# Recent Advances in Fiber-Shaped Electronic Devices for Wearable Applications
## 2 Device Structure of Electronic Fibers  
Compared with conventional planar devices on a two-dimensional (2D) substrate, fiber-shaped devices exhibit good breathability, wet permeability, and hygroscopicity, making them comfortable to wear. The one-dimensional (1D) form of a fiber-shaped device offers excellent flexibility and a light weight, and can accommodate large strain deformations, such as the stretching, 3D bending, and 3D twisting caused by human motion [15]. Moreover, fiber-shaped devices can easily be integrated with other functional fibers using conventional weaving processes. Fiber-shaped devices have mainly been fabricated into four representative structures: coaxial, twisted, three-dimensional layer-by-layer (3D LBL), and woven (Figure 1) [16]. The coaxial structure is a three-dimensional linear structure, like an electrical cable, composed of an inner electrode core surrounded by active materials and outer electrodes (sometimes with an encapsulation layer) (Figure 1a). Solution-based printing is the main fabrication method for coaxial fiber devices, and it can be scaled up to reel-to-reel manufacturing. However, uniformly placing thin films onto flexible and deformable fiber substrates remains challenging. It is also difficult to stack active materials in layers, and to create patterned layers or fully covered films without causing defects to the single fibers. Another design strategy for single-fiber devices is a twisted structure (Figure 1b). Two or more single fibers containing different components are twisted together to fabricate a 1D device in one direction. In general, the twisting process uses both a rotating stage and a rotor. For instance, one end of a functional fiber can be attached to a feeding roller positioned at a specific angle, and the other can be attached to a fiber electrode that is fixed to a motor. The motor in the twister machine begins to move, and the single fibers are plied  
Figure 1: Schematics of representative device structures of (**a–c**) 1D electronic fibers and (**d**) 2D electronic textiles. together. The twisted structure minimizes fiber divergence and increases the interface area between the electrode and the functional fibers. However, the tightness affects the twisting stress; thus, the contact force decreases as the degree of twist decreases. Furthermore, repetitive folding and bending could loosen the twist structure. A 3D LBL structure is similar to a 3D sandwich configuration, in which two or more electrodes are separated by an active layer (Figure 1c). All components are integrated into a single fiber; thus, the 3D LBL structure enables particularly small device dimensions. The fixed electrode layers prevent physical disconnection between the active layer and the electrode. In this way, the 3D LBL single-fiber structure provides better structural stability against external physical deformation than the twisted and interlaced fiber structures [17]. 3D LBL devices can be customized to the needs of particular applications via thermal drawing methods [13]. The geometry and architecture of 3D LBL devices are tunable within a single fiber; moreover, discrete device units can be embedded into the preform. In this regard, monolithic integration of multiple functionalities in a 3D LBL electronic fiber can be demonstrated by increasing the density of devices [18]. Most e-textiles are made by interdigitating and interpenetrating different types of 1D electronic fibers. A representative device structure of 2D e-textiles is shown in Figure 1d. Electronic fibers can be designed to hold diverse patterns in 2D textile structures. Two fiber devices can share fiber electrodes in a textile and integrate different functions. For example, fiber-shaped supercapacitors (or batteries) can be integrated with fibriform solar cells (or nanogenerators) in one fabric for simultaneous energy storage and harvesting [17].