Plasma polymerized hydrogel thin films for applications in sensors and actuators

Abstract

Plasma polymerization was used to produce thermoresponsive, hydrogel films of N-Isopropylacrylamide (NIPAAm) in a single step. Through variation of reactor conditions such as deposition pressure and substrate temperature, physicochemical properties of the hydrogel films such as crosslink density and thus swelling could be controlled. Chemical bonding structures in plasma polymerized NIPAAm were studied using Fourier transform infrared spectroscopy (FTIR). Contact angle goniometry and quartz crystal microbalance with dissipation monitoring were used to confirm the existence of a hydrophilic-hydrophobic transition in plasma polymerized NIPAAm thin films, analogous to the lower critical solution temperature (LCST) transition in linear, uncrosslinked chains. Hydrogen bonding in NIPAAm thin films was found to control the moisture uptake capacity; films prepared at higher substrate temperatures and lower reactor pressures, and hence believed to possess greater crosslink density, showed the highest moisture uptake capacity in ambient humidity. Free volume characteristics of NIPAAm thin films were studied using Doppler broadening energy spectroscopy (DBES). Furthermore, a novel, electrophoretic procedure was conceived to incorporate biomolecules such as antibodies in plasma polymerized NIPAAm films for use as sensing layers in vapor phase, surface acoustic wave sensors.