# Advances in Wearable Smart Chemical Sensors for Health Monitoring
## 2 Wearable Chemical Sensors Based on Different Working Principles
### Wearable Chemical Sensors Based on Thermal Energy  
Thermal energy sensors are eco-friendly technologies that utilize thermoelectric and pyroelectric effects to transform heat into electricity by harnessing energy from human movements. The pyroelectric effect pertains to the alteration of spontaneous polarization within certain polar materials due to temperature fluctuations. By leveraging the temperature variations over time, this effect enables the conversion of heat into electrical energy. The underlying mechanism is that time-dependent temperature fluctuations modify the spontaneous polarization of polar crystals, which subsequently leads to the redistribution of surface charges and the generation of a temporary electric current. During human movement, muscles undergo continuous contraction and relaxation. After exercise, the body's metabolism is enhanced, accompanied by an increased energy consumption, thereby resulting in the production of a substantial amount of heat. The abundance of thermal energy in the environment has sparked significant interest in creating wearable energy harvesters that utilize the process of thermal energy conversion. In recent years, scholars at home and abroad have proposed wearable thermal energy harvesting devices based on the thermoelectric effect and applied them to wearable sensors. Utilizing human body heat to produce electricity is an excellent approach for creating self-sustaining wearable technology. [PERSON] et al. [70] aim to improve upon the limitations of current thermoelectric fibers, films and blocks, which include limited power generation capabilities and a lack of flexibility. A versatile superelastic sponge made of graphene was created for use in wearable electronics and thermal regulation, as depicted in Figure 5a. The sponge exhibits a substantial Seebeck coefficient of 49.2 \(\upmu\)V/K and an extensive compressive strain of 98% as shown in Figure 5b. Following 10,000 rounds of compression, the sponge displayed outstanding mechanical and thermal expansion stability, as shown in Figure 5c. In Figure 5d, the impressive thermoelectric performance and promising applications of thermoelectric sponges in everyday life are demonstrated through the utilization of human body heat. The human body serves as a reliable source of heat, supplying a constant stream of thermal energy. The efficient extraction of heat from the human body shows promise for use in creating wearable technology. [PERSON] et al. [49] introduced a new wearable hydrothermal electric generator (PTEG) powered by solar energy, which was created by combining a solar absorber with high efficiency, a pyroelectric film and a thermoelectric yarn, as shown in Figure 5e. The PTEG is more effective at capturing energy from changing temperatures and temperature differences compared to other wearable pyroelectric and thermoelectric generators mentioned in previous studies, as illustrated in Figure 5f. The PTEG was attached to the top of a hat in real-world scenarios. Following 30 min of exposure to solar radiation, the top layer of the PTEG material reached a temperature of 53.7 degrees Celsius, as illustrated in Figure 5g. The PTEG can efficiently capture solar energy when worn, showing potential as a wearable energy generator for everyday use. [PERSON] et al. [71] presented a novel thermoelectric nano-sensor utilizing the thermoelectric effect to detect mercury ions for the first time, as shown in Figure 5h. The thermoelectric nano-sensor's structure consists of aluminum foil serving as the bottom electrode and a substrate of tellurium nanowires (Te NWs). The thermoelectric nano-sensor utilizes Te NWs as a highly specific substance in the reaction with mercury ions, enabling it to achieve a detection limit of 1.7 nM and a wide linear range (from 10 nm to 1 \(\upmu\)m) for detecting mercury ions with high sensitivity. Upon contact between Te NWs and Hg\({}^{2+}\) ions, a HgTe core/shell structure is formed.