# Recent Advances in Wearable Devices for Non-Invasive Sensing
## 5 Wireless Technologies
### Self-Powered System
#### 5.1.2 Energy Harvesting  
Energy harvesters convert non-electrical energy into electricity, which means that they can generate power sustainably as long as the non-electrical sources are supplied. Photovoltaic cells, biofuel cells, and thermoelectric nanogenerators generate electricity from light, biomolecules, and heat, respectively. Triboelectric and piezoelectric nanogenerators convert mechanical forces into electricity, hence, they have an advantage in that energy sources can be acquired easily and continuously from body movement irrespective of the environmental conditions. In addition, since biofuel cells and thermo-, tribo-, and piezoelectric nanogenerators can generate electricity in proportion to the magnitudes of the non-electrical sources they convert, they also can operate as biosensors [149; 150; 151]. In wearable  
Figure 6: (**a**) Wireless power and data transfer between a smart contact lens and a network analyzer via inductive coupling (reprinted with permission from [144]. Copyright 2017 Springer Nature); (**b**) Smart contact lens with a wireless power transfer circuit and a LED display (reprinted with permission from [15]. Copyright 2018 American Association for the Advanced Science); (**c**) Smart contact lens integrated with a wireless power transfer circuit, LED pixel, and printed solid-state supercapacitors for continuous operation (reprinted with permission from [145]. Copyright 2019 American Association for the Advanced Science); (**d**) Self-powered wearable patch for continuous monitoring of pressure driven by a flexible thin-film thermoelectric generator (reprinted with permission from [146]. Copyright 2020 Elsevier B.V); (**e**) Triboelectric and piezoelectric generator with body movement sensing (reprinted with permission from [147]. Copyright 2017 Royal Society of Chemistry); (**f**) Wireless bioelectronics for multimodal human-machine interfaces via BLE (reprinted with permission from [148]. Copyright 2020 Springer Nature); (**g**) E-tattoos for wireless, personalized multimodal sensing via NFC (reprinted with permission from [17]. Copyright 2019 John Wiley and Sons). systems, energy harvesters should form a conformal contact to human skin such that energy sources from the body can be transferred efficiently to the devices. Therefore, the devices need to possess high softness, flexibility, and stretchability. [PERSON] et al. combined a flexible, thin-film thermoelectric generator (tf-TEG) with a pressure sensor to build a self-powered wearable pressure sensing system, as shown in Figure 6d [146]. The tf-TEG consisted of a flexible heat-absorbing film, a hydrogel heat sink, and thin-film thermoelectric materials. The thin-film thermoelectric materials were made of \(n\)-Bi\({}_{2}\)Te\({}_{3}\) and \(p\)-Sb\({}_{2}\)Te\({}_{3}\), and 25 pairs of the thermoelements were electrically connected in series. The heat absorber was composed of PDMS and boron nitride (BN), which provides high thermal conductivity and low electrical conductivity. The heat sink was formed of hydrogel to maximize the dissipation of heat through the evaporation of water. With conformal attachment onto human skin, the tf-TEG generated a high output voltage of 15.8 mV from body heat at ambient conditions. In addition, the device maintained a high output voltage during 8 h of measurements, and the self-powered pressure system exhibited a high-pressure sensing capability and excellent durability. [PERSON] et al. developed a flexible, transparent, self-powered body movement sensor based on a triboelectric and piezoelectric hybrid generator (TPG) [147]. The TPG consisted of Al-doped ZnO (Al:ZnO) as the transparent electrodes, poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) as a piezoelectric functional layer, and micro-structured PDMS as a friction layer, as shown in Figure 6e. The TPG was designed to convert the applied pressure and skin-based friction energy produced from body movements into electricity.