# Advances in Wearable Smart Chemical Sensors for Health Monitoring
## 2 Wearable Chemical Sensors Based on Different Working Principles
### Wearable Chemical Sensors Based on Energy Storage Devices  
The emergence of the 'flexible' era has made the integration of wearable flexible electronic devices and energy storage devices a popular research topic that is greatly appreciated by society. To address the energy issue, people can continuously collect energy from the environment by harnessing thermal energy, mechanical energy or solar energy to achieve self-powered sensing. To ensure better long-term operation without being affected by the surrounding environment, the development of sensors for energy storage devices is of great importance. Although a series of studies such as the development of planar supercapacitors with interdigitated electrodes constructed on a single substrate have been carried out before to achieve self-powered sensing, the addition of internal energy storage elements may lead to reduced stability and reliability, and even cause problems such as slower response speeds. Through the design of sensor structures and the selection of materials, this impact can be mitigated to enable the continuous operation of the sensors. Generally, numerous self-sustaining sensing gadgets equipped with built-in energy storage components have been effectively developed in this area, yet there remains ample opportunity for further investigation. The incorporation of energy storage units with sensors provides significant advantages for chemical sensor uses [74]. [PERSON] et al. [47] designed a gas sensor using inkjet printing technology, printed super capacitors and solar cells as an energy source to be used as a wearable wristband, as shown in Figure 6a, b. The wearable wristband directly drives the sensor by collecting solar energy and is also able to store it in standby mode in the super capacitor for easy use. The super capacitor with a printed design has a face capacitance of 12.9 mF-cm\({}^{-2}\), while the gas sensor shows a high sensitivity to ethanol and acetone under normal room conditions. Printing technology for product fabrication and integration demonstrates great potential for the scalable and simple fabrication of various wearable devices. [PERSON] et al. [48] designed a portable hybrid self-powered power textile system that generates energy by harvesting outdoor sunlight with random motions to store it in an energy storage unit like that shown in Figure 6c. Both gathered energies can be effortlessly transformed into electricity through the use of fiber-shaped dye-sensitized solar cells (for solar power) and fiber-shaped triboelectric nanogenerators (for kinetic energy from body movements), which are subsequently stored as chemical energy in fiber-shaped supercapacitors, as shown in Figure 6c. Figure 5: Wearable chemical sensors based on thermal energy. (**a**) CPU surface covered with an array of graphene-based thermoelectric (TE) sponge devices for power generation and CPU cooling. (**b**) Seebeck coefficient and conductivity of TE sponges with different porosities. (**c**) Stress–strain curves of TE sponges under cyclic strain from 0% to 30%. (**d**) Potential application scenarios of TE sponges as wearable electronic devices in daily life. (**e**) Schematic of the structural design and working mechanism of a solar-driven pyroelectric hybrid generator. (**f**) The voltage obtained on two series-connected commercial capacitors as a function of charging time. (**g**) Scheme of the solar-driven pyroelectric hybrid generator integrated on the top of the hat. The inset shows the temperature rise under natural solar illumination conditions. (**h**) Structure of a self-powered thermoelectric nanosensor and its sensing process for mercury ion detection. (**i**) Schematic of an organic field effect transistor-based sensor and its fabrication process. (**j**) Structural design of hybrid photothermal electric generator (PTEG). Figure 5d. All fiber-like structures have a broad range of uses in the wearable electronics industry. [PERSON] et al. [75] developed a comprehensive system equipped with versatile sensors for detecting No\({}_{2}\).