Study On Osteogenic Mechanism And Composition-structure Transformation Of Calcium Phosphate Biodegradable Ceramics In Vivo

In the 21st century, biomedical materials endowed with bio-functions can be implanted into the human body, which are able to mobilize self-repair and improve the strength of injured tissue. The search for the "ideal" bone implant material is one of the most developing and promising directions of the biomaterial research. Due to it's similarities in composition and mineral ingredients to the human bone tissue, calcium phosphate materials show excellent biocompatibility and bioactivity, which makes them to the most important bone implant materials.In this study, the interaction between calcium phosphate materials and proteins, cells, and bone tissue were investigated on the basis of material science while penetrating medicine and biochemistry. The observed change in biochemistry and microstructure as response to the implanted calcium phosphate materials as well as the mechanisms of osteogenesis in body provide valuable information for the design of new calcium phosphate bone repair materials, bone tissue engineering scaffold materials, and other clinical application.The research included the study of the human cancellous bone structure, the optimization of the material composition and microstructure, the compounding of p-TCP with Na2O-CaO-MgO-P2O5 glass binder, and the preparation of biodegraded porous calcium phosphate ceramics by the foaming method at 850℃. This ceramics obtained a highly visible porosity, large specific surface, and was three-dimensional interconnected with big, small, and micro-pores at a homogeneous pore distribution. As in vitro experiment testified, the calcium phosphate ceramics (CPC) has better biodegradation properties than other bone implant materials.The absorption of the calcium phosphate ceramic to the protein was investigated by using FT-IR,XPS,SEM, and SDS-PAGE. As the results showed, the proteins were strongly absorbed by the CPC, and a shift of the feature peak of the protein and also a chemical shift in the Ca2p and Ols bind energy of CPC was observed. This indicated that the acidic amino-group and alkaline amino-residue on the proteins' surface bonded to the Ca~2+in the p-TCP crystal by ionic bond and the proteins' alkaline amino groups to the oxygen in PO43- by hydrogen bond and electrostatic attraction. Theabsorption mechanism of the protein in the CPC can be described as following: the protein diffuse to the surface of CPC driven by concentration difference in the solution. Then, the protein conformation performs a micro-restructuring under the electrostatic attraction on the surface, which bond the positive and negative charge to the inverse charge on the CPC surface. This results in the formation of the first irreversible chemical absorption layer. After a period of time, other forces such as the gravity and the electrostatic attraction between proteins lead to the second physical absorption layer; the bond between the proteins and free calcium ions in the solution as well as the protein absorption onto the surface, which also takes part in the bio-mineralization of the bone tissue, lead to the third absorption layer.The in vitro experiments of osteoblast and CPC showed that the CPC has better cell affinity than other bio-materials, which can promote the osteoblast to adhere selectively on the materials and to let the cell proliferate and grow on the surface well. Furthermore, CPC can also promote the osteoblast to transform from the Go/Gi phase to the S phase, accelerate the DNA synthesis and cell proliferation as well as increase the OCN expression of the osteoblast, improve the osteoblast activity, and differentiate into osteocyte.The histological observation showed that the CPC has excellent biocompatibility, an appropriate biodegradation, good bioactivity and osteoconductivity. The degradation-absorption of CPC and the formation of new bone is a dynamic balanced process. The degradation speed matches up to the formation speed of new bone. The form of the new bones is in line with the intramembraiious ossification and the type of bone formation stimulated by calcium phosphate ceramic is intramembranous ossification. The observation of the multinucleate giant cell and it's endocytosis showed that the degradation of the CPC is cell-mediated. During the early period of implantation, manly macrophage cells could be observed on the CPC surface. During the middle and later period, mainly osteoclasts were observed in the interface between the bone tissue and the ceramics. The osleogenesis activity of the osteoblast correlates with the absorbed degradation of osteoclast. The degradation product provided the substance for the formation of new bone. The suitable pore and surface structure of the CPC provided the favored condition for the osteoblast adherence, growth, and migration. The interconnected pores are not only the channel for the nutrition substances and cells to pass in and out, but also provide the free space forthe new bone tissue growth.As the analysis of tetracycline dual-labeled images showed, the CPC tended to increase the osteogenesis speed within the implanted area in metrology. The quantitative analysis of the bio-transformation of inorganic material into the organic bone tissue leaded to a theory of the degradation and formation of new bone in vivo.The interface between CPC and host bone was studied by the electron probe method. The result showed that the CPC is not only a useful scaffold material for the growth of bone, but also takes part in the life cycle of the host bone and ultimately becomes living new bone tissue. Further result also indicated that the process of the biodegradation of materials and formation of new bone is taken place at the same time. This complex process is a slow interdependent and interacting biotransformation.The SEM and TEM pictures showed that the calcium and phosphate produced by CPC lead to the appearance of electron dense layers at the interface between the materials and the bone, which improved the deposition of the crystals such as DCPD, OCP, and HAP. The mineralization of collagens always takes place around the materials. The degradation and mineralization occurs at the same time due to the recognition mechanism of the non-college proteins such as the osteocalin. The collagenous fibres mineralize in order and after the maturing of the bone, the mineral phase was identified as nanoparticles of DOHA, HAP etc. The calcium phosphate ceramics stays in direct contact with the bone. There are many kinds of interfaces between the CPC and the new bone tissue. The calcium phosphate particle arranges parallel at the interface, cleavages into lath-like structure gradually, and changes from micro- into nanoparticle. These nanoparticles diffuse into new bone tissue, entwine around the bone tissue, and ultimately become the mineral substance of the new bone. In the body fluid, p-TCP crystals cleavage along the (001) face due to the formation of a P-NaCaPC<4 layer and the self structure deficiency of P-TCP crystals, which cause the binding force between the crystal face ravage during sintering. This structure is beneficial to the degradation of CPC in vivo.