rigidity of semiconductors, though the
substrates are flexible, the device cannot meet the demand for its comfortability. That is what hydrogel
based on thermogalvanic effect or Soret effect comes into play. Compared with traditional metal or
semiconductor materials, hydrogels such as polyacrylamide (PAM)/carboxymethyl cellulose (CMC)-LiCl
and polyvinyl alcohol (PVA)/tempooxidized bacterial cellulose (TOBC) are soft and often exhibit rather
high thermopower, which allows for the utilization of wearable biosensors [35,36] .

In summary, by integrating biosensors with TE materials, a novel approach to energy recycling is achieved,
which extends device autonomy and reduces the need for frequent battery replacement. In this review, we
aim to provide a comprehensive overview and analysis of recent advances in TE-based sensors for health
and environmental monitoring applications [Figure 1]. Firstly, we discuss the fundamental principles of TE
conversion. Additionally, we assess the practical applications of these sensors in real-world settings,
particularly emphasizing their role in self-powered, wearable devices and IoT-integrated environmental
monitoring systems. Finally, we address the current challenges facing TE sensor deployment, such as
material limitations, miniaturization requirements, and scalability issues, and explore future research
directions to maximize the potential of TE sensors for sustainable, long-term monitoring solutions.

TE EFFECTS FOR SENSING
Seebeck effect
The TE effect, including the Seebeck effect, was first reported by Thomas Johann Seebeck in 1821. This
phenomenon occurs when there is a temperature gradient in conductive materials, leading to the
differential migration of charge carriers [7,37-39] . To specify, when materials are exposed to a temperature
gradient, there is a tendency for high-energy charge carriers, which are more prevalent in the hotter regions,
to diffuse toward the colder regions, resulting in the development of an electrostatic potential difference
across the material [Figure 2A] . Under open-circuit conditions, this process continues until the drift and
[40]
diffusion currents reach equilibrium, leading to the establishment of a Seebeck voltage. Thus, we can
conclude that Seebeck effect allows for the direct conversion of temperature gradient into electrical energy.

The figure-of-merit (ZT) is a dimensionless parameter that quantifies the efficiency of TE materials, given
by [38,40,41] :

Eq (1)