Relaxation Behavior and Structural Evolution of Hydrogels under Long-Term Tensile Loading in Air

Hydrogels are increasingly employed in applications involving tensile loading, such as wearable devices, soft robotics, and biomedical sensors. However, their long-term mechanical behavior in air-exposed environments remains underexplored. In this study, we derive a constitutive model to describe the stress relaxation of Ca-alginate hydrogels under long-term sustained uniaxial loading in air and validate it experimentally. The mechanical responses of the hydrogel encompass initial viscoelastic relaxation, followed by progressive stiffening, culminating in a sharp increase in final stress due to network shrinkage. In the proposed constitutive model, an empirical parameter Q, representing volume-changed energy, accurately captures the evolution of stress over time, effectively correlating dehydration kinetics and shrinkage during long-term loading with the structural changes in the hydrogel. Molecular simulations, LF-NMR, in situ Raman, SAXS, and SEM reveal the long-term tensile loading-dependent structural evolution, including promoting molecular aggregation, shifts in water mobility, and multiscale anisotropy in the hydrogel. These results provide mechanistic insights into the coupled dehydration-deformation processes in hydrogels and establish a predictive phenomenological framework for enhancing hydrogel performance and reliability in air-exposed applications.