Modifying High Modulus Hydrogels for Cell Delivery: Intervertebral Disc Repair with Genipin-Crosslinked Fibrin

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PROJECT SUMMARY Back and neck pain are leading causes of global disability, which account for over $135 billion in healthcare spending. Disabling back pain caused by herniation of the intervertebral disc (IVD) can be alleviated by discectomy, the surgical standard of care that removes herniated IVD tissue. While effective, discectomy does not repair annulus fibrosus (AF) defects caused by the herniation, which can lead to accelerated IVD degeneration, reherniation and recurrent pain. Cell-seeded, adhesive hydrogels are a promising strategy to prevent these complications because they can immediately seal AF defects and deliver cells for long-term healing. Engineering such hydrogels for IVD cell delivery is challenging because soft biomaterials typically used for cell delivery risk herniating in the IVD injury space. On the contrary, high-modulus biomaterials designed to bear high-magnitude spine loads can hinder the healing capacity of encapsulated cells. The overall goal of this research proposal is to uniquely integrate principles of cellular microencapsulation, degradable microbeads (MBs) and high-modulus biomaterials to engineer next-generation biomaterials that promote IVD regeneration and functional repair. Aim 1 will assess the protective capacity and degradation kinetics of oxidized alginate (OxAlg) MBs. Aim 2 will characterize the effects of genipin-crosslinked fibrin (FibGen)-OxAlg construct macroporosity on AF cell phenotype and construct biomechanics. Aim 3 will evaluate the biological and biomechanical repair responses of FibGen-OxAlg. Our global hypotheses are that OxAlg MBs will protect AF cells from FibGen hydrogel crosslinking then degrade (Aim 1). Resultant macroporous FibGen-OxAlg constructs will promote AF cell proliferation and ECM synthesis, leading to enhanced construct biomechanics (Aim 2). This cell delivery strategy will promote biological and biomechanical repair in ex vivo IVD organ culture (Aim 3). This work is significant because it develops an easily translatable tissue engineering strategy to address the critical clinical challenges associated with AF defects; this approach may be broadly applicable to other musculoskeletal tissues that exhibit limited healing and experience high mechanical demands, which strongly aligns with the mission of NIAMS. This proposal is highly innovative because no strategies that repair and regenerate AF defects exist, few published studies use cell-laden MBs as porogens in templated hydrogel constructs, and none use such constructs in IVD repair. Validating the efficacy of this biomaterial strategy in a loaded, large animal IVD organ culture system is innovative and significant because there are few published studies using such a culture system and testing in this manner will accelerate clinical translation. Completion of the proposed aims will provide the candidate with rigorous multidisciplinary training in biomaterial synthesis, cell microencapsulation, biomechanical testing and IVD organ culture. Supplemented with the proposed teaching and mentorship experiences, this fellowship will accelerate the career of a promising PhD Candidate who is strongly committed to musculoskeletal tissue engineering research and education.