Abstract
Three-dimensional hydrogels have emerged as a primary platform for recapitulating the extracellular matrix in vitro, offering independent control over mechanics, crosslink chemistry, and scaffold architecture in a format compatible with three-dimensional cell culture. Realizing their potential as physiologically relevant models requires that hydrogels be permissive, supporting and accommodating the cell-driven remodeling that underlies spreading, migration, and tissue organization. At the molecular scale, permissivity is conferred through crosslinks that respond to cell-generated disruptions. Network connectivity introduces an additional layer of control: a chain held by many connections requires simultaneous disruption of many bonds before it can rearrange. At the macroscale, porous architecture provides a route to permissivity that bypasses molecular and network design constraints entirely, as cells navigate pre-existing void space without engaging the gel phase. Understanding how structural features at each scale govern permissivity is therefore a prerequisite for engineering hydrogels that support fundamental cell behaviors.
This dissertation investigates structural determinants of permissivity across molecular, network, and scaffold length scales using two different hydrogel systems. First, we investigate how unintended norbornene homopolymerization during thiol-norbornene photoclick hydrogel fabrication introduces permanent crosslinks that suppress stress relaxation and cell spreading, and demonstrate that reducing polymer functionality mitigates this effect by lowering network connectivity. We then ask whether those stress relaxation properties associated with permissive character are maintained once hydrogels equilibrate in aqueous media, finding that swelling-induced dilution of dynamic bonding partners collapses the tunable viscoelastic space, and that enforcing proximity of dynamic bonding partners restores dynamic mechanical function. Finally, we investigate whether fibrous particle morphology can encode directional pore organization into granular hydrogel scaffolds, demonstrating that fiber geometry imparts pore directional persistence that is associated with enhanced myotube maturation. Together, these studies identify structural features that impact permissivity at each scale and establish design principles for engineering hydrogels that support three-dimensional cell remodeling and organization.
