Fabrication of Active Surfaces with Metastable Microgel Layers Formed during Breath Figure Templating

Patterned porous surfaces with responsive functionalities are fabricated by a thermoresponsive microgel-assisted breath figure (BF) process. When water droplets submerge into a polystyrene (PS) solution during formation of a porous surface by the bottom-up BF process, poly(N-isopropylacrylamide)-co-acrylic acid (PNIPAm-co-AA) microgels dispersed in the solution spontaneously assemble at the water-organic interfaces like "Pickering emulsions", reinforced by capillary flow. The conformal layer of PNIPAm-co-AA microgels lining the pores appears in images from a scanning electron microscope (SEM) either as a smooth surface layer (L) or as an array of domelike protrusions (D), depending on the conditions at which the sample was dried for SEM. The change between L and D morphology correlates with the volume phase transition behavior of the microgels freely suspended: drying at a temperature below the volume phase transition temperature (VPTT) gives L, and the D morphology is formed by drying at a temperature greater than the VPTT of PNIPAm-co-AA microgels. The morphological transition is shown to accompany a significant change in surface contact angle (CA) relative to a corresponding pore layer made of PS, with L having a CA that is reduced by 85° relative to PS, while the decrease is only 22° for D. Porous structures with morphologically responsive surfaces could find application in biocatalysis or tissue engineering, for example, with functional enzymes sequestered when microgels are collaped and accessible when the microgels are swollen.