Synthesis And Characterization Of Sol-gel Derived Bioactive Coatings On Porous NiTi Alloys

In this thesis study, the exploration of synthesizing the bioactive coating on the surface of porous NiTi alloy via sol-gel method was carried out. The basic features and properties of the coatings were characterized by X-Ray Diffraction and microanalysis approaches (SEM and EDS). The corrosion behavior of the coated porous NiTi alloy in Hanks'solution, the rate of Ni ion release from the coated porous NiTi alloy in simulated body fluid (SBF) solution, the stability of the coating in Tris (hydroxymethyl) aminomethane solution and the bioactivity of the coating were investigated. Further, the release characteristics of gentamicin sulfate loaded on porous NiTi alloy was studied and the biocompatibility of the HA coated porous NiTi alloy was evaluated.First, a uniform anatase TiO₂ layer was dip-coated on porous NiTi alloy via the sol-gel route, and a bilayer composite TiO₂ coating consisting of the porous outer layer and the dense layer beneath was also deposited on porous NiTi alloy by adding polyethyleneglycol (PEG) in sol as pore-forming agent. The surface morphology of the TiO₂-coated porous NiTi alloy and the cross-sectional structure of coatings show that the TiO₂ coatings cover not only the outer surface but also the inner surface of porous NiTi alloy. The corrosion behavior of the samples in Hanks'solution at 37°C shows that the dense TiO₂ coatings increase the corrosion resistance of porous NiTi alloy significantly. In contrast with the dense TiO₂ coating, the porous TiO₂ composite coating can improve the surface bioactivity owing to the increased surface area.Moreover, a hydroxyapatite (HA) coating was successfully deposited on porous NiTi alloy via dip-coating using a sol-gel procedure with triethyl phosphite and calcium nitrate as phosphorus and calcium precursors, respectively. The HA coating was deposited on the external surface and within the pores of the porous NiTi alloy, and the thickness of the HA coating was several microns. Cross-sectional SEM characterization and EDS analysis results of the HA coatings show that the coating on the inner surface of the pores is thicker than that on the outer surface. The results of an immersion test in a Tris solution show that the HA coating possesses excellent stability, and the rates of Ni ion release through the HA coatings on the porous NiTi alloy of different porosity ratios in a simulated body fluid decrease markedly compared with the uncoated alloys. There is also a remarkable increase in the apatite forming ability of the HA coated porous NiTi alloy in an FCS solution and the apatite forming ability on the inner surface is more fasterly because of the roughness.Furthermore, the porous TiO₂/HA/TiO₂ composite coating was fabricated on the surface of porous NiTi alloy by mixed sol method. The release characteristics of gentamicin sulfate loaded on porous NiTi was studied. The results show that the porous NiTi alloy can controll the release of drugs by means of the special pore structure. The drug release shows three phases: in the first stage, the release rate was rapid; in the second stage, the release rate was steady; and in the third stage, the release rate is faster, but the total amount released is not significant.In addition, the biocompatibility of the HA coated porous NiTi alloy was evaluated by cell culture method. The cell shows good adsorption on the HA coating and good adhesion within the pore. Compared with the bare porous NiTi alloy, the HA coating has a larger growth rate of osteoblasts and shows better cell compatibility. Compared with pure Ti and the control group, the osteoblast function on porous NiTi alloy shows inconsistency with the proliferation rate, this indicates that the special pore structure of porous NiTi alloy is the fundamental cause for the osteoblast activity. The results of blood compatibility experiments show that the hemolysis rate of the HA coated porous NiTi alloy is 0.3%, this means that the HA coating has good blood compatibility and will not cause acute hemolysis.