Ingestible, Biofriendly, and Flexible Flour-Based Humidity Sensors with a Wide Sensing Range

Ecofriendly and biofriendly materials are highly appealing in electronic devices with the booming development of the Internet of Things. Humidity sensors perform an indispensable role in various fields such as monitoring industry processes and living systems. In this contribution, for the first time, edible and abundant material-flour has been introduced to the sensing layers for humidity detection. The ionic conductive K₂CO₃ and hygroscopic glycerol (Gly) with biosafety were introduced to the sensing layer to promote a humidity sensing capability and long-time stability. The ingestible Gly-K₂CO₃ flour-based humidity sensor using PLA as substrates displays an excellent sensing performance and wide humidity detecting range. Good linearity between capacitance and relative humidity (RH) has been achieved for the RH range from 6% to 94%. According to a wireless monitoring system, even at 250 kHz, the sensor can still monitor the nuance of the humidity difference between 40% and 43% and exhibited an ultrafast response time (0.3 s) and recovery time (0.7 s). Furthermore, the consistent and stable sensing performance after long-time measurements for 100 days and bending tests for 500 cycles also demonstrated a long-time humidity detecting stability and mechanical flexibility for the Gly-K₂CO₃ flour-based humidity sensor. Humidity sensing mechanisms were thoroughly investigated through SEM-EDX, FTIR, XPS, and complex impedance spectroscopies. The enhanced sensing performance, wide humidity detecting range, and long-time stability are attributed to the ionic conductivity from K₂CO₃ and the hydrogen-bond interaction between H₂O and enormous hydrophilic groups, such as hydroxyl and amino groups in the flour-based networks and binary solvent.