Tough and fully recoverable hydrogels.

In natural cartilage, collagen fibers form the extracellular matrix, while aggrecan entangles with these fibers and provides cartilage with its osmotic properties, which are critical to resist cyclic compressive loads. In this paper, a hydrogel was fabricated via the entanglement of a bio-inspired nanostructure (chondroitin sulfate-coated vinyl silica nanoparticles, CS-SNP) within an agar/poly(acrylamide) double network hydrogel. The highly charged chondroitin sulfate groups provide additional compression resistance within the macromolecular chains, while the solid silica cores anchor these entanglements. The presence of the CS-SNP not only improved the compressive modulus, compressive strength, fracture toughness, and fatigue resistance of this hydrogel, but also ensured the full recovery of all these properties after thermal heating. This tough, fully recoverable, and robust hydrogel is a promising material for applications with strong mechanical requirements.