Durable high-humidity sensing via stepwise spray-coating of phytic acid/polyethylenimine film on porous carbon electrodes

Stepwise-coated PA/PEI film on porous C for stable humidity sensing at 95% RH. • Sensing mechanism elucidated by impedance equivalent circuit modeling. • Real-time airway humidity monitoring demonstrated for a tracheostomy patient. Long-term, reliable, and continuous humidity monitoring in high-humidity environments (> 90% RH) is critical in fields such as healthcare and industrial processing. However, this remains technically challenging for existing humidity sensors. Herein, we present a humidity sensor to address this issue. The sensor is based on a phytic acid/polyethylenimine (PA/PEI) composite, fabricated via a stepwise spray-coating process onto a direct laser writing-carbonized porous carbon interdigitated electrode. This approach allows for the formation of a uniform and durable PA/PEI complex on the porous carbon surface. The strong electrostatic interactions between the components enable this formation, as confirmed by FTIR analysis. Our sensor can operate reliably in the range of 70–95% RH. It exhibits a sensitivity of –491 Ω/%RH at 30 °C (95% RH) and a hysteresis of ∼0.26%, with no performance degradation observed after continuous operation for over one month at 95% RH. The sensing mechanism was elucidated through frequency-dependent impedance analysis and equivalent circuit modeling. It involves proton hopping, ion diffusion, and charge transfer processes. Their relative contributions are modulated by both humidity level and temperature. Furthermore, the sensor was integrated with a 3D-printed cuff and a tracheostomy tube. This allowed us to demonstrate the clinical utility of our sensor for real-time monitoring of the airway humidity in a patient. This work provides a scalable, low-cost sensing platform specifically designed for long-term operation in high-humidity environments. Download: Download high-res image (171KB) Download: Download full-size image