Improving the bioactivity of polymeric bone regenerative scaffolds through physically and chemically incorporated molecules
Creators: name: Wu, Xiaohuan
Publisher: name: No Publisher Supplied
Year: 2020

description: Although natural bone grafts including autografts and allografts are widely used in the clinics for the reconstruction of large bone defects, the development of bone graft substitutes (BGSs) continues to be an area of intense research. To apply the concept of tissue engineering in the development of BGSs, osteogenic growth factors and/or stem cells are incorporated into engineered scaffolds to mimic the bone tissue microenvironment. In the Kohn lab, BGSs based on E1001(1k), an intrinsically osteoconductive tyrosine-based polycarbonate, and beta-tricalcium phosphate (β-TCP) nanocrystals, were custom-fabricated (abbreviated as E1001(1k)/β-TCP scaffolds) and tested in various critical size calvarial models in combination with recombinant human bone morphogenetic protein 2 (rhBMP-2). To test the in vivo performance of this type of BGSs in long bone defect reconstruction, comparison study was conducted between E1001(1k)/β-TCP scaffolds and a clinically used β-TCP-based BGS (i.e., chronOS®) in a critical size sheep tibia model, with both constructs carrying a low dose of rhBMP-2 (1.1 mg per defect). It was found that E1001(1k)/β-TCP scaffolds have comparable in vivo performance to chronOS®. More importantly, new bone regenerated in E1001(1k)/β-TCP scaffolds demonstrated a more physiological morphology, indicating ongoing bone remodeling into compact bone tissue.A critical component in the bone microenvironment is the osteogenic growth factor (GF) phase. Though rhBMP-2 being one of the most studied GFs in the bone regeneration field, adverse effects associated with supraphysiological dosing is reported. Therefore, it is necessary to establish a reliable in vitro-in vivo correlation to determine the proper rhBMP-2 doses as well as to evaluate the efficacy of rhBMP-2 delivery systems. To date, the various study design in examining rhBMP-2 release profile in vitro has precluded comparative analyses. Due to the aforementioned concerns, a systematic evaluation of the most widely used in vitro rhBMP-2 activity assays is reported in Chapter 3. It was found that each model cell line (i.e., W-20-17, MC3T3 and C2C12) has an optimal dose-response range upon rhBMP-2 induction. In addition, a correlation between protein concentration (as measured by enzyme-linked immunosorbent assay) and protein activity (as measured by alkaline phosphatase induction from W-20-17 cells) was established. It was found that the expression system used to produce the rhBMP-2 had the most significant effect on its activity and stability in vitro.In the previous in vivo studies, E1001(1k)/β-TCP scaffolds have been successfully utilized as carriers of rhBMP-2 to assist the healing process of critical size bone defects. However, the scaffold-only treatments typically demonstrate moderate osteoconductivity in vivo due to the lack of biological cues in synthetic polymers. On the other hand, owning to the unique redox chemistry, chelation capability with metal ions and Michael-type addition from thiols and amines, catechol functionality has triggered great interest in the tissue engineering field. In Chapter 4, E1001(1k) analog polymers were functionalized with catechol side chains for enhanced bioactivity. It was found that the modified polymers were able to support mesenchymal stem cell, human osteoblast and MC3T3 cell growth. In addition, the surface anchoring catechol groups assist nano-sized silver deposition on the polymer surface by immersion-coating in silver nitrate solution, and the Ag-decorated surface has excellent antibacterial properties against Escherichia coli and Staphylococcus aureus. The versatile secondary functionality introduced by catechol modification makes the E1001(1k) analog polymers suitable for multiple bone-related applications.