Biodegradable materials as sensitive coatings for humidity sensing in S-band microwave frequencies

Worldwide, electronic waste represents the fastest-growing stream of waste. With an increasing number of connected devices, passive and eco-friendly environmental sensing solutions need to be developed. Wireless passive devices for RFID and sensing exist, however, most of them rely on non-biodegradable materials. Willing to produce entirely green radio-frequency (RF) resonators on a paper substrate, we identify potential biodegradable materials to be used as encapsulation and humidity sensing layers. Resonator encapsulation is mandatory to prevent humidity interaction with the transducer while a sensing layer above the resonator enables a good response to humidity. In this work, the radio-frequency behavior of these materials was characterized when implemented on a 3.3 GHz resonating microstrip structure made of copper on FR4 substrate. The response in resonance frequency while varying the relative humidity (RH) from 20% to 80% was monitored. Beeswax-coated resonators exhibited no change in resonance frequency when exposed to humidity and therefore provided excellent encapsulation properties. 10 μm-thick layers of psyllium, konjac and egg-albumin displayed suitable sensing behavior with suitable frequency shifts above 100 MHz from 20% to 80% RH. Konjac and psyllium showed the best compatibility when coated on the beeswax encapsulant, exhibiting reversibility and low hysteresis when exposed to humidity variations.