Design of Biomaterials Towards Endogenous Bone Regeneration
Bone grafting is one of the most commonly used surgical methods to augment bone regeneration in orthopedic procedure. While using natural bones, such as autograft and allograft are considered as the gold standard techniques, they suffer from numerous drawbacks including scarcity, donor site complications, and potential disease transmission. To overcome these limitations, mineralized poly (ethylene glycol) diacrylate-co-N-acryloyl 6-aminocaproic acid (PEGDA-co-A6ACA) composed of an organic phase and an inorganic, biomineralized phase that recapitulates certain aspects of dynamic mineral environment of native has been developed. The real-world application this biomineralized material in treating bone defects in vivo depends upon a myriad of parameters including scaffold structural parameters (e.g. pore size), mechanical properties (e.g. strength and toughness), and host environments (e.g. age of the recipient). In this dissertation, I explored these biomaterial and biological parameters for biomaterials mediated bone regeneration through leveraging endogenous healing mechanism. First of all, I evaluated the potential of mineralized biomaterials to induce bone repair of a critical-sized cranial defect in the absence of exogenous cells and growth factors. I demonstrated that the mineralized biomaterial alone can support complete bone formation within critical-sized bone defects through recruitment of endogenous cells and neo-bone tissue formation in mice. By providing a bone-specific mineral environment, these biomaterials induce osteogenic commitment of recruited host progenitor cells and support the maintenance of cells relevant for the formation and function of bone tissues, including vascularization of the implant during repair. Based on these findings, I further investigated the effect of the scaffold pore size on in vivo ectopic bone formation. Biomineralized PEGDA-co-A6ACA hydrogels were made to have an interconnected macroporous network with different pore size ranges (45-53 μm, 90-106 μm, 160-180 μm, 212-250 μm or 300-355 μm) and similar overall porosity between 65% to 70%. Using these scaffolds, I evaluated their abilities to promote ectopic bone formation upon subcutaneous implantation in wild-type mice as a function of time. I found that scaffolds with pore sizes larger than 100 μm showed similar bone formation abilities, whereas in scaffolds with pore sizes 45-53 μm, cell infiltration only happened at the peripheral region of the scaffolds. Results from this study revealed that pore size of the scaffolds had a prominent influence on the extent of cell infiltration and bone ingrowth. While such biomaterial-mediated in situ tissue engineering is highly attractive, success of this approach relies largely on the regenerative potential of the recruited endogenous cells, which is anticipated to vary with age of the host. To this end, I investigated the effect of the age of the host on mineralized biomaterial-mediated bone tissue repair using critical-sized cranial defects as a model system. Mice of varying ages, 1-month-old (juvenile), 2-month-old (young-adult), 6-month-old (middle-aged), and 14-month-old (elderly), were used as recipients. I showed that the biomineralized scaffolds support bone tissue formation by recruiting endogenous cells for all groups albeit with differences in an age-related manner. The age of the recipient mice had a significant influence on the quantity and quality of the neo-bone tissues characterized in terms of bone mineral deposition and bone tissue-specific markers, where delayed bone formation and decreased quantity of neo-bone tissue formation were observed in older mice. The real-world applications of the biomineralized materials for aiding bone tissue regeneration are greatly limited by the lack of mechanical strength and toughness of the materials. To enhance the mechanical property of the biomineralized scaffold, I further proposed a double network (DN) hydrogel system with an asymmetric network structure, where the first network is tightly cross-linked by A6ACA with crosslinker N, N'-Methylenebisacrylamide (bisacrylamide), and the second network is loosely crosslinked PEGDA. The effects of bisacrylamide crosslinker concentration (2 mol.%, 4 mol.% and 6 mol.%), and molecular weight (Mn: 3.4 kDa, 6 kDa, 10 kDa, and 20 kDa) of 20 w/v % PEGDA on mechanical properties of the resultant DN-hydrogels were investigated and compared to those of single network (SN) hydrogels of the same composition. Findings from this study showed that increase in crosslinker concentration of the first network was correlated with lower ultimate compressive strain, higher compressive strength, toughness and elastic modulus. Furthermore, DN-hydrogels prepared in this work displayed swelling ratios ranging from 569 ± 20% to 1948 ± 12%. Among all compositions, DN-hydrogel with 6 mol.% bisacrylamide and PEGDA 10 kDa demonstrated the highest compressive strength (3.47 ± 0.35 MPa), highest toughness (0.60 ± 0.03 MJ/m3), and elastic modulus (1.04 ± 0.09 MPa). Using this composition, porous DN-hydrogels with interconnected pore architecture were fabricated through polymethylmethacrylate (PMMA) bead leaching method. Resultant porous hydrogels demonstrated potent biomineralization capabilities, and the matrix-bounded CaP minerals were able to undergo dissolutions. Given the high strength and biomineralization capacity, DN-hydrogels reported here could be useful for developments of tissue engineering scaffolds for bone tissue regenerations. Overall, this dissertation explores different biomaterial designs and biological factors in biomaterial-mediated in vivo bone tissue repair, providing materials insights that are useful to researchers and engineers in designs of biomaterials to leverage endogenous healing mechanism for tissue regeneration and repair.
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