Dual ionic cross-linked double network hydrogel with self-healing, conductive, and force sensitive properties

Conventional chemically cross-linked double network hydrogels (DN hydrogels) are considered as one of the toughest soft materials. However, they lack self-healing properties due to their non-reversible covalent bonds. In this work, we propose a new design strategy to synthesize a dual ionic cross-linked polyethylene glycol (PEG)/poly(acrylamide-co-acrylic acid) (PAMAA) DN hydrogels, consisting of an ionic cross-linked linear chain extension of PEG and a PAMAA-Fe3+ as the first and second physical networks, respectively. Due to the reversible non-covalent bonds in both networks (Fe3+ coordination interactions and hydrogen bonds), the DN hydrogels can achieve high mechanical properties (σf of ∼0.36 MPa and strain of ∼1350%) and complete self-healing ability within 12 h. Moreover, due to the free ions diffusion in the water medium of the DN hydrogels, the as-prepared hydrogels also exhibited electrical conductivity and stretch sensitivity. Our results offer a new design strategy to improve the strength and self-healing ability of DN hydrogels by introducing a dual ionic cross-linking within the network, which we hope will provide an alternative focus for the design of tough and self-healing hydrogels with desirable properties.