Buckling, symmetry breaking, and cavitation in periodically micro-structured hydrogel membranes.

We investigated swelling induced instabilities in porous membranes with a square array of micron-sized circular holes prepared from a pH and temperature dual-responsive hydrogel, poly(2-hydroxyethyl methacrylate-co-N-isopropylacrylamide-co-acrylic acid) (PHEMA-co-PNIPAAm-co-PAA). At room temperature (25 °C), the hydrogel swelled to ∼1.5 to 8 times of its dried volume when pH was increased from 2 to 7. Within this regime, we observed four distinctive morphologies of the hydrogel membrane, including a "breathing" mode of the membrane having circular pore arrays, a buckled pore array of alternating mutually orthogonal ellipses, twisted snap-shut pores forming "S" shaped slits, and cusps formed in local regions that perturbed the 2D periodicity of the hydrogel membrane. Using a 3D confocal imaging technique, we followed the post-buckling behaviors of the porous membranes and investigated the pattern evolution process as a function of pH. Amplification of buckling and symmetry breaking were observed when we increased the pH of the buffer solutions from pH 4.0 to 5.0, leading to the transition from an achiral buckled state (pH 4.0) to a chiral twisted state (pH 5.0) driven by the compaction of the hydrogel domains within the space to completely close the pores. When the pH of the aqueous environment was further increased to 7, star-shaped patterns appeared randomly in the film, where the hydrogel domains were compressed by the adjacent neighbors, thus resulting in out-of-plane deformation. Finally, we demonstrated the temperature-dependent reversible switching of the hydrogel membrane among the chiral twisted state, buckled state, and circular state via changing the temperature between 20 °C and 45 °C.