(617d) Surprises in Injection Force and Flow Rheology of Physical Hydrogels

Materials utilizing supramolecular assembly offer distinct advantages in applications requiring injectability or extrusion, arising from the lack of permanent covalent crosslinks in favor of those which can dynamically rearrange. Dynamically crosslinked materials have found use in biomedical applications such as 3D bioprinting, injectable drug or vaccine delivery formulations, or sprayable hydrogels^1–7. Connecting the molecular features which dictate the dynamics and stability of dynamic crosslinks to downstream bulk mechanical properties has been of significant interest^8–10. Although most rheological testing tools, such as dynamic mechanical analysis or shear rheology, probe low-shear or equilibrium behavior, many biomedical applications require high-shear rate capillary flow, so understanding the material response and pressure required for flow in that regime is needed to optimize formulations and predict operating conditions. Thus, further developing the relationship between benchtop rheological properties and the force or pressure required for injection at high shear is critical for scale-up and implementation of dynamically crosslinked materials.

Dynamic materials may be used to directly deliver therapeutic cells as well as encapsulate biologics which are sensitive to the localized shear stress and inhomogeneities in the flow field^9,11,12, highlighting the need for better characterization of the physics of capillary flow for these set of materials. A suite of rheological and mechanical testing tools exists to probe the linear and low-shear rate (below approximately 100 sec^-1) behavior, but unexpected behaviors may emerge at high shear rates, such as shear banding and wall-slip, even at low Reynolds number^13,14. A critical evaluation of the assumptions used to extrapolate material properties from low-shear-rate measurements, such as viscosity flow sweeps, to high-shear-rate behavior is necessary¹⁵.

In contrast to previous approaches which characterize synthetically complex multicomponent systems, we utilize a simple dynamically crosslinked hydrogel with easily tunable crosslinking dynamics which enables mechanistic coupling of molecular crosslink dynamics and macroscopic mechanical properties. Based on the addition of a small-molecule surfactant, the crosslink dynamics of the network are altered, leading to faster rearrangement kinetics¹⁶. Despite its chemical simplicity, this hydrogel system displays complex elastoviscoplastic behavior, including shear-thinning, yield stress, and high extensibility, similar to other nanostructured soft materials¹⁷.

In this talk, I will demonstrate the use of capillary flow behavior of dynamic hydrogels to explain non-monotonic trends in their injection force with regards to crosslink dynamicity. By connecting measurements from analytical techniques covering a range of shear rates including rheometry, viscometry, and an in situ capillary microscope optical rheometer, we show many that current methods of characterizing flow curves cannot predict observed trends in injection force. Critically, we find that strong wall slip dominates the flow profile at high shear rate, leading to unexpectedly low injection forces, and nonuniform trends in injection force with respect to crosslink dynamics. Finally, we give new criteria for hand injectability based on a rheological model which accounts for power law shear-thinning and wall slip.

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