Light-Actuated Anisotropic Microactuators from CNT/Hydrogel Nanocomposites

Abstract

Over the past decades, functional hydrogels that respond to a variety of mechanical and chemical stimuli with a volume change of more than 100% have been developed. Despite this impressive behavior, practical applications of conventional hydrogels are limited by the need to transform their isotropic swelling/contraction into useful deformations, as well as their slow response times. Here, these challenges are addressed by combining poly(N-isopropylacrylamide) (PNIPAM), a widely used temperature-responsive polymer, with carbon nanotubes (CNTs). To ensure strong PNIPAM-CNT cohesion, the hydrogel is synthesized directly on the CNT surfaces using in-situ redox polymerization. The anisotropy of vertically-aligned CNT forests is used to transform the isotropic (de)swelling of PNIPAM into anisotropic motion. This material combination is particularly attractive because the high optical absorption and heat conductivity of carbon nanotubes converts light irradiation into PNIPAM actuation. A wide variety of CNT-skeleton microstructures are tested to reveal a range of actuation behaviors. We demonstrate fast reversible movement, active switching from low to high light absorption states, lattice shape changes, and good cycling stability.