(171ad) Brittle-to-Ductile Rheology in Composite Hydrogels with a Microfibrous Network

Recent studies have shown that the nonlinear and transient hydrodynamics of extracellular matrices in biological systems, such as increased stiffness, viscous dissipation, stress relaxation, and plasticity can substantially affect cell spreading, proliferation, and differentiation. Here, we report the strain stiffening and softening of composite 0.05 wt% agarose hydrogels composed of 0.038 wt% fluorescent dendritic chitosan nanocolloids dispersed in a water/glycerol solvent. A sol-gel transition occurs when the agarose is cooled below the gel point, during which the chitosan nanocolloids self-assemble into a complex architecture within the agarose. The composite displays increased stiffness while still retaining high fracture strains. More interestingly, the presence of soft dendritic networks results in a complex stiffening-to-softening transition in response to various oscillatory and steady shear strains. The increased stiffness of the biopolymer composite is likely due to the formation of a dual network hydrogel in which chitosan microfibers form a sacrificial load-bearing network within the agarose matrix. The dendritic chitosan network might also suppress non-affine deformation within a softer agarose matrix. At intermediate strains, redistribution of internal stress occurs through temporary disruption in the chitosan network which could explain the observed softening properties. At higher strains, the background agarose matrix dominates the strain response as indicated by network stiffening until fracture. We use a confocal rheometer to visualize the heterogeneous yielding and breakdown of the inter-connected cluster network of chitosan microfibers within the strain softening regime to determine the microstructural mechanism responsible for strain softening. Findings from this study can help understand how cells might respond to the rheologically complex microenvironments that are abundantly found in nature.