A constitutive model for hydrogels with tunable mechanical properties by salting-out

Hydrogels with tunable mechanical properties hold significant potential for applications in various fields. Salting-out has proven to be an effective way for substantially and reversibly regulating the mechanical properties of hydrogels. In this study, we explored the evolution of the mechanical behaviors of Polyacrylamide/Chitosan (PAAm/CS) composite hydrogels with salting-out experimentally, and the results indicate that salting-out treatment can increase the tangent modulus by two orders of magnitude and effectively suppress the strain-softening behavior. However, to date, no constitutive model has been developed that can quantitatively describe the variation of hydrogel mechanical properties with salting-out. To quantify the experimental observations, we developed a micromechanism-based constitutive model. In this model, the stress of hydrogel is contributed by the matrix network and the tunable network, with the latter consisting of cluster domains and connecting domains. Based on the microscopic picture, with further salting-out, the modulus of the cluster domain increases and the volume fraction of the cluster domain decreases synergistically, resulting in the evolution of macroscopic properties. The present model captures the mechanical properties of various hydrogels well, and the finite element analysis exhibits ability to predict complex loading conditions. By designing a novel adjustable phononic crystal structure with PAAm/CS hydrogel, it is demonstrated that the current model will play an important role in guiding the preparation of tunable hydrogels and the application for functional devices.