U.S. patent application number 10/365302 was filed with the patent office on 2003-08-21 for devices configured from strain hardened ni ti tubing.
Invention is credited to Boylan, John F., Boyle, William J., Papp, John E., Patel, Anuja H..
Application Number | 20030158575 10/365302 |
Document ID | / |
Family ID | 25381629 |
Filed Date | 2003-08-21 |
United States Patent
Application |
20030158575 |
Kind Code |
A1 |
Boylan, John F. ; et
al. |
August 21, 2003 |
Devices configured from strain hardened Ni Ti tubing
Abstract
Cold worked nickel-titanium alloys that have linear
pseudoelastic behavior without a phase transformation or onset of
stress-induced martensite as applied to a medical device having a
strut formed body deployed from a sheath is disclosed. In one
application, an embolic protection device that employs a linear
pseudoelastic nitinol self-expanding strut assembly with a small
profile delivery system for use with interventional procedures is
disclosed. Linear pseudoelastic nitinol is used in the medical
device as distinct from non-linear pseudoelastic (i.e.,
superelastic) nitinol. The expandable strut assembly is made from a
small diameter tubing of cold worked nickel-titanium alloys. The
self-expanding struts that deploy the filter element is laser cut
from a large diameter cold worked nickel-titanium alloy, then
joined to the small diameter tubing.
Inventors: |
Boylan, John F.; (Murrieta,
CA) ; Boyle, William J.; (Fallbrook, CA) ;
Papp, John E.; (Temecula, CA) ; Patel, Anuja H.;
(San Jose, CA) |
Correspondence
Address: |
FULWIDER PATTON LEE & UTECHT, LLP
HOWARD HUGHES CENTER
6060 CENTER DRIVE
TENTH FLOOR
LOS ANGELES
CA
90045
US
|
Family ID: |
25381629 |
Appl. No.: |
10/365302 |
Filed: |
February 12, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10365302 |
Feb 12, 2003 |
|
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|
09882930 |
Jun 14, 2001 |
|
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|
6551341 |
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Current U.S.
Class: |
606/200 |
Current CPC
Class: |
A61F 2002/018 20130101;
A61F 2230/0067 20130101; A61L 31/022 20130101; A61F 2230/0006
20130101 |
Class at
Publication: |
606/200 |
International
Class: |
A61M 029/00 |
Claims
What is claimed is:
1. A medical device for use in a body lumen, comprising: a tubular
body formed from small diameter tubing; a plurality of struts
formed from a large diameter tubing and disposed on the tubular
body such that the struts project radially outward in an
unconstrained state; wherein the large diameter tubing includes a
cold formed nickel-titanium alloy, and the nickel-titanium alloy is
in a martensitic phase only regardless of stress applied to the
alloy; and a sheath at least partially enveloping the body and
restraining the struts in a compressed state for delivery and
retrieval of the device to and from the body lumen.
2. The medical device of claim 1, wherein the strut includes a long
beam extending from a wide base, and the tubular body includes
apertures formed therein, and wherein each aperture receives the
wide base of the strut therein.
3. The medical device of claim 2, wherein the long beam of the
strut includes a sloped profile.
4. The medical device of claim 2, wherein the aperture has a
beveled interior edge and the wide base has a matching beveled
edge.
5. The medical device of claim 2, wherein the medical device
includes a sleeve that slidably engages the tubular body and covers
the wide bases of the struts thereby retaining the struts to the
body.
6. The medical device of claim 1, wherein the tubular body has been
heat treated and a hysteresis curve of the nickel-titanium alloy
does not include a stress plateau.
7. The medical device of claim 1, wherein the struts assume a shape
imparted by cold forming.
8. The medical device of claim 7, wherein the cold forming occurs
below the recrystallization temperature of the nickel-titanium
alloy.
9. The medical device of claim 1, wherein the ingot transformation
temperature of the nickel-titanium alloy is set above 37 degrees
C.
10. The medical device of claim 1, wherein the small diameter
tubing includes a nickel-titanium alloy.
11. A medical device for delivery to, deployment within, and
removal from a lumen of a mammalian body, comprising: a tubular
body derived from small diameter tubing having a plurality of
apertures formed therein; a plurality of struts derived from a
large diameter tubing; wherein the apertures receive the struts
therein and in an unstressed state bend away from the tubular body;
wherein the small and large diameter tubing include a cold formed
nickel-titanium alloy, and the nickel-titanium alloy is in a
martensitic phase only regardless of stress applied to the alloy;
and a sheath at least partially enveloping the body and restraining
the struts in a compressed state for delivery and retrieval of the
device to and from the lumen.
12. The medical device of claim 11, wherein the strut is cold
formed to provide a curvature in a profile thereof.
13. The medical device of claim 11, wherein the strut includes a
long beam extending from a wide base, and the wide base includes a
radius of curvature matching a radius of curvature of the tubular
body.
14. The medical device of claim 13, wherein some of the apertures
have a first shape and the wide bases of the struts have a second
shape complementary to the first shape so that at least a portion
of the struts is recessed into the tubular body when attached
thereto.
15. The medical device of claim 11, wherein the strut includes a
long beam extending from a wide base, and the wide base includes a
beveled periphery that wedges against a beveled periphery of the
aperture.
16. The medical device of claim 11, wherein the strut has been heat
treated and a hysteresis curve of the nickel-titanium alloy does
not include a stress plateau.
17. The medical device of claim 11, wherein the nickel-titanium
alloy has received low temperature heat treating and does not
undergo a phase transformation when stressed.
18. The medical device of claim 11, wherein the struts are bonded
to the tubular body.
19. A method for providing a medical device for use in a body
lumen, comprising: providing a small diameter tubing; forming the
small diameter tubing into a tubular body; providing a large
diameter tubing wherein the large diameter tubing includes a cold
formed nickel-titanium alloy wherein the nickel-titanium alloy is
in a martensitic phase only regardless of stress applied to the
alloy; fashioning a plurality of struts from the large diameter
tubing; disposing the struts on the tubular body such that the
struts project radially outward in an unconstrained state; and
providing a sheath that at least partially envelopes the body and
restrains the struts in a compressed state for delivery and
retrieval of the device to and from the body lumen.
20. The method for providing a medical device of claim 19, wherein
the method further comprises providing a small tubing including a
cold formed nickel-titanium alloy wherein the nickel-titanium alloy
is in a martensitic phase only regardless of stress applied to the
alloy.
21. The method for providing a medical device of claim 19, wherein
the strut is cold formed to achieve a curved profile.
22. The method for providing a medical device of claim 19, wherein
method includes low temperature heat treat of the nickel-titanium
alloy.
23. A medical device for use in a body lumen, comprising: a tubular
body formed from small diameter tubing; a plurality of struts
formed from a large diameter tubing and disposed on the tubular
body such that the struts project radially outward in an
unconstrained state; wherein the large diameter tubing includes a
cold formed nickel-titanium alloy that includes heat treating, and
the nickel-titanium alloy is in a martensitic phase only regardless
of stress applied to the alloy; and a sheath at least partially
enveloping the body and restraining the struts in a compressed
state for delivery and retrieval of the device to and from the body
lumen.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates generally to application of
nickel-titanium alloys to medical devices. More precisely, the
present invention is directed to cold worked nickel-titanium alloys
that h
[0002] ave pseudoelastic behavior without a phase transformation or
onset of stress-induced martensite as applied to a medical device
deployed from a sheath.
[0003] Near equi-atomic binary nickel-titanium alloys (nitinol) are
known to exhibit "pseudoelastic" behavior when given certain cold
working processes or cold working and heat treatment processes
following hot working. Generally speaking, "pseudoelasticity" is
the capacity of the nickel-titanium alloy to undergo large elastic
strains on the order of 8 percent or more when stressed and to
substantially fully recover all strain upon removal of the stress.
Substantially full recovery is typically understood to be less than
0.5 percent unrecovered strain, also known as permanent set or
amnesia.
[0004] Pseudoelasticity can be further divided into two
subcategories: "linear" pseudoelasticity and "non-linear"
pseudoelasticity. "Non-linear" pseudoelasticity is sometimes used
by those in the industry synonymously with "superelasticity."
[0005] Linear pseudoelasticity results from cold working only.
Non-linear pseudoelasticity results from cold working and
subsequent heat treatment. Non-linear pseudoelasticity, in its
idealized state, exhibits a relatively flat loading plateau in
which a large amount of recoverable strain is possible with very
little increase in stress. This flat plateau can be seen in the
stress-strain hysteresis curve of the alloy. Linear
pseudoelasticity exhibits no such flat plateau. Non-linear
pseudoelasticity is known to occur due to a reversible phase
transformation from austenite to martensite, the latter more
precisely called "stress-induced martensite" (SIM). Linear
pseudoelasticity has no such phase transformation associated with
it. Further discussions of linear pseudoelasticity can be found in,
for example, T. W. Duerig, et al., "Linear Superelasticity in
Cold-Worked Ni--Ti," Engineering Aspects of Shape Memory Alloys,
pp. 414-19 (1990).
[0006] Because of the useful nature of the nickel-titanium alloy,
some have attempted to change its properties to solve different
design needs. For example, U.S. Pat. No. 6,106,642 to DiCarlo et
al. discloses annealing nitinol to achieve improved ductility and
other mechanical properties. U.S. Pat. No. 5,876,434 to Flomenblit
et al. teaches annealing and deforming nitinol alloy to obtain
different stress-strain relationships.
[0007] Binary nickel-titanium alloys have been used in the medical
field. Many medical device related applications exploit the
non-linear pseudoelastic capabilities of nitinol. Examples include:
U.S. Pat. Nos. 4,665,906; 5,067,957; 5,190,546; and 5,597,378 to
Jervis; and U.S. Pat. Nos. 5,509,923; 5,486,183; 5,632,746;
5,720,754; and 6,004,629 to Middleman, et al.
[0008] Yet another application of nickel-titanium alloys is in an
embolic protection or filtering device. Such embolic filtering
devices and systems are particularly useful when performing balloon
angioplasty, stenting procedures, laser angioplasty, or atherectomy
in critical vessels, particularly in vessels such as the carotid
arteries, where the release of embolic debris into the bloodstream
can occlude the flow of oxygenated blood to the brain or other
vital organs. Such an occlusion can cause devastating consequences
to the patient. While the embolic protection devices and systems
are particularly useful in carotid procedures, they are equally
useful in conjunction with any vascular interventional procedure in
which there is an embolic risk. An embolic protection device that
uses superelastic nitinol recently released to the market by the
Cordis Corporation is known as the ANGIOGUARD.
[0009] What has been needed and heretofore unavailable in the prior
art is a medical device that exploits the benefits of linear
pseudoelastic nitinol. With the use of linear pseudoelastic
nitinol, the mechanical strength of the device is substantially
greater per unit strain than a comparable device made of
superelastic nitinol. Furthermore, smaller component parts such as
struts can be used because of the greater storage of energy
available in a linear pseudoelastic nitinol device.
SUMMARY OF THE INVENTION
[0010] The present invention is generally directed to cold worked
nickel-titanium alloys that have linear pseudoelastic behavior
without a phase transformation or onset of stress-induced
martensite as applied to a medical device having a strut formed
body deployed from a sheath.
[0011] In one preferred embodiment, the present invention is
directed to a medical device for use in a body lumen comprising a
body formed from struts, wherein the body includes a cold formed
nickel-titanium alloy, and the nickel-titanium alloy is in a
martensitic phase when the body is stressed into a first shape and
also when the stress to the body is relieved to assume a second
shape. The present invention further includes a sheath at least
partially enveloping the body in its first shape. The sheath may be
used to transport the device to a targeted location in the
patient's anatomy, to deploy the device, and to retrieve the device
at the end of the procedure.
[0012] The raw nitinol for use in the present invention has been
cold formed and is further cold worked to set the desired expanded
shape. Furthermore, the cold forming and cold working occur below
the recrystallization temperature of the nitinol alloy.
[0013] During its operation, the linear pseudoelastic nitinol
device can be stressed without developing stress-induced martensite
in the alloy. Consistent with this behavior, an idealized
stress-strain curve of the linear pseudoelastic nitinol does not
contain any flat stress plateaus. Furthermore, despite application
of stress, the nitinol alloy does not undergo a phase
transformation from austenite to martensite or vice versa.
[0014] The resulting preferred embodiment device has greater
mechanical strength at any given strain as compared to a device
made of a standard superelastic nitinol. The stress-strain curve of
the present invention linear pseudoelastic nitinol device also
possesses more energy storage capacity. As a result, for a given
desired performance requirement, the present invention linear
pseudoelastic nitinol device allows for smaller struts and
consequently a lower profile useful in crossing narrow lesions.
[0015] Another advantage is that because the present invention uses
linear pseudoelastic nitinol, the underlying alloy can be selected
from a broader range of available materials yet still maintain
consistent, mechanical properties. In other words, there is less
sensitivity to material variations and processing vagaries as
compared to superelastic nitinol. In addition, since the linear
pseudoelastic nitinol has no transformation from martensite to
austenite or vice versa, there is less of an influence by
temperature-related effects.
[0016] There are many specific applications for the present
invention including vena cava filters, septal plugs, just to name a
few. One specific application for the present invention is in a
filtering device and system for capturing embolic debris in a blood
vessel created during the performance of a therapeutic
interventional procedure, such as a balloon angioplasty or stenting
procedure, in order to prevent the embolic debris from blocking
blood vessels downstream from the interventional site. The devices
and systems of the present invention are particularly useful while
performing an interventional procedure in critical arteries, such
as the carotid arteries, in which vital downstream blood vessels
can easily become blocked with embolic debris, including the main
blood vessels leading to the brain. When used in carotid
procedures, the present invention minimizes the potential for a
stroke occurring during the procedure. As a result, the present
invention provides the physician with a higher degree of confidence
that embolic debris is being properly collected and removed from
the patient's vasculature during the interventional procedure.
[0017] An embolic protection device and system made in accordance
with the present invention preferably includes an expandable filter
assembly which is affixed to the distal end of a cylindrical shaft,
such as a guide wire. The filter assembly includes an expandable
strut assembly preferably made from a linear pseudoelastic nitinol,
and includes a number of outwardly biased and extending struts that
are capable of self-expansion from a contracted or collapsed
position to an expanded or deployed position within a patient's
vasculature. A filter element made from an embolic capturing media
is attached to the expandable strut assembly. The filter element
opens from a collapsed configuration to an expanded configuration
via the movement of the expandable struts similar to that of an
umbrella.
[0018] The present invention further contemplates a medical device
for use in a body lumen comprising a tubular body formed from small
diameter tubing, a plurality of struts formed from a large diameter
tubing and disposed on the tubular body such that the struts
project radially outward in an unstressed state, wherein the large
diameter tubing includes a cold formed nickel-titanium alloy, and
the nickel-titanium alloy is in a martensitic phase only regardless
of stress applied to the alloy, and a sheath at least partially
enveloping the body and restraining the struts in a compressed
state for delivery and retrieval of the device to and from the body
lumen.
[0019] With this embodiment, it is no longer necessary to fabricate
an expanded strut assembly from a small tubing that is heat treated
to the expanded state. Rather, the expanded strut assembly starts
out as a large diameter tubing wherein the struts themselves are
formed from a large diameter tubing and assembled inward to the
desired embolic protection device shape. The struts are preferably
laser cut from the large tubing and are joined to the small
diameter tubing such that in their unconstrained and stable state,
they project radially outward thereby accomplishing the same
expanded state without need of heat treatment.
[0020] By using a large diameter, cold worked or strain hardened
nickel-titanium hypotube in the assembly of the expanded strut
assembly, the strain hardened nickel-titanium material has
increased mechanical properties that allow for the design of
thinner walled interventional devices. Processing the
interventional devices from large diameter hypotube allows for
greater design flexibility and the ability to create more intricate
designs, because of the increased surface area of the large
diameter nickel-titanium hypotube. Moreover, a thinner walled
device presents a reduced overall crossing profile and further
improves system trackability through a tortuous anatomy.
[0021] Other features and advantages of the present invention will
become more apparent from the following detailed description of the
invention when taken in conjunction with the accompanying exemplary
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a side elevational view, partially in
cross-section, of an embolic protection device embodying features
of the present invention showing the expandable filter assembly in
its collapsed position within a restraining sheath and disposed
within a vessel.
[0023] FIG. 2 is a side elevational view, partially in
cross-section, similar to that shown in FIG. 1, wherein the
expandable filter assembly is in its expanded position within the
vessel.
[0024] FIG. 3 is a perspective view of an expandable strut assembly
which forms part of the filter assembly of the present invention as
shown in its collapsed position.
[0025] FIG. 4 is a plan view of a flattened section of the
expandable strut assembly shown in FIG. 3 which illustrates one
particular strut pattern.
[0026] FIG. 5 is a set of stress-strain curves for conventional
316L stainless steel, linear pseudoelastic nitinol, and non-linear
pseudoelastic nitinol.
[0027] FIG. 6a is a perspective view of a tubular body cut from a
small diameter tubing with apertures cut therein.
[0028] FIG. 6b is a strut formed from a large diameter tubing.
[0029] FIG. 6c is a perspective view of one embodiment of an
expandable strut assembly after the struts of FIG. 6b have been
joined to the tubular body of FIG. 6a.
[0030] FIG. 6d is a perspective view of the expandable strut
assembly of FIG. 6c with the proximal end in the foreground.
[0031] FIG. 6e is a perspective view of the expandable strut
assembly of FIGS. 6c and 6d, wherein a retainer sleeve is fully
engaged to the body to retain the struts thereto.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0032] The present invention is generally directed to cold worked
nickel-titanium alloys that have linear pseudoelastic behavior
without a phase transformation or onset of stress-induced
martensite as applied to a medical device having a strut formed
body deployed from a sheath. Although the present invention is
applicable to and contemplates numerous medical devices, for the
sake of illustration, the following detail description focuses on
an exemplary embodiment involving a filtering device and system for
capturing embolic debris in a blood vessel created during the
performance of a therapeutic interventional procedure.
[0033] In a preferred embodiment, the present invention medical
device has a body formed from struts, wherein the body includes a
cold formed nickel-titanium alloy, and the nickel-titanium alloy is
in a martensitic phase when the body is stressed into a first shape
and also when the stress on the body is relieved to assume a second
shape. The preferred embodiment further includes a sheath at least
partially enveloping the body in its first shape. The sheath may be
used to transport the device to a targeted location in the
patient's anatomy, to deploy the device, and to retrieve the device
at the end of the procedure.
[0034] Turning now to the drawings, in which like reference
numerals represent like or corresponding elements, FIGS. 1 and 2
illustrate a preferred embodiment embolic protection device 10
incorporating features of the present invention. In the particular
exemplary embodiment shown in FIGS. 1 and 2, the embolic protection
device 10 has a body identified as a filter assembly 12, which
assembly includes an expandable strut assembly 14 and a filter
element 16. The filter assembly 12 is optionally rotatably mounted
or fixed on the distal end of an elongated tubular shaft. The shaft
as shown in FIGS. 1 and 2 is a guide wire 18, for example.
[0035] FIG. 1 also depicts a delivery system having a small
delivery profile P. This reduced profile P is an advantage of the
present invention filter assembly 14 and delivery system
(restraining sheath 46 and recovery sheath 48), and is a result of
the stress-strain hysteresis curve for linear pseudoelastic
nitinol. This novel approach is described more fully below.
[0036] In the side elevational and cross-sectional views of FIGS. 1
and 2, the embolic protection device 10 is positioned within an
artery 20 or other lumen of a patient. This portion of the artery
20 has an area of treatment 22 in which atherosclerotic plaque 24
has built up against the inside wall 26 of the artery 20. The
filter assembly 12 is placed distal to, and downstream from, the
area of treatment 22.
[0037] A balloon angioplasty catheter (not shown) can optionally be
introduced within the patient's vasculature in a conventional
Seldinger technique through a guiding catheter (not shown). The
guide wire 18 is passed through the area of treatment 22 and the
dilatation catheter can be advanced over the guide wire 18 within
the artery 20 until the balloon portion is appositioned directly in
the area of treatment 22. The balloon of the dilatation catheter is
inflated, thereby expanding the plaque 24 against the inside wall
26 of the artery 20. This opens the occlusion, expands the artery
20, and reduces the blockage in the vessel caused by the plaque
24.
[0038] After the dilatation catheter is removed from the patient's
vasculature, a stent 25 (shown in FIG. 2) may be delivered to the
area of treatment 22 using over-the-wire techniques. The stent 25
helps to scaffold and maintain the area of treatment 22, which in
turn help to prevent restenosis from occurring in the area of
treatment 22.
[0039] Any embolic debris 27 that breaks off from the plaque 24
during the interventional procedure is released into the
bloodstream. The embolic debris 27 is carried by blood flow
(indicated by arrows) and is captured by the deployed, i.e.,
unfurled, filter element 16 of the filter assembly 12 located
downstream from the area of treatment 22. Once the interventional
procedure is completed, the filter assembly 12 is collapsed and
removed from the patient's vasculature, taking with it embolic
debris 27 trapped within the filter element 16.
[0040] One exemplary embodiment of the expandable strut assembly 14
is shown in FIGS. 1-2. As can be seen in these figures, the
expandable strut assembly 14 includes a plurality of radially
expandable struts 28 that can move from a compressed or collapsed
position as shown in FIG. 1 to an expanded or deployed position
shown in FIG. 2. The expandable strut assembly 14 is preferably
made from a linear pseudoelastic nitinol alloy so that the struts
28 have a radially outward bias toward the expanded position.
[0041] The expandable strut assembly 14 includes a proximal end 32
which is optionally rotatably attached to the guide wire 18. A
distal end 34 is free to slide longitudinally along the guide wire
18 and can rotate thereabout. The distal end 34 translates along
the guide wire 18 whenever the struts 28 move between the expanded
and contracted positions. A proximal end 32 includes a short
tubular segment or sleeve 36 which has a coil spring formed therein
and which acts as a dampening member or element 38. The function of
the dampening element 38 is explained below. The distal end 34 of
the tubing 30 preferably includes a short segment or sleeve 40
which is slidably and rotatably disposed on the guide wire 18.
[0042] The filter element 16 in one preferred embodiment of the
invention includes a tapered or cone shaped section 50, as seen in
FIGS. 1 and 2. The filter element 16 optionally has a plurality of
openings 53 that allow the blood to perfuse through (indicated by
arrows), yet the openings 53 are small enough that the embolic
debris 27 is captured inside the cone shaped section 50. The filter
element 16 includes a short proximal section 52 which is integral
with the cone shaped section 50 and expands to a substantially
cylindrical shape when the struts 28 of strut assembly 14 are
deployed. An inlet opening 51 located at the short proximal section
52 of cone shaped section 50 collects embolic debris 27, directing
the debris 27 into the filter element 16.
[0043] The short proximal section 52 also functions as a
superstructure to which the filter element 16 and the struts 28 of
the strut assembly 14 can be adhesively or otherwise affixed. At
the opposite end, the filter element 16 has a short distal
cylindrical section 54 which is integral with the remaining
sections of the filter element and is attached to the distal end 34
of the expandable strut assembly 14.
[0044] As best seen in FIG. 1, the filter assembly 12 is maintained
in its collapsed or compressed position through the use of a
restraining sheath 46. The restraining sheath 46 should have
sufficient elasticity to resist the outward bias of the struts 28.
One manner of achieving the required elasticity is through
selection of the proper size and wall thickness for the sheath 46.
Another is through use of the proper elastic material that has
sufficient resilience to resist the expansive forces of the struts
28 held therein. Such sheath materials and designs are known in the
art.
[0045] The guide wire 18 and the restraining sheath 46 have
proximal ends (not shown) that extend outside of the patient. From
outside the patient, it is possible to manipulate the struts 28
into the expanded position by retracting the restraining sheath 46
via its proximal end to expose the struts 28. Since the struts 28
are self-expanding by nature, the withdrawal of the restraining
sheath 46 allows the struts 28 to spring open and the filter
element 16 to unfurl into their expanded positions within the
artery 20. This is depicted in FIG. 2.
[0046] The guide wire 18 optionally includes a small sphere 56
affixed thereto. The small sphere 56 is useful during the delivery
of the embolic protection device 10 into the patient's vasculature.
Specifically, the sphere 56 is approximately as large as the inner
diameter of the restraining sheath 46 and is effectively used as a
nose cone. The nose cone prevents possible "snowplowing" of the
embolic protection device 10 as it is delivered through the
patient's arteries.
[0047] When the embolic protection device 10 is to be removed from
the patient's vasculature, an optional recovery sheath 48 is used
to collapse and recover the filter assembly 12, as shown in FIG. 2.
Generally, this recovery sheath 48 has a slightly larger inner
diameter than the restraining sheath 46 since the struts 28 are now
deployed. Furthermore, the recovery sheath 48 must have sufficient
tensile strength and elasticity at the distal end 47 to be capable
of collapsing the expanded strut assembly 14.
[0048] The collapse of the expandable strut assembly 14 can be
accomplished by holding the guide wire 18 and moving the proximal
end (not shown) of the recovery sheath 48 forward, which moves the
distal end 47 of the sheath 48 over the struts 28. Alternatively,
the recovery sheath 48 can be held stationary while the proximal
end of the guide wire 18 is retracted back to pull the entire
filter assembly 12 into the sheath 48. Upon collapse of the filter
assembly 12, any embolic debris 27 generated and entering the
bloodstream during the interventional procedure remains trapped
inside the filter element 16 and is withdrawn from the bloodstream
when the embolic protection device 10 is removed from the patient's
vasculature.
[0049] The number of struts 28 formed on the expandable strut
assembly 14 can be any number which provides sufficient
expandability within the artery to properly deploy and maintain the
filter element 16 in place. In the embodiment shown in FIGS. 1 and
2, the expandable strut assembly has four self-expanding struts 28.
Likewise, the particular size and shape of each strut 28 can be
varied.
[0050] FIGS. 3-4 show an expandable strut assembly 14 having a
strut pattern formed from an inverted, triangular shape first
portion 60, a substantially straight center section 62, and a
second inverted triangular shaped section 64, which completes the
strut. This particular strut pattern is one preferred design that
provides greater strength in regions of the strut where there would
be a tendency for the strut to break or become weakened. These
regions include the very proximal and distal ends of each strut
which are designed with a wider base. This particular design also
allows the expandable strut assembly 14 to open and close more
uniformly. This is advantageous especially when collapsing the
struts for removal from the patient. Additionally, the center
section 62 allows the struts 28 to expand to a greater volume,
which allows a larger filter element to be placed on the strut
assembly 14, if needed.
[0051] When the precise pattern is cut into the tubing 30, a sleeve
36 which forms the proximal end 32 may optionally be formed into a
helical coil as shown in FIG. 3. The helical coil then functions as
a damping element 38 for the expandable strut assembly 14. As seen
in FIGS. 1 and 2, the sleeve 36 slides over the guide wire 18. The
proximal end 32 of the expandable strut assembly 14 is mounted
between a tapered fitting 42 and an optional radiopaque marker band
44. The tapered end fitting 42 and the marker band 44 affix the
proximal end 32 on to the guide wire 18 to prevent any longitudinal
motion, yet allow for rotation of the filter assembly 12.
[0052] FIG. 4 is a plan view of a rolled out flat sheet of the
tubing 30 used to form the struts 28. Preferably, however, the
tubing 30 is made from nitinol and the initial state of the tubing
is cold worked and fully martensitic in the as-received condition.
A particular design pattern is cut into the thin wall of the tubing
30 in order to form each strut. In the case of the exemplary
embodiment shown in FIG. 3, that pattern consists of truncated
diamond shape apertures 65 which help form the first section 60,
the center section 62 and the triangular shaped section 64. To
create the apertures 65, portions of the tubing 30 are selectively
removed through laser cutting preferably, but etching, stamping, or
other processes are suitable insofar as each particular strut can
be fashioned into a precise shape, width, and length. This
truncated diamond aperture pattern 65 repeats, as seen in FIG. 4,
to provide uniform size to each of the struts 28 formed therein.
Narrow struts such as that shown in FIGS. 1 and 2 can, of course,
be formed as described above.
[0053] Subsequently, the laser cut nitinol tubing 30 is preferably
cold formed and specifically cold worked with no heat treatment
such that it remains in the fully martensitic state. The cold
working proceeds only at temperatures below the recrystallization
temperature of the nitinol alloy. Next, the laser-cut nitinol
tubing 30 is cold worked to its desired expanded size. The desired
expanded size is thus imparted or set into the laser cut tube.
[0054] Alternatively, the tube can be swagged and drawn into the
desired shape and size. Also, the tubing itself may be formed from
nitinol sheet stock rolled into a tube and joined at the seam, then
cold drawn to the desired size. The tube is then laser cut and
processed to ensure that the material remains in a fully
martensitic state.
[0055] Importantly, the laser-cut nitinol tubing 30 is not heat
treated to prevent generation of any loading or unloading plateaus
in the stress-strain curve. In an alternative embodiment, the
nitinol tubing may undergo heat treating for only very limited
durations at low temperatures. The present invention recognizes
that a significant difference between linear pseudoelasticity and
non-linear pseudoelasticity is the absence or presence,
respectively, of stress-induced martensite. It also recognizes that
in order to set a particular shape in nitinol, the nitinol must be
heat treated at a relatively high temperature for a short period of
time. Under normal circumstances, this material would then exhibit
non-linear pseudoelasticity and therefore would undergo a
reversible phase transformation from austenite to martensite. When
setting a shape under standard conditions, for example, 550 degrees
C. for 5 minutes, the nitinol exhibits essentially no springback;
that is, its unconstrained shape after heat treatment is nearly
identical to its constrained shape during heat treatment. The
nitinol does not spring back to its original shape prior to heat
treatment. At the other extreme, linear pseudoelastic nitinol with
no heat treatment has 100 percent springback and always returns to
its original, cold worked shape.
[0056] Springback is a continuous function between no heat
treatment (100 percent springback) and ideal shape setting heat
treatment (approximately zero percent springback). From an
engineering perspective for design of nitinol based pseudoelastic
devices, less springback is more favorable than more springback.
However, in some circumstances, linear pseudoelasticity may be
preferable to non-linear pseudoelasticity. Therefore, the present
invention, in addition to contemplating cold-worked only nitinol,
addresses that regime of heat treatment temperatures and times
within which springback is adequately minimized to successfully
impart a desired shape to the nitinol structure and within which
the nitinol does not develop a stable and reversible martensitic
phase.
[0057] In the preferred embodiment of the present invention, to
achieve the linear pseudoelastic behavior, the binary
nickel-titanium tubing has approximately 55.8 atomic percent
nickel. The tubing must contain a minimum of approximately 38
percent cold working when measured by the reduction in
cross-sectional area, and there is not to be any heat treatment
following final cold reduction. As to the alternative embodiment,
the present invention contemplates accumulated heat treatment of
the tubing of up to 300 degrees C. for up to 5 minutes. Under ideal
conditions, these process parameters should adequately ensure that
the nitinol remains martensitic without a phase change under
stress.
[0058] To illustrate the metallurgical aspects of cold worked
nickel-titanium alloys, FIG. 5 contains the elastic component of
three idealized stress-strain curves for 316L stainless steel,
linear pseudoelastic nitinol, and non-linear pseudoelastic nitinol.
In a preferred embodiment, the expandable strut assembly 14 of the
present invention is formed partially or completely of alloys such
as the linear pseudoelastic nitinol shown in FIG. 5.
[0059] In FIG. 5, in an idealized curve A for a non-linear
pseudoelastic nitinol, the relationship is plotted on x-y axes,
with the x axis representing strain and the y axis representing
stress. The x and y axes are labeled in units of stress from zero
to 320 ksi and strain from 0 to 9 percent, respectively.
[0060] In curve A, when stress is applied to a specimen of a metal
such as nitinol exhibiting non-linear pseudoelastic characteristics
at a temperature at or above that which the transformation of the
martensitic phase to the austenitic phase is complete, the specimen
deforms elastically until it reaches a particular stress level
where the alloy then undergoes a stress-induced phase
transformation from the austenitic phase to the martensitic phase
(i.e., the stress-induced martensite phase). As the phase
transformation progresses, the alloy undergoes significant
increases in strain with little or no corresponding increases in
stress. On curve A this is represented by upper, nearly flat stress
plateau at approximately 70 to 80 ksi. The strain increases while
the stress remains essentially constant until the transformation of
the austenitic phase to the martensitic phase is complete.
Thereafter, further increase in stress is necessary to cause
further deformation. The martensitic metal first yields elastically
upon the application of additional stress and then plastically with
permanent residual deformation (not shown).
[0061] If the load on the specimen is removed before any permanent
deformation has occurred, the martensite specimen elastically
recovers and transforms back to the austenitic phase. The reduction
in stress first causes a decrease in strain. As stress reduction
reaches the level at which the martensitic phase transforms back
into the austenitic phase, the stress level in the specimen remains
essentially constant (but less than the constant stress level at
which the austenitic crystalline structure transforms to the
martensitic crystalline structure until the transformation back to
the austenitic phase is complete); i.e., there is significant
recovery in strain with only negligible corresponding stress
reduction. This is represented in curve A by the lower stress
plateau at about 20 ksi.
[0062] After the transformation back to austenite is complete,
further stress reduction results in elastic strain reduction. This
ability to incur significant strain at relatively constant stress
upon the application of a load and to recover from the deformation
upon the removal of the load is commonly referred to as non-linear
pseudoelasticity (or superelasticity).
[0063] FIG. 5 also has a curve B representing the idealized
behavior of linear pseudoelastic nitinol as utilized in the present
invention. Curve B generally has a higher slope or Young's Modulus
than curve A for the non-linear pseudoelastic nitinol. Also, curve
B does not contain any flat plateau stresses found in curve A. This
stands to reason since the nitinol of curve B remains in the
martensitic phase throughout and does not undergo any phase change.
The same tension and release of stress cycle to generate curve A is
used to generate curve B. To that end, curve B shows that
increasing stress begets a proportional increase in reversible
strain, and a release of stress begets a proportional decrease in
strain. The areas bounded by curves A and B represent the
hysteresis in the nitinol.
[0064] As apparent from comparing curve B to curve A in FIG. 5,
with the use of linear pseudoelastic nitinol, the mechanical
strength of the present invention medical device is substantially
greater per unit strain than a comparable device made of
superelastic nitinol. Consequently, a major benefit is that smaller
component parts such as struts can be used because of the greater
storage of energy available in a linear pseudoelastic nitinol
device. A small profile is one critical factor for crossing narrow
lesions or for accessing remote and tortuous arteries.
[0065] FIG. 5 includes curve C which is the elastic behavior of a
standard 316L stainless steel. Stress is incrementally applied to
the steel and, just prior to the metal deforming plastically,
decrementally released. It is provided here simply for comparison
to curves A and B.
[0066] As mentioned above, the present invention medical device
uses preferably a binary nickel-titanium alloy. In an alternative
embodiment, however, the nickel-titanium may be alloyed with a
ternary element such as palladium, platinum, chromium, iron,
cobalt, vanadium, manganese, boron, copper, aluminum, tungsten,
tantalum, or zirconium.
[0067] FIGS. 6a-6e illustrate a preferred embodiment construction
of the expandable strut assembly 14 wherein large diameter strain
hardened (i.e., cold worked linearly pseudoelastic) nickel-titanium
tubings are used. As mentioned earlier, the use of cold worked
nitinol material increases the device's mechanical properties,
which allows for the design of thinner walled interventional
devices. Also, processing the device from a large diameter hypotube
permits greater design flexibility and the ability to create more
intricate design options because of the increased surface area of
the large diameter nickel-titanium hypotube.
[0068] A problem to be resolved was how to construct an expandable
strut assembly or basket made from tubing yet avoid heat treating
the nickel-titanium alloy. Many designs employ a basket made from
small diameter tubing that is heat treated to set the expanded
shape. In the present embodiment, the deploying struts are made
from a large diameter tubing and attached to a small diameter
tubing. A medical device fashioned from this combination of tubing
sizes avoids or minimizes the need for heat treating steps.
[0069] In various alternative embodiments, the basket shape can be
made by swagging from large diameter tubing. Further, a sheet of
the nitinol material can be processed into a tube, or the sheet can
be formed to create approximately one-half of the desired basket
shape.
[0070] FIG. 6e is a perspective view of a preferred embodiment
expandable strut assembly 70. The expandable strut assembly 70 is
separated into its two major component parts in FIGS. 6a and 6b.
FIG. 6a shows a tubular body 72 formed from a small diameter
tubing. Comparable sizes include 0.5 to 1.0 mm diameter
hypotubes.
[0071] FIG. 6b is a perspective view of a strut 74 fashioned from a
large diameter tubing. Comparable large diameter tubing can be
found in 4 to 50 mm diameter hypotubes. Both the small diameter
tubing and the large diameter tubing are preferable made from the
aforementioned cold worked nickel-titanium alloy. Other materials
known in the art can also be used.
[0072] In FIG. 6a, the tubular body 72 has been laser cut through
processes known in the art to create a particular shape with
apertures or key holes 76 formed therein. The key holes 76 are
dispersed around the circumference of the tubular body 72 and there
are preferably four key holes. Obviously, depending on design,
there can be more or fewer key holes and their locations can be
changed depending upon the assembly location of the strut 74 as
described in further detail below. The tubular body 72 has a
proximal end 82 and a distal end 80. At the distal end 80 there are
preferably four tabs 78 to facilitate mounting of the expandable
strut assembly 70 to the guide wire 18. Furthermore, the key holes
76 are cut or formed so that they have a bevel 84 along the
periphery of the aperture. The bevel 84 along the key bole 76
periphery decreases the opening size towards the interior of the
tubular body 72. Thus, when a complementary part is assembly to
cover the key hole 76, that part tends to wedge into the beveled
opening.
[0073] FIG. 6b is a perspective view of a strut 74 that has been
preferably laser cut from a large diameter tubing. The strut 74 has
a long beam terminating in a wide base 86. Preferably, the wide
base 86 has a periphery that also includes a bevel 84 that
complements the bevel 84 at the key holes 76. In addition, the wide
base 86 is shown with an optional curvature 88. The curvature 88 is
intended to match the curvature of the tubular body 72.
[0074] FIGS. 6c and 6d are alternative perspective views of the
expandable strut assembly 70 showing the distal end 80 and then the
proximal end 82 in the foreground, respectively. As seen in either
drawing, two struts 74 have been assembled to the tubular body 72
in which the wide base 86 mates with the complementary-shaped key
hole 76. Two more struts 74 can be attached to the remaining two
key holes 76, but have been omitted from the drawings for the sake
of clarity of illustration. As seen in FIGS. 6c and 6d, the struts
74 have a profile in the form of an ogee, wherein the long beam
bends outwardly and then bends back toward the tubular body 72.
These curves in the struts 74 are preferably formed by cold working
after their basic shape has been cut from the large diameter
tubing. The curved profile of the struts 74 can of course be
changed to suit the design of the expandable strut assembly and
spring forces needed to deploy the filter element 16. As
distinguished from conventional nitinol material, the present
invention strut preferably incorporates the necessary curves in its
profile through cold working and not through heat setting. With
minimal or no heat treatment, the nickel-titanium alloy has a
stress-strain curve similar to that shown in FIG. 5, in which curve
B lacks a discernible flat stress plateau. As a result, when the
struts 74 are held tightly against the exterior of the tubular body
72 by a delivery sheath or the like, there is no creation of
stress-induced martensite or a phase transformation. Rather, the
nickel-titanium alloy used in the strut 74 remains in its
martensitic phase throughout delivery, deployment, and recovery
from the body lumen.
[0075] Because of the wedging action from the beveled periphery of
the key holes 76 and wide base 86, the strut 74 cannot fall through
the key hole towards the interior of the tubular body 72. The wide
base 86 of the strut 74 is joined to the key holes 76 by use of
glue, solder, or the like. To further secure the struts 74 to the
tubular body, there is an optional sleeve 90 that slides over the
proximal end 82 of the tubular body thus holding the wide bases 86
inside their respective key holes. This is shown in the perspective
view of FIG. 6e.
[0076] In an alternative embodiment, as seen in FIG. 6e, a tubular
shape inner sleeve 94 that fits inside the tubular body 72 can be
used to hold the struts 74 in place. The inner sleeve 94 as well as
the outer sleeve 90 can be made from stainless steel, a rigid
plastic such as polyamide, or similar material known in the
art.
[0077] Also shown in FIGS. 6c and 6d are two alternative
embodiments of the wide base 86 in which the curvature 88 has been
reduced to the radius of the small diameter tubular body 72 thereby
conforming to the surface profile. On the other hand, a wide base
92 has not been conformed into the radius of the small diameter
tubular body 72. Either configuration for the wide bases 86, 92 are
contemplated, with the wide base 86 having curvature 88 being the
preferred design because the fitted joint involves less tolerance
and the strut 74 extends from a more stable platform.
[0078] The strut 74 is either left at the large tube diameter or it
may be cold and/or heat formed to the curved shape shown. Heat
forming at the wide base 86 is possible even to maintain the
material in the martensitic state because during delivery,
deployment, and recovery, the wide base 86 does not undergo any
bending. If it is left in the large tube diameter shape, it is
flexed into the position shown in FIGS. 6c and 6d by the sleeve 90
pressing against the wide base 86 against the key hole 76.
[0079] Typically, the small diameter tubing is laser cut from a
hypotube having a 0.026 inch diameter. It is then expanded and heat
set at the fully expanded state to create the expanded strut
configuration. By using parts fashioned from tubing of two
different diameters as in the present invention, the need for
performing an expansion and heat set are eliminated. Thus, a 4.0 mm
device is cut from approximately 4.5 mm tubing, and a 5.0 mm device
is cut from approximately 5.5 mm tubing, etc.
[0080] While the present invention has been illustrated and
described herein in terms of linear pseudoelastic nitinol filter
assembly of an embolic protection device and its delivery system,
it is apparent to those skilled in the art that the present
invention can be used in other instances. Other modifications and
improvements may be made without departing from the scope of the
present invention.
* * * * *