U.S. patent application number 10/443231 was filed with the patent office on 2003-11-06 for tissue supporting devices.
Invention is credited to Brown, Brian J., Burmeister, Paul H., Euteneruer, Charles L., Fordenbacher, Paul J., Vrba, Anthony C..
Application Number | 20030208263 10/443231 |
Document ID | / |
Family ID | 22930175 |
Filed Date | 2003-11-06 |
United States Patent
Application |
20030208263 |
Kind Code |
A1 |
Burmeister, Paul H. ; et
al. |
November 6, 2003 |
Tissue supporting devices
Abstract
A new multiple component stent arrangement which allows for
initial self-expansion and subsequent deformation to a final
enlarged size.
Inventors: |
Burmeister, Paul H.; (Maple
Grove, MN) ; Euteneruer, Charles L.; (St. Michael,
MN) ; Brown, Brian J.; (Hanover, MN) ;
Fordenbacher, Paul J.; (Minneapolis, MN) ; Vrba,
Anthony C.; (Maple Grove, MN) |
Correspondence
Address: |
VIDAS, ARRETT & STEINKRAUS, P.A.
6109 BLUE CIRCLE DRIVE
SUITE 2000
MINNETONKA
MN
55343-9185
US
|
Family ID: |
22930175 |
Appl. No.: |
10/443231 |
Filed: |
May 21, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10443231 |
May 21, 2003 |
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09427291 |
Oct 26, 1999 |
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09427291 |
Oct 26, 1999 |
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09172590 |
Oct 14, 1998 |
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6451052 |
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09172590 |
Oct 14, 1998 |
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08737492 |
Mar 19, 1997 |
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6582461 |
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08737492 |
Mar 19, 1997 |
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PCT/US95/06228 |
May 18, 1995 |
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Current U.S.
Class: |
623/1.19 ;
623/23.7 |
Current CPC
Class: |
A61F 2/90 20130101; A61F
2002/9155 20130101; A61F 2250/0042 20130101; A61L 31/022 20130101;
A61F 2/88 20130101; A61F 2/844 20130101; A61F 2220/0075 20130101;
A61F 2210/0076 20130101; A61F 2002/91533 20130101; A61F 2/915
20130101; A61F 2210/0019 20130101; A61F 2002/91575 20130101; A61F
2002/91558 20130101; A61F 2/91 20130101; A61F 2250/0029
20130101 |
Class at
Publication: |
623/1.19 ;
623/23.7 |
International
Class: |
A61F 002/06 |
Claims
What is claimed:
1. As a tissue supporting device a constrainable, self-expanding
member of generally tubular shape comprised of first and second
portions; first portion being of a resilient material; the second
portion being of a deformable and substantially less resilient
material than the first portion; the member being constrainable to
a deployable diameter in preparation for insertion into a patient;
the device being self-expanding when unconstrained to an initially
deployed diameter due to the resiliency of the first portion; the
portions being so associated with respect to each other and the
member such that the device may be further deformed due to the
deformability of the second portion by an external force to
radially enlarge the member to an enlarged fully deployed diameter
for providing permanent tissue support.
2. The device of claim 1 wherein the first and second portions are
of metal.
3. The device of claim 2 wherein the first portion is a spring
metal and the second portion is an annealed metal.
4. The device of claim 1 wherein the first and second portions are
in the form of layers.
5. The device of claim 1 wherein the first and second portions are
discrete portions in the circumference of the device body.
6. The device of claim 1 wherein the first and second portions are
of shape memory alloy, austenite and martensite, respectively.
7. The device of claim 1 wherein the first and second portions are
strands.
8. The device of claim 1 wherein the first and second portions are
of a shape memory alloy.
9. A permanent self-expanding stent having a generally tubular body
of a predetermined fabricated diameter comprised, at about normal
body temperatures, of a shape-memory, superelastic, austenitic
alloy portion and a shape memory, martensitic alloy portion, the
superelastic austenitic alloy portion having a transition
temperature from martensitic to austenitic less than body
temperature while the martensitic alloy portion has a transition
temperature from martensitic to austenitic substantially greater
than body temperature, the martensitic alloy portion and
superelastic austenitic alloy portion being constructed, arranged
and associated with respect to each other in comprising the stent
such that the two alloy portions act in combination to allow, upon
transformation of the austenitic alloy portion to martensitic at a
temperature below the transition temperature, constraint of the
stent to a deployment diameter smaller than the predetermined
fabricated diameter and upon transformation of the austenite alloy
portion from martensite back to austenite to self-expand the stent
back to about the predetermined fabricated diameter at temperatures
in excess of the transition temperature of the austenitic
superelastic portion, the shape memory of the superelastic
austenitic portion tending to form the stent to a larger diameter
due to its shape memory but being restrained therefrom by the
martensitic alloy portion whereby the austenitic alloy portion can
be deformed by external force without plastic deformation along
with the martensitic portion to an enlarged stent diameter beyond
that of the self-expanded diameter.
10. The stent of claim 9 wherein the first and second portions are
in the form of layers in overlying relationship.
11. The stent of claim 9 wherein the first and second portions are
different phases in an alloy.
12. The stent of claim 9 wherein the first and second portions are
in the form of strands.
13. The stent of claim 9 wherein the first and second portions are
in the form of longitudinally arranged interconnected alternating
rings.
14. The stent of claim 9 comprised of a plurality of cable-like
strands and wherein each strand is comprised of a plurality of
wires some of which are of the first portion and some of which are
of the second portion.
15. A permanent self-expanding stent having a generally tubular
body of a predetermined fabricated diameter-parent shape,
comprised, at about normal body temperatures, of a shape-memory,
superelastic, austenite phase portion and a shape memory,
martensite phase portion, the superelastic austenite phase portion
having a transition temperature from martensitic to austenitic less
than body temperature while the martensite phase portion has a
transition temperature from martensitic to austenitic substantially
greater than body temperature, the martensite phase portion and
superelastic austenite phase portion being constructed, arranged
and associated with respect to each other in comprising the stent
such that the two portions act in combination to allow, upon
transformation of the austenite phase portion to martensite,
constraint of the stent to a deployment diameter smaller than the
predetermined fabricated diameter and upon transformation of the
austenite phase portion from martensite back to austenite to
self-expand the stent back toward the predetermined fabricated
diameter at temperatures in excess of the transition temperature of
the austenite superelastic portion, the shape memory of the
superelastic austenitic portion tending to form the stent to the
fabricated diameter parent shape due to its shape memory but being
restrained therefrom by the martensite portion whereby the
austenite portion recovery back toward the fabricated diameter can
be assisted by external force along with the deforming of the
martensitic portion without slip deformation to an enlarged stent
diameter beyond that of the restrained self-expanded diameter.
16. As a tissue supporting device, a constrainable, self-expanding
member of generally tubular shape comprised of first and second
portions of nickel-titanium shape-memory alloy; the first portion
alloy having martensitic and austenitic superelastic shape memory
metallurgical states and a transition temperature therebetween, the
transition temperature being at less than body temperature; the
second portion alloy having martensitic and austenitic
metallurgical states and a transition temperature therebetween, the
transition temperature being substantially higher than body
temperature, said first portion alloy being transformable from
austenitic to the martensitic state when cooled below its
transition temperature so as to render both alloy portions in the
martensitic state whereby the member is constrainable to a
deployable diameter in preparation for insertion into a patient,
during which the first portion alloy may transform to the
austenitic state while constrained, the second portion alloy being
and remaining in the martensitic state; the stent being
self-expanding at body temperature when unconstrained to an
initially deployed diameter due to the first portion alloy being in
the austenitic state and the second portion alloy being in the
martensitic state, the alloy portions being so associated with
respect to each other and the member such that the second portion
alloy restrains the first portion so that the member assumed the
initially deployed diameter, because of the restriction of the
austenitic superelastic alloy from the full exercise of its shape
memory and whereby the alloy portions may be further deformed by an
external force to radially enlarge the member to an enlarged fully
deployed diameter for providing permanent tissue support.
17. A self-expanding stent comprised of at least two components
arranged for coaction, the first component being substantially
austenite and the second being substantially martensite.
18. The stent of claim 16 wherein the first component is a nitinol
alloy.
19. The stent of claim 16 wherein the first component is
superelastic and the second component is any deformable
material.
20. As a tissue supporting device, a constrainable, self-expanding
member of generally tubular shape comprised of nickel-titanium
shape memory alloy containing components of both martensite and
austenite phases, the transition temperature being at about body
temperature, said alloy being transformable to the fully
martensitic state when cooled below its transition temperature so
as to render it to the martensitic state whereby the member is more
easily constrainable to a deployable diameter in preparation for
insertion into a patient; the stent being self-expanding at body
temperature when unconstrained to an initially deployed diameter
due to a portion of the alloy being in the austenitic state and a
portion of the alloy being in the martensitic state, the alloy
portions being so associated with respect to each other and the
member such that the member assumes the initially deployed
diameter, upon .self-expansion and the alloy portions may be
further deformed by an external force to radially enlarge the
member to an enlarged fully deployed diameter for providing
permanent tissue support.
21. A permanent self expanding stent having a generally tubular
body of a predetermined fabricated diameter-parent shape,
comprised, at about normal body temperatures, of a shape-memory,
superelastic, austenite phase portion and a shape memory martensite
phase portion, the superelastic austenite phase portion having a
transition temperature from martensitic to austenitic less than
body temperature while martensite phase portion has a transition
temperature from martensitic to austenitic substantially greater
than body temperature, the martensite phase portion and
superelastic austenite phase portion being constructed, arranged
and associated with respect to each other in comprising the stent
such that the two portions act independently to allow, upon
transformation of the austenite phase portion to martensite,
constraint of both of the original phase portions of the stent to a
deployment diameter smaller than the predetermined fabricated
diameter and upon transformation of the austenite phase portion
from martensite back to austenite to self-expand the stent back to
the austenite phase portion predetermined fabricated diameter at
temperatures in excess of the transition temperature of the
austenite superelastic portion, the shape memory of the
superelastic austenitic portion tending to form the austenitic
portions of the stent to the fabricated diameter parent shape due
to its shape memory, with the martensitic portions remaining in the
deployment shape, additional recovery back toward the stent
fabricated diameter parent shape can be assisted by an external
force deforming the martensitic portion without slip deformation to
an enlarged stent diameter beyond that of the self-expanded
austenitic portion diameter, but not greater than the stent
fabricated diameter parent shape.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to tissue supporting devices in
general and most particularly to vascular stents for placement in
blood vessels. A primary feature of the devices of this invention
is that they are expandable within the body.
[0002] In the past, such devices have been provided for
implantation within body passageways. These devices have been
characterized by the ability to be enlarged radially, often having
been introduced into the desired position in the body as by
percutaneous techniques or surgical techniques.
[0003] These devices are either expanded mechanically, such as by
expansion of a balloon positioned inside the device, or are capable
of releasing stored energy to self-expand themselves within the
body.
[0004] The materials which have been used to make up these devices
have included ordinary metals, shape memory alloys, various
plastics, both biodegradable and not, and the like.
[0005] This invention is concerned with the use of these materials
in a new multiple component arrangement which allows for initial
self-expansion and subsequent deformation to a final enlarged
diameter in the body.
[0006] Balloon expandable stents do not always expand uniformly
around their circumference. As a result, healing may not take place
in a consistent manner. If the stent is coated or covered,
non-uniform expansion may tear the covering or coating.
Additionally, long stents of this type may require long balloons
which can be difficult to handle, difficult to size, and may not
offer ideal performance in tortuous passages in blood vessels and
the like.
[0007] Thus, when addressing such issues, self-expandable stents
have been thought to be generally more desirable. Unfortunately,
one cannot control the degree of expansion and hence the degree of
embedment in the vessel wall. It has been determined that a stent
must be embedded to some degree to be clinically satisfactory.
[0008] The stents of the present invention provide the best
features of both of these types of stents without their
drawbacks.
SUMMARY OF THE INVENTION
[0009] The tissue supporting devices of this invention are
generally cylindrical or tubular in overall shape and of such a
configuration as to allow radial expansion for enlargement. They
are often referred to herein in the general sense as "stents".
Furthermore, the devices are comprised of at least one component,
element, constituent or portion which exhibits a tendency to
self-expand the device to an expanded size and at least one other
component, element, constituent or portion which is deformable so
as to allow an external force, such as a balloon positioned within
the body of the device, to further expand it to a final, larger
desired expanded size. The terms "component", "element",
"constituent" and "portion" are often referred to herein
collectively as "portion".
[0010] Preferably, the devices of the invention are made of metal
and most preferably of shape memory alloys.
[0011] In one embodiment, a first portion is a resilient
spring-like metal for self-expansion and a second portion is a
deformable metal for final sizing. In a more preferred shape memory
embodiment, a first portion is a self-expanding austenitic one and
a second is a martensitic one capable of deformation. In the case
of shape memory embodiments the "portions" may be discrete or
merely different phases of an alloy.
[0012] The most preferred embodiment of the invention is a stent,
preferably of shape memory alloy. The most preferred shape memory
alloy is Ni--Ti, although any of the other known shape memory
alloys may be used as well. Such other alloys include: Au--Cd,
Cu--Zn, In--Ti, Cu--Zn--Al, Ti--Nb, Au--Cu--Zn, Cu--Zn--Sn,
Cu--Zn--Si, Cu--Al--Ni, Ag--Cd, Cu--Sn, Cu--Zn--Ga, Ni--Al, Fe--Pt,
U--Nb, Ti--Pd--Ni, Fe--Mn--Si, and the like. These alloys may also
be doped with small amounts of other elements for various property
modifications as may be desired and as is known in the art.
[0013] The invention will be specifically described hereinbelow
with reference to stents, a preferred embodiment of the invention
although it is broadly applicable to tissue support devices in
general.
BRIEF DESCRIPTION OF THE FIGURES
[0014] FIG. 1 is a braided stent according to one embodiment of
this invention.
[0015] FIG. 2 is a graph showing the martensitic/austenitic
temperature transformation curve and the superelastic area of a
shape memory alloy.
[0016] FIGS. 3 is an end view of a layered stent having two
discrete components according to one aspect of this invention.
[0017] FIGS. 4a and 4b are graphs showing the
martensitic/austenitic temperature transformation curves of the
layers in the stent of FIG. 3.
[0018] FIGS. 5a and 5b are views of another embodiment of the
invention comprised of alternating rings of shape memory alloy.
[0019] FIG. 6 is a showing of a stent fragment of a braided version
of a shape memory stent of this invention.
[0020] FIG. 7 is a graph showing a temperature window for a shape
memory alloy to be used in yet another stent version of this
invention.
[0021] FIG. 7a is a graph showing expansion of a stent with
temperature.
[0022] FIG. 7b is a graph of the same type, the stent having been
cold-worked.
[0023] FIG. 7c is a graph of the same type, the stent having had
pseudoelastic prestraining.
[0024] FIG. 7d is a graph of the same type, the stent having
amnesia inducement.
[0025] FIGS. 8-11 show various expandable configurations (closed
and open) illustrated in fragment which may be used in the stents
of this invention. FIGS. 9a and 9b show a preferred embodiment of
an articulated stent.
[0026] FIG. 12 shows another version of an expandable stent of the
invention.
[0027] FIG. 13 shows yet another version of a stent which may be
used with the invention.
[0028] FIG. 14 is a schematic showing of a braided stent made up of
a plurality of strands.
[0029] FIG. 15 is a detail of a single strand from the stent of
FIG. 14 showing that the strand is made up of a plurality of wires
of two different types.
[0030] FIG. 16 is a cross-sectional view taken along line 16-16 of
FIG. 15 showing the two different types of wire.
DETAILED DESCRIPTION OF THE INVENTION
[0031] Preferred embodiments of this invention are described below
with particular reference to the accompanying drawing Figures.
[0032] Referring first to the embodiment shown in FIG. 1, a stent
10 is shown comprised of braided or interwoven metal strands 12 and
14. Strands 12 are of a resilient spring-like metal such as spring
steel, Elgiloy for example. Preferably, strands 12 are spirally
extending in the same direction, spiraling to the right as seen in
FIG. 1. Strands 14 are of a deformable or annealed metal such as
stainless steel and are preferably spiraled in the opposite
direction as strands 12, as shown in FIG. 1.
[0033] Given such a stent construction of two components i.e.,
strands 12 and. 14, it can be seen that stent 10 may be readily
loaded on a catheter as by placing it over an uninflated balloon on
a balloon catheter and compressing it tightly around the balloon
and then placing a sheath over the stent to hold it in place during
the transluminal placement procedure. Once in place, the sheath is
removed, for example slid back, to expose the stent, allowing it to
self-expand by force of the resilient strands 12 to substantially
assume a self-expanded shape/size. Some self-expansion may be
restrained if held back by strands 14. To finally adjust the size
of the stent, the balloon may be expanded by inflation from within
the stent to exert an outward radial force on the stent and further
enlarge it by stretching and deforming the deformable metal of
strands 14. This may be aided by building into strands 14, a series
of readily deformable structures or means such as bends or kinks 16
as shown in FIG. 1. It can be seen that a permanent adjustable size
beyond the self-expanded size may be obtained with this embodiment.
It is to be noted that many configurations other than braided may
be readily devised to take advantage of this two component concept,
including various of the subsequent configurations -described
hereinbelow. Also, it should be noted that, although not preferred,
the stent may be initially deployed without a balloon; the balloon
following on a separate catheter.
[0034] Referring now to subsequent features, other preferred
embodiments of the invention will be described which make use of
shape memory alloys and some of their unique properties, primarily
their special types of deformation i.e., shape memory deformation
in martensite and/or superelastic deformation in austenite.
[0035] The term "superelasticity" is used to describe the property
of certain shape memory alloys to return to their original shape
upon unloading after a substantially deformation while in their
austenitic state. Superelastic alloys can be strained while in
their austenitic state more than ordinary spring materials without
being plastically deformed. This unusually large elasticity in the
austenitic state is also called "pseudoelasticity", because the
mechanism is nonconventional in nature, or is also sometimes
referred to as "transformational superelasticity" because it is
caused by a stress induced phase transformation. Alloys that show
superelasticity also undergo a thermoelastic martensitic
transformation which is also the prerequisite for the shape memory
effect. Superelasticity and shape memory effects are therefore
closely related. Superelasticity can even be considered part of the
shape memory effect.
[0036] The shape memory and superelasticity effects are
particularly pronounced in Ni--Ti alloys. This application will
therefore focus on these alloys as the preferred shape memory
alloys. The shape memory effect in Ni--Ti alloys has been described
many times and is well known.
[0037] In near-equiatomic Ni--Ti alloys, martensite forms on
cooling from the body centered cubic high temperature phase, termed
austenite, by a shear type of process. This martensitic phase is
heavily twinned. In the absence of any externally applied force
transformation takes place with almost no external macroscopic
shape change. The martensite can be easily deformed by a "flipping
over" type of shear until a single orientation is achieved. This
process is also called "detwinning".
[0038] If a deformed martensite is now heated, it reverts to
austenite. The crystallographic restrictions are such that it
transforms back to the initial orientation thereby restoring the
original shape. Thus, if a straight piece of wire in the austenitic
condition is cooled to form martensite it remains straight. If it
is now deformed by bending, the twinned martensite is converted to
deformed martensite. On heating, the transformation back to
austenite occurs and the bent wire becomes straight again. This
process illustrates the shape memory deformation referred to
above.
[0039] The transformation from austenite to martensite and the
reverse transformation from martensite to austenite do not take
place at the same temperature. A plot of the volume fraction of
austenite as a function of temperature provides a curve of the type
shown schematically in FIG. 2. The complete transformation cycle is
characterized by the following temperatures: austenite start
temperature (A.sub.s), austenite finish temperature (A.sub.f), both
of which are involved in the first part (1) of an increasing
temperature cycle and martensite start temperature (M.sub.s) and
martensite finish temperature (M.sub.f), both of which are involved
in the second part (2) of a decreasing temperature cycle.
[0040] FIG. 2 represents the transformation cycle without applied
stress. However, if a stress is applied in the temperature range
between A.sub.s and M.sub.d, martensite can be stress-induced.
Stress induced martensite is deformed by detwinning as described
above. Less energy is needed to stress induce and deform martensite
than to deform the austenite by conventional mechanisms. Up to
about 8% strain can be accommodated by this process (single
crystals of specific alloys can show as much as about 25%
pseudoelastic strain in certain directions). As austenite is the
thermodynamically stable phase at temperatures between A.sub.s and
M.sub.d under no-load conditions, the material springs back into
its original shape when the stress is no longer applied.
[0041] It becomes increasingly difficult to stress-induce
martensite at increasing temperatures above Af. Eventually, it is
easier to deform the material by conventional mechanisms (movement
of the dislocation, slip) than by inducing and deforming
martensite. The temperature at which martensite can no longer be
stress-induced is called M.sub.d. Above M.sub.d, Ni--Ti alloys are
deformed like ordinary materials by slipping.
[0042] Additional information regarding shape memory alloys is
found in the following references, all of which are incorporated
fully herein by reference:
[0043] "Super Elastic Nickel-Titanium Wires" by Dieter Stockel and
Weikang Yu of Raychem Corporation, Menlo Park, Calif., copy
received November 1992;
[0044] Metals Handbook, Tenth Edition, Vol. 2, Properties and
Selection: Nonferrous Alloys and Special-Purpose Materials, "Shape
Memory Alloys" by Hodgson, Wu and Biermann, pp. 897-902; and,
[0045] In Press, Titanium Handbook, ASM (1994), Section entitled
"Structure and Properties of Ti--Ni Alloys by T. W. Duerig and A.
R. Pelton.
[0046] Since the most preferred shape memory alloy is Ni--Ti, the
martensitic state of this alloy may be used to advantage in the two
component concept of this invention.
[0047] For example, with reference to FIG. 3, a layered
construction may be provided in a stent 30 (shown in end view)
which is generally a hollow cylindrical or tubular body in shape
but which may be formed in a wide variety of specific
configurations or patterns to foster radial expansion of the body
as exemplified in FIGS. 1, 5, 6 and in subsequent FIGS. 8-11.
[0048] Stent 30 is comprised of at least two layers 32 and 34, one
of which 32 is a Ni--Ti alloy (50.8 atomic wt. % Ni, balance Ti,
transition temperature of A.sub.f=0.degree. C.) and normally in the
austenitic state, the other of which 34 is a Ni--Ti (49.4 atomic
wt. % Ni, balance Ti, transition temperature A.sub.f=60.degree. C.)
and normally in the martensitic state. Preferably, the inner layer
is 32 and the outer layer is 34. However, this may be reversed and
also a plurality of layers, alternating or otherwise, may be
utilized in this particular embodiment. Stent 30 is made to a
fabricated size and shape (parent shape) which provides austenitic
layer 32 its parent shape and size i.e., its superelastic high
temperature shape and size. Obviously, in its as fabricated
condition, the Ni--Ti alloy of austenitic layer 32 is selected so
as to have a transition temperature range between its austenitic
and martensitic states which is lower than body temperature as to
ensure that in the body and at body temperatures the austenitic
state will always prevail.
[0049] On the other hand, martensitic layer 34 is of a Ni-Ti alloy
having a transition temperature range significantly greater than
body temperature so as to ensure that under body conditions the
martensitic state will always prevail and the alloy will never
transform to austenite in stent use. This is shown in the graphs of
FIGS. 4a and 4b which demonstrate the relative transition
temperatures of layers 32 and 34, respectively for purposes of this
invention. It can be seen from these graphs that the normal
condition of layer 32 (FIG. 4a) at body temperatures and higher is
the austenitic state while the normal condition of layer 34 (FIG.
4b) at body temperatures is martensitic.
[0050] To manufacture the layered construction, one may make the
austenitic portion with any standard metallurgical technique and
vapor deposit the martensitic portion on its surface. Other
manufacturing techniques such as diffusion bonding, welding, ion
beam deposition, and various others will be apparent to those
familiar with this art.
[0051] Such a stent may be compressed or constrained (deformed to a
small diameter) onto a balloon catheter as described for the
previous embodiment and captured within a sheath. During the
constrainment, austenitic layer 32 may stress induce to a
martensitic state. In the alternative, the stent may be cooled
below the transition temperature of layer 32 to facilitate its
deformation and constrainment. Martensitic layer 34 merely
undergoes deformation. Thus the stent may be "loaded" onto a
balloon catheter. However, with temperature changes occurring up to
body temperature, layer 32 will remain martensite until the
constraint is removed. When released in place in the body, stent 30
will expand to a percentage of its self-expanded size and shape due
to the transformation of layer 32 from martensite to austenite at
which point the balloon may be used to radially expand the stent to
a greater permanent diameter by deforming martensitic layer 34. On
the other hand, initial deployment can take place without a balloon
which may be separately inserted after deployment.
[0052] The two component concept of the invention in the layered
embodiment of FIG. 3 requires both the martensitic and austenitic
phase characteristics of shape memory alloy(s) in the two discrete
components 32 and 34.
[0053] Preferably, the stent is fabricated in such a way that the
austenitic layer 32 tends to self-expand stent 30 to a
predetermined fabricated diameter (parent shape). The martensitic
layer 34 tends to hold back this self-expansion, preventing full
expansion. For example, the stent may only be able to self-expand
to 75% of its full possible diameter (as determined by the
austenitic layer). Therefore, expansion beyond 75% is accomplished
by an applied external force, as by the balloon inside the stent.
It is suggested that the stent not be expanded beyond its normal
fabricated diameter for the austenitic layer 32 will have the
tendency of making the stent diameter smaller as it tries to
recover its fabricated diameter (parent shape). If the stent is
subjected to a temperature above body temperature and above the
transition temperature of the martensitic layer (which is
clinically unlikely), the stent will self-expand to the fabricated
diameter only. Depending on design size there are thus provided
permanent stents capable of fulfilling any needed range of sizes
with an adjustable sizing capability.
[0054] As is known in the art, the desired properties of the shape
memory alloys required for use in this invention may be obtained by
alloy composition and working and heat treatment of the alloys, in
various combinations or singly.
[0055] Manufacturing techniques influence the phase characteristics
of the material. Alloy composition, work history, and heat
treatment all influence the final characteristics. At a specific
operating temperature, say body temperature, the austenite phase
material will have a transition temperature below body temperature
(i.e., A.sub.f=0.degree. C.). The material is capable of taking
high strains and recovering after the load is released. The
martensite phase material will have a higher transition temperature
than body temperature (i.e., A.sub.f=60.degree. C.), and is
characteristically soft and pliable and retains the deformed shape
after load removal. This martensite deformation is caused by
detwinning, not the typical plastic deformation, or yielding, of
crystal slip.
[0056] With reference to FIGS. 5 and 6, other stent constructions
are shown which are similar to the layered version of FIG. 3 in so
far as utilization of the two component concept of this invention
is concerned.
[0057] FIGS. 5a and 5b shows a stent 50 made up of alternating
expandable rings 52 and 54 of austenitic and martensitic alloys,
respectively, analogous to layers 32 and 34 of the FIG. 3
embodiment. Rings 52 and 54 for example are interconnected by strut
members 56 which may be of any material capable of rigidly holding
the rings together. Other interconnector means may be used. As can
be seen in FIG. 5b, the placement of strut members 56 does not
require them to take part in the radial expansion of the stent and
they can therefore be of a relatively ordinary material such as
stainless steel.
[0058] Referring now to FIG. 6, a braided or interwoven
construction is shown similar in construction to that of the
embodiment of FIG. 1. In this embodiment, strands 62 extending to
the right in FIG. 6 are an alloy in the austenitic state whereas
strands 64 extending to the left in FIG. 6 are an alloy in the
martensitic state.
[0059] Referring now to the graph of FIG. 7, it is demonstrated
that the two component concept of the invention may be embodied in
two phases, i.e., components of a single shape memory alloy and
need not be in the form of two discrete components such as layers,
members, wires, etc. In the graph of FIG. 7, it can be seen that an
alloy composition can be selected such that it has a phase
transition temperature window that bounds the proposed operating
temperatures of the stent, such as the normal body temperature
range. Within this transitional window or zone, the material
undergoes the phase transition and is effectively compositionally
comprised of a ratio of austenitic to martensitic phase depending
on the temperature of the stent. This ratio should be selected so
as to provide sufficient radial force from the austenite phase
while still allowing for further expansion of the martensite phase
with a mechanical expansion means such as a balloon. Selecting body
temperature as the operating temperature, a Ni-Ti alloy of about
50/50 atomic wt. % composition (range about 49/51%) will provide an
acceptable "window" for this embodiment, the two components are the
austenite and martensite phases of the nitinol.
[0060] The method of making a stent may be described as follows.
Age the shape memory material (Ni Ti) until body temperature falls
somewhere within the transformation window. Therefore the stent
will not fully recover to its high temperature shape at body
temperature. An example of this technique is described below.
[0061] A stent of tubular 50.8% Ni balance Ti was prepared having a
1.5 mm diameter. It was substantially all austenite at room
temperature, the A.sub.f being about 15-20.degree. C. and therefore
being superelastic at room temperature. The stent was cooled to
below room temperature to form substantially all martensite and
mechanically expanded to 4.7 mm in diameter. It was maintained at
the 4.7 mm in diameter and heat treated at 500.degree. C. for 30
minutes and water quenched. Finally, it was again cooled to below
room temperature to form substantially all martensite and
compressed to a diameter of 1.5 mm. After deployment and at body
temperature the stent has a diameter of 3.5 mm. At about 70% of
full expansion, i.e., about 40.degree. C. it had a diameter of 4.5
mm and at 42.degree. C. it had a fully expanded diameter of 4.7
mm.
[0062] This method works fairly well, but due to the slope of the
temperature versus diameter plot being extremely vertical at body
temperature, a small change in body temperature, or manufacturing
control, can have a large impact on the actual self expansion
diameter. As can be seen from FIG. 7, the slope of the line between
A.sub.f and A.sub.s is rather steep with small changes in
temperature leading to large changes in percent austenite and
consequently large changes in diameter of a stent made of such an
alloy. FIG. 7a shows a temperature versus diameter plot. Three
methods of modifying the slope of the line on the temperature
versus diameter graph are cold work, pseudoelastic prestraining,
and amnesia inducement, illustrated in FIGS. 7b, 7c and 7d,
respectively.
Cold Work
[0063] Residual cold work in nitinol draws out or masks the point
of Af on the diameter versus the temperature curve. This is seen by
the sluggish increase in diameter as temperature increases in the
last 20-30% of recover. By utilizing the effects of cold work, the
effects of temperature change on diameter can be reduced in the
last 20 to 30% of stent expansion. Shown in FIG. 7b is an example
of a temperature versus diameter plot for a cold worked part. FIG.
7a above shows an example of a part without cold work.
Pseudoelastic Prestraining
[0064] Utilizing the effects of pseudoelastic prestraining (S.
Eucken and T. W. Duerig, ACTA Metal, Vol. 37, No. 8, pp 2245-2252,
1989) one can create two distinct plateaus in the stress-strain
behavior. This difference in stress strain behaviors can be
directly linked to two distinct A.sub.f temperatures for the two
plateaus. By placing the transition between the two plateaus at the
transition from self expanding to balloon expanding, i.e., 70%, one
can control the characteristics of the stent at body temperature.
The goal would be to place the A.sub.f temperature for the first
plateau (from maximum compression to 70% expansion) below body
temperature such that the stent has self expanding characteristics.
The A.sub.f temperature for the second plateau would be above body
temperature such that there is no additional self expansion in this
region (70 to 100% expansion) a mechanical device, like a balloon,
can then be used to custom size the stent between 70% and 100% of
the high temperature shape. Results of such a technique is shown in
FIG. 7c.
Amnesia Inducement
[0065] One of the characteristics of nitinol is cycle amnesia. This
was also discussed about in the article referred to immediately
above. As nitinol is cycled from its heat set shape as shown in
FIG. 7d, there is an increase in the amount of amnesia to recover
to the heat set shape with each cycle. As long as this amnesia is
not caused by permanent plastic deformation, the amnesia can be
removed by heating the part above M.sub.d. This shows there is
martensite left in the part after cycling which is preventing full
recovery in the austenite phase (just above A.sub.f). This presence
of non recoverable martensite (below M.sub.d) is what may be used
for the balloon expansion region of the stent.
[0066] FIGS. 8-11 represent examples of various expandable
configurations (a=closed, b=expanded) which may be incorporated
into the devices of this invention. The version shown in FIGS. 10a
and 10b may be modified as shown in FIGS. 10c and 10d (closed and
open, respectively) by omitting portions (indicated at 100 in FIGS.
10c and 10d) as to render the stent flexible for articulation. This
may be done to other of the structures as well to improve
flexibility.
[0067] Yet another version of a device incorporating the two
component concept of the invention is shown in FIG. 12. In this
embodiment, a fragment of a stent 110 is shown. The stent includes
a self-expanding component 112 and a deformable, external force
expandable component 114. Self expanding component 112 may be
resilient spring-like metal such a stainless steel or it may
preferably be a shape memory alloy in the austenitic state.
Component 114 may be any deformable metal or the like such as
annealed stainless steel or preferably a shape memory alloy in the
martensitic state. The two components may simply be mechanically,
welded or bonded together. Functions and operations are as
described hereinabove.
[0068] Referring to FIG. 13 a version analogous to the embodiment
of FIG. 12 is shown in which the two component concept is again
embodied as different zones or portions of a single metal
material.
[0069] As shown in FIG. 13, a stent 120 (fragment showing) is of a
self-expanding component 122 and a deformable component 124, both
of which may be a single metal as spring steel or austenitic Ni--Ti
which has been appropriately treated with respect to component 124
as by localized heat treatment or the like to alter the
characteristics of the material of the 122 component so as to
render it deformable or martensitic, depending on whether it is
merely resilient or is austenitic. Again, function and operation
are the same as with other embodiments.
[0070] Referring now to FIGS. 14-16, a multi-strand braided stent
is shown in FIG. 15. Each strand 150 in the stent is a micro-cable.
That is, each strand is made up of a plurality of wires 152 and 154
as is seen in FIGS. 15 and 16. Each of the wires 152 and 154
consists of two different nitinol alloys as seen best in FIG. 16,
or one nitinol and one ordinary metal such as stainless steel,
platinum or tantalum. The latter two would provide enhanced
radiopacity. One nitinol alloy wire 154 has an austenitic finish
(A.sub.f) temperature less than body temperature. The other wire
152 could be nitinol having an A.sub.s (austenitic start) greater
than body temperature. Also, it could be an ordinary metal.
Additionally, one or more of the strands may be of a biodegradable
material such as a plastic or may be of a material including an
absorbable drug.
[0071] Since the two alloys are stranded into micro-cable one does
not have to engage in selective, discrete heat treating methods to
produce both shape memory and martensitic effects.
[0072] Radiopaque portions or coatings may be included on any parts
of these stents as is known in the prior art.
[0073] While this invention may be embodied in many different
forms, there are described in detail herein specific preferred
embodiments of the invention. This description is an
exemplification of the principles of the invention and is not
intended to limit the invention to the particular embodiments
illustrated.
[0074] The above Examples and disclosure are intended to be
illustrative and not exhaustive. These examples and description
will suggest many variations and alternatives to one of ordinary
skill in this art. All these alternatives and variations are
intended to be included within the scope of the attached claims.
Those familiar with the art may recognize other equivalents to the
specific embodiments described herein which equivalents are also
intended to be encompassed by the claims attached hereto.
* * * * *