| Pure titanium is widely used in the biomedical field due to its excellent biocompatibility, good corrosion resistance and elongation, and it can help to form the osteogenesis between the bone and implant. However, the mechanical strength of the pure titanium was lower than that of titanium alloy which would lead to the fracture of dental implant in clinical practice. Therefore, the mechanical properties of titanium need to be improved. Equal channel angular pressing(ECAP) can improve the mechanical properties of the material through changing the grain sizes of the metal without introducing any harmful elements. The previous studies of our group demonstrated that the mechanical properties of titanium have been improved obviously by ECAP when the grain sizes were reduced from 30μm to 300-400 nm after grain refinement. The hardness of the material increased from 1496 MPa to 2458 MPa, dynamic and static compressive yield strength increased from 900 MPa, 300 MPa to 1240 MPa, 772 MPa, respectively. These data provide the foundation for the application of ultrafine-grained titanium(UFG) as implant material in the clinical. In recent years, there are some reports on the biocompatibility of UFG titanium, and these studies have shown that UFG titanium can promote adhesion and spreading of fibroblast and osteoblast. But there are less reports on the biocompatibility of UFG titanium after treatment of micro-arc oxidation which would improve osseointegration rate and reduce the risk of the failure.In this study, UFG titanium prepared by ECAP was processed by micro-arc oxidation(MAO) to prepare titania coatings. Surface morphology and cross-sectional morphology of coating were observed, and the composition, thickness, roughness of titania coatings and bonding strength between ultrafine-grained Ti and coatings were investigated to evaluate the application of UFG titanium processed by micro-arc oxidation method. Based on the above studies, biological security was performed for UFG titanium and these can provide experimental evidence for application of UFG titanium in the field of oral implantology.The study included two parts: 1. The micro-arc oxidation of UFG titanium and properties of titania coating 1) Preparation of UFG titanium specimen processed by ECAPPure titanium was processed by four passes of ECAP at room temperature and the material was wire-cut into slice of speciment. The results showed that UFG titanium was produced in the die with a channel angle of 120? in route BC for four passes at room temperature. The billets of UFG titanium had a smooth surface without surface defects. The grain sizes of the material were reduced from 30μm to 0.3μm-0.4μm which lead to enhance the strength. 2) Micro-arc oxidation of UFG titaniumThe UFG titanium by ECAP pressing was put into the anode of the micro-arc oxidation equipment to prepare the oxidation coating. The processing time was 5, 10, 15 minutes respectively. The results showed that the surface of UFG titanium was formed a layer uniform titania coating after micro-arc oxidation treatment, which changed from gray to gray-white. Titania coating was combined tightly with UFG titanium substrates. The study proved that the method can be applied for treatment of UFG titanium. 3) Morphology characterization of titania coating on UFG titanium materialThe surface topography and cross-section morphology of oxide coating on UFG titanium were analyzed by scanning electron microscopy and three-dimensional topography scanner. The results showed that the surface of MAO coating was porous. The MAO coating consisted of two parts: the inner structure was compact and the external structure was porous. The external structure included roughly circular or elliptical shape micropores which was convenient for adhesion and growth of osteoblast. As the treatment time increased, the thickness of external structure was increased and the diameter of pore was increased. 4) The physical and chemical properties of UFG titanium after micro-arc oxidationThe composition of MAO coating on UFG titanium were analyzed by X-ray spectrometer, the results showed that the main elements of coating were O, Ti, in addition to a small amount of P, Si, C. There was no significant difference in the proportion of each element between UFG titanium and pure titanium in different times. The thickness of coating was analyzed by scanning electron microscopy(SEM). The results showed that the thickness of the MAO coating on UFG titanium was larger than that on the pure titanium. As the processing time increased, the thickness of the oxide coating was increased. The roughness was measured by three-dimensional topography scanner. The bonding strength between UFG titanium and coatings was investigated through automatic scratch tester. The results showed that there was no significant difference in the roughness and bonding strength between UFG titanium and pure titanium. However, with the processing time increased, the roughness of experimental group and control group was increased, while the bonding strength was decreased. 2. The study on biological security of UFG titanium after micro-arc oxidationThe hemolysis test, cytotoxicity test, acute toxicity test and oral mucous membrane irritation test were performed to evaluate the biological safety of MAO coating on UFG titanium according to the criteria of biological evaluation of medical devices. 1) Hemolysis test: Hemolysis rates of MAO coating was measured using fresh rabbit blood. The results showed that hemolysis rate of UFG titanium by micro-arc oxidation was lower than 5%, indicating that the UFG titanium by micro-arc oxidation retain good blood compatibility and meet the clinical requirements. 2) Cytotoxicity test: The cytotoxicity of extracts of MAO coating on UFG titanium was evaluated through using fibroblasts. The data showed the cell growth well and the number was increased. The cytotoxicity of UFG titanium was rated as grade 0, which indicated that there was no influence on cell, and MAO coating on UFG titanium had good biocompatibility. 3) Acute toxicity test: The acute toxicity of extracts of MAO coating on UFG titanium was detected by oral gavage method in mice. The results showed that mice behaved no toxic response from macro and micro aspects, suggesting that UFG titanium after micro-arc oxidation treatment was no toxic effects on the body in a short period. 4) Oral mucous membrane irritation test: The golden hamsters as experimental subjects were investigated for two weeks. The phenomenon such as congestion, edema, erosion were no observed in the oral mucosal, and there was a small amount of infiltration of inflammatory cells in tissue sections, suggesting that the UFG titanium with micro-arc oxidation had no adverse stimulus for oral mucosal. Conclusion: 1. UFG titanium materials were successfully prepared through four-pass processing of equal channel angular pressing(ECAP). The coating of UFG titanium was formed by micro-arc oxidation technology. The inner layer of coating was dense and external structure was porous, and surface contains circular or elliptical shape micropores which was convenient for growth of cells. The binding strength between coating and substrate was firm. These proved data for the application of micro-arc oxidation process for UFG titanium. 2. The biological security demonstrated that UFG titanium by treatment of micro-arc oxidation was excellent blood compatibility and non-cytotoxic. No toxic effect was found for the body in the short term and no stimulating effect of the oral mucosa was occurred. These indicate UFG titanium exhibited excellent biological security. The results also provide evidence for the applications of UFG titanium in the implant field. |
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