# Advances in Wearable Smart Chemical Sensors for Health Monitoring
## 2 Wearable Chemical Sensors Based on Different Working Principles
### Wearable Chemical Sensors Based on Triboelectric Nanogenerators  
Mechanical energy is a green and ubiquitous natural resource [57]. TENGs, a novel innovation for transforming scattered, unpredictable, low-frequency mechanical power into electricity, are gaining attention from the general public because of their cost-effectiveness, variety of material options, uncomplicated design and high efficiency in energy conversion. Consequently, they are now extensively employed in wearable electronic gadgets [58]. Nowadays, triboelectric nanogenerators based on four working modes, namely, sliding friction, contact separation, single electrode and independent layers, have been applied in the fields of e-skin, health management and the environment as well as sports training. The rapid development of triboelectric nanogenerator technology is closely related to the chemical modification of friction layer materials [59; 60]. Cellulose, currently one of the most abundant renewable natural polymers on Earth, is characterized by a wide applicability, excellent cost-effectiveness and superior flexibility. As a result, it has emerged as one of the most promising friction materials for TENGs in recent years. Owing to its high proneness to electron loss, it is frequently utilized as a positive friction material. Nevertheless, numerous materials within the cellulose family, including cellulose, cellulose nanocrystals and cellulose nanofibers, suffer from drawbacks such as poor flexibility, complex manufacturing processes and performance limitations. These shortcomings restrict their application as negative friction materials in TENGs. Consequently, to address these issues, [PERSON] et al. [61] have conducted relevant research and prepared ionic conductive elastomers with dual crosslinked networks through the in situ polymerization of deep eutectic solvents that are polymerizable. In Figure 2a, the creation process and a diagram of the interconnected structure of ionic conducting elastomers derived from cellulose-based polymerizable deep eutectic solvents (PDESs) are shown. Figure 2b demonstrates that the mechanical strength and toughness were more than doubled by the formation of the dual crosslinked network. Cellulose plays a role in creating a strong interconnected network held together by hydrogen bonds, enhancing the elastomer's ability to recycle, protect against frost and resist solvents. Figure 2c illustrates the elastomer's ability to stretch and flex effortlessly at \(-20\)\({}^{\circ}\)C without any signs of damage, showcasing its excellent resistance to frost. The conductive elastomers' resistance to solvents was assessed by immersing the elastomer samples in various solvents at room temperature for 12 h, including petroleum ether, dichloromethane, tetrahydrofuran, ethyl acetate, acetone and methanol. After the immersion period, the elastomer samples maintained their original structure, except for those in the methanol. This change was due to methanol molecules forming new hydrogen bonds with acrylic acid (AA) and choline chloride (ChCI), disrupting the ion-conducting elastomers' hydrogen-bonded cross-linking network, as depicted in Figure 2d. As shown in Figure 2e,f, the wearable sensing capabilities of conductive elastomers based on PDES were investigated. It can be used to determine the wrist bending angle and different finger movements. This study paves the way for the development of promising strategies for the fabrication of sustainable and multifunctional flexible electronics for use in harsh environments [62].