Creasing instability of surface-attached hydrogels

The unidirectional expansion of a thin surface-attached polymer gel upon swelling by solvent generates a biaxial compressive stress within the gel. For sufficiently large stresses, a mechanical instability can occur in which the free surface locally buckles and folds against itself to form creases. This instability has important implications for the design of biomaterials, smart surfaces, and sensors, since it places a fundamental limit on the amount of swelling that a surface-attached polymer layer may undergo without forming topographical features. However, while this instability was first observed more than a century ago, the amount of compression necessary to form creases has never been systematically studied. Using a model system of poly(acrylamide-co-sodium acrylate) hydrogels, we establish that the onset of creasing corresponds to an effective linear compressive strain of ∼ 0.33, or a change in thickness by a factor of ∼ 2. Remarkably, this value varies only slightly with modulus over a range of ∼ 0.6-24 kPa and is independent of gel thickness from 3 μm-1 mm, in excellent agreement with theoretical predictions. This instability is reversible, with creases disappearing as the degree of swelling is lowered, but surfaces exhibit a significant memory for crease locations when subsequently re-swelled.