Humidity-Responsive Photopolymer Actuators: Sorption, Swelling, Geometry, Hysteresis and Environmental Stability

Humidity-responsive photopolymer actuators convert water sorption into differential swelling, modulus change or internal stress. Their response depends on equilibrium uptake, diffusion kinetics, geometry and the path by which relative humidity is changed. Printed anisotropy and post-cure can alter free volume and hydrophilicity, while bilayer or gradient designs amplify small material strains into large curvature. Qualification therefore requires sorption and actuation to be measured together. Mass uptake, dimensional strain, curvature, response time and hysteresis over controlled adsorption-desorption cycles establish whether the device is reversible or merely history dependent. The evidence supports an operating humidity window, a geometry-specific response-time limit and a cycle-stability requirement tied to retained dry and wet reference states. ER-373.