Shape memory alloy articles with improved fatigue performance and methods therefore

Abstract

Articles made of shape memory alloys having improved fatigue performance and to methods of treating articles formed from shape memory alloy materials by pre-straining the articles (or desired portions of the articles) in a controlled manner so that the resultant articles exhibit improved fatigue performance. The shape memory articles are preferably medical devices, more preferably implantable medical devices. They are most preferably devices of nitinol shape memory alloy, most particularly that is superelastic at normal body temperature. The pre-straining method of the present invention as performed on such articles includes the controlled introduction of non-recoverable tensile strains greater than about 0.20% at the surface of a desired portion of a shape memory alloy article. Controlled pre-straining operations are performed on the shape-set nitinol metal to achieve non-recoverable tensile strain greater than about 0.20% at or near the surface of selected regions in the nitinol metal article.

We claim: 
     
         1 . A self-expanding implantable medical device comprising a frame including nitinol that exhibits superelastic behavior at body temperature, the frame of said self-expanding implantable medical device being expandable from a constrained delivery profile to an expanded operative profile, wherein the frame is pre-strained by the application of a pre-straining bending force to an extent that selectively induces at least about 8.0% tensile strain at or near only a surface of at least one selected location on the frame during circumferential compaction of the frame to a smaller size, wherein following release of the pre-straining force, the frame circumferentially expands to, or near to, its geometry prior to the application of the pre-straining forces. 
     
     
         2 . A self-expanding implantable medical device according to  claim 1  wherein the pre-straining results in at least about 8.5% tensile strain. 
     
     
         3 . A self-expanding implantable medical device according to  claim 2  wherein the pre-straining bending force is applied in a controlled manner. 
     
     
         4 . A self-expanding implantable medical device according to  claim 1  wherein the pre-straining bending force is applied in a controlled manner. 
     
     
         5 . A self-expanding implantable medical device comprising a frame including nitinol that exhibits superelastic behavior at body temperature, the frame of said self-expanding implantable medical device being expandable from a constrained delivery profile to an expanded operative profile, wherein the frame is pre-strained by the application of a pre-straining torsional force to an extent that selectively induces at least about 8.0% tensile strain at or near only a surface of at least one selected location on the frame during circumferential compaction of the frame to a smaller size, wherein following release of the pre-straining force, the frame circumferentially expands to, or near to, its geometry prior to the application of the pre-straining forces. 
     
     
         6 . A self-expanding implantable medical device according to  claim 5  wherein the pre-straining results in at least about 8.5% tensile strain. 
     
     
         7 . A self-expanding implantable medical device according to  claim 6  wherein the pre-straining torsional force is applied in a controlled manner. 
     
     
         8 . A self-expanding implantable medical device according to  claim 5  wherein the pre-straining torsional force is applied in a controlled manner. 
     
     
         9 . A self-expanding implantable medical device comprising a frame including nitinol that exhibits superelastic behavior at body temperature, the frame of said self-expanding implantable medical device being expandable from a constrained delivery profile to an expanded operative profile, wherein the frame is pre-strained by the application of a pre-straining bending force to an extent that selectively induces at least about 0.40% non-recoverable tensile strain at or near only a surface of at least one selected location on the frame during circumferential compaction of the frame to a smaller size, wherein following release of the pre-straining force, the frame circumferentially expands to, or near to, its geometry prior to the application of the pre-straining force. 
     
     
         10 . A self-expanding implantable medical device according to  claim 9  wherein the pre-straining bending force is applied in a controlled manner. 
     
     
         11 . A self-expanding implantable medical device according to  claim 9  wherein the pre-straining results in at least about 0.45% non-recoverable tensile strain. 
     
     
         12 . A self-expanding implantable medical device according to  claim 11  wherein the pre-straining bending force is applied in a controlled manner. 
     
     
         13 . A self-expanding implantable medical device according to  claim 9  wherein the pre-straining results in at least about 0.50% non-recoverable tensile strain. 
     
     
         14 . A self-expanding implantable medical device according to  claim 13  wherein the pre-straining bending force is applied in a controlled manner. 
     
     
         15 . A self-expanding implantable medical device according to  claim 9  wherein the pre-straining results in at least about 0.60% non-recoverable tensile strain. 
     
     
         16 . A self-expanding implantable medical device according to  claim 15  wherein the pre-straining bending force is applied in a controlled manner. 
     
     
         17 . A self-expanding implantable medical device according to  claim 9  wherein the pre-straining results in at least about 0.80% non-recoverable tensile strain. 
     
     
         18 . A self-expanding implantable medical device according to  claim 17  wherein the pre-straining bending force is applied in a controlled manner. 
     
     
         19 . A self-expanding implantable medical device according to  claim 9  wherein the pre-straining results in at least about 1.0% non-recoverable tensile strain. 
     
     
         20 . A self-expanding implantable medical device according to  claim 19  wherein the pre-straining bending force is applied in a controlled manner. 
     
     
         21 . A self-expanding implantable medical device according to  claim 9  wherein the pre-straining results in at least about 1.5% non-recoverable tensile strain. 
     
     
         22 . A self-expanding implantable medical device according to  claim 21  wherein the pre-straining bending force is applied in a controlled manner. 
     
     
         23 . A self-expanding implantable medical device according to  claim 9  wherein the pre-straining results in at least about 2.0% non-recoverable tensile strain. 
     
     
         24 . A self-expanding implantable medical device according to  claim 23  wherein the pre-straining bending force is applied in a controlled manner.