Dynamic Switching of Helical Microgel Ribbons

We report on a microscopic poly(N-isopropylacrylamide) hydrogel ribbon, coated by a thin gold layer, that shows helical coiling. Confined swelling and shrinkage of the hydrogel below and above its characteristic volume phase transition leads to a temperature actuated reversal of the sense of the helix. The extent and the shape of the winding are controlled by the dimensions and mechanical properties of the bilayer ribbon. We focus on a cylindrical helix geometry and monitor the morphing under equilibrium and nonequilibrium conditions, that is, when the temperature changes faster than the volume (millisecond range). For slow temperature variations, the water release and uptake follow the equilibrium transition trajectory determined by the time needed for the diffusion of water into and out of the microscopic gel. Much faster variations of the temperature are accomplished by internal heating of embedded gold nanorods by IR-light irradiation. This causes elastic stresses that strongly affect the motions. This method enables well reproducible deviations from the equilibrium transition path and allows us to control rather precisely the spatiotemporal transformation in a cyclic repetitive process. Actuation and response are sensitive to small variations of temperature and composition of the aqueous sol in which the gel is immersed. The principle as described may be used to detect specific analytes that bind either to the surface of the gold layer or within the gel and can modify the interaction between the water and the gel. The reported nonequilibrium morphing implies that the system dissipates energy and may also be able to perform work as required for a microscopic motor.