# Recent Advances in Fiber-Shaped Electronic Devices for Wearable Applications
## 1 Introduction  
Electronic fiber is a building block of electronic textiles (e-textiles) for developing wearable electronics. In practical applications, fiber-shaped devices have attracted great attention as a potential alternative to conventional planar-type electronic devices. Because of their structural features, which enable them to be sewn into various fabrics, electronic fibers are an ideal device platform for realizing the three-dimensional (3D) deformability, light weight, breathability, washability, and comfort required for e-textiles [1, 2, 3]. Their one-dimensional (1D) shape allows fiber devices to maintain their electronic functions under various kinds of mechanical deformation and stimuli. Moreover, electronic devices with different functionalities can be fabricated onto a 1D substrate, and monofunctional fibers can be woven together into an integrated device or e-textile [4]. The use of such fiber technologies will allow various electronic systems for computing, information technology, and communications to be easily incorporated in e-textiles, which can accommodate diverse functional fibers into an unlimited number of structures. For example, smart integrated textiles can be used to process and digitize mechanical, chemical, electrical, and thermal data gathered by fiber units that sense and react to the human body and the environment [5].  
Recently, fiber electronics have advanced rapidly with the development of flexible electronics, stimuli-responsive sensors, and soft electronic materials. Unlike conventional electronic devices, electronic fibers require diverse fabrication methods, such as fused printing, spinning, electrodeposition, chemical vapor deposition, casting, rolling, molding, and thermal drawing [1, 6, 7, 8, 9, 10, 11, 12, 13]. These scalable fabrication processes have themselves been developed to achieve more precise patterning and uniform deposition of the active materials. Improvements in fiber technology have allowed basic device units to be produced in fiber form. The shape, composition, and architecture of electronic fibers can be adjustedby using suitable soft materials and fabrication processes to optimize function and performance [4,14]. With the rise of smart clothing and artificial intelligence (AI) technology, functional fibers with sensing, computing, memory, energy storage, energy-harvesting, and display capabilities have attracted much interest in the fiber industry [5]. In response to the demand for deformable devices, various types of fiber-shaped electronic components have been successfully developed, including transistors, memory devices, memristors, artificial synapses, sensors, light-emitting diodes (LEDs), and energy devices. To investigate the practical applications of these fiber-shaped devices, researchers have woven them into fabrics and integrated them into textiles [5].  
In this review, we summarize recent research in electronic fibers with fiber-shaped energy storage and harvesting devices, and smart integrated electronic textile systems. We introduce the structures and structural features of electronic fibers in Section 2. The potential applications of and feasible approaches to fiber-shaped devices, including newly developed fabrication techniques and production processes, are explained in Section 3 to highlight the research and development still required to realize smart fiber systems. In Section 4, we discuss recent advances in multi-functional fibers and e-textiles. Finally, we consider the challenges and issues that need to be solved in the development of fiber-shaped electronic devices and smart textile systems.