# Fiber-Based Sensors and Energy Systems for Wearable Electronics
## 3 Fiber-Based Sensors
### Tactile Sensing Fibers
Among many carbon-based materials used to make conductive textiles, carbon nanotubes (CNTs) are most popularly selected for their high surface area, high electrical conductivity, and high stability [65; 74; 75; 76; 77; 78]. A strain sensor based on a conductive cotton fabric was made by dip-coating a pristine cotton fabric into a solution of single-walled CNT (SWCNT) (Figure 1a) [30]. This sensor was able to sense the strain up to 100%, and due to its high flexibility and stretchability, this fabric sensor could be twisted and knotted. Furthermore, it was possible to wash the sensor using a washing machine and detergent eight times with an acceptable change in resistance, implying potential applicability of this sensor as a wearable device. Polymer fibers have also been utilized to fabricate textile tactile sensors. [PERSON] et al. fabricated a strain sensor by dip-coating polyurethane (PU) fiber-knitted fabric into silver nanowire (AgNW) solution [31]. This PU fabric sensor showed a good stretchability of up to 140% and a maximum GF of 10. What's more, the flexibility and stretchability of this fabric sensor allowed it to be weaved into an e-textile for 2D force mapping. Other fibers and active materials have also been used, such as cellulose acetate fiber (CAF) and reduced graphene oxide (RGO) [79]. Conductive composite fibers have also been fabricated as resistive tactile sensors. These fibers with endogenous conductivity are usually fabricated with an elastic polymer incorporated with conductive fillers, via spinning methods. Melt-spinning is popularly used for its economical fabrication of conductive fibers. [PERSON] et al. fabricated melt-spun polycarbonate (PC) and multi-walled carbon nanotubes (MWCNT) into a fiber  
Figure 1: Fiber-based resistive tactile sensors. (**a**) (i) Natural fabrics can be made into tactile sensors through dip-coating. Pristine cotton fabric was dipped into a solution of multilayer graphene. (ii) The behavior of this sensor’s resistance in response to strain. (iii) The cotton fabric-based tactile sensor was attached to a human wrist to monitor wrist bending. Reproduced with permission from [30], Copyright (2020), American Chemical Society. (**b**) (i) Conductive composite fibers can be fabricated through spinning methods. SBS-AgNW mixture was wet-spun into a conductive composite fiber (left). Cross-sectional image of SBS-AgNW fiber (right). (ii) The behavior of SBS-AgNW fiber’s resistance in response to strain. Reproduced with permission from [63], Copyright (2015), Wiley. (iii) A fibrous sensor was attached to a textile using a silicone film. Reproduced with permission from [41], Copyright (2008), MDPI. with a diameter of around 300 \(\upmu\)m [40]. This fiber strain sensor showed limited stretchability (less than 20% stretching before breakage) and conductivity of 2 \(\times\) 10\({}^{-5}\) S/c, but the maximum GF was relatively high at 16. In another study, thermoplastic elastomer (TPE) was mixed with CB and melt-spun into a fiber with a diameter of 315 \(\upmu\)m [41]. This fiber was capable of sensing strain up to 80% with a higher maximum GF of 20 and conductivity 1 \(\times\) 10\({}^{-3}\) S/cm. This sensor was with stood eight times of washing with detergent without any significant conductivity loss. Moreover, the researchers were able to attach 21 fiber sensors onto a t-shirt of a subject using silicone films to classify the wearer's body position. Inherently, melt-spinning limits the concentration of the fillers, resulting in low conductivity [40,69] Wet-spinning is another conductive composite fiber fabrication method that allows outstanding dispersion of filler into the fiber with good mechanical properties [69,70]. In a study by [PERSON] et al. a mixture of styrene-butadiene-styrene (SBS) and AgNW was wet-spun a conductive fiber (Figure 1b) [34]. In addition to the high electrical conductivity of 2450 S/cm, this fiber was also very stretchable up to 900% and able to sense strain up to 220% with a maximum GF of 15.