METHOD OF MAKING SHAPE MEMORY ALLOY ARTICLES WITH IMPROVED FATIGUE PERFORMANCE

• US 20100319815A1
• Filed: 08/31/2010
• Published: 12/23/2010
• Est. Priority Date: 05/02/2003
• Status: Active Grant

• Current Assignee: W. L. Gore & Associates (W.L. Gore & Associates)
• Original Assignee: W. L. Gore & Associates (W.L. Gore & Associates)
• Inventors: Steele, Robert R., Lasley, Christopher C., Dooley, Bret A., Mitchell, Michael R., Tittelbaugh, Eric M.
• Granted Patent: US 8,177,927 B2
• Time in Patent Office: Days
• US Class Current: 148/519
• CPC Class Codes: A61B 2017/00867 shape memory effect
• CPC Class Codes: A61C 2201/007 using shape memory effect
• CPC Class Codes: A61F 2/82 Devices providing patency to, or preventing collapsing of, tubular structures of the body, e.g. stents stent-grafts for tubular structures of the body other than blood vessels A61F2/04; stent-grafts for blood vessels A61F2/07; instruments specially adapted for placement or removal of stents or stent-grafts A61F2/95; for closing wounds, or holding wounds closed A61B17/04 - A61B17/115; dilators A61M29/00
• CPC Class Codes: A61L 2400/16 Materials with shape-memory or superelastic properties
• CPC Class Codes: A61L 31/022 Metals or alloys
• CPC Class Codes: A61L 31/121 having an inorganic matrix
• CPC Class Codes: A61L 31/14 Materials characterised by their function or physical properties , e.g. injectable or lubricating compositions, shape-memory materials, surface modified materials
• CPC Class Codes: C22F 1/006 Resulting in heat recoverable alloys with a memory effect

Abstract

A method of making 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. The pre-straining operations result in a significant increase in fatigue life of the selectively treated regions and an overall improvement in the fatigue performance of the device.

Claims

1. A method of making a nitinol implantable medical stent with improved fatigue resistance comprising:

a) providing an implantable medical stent having an original geometry at body temperature and comprised of shape-set nitinol metal exhibiting superelastic behavior at body temperature; and

b) pre-straining the shape-set nitinol metal 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 nitinol implantable medical stent while maintaining a significant portion of a transverse cross-section of the at least one selected location within its superelastic material limit;

c) wherein following release of the pre-straining force, the implantable medical stent returns to, or near to, its original geometry at body temperature.

2. A method according to claim 1 wherein the implantable medical stent is pre-strained at a specific temperature and subsequently exposed to other temperatures in a constrained condition thereby inducing the tensile strain to at least about 8.0% at or near only the surface of the at least one selected location on the nitinol implantable medical stent.

3. A method according to claim 1 wherein the implantable medical stent comprises a composite material having a superelastic nitinol metal core and an outer surface material with lower recoverable strain capability than the superelastic nitinol metal core.

4. A method according to claim 3 wherein the outer surface material with lower recoverable strain capability comprises a nitinol material with an A_f greater than 37°

C.

5. A method according to claim 3 wherein the outer surface material with lower recoverable strain capability comprises a material with lower resilience than the core material.

6. A method according to claim 3 wherein the outer surface material with lower recoverable strain capability comprises stainless steel.

7. A method according to claim 1, wherein the implantable medical stent is subjected to surface modification to decrease the recoverable strain limit prior to pre-straining.

8. A method according to claim 1 wherein the pre-straining results in at least about 8.5% tensile strain.

9. A method according to claim 1 wherein the pre-straining results in at least about 9.0% tensile strain.

10. A method according to claim 1 wherein the pre-straining results in at least about 9.5% tensile strain.

11. A method according to claim 1 wherein the pre-straining results in at least about 10.0% tensile strain.

12. A method according to claim 1 wherein the pre-straining results in at least about 10.5% tensile strain.

13. A method according to claim 1 wherein the pre-straining results in at least about 11.0% tensile strain.

14. A method according to claim 1 wherein the pre-straining results in at least about 11.5% tensile strain.

15. A method according to claim 1 wherein the stent is a stent-graft.

16. A method according to claim 2 wherein the stent is a stent-graft.

17. A method according to claim 3 wherein the stent is a stent-graft.

18. A method according to claim 4 wherein the stent is a stent-graft.

19. A method according to claim 5 wherein the stent is a stent-graft.

20. A method according to claim 6 wherein the stent is a stent-graft.

21. A method according to claim 7 wherein the stent is a stent-graft.

22. A method according to claim 8 wherein the stent is a stent-graft.

23. A method according to claim 9 wherein the stent is a stent-graft.

24. A method according to claim 10 wherein the stent is a stent-graft.

25. A method according to claim 11 wherein the stent is a stent-graft.

26. A method according to claim 12 wherein the stent is a stent-graft.

27. A method according to claim 13 wherein the stent is a stent-graft.

28. A method according to claim 14 wherein the stent is a stent-graft.

29. A method of making a nitinol implantable medical filter device with improved-fatigue resistance comprising:

a) providing an implantable medical filter device having an original geometry at body temperature and comprised of shape-set nitinol metal exhibiting superelastic behavior at body temperature; and

b) pre-straining the shape-set nitinol metal 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 nitinol implantable medical filter device while maintaining a significant portion of a transverse cross-section of the at least one selected location within its superelastic material limit;

c) wherein following release of the pre-straining force, the implantable medical filter device returns to, or near to, its original geometry at body temperature.

30. A method according to claim 29 wherein the implantable medical filter device is pre-strained at a specific temperature and subsequently exposed to other temperatures in a constrained condition thereby inducing the tensile strain to at least about 8.0% at or near only the surface of the at least one selected location on the nitinol implantable medical filter device.

31. A method according to claim 29 wherein the implantable medical filter device comprises a composite material having a superelastic nitinol metal core and an outer surface material with lower recoverable strain capability than the superelastic nitinol metal core.

32. A method according to claim 31 wherein the outer surface material with lower recoverable strain capability comprises a nitinol material with an A_f greater than 37°

C.

33. A method according to claim 31 wherein the outer surface material with lower recoverable strain capability comprises a material with lower resilience than the core material.

34. A method according to claim 31 wherein the outer surface material with lower recoverable strain capability comprises stainless steel.

35. A method according to claim 29, wherein the implantable medical filter device is subjected to surface modification to decrease the recoverable strain limit prior to pre-straining.

36. A method according to claim 29 wherein the pre-straining results in at least about 8.5% tensile strain.

37. A method according to claim 29 wherein the pre-straining results in at least about 9.0% tensile strain.

38. A method according to claim 29 wherein the pre-straining results in at least about 9.5% tensile strain.

39. A method according to claim 29 wherein the pre-straining results in at least about 10.0% tensile strain.

40. A method according to claim 29 wherein the pre-straining results in at least about 10.5% tensile strain.

41. A method according to claim 29 wherein the pre-straining results in at least about 11.0% tensile strain.

42. A method according to claim 29 wherein the pre-straining results in at least about 11.5% tensile strain.