Crosslink Dynamics Control Injection Force and Flow Profiles of Non-Covalent Gels

Dynamically cross-linked hydrogels have found remarkable utility in 3D bioprinting and injectable slow-release delivery of pharmaceutics and vaccines, applications which take advantage of their ability to flow through needles or extrusion devices with relative ease. Predicting the force required to inject or extrude dynamic hydrogel materials is paramount to their application. Here, we report an injectable dynamic hydrogel with facile tunability of the cross-link exchange rate using a small-molecule surfactant. We found surprising nonmonotonic trends in injection force with respect to cross-link dynamics, not explained by traditional flow rheology measurements. By using an optical in situ capillary rheometer, we found that wall slip is the dominant mechanism influencing flow profiles, rather than shear banding, as has been reported in previous studies of dynamically cross-linked hydrogels. We hypothesize that the main driver of flow rheology and injection force for these materials is the combination of a lower slip distance and bulk stiffness with increasing dynamicity, balancing the bulk cohesive force with adhesive forces to the wall. Finally, by developing new models which describe the flow behavior of noncovalent gels, we present new criteria for extrudability, enabling better predictions for operating conditions of downstream applications of dynamic hydrogels.