Investigation On Metallic-ion Release Behavior Of Nb-60Ta-2Zr Alloy Exposed To Simulated Physiological Medium

Metallic implants exposed to human physiological environment usually suffer from corrosion and wear,causing the release of metallic ions.To some extent,specific metallic ions at a controllable concentration can exhibit positive healing effects.However,excess accumulation of released metallic ions in human body may cause many adverse biological effects,and directly affects the biocompatibility of metallic implants.It is well known that the metallic-ion release behavior of metallic implants is closely related to its corrosion resistance,which can be assessed by universal and practical electrochemical measurements.Base on the above background,investigating the metallic-ion release kinetics of metallic implants in simulated physiological medium and revealing the correlation between metallic-ion release and electrochemical behavior would provide an insight into understanding the corrosion behavior of metallic implants.As such,this will contribute to achieve controllable release of metal ions,then offering a guide to accelerate the further development of metallic implants.It is reported that traditional biomedical metals for implanted stents,such as 316L stainless steel and Co-Cr alloys,has potential biosafety problem during long-term implantation.Toxic ions,such as Ni2+、Cr6+、Co2+ can be greatly accumulated in human body.Besides,these traditional biomedical metals have high magnetic susceptibility(χv),which would exhibit artifacts during magnetic resonance imaging(MRI)and cause misdiagnosis.By comparison,Nb-60Ta-2Zr(wt.%)alloy has low χv and shows desirable MRI compatibility.In addition,this alloy has excellent biocompatibility,corrosion resistance and qualified mechanical properties,becoming a promising MRIcompatible candidate material for vascular stents.In this thesis,Nb-60Ta-2Zr alloy was selected to investigate metallic-ion release behavior and kinetic characteristics.The correlation between metallic-ion release and electrochemical behavior was further examined.Commercially pure(CP)Nb,Ta and Ti were also investigated for comparison.The main conclusions are as follows:(i)The metallic-ion release behavior and kinetic characteristics of Nb-60Ta-2Zr alloy exposed to simulated physiological medium are illustrated.The metallic-ion release process of Nb-60Ta-2Zr alloy is apparently characterized by two distinct phases.In stage I,the release rate attenuates exponentially with exposure time.While in subsequent stage II,metallic-ion release substantially maintains at a dynamically steady state.The time-dependent evolution of released metallic-ion concentration follows an empirical master equation as expressed as Ci=α·[1-exp(-τ/b)]+c ·τ.Moreover,the established kinetic equation works well to quantitatively characterize the kinetic behavior of metallic-ion release for other metallic implants in exposure to the simulated physiological medium.In addition,the total quantity of metallic ions released from Nb-60Ta-2Zr alloy is comparable to that of CP Ta,but is one order of magnitude lower than that of CP Nb and CP Ti by ICP-MS analysis.After normalizing with individual composition in the alloy,released Nb ions quantity of Nb-60Ta-2Zr alloy remains one order of magnitude lower than that from CP Nb,indicating that alloying with Ta in the Nb-based solid solution has an effect to mitigate the release of Nb.(ii)The passive film formed on Nb-60Ta-2Zr alloy in exposure to simulated physiological medium for 21 days is mainly composed of completely oxidized Nb2O5 and Ta2O5,together with small amounts of ZrO2 and suboxides of Nb and Ta by XPS analysis.The cation fractions of Nb and Ta in the passive film are nearly the same as their individual concentrations in the alloy.Furthermore,the released ion concentration of alloying element gradually saturates to the level of respective nominal composition in the alloy.It is indicated that there’s no remarkable preferential oxidation and dissolution of elements Nb and Ta after long-term exposure.(iii)The correlation between electrochemical behavior and metallic-ion release behavior of the investigated metals is revealed.The corrosion current density(icorr)of Nb-60Ta-2Zr alloy,CP Nb,CP Ta and CP Ti exposed to simulated physiological medium for 1 day are on the same order of magnitude,while the metallic-ion release rates of Nb-60Ta-2Zr alloy and CP Ta are one order of magnitude lower than those for CP Nb and CP Ti.Besides,icorr of Nb-60Ta-2Zr alloy does not exhibit remarkable change with extended exposure time,which are opposed to the above metallic-ion release kinetics analysis.Therefore,icorr is not a qualified indicator with a one-to-one correlation with metallic-ion release,at least in the current study.Similarly,there is no obvious corresponding relationship between open-circuit potentials and metallic-ion release.In addition,the passive film resistance of Nb-60Ta-2Zr alloy increases with exposure time.It is noteworthy that the concentrations of released metallic ions from these metals manifest a reverse ranking sequence based on the magnitude of the passive film resistance.