LAUSR

A transparent and flexible cellulose/KOH composite ionic film (CKF) is fabricated as humidity sensor. CKF exhibits high optical transmittance (87.14 % at 550 nm), which has rarely been reported among humidity sensors, due to the small pore size of 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 varies over 200 times, response/recovery times of 6.0/10.8 s which are shorter than majority of the reported values, as well as hysteresis error of 0.57 % which is significantly less than literatures. Furthermore, CKF is insensitive to both temperature (10~70 °C) and pressure (0~120 kPa), indicating high selectivity as humidity sensors. In both the noncontact 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 of human skin, and demonstrate the potential of CKF-based moisture detector as a promising modular component in integrated intelligent wearable equipment.