Progress in the mechanical enhancement of hydrogels: Fabrication strategies and underlying mechanisms

Hydrogels have become ideal materials in the nascent applications of tissue engineering, soft robots, drug delivery, and so forth. However, compared with biological tissues, the inherent heterogeneous microstructure and low density of polymer chains make hydrogels mechanically weak, severely limiting their use as structural materials. In recent decades, in order to meet the mechanical requirements of load-bearing biomaterials, significant research effort have been devoted to improving the mechanical parameters of hydrogels. To achieve this goal, fiber/fabric reinforced hydrogels, double network hydrogels, supramolecular-interaction-based hydrogels, hydrogels with well-aligned microstructures, and solvent induced robust hydrogels have been investigated. In this review, the fabrication strategies, the relationships between the structure and the mechanical properties of the resulting hydrogel, and the underlying enhancement mechanisms in various classes of hydrogel have been summarized. Here, the mechanisms behind these strategies rely on creating the mechanically effective networks, which are achieved by introducing a rigidly reinforced phase, synergistic network with distinctive features, sacrificial bonds, oriented hierarchical structures, or increased supramolecular interactions. Despite significant achievements toward strong and tough hydrogels, considerable important challenges remain, such as simultaneously achieving high strength, toughness, and high water content of hydrogels. It is believed that the already proposed strategies will push the development of hydrogels.