Swelling-Induced Surface Patterns in Hydrogels with Gradient Crosslinking Density

Graphical Abstract

The dynamic evolution of a wide range of surface patterns, including a highly ordered hexagonal pattern in transit to peanut shape, lamellar, and random worm-like structures, is reported in poly (hydroxyethyl methacrylate) hydrogel films with a modulus gradient through film depth. The order of the patterns is determined by the crosslinker concentration and the characteristic wavelength is proportional to the initial gel thickness.

Abstract

Hydrogels with controlled surface patterns are useful for a range of applications, including in microdevices, sensors, coatings, and adhesives. In this work, a simple and robust method to generate a wide range of osmotically driven surface patterns, including random, lamellar, peanut, and hexagonal structures is developed. This method does not require the use of organic solvents for swelling, pre-patterning of the film surface, or coating of a second layer on the gel. The patterns are fabricated by exposing a photocurable formulation to light while open to air and then swelling, using oxygen inhibition of the radical polymerization at the surface to create a gradient of crosslinking with depth, which was confirmed by measuring the double bond conversion at the surface, surface mechanics, and molecule diffusion into the network. The modulus gradient, and hence osmotic pressure, is controlled by the crosslinker concentration, and the characteristic size of the patterns is determined by the initial film thickness. The patterns are stable in both swollen and dry states, creating a versatile approach that is useful for diverse polymers to create complex patterns with long-range order.