Research On Degradation Behavior And Biocompatibility In Vitro Of Bio-Degradable Mg-Zn-Ca-Zr Alloys

With the improvement of living standards and the increase of the elderly population,the incidence of cardiovascular diseases has shown an obvious upward trend.Heart stent implantation surgery has become the most effective measure to treat cardiovascular diseases.In recent years,magnesium alloy has unique advantages and has been widely studied as a cardiovascular implant material.The outstanding characteristics of magnesium as a scaffold material:（1）The density of magnesium（1.74 g/cm³）is similar to that of human bone（1.8-2.1g/cm³）;（2）Magnesium has mechanical properties similar to human bone,which can effectively avoid stress shielding;（3）Magnesium’s natural degradability can avoid secondary surgery to reduce the suffering of patients;（4）Magnesium is the fourth element in the human body,and magnesium ions are beneficial to the human body’s micro-release.Due to its excellent natural properties,magnesium is expected to become a biodegradable medical cardiovascular stent material.In this paper,Zn,Ca and Zr are selected as alloying elements,and Mg-x Zn-0.5Ca-0.4Zr（x=2、3、4 and 5 wt.%）alloy is designed to be melted by conventional casting method;And then,Mg-4Zn-0.5Ca-0.4Zr alloy is subjected to T4,T6 and extrusion process to obtain magnesium alloys with different microstructures,respectively.Through microscopic morphology,weightloss,hydrogen evolution experiment,electrochemical experiment,blood compatibility and cytotoxicity to reflect alloy’s degradation behavior and biocompatibility.By studying the degradation behavior of magnesium alloy in a simulated environment in vitro,the interaction relationship between the degradation rate and the microstructure of the alloy is clarified.The relevant research conclusions are as follows:（1）Microstructure change of As-cast Mg-x Zn-0.5Ca-0.4Zr alloys:when x=2,the alloy phase composition includesα-Mg and Ca₂Mg₆Zn₃ phase;However,when x=3,4 and 5,the alloy phase is consisted ofα-Mg and Mg₇Zn₃ phases;In addition,with the increase of Zn content,the average grain size of the alloys gradually decreases as well.（2）Corrosion mechanism of as-cast Mg-x Zn-0.5Ca-0.4Zr（x=2、3、4 and 5 wt.%）alloy:Both Ca₂Mg₆Zn₃ phase and Mg₇Zn₃ phase have higher corrosion potentials than magnesium matrix.Mg₇Zn₃ phase acts as a cathode while magnesium matrix acts as an anode to accelerate matrix corrosion during galvanic corrosion occurrance;In addition,different phase morphologies have different effects during alloy corrosion.When x=4,the continuous Mg₇Zn₃ phase distributed along the grain boundary can hinder the spread of corrosion.The experience results indicate that the Mg-4Zn-0.5Ca-0.4Zr alloy has the best corrosion resistance（CR=0.815±0.078 mm/y）.（3）The heat-treated Mg-4Zn-0.5Ca-0.4Zr alloy phase is composed ofα-Mg and Mg₇Zn₃phases.Heat treatment makes the Mg₇Zn₃ phase solid dissolve into the magnesium matrix,weakening the corrosion effect of galvanic corrosion on the magnesium matrix,and the heat-treated alloy is resistant Corrosion resistance is improved;among them,T6（5h）alloy has the best corrosion resistance（CR=0.612±0.050 mm/y）.The extruded Mg-4Zn-0.5Ca-0.4Zr alloy’s phase composition is stillα-Mg and Mg₇Zn₃ phase,the grains are obviously refined and the amount of Mg₇Zn₃ precipitation is reduced;because the corrosion potential at the grain boundaries is slightly higher than that of the magnesium matrix.The finer the grain,the better the corrosion resistance of the alloy;among them,the corrosion resistance of the extruded E16-1 alloy is the best（CR=0.425±0.040 mm/y）.（4）The interaction relationship between degradation rate and biocompatibility:the hemolysis rate of the as-cast,heat-treated and extruded Mg-4Zn-0.5Ca-0.4Zr alloy is less than 5%,and the hemolysis rate of the extruded state is the lowest;in addition,the L929cytotoxicity test shows that the relative cell growth rate（RGR）is greater than 75%,and the RGR value of the extruded alloy is greater than 100%.Note:the better the corrosion resistance of the alloy,the lower the hemolysis rate,and the more beneficial it is to cell proliferation.