Engineering Microporous Annealed Particle Scaffolds as a Chondrogenic Niche for Microfracture-Based Articular Cartilage Repair

One of the promising aspects of regenerative medicine is the potential to functionally restore previously irreparable tissues by stimulating self-healing mechanisms within the body. Stem cells are at the forefront of regenerative medicine due to their unlimited self-renewal as well as their capacity to differentiate into other cell types. The regenerative potential of these endogenous super cells can be harnessed for various tissue applications by carefully designing instructional biomaterial constructs. Cartilage, for example, has limited regeneration potential due to its avascular nature, making it an enticing application of stem cell-modulating biomaterials. Currently, patients with diseased or damaged cartilage, such as osteoarthritis, are lacking meaningful options for repair. One approach to articular cartilage regeneration, microfracture surgery, entails the creation of carefully placed holes (i.e., microfractures) in the bone beneath the damaged tissue, releasing blood and regenerative stem cells into the cartilage defect. Perhaps due to the limited number of stem cells at the defect site or the absence of potent chondrogenic cues, the neocartilage produced by this technique is typically fibrotic in nature, making it brittle and prone to failure. Recent studies involving the application of biomaterials immediately following microfracture generation have shown the potential to produce a more desirable hyaline-like tissue. Nonetheless, existing scaffolds still have ample room for improvement, as they typically lack effective defect filling, stem cell recruitment and expansion, and/or sufficient chondrogenic stimuli.

We believe that our biomaterial platform, microporous annealed particle (MAP) scaffold, is an excellent candidate for improving microfracture-based articular cartilage regeneration. MAP scaffolds are comprised of an injectable slurry of individual hydrogel microspheres that undergo a secondary crosslinking step in situ to form structurally stable constructs with interconnected cell-scale pores. Due to its injectable nature, the MAP platform inherently grants microfracture-related benefits such as arthroscopic application, complete defect filling, and enhanced tissue infiltration. Furthermore, poly(ethylene glycol) (PEG)-based MAP hydrogels can easily be physically and chemically tuned to modulate stem cell behavior.

In this dissertation, we present a biomimetic scaffold loaded with chondrogenic cues engineered to promote stem cell recruitment, proliferation, and chondrodifferentiation. We first mechanically tune MAP hydrogels to mimic the stiffness of developmental cartilage tissue. We then investigate how MAP microparticle diameter and corresponding scaffold pore architecture affect stem cell proliferation and migration. Next, we incorporate chondroitin sulfate (CS), the most common glycosaminoglycan in cartilage, into our hydrogels at a biologically relevant concentration and study its effects on stem cell proliferation, migration, and chondrodifferentiation. Finally, we covalently tie a chondrogenic growth factor, which doubles as a stem cell chemoattractant, into our CS-laden microgels and evaluate the chondrogenic potential of our carefully tailored MAP scaffolds. Overall, we believe the work in this dissertation establishes techniques that are useful for the application of MAP to various microenvironments and unlocks the translational potential of MAP scaffolds as a regenerative platform for microfracture-based articular cartilage repair.