# Fiber-Based Sensors and Energy Systems for Wearable Electronics
## 1 Introduction  
The demand for personal devices capable of biological and environmental sensing is greater, now more than ever. In particular, wearable devices have been receiving much attention, due to their ability to sense the physiological and environmental states while being worn [1; 2; 3; 4]. These wearable devices can aid individuals who are exposed to physical injuries or hazardous environments by detecting physical impact, biofluid-based biomarkers, or environmental cues [5; 6; 7; 8]. For these devices to perform their function while being worn, they need to be flexible, stretchable, and durable [9; 10; 11; 12; 13]. These mechanical requirements have shifted the recent attention to fibers. A paper review and analysis from the author's side shows that the number of publications whose title includes \"electronic fibers\" has risen by more than ten folds in the last decade compared to the previous decade indicating the increased academic interest in fiber-based electronics. The development of the electronic fibers became possible, due to the convergence of rapidly advancing fields of nanotechnology, electronics, and material engineering that has granted functionalities to conventionally passive fibers. For example, incorporating active materials, such as metals, conductive polymers, and carbon-based nanomaterials into fiber created conductive fibers that can bea building block of wearable electronic devices [14; 15; 16; 17; 18]. These fiber-based wearable devices, compared to conventional planar wearable devices, have several benefits. In addition to the flexibility and stretchability of many current wearable devices, fiber-based wearable devices also offer breathability, the ability to conform to complex body structures, and resistance to wear and even washing [1; 5; 12]. Moreover, fiber materials can be carefully selected according to their physical and chemical properties, and various combinations of active materials offer different functionalities, such as high elasticity, conductivity, and resistivity to different stimuli. Further, fiber-based textile manufacturing has been used as a popular and well-established technique for decades, exploiting the hierarchical frame, which comprises fiber, yarn, and fabric, suggesting the ease of scalability and integration into conventional garments. Therefore, recent attention has been focused on fiber-based electronics capable of physiological and environmental sensing. The fiber-based electronics are not limited to wearable sensors. Recently, fiber-based energy-storage systems have been receiving increasing attention, due to their self-powering potential for complete wearability [19; 20; 21]. In this review, various types of fiber-based sensors with motion and accident detection [2; 22], biomarker detection [11; 23], and environment detection [24; 25] capabilities are explored, along with popular fabrication methods, such as coating, spinning, and thermal drawing process (TDP). In addition, fiber-based batteries and supercapacitors are investigated. Lastly, current trends and challenges of fiber-based wearable devices are discussed.