Thermoresponsive Chemical Connectors Based on Hybrid Nanowire Forests

Abstract

Microand nanostructured surfaces found in biological systems have inspired the fabrication of functional materials with unique optical, chemical, and mechanical properties. Inspired by the nanofibrillar structures of gecko adhesives, we recently reported self-selective, chemical connectors (i.e., fasteners) based on interpenetrating nanowire (NW) forests, which primarily use the highly tunable van der Waals (vdW) interactions to enable efficient binding of components at both macroand microscales. Of particular interest to certain practical applications are programmable fasteners that can change their adhesion properties on command, for example, in response to external stimuli. In this regard, herein, we report programmable NW fasteners that reversibly change their wet adhesion strength in response to a thermal change of the environment. The thermoresponsive NW fasteners are based on core/multishell hybrid NW forests with an outer shell of poly(N-isopropylacrylamide) (PNIPAM). PNIPAM is a thermoresponsive hydrogel with a lower critical solution temperature (LCST) of approximately 32 8C in water. Specifically, at room temperature, PNIPAM absorbs water, resulting in the swelling of the polymer and hydrophilic surface properties. However, PNIPAM shrinks at temperatures higher than the LCSTand transforms to a hydrophobic state. We utilized this well known property of PNIPAM in conjunction with high aspect ratio nanofibrillar structures to enable programmable fasteners with tunable properties. The fabrication procedure for the thermoresponsive NW fasteners is outlined in Figure 1a. First, Ge/parylene core/ shell NW forests were prepared by growing Ge NW forests