A thin, deformable member may be fixed at one end, while another portion of the member rests on a hydrogel substance whose thickness changes depending on a characteristic of a liquid that permeates the hydrogel. When the hydrogel changes thickness and causes part of the member to tilt, a reflective surface on the member may reflect light in a different direction. Appropriate sensors may detect the change in the direction of the reflected light, allowing determination of the change in thickness, which in turn permits determination of the relevant characteristic of the liquid.