# Fiber-Based Sensors and Energy Systems for Wearable Electronics
## 3 Fiber-Based Sensors
### Photo-Sensing Fibers  
Photosensors are used for imaging or scanning, ultraviolet (UV) radiation monitoring, and optical communication in medical, military, and security systems. However, photosensors with lenses or beam splitters have a large size, heavy weight, and limited sustainability and field of view [114]. The fiber-based photosensors do not require a lens and can be woven into a lightweight and low optical density textiles to detect the direction and wavelength of an electromagnetic field over a large area. On the other hand, excessive exposure to UV rays can lead to erythema, cataracts, and skin cancer, so UV monitoring is very necessary for people who stay outdoors for long periods of time [115]. The fiber-based UV sensors can be lightly worn on the wrist and can provide the degree of UV light exposure  
Figure 3: Chemical sensing fiber. (**a**) Cross-sectional SEM image of the chemiluminescent fiber. (**b**) Sensitivity to peroxide vapor down to 10 ppb. Reproduced with permission from [55], Copyright (2012), Wiley. (**c**) Schematics of MOFs/MWCNT fiber. (**d**) Sensitivity to NO2 down to 0.1 ppm under bending. Reproduced with permission from [47], Copyright (2017), American Chemical Society. (**e**) Schematic of the fibers integrated into a textile. (**f**) The calibration plot of the chronoamperometric responses of the fiber up to 200% strain. Reproduced with permission from [39], Copyright (2019), American Chemical Society. in real-time. These fibers typically are fabricated as a photoconductive semiconductor p-n diode junction contacted by adjacent electrodes. The first photo-sensing fiber for imaging was fabricated by the TDP in 2004 [50]. The fiber was composed of amorphous chalcogenide glass (As\({}_{40}\)Se\({}_{50}\)Te\({}_{10}\)Sn\({}_{5}\)) core in contact with Tin microwires and enclosed with a transparent insulating polymer (Figure 4a). Chalcogenide glasses have photo-sensing capability that generates an electron-hole pair upon exposure light in visible and Infrared (IR) ranges and the current flows through the microwires. The fibers were flexible enough to be woven into the spectrometric textile (Figure 4b). More complex functions can be performed by combining multiple optoelectronic within a single fiber. The eight detectors arranged in the optical fiber could extract information about the direction and wavelength of incident radiation over a visible spectrum (Figure 4c,d) [57]. A 32 by 32 array measuring 0.1 square meters reconstructed diffraction images of polychromatic illumination for all rotation angles of the object. However, the amorphous semiconductor core has difficulties in achieving high light absorption and effective contact with the electrode. Another approach relied on the mechanism of sonochemical nanowire growth in amorphous selenium (Se) domains [116]. This method could determine the phase and direction of crystalline nanowires that make direct contact with the electrodes and maintain the sensitivity, light absorption, and response speed for effective photo-sensing. Because of their complex structure, fiber sensors for imaging or scanning are mostly synthesized through thermal drawing. Photo-sensing fibers can also be used for optical communications. [PERSON] et al. integrated light-emitting, and photo-sensing p-i-n diodes into optical fibers via thermal drawing [51]. The commercialized semiconducting diodes were inserted in pockets, and thermal drawing was performed while feeding two wires to the fiber. A three-megahertz two-way optical communication was demonstrated between the fabrics containing light-emitting and photo-sensing fibers. UV monitoring sensors utilize the change in conductivity by adsorption/desorption between oxygen molecules and the holes of semiconductor ZnO [117; 118; 119], gallium nitride (GaN) [120; 121], titanium dioxide (TiO\({}_{2}\)) [122], and CNT [123]. [PERSON] et al. fabricated the Kevlar fiber coated with ZnO nanowires by plasma etching, which directly responded to UV exposure from 0.2 to 1 mW cm\({}^{-2}\)[124]. [PERSON] et al.