Gel-Gel Interface Engineering for the Synthesis of Anisotropic Hydrogels with Designable Polymer Orientations.

Anisotropic hydrogels with designable structural complexity can exhibit sophisticated properties reminiscent of those found in living organisms. However, conventional synthetic methods typically require specific anisotropic additives and complicated processes, limiting design flexibility. Here, a simple and versatile strategy to synthesize anisotropic hydrogels with photo-designable orientations of polymer networks is developed by harnessing a "gel-gel interface," which can be generated through the intentional adhesion between hydrogels. This strategy originates from the serendipitous discovery: when a gel-gel interface is intentionally introduced into a hydrogel through a two-step polymerization, the polymer networks near the interface are spontaneously aligned perpendicular to the interface, as a result of the simultaneous swelling and fixation process during gel-to-gel adhesion. By employing a photo-initiator system to control the gel-gel interface, anisotropic hydrogels with both 2D and 3D designed polymer orientations, as well as anisotropic hydrogels with thermally switchable polymer orientations, are successfully synthesized. The gel-gel interface has long been regarded as merely an undesirable byproduct of gel adhesion, while the complementary bulk region of hydrogels has been the primary focus in constructing anisotropic hydrogels. In contrast, this work demonstrates the utility of the gel-gel interface, expanding design possibilities for next-generation hydrogels with designable structural complexity.