# Recent Advances in Fiber-Shaped Electronic Devices for Wearable Applications
## 3 Device Applications
### Electronic Devices
#### 3.1.1 Transistors
One of the main issues is that 1D FETs still require sophisticated fabrication techniques that demand a vacuum process such as thermal evaporation, sputtering, or atomic layer deposition, which is unsuitable for commercialization [30]. Moreover, the high operating voltages required by OFETs, along with their low conductance values and unstable electrode interconnections, need to be resolved. In this regard, a new device design strategy has been developed for high-performance 1D FETs. [PERSON] et al. fabricated fibrous OFETs with a twisted structure and a solid ion-gel electrolyte (Figure 2c) [30]. The source and drain (S/D) fiber electrodes were coated with an organic semiconductor and twisted together. The twisted assembly of electrode fibers was then surrounded by an ion gel, and the gate wire was then wound around that [30]. The resulting fibrous OFETs achieved milliampere-level output current and a good on/off ratio of 10\({}^{5}\) at low gate voltages (below \(-\)1.3 V) [30]. Their work reveals a promising structural strategy that could help overcome the current limitations of coaxial fiber FETs. As another class of fiber transistors for wearable electronic devices, fiber-shaped organic electrochemical transistors (OECTs) have been explored because they can simplify the complex manufacturing processes required for coaxial fiber OFETs [24,31]. The OECTs use an electrolyte instead of an insulating layer of FETs, and therefore they do not need a smooth fiber substrate [31]. By applying a gate voltage, ions are injected from the electrolyte into the top surface or inner part of the semiconducting film, thereby doping the channel [32]. However, OECTs operated by doping/de-doping conducting polymers can work only in the depletion mode, and the response time is longer than that of OFETs due to slow ion transport in the ionic liquid [32]. [PERSON] et al. demonstrated electric double-layer capacitor-gated (EDLC) transistors on sputter-coated metal fibers using poly(3-hexylthiophene) (P3 HT) and imidazolium ionic liquid [11]. In the EDLC-OFETs, the channel conductivity was modulated by electrolyte polarization upon exposure to an electric field [11]. Therefore, the demonstrated EDLC transistors operated below 1 V and exhibited large current densities and improved switching speeds [11]. [PERSON] et al. also demonstrated CNT transistors on linen threads, using a colloidal silica-based ionic liquid gel that induced all-around electrostatic gating (Figure 2d) [26]. The thread substrate was knotted with S/D Au wire, and then the P3 HT (or CNT) semiconductors and ionogel were sequentially deposited by drop casting onto the thread [26]. These ionogel-gated transistors were applied as a switch and a multiplexed diagnostic device with simple logic gates (NAND, NOR, and NOT) [26]. Electrochemical and electrolyte-gated transistors can easily be integrated into woven circuitry in textiles and operated at low voltages. However, the low reliability in switching behavior under chemical and bias stress must be improved before their application will be practical. For this reason, highly reliable electrolyte-based materials should be developed.