Figure 1

Synthetic process and schematic structure of the polydopamine–polyacrylamide (PDA–PAM) hydrogel. (a) The PDA–PAM hydrogel was formed by a facile two-step process. Dopamine (DA) was polymerized by oxygen to form PDA under an alkali environment. After that, acrylamide (AM) was polymerized and crosslinked to form a polymer network to obstruct external oxygen and inhibit the oxidation of catechol groups. Photo (1): without DA prepolymerization, the hydrogel could not form. Photo (2): after DA prepolymerization, the hydrogel was cured. Photo (3): the hydrogel firmly adhered on the author’s arm. (b) Schematics of the molecular structure of the PDA–PAM hydrogel. The hydrogel was designed based on a single PAM network that was hybrid-crosslinked with covalent (N,N’-methylene bisacrylamide (BIS), gray circles) and reversible non-covalent bonds from PDA (black ovals). (1) The PDA chains were linked to the PAM network via interactions between free catechol groups and the amino groups of PAM (green triangles). (2) Reversible noncovalent bonds formed between the catechol groups of the PDA chains, including π–π stacking and hydrogen bonds, and were diffused uniformly in the PAM network. These reversible bonds contributed to the toughness and self-healing of the single network hydrogel.