Printed Humidity Sensors from Renewable and Biodegradable Materials

Increasing environmental concerns raised by the accumulation of electronic waste draws attention to the development of sustainable materials for short-lived electronics. In this framework, printed capacitive humidity sensors and temperature resistive detectors composed exclusively of biodegradable materials: shellac, carbon-derived particles, and egg-albumin are reported. The sensor platform comprises interdigitated electrodes serving as a capacitive transducer for humidity sensing, and a serpentine used as a resistive temperature detector. Both the interdigitated and serpentine electrodes are manufactured by screen-printing carbon ink on a shellac substrate. The humidity sensors are constructed by drop-coating egg albumin on the interdigitated carbon electrodes and the temperature detector is prepared by encapsulating the serpentine design with shellac. Shellac is shown to be a biodegradable alternative to hydrophilic cellulose-derived substrates, with the capacitive humidity sensors demonstrating a sensitivity of 0.011% RH-1. The response and recovery times on shellac are 12 and 20 times faster than on cellulose-based substrate, and the serpentine resistive temperature detectors have a temperature coefficient of 5300 ppm K-1. At the end of their service-life, the sensors produced are home compostable and can be environmentally friendly disposed, potentially enabling their future use for sustainable and environmentally friendly smart-packaging, agricultural sensing, or point-of-care testing.