Spatiotemporal toughness modulation in hydrogels through on-demand cross-linking

Tough hydrogels are promising for soft robotics, bioelectronics, and tissue adhesives due to their exceptional resilience and biocompatibility, yet precise spatiotemporal control of their mechanics remains challenging. Here, we present a hydrogel platform that enables spatiotemporal modulation of toughness through a latent ionic cross-linking mechanism. By embedding calcium carbonate (CaCO₃) microparticles in alginate/polyacrylamide double-network hydrogels, we create a system where localized calcium release and thus ionic cross-linking can be programmed in both space and time. Spatial control is achieved by direct ink writing of CaCO₃, while temporal activation is triggered by glucono-δ-lactone, a biocompatible acidifier that releases calcium on demand. This strategy allows user-defined tuning of stiffness and toughness, enabling fabrication of three-dimensional (3D) hydrogels with tailored mechanical profiles. The resulting materials offer a versatile platform for anisotropic impact shielding, directional strain sensing, and 3D-printed tissue adhesives, representing a paradigm shift for adaptive, reconfigurable, and multifunctional soft materials.