(558c) Leveraging Crosslinking Chemistry to Tune Shear Thickening in Dynamic Covalent Hydrogels

Hydrogels crosslinked through dynamic covalent bonds have garnered recent interest as injectable materials with potential applications for additive manufacturing, biomaterials, and enhanced oil recovery. Due to the reversibility of the crosslinks, the hydrogels are reconfigurable, such that they can flow under the shear applied during injection and self-heal at the site of delivery. Though injectable dynamic covalent hydrogels with a range of crosslinking bond exchange kinetics have been characterized under small amplitude oscillatory shear, there remains a need to characterize the material properties under continuous shear, which is more representative of injection. Further, to systematically design dynamic covalent hydrogels tailored for specific applications, there is a need to understand how the material properties are affected by the crosslinking bond exchange kinetics. Toward these ends, we synthesized poly(ethylene glycol)-based hydrogels with boronic ester, thia-conjugate addition, and hydrazone crosslinks, by which the crosslinking bond exchange kinetics are tuned across three orders of magnitude, in turn affecting the rheological properties of the hydrogels. Under small amplitude oscillatory shear, we confirmed that the hydrogels with different crosslinking chemistries provide access to a range of moduli (50-750 Pa) and relaxation times (0.5-1000 s). Meanwhile, under steady shear, the hydrogels shear thickened at low shear rates, with the onset and extent of shear thickening dependent on the crosslinking bond exchange kinetics and concentration of the particular hydrogel formulation. Though edge fracture precluded characterization at high shear rates, we also showed that the hydrogels are injectable and self-healing. Altogether, the relationships between crosslinking bond exchange kinetics and nonlinear rheological properties will help in further advancing the design and application of dynamic covalent hydrogels as injectable materials.