Patterning Porosity in Hydrogels by Arresting Phase Separation.

Independent control over phase separation and photo-cross-linking allows the structure and porosity of hydrogels to be patterned in a single photolithographic step. This observation is based upon a temperature-triggered spinodal decomposition of a ternary mixture of water, salt, and polymer into a salt-rich aqueous phase and a polymer-rich phase. Importantly, subsequent exposure to light arrests the phase separation, allowing the porosity state to be frozen in a cross-linked hydrogel network. Tuning the delay between the application of heat and illumination allows the pore size to be tuned between 400 nm and 4 μm. By utilizing gray-scale photomasks, a single process can be used to define regions of pure hydrogel, porous hydrogel with a programmed average pore size, and blank substrate with no hydrogel. In addition to representing a combination of top-down and bottom-up processes that enables the realization of complex samples, the simplicity of this process and the versatility of the resultant patterns could provide a useful capability for the definition of hydrogel samples for the development of advanced biomaterials.