Preparation Of Antimicrobial Bioactive Coatings Developed On Magnesium Alloys And Study On The In Vitro Biocompatibility

Magnesium element is one of the necessary elements of human body. In addition, magnesium alloys have good mechanical property and biocompatibility. Therefore, magnesium alloys show broad potential applications as medical implant material, which has been taken notice by some researchers. However, the corrosion rate of magnesium alloy is too fast in the complex environment of human body, which becomes the bottleneck of their clinical applications. Magnesium alloys can be modified by micro arc oxidation (MAO) technology, which can greatly improve the corrosion resistance and biocompatibility of magnesium alloys. This is very important for the study for medical implant materials. Anodic coatings containing calcium phosphate salts have excellent bioactivity. At the same time, in view of poor corrosion resistance of anodic coatings obtained in the solution containing inorganic phosphates, phytic acid that widely exists in plant seeds was used in the paper as the basic electrolyte. By comparing the influence of different calcium salts on the properties of anodic coatings, the electrolytes with high contents of calcium and phosphorus elements were obtained. The orthogonal experiment was used to explore the best technological parameters and study the formation mechanism of micro arc coatings. The specific research results are achieved as follows:(1) In a base solution containing 28 g/L ammonium hydrogen fluoride and 20 g/L of phytic acid (pH=5.10), CaO, Ca(CH3COO)2, Ca(H2PO4)2,EDTA-CaNa and glycerol phosphate calcium were separately added and micro arc coatings were prepared on AZ91. The results show that the roughness and diameter of micro pores of anodic coatings obtained in different calcium salts are different. In addition, after addition of calcium salts, the Ca content of anodic coatings can increase to some extent. Especially EDTA-CaNa can considerably increases the Ca content of anodic coatings. By means of surface morphology observation, anodic coatings prepared in the solution containing EDTA-CaNa are more uniform, indicating that EDTA-CaNa is one kind of ideal calcium-containing electrolytes. EDTA-CaNa concentration was positively correlated with the calcium content in the oxide coating, but negatively correlated with phosphorus content. With the increase of the concentration, the number and diameter of micro pores on anodic coatings decrease, but the compactness of anodic coatings becomes poor. The calcium content in the membrane layer increases to 1.82 at.%, while the phosphorus content decreases to 3.15 at.%.(2) Surface morphology, element component and phase structure of anodic coatings developed in the solution containing 28 g/L NH4HF2+2O g/L phytic acid+ 14 g/L EDTA-CaNa on AZ91 magnesium alloys were investigated by using SEM, EDS, XRD and XPS.In addition, the antimicrobial properties of the anodized samples were measured. The results show that anodic coatings are mainly composed of Mg, MgO, and MgF2 as well as a small amount of Mg(OH) 2 and Ca (OH) 2. Phytates were not found in the coatings. The antibacterial rates of the oxidized samples with Escherichia coli and Staphylococcus aureus are larger than 99.99% and 99.98% respectively, which have excellent antibacterial effects.(3) In a base solution containing 28 g/L ammonium hydrogen fluoride and 20 g/L phytic acid, the influences of the ammonium hydrogen fluoride, phytic acid, EDTA-CaNa and treating time on the coating formation of calcium and phosphorus elements were investigated by using the orthogonal experiment of four factors with three levels. The order of affecting the calcium content in micro arc coatings is EDTA-CaNa concentration> ammonium hydrogen fluoride concentration> phytic acid concentration> treating time. The sequence of influencing phosphorus content in the oxidation film is phytic acid concentration> EDTA-CaNa concentration> ammonium hydrogen concentration> treating time. It suggests that the concentrations of calcium salt and phosphate salt are helpful the increase of the calcium and phosphorus contents in anodic coatings.(4) Anodic coatings prepared in the solution containing silicate on Mg-1.0Ca alloy were characterized by using SEM, XRD, the hydrogen evolution experiment and MTT method. SEM and XRD analyses show that the oxide coating is a typically porous structure and contain MgO, MgSiO3 and CaSiO3, etc. After soaking in simulated body fluid for 72 hours, the contents of Ca and P in anodic coatings increase significantly, indicating the anodized samples achieve excellent bioactivity. Hydrogen evolution experiment shows that MAO treating can greatly improve the corrosion resistance of Mg-1.0Ca alloy. The cytotoxicity grades of Mg-1.0Ca alloy before and after MAO treatment are all 1, showing that the Mg-1.0Ca alloy and anodic coatings have good biocompatibility.