Degradable and Printed Microstrip Line for Chipless Temperature and Humidity Sensing

Research on chipless and passive architectures for environmental sensing is generating high interest because they do not require any semiconductor components or batteries to operate, thus resulting in an eco-friendlier footprint. This study demonstrates a printed microstrip line with multiple resonators using biodegradable materials to continuously monitor temperature and relative humidity (RH). Constructed with a paper substrate and printed zinc conductive lines, and encapsulated with beeswax to protect against the interference of humidity, the microstrip line integrates spiral-shaped resonators. One resonator operates at 1.2 GHz for temperature sensing, while another, coated with konjac glucomannan serves for relative humidity sensing at 2 GHz. The multi-resonating features allow for a simultaneous assessment of temperature and humidity. The microstrip line displays a linear sensitivity to temperature of −1.35 MHz °C−1 and a non-linear relative humidity sensitivity ranging between −0.8 and −8 MHz/%RH from 30% to 70% RH. Its degradation in a lab-made compost for 70 days shows the removal of the transducing layer in 7 days and degradation of the cellulosic substrate starting after 5 weeks. The developed environmental sensing devices are notably promising for future applications in smart packaging and the tracking of goods aiming at the minimization of electronic waste.