Mechanism of Shear Thickening in Dynamic Covalent Hydrogels

Hydrogels with dynamic covalent linkers have garnered intense interest as extracellular matrix (ECM) mimics and injectable delivery vehicles due to their tailorable viscoelasticity, stress relaxation, and self-healing behavior. While dynamic covalent hydrogels with a range of bond exchange timescales have been developed, their properties under flow are less well studied. In this context, we have developed synthetic multi-arm poly(ethylene glycol) (PEG) hydrogels with three different dynamic covalent linking chemistries: boronic ester, hydrazone, and thia-conjugate addition bonds. This suite of dynamic covalent linkages allows control over the bond exchange kinetics across three orders of magnitude, which dictates hydrogel viscoelasticity under small amplitude oscillatory shear. Interestingly, the hydrogels exhibit non-monotonic flow curves under steady shear, with shear thickening behavior that depends on the crosslinking bond exchange kinetics and polymer concentration. To probe the mechanism of shear thickening, reversible shear rate sweeps were performed, as well as absorbance measurements at different oscillatory strains. Our data point to non-Gaussian chain stretching as a primary driver of shear thickening behavior. Overall, these results provide insight to the molecular and structural characteristics that govern dynamic covalent PEG gelation, mechanics, and flow, while also expanding the types of scaffolds applicable to tissue engineering and therapeutic delivery.