Low-resistance silver microparticle-HEMA-PEGDA composites for humidity sensing

Abstract

We present a printable smart material that undergoes a large and reversible change in resistance in response to changes in relative humidity (RH). This material is a conductive polymer composite (CPC), comprised of commercial silver (Ag) microparticles that provides high conductivity to the films, and of a hygroscopic polymer matrix (polyhydroxyethylmethacrylate-co-polyethylene(glycol)diacrylate (HEMA-co-PEGDA)) that responds to humidity by undergoing an increase in volume (through swelling). We characterized the electrical resistivity and swelling behavior of the CPCs, formed sensors comprised of either thin films or patterned lines, and investigated how RH affects the electrical resistivity of the composite films for different compositions and geometries. To characterize the material responsive behavior, a data acquisition system was used to measure changes in resistance while the RH was controlled in a humidity chamber. When the humidity was increased from 25% to 95% RH, the resistance of the films increased up to 650%. Composites with a higher fraction of HEMA in the matrix were found to have both lower recovery times and less hysteresis than composites in which the matrix was pure PEGDA. Such materials can form the basis of inexpensive, printable humidity sensors.