# Fiber-Based Sensors and Energy Systems for Wearable Electronics
## 5 Conclusions and Perspectives  
Fiber-based wearable devices have attracted the attention of many researchers, due to the intrinsic flexible, stretchable, durable, and breathable properties of fibers. Particularly, fiber-based sensors capable of measuring physiological and environmental states, while being comfortably worn hold great potential to be ideal wearable devices. Various types of sensors have been fabricated with fibers, including tactile sensors, chemical sensors, photosensors, and temperature sensors. These flexible and stretchable fiber-based sensors can be resistant to wear and wash and integrated into conventional garments, suggesting the ease of scalability. Furthermore, recent advancements in fiber-based energy-storage systems present possibilities to remove external power sources and fabricate self-powering wearable devices. Although fiber-based devices hold great potentials to be ideal wearable devices, they are not without limitations. Currently, there exist four major limitations of fiber-based wearable devices, which includes (1) performance, (2) wearability, (3) fabrication, and (4) integration (Figure 9). Figure 8: Schematic illustration of representative fiber-based supercapacitors. (**a**) Carbon-based supercapacitor fiber. Reproduced with permission from [146], Copyright (2013), Wiley. (**b**) Metal oxide-based supercapacitor fiber. Reproduced with permission from [49], Copyright (2017), Elsevier Ltd. (**c**) The longest supercapacitor fiber fabricated by the TDP. Reproduced with permission from [53], Copyright (2020), Wiley. 1. _Performance:_ The performance of fiber-based electronics may deteriorate, due to temperature, humidity, and gas in the air that is also the limitation of current wearable electronics. This type of performance degradation can be prevented through suitable packaging technologies by protecting the internal materials of the device from H2O or O2 in the air or preventing chemical reactions with active materials. However, in the case of fiber electronics, the packaging materials also need excellent flexibility and stretchability in order to maintain the performance without failure after bending or stretching deformation. While high-density polyethylene and polypropylene are being used as packaging materials because of their high flexibility and good processability, high moisture permeability makes them unsuitable as a package material. Therefore, maintaining the performance of fiber-based electronics through packaging with sufficient stability against external environments, excellent durability, and mechanical properties is an important challenge. 2. _Wearability:_ Ideal wearable devices need to be worn with comfort and minimal interference with body movements. To achieve this, the devices need to be flexible, stretchable, and breathable. Fibers' inherent flexibility and breathability have made them attractive building blocks of wearable devices. However, while stretchability is a critical factor for wearability, apart from strain sensors, many fiber-based wearable sensors, and energy-storage systems showed only limited stretchability or did not report it. The durability of the device is also of great importance for wearability and can be evaluated using cycling loading-unloading and washing tests. Currently, the washability remains to be a limitation as only a handful of fiber-based sensors and energy-storage systems reported acceptable performance change after a few washing cycles. 3. _Fabrication:_ In this review, we introduced popular fabrication methods for fibers, but all have unique disadvantages. For example, dip coatings have poor mechanical properties, due to weak interfacial bonding and it is not suitable for industrial manufacturing, due to the increase in manufacturing time and cost. Considering it, thermal drawing can be an ideal method for its production effectiveness. Thermal drawing can produce kilometers of fiber from centimeters of preforms at once.