A Study Of Electropolishing Surface Modifications On NiTi Shape-Memory Alloy

NiTi stents have become one of the main devices to treat vascular stenosis. As a long-term implant biomedical material, NiTi stents are required to have reliable mechanical property and good biocompatibility, especially. It has been shown that surface quality has a significant influence on the biocomtibility, so the surface trentment is very importment. Electropolishing (EP) has been widely used in stent post-trearment for its special characteristics. However, the polishing conditions for NiTi alloy are very strict, and for the business competition, the electropolishing detalils have been reported little.In this paper, we presented a modified electrolyte that added A,B material to the widely used CH3COOH-HCIO4 solution, and investigated the various impact factors of the electropolishing. Used scanning electro microscope (SEM), atomic force microscope (AFM), X-ray photoelectron spectroscopy (XPS), nonoindentation tests, tensile test, potentiodynamic polarization test and homocompatibility experiments to verify the surface characterization, mechanics property, corrosiom resistance and blood compatibility of the EP NiTi. At last, we polished two types ofNiTi stents, and investigated its' surface topography.The results shown that current density was the key factor that can strongly influence the surface quality and the next importment factors in turn were electrolyte temperature, electro gap and polishing time. The optimal parameter set of electropolishing NiTi was 0.6 A/cm~2 current density, 0℃temperature, 2 cm electrode gap and 45 seconds polishing time.The surface of EP NiTi was smooth and uniform, and the surface roughness and lightess were close to mechanical polished (MP) NiTi sample. The XPS results showed that the EP NiTi surface was predominantly covered by TiO2, the Ti/Ni atomic ratio is 27.6 and indicating a biocompatibility safer versus that of 3.1 for MP NiTi. The surface hardness for EP NiTi was reduced while the tensile peoperty was not changed. Measurements using potentiodynamic polarization in Hanks' solution showed that no pitting occurred for electropolished sample even through the applied potential increased up to 1.5V (SCE), indicating an excellent corrosion ressitance. The Ni ion release of EP NiTi in Hanks' solution was less than MP NiTi sample, which was in agreement with the corrosion tests. Homocompatibility experiments revealed that the EP NiTi surface was hydrophobic. The adhered amount of platelet was less on the EP NiTi surface and the time of blood clot was short. At last, we polished the knitted and layser-cutting NiTi stents with the proposed parameters. The initial surface roughness was relatively low for knitted stents, so the polished results were good. As for laser-cutting stents, preparation by picking was necessary before electropolishing. The EP NiTi stents surfaces after picking were smooth, which was similar to the foreign product.