Dual-Polarized Keratin-Based UWB Chipless RFID Relative Humidity Sensor

In this paper, we realize a non-invasive, bio-compatible, reliable and compact sensor to measure relative humidity (RH) levels by depositing keratin bio-polymer on a dual-polarized ultra wide band (UWB) chipless radio frequency identification (RFID) tag. The bio-compatible and hydrophilic properties of keratin make the proposed sensor an appropriate solution for monitoring RH level in different food and pharmaceutical industries, where non-toxic materials are in high demand. The proposed sensor comprises two main parts: a three-armed Fermat spiral resonator and the keratin bio-polymer mounted on the surface of the resonator. Keratin is extracted through the alkaline hydrolysis process and analyzed using X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR) methods. The three-armed Fermat spiral resonator acts as a scatterer, generating both co- and cross-polarized resonant responses when interrogated by a signal in a UWB frequency range. Having a cross-polarized response increases the robustness of the proposed sensor in the presence of interference and helps enhance the reliability of the sensor by offering three additional parameters. Amplitude, quality factor, and resonant frequencies are three main parameters that have been extracted from the responses of the proposed RH sensor at both co- and cross-polarization by modelling its responses as a function of frequency. To examine the performance of the sensor, laboratory measurements are carried out in the Esky box. The results show that the proposed sensor can be used to monitor RH changes from 54% to 74% at 26° Celsius.