# Advances in Wearable Smart Chemical Sensors for Health Monitoring
## 2 Wearable Chemical Sensors Based on Different Working Principles
### Wearable Chemical Sensors Based on PENG  
PENGs are tools that harness piezoelectric effects to detect the electrical charge generated when pressure is applied to a piezoelectric substance. Utilizing electromechanical energy conversion principles, these devices offer benefits such as rapid response rates, a strong parametric response, compact design, durable construction and efficient output, making them popular in wearable sensor technology. PENGs are one of the most popular power sources for wearable chemical sensors. [PERSON] et al. [64] reported a robust superhydrophobic antimicrobial self-powered PENG sensor. As shown in Figure 3a, this figure demonstrates the preparation process of the PENG sensor, which consists of a flexible electrostatically spun poly (vinylidene fluoride-co-tetrafluoroethylene) piezoelectric nanofiber membrane and sprayed fork-finger carbon nanotube electrodes, which are subsequently encapsulated by cross-linked conformal hydrophobic nano-coatings wrapped around each fiber by initiated chemical vapor deposition (iCVD). The acquired sensors are capable of precisely tracking a range of human activities, including respiration and various motions. iCVD nano-coatings offer superior sensor protection, including self-cleaning properties, super hydrophobicity and over 90% resistance to bacterial fouling, as shown in Figure 3b,c. These durable ultra water-repellent self-sustaining PENG sensors are anticipated to have broader real-world uses in mon itoring human behavior, advanced medical systems and interactions between humans and computers. [PERSON] et al. [46] proposed a flexible PENG powered room-temperature ammonia gas detector utilizing a gold-molybdenum selenide (Au-MoSe\({}_{2}\)) compound and a two-dimensional (2D) semiconductor MoS\({}_{2}\) layer (Figure 3d). The synthesis of Au NPs was achieved by heating 0.12 mmol of HAuCl\({}_{4}\)-3H\({}_{2}\)O under continuous stirring, during which sodium citrate solution was added. Subsequently, 10 mL of the prepared gold solution and 100 mg of MoSe\({}_{2}\) were mixed by stirring and then centrifuged in deionized water for the synthesis process. Eventually, the resulting Au-MoSe\({}_{2}\) sample manifested as a metallic state within the composite material. The ammonia gas sensor also exhibited excellent mechanical reliability (Figure 3e) and outstanding long-term stability (Figure 3f). Moreover, the PENGs utilizing MoS\({}_{2}\) can capture kinetic energy from body movements and show promising prospects for integration into autonomous wearable gadgets. [PERSON] et al. [65] proposed a self-sustaining flexible humidity sensor utilizing a PVA/MXene nanofiber film and MoSe\({}_{2}\) piezoelectric nanogenerator, as shown in Figure 3g. Utilizing a single-layer MoSe\({}_{2}\) PENG, the humidity sensor on a flexible PET substrate, which is constructed using electrospun PVA/MXene nanofibres, demonstrates  
Figure 3: Wearable chemical sensors based on PENGs. (**a**) The preparation process for self-powered PENG sensor. (**b**) Output voltage signals of the original sensor and the sensor after exposure to RH90% for 168 h, artificial sweat immersion for 72 h and artificial rain immersion for 72 h. (**c**) Antifouling rates of pristine PVT\(-\)C and PVT\(-\)C\(-\)PFE against _E. coli_ and _Staphylococcus aureus_ determined by the plate counting method. (**d**) Schematic of the self-powered NH\({}_{3}\) sensor based on MoS\({}_{2}\) sheet piezoelectric nanogenerator drive. (**e**) Mechanical reliability test of PEAS based on Au-MoSe\({}_{2}\) composite exposed to 20 ppm NH\({}_{3}\). (**f**) Long-term stability test of Au\(-\)MoSe\({}_{2}\)-based PEAS exposed to 20, 50 and 100 ppm of NH\({}_{3}\) for 7 weeks. (**g**) Schematic of the fabrication of the PVA/MXene humidity sensor. (**h**) Resistance response and recovery curves of the PVA/MXene sensor over time between 11% RH and 97% RH. (**i**) Humidity hysteresis curve of the PVA/MXene nanofiber thin film sensor. (**j**) Repeatability of the self-powered PVA/MXene sensor. a significant response of 40%.