Lyotropic “Salty”\nTuning for Straightforward\nDiversification and Anisotropy in Hydrogel Actuators

The specific ion effect (SIE), the control of polymer\nsolubility\nin aqueous solutions by the added ions, has been a phenomenon known\nfor more than a century. The seemingly simple nature of the ion–polymer–water\ninteractions can lead to complex behaviors, which have also been exploited\nin many applications in biochemistry, electrochemistry, and energy\nharvesting. Here, we show an emerging diversification of actuation\nbehaviors in “salty” hydrogel and hydrogel-paper actuators.\nSIE controls not only the dehydration speeds but also the water diffusion\nand mechanical properties of the gels, leading to composite actuation\nbehavior. Most reported thermally activated hydrogel actuators suffer\nfrom expensive precursors or complex fabrication processes. This work\naddresses these issues by using a physicochemical effect displayed\nwithin an inexpensive gel with common salts. SIE-controlled anisotropic\nactuation in geometrically different systems provides a demonstration\nof how such physicochemical effects can lead to higher complexity\nin basic soft material design and hydrogel soft robotics.