Fabrication and Characterization of Collagen-Apatite Composites for Bone Tissue Engineering

Due to the high demand of scaffolds for treating bone fractures every year, biomimetic coatings and scaffolds resembling the composition and structure of natural bone are of keen interest of bone tissue engineers. In this research, a series of biomimetic coatings and scaffolds have been developed, including biomimetic apatite coatings, biomimetic collagen-apatite composite coatings, intrafibrillar calcified collagen fibrils, intrafibrillar silicified collagen fibrils, and intrafibrillar calcified collagen scaffolds. First, a crack-free apatite coating was deposited on the surface-treated Ti6Al4V substrate using a biomimetic process, in which the surface-treated titanium substrate was immersed in a modified simulated body fluid (m-SBF). Dual-beam focused ion beam/scanning electron microscopy (FIB/SEM) was used to characterize the cross-sectional microstructures of the ceramic coating. Cross-sectional SEM images and TEM images revealed that crack-free apatite coatings have been achieved by adjusting the pH of the m-SBF. The coating formed demonstrates a gradient porous structure, which is dense close to the substrate and then it becomes more and more porous towards the surface of the coating. Second, a biomimetic collagen-apatite composite coating was also successfully Changmin Hu-University of Connecticut, 2018 formed on the surface-treated Ti6Al4V alloy using collagen-containing m-SBF. During the composite coating formation, collagen and apatite co-precipitate to form the composite coatings. Dual-beam FIB/SEM was used to characterize the cross-sectional microstructures of the composite coatings. The result indicates that the cross-section of the collagen-apatite composite coating also exhibits a gradient porous structure, and that collagen-apatite composite coating is thinner and less porous than the pure apatite coating. Third, inspired by the structure of natural bone, collagen fibrils with intrafibrillar calcification were prepared for bone tissue engineering. Poly(acrylic acid) (PAA) was used as a sequestration analog of non-collagenous proteins (NCPs) for stabilizing amorphous calcium phosphate (PAA-ACP) nanoprecursors to infiltrate into collagen fibrils. Additionally, sodium tripolyphosphate (TPP) was applied as a templating analog of NCPs to regulate the orderly deposition of minerals within the gap zone of collagen fibrils. The effect of PAA concentration on the intrafibrillar mineralization of reconstituted collagen fibrils was also investigated. Fourth, silicon is an essential element contributing significantly to the health of bone, and great efforts have been made to produce silica-containing biomaterials. Poly (allylamine) hydrochloride (PAH) was used as an analog of zwitterionic proteins to stabilize silicic acid (SA) to produce fluidic silica (PAH-SA) nanoprecursors, and TPP was used as a templating analog of zwitterionic proteins. Silicified collagen fibrils with core-shell, twisted and banded structures were produced by controlling the zwitterionic proteins analogs. Intrafibrillar silicified collagen fibrils were produced Changmin Hu-University of Connecticut, 2018 using PAH-SA as fluidic silica nanoprecursors and TPP as the templating analog to modulate the deposition of silica within the gap zone of collagen fibrils. Fifth, with the success in reproducing intrafibrillar mineralized collagen fibrils, biomimetic collagen/apatite scaffolds consisting of intrafibrillar mineralized collagen fibrils were prepared via a bottom-up approach, resembling the composition and structure of natural bone from the nanoscale to the macroscale. At the macro-scale, unidirectional macro-pores are aligned across the entire scaffold, and at the micro-scale, each layer is comprised of aligned and well stacked lamella. Finally, at the nano-scale, apatite minerals deposit within the gap zone of collagen fibrils and orient along the long axis of these fibrils. The biomimetic collagen-apatite scaffolds demonstrate a good biocompatibility in vitro, indicating a great potential to be used for bone tissue engineering applications. In summary, a novel methodology has been developed for the preparation and characterization of biomimetic coatings and scaffolds with a hierarchical structure, demonstrating a great potential for bone tissue engineering.