Tough hydrogels with different toughening mechanisms and applications. Zhengyu Xu, Yanru Chen, Yi Cao, and Bin Xue review the design and mechanical properties of various tough hydrogels and their applications in tissue engineering, flexible electronics, and other fields. Hydrogels are biocompatible but often lack the mechanical properties of natural materials. To address this, researchers have developed tough hydrogels with high strength, stretchability, toughness, and fast recovery. These hydrogels are designed using various strategies, including sacrificial bonds, hierarchical architecture, network topology, and force-triggered length release. Sacrificial bonds, such as those in double-network hydrogels, allow energy dissipation and enhance toughness. Hierarchical architectures, like those with multi-scale structures, improve mechanical properties. Network topology, such as entanglements and molecular sliding, also contributes to toughness. Force-triggered length release mechanisms enable hydrogels to dissipate energy and recover quickly. These tough hydrogels have promising applications in tissue engineering, flexible electronics, and other fields. The review discusses the design principles, toughening mechanisms, and future development directions of tough hydrogels.Tough hydrogels with different toughening mechanisms and applications. Zhengyu Xu, Yanru Chen, Yi Cao, and Bin Xue review the design and mechanical properties of various tough hydrogels and their applications in tissue engineering, flexible electronics, and other fields. Hydrogels are biocompatible but often lack the mechanical properties of natural materials. To address this, researchers have developed tough hydrogels with high strength, stretchability, toughness, and fast recovery. These hydrogels are designed using various strategies, including sacrificial bonds, hierarchical architecture, network topology, and force-triggered length release. Sacrificial bonds, such as those in double-network hydrogels, allow energy dissipation and enhance toughness. Hierarchical architectures, like those with multi-scale structures, improve mechanical properties. Network topology, such as entanglements and molecular sliding, also contributes to toughness. Force-triggered length release mechanisms enable hydrogels to dissipate energy and recover quickly. These tough hydrogels have promising applications in tissue engineering, flexible electronics, and other fields. The review discusses the design principles, toughening mechanisms, and future development directions of tough hydrogels.