A tissue engineering approach to bone regeneration using novel hybrid scaffolds based on natural chitosan and synthetic poly(lactide-co-glycolide)

Reconstruction and regeneration of fractured or diseased bone has been a common human health care issue. Transplantation of autogenous or allogenous bone tissues for the regeneration of bone defects, although have saved innumerable lives and improved life quality, suffers from a number of limitations such as donor shortage, as well as donor site morbidity associated with autografting, and risks of disease transmission with allografing. Bone tissue engineering represents a novel, promising strategy that applies the biological, chemical and engineering principles toward the repair, restoration and regeneration of living tissues by using biomaterials, cells, and factors alone or in combination.; This dissertation thus presented a scaffold based tissue engineering approach to bone regeneration using a combination of two types of degradable biomaterials, natural biopolymer chitosan and synthetic polymer poly(lactide- co-glycolide) (PLAGA). This combinative approach integrated two individual polymers into one scaffolding system and led to the development of novel bone tissue engineering scaffolds that are mechanically competent and functionalizable. Sintered microsphere scaffolds containing both chitosan and PLAGA were developed. The sintering conditions were optimized, and the sintered chitosan/PLAGA microsphere scaffolds were shown to have appropriate mechanical properties, suitable microstructures for load-bearing bone tissue engineering applications. Chitosan/PLAGA scaffolds supported MC3T3-E1 osteoblast-like cell proliferation and enhanced phenotypic expression of the cells. Through the functionality of chitosan, a novel scaffold that exhibits bioactivity was developed by ionic immobilization of a biomolecule heparin on the chitosan/PLAGA scaffolds. Chitosan/PLAGA scaffolds showed the capacity to retain heparin molecules on the scaffold surface. Heparin immobilization, without affecting the mechanical properties and microstructures of the scaffolds, influenced the osteoblastic cell functions in a dose-dependent manner. A stimulatory effect of immobilization of low dose heparin on the proliferation and differentiation of MC3T3-E1 cells was demonstrated. The biodegradability of the newly designed chitosan/PLAGA scaffolds was investigated. Chitosan/PLAGA scaffolds were shown to be biodegradable with a slower degradation rate than pure PLAGA scaffolds. Finally, the bone forming capacity of chitosan/PLAGA scaffold, heparin immobilized chitosan/PLAGA scaffold, and these scaffolds loaded with recombinant human bone morphogenetic protein-2 (rhBMP-2) was evaluated using a rabbit ulnar defect model. The in vivo study demonstrated that the synergistic effect of both heparin and rhBMP-2 promoted fracture healing and mineralized bone formation of defects. The work described in this dissertation provided strong evidence that the chitosan/PLAGA sintered microsphere scaffold can be a potential candidate for load-bearing bone tissue engineering applications. More importantly, the strategy of introducing reactive chitosan component to PLAGA scaffolds can lead to the development of novel functional scaffolding systems that exhibit bio-recognition and specificity via physically or covalently attachment of desirable bioactive molecules, peptide sequences or proteins. The hybrid scaffolding systems can therefore potentially serve as superior candidates for bone tissue engineering applications.