Double network hydrogels (DN gels) are unique interpenetrating polymer networks consisting of two kinds of polymer networks with strong asymmetric structure. Compared to single network hydrogels, DN gels exhibit extremely high strength (fracture tensile stress of 1 ~ 10 MPa and strain of 1000% ~ 2000%) and toughness (tearing fracture energy of 10² ~ 10³ J ·m^-2), due to their contrasting network structures where the first, brittle polyelectrolyte network is strongly entangled and interpenetrated with the second, soft, neutral polymer network. Fundamental understanding of the fracture process and toughening mechanisms of DN gels is critical for rational design of the next-generation of tough DN gels with desirable mechanical properties. Some DN gels illustrate large hysteresis, yielding/necking and softening phenomena, which can't be well interpreted by classical Lake-Thomas theory. Based on these experimental facts, Brown and Tanaka had suggested a "Damage Zone" model to explain the extraordinary high toughness of DN gels. Recently, "sacrificial bonds" theory, which proposed by Gong's group, has been well applied to design and prepare high toughness DN gels with novel nano-/microstructures. In the present of review, we focus on the toughening mechanisms of DN gels. The latest finding in this field are summarized, and the effect factors on toughness are discussed. In the end, the problems and research directions of the mechanisms of DN gels are pointed out.

Contents
1 Introduction
2 Experimental facts for toughening mechanisms of double network hydrogels
2.1 Hysteresis
2.2 Yielding and necking
2.3 Softening
3 Toughening mechanisms of double network hydrogels
3.1 Brown-tanaka model
3.2 Sacrificial bonds theory
4 Influence factors on toughness of double network
4.1 First network
4.2 Second network
4.3 Between the two network
4.4 Others
5 Conclusion and perspective