Fundamentals of double network hydrogels.

Double network (DN) hydrogels as promising soft-and-tough materials intrinsically possess extraordinary mechanical strength and toughness due to their unique contrasting network structures, strong interpenetrating network entanglement, and efficient energy dissipation. It has been ∼11 years since the first PAMPS-PAAm DN hydrogel was developed, but the research and development of new DN hydrogels are still at a very early stage. A vast number of network monomers available in the current chemical inventory provide the possibility to design new DN gels and to explore the fundamental relationship of DN gels among network structures, mechanical properties, and toughening mechanisms, which help to derive new design principles for the next-generation tough hydrogels. In this review, we strive to highlight the development and fundamentals of DN gels covering from preparation methods, network structures, to toughening mechanisms over the last decade.