Creation of Double Network Hydrogels with Extremely High Strength and Its Anomalous Fracture Mechanism

Hydrogels have attracted much attention as potential soft and wet materials because of their unique properties, such as low surface friction and volume phase transition. However, conventional hydrogels are mechanically too weak and unsuitable for practical uses. Inspired by the structure of articular cartilage, a natural tough hydrogels, we have created novel hydrogels with extremely high mechanical strength. The gels are a kind of interpenetrating networks (IPN) consisting of rigid polyelectrolyte and soft neutral polymers, and show a compressive fracture stress of 60 MPa, 3000% of tensile strain and 2500 J/m(2) of fracture energy. Such mechanical properties cannot be explained by the standard theories about fracture of soft polymer systems. In this paper, we discuss, based on our recent papers, the synthesis conditions to obtain the tough hydrogels and physical mechanism under the high toughness. The essential point is that partial breakage of the rigid component causes formation of yielding zone around crack tip, which leads to a large dissipation for the crack propagation. We also describe several results on tissue engineering application of the hydrogels.