Biodegradable CMC-Based Chipless RFID Sensor for Wireless Humidity Monitoring

Chipless and passive radio frequency (RF) sensors for environmental monitoring attract tremendous interest because they can operate without semiconductor components or batteries. However, they rely on nonbiodegradable synthetic materials. In this work, we leverage the intrinsic sensing capability and self-standing nature of carboxymethyl cellulose (CMC) film to develop a single-layer miniaturized RF humidity sensor. The device consists of a compact resonant antenna whose frequency response is governed by the humidity-dependent dielectric constant and loss tangent of the CMC film. The antenna is optimized using Ansys HFSS and then fabricated onto the CMC film substrate via aerosol jet printing. Morphological studies confirm high fidelity of the antenna patterning, as well as strong adhesion to the film. From the electrical characterization, it can be noted that there is a clear resonance in the measured transmission spectrum (S11) at 2.65 GHz, with an amplitude of –13.92 dB. Later, under controlled environmental conditions ranging from 30 to 90% RH and at ambient temperature, a monotonic downshift trend of the resonance frequency is obtained. The corresponding sensitivity ranges between 3.02 and 6.16 MHz/%RH. Furthermore, the sensor exhibits only 14.4 MHz hysteresis at 60% RH during humidification-dehumidification testing. Moreover, degradation is 83% within 20 days.