A Flexible Wireless Pressure Sensor Based on UHF RFID Technology

Passive wireless flexible sensors are attractive in the areas of wearable electronics, healthcare services, and smart robotics due to the advantages of simplified electrical connection, no requirement for power source, and long lifetime. In this article, a wireless passive flexible pressure sensor based on ultrahigh-frequency (UHF) radio frequency identification (RFID) technology is proposed. The sensor consists of three layers, including a flexible RFID tag, an absorptive layer of ferrite film, and a compressive separation sponge in between. As an external load applies on the sensor, the sponge layer gets compressed, and the separation distance between the RFID tag and ferrite film reduces. As a result, the strength of the backscattering signal received by an RFID reader decreases due to the increment on the absorption effect from the ferrite film. Importantly, this is the first passive UHF pressure sensor adopting a tag-sponge-ferrite film structure, providing a compact solution to wireless pressure detection in the UHF band. Theoretical simulations have been conducted to analyze the influence of the thickness of the separation layer on the strength of the backscattering signal of the sensor, from which the optimal design of the passive pressure sensor has been defined. The sensor shows a high sensitivity of −9.49 dBm/kPa in the pressure range of 1.39–2.57 kPa, and environmental stabilities (within 0.91%) to temperature changes (25 °C–45 °C) and relative humidity (RH) variations (43%RH–82%RH). Importantly, the wireless transmission distance between the sensor and the reader is more than 70 cm, which is 16.67 times higher than a single LC-based wireless pressure sensor. This facilitates to broaden the potential usages of wireless passive pressure sensors in the fields of wearable electronics. To demonstrate the utility, the sensor has been integrated with a cuff and a cushion to wirelessly monitor bandage pressures and body movements.