"Adaptable sliding hydrogels as a dynamic 3D cell niche for enhancing stem cell-based cartilage tissue engineering"
Hydrogels are versatile water-swollen polymeric networks that can be designed to mimic the mechanical and physical properties of extracellular matrix (ECM). With widespread use as 3D cell culture scaffolds, viscoelastic materials have been leveraged to recapitulate the stress relaxation behavior of native tissue ECM in response to applied forces and are typically formed via reversible covalent crosslinking, polymer entanglement, or weak physical interactions. In the context of cartilage regeneration, model hydrogel systems such as alginate and collagen have shown that increasing stress relaxation can enhance mesenchymal stromal cell (MSC) chondrogenesis and chondrocyte-based cartilage formation. However, the influence of stress relaxation on MSC-based cartilage regeneration has yet to be studied across a broad range of stress relaxation timescales or in the absence of confounding biochemical cues.
To address the above gaps in knowledge, the overall goal of this thesis is to develop an adaptable sliding hydrogel (ASGs) with tunable stress relaxation and plasticity and to investigate its effect on MSC fate and cartilage regeneration in 3D. Part I focuses on material development and characterization of the dynamic mechanical properties of ASG, namely stress relaxation and plasticity. To achieve a range of tunable stress relaxation and plasticity that are distinct from existing dynamic hydrogels used for MSC differentiation, reversible hydrazone crosslinks with varied kinetics were introduced into a polyethylene glycol (PEG)–based sliding hydrogel (SG) platform, yielding ASG. Part II focuses on assessing MSC cell fates related to cartilage regeneration including viability, apoptosis and differentiation. We found that increasing stress relaxation and plasticity in ASG promotes rapid and robust cartilage formation by human MSCs, reducing apoptosis and improving cell viability over time. Part III focuses on elucidating the underlying molecular mechanisms through which MSCs sense and respond to tunable dynamic matrix properties that contribute to enhanced cartilage regeneration. Mechanistically, ASG facilitates local matrix remodeling and enables MSCs to form “pericellular pockets” in 3D that correlate with enhanced nascent extracellular matrix deposition and reorganization, integrin signaling, and nuclear dynamics.
In summary, this thesis establishes ASG as a novel biomaterial tool that supports robust MSC-based cartilage regeneration in 3D and identified pericellular pocket formation as a unique mechanism through which MSCs sense and respond to stress relaxation and plasticity. Overall, the ASG platform is a mechanically dynamic cell scaffold that can be harnessed to elucidate how tuning dynamic mechanical cues modulate stem cell differentiation towards multiple lineages. The findings from this research also help inform the design of next-generation scaffolds that harness hydrogel mechanics to accelerate the regeneration of complex tissues.