Highly tough and fast recoverable double network hydrogels

Hydrogels consist of a three-dimensional network structure and represent a class of wet and soft materials. It is well known that double network hydrogels exhibit excellent mechanical properties, offering potential applications not only in articular cartilage. Articular cartilage is a dense connective tissue of collagen fibers constituting a semi-circular frame combined with the bone. Here, we demonstrate a simple strategy to synthesize the CS-g-PAA/PEI-0.10 double network hydrogels, with varying contents of ethylene imine polymer (PEI) in the chitosan /acrylic acid (CS/AA) solution. The environmental SEM imaging reveals that the CS-g-PAA/PEI-0.10 double network hydrogels have the homogeneous porous network and the excellent mechanical properties under compression. The underlying mechanism is that the second network acts as the sacrificial network to make the first network rapidly recoverable, allowing the compressive stress to fast release in seconds. These favorable mechanical properties enable the high application potentials in articular cartilage.