In Vitro Degradation Behavior And Biocompatibi Lity Of Yb–containing ZK60 Magnesium Alloys

Magnesium alloys have been regarded to be a promising candidate for the medical implantable material due to its high specific strength,environmental friendliness,and excellent biocompatibility.However,the rapid corrosion rate in biological environments greatly limits its wide application as biodegradable material.The question is still open as to how to decrease the corrosion rate and preserve the mechanical integrity of magnesium alloys in service.In the current investigation,a rare earth element Yb containing ZK60 magnesium alloy with much improved mechanical properties was adopted to explore its potential use as biodegradable material.A systematic study was conducted in simulated body fluid(SBF)focusing on the biocorrosion behavior and biological toxicity of ZK60 alloys with different Yb concentrations and heat treatments,to be precise,the solution(T4)and aging(T6)treatments.This research sheds a light on the influence of Yb on the biocorrosion and biocompatibility of magnesium alloys and provides fundamental understanding for the development of novel Yb-containing magnesium alloys with excellent mechanical properties and favorable corrosion rate.The main conclusions are summarized as follows:The effect of Yb concentration on the biocorrosion and biocompatibility of ZK60-T4 alloys was investigated.It was found that Yb could effectively refine the microstructure of the matrix and thus improve the mechanical properties.The electrochemical examinations and immersion tests showed that Yb solute could significantly improve the biocorrosion resistance of ZK60 alloy after solution treatment.The ZK60–1.0wt.% Yb alloy exhibited the most favorable corrosion rate of ~1.31 mm / year,which was mainly attributed to the almost clear grain boundaries and moderate grain sizes after the T4 treatment.In vitro biocompatibility examinations indicated that the ZK60-T4 alloys with different Yb concentrations did not display obvious cytotoxicity,and the hemolysis rates were less than 0.1%,exhibiting excellent biological safety.The effect of ytterbium concentration on the biocorrosion and biocompatibility of ZK60-T6 alloys was very analogous to those of ZK60-T4 counterparts.It was obvious that Ybaddition could stabilize the mg matrix phases with improved size and distribution,thus enhancing the mechanical properties,especially for the yield strength.The electrochemical examinations and immersion tests suggested that Yb solute could considerably improve the biocorrosion resistance of ZK60 alloy after aging treatment.Although the corrosion resistance was slightly deteriorated after T6 treatment,the aged ZK60–2.0 Yb alloy still exhibited a favorable corrosion behavior,which was mainly ascribed to the corrosion barrier effect of a more compact and uniform protective film induced by the dispersed nano-scale precipitates.The in vitro biocompatibility tests indicated that the ZK60-T6 alloys with different Yb concentrations did not display obvious cytotoxicity,and the hemolysis rates were less than2 %,exhibiting favorable biological safety.The density of precipitates was increased with extended aging time in ZYbK620-T6 alloys,and the morphology gradually evolved from chain-like to dendritic.Many nano-sized precipitates appeared inside the peak-aged grains.The electrochemical examinations and immersion tests manifested that the biocorrosion behavior was closely correlated to the aging time.The peak-aged alloy showed favorable corrosion resistance due to the uniform corrosion mode stemmed from the dense and well-dispersed nanoprecipitates.But the over-aged samples exhibited severe micro-galvanic effect and local corrosion due to the appearance of coarse precipitates at grain boundaries.The in vitro biocompatibility tests showed that the precipitates with different sizes and distribution posed very limited negative influence on the cytotoxicity,but the hemolysis rate increased gradually with the extension of the aging time.According to this study,it was demonstrated that Yb solute could decrease the biocorrosion rate with improved mechanical properties and favorable biocompatibility of the ZK60 base alloy.Given the favorable comprehensive performance in mechanical,biocorrosion behaviors and biological safety,the T6 treated ZK60–2.0 Yb alloy may be considered as a promising candidate for biomedical applications.