New stand-alone hygrometers based on the moisture-driven bending of a nanoclay and polyimide bilayer

Abstract

Natural bentonite (BNT) nanoclay and a polyimide (PI) film were used to fabricate durable bilayer-type actuators driven by airborne moisture. The BNT/PI actuators, optimized for the layer thickness and dimensions, displayed a linear response over the relative humidity (RH) range of 4 – 92% with the curvature sensitivity of 0.35 cm−1%RH−1. It also showed good selectivity to moisture against common organic solvent vapors and excellent reproducibility for up to 200 consecutive operations. Systematic investigations on the materials properties for the individual layers revealed that the bending occurred upon the adsorption of water vapor because the BNT layer underwent greater volume expansion and loss of stiffness than the PI layer. These BNT/PI actuators were successfully applied to create three different prototypes of user-friendly, stand-alone hygrometers: a clock-like hygrometer, an emoticon-display hygrometer, and a humidity-switch-based LED hygrometer.