# Fiber-Based Sensors and Energy Systems for Wearable Electronics
## 3 Fiber-Based Sensors
### Chemical Sensing Fibers  
Chemicals present in our body or surroundings provide countless information. Accordingly, chemical sensors with continuous real-time monitoring of chemical markers can be developed into wearable devices. These chemical sensors are categorized to be environmental scanning or health monitoring. Chemical sensors for environmental scanning can mimic the olfactory system by detecting and quantifying local exposure to hazardous vapors, such as nitrogen dioxide (NO\({}_{2}\)) [85; 86; 87], ammonia (NH\({}_{3}\)) [88], and hydrogen chloride (HCl) [89]. Chemical sensors for healthcare typically detect the markers in sweat [90; 91; 92; 93], tears [94; 95; 96], saliva [97; 98], and respiration [99; 100; 101]. In particular, fiber-based chemical sensors are able to be used in sports apparel to detect pH, cortisol, glucose, and sodium ions in sweat. Sweat can provide a simple and convenient medical examination in a non-invasive manner [102]. The fiber-based sweat sensors have the advantages of being comfortable, breathable, and reduced risk of skin contamination compared to the patch-type, which is directly attached to the skin. In the aspect of sensing mechanism, chemical sensing fibers have been fabricated based on the (1) chemiluminescence, (2) chemiresistive, and (3) electrochemical mechanism. Various sensing materials have been studied for vapor sensing fibers based on chemiluminescence and chemiresistive mechanisms. Chemiluminescent sensing has advantages in terms of measurement simplicity, portability, and low cost [103]. In a pre-existing chemiluminescent-based approach, luminescence signals are collected through a thermal-drawn optical fiber and transmits it to a photodetector at the end of the fiber [104]. In order to increase sensitivity and optical transmission, [PERSON] et al. fabricated an optical fiber with photoconductive detectors embedded along the fiber axis to detect the emitting light in the hollow core, as shown in Figure 3a [55]. The peroxide vapor reacts with the sensing agent coated on the hollow core to emit light, and the electrical signal is recorded by an electrode in contact with an amorphous Se\({}_{97}\)S\({}_{3}\)-based photodetector. As shown in Figure 3b, the fiber can detect a low concentration of NO\({}_{2}\) at 10 parts-per-billion (ppb). Early flexible chemisesistive gas sensors were synthesized using metal oxides and conductive polymers [105; 106; 107]. However, due to the decrease in sensitivity of these sensors at a high temperature [108], carbon-based materials or metal-organic frameworks (MOFs) have been used for chemiresistive fiber via chemical vapor deposition (CVD) or dip-coating. MOFs are attracting attention as sensor materials, due to their high porosity, structural flexibility, and large surface area [109]. [PERSON] et al. developed the hybrid fiber in which ZIF-67-Co MOFs combined with highly conductive MWCNTs [47]. The MOFs/MWCNT sheet synthesized by CVD is twisted into the fiber, which has a negative resistance response to NO\({}_{2}\) (Figure 3c,d). Although the sensitivity of fibers synthesized by CVD is quite high, the long manufacturing time can limit mass production. [PERSON] et al. demonstrated the RGO-based fiber fabricated by dip-coating with biomolecular glue, which showed a response to NO\({}_{2}\) gas with a low detection limit and excellent electrical conductivity retention at a bending radius of 1 mm [110]. As it is woven with other fibers with a built-in electric circuit, LED light is turned on by NO\({}_{2}\) exposure. In order to further increasing the sensitivity, efforts are being made to increase the surface area of the active materials. Recently, metal oxides, which can increase the active site area and charge transfer coefficient by absorbing oxygen atoms, such as zinc oxide (ZnO), have been used as coating materials by making mesoporous structures through the calcination process [32].