Mechanically strong hydrogels with reversible behaviour under cyclic compression with MPa loading

Hydrogels are fascinating materials with high water content and low surface friction that can be tailored for numerous applications. However, their practical application is often hampered by an intrinsic mechanical weakness. Strategies for mechanically strong hydrogels have been developed, with double network (DN) hydrogels reaching remarkable compression resistance, although so far with loss of these properties under repeated compression. In this study we present DN hydrogels composed of cross-linked six arm star-shaped poly(ethylene oxide-stat-propylene oxide) (sPEOPO) primary networks and polyacrylamide (PAAm) as secondary network. These hydrogels possess high water content (>90 w/w%) and high compression strength of up to 5.6 MPa. Most importantly, they show a fully reversible behaviour in repeated loading-unloading experiments with 1 MPa maximal stress, which we attribute to the partial healing capacity of the primary network and the overall macroporous morphology of the gels.