Poroelastic swelling kinetics of thin hydrogel layers: comparison of theory and experiment

Thin poly(N-isopropylacrylamide) (PNIPAM) hydrogels were allowed to swell under two conditions: as freestanding layers and as substrate-attached layers. Through a combination of particle tracking and defocusing methods, the positions of beads embedded within the gels were monitored over time via fluorescence microscopy, providing a convenient method to track the kinetics of swelling for layers with thicknesses of the order 100 µm. These data are compared with the predictions of linear poroelastic theory, as specialized for polymer gels. This theory, along with a single set of material properties, accurately describes the observed swelling kinetics for both the freestanding and substrate-attached hydrogels. With the additional measurement of the substrate curvature induced by the swelling of the substrate-attached hydrogels, these experiments provide a simple route to completely characterize the material properties of the gel within the framework of linear poroelasticity, using only an optical microscope.