Development of a Highly Flexible and Durable Fiber-Shaped Temperature Sensor Based on Graphene/Ni Double-Decked Layer for Wearable Devices

In this study, we approached a new way to develop inexpensive, lightweight, highly flexible temperature sensors offering greater repeatability and linearity response to temperature. A polyethylene terephthalate (PET) monofilament fiber and our developed graphene (G), synthesized via an easy and low-cost technique and deposited over a Ni layer (G/Ni), were used as the substrate and temperature-sensitive layer, respectively. The fabricated G/Ni-based temperature device (D-1) demonstrated a positive temperature coefficient of resistance (TCR) of 0.0108 (°C^-1) with high repeatability and linearity of response to the temperature. D-1 retained 75% of the TCR characteristic after 10,000 bending cycles with a bending radius of 11.42 mm. It also maintained its resistance when applied in an environment where the humidity level changed from 40% to 90%. Its resistance was constant when it was exposed to O₂ gas, N₂ gas, and different alcoholic vapor environments for 20 min. Moreover, the TCR performance of D-1 before and after the bending tests were compared with those of devices based on commercially available graphene oxide (GO) coated over Ni (GO/Ni) (D-2) and bare Ni thin film (D-3). D-1 showed a greater TCR (0.0108 °C^-1) response with high linearity (R2 = 0.9938) than D-2 and D-3, which exhibited TCR responses of 0.0032 °C^-1 (with an R2 of 0.9752) and 0.0023 °C^-1 (with an R2 of 0.9271), respectively. Finally, the response and recovery times of D-1 were measured to be of about 13 and 22 s, respectively.