Crosslink Strength Governs Yielding Behavior in Dynamically Crosslinked Hydrogels

Summary Yielding of dynamically crosslinked hydrogels, or transition between a solid-like and liquid-like state, allows facile injection and utility in translational biomedical applications including delivery of therapeutic cells. Unfortunately, characterizing the time-varying nature of the transition has not been attempted, nor are there design rules for understanding the effects of yielding on encapsulated cells. Here, we unveil underlying molecular mechanisms governing the yielding transition of dynamically crosslinked gels currently being researched for use in cell therapy. We demonstrate through nonlinear rheological characterization that the network dynamics of the dynamic hydrogels dictate the speed and character of their yielding transition. Rheological testing of these materials reveals unexpected elastic strain stiffening during yielding, as well as characterizing the rapidity of the yielding transition. A slower yielding speed explains enhanced protection of directly injected cells from shear forces, highlighting the importance of mechanical characterization of all phases of yield-stress biomaterials. Significance Statement Many direct-injection cell therapies suffer from poor viability of injected cells due to membrane disruption by shear stresses during injection. Injectable hydrogels with dynamic crosslinks can increase viability during injection, utilizing the ability of these materials to undergo a reversible solid-to-liquid shear-rate dependent yielding transition. Much effort has been applied to understanding the yielding transition of these materials, yet design rules for connecting the character of yielding to viability of injected cells has not been established. Here, we apply rheological testing to show that faster network dynamics result in a more gradual yielding transition, affording greater protection to cells during injection. This work introduces a new rheological protocol for the development of injectable hydrogels for cell therapy.