A Printed Paper-based RFID Tag for Wireless Humidity Sensing

The trend toward low-cost, lightweight, and flexible RFID tags capable of sensing is growing. This work proposes a printed paper-based RFID tag for wireless humidity sensing. The proposed RFID tag, which does not require an integrated chip, is a parallel inductive-capacitive (LC) resonator whose interdigitated capacitor is coated with Polyvinyl Alcohol (PVA). Variation in the quality factor and resonant frequency of the tag in response to change of humidity can be remotely tracked with an interrogator coil inductively coupled to the RFID tag. To make the whole tag easy to print on a low cost flexible paper based substrate, a new configuration for the spiral inductor coil is proposed. It is experimentally shown that a change in relative humidity from 86% to 91 % results in a shift of notch frequency and an increase in magnitude at notch frequency of the reflection coefficient of the interrogator by 6 kHz and 0.07 dB, respectively. Finally, the long-term stability of the printed RFID tag in measuring humidity is demonstrated.