Replica molding of high-aspect-ratio hydrogel pillar arrays and their stability in air and solvents.

High aspect-ratio hydrogel pillars are attractive in applications, such as tissue engineering, actuation, and sensing. By replica molding from respective partially polymerized precursor solutions, followed by photocross-linking with ethylene glycol dimethacrylate (EGDMA), we successfully fabricated three kinds of high-aspect-ratio (up to 12) hydrogel pillar arrays, including poly(hydroxyethyl methacrylate) (PHEMA)-based, poly(hydroxyethyl methacrylate-co-N-isopropylacrylamide) (PHEMA-co-PNIPA)-based, and poly(ethylene glycol dimethacrylate) (PEGDMA) systems. In the dry state, all hydrogel pillars were mechanically robust and maintained their structural integrity. When exposed to water, PHEMA-co-PNIPA conical pillar array was wetted and swollen by water, which drastically decreased its Young's modulus. The combination of reduction in stiffness and capillary force between pillars caused PHEMA-co-PNIPA conical pillars to collapse on the substrate after drying from water in air. In comparison, highly cross-linked PEGDMA conical pillars were not wetted by water and maintained high stability since their Young's modulus exceeded the critical modulus required for pattern collapse by capillary force. When exposed to a lower surface energy solvent, ethanol, however, the PEGDMA conical pillars surface became wettable and the pillars collapsed after drying due to capillary force. Depending on the pillar array geometry, PEGDMA pillars dried from ethanol collapsed either randomly in the case of conical pillar array or in groups of four in the case of more densely packed circular pillars.