Effective porosity and fluid flow in macroporous ultrasoft hydrogels: An experimental characterization

Hydrogels are the preferred materials for applications mimicking soft tissues due to their high water content and tunable mechanical properties. The state of the water in these hydrated networks governs their response to mechanical loading through coupled interstitial flow and large deformations of the solid network. Reliable experimental methods for quantifying the fraction of mobile fluid during mechanical deformation remain limited. Within the theoretical framework of mixture theory, we describe hydrogels as hydrated biphasic media consisting of a deformable incompressible solid matrix and a mobile fluid phase. We developed a mechanical testing protocol that enables the experimental separation of solid and fluid contributions under loading. The method is demonstrated using biocompatible and highly versatile hydrogel phantoms of varying compositions. Controlled, incremental drained confined compression of the hydrogel samples results in free-water fractions of approximately 40%, 60%, and 77%, reflecting the systematic influence of the polymer content on the porosity and fluid mobility. Comparison with cryo-SEM-derived surface porosity reveals statistically significant differences and highlights the scale-dependent sensitivity of surface measurements compared to bulk measurements. This study introduces a new mechanical method for quantifying the free-water fraction in macroporous, ultrasoft, highly hydrated biomaterials. Furthermore, the multi-step protocols enable the separation of dissipative, fluid-related relaxation from the equilibrium response of the solid skeleton, allowing direct calibration of constitutive models for macroporous soft solids. The proposed method provides a reliable basis for the development and optimization of hydrogels for applications where fluid transport is critical, such as neural interfaces, bioelectronic platforms, and tissue-engineered constructs. Statement of significance Water in hydrogels strongly determines their mechanical and transport behaviors, but quantitative measures of the mobile fluid fraction are lacking. We developed a new mechanical testing protocol that enables the experimental separation of solid and fluid contributions under loading in ultrasoft macroporous biomaterials. Validated on different phase-templated composite hydrogels and benchmarked against cryo-SEM-derived porosity, the method provides an experimentally derived effective porosity that can be used in biphasic constitutive models. This work bridges imaging and continuum modeling, with the goal of improving predictive simulations and informing design choices. Furthermore, it serves as basis to optimize the engineering of soft devices used for controlled fluid delivery, responsive scaffolds, and surface-conforming interfaces.