Shape memory articles and methods for controlling properties

Abstract

Methods for controlling properties of structural elements of implantable medical devices, where the structural elements contain shape memory alloys (SMAs) include promoting or inhibiting in vivo formation of R-phase crystal structure or converging or separating the R-phase from the austenite phase.

What is claimed is: 
     
         1 . A method for increasing fatigue performance of a structural element of an implantable medical device under displacement-controlled fatigue conditions, wherein the element comprises a shape memory alloy (SMA), the method comprising:
 manipulating the SMA to (i) promote R-phase formation following implantation in a subject, or (ii) cause the R-phase and austenite phase to converge.   
     
     
         2 . The method of  claim 1 , wherein manipulating the SMA further results in one or both of a decrease in modulus or an increase in ductility. 
     
     
         3 . The method of  claim 1 , wherein manipulating the SMA comprises heat-treating the SMA to promote R-phase formation following implantation or to cause the R-phase and austenite phase to converge. 
     
     
         4 . The method of  claim 3 , wherein the heat-treating comprises heating the SMA at a temperature of 450° C. or greater. 
     
     
         5 . The method of  claim 3 , wherein the heat-treating comprises heating the SMA at a temperature of 500° C. or greater. 
     
     
         6 . The method of  claim 3 , wherein the heat-treating comprises heating the SMA at a temperature in a range from 450° C. to 550° C. 
     
     
         7 . The method of  claim 6 , wherein the heat-treating is performed on the SMA without stress/strain tuning of the SMA. 
     
     
         8 . The method of  claim 6 , further comprising subjecting the SMA to stress/strain tuning during the heat-treating. 
     
     
         9 . The method of  claim 1 , wherein the structural element is formed to have a length-to-depth ratio of 8:1 or less. 
     
     
         10 . The method of  claim 1 , wherein the SMA comprises Ni and Ti. 
     
     
         11 . A method of  claim 10 , wherein the Ni and Ti are substantially equiatomic. 
     
     
         12 . A method of  claim 1 , wherein the structural element comprises a heart valve frame or a component thereof. 
     
     
         13 . An implantable medical structural scaffold, comprising:
 a shape memory alloy (SMA) material,   where the SMA material comprises an R-phase and an austensite phase transition temperature overlap.   
     
     
         14 . The implantable medical structural scaffold of  claim 13 , wherein the structural element comprises a heart valve frame or a component thereof. 
     
     
         15 . A method for increasing the modulus of a structural element of an implantable medical device, wherein the element comprises a shape memory alloy (SMA), the method comprising:
 manipulating the SMA to (i) inhibit R-phase formation following implantation in a subject, or (ii) cause the R-phase and austenite phase to separate.   
     
     
         16 . The method of  claim 15 , wherein manipulating the SMA further results in one or both of a decrease in ductility and enhanced force-controlled fatigue performance. 
     
     
         17 . The method of  claim 15 , wherein manipulating the SMA comprises heat-treating the SMA to inhibit R-phase formation following implantation or to cause the R-phase and austenite phase to separate. 
     
     
         18 . The method of  claim 15 , wherein the heat treatment comprises heating the SMA at a temperature in a range from 300° C. to 525° C. 
     
     
         19 . The method of  claim 15 , wherein the structural element is formed to have a length-to-depth ratio of 8:1 or greater.