Method for imparting improved fatigue strength to wire made of shape memory alloys, and medical devices made from such wire

• US 9,272,323 B2
• Filed: 02/28/2013
• Issued: 03/01/2016
• Est. Priority Date: 10/31/2008
• Status: Active Grant

• Current Assignee: W. L. Gore & Associates (W.L. Gore & Associates)
• Original Assignee: W. L. Gore & Associates (W.L. Gore & Associates)
• Inventors: Schaffer, Jeremy E.
• Primary Examiner(s): WYSZOMIERSKI, GEORGE P
• Application Number:
  US13/780,238
• Publication Number:
  US 20130174631A1
• Time in Patent Office: 1,097 Days
• Field of Search: None
• US Class Current: 1/1
• 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 31/14 Materials characterised by their function or physical properties , e.g. injectable or lubricating compositions, shape-memory materials, surface modified materials
• CPC Class Codes: A61N 1/05 for implantation or insertion into the body, e.g. heart electrode A61N1/06 takes precedence
• CPC Class Codes: B21F 9/00 Straining wire straining prestressing wires for concrete E04G21/12; connections or attachments adapted for straining F16G11/00; bundling machines or tools B65B13/00
• CPC Class Codes: C22C 19/00 Alloys based on nickel or cobalt
• CPC Class Codes: C22C 19/007 with a light metal (alkali metal Li, Na, K, Rb, Cs; earth alkali metal Be, Mg, Ca, Sr, Ba, Al Ga, Ge, Ti) or B, Si, Zr, Hf, Sc, Y, lanthanides, actinides, as the next major constituent
• CPC Class Codes: C22F 1/006 Resulting in heat recoverable alloys with a memory effect

Abstract

Wire products, such as round and flat wire, strands, cables, and tubing, are made from a shape memory material in which inherent defects within the material are isolated from the bulk material phase of the material within one or more stabilized material phases, such that the wire product demonstrates improved fatigue resistance. In one application, a method of mechanical conditioning in accordance with the present disclosure isolates inherent defects in nickel-titanium or NiTi materials in fields of a secondary material phase that are resistant to crack initiation and/or propagation, such as a martensite phase, while the remainder of the surrounding defect-free material remains in a primary or parent material phase, such as an austenite phase, whereby the overall superelastic nature of the material is preserved.

Claims

1. A method, comprising the steps of:

providing a wire product made of a shape-set, nickel-titanium shape memory material, a majority of the shape memory material in an austenitic phase at a human body temperature;

mechanically conditioning the wire product by;

applying an engineering stress between 700 MPa and 1600 MPa; and

releasing the applied engineering stress such that said wire product has a fatigue endurance such that said wire survives 10⁶ cycles at a 1.90% peak-to-peak strain amplitude; and

incorporating the wire product into a medical device.

2. The method of claim 1, wherein said mechanical conditioning step occurs prior to said incorporation step.

3. The method of claim 1, wherein said mechanical conditioning step occurs after said incorporation step.

4. The method of claim 1, wherein said mechanical conditioning step comprises:

applying an engineering stress between 900 MPa and 1450 MPa;

and releasing the applied engineering stress.

5. The method of claim 1, wherein said mechanical conditioning step comprises:

applying an engineering stress between 1100 MPa and 1350 MPa;

and releasing the applied engineering stress.

6. The method of claim 1, further comprising repeating said mechanically conditioning step at least once.

7. The method of claim 1, wherein said mechanical conditioning step is conducted below a martensite deformation temperature (Md) of the nickel-titanium shape memory material.

8. The method of claim 1, wherein said mechanical conditioning step further comprises:

applying the stress to the wire product in an environment having a temperature T, wherein

T=Af±

50°

C.,

wherein Af is the austenite transformation finish temperature of the nickel-titanium shape memory material.

9. The method of claim 1, wherein the wire product is selected from the group consisting of wire having a circular cross-section, wire having a non-circular cross-section, cable, coil, and tubing.

10. A method of conditioning a wire made of a shape-set, nickel-titanium shape memory material, a majority of the shape memory material in an austenitic phase at a human body temperature, the method including the steps of:

applying an engineering stress to the wire between 700 MPa and 1600 MPa; and

releasing the applied engineering stress such that an isothermally non-recoverable strain

of the wire is less than 1% and such that said wire has a fatigue endurance such that said wire survives 10⁶ cycles at a 1.90% peak-to-peak strain amplitude.

11. The method of claim 10, wherein said steps of applying an engineering stress and releasing the applied engineering stress are conducted below a martensite deformation temperature (M_d) of the nickel-titanium shape memory material, and in an environment having a temperature T, wherein

T=A_f±

50°

C.

12. The method of claim 10, wherein said step of applying an engineering stress comprising applying an engineering stress to the entirety of the wire.

13. The method of claim 10, wherein the wire has an active austenitic finish temperature (Ar) of between 280K and 298K.

14. The method of claim 10, wherein the applied engineering stress comprises an engineering stress between 900 MPa and 1450 MPa.

15. The method of claim 10, wherein the applied engineering stress comprises an engineering stress between 1100 MPa and 1350 MPa.

16. The method of claim 10, further comprising the step of repeating said steps of applying an engineering stress and releasing the applied engineering stress at least once.

17. The method of claim 10, wherein a length oftime elapses between said step of applying an engineering stress and said step of releasing the applied engineering stress, the length of time selected to produce said isothermally non-recoverable strain of the wire ofless than 1%.

18. The method of claim 17, wherein:

said step of applying an engineering stress to the wire comprises raising the stress level via a strain-controlled ramp up to between 700 MPa and 1400 MPa, and then holding the wire at said stress level the length of time, and

said step of releasing the applied engineering stress comprises a strain-controlled ramp down to zero engineering stress.

19. The method of claim 10, wherein said step of releasing the applied engineering stress is performed such that said isothermally non-recoverable strain of the wire is less than 0.25%.

20. The method of claim 10, wherein the wire is subjected to engineering strain of between 8% and 14% during the step of applying an engineering stress.

21. The method of claim 10, wherein said step of applying an engineering stress to the wire comprises applying a tensile force.