# Fiber-Based Sensors and Energy Systems for Wearable Electronics
## 3 Fiber-Based Sensors
### Tactile Sensing Fibers
Although a cyclic loading test with 1000 cycles revealed limited durability, the high stretchability and wide range of sensing allowed the researchers to attach these SBS-AgNW fiber sensors onto latex gloves to monitor finger joint movements and decipher sign language. In a more recent study by [PERSON] et al., Ecoflex silicone, and MWCNT based core-sheath fiber was fabricated through the coaxial wet-spinning method, and its applicability as a wearable strain sensor was tested [35]. The conductive core was spun from an MWCNT-dispersed silicone solution, and the insulation sheath was spun from a pure silicone solution. Although its conductivity of 1 \(\times\) 10\({}^{-2}\) S/cm was lower than that of SBS-AgNW fiber, it showed a high stretchability of 300% and a maximum GF of 1378. Furthermore, its resistance was stable during 10,000 cycles of 100% strain, indicating excellent durability. The researchers then weaved silicone-MWCNT fiber sensors into a fabric glove to detect finger motions. Conductive polymers have also been utilized as conductive fillers. One study utilized PU and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) to fabricate a conductive fiber via wet-spinning [36]. This fiber showed a relatively good electrical conductivity at 9 S/cm coupled with a high maximum GF of 10,000 and a wide strain sensing range (0-200%). Electrospun nanofibers can also be made into strain sensors. [PERSON] et al. electrospun a mixture of Polyvinyl alcohol (PVA) and PEDOT:PSS and encapsulated the electrospun nanofibers between Kapton and Polydimethylsiloxane (PDMS) [44]. The resultant conductive nanofiber-based strain sensor had a limited sensing range between \(-\)1.2% and 1.2%, but it showed a relatively high GF of 396 and a fast response time. Additionally, the researchers attached the PEDOT-PVA fiber sensor onto a finger to detect the finger bending while powering it with solar cells. Recently, increasing attention has been given to piezoelectric textile textile sensors. These sensors are able to transform mechanical stimuli into electrical energy, providing simultaneous sensing and self-powering capabilities. In one study by [PERSON] et al., electrospun polyvinylidene fluoride (PVDF) and Barium titanate (BaTiO\({}_{3}\)) mixture was used to fabricate piezoelectric nanofibers [45]. The PVDF-BaTiO\({}_{3}\) nanofibers comprised a pressure-sensor capable of detecting up to 40 kPa. This sensor's sensibility was measured to be 17 mV/kPa, and its piezoelectricity remained stable over 1750 cycles of pressure loading, boasting its good durability. Similarly, [PERSON] et al. coaxially electrospun a pressure-sensing nanofiber with PVDF-BaTiO\({}_{3}\) as a core and PVDF and graphene oxide (PVDF-GO) as a conductive sheath [46]. This nanofiber-based sensor showed a wider sensing range (80-230 kPa) and higher sensitivity (10.89 mV/kPa). Furthermore, an array of these textile sensors was able to create a 2D map of the pressed objects and monitor various joint movements when attached to a human arm. The above-mentioned examples utilized piezoelectric nanofibers to fabricate planer textile sensors. Furthermore, those sensors employed piezoelectric ceramics. While piezoelectric ceramics provide high piezoelectric outputs, they are mechanically hard, lowering their flexibility and stretchability [26]. On the other hand, there are efforts to create single piezoelectric fiber-based sensors using piezopolymers with an inherent flexibility that can further capitalize on the mechanical merits of the fibers and their potential to be integrated into conventional garments. [PERSON] et al. reported a simple melt-spinning method for fabricating a piezoelectric fiber with a core-sheath structure with a diameter of 60 \(\upmu\)m [42]. The conductive core was composed of CB and polyethylene, and PVDF sheath that acted as a piezoelectric layer. The researchers weaved these fibers into a textile, which in turn was sewn into a bag strap to harvest energy as the wearer walked on stairs.