Double network hydrogels with extremely high toughness and their applications

Polymer hydrogels attract attention as excellent soft and wet materials. However, common hydrogels are mechanically too soft and brittle to be used as load-bearing substances. By mimicking the structure of the articular cartilage, which is one of the native tough hydrogels, double network (DN) hydrogel with extremely high mechanical performance has been developed. Having high water content (about 90 wt%), DN gels consist of two types of polymer components with opposite physical natures: the minor component (the first network) abundantly cross-linked polyelectrolytes, and the major component (the second network) comprised of poorly cross-linked neutral polymers. Under suitable conditions, DN gels exhibit 0.1-1 MPa of elastic modulus, 20-60 MPa of compressive fracture stress, 1,000-2,000% of tensile strain, and 100-1,000 J m(-2) of fracture energy. These excellent mechanical properties are comparable to those of rubber and natural bio-tissues. This paper reviews the main principle of DN gels, including their preparation method, mechanical feature, and toughening mechanism. The processability and the applicability of DN hydrogels as biomaterials and as conductive materials are also discussed.