Hydrogels are three-dimensional hydrophilic polymer networks used in many areas, such as soil amelioration, medicine, hygiene, biomedical applications and so on. For practical utilities, most hydrogels must be able to operate against an external load, and thus hydrogels with high mechanical strength are required. However, the industrial and biomedical applications of ordinary hydrogels made from synthetic sources are strongly limited by their poor mechanical properties. In contrast, many biological gels com...