Self-Wrinkling in Polyacrylamide Hydrogel Bilayers.

Swelling of a gel film attached to a soft substrate can induce surface instability, which results in the formation of highly ordered patterns such as wrinkles and folds. This phenomenon has been exploited to fabricate functional devices and rationalize morphogenesis. However, obtaining centimeter-scale patterns without immersing the film in a solvent remains challenging. Here, we show that wrinkles with wavelengths of up to a few centimeters can be spontaneously created during the open-air fabrication of film-substrate bilayers of polyacrylamide (PAAm) hydrogels. When the film of an aqueous pregel solution of acrylamide prepared on the PAAm hydrogel substrate undergoes open-air gelation, hexagonally packed dimples initially emerge on the surface, which later evolve into randomly oriented wrinkles. The formation of such self-organized patterns can be attributed to the surface instability resulting from autonomous water transport in the bilayer system during open-air fabrication. The temporal evolution of the patterns can be ascribed to an increase in overstress in the hydrogel film due to continued water uptake. The wrinkle wavelength can be controlled in the centimeter-scale range by adjusting the film thickness of the aqueous pregel solution. Our self-wrinkling method provides a simple mechanism for the generation of swelling-induced centimeter-scale wrinkles without requiring an external solvent, which is unachievable with conventional approaches.