Robust bonding and one-step facile synthesis of tough hydrogels with desirable shape by virtue of the double network structureNote: it should be noted that in our previous papers,8,9,25,26G was calculated by using a different expression, which is not proper according to the definition

Robust bonding of a hydrogel in aqueous environment, either to another hydrogel or to a solid, is one of the major unsolved issues for the practical applications of hydrogels in various fields. Here we report robust bonding between a pair of hydrogel sheets, containing over 90 wt% of water, by applying the double-network (DN) structure. In the optimal condition, the peeling energy of the united gel sheets reaches 1200 J m −2 , which is comparable to the bulk fracture energy of a normal type of tough DN gels. This hydrogel bonding technique is also applied to form tough bonding between hydrogel and plastic plates. Furthermore, based on this technique, we have developed a facile method to synthesize robust double network hydrogels with any desirable free-shape from micro-gel precursors. These novel techniques will substantially merit the applications of the tough hydrogels in various fields, such as an artificial meniscus. A pair of hydrogel sheets was toughly bonded to each other by introducing the double network structure into the interface between the sheets.