Optically controllable materials: potential valves and actuators in microfluidics and MEMS

Composite materials consisting of optically active nanoparticles embedded within a thermally sensitive polymer selectively collapse when irradiated by light that matches the peak absorbtion wavelength of the nanoparticles. A copolymer of N-isopropylacrylamide and acrylamide exhibits a lower critical solution temperature (LCST) that is dependent on the relative amounts of each monomer in the polymer. Raising the temperature of the copolymer above the LCST initiates a rapid, reversible collapse. Optically active nanoparticles have been incorporated into NIPAAm/AAm hydrogels for the purpose of initiating a temperature increase via targeted absorption of near IR and green light. Gold nanoshells consist of a thin layer of gold surrounding a silica core, and altering the core/shell ratio allows the absorption of the nanoshells to be tuned over the visible and near IR spectrum. Gold colloid absorbs green light strongly at 532 nm. Two sets of composite hydrogels were fabricated, each containing one of the two nanoparticles. The nanoshell-composite hydrogels collapse in response to near-infrared irradiation but do not react to green light. The opposite behavior occurs for the colloid-composite hydrogels. This independent optical addressability should prove useful in a wide range of applications such as microfluidics and MEMS.