Femtosecond laser writing of plasmonic nanoparticles inside PNIPAM microgels for light-driven 3D soft actuators

Herein, three-dimensional (3D) incorporation of plasmonic Ag nanoparticles was performed inside temperature-responsive poly(N-isopropylacrylamide) microgels using near-infrared femtosecond laser multi-photon reduction. The nanoparticles, formed by laser writing at lower doses, exhibited intense plasmonic absorption in the gels around 420 nm wavelength. Light-induced local shrinking of up to 86%, under assumption of isotropic shrinkage, in volume was achieved by the efficient photothermal conversion of Ag nanoparticles. Such shrinkages and deformation speeds strongly depended on the geometric design and 3D layout of the laser writing patterns of Ag nanoparticles inside the microgels. In particular, femtosecond laser incorporation enhanced the recovery speed by more than twice in comparison with the gels containing nanoparticles over the entire region. Laser direct incorporation allows for the control of the 3D position and extent and response speeds of gel deformation.