Macroscale double networks: highly dissipative soft composites

Hydrogels contain large amounts of water, making them useful in biomaterial applications. However, their inherent softness prevents their direct use in load-bearing applications. By incorporating toughening mechanisms through the double network concept, the mechanical properties of hydrogels have been greatly improved. In this Focus Review, our goal is to consider recent attempts to achieve hydrogel composites with further improved strength and toughness that could lead to the development of prosthetic biomaterials. We outline the way in which the double network concept improves the mechanical properties of gels and the specific mechanical traits that are enabled. We next review the current literature on soft composites, noting that the reinforcement mechanisms often differ from the double network concept, and summarize the types of properties that these materials can achieve. We also highlight the difficulties of working with hydrogels versus simple elastomers. Finally, we look at a recent subset of materials that utilize a mechanism analogous to the double network concept to achieve toughening on the macroscale. Macroscale double networks provide a unique opportunity to improve the mechanical properties of all soft materials for a wide range of applications. Sacrificial bonds break to dissipate energy and can increase the toughness of materials. Incorporating sacrificial bonds into hydrogels through the double network process enabled the first extremely tough hydrogels. In this Focus Review, we discuss the nature of sacrificial bonds, and how they can be used on the macroscale to enable tough soft composite materials. By matching the essence of the double network concept, we can make tough materials from macroscale composites for biomedical and engineering applications.