Study On Corrosion Behavior Of Magnesium Alloy For Biliary Stent

In recent years,magnesium alloy as a new type of medical metal has attracted common attention in the fields of biomedical science because of its excellent mechanical properties,good biocompatibility,and distinct biodegradability.In terms of stent design for the interventional therapy,current research of implant magnesium alloy is mostly focused on cardiovascular stents.However,there are few studies on the feasibility of stent implantation for other human lumens.In this research,the feasibility of magnesium and magnesium alloy as biliary stents was investigated by experimental and numerical simulation methods.Excellent comprehensive performance of Mg-3Sn-0.5Mn,Mg-3Sn-1Zn-0.5Mn and pure magnesium were selected as experimental materials.By means of electrochemical testing and in vitro immersion testing,the corrosion degradation properties of the three materials were ivestigated in modified simulated body fluid（m-SBF）and human bile with various p H levels,and a corrosion dynamics model of magnesium alloy corrosion was established.Nonparametric shape optimization of the minimum structural element of the two-dimensional（2D）stent was carried out,and the three-dimensional（3D）model of the entire biliary stent was obtained.On base of the component transmission module of computational fluid dynamics（CFD）,the corrosion behavior of the biliary stent was numerically simulated.Results of the electrochemical testing in m-SBF,show that the corrosion resistance of Mg-3Sn-0.5Mn and Mg-3Sn-1Zn-0.5Mn is much better than that of pure magnesium,and the corrosion property of Mg Sn Zn Mn alloy is the best for adding of Zn element in the alloy.The plate-like corrosion products of magnesium alloy were observed by SEM.With the p H of m-SBF increases,the corrosion rate of the three materials gradually decreases,and the corrosion product layer becomes more uniform and complete.Based on energy dispersive spectrometer（EDS）,X-ray diffraction（XRD）,and infrared spectroscopy（FI-TR）analysis,it is found that the corrosion products contain a small amount of crystalline calcium phosphate,amorphous Mg（OH）₂,calcium magnesium phosphate and carbonate.The changes of corrosion rate of pure magnesium,Mg-3Sn-0.5Mn and Mg-3Sn-1Zn-0.5Mn in human bile are consistent with that in m-SBF.The corrosion rate is greatly decreased for the organic macromolecule in corrosion products increasing the ability to hinder the transport of the ion.Due to the presence of bile acid and lecithin in human bile,there is no crystalline calcium phosphate in corrosion products.The characterization results predict that the corrosion products of magnesium and magnesium alloys have no possibility of forming gallstones in human bile.A corrosion dynamics model of Mg-3Sn-1Zn-0.5Mn in bile was established based on the mass transfer theory and the results of electrochemical corrosion.In this paper,the minimum structural element was optimized during the two analysis steps of crimping and expansion by finite element method（FEM）.The value of maximum principal strain was reduced by 41.04%and 42.19%in the process of crimping and expansion.From the simulation results of hydromechanics,the corrosion rate of the stent at the inlet of the model was higher,the stent near the bile corroded first,and there was no concentrated corrosion.The range with the highest corrosion rate is the corner of the stent,where the corrosion rate is 4×10^-7 g·（cm²s）^-1,which accounts for 0.11%of the stent surface,and the area where the corrosion rate is below 9×10^-8 g·（cm²s）^-1 accounts for 52.2%.