Controlled switching of the wetting behavior of biomimetic surfaces with hydrogel-supported nanostructures

An important feature of biological systems is their response to external stimuli with subsequent changes in properties and function. The ability to "engineer'' adaptiveness into next-generation materials is becoming a key requirement and challenge in chemistry, materials science and engineering. Recently we have described new hybrid nano/microstructures capable of dynamic actuation by a hydrogel "muscle''. Here we demonstrate the application of a variation of such biomimetic surfaces in controlled reversible switching of the surface wetting behavior. Arrays of rigid nanostructures were integrated with responsive hydrogel films by performing in situ polymerization in microscopic confinement of two surfaces. The attachment of hydrogel was achieved through a multifunctional polymeric anchoring layer. Using two different attachment strategies, several designs involving an array of either attached or free-standing nanocolumns embedded in the hydrogel film are described. We demonstrate a superhydrophobic-hydrophilic transition (so-called "direct response'') or a hydrophilic superhydrophobic transition ("reverse response''), respectively, upon the exposure of these two structures to water. We show that all the changes in the wetting behavior are reversible and the structures return to their original superhydrophobic or hydrophilic state upon drying. The ability to design surfaces with reversible changes in their wetting behavior may have exciting applications as "smart,'' responsive materials with tunable water-repelling or water-attracting properties.