Thermally induced shape-shifting of micrometer scale polymeric fibers and meshes

Microscale shapes-shifting systems that perform a geometry change through active deformation can be found in a variety of natural systems on the microscale level. For example, muscle cells microscopically deform upon an electric stimulus, which can lead, eventually, to motion in the macroscale. Fabrication of synthetic microscale shape-shifters that exhibit similar behavior requires control of the chemistry, and in particular development of stimuli-responsive materials, but also demands control over the morphology of the system and over the arrangement and structuring in the microscale level.

In this work, we present the fabrication of cross-linked poly(N-isopropylacrylamide) (PNIPAAM) hydrogel fibers with a diameter of 1 µm via the electrospinning approach. The PNIPAAM fibers exhibit a significant reduction in volume when heated above 32°C and consequently a reversible and substantial change in fibers’ dimensions and in their alignment. We demonstrate the use of such fibers for obtaining 2D thermo-responsive meshes, which exhibit material memory and a fast transition between two well-defined spatial arrangements as a function of the temperature of the system. Examination of the shape-shifting behavior by following the strain of the fibers and the shape-shifting velocity throughout the process indicates a significant change from the behavior of bulk 2D structures of similar dimensions. Such systems, which can shape-shift in the microscale in a fast, controlled, and well-defined manner, can lead to new interactive materials including micro thermal mechanical systems, soft robotics, and synthetic muscles.