Modulation of the secondary structures of peptides gives rise to a range of natural peptide-based soft materials with outstanding mechanical properties. Here, we discuss detailed insights on how the secondary structure of tailor-made polypeptides governs the mechanical properties of polymeric hydrogels. To this end, we developed a series of polymeric hydrogels cross-linked by poly­(3-propyl-acrylate-glutamine) with tailorable secondary structuresα-helices and random coils. Interestingly, the hydrogels cross-linked by the α-helical cross-linker exhibit a high tensile strength and toughness because of the cooperative intramolecular hydrogen bonding along the polypeptide backbone. Furthermore, the α-helices endow the hydrogels with high resilience and rapid recovery because of their reversible cooperative hydrogen bonding. In contrast, the hydrogels cross-linked by the random coil cross-linker show inferior tensile strength and toughness. Our study establishes quantitative nanoscale/macroscale structure–property relationships in polymeric hydrogels and has important implications for the rational design of soft materials using polypeptides with a specific secondary structure.

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including ionic bond, [4][5][6][7][8][9] hydrogen bond, [10][11][12][13][14][15] coordination, [16][17][18] microcrystalline, [19][20][21] and hydrophobic interaction. [22][23][24][25] Although these non-covalent bonds can significantly improve the extensibility and self-recovery of hydrogels, they usually dramatically reduce the toughness. Covalent cross-linkers are always required for reaching high toughness, whereas the permanent fracture of covalent cross-links will lead to irreversible structure damage and significant decline of toughness and mechanical strength. Without the help from covalent cross-linkers, what kind of multiple physical cross-linker system can endow mono-network hydrogels with both rapid self-recovery and high toughness still remains a challenge.Our strategy is inspired by the nacre of abalone shell, which is constituted by alternating layers of CaCO 3 and biopolymers. The hard CaCO 3 layer provides strength and hardness and the soft biopolymer layer provides adhesion and ductility. The cooperative effect of two components leads to eminent strength and toughness. [26,27] This naturally suggests a new strategy to construct high toughness materials via combining both weak and strong non-covalent cross-linkers in one polymeric network. Polyacrylic acid will be an appropriate candidate for the versatile functions of carboxyl group to realize two different interactions in one polymeric network. First, carboxyl groups are able to form ionic bond with alkaline group, such as primary amine and pyridine. [28] Second, carboxyl groups can coordinate with metal ions, such as Al 3+ and Fe 3+ . [29] In this work, we present a new class of polyacrylic acid hydrogels cross-linked by coordinate bonds with Al 3+ and ionic bonds with triamine (diethylenetriamine, DETA). Compared to Al 3+ or DETA cross-linked hydrogels, this dual physically cross-linked hydrogel become super tough and the toughness of hydrogels can be recovered rapidly. In addition, the mechanical properties can be tuned over a wide range by using different molar ratios of cross-linkers. Experimental Section MaterialsDETA, acrylic acid (AAc), ammonium persulfate (APS), and aluminum chloride (AlCl 3 ) were purchased from Aladdin Many biological tissues including cartilage and tendons are composed of polymeric hydrogels, which exhibit high toughness and rapid self-recovery. However, developing hydrogels with both high toughness and rapid recovery remains a challenge. Inspired by the nacre of abalone shell, two non-covalent cross-linkers (Al 3+ and diethylenetriamine [DETA]) with different relaxation times are introduced into a polyacrylic acid network. Compared with mono cross-linked hydrogels, the dual physical cross-linked hydrogel exhibits both high toughness (work of extension at fracture up to 8.0 MJ m −3 ) and rapid self-recovery ability without loss of extensibility. Strong carboxylate-DETA ionic cross-links with longer relaxation time endow the hydrogels with high strength and help to localize the reformation of carboxylate-Al 3+ coordi...