Preparation And Characterization Of The HA/Collagen Composite Coating And Its Formation Mechanism

Hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂, HA) is the main mineral component of human hard tissues. It has great bioactivity and biocompatibility,and after being implanted, it can form a kind of osteal bond with the organic tissues within a short time. Collagen existing in the extracellular matrix is an essential component of the connective tissue and it has excellent biocompatibility, complete biodegradability, low immunity and low antigenicity. And it can protect and support human tissue, enhance the tensile strength of bond and it shows special biomechanics. Natural bone consists of a kind of inorganic/organic composite assembled by ordered nano-HA and collagen protein, in which the HA crystal diameter is about 20 nm and its length is about 60 nm. The c-axis of the HA crystal grow along the collagen 3D helical axis and it has special nano-micro two level of structure. At present, some groups have done a mass of research about HA/collagen composites by using coprecipitation methods, biomimic methods, and so on. But these composites cannot meet the needs of the heavy load part in the body completely. In this work, we mimic the basic composition and structure of bone and combine the advantages of the HA coating and the Ti substrate to achieve an excellent bioactive and mechanical bone substitute material.In this work, an electrochemical co-deposition technique in low concentration system has been developed to prepare HA/collagen and HA/chitosan composite coatings on Ti substrate. Meanwhile, we also tried to change reaction conditions to find formation mechanism of the nano-micro porous structure, which is explained by a proposed concept of"Bubble Template". The main work is summaried as follows:1. The electrolyte contained 10-4 mol/L CaCl₂ and NaH₂PO₄ and 0.1 mol/L NaCl with Ca/P ratio 1.67, in which collagen concentration was 0.1⁰.3mg/mL and pH value was adjusted to 6.0⁶.3 by 0.1 mol/L NaOH. The SEM, AFM, XRD, FTIR and XPS spectroscopic measurements were performed to characterize the morphologies, chemical composition and crystalline structures of the composite coating. It is found that the composite coating exhibited a homogeneous and porous morphology with nano-micro structure. Compared with the pure HA coating, the peak intensity of the (002) crystal face of the composite coating in the XRD measurements is greatly weakened, due to the block effect of collage during the nucleation and growth of HA crystallizing process. The FTIR spectroscopy exhibits the typical peaks of the collagen: the amideâ… band and the amideâ…¡band, and their peaks derives a little from the same peaks of the pure collagen, indicating that there is a kind of chemical bond between collagen and HA. The XPS results indicate that the C and N atom content increases greatly. The cell culture results in vitro and MTT tests reveal that, compared with pure HA and pure Ti, the HA/collagen composite coating exhibits best biological properties. And the bonding strength of the hybrid coating is improved markedly.2. The effects of substrate materials, additives, reaction temperature and deposition time on nano-micro porous structure were also studied. The addition of additives(collagen, BSA and chitosan) makes the viscosity of the electrolyte increased and from silicon slice with gold film, Ge wafer to pure Ti plate, the smoothness decreases in sequence. The experiments indicate that viscosity of the electrolyte, the smoothness of the substrate, reaction temperature and deposition time play important roles in the formation of nano-micro porous structure. A concept of"Bubble Template"is proposed to explain the formation mechanism of nano-micro porous structured HA/collagen coating.3. The composite coating of HA/chitosan has also been prepared by electrochemical co-deposition method. The preliminary characterization has been carried out by SEM and AFM images, and it is shown that the HA/chitosan composite coating has ordered uniform porous tow-level structure. The EDS spectrum and the sharp peaks of the XRD patterns indicate that the main components is high crystal HA. The bonding strengths between the HA/chitosan coating and the titanium substrate has been improved in a certain extent.