Inkjet-Printed Humidity Sensors with Black Soldier Fly Melanin

With the increasing material and energy demand for the fabrication of sensors for the Internet of Things, a move towards more circular economies and the use of sustainable materials with efficient fabrication methods is necessary. Among these sensors, humidity sensors are useful tools for monitoring atmospheric conditions in fields ranging from industry and agriculture to healthcare.<br/>In this study, the bio-sourced and biodegradable materials melanin and choline chloride were investigated for the inkjet printing of flexible humidity sensors. Specifically, an aqueous dispersion of black soldier fly melanin – a by-products of food waste management – was optimized for printing with a cosolvent and mixed with the biocompatible salt choline chloride. The composite was printed onto electrodes on different flexible substrates. Characterization by impedance spectroscopy showed that the addition of choline chloride increased the ion concentration and AC conductivity by more than three orders of magnitude, resulting in a significant improvement in sensing performance and reduced hysteresis compared to pure melanin sensors at frequencies up to 1 kHz. The devices exhibited fast detection and recovery times (~ 0.8 ± 0.3 s) with a 170 ± 40-fold decrease in impedance for relative humidity changes from 30 % to 90 %.<br/>Overall, inkjet printing – as a material-efficient deposition method – and the use of bio-based by-products reduced the embodied energy of the sensor and enhanced its sustainability compared to conventional electronics. In addition, concepts for reusability and degradability were demonstrated that allow for the use of the sensor in wearable or agricultural applications.