All-printed wearable humidity sensor with hydrophilic polyvinylpyrrolidone film for mobile respiration monitoring

In this study, a capacitive-type wearable humidity sensor comprising a polyvinylpyrrolidone (PVP) film, as active sensing dielectric, and carbon nanotube (CNT) random networks, as electrodes, all-printed on flexible substrates in ambient air, is demonstrated for real-time respiration monitoring. The PVP-based humidity sensor achieved a high sensing response of 3.0 +/- 0.15 nF/RH%center dot cm(2) (where RH stands for relative humidity) and a sensitivity of 6.4 +/- 0.1 pF/%RH in the RH range of 40-90%. A significantly small hysteresis window, fast response/recovery (similar to 30 ms), and high reliability were noted. The sensing mechanism of the capacitive response toward humidity was investigated, and it is attributed to continuous proton hopping through water molecule layers. The operational stability of the proposed humidity sensor under temperature variations and mechanical deformation, was investigated to address existing issues related to wearable electronics. Finally, the feasibility of a mobile respiration monitoring device is demonstrated by developing an armband-type wireless-communication monitoring system equipped with the wearable humidity sensor. Real-time biophysical information can be analyzed by monitoring the changes in the respiration intensity and frequency, which were wirelessly transmitted to the user's mobile interface. We note that very few studies have focused on all-printed wearable humidity sensors consisting of pristine PVP and immobilized CNT random networks for real-time mobile respiration monitoring of human breath regardless of bent and relaxed state.