Water-Developable Ionic Liquid Polymer Enables Humidity-Sensing and Microscale Patterning

Abstract

Polymeric ionic liquids (i.e., PILs) have diverse sensing capabilities due to their potentially high ionic conductivity and tunable chemical response. However, integrating them into miniaturized sensor components often requires harsh nonaqueous developers, limiting eco-friendly microfabrication. To overcome this challenge, we designed a system that utilizes a UV-active thiol–ene click reaction to cross-link a photopatternable PIL, PAGE-TFSI–EMIm+, that enables device patterning under 365 nm exposure. This specific design leverages the material’s ionic nature to enable fully aqueous development, bridging the gap between precise microfabrication and processing with a lower environmental footprint in the development stage. The resulting films were successfully patterned with 12.8 μm resolution. As an independent proof-of-concept for the functional application of these materials, an impedance-type humidity sensor was fabricated by integrating interdigitated electrodes with patterned 3 mm-diameter PAGE-TFSI–EMIm+ films. The sensor demonstrated excellent sensitivity with low hysteresis (2.5% relative humidity (RH)) across a broad humidity range of 0–78% RH. Equivalent circuit fitting of electrochemical impedance spectroscopy (EIS) results suggest that the sensing mechanism is primarily governed by the variable ionic conductivity with humidity within the film. This approach underscores the potential of photopatterned PILs for advanced sensor architectures.