Static and dynamic operation of metal-coated hydrogel cantilever humidity sensors based on hygroscopic mismatch

Hydrogels, intrinsically sensitive materials to water molecules (i.e. moisture), have been mostly employed as a humidity sensing layer on top of elastic mechanical structures such as microcantilevers [1] where the hydrogel layer is relatively thin compared to the underlying structure. Until now, there has been no report attempting to make a humidity sensor based on relatively thick hydrogel microstructures with thin elastic coating that are preferred for increasing the curvature upon stress-induced bending and the frequency shift upon the volumetric mass uptake. Here, we fabricate metal-hydrogel composite microcantilevers, thin metal layer (150 nm copper) on two orders of magnitude thicker hydrogel (10 μm polyethyleneglycol diacrylate; PEGDA), to explore static and dynamic operation of microcantilever humidity sensors based on hygroscopic mismatch up to the relative humidity of 40% for the first time.