Flexible and Transparent Cellulose-Based Ionic Film as a Humidity Sensor

A transparent and flexible cellulose/KOH composite ionic film (CKF) is fabricated as a humidity sensor. CKF exhibits high optical transmittance (87.14% at 550 nm), which has rarely been reported among humidity sensors as a result of the small pore size of the cellulose matrix caused by water-evaporation-induced dense packing and uniform distribution of amorphous KOH via simply soaking–drying. CKF also possesses flexibility and robust mechanical property. The conductive CKF shows fast and reversible real-time response to relative humidity (RH) in the 11.3–97.3% RH range with conductance varying over 200 times, response/recovery times of 6.0/10.8 s, which are shorter than the majority of the reported values, as well as a hysteresis error of 0.57%, which is significantly less than that reported in the literature. Furthermore, CKF is insensitive to both the temperature (10–70 °C) and pressure (0–120 kPa), indicating high selectivity as humidity sensors. In both the non-contact fingertip moisture detection and breathing rate detection, the flexible and transparent CKF-based humidity sensor responds favorably to RH change. Moreover, a flexible and transparent CKF-based wearable skin moisture detector is assembled to measure the moisture of human skin in different situations, whose measurement is very close to the commercial detector. The results offer real-time moisture information on human skin and demonstrate the potential of a CKF-based moisture detector as a promising modular component in integrated intelligent wearable equipment.