Remote Monitoring of Skin Temperature Through a Wristband Employing a Printed VO Sensor

Abstract

The need for highly sensitive, environmentally stable, mechanically flexible, and low-cost temperature sensors for on-body measurements has been increasing with the wide adoption of personal healthcare-Internet-of-Things (H-IoT) devices. Printed electronics (PE) is a good platform for such sensors because it enables the realization of flexible devices through simple and rapid methods at a relatively low cost. However, previously reported printed temperature sensors suffer from poor sensitivity and/or environmental instability. In this article, we report a custom tungsten (W)-doped vanadium dioxide (VO2) ink-based screen-printed temperature sensor having the highest temperature coefficient of resistance (TCR) of 2.78% C-1 with a resolution of 0.1 °C between 30 °C and 40 °C. To protect it from environmental effects, a fluoropolymer-basedpassivation layer is added for accurate temperature readings even in 90% relative humidity. The sensor is printed on a flexible substrate and shows minimal deterioration in performance over 1000 bending cycles. For wearability and remote monitoring, the sensor is integrated with a custom Bluetooth low energy (BLE) wireless readout in the form of wristband. The BLE readout comprises an ultrathin and flexible patch antenna optimized for both BLE bandwidth (BW) and human wearability. It demonstrates a minimal specific absorption rate (SAR) value of only 0.068 W/kg, making it safe to wear. Despite the antenna’s thin structure (0.004λ), it has a gain of 1.65 dBi, enabling an excellent communication range. The proposed wristband is tested on ten volunteers and under daily activities, which shows promising results with a maximum error of 0.16 °C with reference to those of a commercial thermometer.