Living tissues, such as muscle, autonomously grow and remodel themselves to adapt to their surrounding mechanical environment through metabolic processes. By contrast, typical synthetic materials cannot grow and reconstruct their structures once formed. We propose a strategy for developing “self-growing” polymeric materials that respond to repetitive mechanical stress through an effective mechanochemical transduction. Robust double-network hydrogels provided with a sustained monomer supply undergo self-growth, and the materials are substantially strengthened under repetitive loading through a structural destruction-reconstruction process. This strategy also endows the hydrogels with tailored functions at desired positions by mechanical stamping. This work may pave the way for the development of self-growing gel materials for applications such as soft robots and intelligent devices.

Double-network (DN) gels and elastomers, which consist of two (or more) rubbery polymer networks with contrasting physical properties, have received significant attention as they are extremely tough soft materials. The 1 st network of tough DN materials should be more brittle and weaker than the 2 nd network. In this paper, we reexamined the structural requirements of the covalently-cross-linked 1 st network of tough DN materials and established a non-prestretching strategy. While prestretching of network strands has been considered necessary for preparation of the brittle and weak 1 st network, we found that a non-prestretched network having a short strand length and low strand density can be used as the brittle and weak 1 st network for preparation of both tough DN gels and elastomers. This work can further expand the chemical and mechanical diversity of DN materials.