| Current coronary stents that have inputted clinical application are made of non-degradable materials, which stay in the body for long-term cause many issues, such as restenosis, thrombosis and other issues blocking. Magnesium alloy is an ideal material for degradable cardiovascular stents because its excellent mechanical properties and biocompatibility, and it can degrade in human body, which release magnesium ions that are necessity. But the poor corrosion resistance property of magnesium alloy and in vivo rapid degradation make stents’effective service period become short, which can’t meet the clinical requirement, thus the improving of corrosion resistance property of magnesium alloy is particularly important. Organic coatings have excellent biocompatibility and facilitate to carry drags. Chitosan(CTS) that has both hydroxyl and amino groups is a nontoxic, biodegradable, and biocompatible natural polymer and can be used in a wide range of applications. The introduction of sulfonic acid groups can obtain heparinoid substances, which can greatly improve their water-soluble property, and sulfonated chitosan show anti-coagulant in medicine. In this study, chitosan coatings, sulfonated chitosan coatings and composite coatings were fabricated on magnesium alloy by dipping method. The coatings were characterized and the corrosion property of the substrates and treated samples were investigated.The experiments show that chitosan coatings fabricated on magnesium alloy by different concentrations of chitosan solution which have different viscosity are different in density and surface roughness. Because concentration of0.5%chitosan solution is relatively thin, the coatings have poor density and pores, which prone to cause micro-galvanic effect. The chitosan coatings fabricated by concentration of1.5%chitosan solution have bad surface roughness, big swelling ratio and is liable to flake off because of its higher viscosity. The chitosan coatings fabricated by concentration of1.0%chitosan solution have good overall properties, and can effectively protect magnesium alloy. The thickness of coatings can be controlled by changing the dipping number. While the dipping number is3,5and7times, the thickness of chitosan coating is70μm,85μm and100μm.The bonded area of substrate and coating is increased after the acidification of magnesium alloy, and the hydrogen evolution reaction caused by acid solvent is weakened, too. The introduction of a large number of active-OH by acidification layers enhance the bonding strength of substrate and coatings, and improve the corrosion resistance of magnesium alloy. But the formation of acidification layers consumes a certain amount of substrate material, so the control of acidification is essential.The experiments show that Si-O covalent bond of silane coupling agent and magnesium alloy can enhance the bonding strength of coatings and substrates. Compared to the silane coupling agent with the use of ethanol as solvent, silane coupling agent with the use of deionized water as solvent show better performance, but it is likely to cause the large areas of shedding of coatings. The films produced by coupling agent solution in ethanol as solvent have excellent overall properties, and pre-activation treatment can provide more contact area and-OH for silane films.As the aldehyde group on GA(glutaraldehyde) could react with the amino group on chitosan formed the-C=N-group, each chitosan chain are connected into network, so the coating-forming properties of chitosan becomes better. Compared to chitosan coatings, crosslinked chitosan coatings are denser and more smooth without micro-porouses, which made the coated substrate had an excellence performance. The swelling ratio of crosslinked chitosan coatings is greatly reduced, which is why coatings are not tilted. As chitosan coatings are crosslinked, the filiform corrosion of magnesium alloy may be increased, and the activity of chitosan is affected, so the addition of should be strictly controlled.As the tendency of the adsorption of metal ions, sulfonic acid groups grafted on chitosan like the distribution of adsorption sites on the chitosan coating, which can enhance the bond strength and improve corrosion resistance of magnesium alloy. The structure of sulfonated chitosan is very similar to the structure of the heparin. In the infrared spectroscopy the peaks at the wave number804.2cm-1and1224.6cm-1mean that the sulfonic acid groups are successfully grafted at3and6bit, so it has good anticoagulant property.The conjunctiva power of layers of chitosan/sulfonated chitosan self-assembled is the electrostatic force between the amino and sulfonic acid and molecular chain winding force. While maintaining the good anticoagulant property of sulfonated chitosan, layers of chitosan/sulfonated chitosan self-assembled can greatly improve the corrosion resistance of magnesium alloy. Open circuit potential of sample is about-0.9V, and the growth of pH value is not more than0.4after immersed in simulated body fluid for120h, which reflect superior protective feature.The better hydrophilicity means the better anticoagulant performance. The hydrophilic of pure chitosan film is bad, and its contact angle is83°. As the pretreatment of magnesium alloy surface can improve the hydrophilic of chitosan film on magnesium alloy surface, the contact angle of coupleded chitosan film is76.5°, and the contact angle of crosslinked chitosan film is52.5°. Sulfonated chitosan film has perfect hydrophilic, and its contact angle is27°. The contact angle of layers of chitosan/sulfonated chitosan self-assembled is57°. |
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