Fig. 4. Structure and properties of HN-PH_n HN hydrogels cross-linked by the peptide-Zn²⁺ coordination complexes.

(A) Schematic illustration of the network structure of HN-PH_n hydrogels. The network comprises covalent bonds as the primary cross-linkers and ligand-metal interactions as the secondary cross-linkers. (B) Optical images of the HN-PH₁ (top), HN-PH₃ (middle), and HN-PH₆ (bottom) hydrogels under a compression-relaxation cycle. The HN-PH₁ and HN-PH₃ gels were fractured, whereas the HN-PH₆ gel was almost fully recovered. (C) Optical images of the HN-PH₆ gel under an extreme compressive condition (compressed to >70% strain for 100 times at 1.6 Hz). (D) Optical images of the HN-PH₆ gel under an extreme tensile condition (stretched to >150% strain for 100 times at 1.6 Hz). (E) Optical image of the HN-PH₆ gel twisted into a spiral shape. (F) Optical images of the HN-PH₆ gel compressed with a sharp blade and relaxed. No detectable cut was observed on the gel. PAM, polyacrylamide. Photo credits: Wenxu Sun, Nanjing University.