The development of tough hydrogels with high strength and resilience has become a major scientific interest in recent years. As conventional hydrogels based on single networks (SN) are usually very soft or (if stiff then) brittle, tough hydrogels can largely expand the application potential of these materials.
Among several strategies, the creation of double networks (DNs) has proven particularly effective for achieving high toughness in hydrogels. DN hydrogels typically consist of a rigid first network, often based on a highly swelling polyelectrolyte, which serves as a sacrificial structure to dissipate mechanical energy applied to the hydrogel. Surprisingly, the influence of the charge density on the mechanical performance of DN hydrogels remained unexplored.
In our contribution to the field, we synthesized a range of double-network hydrogels from charged and neutral acrylic monomers, which exhibited varying combinations of strength, deformability, and mechanical reversibility. Based on these observations, we suggested that the mechanical performance of DN hydrogels can be controlled by the charge density of the first network independently of its (covalent) crosslinking density.
We took steps to validate this hypothesis by designing poly(2-oxazoline)-based hydrogels in which the charge density was precisely controlled using a thiol-ene “click” reaction. This approach yielded tough hydrogels with nearly complete mechanical reversibility.
Our findings suggest that this strategy for DN hydrogel design is broadly applicable to other systems, opening new pathways for the development of tough hydrogels with tunable mechanical properties. Furthermore, we believe our results could contribute to a deeper understanding of hydrogel toughening mechanisms.