Double-Network Tough Hydrogels: A Brief Review on Achievements and Challenges

Abstract

This brief review attempts to summarize research advances in the mechanical toughness and structures of double-network (DN) hydrogels. The focus is to provide a critical and concise discussion on the toughening mechanisms, damage recoverability, stress relaxation, and biomedical applications of tough DN hydrogel systems. Both conventional DN hydrogel with two covalently cross-linked networks and novel DN systems consisting of physical and reversible cross-links are discussed and compared. Covalently cross-linked hydrogels are tough but damage-irreversible. Physically cross-linked hydrogels are damage-recoverable but exhibit mechanical instability, as reflected by stress relaxation tests. This remains one significant challenge to be addressed by future research studies to realize the load-sustaining applications proposed for tough hydrogels. With their special structure and superior mechanical properties, DN hydrogels have great potential for biomedical applications, and many DN systems are now fabricated with 3D printing techniques.

Keywords: 3D printing; double network; recoverability; stress relaxation; toughness.

Figure 1

Schematic of Lake–Thomas theory. The black dots represent the carbon atoms, while the lines connecting two adjacent black dots represent the carbon–carbon bonds in the polymer strand. When the crack propagates and breaks one carbon–carbon bond, the bond energy of all backbone bonds will be dissipated. The fracture toughness of the broken strand is enhanced by the number of backbone bonds in the strand.

Figure 2

Damage process of DN hydrogels. The multiple damages in the 1st network dissipate large amounts of energy and lead to irreversible hysteresis in load–unload tests.

Figure 3

Schematic shows that (a) alginate chains are ionically chelated by Ca²⁺, and (b) under mechanical stress, the alginate–Ca²⁺ connections are pulled apart, and the fracture of these physical bonds dissipates mechanical energy; (c) upon removal of mechanical stress, the ionic cross-links are able to recover.

Figure 4

Schematics of stress relaxation tests for covalently cross-linked and physically cross-linked hydrogels. In the test, the hydrogel samples are stretched to a set strain and held to observe if the stress decays as a function of time. Physically cross-linked hydrogels exhibit substantial stress relaxation (mechanical instability). Covalently cross-linked hydrogels demonstrate small stress relaxation.