Abstract
As a new type of biodegradable implant material, magnesium based metals have gained more and more studies and attention for medical applications due to their good mechanical properties, inherent characteristics of biocompatibility and biodegradation in the physiological environment. However, problems such as local alkalization, excessive hydrogen release and earlier failure of implants, caused by the poor corrosion resistance in body fluid which has limited their clinical applications. In this dissertation,bioactive coatings containing Ca-P with self-sealing structures were fabricated on the surface of pure magnesium using micro-arc oxidation technique (MAO) in a specific calcium hydroxide based electrolyte system. And bare magnesium was used as a comparison, the affects of voltage and electrolyte concentration ratio on coatings microstructure, chemical composition, electrochemical corrosion properties and in vitro degradation behavior were systematically studied. In addition,in vivo corrosion resistance of the MAO-coated implants was studied by preliminary animal experiments. The main conclusions were drawn as follows:
It was found that coatings obtained at different voltages showed similar surface morphologies that the majority of micro-pores were filled with compound particles. The thickness of the coatings was increased with increase of the applied voltage. Both the porous structure and the compound particles had the same chemical compositions that were mainly composed of O,Mg,F,Ca and P while only a little crystallized MgO were detected in the coatings. XPS analysis indicated that the MAO coatings were composed of MgF2,MgO2Mg3(PCU)2,Ca2P2〇7 and CaHPCU. Both electrochemical tests and in vitro immersion tests manifested a significant increase in corrosion resistance for samples owing to the self-sealing MAO coatings on the surfaces. Besides,the coating with higher thickness obtained at 450V of applied voltage behaved the superior corrosion resistance. The coatings suffered significant degradation during the immersion in Hank's solution. And EDS analysis showed that appreciable Ca and P contents on the surfaces of the coatings after immersion process.
When Ca(OH)2 and (NaPO3)6 were at the concentration of 1.2g/L and 4g/L respectively,the obtained coating had the best stability in corrosive environment, and the pH values of PBS solution were reduced and maintained at the lowest levels, indicated this coating had provided the most effective protection for the magnesium substrate. Besides, the existence of the MAO coatings effectively inhibited the corrosion of the magnesium substrate in PBS solution which ensured the mechanical integrity to a certain extent. Many strip-shaped particulate corrosion products mainly composed of O,Mg and P were found on the surfaces of the MAO-coated samples after immersion in PBS solution, which might be the mixture of Mg(OH)2 and magnesium phosphate.
The preliminary animal experiments indicated that the in vivo corrosion properties of magnesium implants were significantly improved by this Ca-P containing MAO coatings,which prevented the implants from rapid degradation in the initial stage of implantation and maintained the initial shape of the implants over a longer period. Thus, the strength, shape and other physical and chemical properties of the implants would be guaranteed during the service period.
Key words: pure magnesium, micro-arc oxidation, self-sealing structure,corrosion resistance,degradation behavior
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