# Fiber-Based Sensors and Energy Systems for Wearable Electronics
## 3 Fiber-Based Sensors
### Tactile Sensing Fibers
Importantly, it should be noted that the tactile sensing capability of this fiber was not tested, and the limited stretchability of 20% was suggested. For wearable sensors, stretchability is an important factor that influences the comfort and durability of the device. In a study by [PERSON] et al., a piezoelectric fiber capable of pressure and strain sensing was made by wrapping electrospun polyvinylidene fluoride trifluoroethylene (PVDF-TrFE) mat between an inner electrode (silver-coated nylon yarn) and an outer electrode (CNT sheet) (Figure 2a) [43]. Although this fiber was thicker with a diameter of 320 \(\upmu\)m, it boasted enhanced stretchability of 50% and pressure-sensing capability up to 160 kPa. This fiber's flexibility and stretchability allowed it to be knotted, coiled, and weaved into the fabric. The coiled PVDF-TrEF fibers preserved their sensitivity during a cyclic loading test with 400,000 cycles of 50% strain. Lastly, capacitive sensors composed of two opposite electrodes and one dielectric layer between the electrodes have been developed. Direct current can generate capacitance between the two electrodes whose amplitude depends on the geometric parameters, such as the distance and the area between the two electrodes. Therefore, a tactile stimulus shifts  
Figure 2: Fiber-based piezoelectric and capacitive tactile sensors. (**a**) Piezoelectric tactile fiber sensor was fabricated with an electrospun PVDF-TrFE fiber wrapped with silver-coated nylon yarn. This fiber sensor was flexible enough to be yarn and made into a textile. Reproduced with permission from [43], Copyright (2015), Wiley. (**b**) Core-spinning with silver fiber and cotton fiber. This fiber was integrated into cotton fabric. Reproduced with permission from [80], Copyright (2019), Wiley. (**c**) (i) Multicore-shell printing with dielectric elastomer and ionically conductive fluid. (ii) This fiber sensor was sewn onto pants to detect walking. Reproduced with permission from [81], Copyright (2015), Wiley. (iii) Cotton fabric based capacitive tactile sensors were attached to a robotic hand. Reproduced with permission from [80], Copyright (2020), American Chemical Society. the sensor's geometry and capacitance. This change in capacitance is measured and utilized to determine the amplitude of the applied stimuli. The capacitive sensors tend to show a linear sensitivity and better durability, especially compared to resistive sensors [13; 22; 82]. [PERSON] et al. made an electric layer with silk fabric and AgNWs coated on the outside and separated by a polyester fabric dielectric layer [83]. This fabric-based sensor was capable of detecting pressure up to 5 kPa with 0.3 GF and stretching up to 25%. Furthermore, this sensor was used to create a 2D map of the applied pressure and attached to a volunteer's arm to recognize the movement of an arm. Other studies reported fiber-based capacitive tactile sensors [40; 54; 81; 82; 84]. [PERSON] et al. fabricated elastomeric fibers through a melt extrusion method with a hollow core, in which eutectic gallium indium (EGaln) was filled and acted as an electrode [84]. Two of these fibers were twisted together to form two EGaIn electrodes separated by the dielectric elastomeric outer layer. This capacitive sensor showed a GF of 0.82 and stretchability up to 100%. This fiber sensor was thicker in diameter (diameter of 850 \(\upmu\)m), compared to other fiber-based tactile sensors. Nonetheless, this sensor was capable of detecting not only strain and pressure, but also torsion. Twisting two electrode-fibers together has also been utilized in other studies. In a study by [PERSON] et al. silver-nylon fiber as an electrode was core-spun with cotton fiber that acted as a dielectric layer (Figure 2b) [54]. Similarly, the researchers twisted core-spun fibers, and the resultant tactile-sensor fiber showed a diameter less than 600 \(\upmu\)m. The GF was reported to be 0.7, and the strain detection range was limited to less than 18%.