Poroviscoelastic characterization of polyacrylamide–alginate hydrogels across a broad range of length scales using torsional and depth sensing indentation tests

Abstract

The poroviscoelastic behavior of polyacrylamide (PAAm) and PAAm/alginate hydrogels is investigated through torsional and spherical indentation tests across multiple scales. The results reveal that both hydrogel types exhibit mechanical responses governed by viscoelastic and poroelastic mechanisms, with their relative contributions depending on contact scale and time. Torsional tests, which exclude fluid flow, showed purely viscoelastic relaxation, with increased stiffness and reduced relaxation in hydrogels with higher crosslinking density or alginate content. In indentation tests, relaxation time increased with contact size, indicating strong poroelastic influence. A constrained fitting model was applied to decouple both contributions and extract key material parameters such as diffusivity and relaxation modulus. Increased crosslinking enhanced both stiffness and diffusivity, while alginate addition improved stiffness but reduced diffusivity and Poisson's ratio. The consistency between results from both testing modes validates the proposed separation approach. Overall, this work demonstrates that multiscale poroviscoelastic characterization enables accurate understanding of the mechanical behavior of complex hydrogels.