Flexible and transparent humidity sensors based on hyperbranched poly(ionic liquid)s for wearable sensing

The rapid development of wearable electronics is driving the exploration of flexible and transparent humidity sensors. However, the limited humidity response and mechanical flexibility of sensitive materials hinder the normal operation of the device. In this research, flexible humidity sensors with a fast response time (1 s) were prepared using transparent imidazolium-based hyperbranched poly(ionic liquid)s (HPIL). The sensor performance was improved by regulating the counter anions with different hydrophobicity, such as trifluoromethanesulfonimide ion (TFSI-). The HPIL-TFSI sensor featured an improved response under low humidity condition (6%∼33% RH) with good linearity, excellent bending humidity response, mechanical stability, and long-term stability over a wide humidity range from 6% to 98% RH. Moreover, accurate sensing was achieved for respiratory monitoring, non-contact sensing, and detecting moisture on patched skin. The HPIL-TFSI sensor paves the way for the production of high-performance wearable sensor platforms.