Strain Softening-Stiffening of Composite Hydrogels with Microfibrous Networks

Understanding the transient rheology of biopolymer hydrogels is crucial for applications in tissue engineering, pharmaceuticals, food science, biosensors, and 3D printing. Single-component hydrogels exhibit nonlinear rheological responses including strain softening-stiffening transition due to microstructure deformation, such as fiber bending, buckling, and sliding. However, the connection between the rheological behaviors of multicomponent hydrogels and the corresponding dynamic microstructure evolution remains poorly understood. In this study, we investigate a composite hydrogel system comprising two contrasting networks: a strain-stiffening agarose matrix and a strain-softening network of chitosan dendritic particles. The chitosan particles form microfibrous networks via van der Waals interactions in the composite hydrogels, and connect with the agarose networks through hydrogen bonding. Remarkably, the composite hydrogels exhibit a strain softening-stiffening transition with low agarose concentration (< 1 mg/mL), with the chitosan networks reinforcing the hydrogels without compromising fracture strain. Using confocal rheometry, we link the real-time microstructure evolution of the composite hydrogels with the rheological responses, revealing that the strain softening-stiffening transition arises from the interplay between agarose fiber stretching and chitosan fiber unbending. These findings provide new insights into the design of multifunctional hydrogels with tunable mechanical properties, advancing their potential in biomedical and industrial applications.