Smart Polymer Interfaces—From Biologically Inspired to Biologically Applied Designs

Nature has perfected ways to regulate efficient spatiotemporal functionality in biological systems through molecular designs of environmentally responsive materials. To accommodate the required functions, system components must coordinate changes in their shapes, mechanics, and dynamics. We take advantage of concepts borrowed from nature to design polymer interfaces with tunable functionality. Specifically, we investigate composite polymeric materials that emulate topographic and mechanical properties of biological systems on submicron scales, with the goal of using these synthetic systems in applications requiring controllable interfacial properties. For example, coating lithographically patterned silicon substrates with thermoresponsive poly(N-isopropylacrylamide) (or pNIPAM) films enables the fabrication of scaffolds with soft, nanostructured interfaces and tunable topographies. Above the LCST of pNIPAM, the films assume a collapsed structure and conform to the topography of the patterned substrate. However, below LCST the film swells in unusual ways depending on the confinement effects imposed by the substrate features. AFM measurements on these scaffolds show that variations in their Young modulus under different conditions mimic the mechanical properties of cell cytoskeletons and intercellular biopolymer networks. Complementary neutron reflectometry studies illustrate that the observed mechanical properties are commensurate with the hydration of the polymer films in and out of confinement. These composite scaffolds have wide-ranging applications, from directed cell growth to biosensors. In another example, we investigate the functionalization of nanochannels with pH-responsive polymer brushes, i.e. poly(dimethylaminoethyl methacrylate), for applications requiring regulated fluid flow and particle selectivity – two common biological functions. High-resolution specular and off-specular neutron reflectometry studies show that changes in the brush conformations in response to variations in the solution pH and ionic strength yield controllable channel gating. These systems offer advanced capabilities in tunable nanofluidic devices, particle filtration, and molecular sorting.