One-step synthesis of PHEMA hydrogel films capable of generating highly ordered wrinkling patterns

Poly(2-hydroxyethyl methacrylate) (PHEMA) films with gradient crosslinking density are the only hydrogel films capable of generating regular wrinkling patterns upon swelling, however, they were synthesized via a complicated two-stage photopolymerization. In an effort to develop a simple synthesis method, we found that one-step UV-curing of HEMA liquid always results in films with corrugated surface, which is a common issue when curing liquid prepolymers. We further found that the introduction of a physical network in the prepolymer solution could prevent the surface from corrugating during UV curing. For this purpose, linear PHEMA was added into the prepolymer solutions, and PHEMA films with a smooth surface were successfully synthesized by a simple one-step UV curing. The resulting films generate various swelling-induced wrinkling patterns, including random worms, peanuts and long-range ordered hexagons, depending on the contents of crosslinker and linear PHEMA in the prepolymer solution. Particularly, large area, highly ordered, honeycomb-like wrinkling patterns were obtained from films with proper content of crosslinker and linear PHEMA. These regular patterns are expected to find applications such as the fabrication of uniform multicellular spheroids.