# Recent Advances in Fiber-Shaped Electronic Devices for Wearable Applications
## 3 Device Applications
### Energy-Harvesting/Storage Devices  
Stiff and rigid batteries and energy harvesting devices hinder the development of wearable devices, which require flexibility [68]. Batteries with high capacity, a long-term cycling life, and energy-generating capabilities are also needed to build higher-level wearable electronic systems that include logic circuits [68]. Large-scale quantitative data transfer and computing consume a great deal of power. Therefore, it is important to develop deformable and durable energy harvesting/storage devices with high energy density. To meet the requirements of wearables, fibriform energy harvesting and storage devices have been demonstrated during the past decade. In this section, we describe recent efforts to develop fiber-shaped energy harvesting and storage devices. Given that portable batteries have gradually become important in the wearable device industry, fiber-shaped batteries should have high capacities, long lifetimes, and excellent mechanical robustness. Recently, various shapes and materials have been used to realize fiber batteries. Both fiber-shaped lithium- and sodium-ion batteries with 3D interconnected hexagonal structures have been developed, using exclusive ion transport and efficient pseudocapacitive charge storage [69]. [PERSON] et al. developed a washable zinc-ion battery (ZIB) using double-helix yarn electrodes and a polyacrylamide electrolyte [70]. The yarn ZIB exhibits a high specific capacity (302 mAh g\({}^{-1}\)) and volumetric energy density (54 mWh cm\({}^{-1}\)), as well as excellent knittability and stretchability (up to 300% strain) [70]. Owing to its tailorable properties, long-yarn ZIB was woven into a textile that was used to power a flexible LED belt [70]. High-capacity aqueous zinc-ion battery fibers were also reported by [PERSON] et al. (Figure 7a) [69]. The fibrous Zn-ion batteries, composed of V\({}_{6}\)O\({}_{13}\)/CNTs, can harvest energy from ambient air to recharge without an additional power supply (Figure 7b) [69]. The resulting battery fibers presented a high specific capacity (371 mAh g\({}^{-1}\) at 200 mA g\({}^{-1}\)), stable switching (>5000 cycles at 5 A g\({}^{-1}\)) and recharging of \(\sim\)60% upon exposure to air (Figure 7c) [69]. The self-charging battery fiber also acted as a strain sensor in an integrated wearable fingertip device (Figure 7d) [69]. For sustainable and self-sufficient use, energy harvesting technologies that can generate electric energy from the human body and its surroundings are required [71; 72]. TENGs convert mechanical energy into electricity by triboelectrification and electrostatic induction. Wearable TENGs can collect low-frequency and irregular mechanical energy from body motion [73]. In previous research, fiber TENGs used thread-like metal wires, but those TENGs exhibited mechanical limitations such as low stretchability and elasticity. Although designing 2D or 3D textile architectures by knitting, weaving, or braiding different fiber TENGs can improve their deformability, the individual fibers remain mostly undeformed [73]. [PERSON] et al. fabricated intrinsically stretchable fiber TENGs (>650% strain) that can harvest mechanical and electromagnetic energy (Figure 7e-g) [73]. Poly(styrene-b- (ethylene-co-butylene)-b-styrene) hollow elastomeric fibers filled with liquid metal EGaIn produced energy through triboelectricity (160 V m\({}^{-1}\) and \(\sim\)360 \(\mu\)W m\({}^{-1}\)) and induced the electrification of the liquid metal (\(\pm\) 8 V m\({}^{-1}\) at 60 Hz and \(\sim\)8 \(\mu\)W m\({}^{-1}\)) [73]. The fibers acted as textile power supplies and sensors self-powered by touch and motion (Figure 7h) [73]. To integrate various functions in a single fiber and realize versatile energy fibers, [PERSON] et al. fabricated a multifunctional coaxial energy fiber for energy generation, storage, and use [74]. The energy fiber comprises a fiber-shaped TENG, supercapacitor, and pressure sensor in a coaxial geometry [74].