Bioerodible magnesium alloy microstructures for endoprostheses

Abstract

A bioerodible endoprosthesis includes a bioerodible magnesium alloy. The bioerodible magnesium alloy has a microstructure including equiaxed Mg-rich solid solution-phase grains having an average grain diameter of less than or equal to 5 microns and second-phase precipitates in grain boundaries between the equiaxed Mg-rich solid solution-phase grains. The beta-phase precipitates have an average longest dimension of 0.5 micron or less. The microstructure can be produced by one or more equal-channel high-strain processes.

What is claimed is: 
     
         1 . A bioerodible endoprosthesis comprising:
 a bioerodible magnesium alloy comprising magnesium and one or more additional alloying elements, wherein the alloy has a microstructure comprising equiaxed Mg-rich solid solutionphase grains having an average grain diameter of less than or equal to 5 microns and continuous or discontinuous second-phase precipitates in grain boundaries between the Mg-rich solid solution-phase grains, the second-phase precipitates having an average longest dimension of 0.5 micron or less.   
     
     
         2 . The endoprosthesis of  claim 1 , wherein the second-phase precipitates are primarily centered upon the gran boundaries and do not extend into a Mg-rich solid solution phase grain interior by more than 1 micron from the grain boundary when viewed at 200-500× magnification on a metallography plane. 
     
     
         3 . The endoprosthesis of  claim 1 , wherein the equiaxed Mg-rich solid solution phase grains have an average grain diameter of less than or equal to 1 micron and the second-phase precipitates have an average longest dimension of 0.2 microns or less. 
     
     
         4 . The endoprosthesis of  claim 1 , wherein less than 50% of the equiaxed Mg-rich solid solution-phase grains have twin bands. 
     
     
         5 . The endoprosthesis of  claim 1 , wherein less than 15% of the equiaxed Mg-rich solid solution-phase grains have twin bands. 
     
     
         6 . The endoprosthesis of  claim 1 , wherein the bioerodible magnesium alloy includes beta-phase precipitates outside the grain boundaries, wherein at least 50% of the total amount of beta-phase precipitates are located in grain boundaries between the equiaxed Mg-rich solid solution-phase grains. 
     
     
         7 . The endoprosthesis of  claim 1 , wherein the bioerodible magnesium alloy includes beta-phase precipitates outside the grain boundaries, wherein at least 65% of the total amount of beta-phase precipitates are located in grain boundaries between the equiaxed Mg-rich solid solution-phase grains. 
     
     
         8 . The endoprosthesis of  claim 1 , wherein the bioerodible magnesium alloy includes beta-phase precipitates outside the grain boundaries, wherein at least 80% of the total amount of beta-phase precipitates are located in grain boundaries between the equiaxed Mg-rich solid solution-phase grains. 
     
     
         9 . The endoprosthesis of  claim 1 , wherein the alloy has an elastic modulus of between 39 GPa and 44 GPa, a 0.2% offset yield strength of between 150 MPa and 350 MPa, an ultimate tensile strength of between 250 MPa and 400 MPa, and a tensile reduction in area of at least 30% 
     
     
         10 . The endoprosthesis of  claim 1 , wherein the bioerodible magnesium alloy comprises aluminum. 
     
     
         11 . The endoprosthesis of  claim 10 , wherein the second-phase grain boundary precipitates comprise Mg 17 Al 12 . 
     
     
         12 . The endoprosthesis of  claim 1 , wherein the bioerodible magnesium alloy comprises zinc, calcium, manganese, neodymium, tin, yttrium, cerium, lanthanum, gadolinium, or a combination thereof. 
     
     
         13 . The endoprosthesis of  claim 1 , wherein the bioerodible magnesium alloy comprises between 5 and 11 weight percent aluminum, between 0.1 and 3.0 weight percent zinc, up to 0.3 weight percent manganese, and between 0.6 and 1.5 weight percent neodymium, and balance magnesium. 
     
     
         14 . The endoprosthesis of  claim 1 , wherein the endoprosthesis is a stent comprising a plurality of struts, wherein the struts have a width to thickness ratio of less than 1.2. 
     
     
         15 . The endoprosthesis of  claim 1 , wherein the endoprosthesis has a surface finish having an R a  surface roughness of less than 0.5 microns. 
     
     
         16 . The endoprosthesis of  claim 1 , wherein the fully manufactured non-sterile or sterile finished product bare bioerodible magnesium alloy endoprothesis has a mass loss of less than 10% after 28 days of continuous immersion in non-flowing, agitated Simulated Body Fluid at 37° C., where the Simulated Body Fluid has a volume of at least 10 times an initial volume of the stent. 
     
     
         17 . A method of processing a bioerodible magnesium alloy containing at least 85 weight percent magnesium for a stent comprising:
 forming an ingot or billet comprising a magnesium alloy, the magnesium alloy comprising magnesium and one or more alloying elements; and   performing at least one high-strain process on the ingot or billet to form a microstructure comprising equiaxed Mg-rich solid solution-phase grains having an average grain diameter of less than or equal to 5 microns and continuous or discontinuous second-phase precipitates in grain boundaries between the equiaxed Mg-rich solid solution-phase grains, the second-phase precipitates having an average longest dimension of 0.5 micron or less.   
     
     
         18 . The method of  claim 17 , further comprising holding the ingot or billet at a temperature of between the solvus and liquidus boundaries of the phase diagram for at least 2 hours to homogenize the ingot or billet before preforming the at least one high-strain process on the ingot or billet. 
     
     
         19 . The method of  claim 17 , wherein the at least one high-strain process is an equal-channel high-strain process preformed at a temperature of less than 400° C. 
     
     
         20 . The method of  claim 19 , wherein the ingot or billet is processed through at least two equal-channel high-strain processes at different temperatures, wherein a first equal-channel high-strain process occurring at a first time is performed at a higher temperature than a second equal-channel high-strain process occurring at a second time after the first time, wherein the first equal-channel high-strain process is performed at a temperature of between 250° C. and 400° C. and the second equal-channel high-strain process is performed at a temperature of between 150° C. and 300° C.