Wearable humidity sensors based on bar-printed poly(ionic liquid) for real-time humidity monitoring systems

Abstract

Wearable chemical sensors can provide crucial human-physiology information by monitoring humidity in various environments. Capacitive-type humidity sensors based on poly(ionic liquid) (PIL) as the sensing material, which was synthesized via the polymerization of an ionic liquid (IL) were developed. The PIL films fabricated through a facile bar-printing process in ambient air, exhibited outstanding capacitive characteristics over a broad relative humidity (RH) range of 10%−80% at frequencies between 100 Hz and 1 MHz. Notably, high sensitivity (1.8 nF/%RH), good linearity (R2 = 0.980), low hysteresis, and fast response (~20 ms) of the developed humidity sensors were achieved. The corresponding sensing mechanism of the PIL for capacitive-type humidity sensors was studied by investigating the chemisorption of water molecules on the PIL surface and proton hopping through multiple layers of water molecules physisorbed on chemisorbed water molecules through single hydrogen bonding. In addition, the operational stability of the wearable humidity sensors developed in this study was investigated by analyzing the capacitive characteristics of the PIL sensing material at various temperatures (20–60 °C), even under a bending state and after 20,000 bending-stress cycles with a strain of 30%. Finally, a real-time humidity monitoring system integrated with signal processing and wireless communication modules was demonstrated by analyzing real-time capacitive signals acquired from a wearable humidity sensor attached to a human wrist and by visualizing processed humidity information on a user’s smartphone.