Biomimetic polymeric film wrinkles reversibly like skin

Stay in the bathtub or pool a little too long or keep a water-soaked bandage wrapped on your finger for an extended period, and your skin will look like a shriveled-up prune. That water-induced wrinkling process has inspired materials scientists at the University of Connecticut to design a series of polymeric materials that wrinkle in a controllable way upon exposure to humidity (Adv. Mater. 2017, DOI: 10.1002/adma.201700828). The advance deepens understanding of wrinkling dynamics and may lead to moisture sensors, optical coatings, light diffusers, and other devices in which functions are triggered by changes in humidity. To make the materials, Songshan Zeng, Luyi Sun, and coworkers prepared bilayers consisting of a stiff hydrophilic film of polyvinyl alcohol (PVA) tightly bound to a flexible hydrophobic film of polydimethylsiloxane. By varying the degree of PVA cross-linking and the thickness of the layers, the team designed samples that wrinkled in distinct ways because of moisture-induced stiffening and swelling. One of the materials wrinkled in a fully reversible way when humidified and later dried. Another material wrinkled but did not unwrinkle when dried. A third material wrinkled, then unwrinkled when it was dried but did not rewrinkle when humidified.