Combining both chemical and physical cross-links in a double-network hydrogel (DN gel) has emerged as a promising design strategy to obtain highly mechanically strong hydrogels. Unlike chemically cross-linked DN gels, little is known about the fracture process and toughening mechanisms of hybrid chemically physically linked DN gels. In this work, we engineered tough hybrid DN gels of agar/polyacrylamide (Agar/PAAm) by combining two types of cross-linked polymer networks: a physically linked, first agar network and a chemical-linked, second PAAm network. The resulting Agar/PAAm exhibited high stiffness of 313 kPa and high toughness of 1089 J/m2. We then specifically examined the effect of the first agar network on the mechanical properties of hybrid Agar/PAAm gels. We found that by controlling agar concentrations above a critical value, the physically linked agar network can simultaneously enhance both stiffness and toughness of Agar/PAAm DN gels, as evidenced by a linear relationship of elastic modulus and tearing energies of the gels as the increase of agar concentration. This toughening behavior is different from that of chemically linked DN gels. Complement to chemically linked DN gels, this work provides a different view for the design of new stiff and tough hydrogels using hybrid physical and chemical networks.

Injectable hydrogels are increasingly popular among researchers because of their in situ formability, in situ drug delivery, high targeting, and the ability to allow uniform incorporation of therapeutic molecules and/or...

Hydrogels have a wide range of applications in the fields of biomedicine, flexible electronics, and bionics. In this study, injectable and self-healable hydrogels were first prepared based on a dynamic covalent CC bond formed via the Knoevenagel condensation reaction between poly­(ethylene glycol) dicyanoacetate and water-soluble poly­(vanillin acrylate). Three kinds of catalysts (phosphate buffer, zeolitic imidazolate framework-8, and tertiary amine) were used in Knoevenagel condensation for preparing hydrogels. All hydrogels in this study could be formed in situ, and their gelation time ranged from seconds to minutes. The properties and application of hydrogels could be customized according to the type of catalyst employed. 3-[4,5-Dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) results indicated that all the components and hydrogels exhibited low toxicity, and the hydrogels could be used as 3D cell culture scaffolds. Because of the dynamic covalent CC bond formed by Knoevenagel condensation, the resultant hydrogels were found to be dynamic and showed good self-healing properties. This work presents a new dynamic covalent chemistry for the preparation of self-healable materials.