Repulsion of Polar Gels From Water: Hydration-Triggered Actuation, Self-Folding, and 3D Fabrication

Synthetic materials that mimic the ability of natural occurring features to self-actuate in response to different stimuli have wide applications in soft robotics, microdevices, drug delivery, regenerative medicine, and sensing. Here, unexpected and counter-intuitive findings are presented in which a strongly polyelectrolytic hydrogel repels from strong polar solvents upon partial exposure (e.g., partial hydration by water). This repulsion drives the actuation and self-folding of the gel, which results in rapid formation of different three-dimensional shapes by simply placing the corresponding two-dimensional films on water. A detailed investigation into the role of hydrogel chemistry, pH, and morphology on hydration-triggered actuation behavior of the gels and their nanocomposites is described. Finally, a computational model is developed in order to further elucidate mechanisms of actuation. Modeling partial hydration as a repulsive driving force, it tracks the evolution of the shape of the thin film that results from restoring elastic forces. Taken together, the results indicate that an interplay between elastic and Coulombic repulsive forces leads to seemingly unexpected behavior of actuation of strongly polyelectrolytic gels away from polar solvents, leading to a novel and simple fabrication strategy for diverse 3D devices.