Swelling-induced instabilities in microscale, surface-confined poly(N-isopropylacryamide) hydrogels

A hydrogel is a three-dimensional hyperelastic polymer network that swells to a specific volume upon exposure to a penetrating solvent. If mechanical constraints interfere with the swelling process, anisotropic compressive stresses are generated, which may manifest in local or global instabilities. Herein, we employ confocal microscopy for the in situ, three-dimensional study of micron-scale hydrogels that are pinned to a solid substrate. Depending on the initial geometry of the hydrogel, four general modes of swelling-induced deformation were found: lateral differential swelling, local sinusoidal edge buckling, bulk sinusoidal buckling, and surface creasing. The transition between local edge buckling and bulk buckling is consistent with linear elastic theory; however, linear theory cannot be used to predict many details of the swollen structures. Whereas global buckling has a well-defined wavelength that depends on height of the hydrogel structure, edge buckling appears to be independent of height and depends on sample history. Moreover, edge buckling can appear in globally buckled structures, suggesting two different mechanisms for the two instabilities.