We synthesized a thin layer of cross-linked poly(ethylene glycol) covalently attached to the surface of an elastomeric polymer. Immersing the polymer in water caused the hydrogel to absorb water and swell, which created strain on the elastomeric back-layer. The interplay between expansion of the hydrogel and the distortion of the elastomer created the spontaneous buckling of the poly(ethylene glycol) layer, which formed a beautiful pattern of wrinkles. The physical phenomenon responsible for this buckling behavior emerges in many places in nature, including: fingerprints and the wrinkles that form on the petals of flowers and the edges of leaves.
[Image courtesy of Ye Jin Eun and Douglas B. Weibel; University of Wisconsin-Madison]