U.S. patent application number 14/226503 was filed with the patent office on 2014-07-24 for shape memory alloy articles with improved fatigue performance and methods therefore.
This patent application is currently assigned to W. L. Gore & Associates, Inc.. The applicant listed for this patent is W. L. Gore & Associates, Inc.. Invention is credited to Bret A. Dooley, Christopher C. Lasley, Michael R. Mitchell, Robert R. Steele, Eric M. Tittelbaugh.
Application Number | 20140207228 14/226503 |
Document ID | / |
Family ID | 33310500 |
Filed Date | 2014-07-24 |
United States Patent
Application |
20140207228 |
Kind Code |
A1 |
Dooley; Bret A. ; et
al. |
July 24, 2014 |
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.
Inventors: |
Dooley; Bret A.; (Flagstaff,
AZ) ; Lasley; Christopher C.; (Flagstaff, AZ)
; Mitchell; Michael R.; (Flagstaff, AZ) ; Steele;
Robert R.; (Flagstaff, AZ) ; Tittelbaugh; Eric
M.; (Flagstaff, AZ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
W. L. Gore & Associates, Inc. |
Newark |
DE |
US |
|
|
Assignee: |
W. L. Gore & Associates,
Inc.
Newark
DE
|
Family ID: |
33310500 |
Appl. No.: |
14/226503 |
Filed: |
March 26, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13545750 |
Jul 10, 2012 |
8709177 |
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14226503 |
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12873191 |
Aug 31, 2010 |
8216396 |
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13545750 |
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10428872 |
May 2, 2003 |
7789979 |
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12873191 |
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Current U.S.
Class: |
623/1.18 |
Current CPC
Class: |
A61C 2201/007 20130101;
A61F 2/82 20130101; C22F 1/006 20130101; A61B 2017/00867 20130101;
A61L 31/14 20130101; A61L 31/121 20130101; A61L 31/022 20130101;
A61L 2400/16 20130101 |
Class at
Publication: |
623/1.18 |
International
Class: |
A61F 2/82 20060101
A61F002/82 |
Claims
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.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to the field of shape memory
alloys, medical articles made from shape memory alloys and more
specifically to shape memory alloy articles having improved fatigue
resistance, and methods of making such articles.
BACKGROUND OF THE INVENTION
[0002] Shape memory alloys have been used for a variety of
applications since the discovery of shape memory transformation by
Chang and Reed in 1932. Nitinol, the near-equiatomic alloy of
nickel and titanium (optionally containing ternary, quaternary or
more elements such as copper, chromium, iron, vanadium, cobalt or
niobium) that thus far offers the most useful shape memory and
superelastic properties, was discovered by Buehler and his
colleagues in 1962.
[0003] Nitinol has proven to be adequately biocompatible for a
variety of medical device applications, including implantable
applications. It has been used for orthodontics, in a variety of
orthopedic devices, for filter devices in various body conduits and
for stent devices for maintaining patency of various body conduits,
particularly those of the vasculature. These stent devices
(including stent-grafts, i.e., stents provided with a flexible
covering of a vascular graft material such as porous expanded
polytetrafluoroethylene) are generally inserted into a body conduit
at a site remote from the intended implantation location, and
transported to the desired location by a catheter or similar
device. They are usually inserted in a collapsed or compacted state
to enable their movement through the body conduit to the desired
implantation site, at which location they are expanded to the
desired size to interferably fit within the conduit and hold the
conduit open at that location. While these devices are most often
used for cardiac applications, they are also used for the repair of
thoracic and abdominal aortic aneurysms and for peripheral and
carotid applications.
[0004] Many of these stent devices are made from materials intended
to be expanded by the application of a force applied internal to
the tubular device, usually by the inflation of a catheter balloon
on which the device was mounted for insertion into the body
conduit. These balloon-expandable devices are most often made from
a plastically deformable material such as a stainless steel. Many
other stents are made from shape memory materials, particularly
nitinol, and take advantage of the shape memory or superelastic
properties so that they may be implanted simply by releasing the
constrained, compacted device and allowing it to self-expand at the
desired implantation site.
[0005] Stent devices should be adequately flexible to enable them
to be delivered through bends in the sometimes-tortuous pathways of
a body conduit. They may also need to be adequately flexible to
conform to bends in the body conduit at the implantation site, and
to be able to accommodate movement of the body conduit. This is
particularly true in the vasculature, where a vessel often changes
dimension as a function of systole and diastole. These devices
consequently should also have good fatigue resistance.
[0006] Shape memory materials can exhibit pseudoelastic
(superelastic) behavior, allowing the material to recover a
significant amount of strain due to the reversible, isothermal
metallurgical phase transformations by changes in the state of
stress. The superelastic behavior is characterized by a linear
elastic and a nonlinear pseudoelastic stress-strain response
allowing the material to recover a significant amount of strain due
to the reversible austenitic-martensitic phase transformation.
Conventional nitinol materials can typically recover principle
strains on the order of up to 8% (see "Nitinol Medical Device
Design Considerations" by Philippe P. Poncet, SMST-2000:
Proceedings of the International Conference on Shape Memory and
Superelastic Technologies, pp. 441-455). The superelastic behavior
of nitinol allows for the design of devices that exert a relatively
constant stress over a wide range of strains or shapes. This unique
behavior has been utilized in the design of many implantable
medical devices such as stents and stent-grafts.
[0007] The phase stability of nitinol is a function of both
temperature and stress. The phase stability in the unstressed state
is characterized by the transformation temperatures M.sub.f,
M.sub.s, A.sub.s, and A.sub.f. Martensite is the stable phase at
temperatures below M.sub.f, the martensitic finish temperature.
Upon heating, the martensitic structure begins a reversible
thermoelastic phase transformation to austenite when the
temperature reaches A.sub.s, the austenitic start temperature. The
transformation to austenite is completed when the temperature
reaches A.sub.f, the austenitic finish temperature. Upon cooling
the austenite, the material begins to transform to martensite at a
temperature equal to M.sub.s, the martensitic start temperature,
and completes its transformation to martensite at a temperature
equal to M.sub.f, the martensitic finish temperature.
[0008] The shape memory effect of nitinol is demonstrated by
shaping the material in the relatively high-temperature austenitic
phase and setting the shape by an appropriate heat treatment. Upon
cooling the material below the martensitic transformation
temperature, the material can be deformed to a second shape
configuration while in the martensitic state. Upon heating to
temperatures above the austenitic transformation temperature the
material will return to its original shape configuration.
Conventional nitinol materials can typically recover up to 8%
strain by this shape memory effect (reference ASM Handbook, Volume
2, Shape Memory Alloys, Darel Hodgson et al., page 899).
[0009] The superelastic effect of nitinol is demonstrated by the
application of stress to the nitinol material at temperatures above
the austenitic transformation temperature, and below M.sub.d, the
maximum temperature at which stress-induced martensite can be
formed. The initial application of stress in this case causes the
austenitic structure to deform in the classical Hookean linear
elastic manner until a critical stress is achieved. The application
of stress beyond this critical stress results in a nonlinear
stress-strain response due to the isothermal reversible
transformation to martensite. Upon removal of the applied stress,
the material can reversibly transform back to austenite, returning
to its original shape. As noted previously, conventional nitinol
materials can recover approximately 6-8% strain by this
superelastic effect.
[0010] The alternating in-vivo load conditions (due to changes such
as between systole and diastole) often limit the design of medical
devices such as stents and stent-grafts due to the fatigue
capability of nitinol materials. Improvements in the fatigue
performance of nitinol are desirable to provide an increased
fatigue life and fatigue life safety factor and to increase design
flexibility for implantable medical devices that include
nitinol.
[0011] Various publications describe the fatigue resistance of
devices made from shape memory materials. European Patent
Application EPI 170393 describes a method for improving fatigue
performance of actuators made from materials that have shape memory
effect. The process includes introducing significant cold work,
applying stress in the expected loading direction, and heating
above the recrystallization temperature for short times to create a
uniform, fine-grained, microstructure.
[0012] According to a published article, "Cyclic Properties of
Superelastic Nitinol: Design Implications" (SMST-2000: Proceedings
of the International Conference on Shape Memory and Superelastic
Technologies, D. Tolomeo, S. Davidson, and M. Santinoranont, pp.
471-476) strain-controlled fatigue tests were conducted with
various pre-strain conditions up to 6% pre-strain. Samples were
subjected to strains up to 6%, then unloaded to a specified cyclic
displacement. The endurance limits for different pre-strain values
remained relatively constant.
[0013] A published article titled "Effect of Constraining
Temperature on the Postdeployment Parameters of Self-Expanding
Nitinol Stents" (SMST-2000: Proceedings of the International
Conference on Shape Memory and Superelastic Technologies, Martynov
and Basin, pp. 649-655) describes the evaluation of retaining
temperature on the post deployment parameters of 28 mm aortic-size
stents having a typical diamond shaped stent cell structure. The
article states that "The maximum deformation of any stent element
in the fully compressed state (when the stent is packed into a
delivery catheter) should not exceed the available reversible
deformation limit, which is about 6 to 8%, depending on the
material used."
[0014] In another published article, "Fatigue and Fracture Behavior
of Nickel-Titanium Shape Memory Alloy Reinforced Aluminum
Composites," authors Porter and Liaw describe an aluminum matrix
composite reinforced with discontinuous nitinol particulates by
powder metallurgy processing. The reinforced composite material is
cold rolled at minus thirty degrees centigrade (-30.degree. C.).
Upon re-heating, the nitinol transforms to austenite creating
residual internal stresses around each particle to strengthen the
material. Improved fatigue life was observed compared to the
unreinforced control matrix material.
[0015] An article entitled "The Study of Nitinol Bending Fatigue"
(W. J. Harrison and Z. C. Lin, SMST-2000, Proceedings of the
International Conference on Shape Memory and Superelastic
Technologies) describes fatigue testing of nitinol samples
subjected to alternating strain to simulate the effects of changing
strain resulting from systole and diastole, and optionally
subjected to an additional constant strain (mean strain) that would
be expected to result from the interference between an expanded
stent and the vessel into which it has been fitted. The samples
tested were cut from nitinol tubing. The samples showed good
fatigue life, with the fatigue life being greater for samples
exposed to higher mean strain. This result suggests that that the
samples had apparently been cut at their small diameter (i.e., the
"compacted" diameter appropriate for insertion of such a device
into a body conduit) and subsequently expanded to a larger diameter
at which they were tested, as opposed to having been cut at the
larger, expanded diameter and then compressed slightly to create
the mean strain.
SUMMARY OF THE INVENTION
[0016] The present invention relates to 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.
[0017] 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 (approximately
37.degree. C.).
[0018] Implantable medical devices are those devices that are
intended to remain within a living body for periods of 24 hours or
longer.
[0019] The shape memory alloy articles may be produced from
materials of various shapes, such as wire of various transverse
cross sectional shapes including circular, elliptical, square,
rectangular, etc. Alternatively, the articles may be made by
machining precursor forms such as sheets, tubes or rods, as by
electrical discharge machining (EDM), laser cutting, chemical
milling, or the like.
[0020] 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 of the shape-set nitinol metal
are performed 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. The pre-straining treatments described in this
invention are useful for increasing the fatigue life safety factor
of current nitinol-based medical devices and for incorporating into
the design of future implantable medical devices that include
nitinol, thereby providing additional design flexibility.
[0021] Controlling the amount of pre-strain involves pre-straining
the shape memory metal by the controlled application of bending,
torsional or a combination of these and/or other forces at
pre-determined temperatures. These amounts of pre-strain (resulting
in at least about 0.20% non-recoverable strain) may be calculated
by analytical methods such as finite element analysis or the like,
in conjunction with the material's loading and unloading
behavior.
[0022] Non-recoverable tensile strain is intended to mean the
permanent set, i.e., the plastic deformation that remains upon
releasing the tensile pre-strain or stress, arising from the
displacement of atoms to new lattice sites, as determined by
representative material stress-strain (loading and unloading)
behavioral properties, or as measured by techniques such as
microhardness testing, x-ray diffraction, backscatter electron
Kikuchi patterns, synchrotron radiation, convergent beam electron
diffraction or the like.
[0023] The method of this invention involves pre-straining articles
such that targeted surface regions are subjected to tensile
pre-strains exceeding the recoverable strain limit of the material
(typically 6%-8% strain), while maintaining a significant portion
of the subsurface area (less affected by the pre-strain) within the
superelastic material limit. Tensile pre-strains of this type may
be induced by the application of forces such as bending or
torsional forces. Upon removing the pre-straining force, the
lesser-affected superelastic subsurface region of the article
allows the bulk article to recover a significant level of strain,
such that the article, following the removal of the pre-straining
force, returns to or near to its original geometry.
[0024] This process thus results in desired local surface regions
of the pre-strained article being in a state of compression. A
residual compressive stress state has thus been induced at the
targeted surface region. The result is a significant improvement in
fatigue performance of targeted regions of the article subjected to
this pre-straining operation due to the introduction of residual
compressive surface stresses.
[0025] The process of inducing compressive residual surface
stresses at desired locations by the controlled pre-straining
operation of the present invention, may also produce a concomitant
surface region which is subjected to compression, on the side of
the article opposite the targeted region subjected to tension
during the pre-straining operation. The compressive strains
introduced on the regions opposite the targeted regions may also
exceed the recoverable strain limit of the material, resulting in a
residual state of tension at these regions upon removal of the
pre-straining load. The end result of the pre-straining operation
disclosed in this invention is the improvement in fatigue
performance at the targeted regions of the medical article, thus
resulting in a more fatigue resistant device. This operation can
thus be applied to specifically chosen regions of a medical device
where service fatigue loading is most severe and improved fatigue
performance is desired, or over the entire surface region of the
article.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] FIG. 1A shows a perspective side view of a nitinol alloy
wire of circular cross-section subjected to a pre-straining
operation by a controlled bending operation.
[0027] FIG. 1B shows a transverse cross-sectional view taken
through the wire of FIG. 1A indicating representative strain
contours for the selectively treated cross-sectional area of the
wire.
[0028] FIG. 1C shows a view of a shape-set nitinol wire specimen:
test specimens are pre-strained following the shape-set heat
treatment, while control specimens are not.
[0029] FIG. 2 shows stress-strain curves for nitinol wires
subjected to pre-straining that results in non-recoverable tensile
strain of less than about 0.20% and for inventive wires subjected
to tensile pre-straining at 37.degree. C. that results in
non-recoverable tensile strain of greater than about 0.20%.
[0030] FIG. 3 shows stress-strain curves for nitinol wires
subjected to pre-straining that results in non-recoverable tensile
strain of less than about 0.20% and for inventive wires that
results in non-recoverable tensile strain of greater than about
0.20%, loaded at -30.degree. C. in tension followed by unloading at
-30.degree. C. from various pre-strain levels, and heated in the
stress-free state to 37.degree. C.
[0031] FIG. 4 shows stress-strain curves for nitinol wires loaded
at -30.degree. C. in tension, heated to 37.degree. C. while
maintained at various pre-strain levels, followed by unloading at
37.degree. C. from the various pre-strain levels.
[0032] FIG. 5 shows a graph of the non-recoverable strain achieved
in nitinol wire when subjected to various methods of tensile
pre-straining.
[0033] FIG. 6 shows stress-strain curves for nitinol wires
subjected to tensile pre-straining treatments at various elevated
temperatures.
[0034] FIG. 7 shows a fitted Weibull fatigue survivability plot for
a group of nitinol wire samples provided with tensile pre-straining
treatment in accordance with embodiments of the present invention,
compared to a non pre-strained control group when both groups were
subjected to an axial fatigue test.
[0035] FIG. 8 shows a fitted Weibull fatigue survivability plot for
a group of nitinol wire samples provided with bending pre-straining
treatment in accordance with embodiments of the present invention,
compared to the non pre-strained control group when both groups
were subjected to a flexural fatigue test.
DETAILED DESCRIPTION OF THE INVENTION
[0036] The present invention relates to methods of treating
implantable medical device components formed from nitinol materials
(such as nitinol wire) so that the resultant device exhibits
improved fatigue performance. This invention identifies methods to
induce non-recoverable tensile strain greater than about 0.20%
strain at or near the surface of selected regions of the nitinol
metal by controlled pre-straining processes. The non-recoverable
tensile strain may thus be greater than about 0.25%, 0.3%, 0.35%,
0.4%, 0.45%, 0.5%, 0.6%, 0.7%, 0.8%, 1.0%, 1.25%, 1.5%, and
2.0%.
[0037] Implantable medical devices are typically designed such that
the maximum deformation of any portion of the nitinol material does
not exceed the available reversible deformation limit, typically 6%
to 8% strain, after the shape setting treatments ("Effect of
Constraining Temperature on the Postdeployment Parameters of
Self-Expanding Nitinol Stents," SMST-2000: Proceedings of the
International Conference on Shape Memory and Superelastic
Technologies, Martynov and Basin). The purpose of maintaining
maximum principle strains below the reversible deformation limit
after the shape setting process has been completed is to ensure
that the device will preserve its original shape.
[0038] The reversible deformation limit is defined as the maximum
strain a material can undergo without inducing non-recoverable
strain (i.e., permanent set) greater than about 0.20%.
[0039] The methods of this invention involve the controlled
pre-straining of desired portions of nitinol articles such that
targeted surface regions are subjected to tensile strains exceeding
the about 0.20% recoverable strain limit of the material, while
maintaining a significant portion of the cross-section within the
superelastic material limit. Upon removing the pre-straining force,
the superelastic region of the structure allows the bulk structure
to recover significant levels of strain such that the structure
returns to, or near to, its original geometry. This process thus
results in desired regions of the pre-strained material that had
been subjected to tensile strains beyond their recoverable limit to
be in a state of compression. A residual compressive stress state
has thus been induced at the targeted surface regions. This process
results in a significant improvement in fatigue performance of
targeted regions of the structure subjected to this pre-straining
operation due to the introduction of residual compressive surface
stresses. The controlled process of pre-straining can be
accomplished by flexural loading, torsional loading, or any
combination of loading conditions designed to induce
non-recoverable tensile strains greater than about 0.20% at or near
the surface of fatigue-critical locations of a shape-set, nitinol
containing implantable medical device.
[0040] An example of a pre-straining method included as an
embodiment of this invention is illustrated in FIGS. 1A and 1B.
FIG. 1A shows a nitinol wire of circular cross-section being
deformed by a controlled bending pre-straining operation, indicated
by the arrows. The transverse cross-section of FIG. 1B shows
constant strain contours of the indicated cross-section of the wire
during the pre-straining, bending operation in accordance with FIG.
1A, with the region showing arcuate iso-strain contours near
location A representing the region subjected to non-recoverable
tensile strains greater than about 0.20%. In FIG. 1B, the region
showing the iso-strain contours near location B is subjected to
compressive strains. Upon removal of the pre-straining force, the
superelastic nature of the bulk material (i.e., generally the
material outside of the regions indicated by the iso-strain
contours) forces the material to return to, or near to, its
original shape. This operation thus induces residual compressive
stresses at or near the surface location A and residual tensile
stresses at or near the surface location B. The result of this
pre-straining operation is an improvement in the fatigue
performance of the treated region A. This pre-straining method may
thus be applied in a controlled manner to treat desired
fatigue-critical locations of a device.
[0041] FIG. 1C shows a view of a shape-set nitinol wire specimen.
Wire specimen 10 is formed around pins 11, 12, and 13 and includes
about 1.25 turns of the wire around each of pins 11 and 13 as
shown, to create loops 14. The two opposing loops (14) are wound in
opposite directions (i.e., clockwise and counter-clockwise). The
distance between pins 11 and 12 is defined as dimension "A" with
dimension "B" (partially defining the location of pin 12, parallel
to dimension "A") being half of dimension "A". Dimension "C"
finally defines the location of pin 12 as the distance pin 12 is
located above a line between pins 11 and 13. Pins 11 and 13 are of
equal diameter. The diameter of pin 12 is chosen to provide the
desired radius at the apex 15 of the wire specimen. After being
formed as shown, these wire specimens 10 are subjected to shape-set
heat treatment prior to fatigue testing. Test specimens are
pre-strained following the shape-set heat treatment, while control
specimens are not.
[0042] FIG. 2 shows stress-strain curves for nitinol wire samples
loaded at 37.degree. C. in tension, followed by unloading at
37.degree. C., from various pre-strain levels. It is noted that the
particular stress-strain response is dependent upon such factors as
alloy composition and thermal and mechanical process histories. The
resultant non-recoverable tensile strain (i.e., permanent set)
increases with increasing pre-strain level. Information obtained
from this type of family of stress-strain curves, in conjunction
with analytical procedures such as finite element analysis, can be
utilized to develop an appropriate temperature, controlled
pre-straining (bending) process. This process is designed to induce
non-recoverable tensile strain levels greater than about 0.20% at
desired, fatigue-critical surface locations of a shape-set nitinol
structure. It is apparent this process can be developed for other
temperatures as well.
[0043] FIG. 3 shows a representative family of stress-strain curves
for nitinol wire samples which have been loaded at -30.degree. C.
in tension, followed by unloading at -30.degree. C. from various
pre-strain levels, and heated in the stress-free state to
37.degree. C. The resultant non-recoverable tensile strain
increases with increasing pre-strain level. This family of
stress-strain curves, in conjunction with analytical procedures
such as finite element analysis, can similarly be utilized to
develop an appropriate temperature, controlled pre-straining
process designed to induce non-recoverable tensile strain levels
greater than about 0.20% at fatigue-critical surface locations of a
shape-set nitinol structure.
[0044] FIG. 4 shows another representative family of stress-strain
curves for nitinol wire samples loaded to various pre-strain levels
at -30.degree. C. in tension, heated to 37.degree. C. while
maintained at their respective pre-strain condition, followed by
unloading at 37.degree. C. from their respective pre-strain
condition. The resultant non-recoverable tensile strain increases
with increasing pre-strain level. This family of stress-strain
curves, in conjunction with analytical procedures such as finite
element analysis, can similarly be utilized to develop an
appropriate temperature, controlled pre-straining process designed
to induce non-recoverable tensile strain levels greater than about
0.20% at fatigue-critical surface locations of a shape-set nitinol
structure.
[0045] FIG. 5 shows a plot of non-recoverable tensile strain as a
function of tensile pre-strain level for the various controlled
pre-straining procedures described in FIGS. 2-4. Curve A describes
samples subjected to tensile pre-strain at 37.degree. C. and then
unloaded at 37.degree. C. (as shown in FIG. 2). Curve B describes
samples subjected to tensile pre-strain at -30.degree. C., unloaded
at -30.degree. C. and then heated to 37.degree. C. (as shown in
FIG. 3). Curve C describes samples subjected to pre-strain at
-30.degree. C. and then heated to 37.degree. C. in the pre-strained
condition, and subsequently unloaded at 37.degree. C. (as shown in
FIG. 4). This type of plot, in conjunction with analytical
procedures such as finite element analysis, can be utilized to
develop an appropriate temperature, controlled pre-straining
process designed to induce non-recoverable tensile strain levels
greater than about 0.20% at fatigue-critical locations of a
shape-set nitinol structure.
[0046] Additional techniques may be utilized to decrease the
recoverable strain limit for given pre-strain levels to allow for
the introduction of the desired non-recoverable tensile strain at
the fatigue-critical locations. These techniques can be
incorporated to allow for the introduction of non-recoverable
tensile strains at relatively low pre-strain levels (less than 6%
to 8% pre-strain). Such techniques include but are not limited to,
chemical compositional alloy modifications, thermal and mechanical
process history modifications, surface modification techniques such
as laser surface treatments, or the like.
[0047] FIG. 6 shows a family of stress-strain curves for nitinol
wire samples loaded in tension to 6% pre-strain and unloaded at
various temperatures. The resultant non-recoverable tensile strain
is shown to increase with increasing temperature. This provides an
example of one technique, by using an elevated temperature
pre-straining, which can be used to create relatively high (greater
than about 0.20%) non-recoverable strains at relatively low
pre-strain levels. This type of plot can be used, in conjunction
with analytical procedures such as finite element analysis, to
develop an appropriate temperature, controlled pre-straining
process designed to induce non-recoverable tensile strain levels
greater than about 0.20% at fatigue-critical locations of a
shape-set nitinol structure. The use of elevated temperature,
controlled pre-straining processes can be utilized to induce
non-recoverable tensile strain levels greater than about 0.20% at
fatigue-critical locations of a shape-set nitinol structure.
[0048] Another technique to provide for the induction of
significant (greater than about 0.20%) non-recoverable tensile
strains at relatively low pre-strain levels includes the use of a
composite structure consisting of a superelastic nitinol core
material and an outer surface material with limited recoverable
strain capability. The outer material may include a nitinol
material with an A.sub.f greater than 37.degree. C., preferably a
nitinol material with an A.sub.s greater than 37.degree. C.
Alternatively, the outer surface material may also be stainless
steel, or any other material with a lower recoverable strain limit
than the nitinol core material. The use of such a composite
material can allow the introduction of significant non-recoverable
tensile surface strains at relatively low pre-strain levels. The
induction of non-recoverable tensile surface strains greater than
about 0.20% at of near the surface may be introduced by
pre-straining the material by bending pre-straining, torsional
pre-straining, or a combination of complex pre-strain loading
conditions.
[0049] The process of inducing compressive residual surface
stresses by the pre-straining operations described herein may also
produce a concomitant surface region which is subjected to
compressive strains, occurring on the opposite surface region of
the targeted region subjected to tension, during the pre-straining
operation. The compressive strains introduced on the regions
opposite the targeted regions may also exceed the recoverable
strain limit of the material, resulting in an undesirable residual
state of tension at these regions which may result in reduced
fatigue life.
[0050] FIG. 1C shows the test specimen in a relaxed condition,
wherein apex 15 contains no significant residual stresses.
Following controlled pre-straining caused by moving pins 11 and 13
closer together, the outer radius of apex 15 of the specimen as
shown in FIG. 1C will be in the state of residual compression
stress while the inner radius will be in the state of residual
tensile stress. This method of pre-straining is thus desired when
the critical fatigue location is the outer radius of apex 15.
Alternatively, if the fatigue-critical location is the inner radius
of apex 15, pre-straining is accomplished by moving pins 11 and 13
further apart. In service, the fatigue-critical location is one
that has been previously pre-strained in tension, thus inducing
residual compressive stress at that fatigue-critical location.
[0051] The end result of the pre-straining operation disclosed in
this invention is the improvement in fatigue performance at
targeted regions of the medical device structure, thus resulting in
a more fatigue resistant device. This operation can thus be applied
to specific medical device structure regions where service fatigue
loading is most severe and improved fatigue performance is desired,
or over the entire surface region of the structure.
[0052] In another aspect of the present invention, it is noted that
it is not uncommon for nitinol articles including implantable
medical articles to be subjected to surface modification by various
methods such as electropolishing and shot peening. These methods
are known to reduce any non-recoverable strain at the surface of
these articles. Consequently, it is appropriate that any desired
surface modification is performed prior to the controlled
pre-straining operations as taught by the method of the present
invention.
Example 1
[0053] Axial fatigue tests were conducted using superelastic
nitinol wire samples subjected to different tensile pre-strain
conditions. The nitinol wire (Fort Wayne Metals, Fort Wayne, Ind.,
nominal diameter 0.305 mm) utilized for these tests was
electropolished to a diameter of 0.300 mm and heat treated in air
to obtain a straight configuration and to impart superelastic
behavior at 37.degree. C. (A.sub.f<37.degree. C.) with a
permanent set of less than 0.20% when loaded to 6% strain and
unloaded at 37.degree. C.
[0054] An Instron servohydraulic test machine (Canton, Mass., model
no. 8841) was used for the axial fatigue testing. The testing was
performed in an air thermal chamber set at 37.degree. C.
(+/-1.degree. C.). Wavemaker software (Fast Track 2, Wavemaker
Editor/Runtime, version 7.0.0, provided by Instron) was used to
generate and execute the axial fatigue tests using a displacement
controlled sine waveform. Test specimen gauge length was 100 mm,
held with flat-faced grips (Instron PN 2716-016). Five specimens
were pulled to 104 mm length (4% mean strain), and cycled .+-.0.5
mm (0.5% alternating strain) at cyclic frequencies until failure by
fracture, as shown in Table 1. Three additional specimens were
pulled to 108 mm length (8% pre-strain), released to 104 mm length
(4% mean strain), and cycled .+-.0.5 mm (0.5% alternating strain),
at a cyclic frequency of 12 Hz until fracture (Table 2). An
additional three specimens were pulled to 106 mm length (6%
pre-strain), released to 104 mm length (4% mean strain), and cycled
.+-.0.5 mm (0.5% alternating strain), at a cyclic frequency of 12
Hz until fracture (Table 3).
[0055] Test results as presented in Tables 1-3 show an increase in
fatigue life with an increase in pre-strain level.
TABLE-US-00001 TABLE 1 No pre-strain, 4% mean strain, 0.5%
alternating strain cyclic Specimen frequency CTF 1 8 Hz 3,852 2 8
Hz 2,998 3 15 Hz 3,383 4 12 Hz 3,868 5 12 Hz 3,988 mean CTF: 3618
cycles
TABLE-US-00002 TABLE 2 8% pre-strain, 4% mean strain, 0.5%
alternating strain cyclic Specimen frequency CTF 6 12 Hz 9,266 7 12
Hz 9,779 8 12 Hz 9,533 mean CTF: 9526 cycles
TABLE-US-00003 TABLE 3 6% pre-strain, 4% mean strain, 0.5%
alternating strain cyclic Specimen frequency CTF 9 12 Hz 6,185 10
12 Hz 7,520 11 12 Hz 7,541 mean CTF: 7082 cycles
[0056] The axial fatigue test results are summarized in FIG. 7,
showing a fitted Weibull distribution fatigue survival plot
comparing the fatigue lives for different groups of nitinol wire
samples (plotted as proportion of survivors within each group
versus number of cycles to failure, or CTF).
Example 2
[0057] Flexural fatigue tests were conducted using superelastic
nitinol wire (Fort Wayne Metals, Fort Wayne, Ind., nominal diameter
0.323 mm) samples subjected to different tensile pre-strain
conditions. The nitinol wire used for these tests was
electropolished to a diameter of 0.321 mm.
[0058] Thirty wire test specimens were formed into the shape
described in FIG. 1C, by winding the wire around the 0.79 mm
diameter stainless steel pins 11, 12 and 13 of the heat treatment
fixture, as shown in FIG. 1C. All test specimens were heat treated
in air while on the fixture to set the test sample geometry
configuration and to impart superelastic behavior at 37.degree. C.
(A.sub.f<37.degree. C.) with a permanent set of less than 0.20%
when loaded to 6% strain and unloaded at 37.degree. C. Dimension
"A" between pins 11 and 13 (center-to-center) was 13.72 mm, while
dimension "B" was half of dimension "A". Dimension "C" was 5.08 mm.
The support loops 14 at the ends of each sample 10 were of an
inside diameter that conforms to the diameter of pins 11 and 13.
The apex 15 of each test specimen 10 was formed to a radius (at the
inside radius of the apex bend) that conformed to the diameter of
pin 12.
[0059] Prepared test specimens were divided into three separate
groups (10 samples per group): a control group (Group 1: no
pre-strain), a room temperature pre-strain group (Group 2), and a
cold pre-strain group (Group 3). Each sample from Group 2 was
pre-strained by placing the sample eyelet support loops (14) onto
the same pin (11) to pre-strain the test sample apex (15) at room
temperature. The test specimens were kept at this condition for 2
hours at room temperature and then removed. Group 3 samples were
placed into a bath mixture of dry ice and 100% isopropyl alcohol,
with a submersed thermocouple to monitor bath temperature. The
samples were then pre-strained while submersed in the bath,
following the same pre-strain procedure described for Group 2. The
bath temperature ranged from -34.degree. C. to -14.degree. C.
during the pre-straining procedure. The samples were removed from
the bath while in the constrained condition and placed in room
temperature air for 2 hours prior to removal of the pre-strain
constraint. Group 1 was not pre-strained, and served as control
samples for the subsequent fatigue tests. The maximum principle
tensile pre-strain level at the fatigue-critical location (outside
radius surface of the apex) was calculated to be approximately
8.5%. This maximum principle pre-strain level was calculated by
applying standard engineering mechanics formulas (straight and
curved beam deflection equations, from "Roark's Formulas for Stress
& Strain," 6.sup.th edition, McGraw Hill, New York, N.Y.) to
the specimen geometry.
[0060] Fatigue tests were conducted using a fatigue tester designed
and built for the purpose of conducting cyclic, deflection
controlled, fatigue testing of apical wire samples of the
previously described geometry. The tester is designed to
accommodate up to forty test samples. Wire fatigue test samples
were loaded onto the fatigue test apparatus by placing the test
sample support loops onto 0.79 mm diameter stainless steel pins of
the fatigue tester. The tester was set to alternate test pin
deflections from 9.20 mm and 10.16 mm (i.e., dimension "A" of FIG.
1C alternated between 9.20 mm and 10.16 mm). These deflections were
selected to achieve a maximum principle mean tensile strain of 2.5%
and an alternating strain of 0.3% at the outside radius of the test
specimen apex. The maximum principle strains for these deflections
was calculated by applying standard engineering mechanics formulas
(straight and curved beam deflection equations, from "Roark's
Formulas for Stress & Strain," 6.sup.th edition, McGraw Hill,
New York, N.Y.) to the specimen geometry.
[0061] These deflections were set-up using a telescoping dial depth
gage and gage blocks. All 30 specimens were mounted on the tester,
with test and control samples being placed alternately along the
test fixture. The fatigue testing was performed in a
37.+-.1.degree. C. water bath and at a cyclic frequency of
approximately 18 Hz.
[0062] The flexural fatigue test results are summarized in FIG. 8,
showing a fitted Weibull distribution fatigue survival plot
comparing the fatigue lives for different groups of nitinol wire
samples (plotted as proportion of survivors within each group
versus number of cycles to failure by fracture at the apex, or
CTF). Data are presented in Tables 4-6 for Groups 1-3 respectively.
Various specimens from Groups 2 and 3 survived the 40 million cycle
length of the tests as noted. The fatigue test results demonstrate
an improvement of approximately three orders of magnitude in the
mean fatigue lives for the pre-strained sample groups. This example
demonstrates the significant improvement in the fatigue performance
of nitinol (particularly nitinol wire) when subjected to a
pre-straining treatment.
TABLE-US-00004 TABLE 4 Controls, No Pre-Strain Specimen CTF 1
13,002 2 17,004 3 20,000 4 20,000 5 23,006 6 24,002 7 24,002 8
24,002 9 29.006 10 37,002
TABLE-US-00005 TABLE 5 Room Temperature Pre-Strain Specimen CTF 1
125,055 2 1,300,000 3 4,148,832 4 4,246,188 5 12,408,376 6
40,000,000+ 7 40,000,000+ 8 40,000,000+ 9 40,000,000+ 10
40,000,000+
TABLE-US-00006 TABLE 6 Cold Pre-Strain Specimen CTF 1 755,022 2
2,229,536 3 2,399.999 4 2,481,166 5 2,817,037 6 7,723,746 7
8,242,257 8 9,278,477 9 40,000,000+ 10 40,000,000+
[0063] While the principles of the invention have been made clear
in the illustrative embodiments set forth herein, it will be
obvious to those skilled in the art to make various modifications
to the structure, arrangement, proportion, elements, materials and
components used in the practice of the invention. To the extent
that these various modifications do not depart from the spirit and
scope of the appended claims, they are intended to be encompassed
therein.
* * * * *