Lyotropic "Salty" Tuning for Straightforward Diversification and Anisotropy in Hydrogel Actuators.

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