Tough and self-recoverable hydrogel membranes with micrometer-scale thickness are promising for biomedical applications, which, however, rarely be realized due to the intrinsic brittleness of hydrogels. In this work, for the first time, by combing noncovalent DN strategy and spin-coating method, we successfully fabricated thin (thickness: 5-100 µm), yet tough (work of extension at fracture: 10 5 -10 7 J m −3 ) and 100% self-recoverable hydrogel membranes with high water content (62-97 wt%) in large size (≈100 cm 2 ). Amphiphilic triblock copolymers, which form physical gels by self-assembly, were used for the first network. Linear polymers that physically associate with the hydrophilic midblocks of the first network, were chosen for the second network. The internetwork associations serve as reversible sacrificial bonds that impart toughness and self-recovery properties on the hydrogel membranes. The excellent mechanical properties of these obtained tough and thin gel membranes are comparable, or even superior to many biological membranes. The in vitro and in vivo tests show that these hydrogel membranes are biocompatible, and postoperative nonadhesive to neighboring organs. The excellent mechanical and biocompatible properties make these thin hydrogel membranes potentially suitable for use as biological or postoperative antiadhesive membranes.

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Living organisms share the ability to grow various microstructures on their surface to achieve functions. Here we present a force stamp method to grow microstructures on the surface of hydrogels based on a force-triggered polymerisation mechanism of double-network hydrogels. This method allows fast spatial modulation of the morphology and chemistry of the hydrogel surface within seconds for on-demand functions. We demonstrate the oriented growth of cells and directional transportation of water droplets on the engineered hydrogel surfaces. This force-triggered method to chemically engineer the hydrogel surfaces provides a new tool in addition to the conventional methods using light or heat, and will promote the wide application of hydrogels in various fields.

The hydrogel chemical structure at the gel-solution interface is important towards practical use, especially in tough double network (DN) hydrogels that have promising applications as structural biomaterials. In this work, we regulate the surface chemical structure of DN hydrogels and the surfacebulk transition by the molding substrate used for the synthesis of the 2 nd network. To characterize the surface and bulk structure, we combined ATR-FTIR and a newly developed microelectrode technique that probes the electric potential distribution within a hydrogel. We found that the polymerization on a repulsive substrate leads to the formation of a thin layer of 2 nd network on the surface of DN hydrogels, which makes the surface different from the bulk. By controlling the 2 nd network polymerization conditions and molding substrate, the surface-bulk transition region can be regulated, so that either only the 2 nd network or both networks are present at the DN hydrogel surface. Through these findings we gained a new insight on the structure formation at the DN hydrogel surface, and this leads to easy regulation of the hydrogel surface structure and properties.

Chitin is a biopolymer which has been proved to be a candidate as biomedical materials, yet the weak mechanical properties limited their potentials seriously. In this work, a chitin-based doublenetwork (DN) hydrogel was designed as a potential superficial repairing material. The hydrogel was synthesized through double-network (DN) strategy composing hybrid regenerated chitin nanofibers (RCNs)-poly (ethylene glycol diglycidyl ether) (PEGDE) as the first network and polyacrylamide (PAAm) as the second network. The hybrid RCNs-PEGDE/PAAm DN hydrogel was strong and tough, possessing Young's modulus (elasticity) E 0.097 ± 0.020 MPa, fracture stress σf 0.449 ± 0.025 MPa, and work of fracture Wf 5.75 ± 0.35 MJ• m -3 . The obtained DN hydrogel was strong enough for surgical requirements in the usage of soft tissue scaffolds. In addition, the chitin endowed the DN hydrogel with good bacteria resistance and accelerated fibroblast proliferation, which NIH3T3 cell number increased nearly 5 times within 3 days.Subcutaneous implantation studies showed that the DN hydrogel did not induce inflammation after 4 weeks, suggesting a good biosafety in vivo. These results indicated that the hybrid RCNs-PEGDE/PAAm DN hydrogel had great prospect as rapid soft tissue repairing materials.