Mechanics of Physically Cross-Linked Hydrogels: Experiments and Theoretical Modeling

The remarkable ductility and enhanced toughness of metal-ligand-based hydrogels caused by physical cross-links that improve their mechanical properties have proven the efficacy of hydrogels in various engineering applications. Here, we bring the first comprehensive investigation of hydrogels under bulge testing. The multiaxial response of these materials is crucial for enhanced durability and load-bearing capability. In this study, we derive a hyperelastic constitutive model with a description of failure and validate it experimentally. The latter model is further used to analyze cavitation in these materials. This study demonstrates that incorporating imidazole-Ni²⁺ metal-ligand cross-links can significantly enhance several mechanical properties. For instance, increasing the imidazole content from 40 to 70 mol % improves the elastic modulus by 400% and the ultimate equibiaxial stress by 80%. The detailed experimental investigation reveals that the inflation of these hydrogels strongly depends on structural evolution. The current study paves the way for the development of novel experimental techniques and constitutive models to fine-tune the mechanical properties of hydrogels as per user requirements.