U.S. patent application number 13/113361 was filed with the patent office on 2011-10-20 for medical devices and methods of making the same.
This patent application is currently assigned to Boston Scientific Scimed, Inc.. Invention is credited to Verivada Chandrasekaran, Christopher Torres Molina, Karl Morris Schmidt.
Application Number | 20110257730 13/113361 |
Document ID | / |
Family ID | 29250322 |
Filed Date | 2011-10-20 |
United States Patent
Application |
20110257730 |
Kind Code |
A1 |
Chandrasekaran; Verivada ;
et al. |
October 20, 2011 |
MEDICAL DEVICES AND METHODS OF MAKING THE SAME
Abstract
An endoprosthesis, such as a stent, having a layer that can
enhance the biocompatibility of the endoprosthesis, and methods of
making the endoprosthesis are disclosed.
Inventors: |
Chandrasekaran; Verivada;
(Mercer Island, WA) ; Schmidt; Karl Morris;
(Seattle, WA) ; Molina; Christopher Torres;
(Redmond, WA) |
Assignee: |
Boston Scientific Scimed,
Inc.
Maple Grove
MN
|
Family ID: |
29250322 |
Appl. No.: |
13/113361 |
Filed: |
May 23, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12703838 |
Feb 11, 2010 |
7967854 |
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13113361 |
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11738229 |
Apr 20, 2007 |
7682649 |
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12703838 |
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10629934 |
Jul 29, 2003 |
7297157 |
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11738229 |
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10263212 |
Oct 2, 2002 |
6638301 |
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10629934 |
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Current U.S.
Class: |
623/1.34 |
Current CPC
Class: |
C23C 28/023 20130101;
C23C 28/34 20130101; Y10T 428/31573 20150401; A61F 2250/0032
20130101; A61L 31/088 20130101; A61B 90/39 20160201; C23C 28/325
20130101; A61F 2002/91566 20130101; C23C 8/10 20130101; A61F
2250/0098 20130101; Y10T 428/31692 20150401; A61L 31/18 20130101;
A61F 2002/91525 20130101; C23C 14/46 20130101; Y10T 428/12
20150115; C23C 28/36 20130101; C25D 5/48 20130101; Y10T 428/12771
20150115; C23C 28/322 20130101; C23C 28/42 20130101; C23C 28/028
20130101; Y10T 428/31576 20150401; A61F 2/915 20130101; C23C 28/345
20130101; C23C 28/341 20130101; Y10T 428/2995 20150115; Y10T
428/31544 20150401; C23C 14/16 20130101; A61F 2002/91575 20130101;
C23C 28/321 20130101; Y10T 428/31551 20150401; A61F 2/91 20130101;
A61F 2002/91533 20130101; C23C 28/00 20130101; C23C 28/021
20130101; Y10T 428/31678 20150401 |
Class at
Publication: |
623/1.34 |
International
Class: |
A61L 27/50 20060101
A61L027/50 |
Claims
1. A medical implant, comprising: a member including at least a
first portion, the first portion comprising an oxide-rich surface
layer and an alloy of a radiopaque material and a second material,
the oxide-rich surface layer defining an outer surface of the
member, the oxide-rich surface layer comprising an oxidized form of
the alloy.
2. The medical implant of claim 1, wherein the member comprises a
second portion disposed inwardly of the first portion.
3. The medical implant of claim 2, wherein the first portion
includes a first layer consisting essentially of the radiopaque
material, a second layer comprising the alloy disposed outwardly of
the first layer, and the oxide-rich surface layer disposed
outwardly of the second layer.
4. The medical implant of claim 3, wherein the first layer is more
radiopaque than the second portion.
5. The medical implant of claim 2, wherein the second portion
comprises a material selected from the group consisting of
stainless steel and nickel-titanium alloy.
6. The medical implant of claim 2, further comprising a third
portion between the first portion and the second portion.
7. The medical implant of claim 1, wherein the radiopaque material
is selected from the group consisting of gold, platinum, palladium,
and tantalum.
8. The medical implant of claim 1, wherein the second material is
selected from the group consisting of titanium, chromium,
palladium, niobium, and silicon.
9. The medical implant of claim 1, further comprising a polymeric
layer on the member.
10. The medical implant of claim 1, further comprising a
drug-releasing layer on the member.
11. The medical implant of claim 1, wherein the alloy comprises a =
concentration gradient of the radiopaque material.
12. The medical implant of claim 11, wherein the concentration of
the radiopaque material increases as a function of distance from
the outer surface.
13. The medical implant of claim 11, wherein the concentration
gradient varies substantially linearly along a thickness of the
first portion.
14. The medical implant of claim 1, wherein the radiopaque material
is capable of attenuating an incident X-ray beam by more than about
70%.
15. The medical implant of claim 1, wherein the radiopaque material
is gold and the second material is titanium.
16. The medical implant of claim 1, wherein the alloy has a greater
oxidation potential than the radiopaque material alone.
17. The medical implant of claim 1, wherein the medical implant is
an orthopedic implant.
18. The medical implant of claim 1, wherein the medical implant is
a stent or stent-graft.
19. The medical implant of claim 1, wherein the member is a tubular
member defined by struts and openings.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. application Ser.
No. 12/703,838, filed Feb. 11, 2010, which is a continuation of
Ser. No. 11/738,229, filed Apr. 20, 2007, now U.S. Pat. No.
7,682,649, which is a divisional of U.S. application Ser. No.
10/629,934, filed on Jul. 29, 2003, now U.S. Pat. No. 7,297,157,
which is a continuation of U.S. application Ser. No. 10/263,212,
filed on Oct. 2, 2002, now U.S. Pat. No. 6,638,301, the entire
contents of which are hereby incorporated by reference.
TECHNICAL FIELD
[0002] The invention relates to medical devices, such as, for
example, stents and stent-grafts, and methods of making the
devices.
BACKGROUND
[0003] The body includes various passageways such as arteries,
other blood vessels, and other body lumens. These passageways
sometimes become occluded or weakened. For example, the passageways
can be occluded by a tumor, restricted by plaque, or weakened by an
aneurysm. When this occurs, the passageway can be reopened or
reinforced, or even replaced, with a medical endoprosthesis. An
endoprosthesis is typically a tubular member that is placed in a
lumen in the body. Examples of endoprosthesis include stents and
covered stents, sometimes called "stent-grafts".
[0004] Endoprostheses can be delivered inside the body by a
catheter that supports the endoprosthesis in a compacted or
reduced-size form as the endoprosthesis is transported to a desired
site. Upon reaching the site, the endoprosthesis is expanded, for
example, so that it can contact the walls of the lumen.
[0005] The expansion mechanism may include forcing the
endoprosthesis to expand radially.
[0006] For example, the expansion mechanism can include the
catheter carrying a balloon, which carries a balloon-expandable
endoprosthesis. The balloon can be inflated to deform and to fix
the expanded endoprosthesis at a predetermined position in contact
with the lumen wall. The balloon can then be deflated, and the
catheter withdrawn.
[0007] In another delivery technique, the endoprosthesis is formed
of an elastic material that can be reversibly compacted and
expanded, e.g., elastically or through a material phase transition.
During introduction into the body, the endoprosthesis is restrained
in a compacted condition. Upon reaching the desired implantation
site, the restraint is removed, for example, by retracting a
restraining device such as an outer sheath, enabling the
endoprosthesis to self-expand by its own internal elastic restoring
force.
[0008] To support a passageway open, endoprostheses are sometimes
made of relatively strong materials, such as stainless steel or
Nitinol (a nickel-titanium alloy), formed into struts or wires.
These materials, however, can be relatively radiolucent. That is,
the materials may not be easily visible under X-ray fluoroscopy,
which is a technique used to locate and to monitor the
endoprostheses during and after delivery. To enhance their
visibility (e.g., by increasing their radiopacity), the
endoprostheses can be coated with a relatively radiopaque material,
such as gold. Because the endoprostheses are typically kept in the
body for a relatively long time, it is desirable that they have
good biocompatibility.
SUMMARY
[0009] The invention relates to methods of making medical devices,
such as, for example, stents and stent-grafts, and methods of
making the devices. More particularly, the invention features an
endoprosthesis, such as a stent, having a layer that can enhance
the biocompatibility of the endoprosthesis.
[0010] In one aspect, the invention features a stent including a
member having a first portion, and a second portion disposed
outwardly of the first portion. The second portion is more
radiopaque than the first portion and has a first layer including a
radiopaque material, and a second layer defining an outer surface
of the member and including the radiopaque material and a second
material.
[0011] Embodiments may include one or more of the following
features. The second layer includes an alloy of the radiopaque
material and the second material. The radiopaque material is
selected from the group consisting of gold, platinum, palladium,
and tantalum. The second material is selected from the group
consisting of titanium, chromium, palladium, niobium, and silicon.
The first portion includes a material selected from the group
consisting of stainless steel and nickel-titanium alloy.
[0012] The first portion can be the innermost portion of the
member, and/or contact the second portion.
[0013] The stent can further include a third portion between the
first portion and the second portion, a polymeric layer on the
member, and/or a drug-releasing layer on the member.
[0014] In another aspect, the invention features a stent including
a member having a first portion having a first layer including a
radiopaque material, and a second layer defining an outer surface
of the member and including the radiopaque material and a second
material.
[0015] In another aspect, the invention features a stent including
a member having a first portion, and a second portion disposed
outwardly of the first portion. The second portion is more
radiopaque than the first layer and includes a first layer having a
radiopaque material, and a second layer including the radiopaque
material and defining an outer surface of the member, the second
layer having a lower oxidation potential than an oxidation
potential of the first layer.
[0016] Embodiments may include one or more of the following
features. The radiopaque material is selected from the group
consisting of gold, platinum, palladium, and tantalum. The second
layer includes an alloy of the radiopaque material and a second
material. The second material is selected from the group consisting
of titanium, niobium, palladium, chromium, and silicon.
[0017] The first portion can include a material selected from the
group consisting of stainless steel and a nickel-titanium alloy.
The first portion can be the innermost portion of the member. The
first portion can contact the second portion.
[0018] The first and second portions can have different
compositions.
[0019] The stent can further include a polymeric layer on the
member and/or a drug-releasing layer on the member.
[0020] In another aspect, the invention features a stent having a
member having a first portion including a first layer comprising a
radiopaque material, and a second layer comprising the radiopaque
material and defining an outer surface of the member. The second
layer has a lower oxidation potential than an oxidation potential
of the first layer.
[0021] In another aspect, the invention features a stent having a
member including a first portion having a concentration gradient of
a radiopaque material, the first portion defining an outer surface
of the member.
[0022] Embodiments may include one or more of the following
features. The concentration of the radiopaque material increases as
a function of distance from the outer surface. The concentration
gradient varies substantially linearly along a thickness of the
first portion. The radiopaque material is selected from a group
consisting of gold, platinum, palladium, and tantalum. The first
portion is formed of an alloy including the radiopaque material and
a second material. The member further includes a second portion
disposed inwardly of the first portion, the second portion being
more radiolucent than the first portion.
[0023] In another aspect, the invention features a method of making
a stent including a member. The method includes forming an outer
layer on the member having a radiopaque material and a second
material, and oxidizing a portion of the outer layer.
[0024] Embodiments may include one or more of the following
features. Oxidizing the portion includes forming an oxide or a
nitride from the outer layer. The method further includes forming a
radiopaque layer having the radiopaque material. The outer layer is
formed with a compositional gradient.
[0025] The outer layer is formed by a process selected from the
group consisting of physical vapor deposition, chemical vapor
deposition, and electrodeposition.
[0026] Oxidizing the portion of the outer layer can be performed by
electropolishing, by heating the outer layer in an oxidizing
environment, and/or by ion implanting oxygen in the outer layer and
heating the outer layer.
[0027] The method can further include forming a polymeric layer on
the outer layer, and/or forming a drug-releasing layer on the outer
layer.
[0028] Other aspects, features and advantages of the invention will
be apparent from the description of the preferred embodiments
thereof and from the claims.
DESCRIPTION OF DRAWINGS
[0029] FIG. 1 is a perspective view of an embodiment of a
stent.
[0030] FIG. 2 is a schematic, cross-sectional view of the stent of
FIG. 1, taken along line 2-2.
[0031] FIG. 3 is a schematic, cross-sectional view of a strut of an
embodiment of a stent.
[0032] FIG. 4 is a schematic, partial cross-sectional view of a
strut of an embodiment of a stent.
[0033] FIG. 5 is a schematic diagram of an embodiment of an ion
beam assisted deposition system.
[0034] FIG. 6 is a plot of material concentration as a function of
time.
[0035] FIG. 7 is a table of parameters for an ion beam assisted
deposition process.
[0036] FIG. 8 is a table of parameters for an ion beam assisted
deposition process.
[0037] FIG. 9 is a table of parameters for an ion beam assisted
deposition process.
DETAILED DESCRIPTION
[0038] FIG. 1 shows a support 12 carrying a stent 10, which is in
the form of a tubular member defined by struts 11 and openings 13.
Depending on the type of stent 12 (e.g., balloon-expandable or
self-expandable), support 12 can be a balloon catheter or a
catheter shaft. Referring to FIG. 2, stent 10 includes multiple
cross-sectional portions. In particular, struts 11 of stent 10 are
formed of a relatively radiolucent core 14 surrounded by a
relatively radiopaque portion 16. Radiopaque portion 16 includes a
radiopaque layer 18, e.g., made of gold, and a layer 20, e.g., made
of a gold-titanium alloy, that can enhance the biocompatibility of
stent 10. For example, layer 20 can be passivated to provide stent
10 with a relatively inert outer surface.
[0039] In general, stent 10 can be formed by coating a relatively
radiolucent stent with a radiopaque material, such as gold or
platinum, to form layer 18. Layer 20 is then formed on the
radiopaque material. Layer 20 can be formed on the pre-formed
radiopaque layer 18 and/or formed from a portion of the radiopaque
layer. Layer 20 is then passivated, e.g., by forming a layer of an
oxide or nitride on layer 20 or by converting layer 20 to an oxide
or a nitride.
[0040] Core 14 is generally formed of one or more core material
selected to provide stent 10 with certain physical and mechanical
properties. For example, the core material is selected to provide
stent 10 with sufficient hoop strength and radial strength so the
stent can maintain a body vessel open. Suitable core materials
include stainless steel (e.g., 316L stainless steel), Nitinol
(e.g., for self-expandable stents), other titanium alloys, tantalum
alloys, zirconium alloys, and/or niobium alloys. At the same time,
it is also desirable to reduce (e.g., minimize) differences or
mismatch in mechanical properties (e.g., stiffness) between the
stent and the body vessel. The mechanical mismatch can cause, for
example, inflammation and/or re-occlusion of the vessel. One method
of reducing mechanical mismatch is to form the stent with less
material (e.g., by forming smaller struts 11), thereby
approximating the compliancy or resiliency of the vessel. However,
reducing the amount of core material in stent 10 can also reduce
the radiopacity of the stent.
[0041] To increase the radiopacity of stent 10, the stent includes
radiopaque portion 16 disposed over core portion 14. Portion 16
includes radiopaque layer 18, which is formed with a radiopaque
material. The radiopaque material can be any material with a
density and/or linear absorption coefficient sufficient to enhance
the radiopacity of stent 10. In embodiments, the radiopaque
material has a density and/or linear absorption coefficient to
attenuate an incident X-ray beam. In some cases, the radiopaque
material has a density of equal to or greater than about 10 g/cc.
Examples of radiopaque materials include gold, platinum, palladium,
tantalum, iridium, cobalt, titanium, tungsten, stainless steel,
Nitinol, and metal alloys containing a sufficient percentage of
heavy elements. Radiopaque layer 18 can be, for example, up to
about 8 microns thick, e.g., about 6-8 microns, thick. Methods of
forming radiopaque layer 18 include, for example,
electrodeposition, physical vapor deposition (e.g., sputtering),
chemical vapor deposition, galvanizing, and/or dipping (e.g., in
molten material).
[0042] In some cases, however, the radiopaque materials do not have
a desired level of biocompatibility and/or the biocompatibility of
the material is unknown (e.g., in the long term). It is believed,
for example, that gold may affect (e.g., catalyze) electron
transfer in certain undesirable reactions in the body. Accordingly,
radiopaque portion 16 includes a relatively inert layer 20 disposed
over radiopaque layer 18.
[0043] Layer 20 enhances the biocompatibility of stent 10 by
providing the stent with a layer (as shown, an outer layer) that
can be passivated, e.g., more easily than radiopaque layer 18. For
example, layer 20 is capable of reacting (e.g., oxidizing) and
forming products, such as oxides, nitrides, and/or carbides, that
are more inert, and therefore, more biocompatible, than the
material(s) in radiopaque layer 18. Relative to radiopaque layer
18, layer 20 has a lower oxidation potential, i.e., can be more
easily oxidized to form a biocompatible product.
[0044] In some embodiments, layer 20 includes a mixture (here, an
alloy) of the radiopaque material(s) in radiopaque layer 18 and one
or more alloying material. The alloying material can be any
material capable of forming a mixture with the radiopaque
material(s), and forming a product that is more easily passivated
than the radiopaque material(s). The alloying material can be, for
example, tantalum, titanium, niobium, zirconium, chromium, silicon,
rhodium, iridium, platinum, and/or palladium. Any of the alloying
materials can be used with any of the radiopaque materials
described above.
[0045] As an example, for a gold radiopaque layer 18, the alloying
material can be titanium.
[0046] In this example, layer 20 includes an alloy of
gold-titanium, such as Au.sub.0.30Ti.sub.0.70, which can be more
easily passivated than gold. That is, relative to gold, the
gold-titanium alloy can more easily form or be converted to a
product, e.g., an oxide, that is relatively inert and
biocompatible. In embodiments, for the alloy of gold-titanium
(Au.sub.xTi.sub.y) x can range from about 0-30%, and y can range
from about 70-100%. For example, x can be equal to or greater than
about 0%, 5%, 10%, 15%, 20%, or 25%, and/or equal to or less than
about 30%, 25%, 20%, 15%, 10%, or 5%. In embodiments, the
concentration of titanium, y, can be equal to or greater than about
70%, 75%, 80%, 85%, 90%, or 95%, and/or less than or equal to 100%,
95%, 90%, 85%, 80%, or 75%. Layer 20 can be up to about 10 microns
thick, e.g., about 0.1-10 microns thick. Ternary (e.g., Au--Ti--Cr)
or higher mixtures or alloy systems can be formed.
[0047] In some embodiments, layer 20 can be formed on a pre-formed
radiopaque layer 18.
[0048] For example, after radiopaque layer 18 is formed, modified
layer 20 can be applied on the radiopaque layer by physical vapor
deposition, including sputtering and ion beam assisted deposition,
chemical vapor deposition, or electrodeposition. Layer 20 can also
be formed by forming layers, e.g., alternating layers, of the
radiopaque material and the alloying material on layer 18 in a
predetermined ratio, and heating the layers (e.g., at elevated,
annealing temperatures) to form the alloy by diffusion.
[0049] Alternatively or in addition, layer 20 can be formed from a
portion of a formed radiopaque layer 18. That is, a portion of the
radiopaque layer 18 can be converted to layer 20. For example, a
gold-titanium layer 20 can be formed by implanting titanium ions
into a formed gold radiopaque layer 18, and annealing the
radiopaque layer. As a result, a certain thickness of the
radiopaque layer (e.g., in the sub-micron range) is converted to an
alloyed modified layer that can be passivated. In another example,
a layer of alloy material, e.g., Ti, can be deposited on radiopaque
layer 18, e.g., Au, and the layers can be heated, e.g., annealed,
to form an alloy, e.g., Au--Ti.
[0050] It should be noted that while FIG. 2 shows radiopaque layer
18 and layer 20 as two discrete, well-defined layers, in some
embodiments, the interface between the layers is not well defined.
As a result, the endoprosthesis can be formed with good adhesion
and high durability (e.g., reduced risk of flaking) Corrosion from
contact of dissimilar material can also be reduced. The interface
may not be well defined, for example, when modified layer 20 is
formed from a formed radiopaque layer 18.
[0051] In some embodiments, radiopaque portion 16 does not include
an interface between two layers. Referring to FIG. 3, a strut 22 of
a stent is formed of a relatively radiolucent core 24 surrounded by
a relatively radiopaque layer 26. Core 24 is generally the same as
core 14 described above. Radiopaque layer 26 includes one or more
radiopaque material and one or more alloying material, as described
above. In addition, radiopaque layer 26 is formed having a
compositional gradient in which the concentration(s) of the
alloying material(s) and/or the radiopaque material(s) varies along
the thickness of layer 26 (arrows A and B). As an example, for a
radiopaque layer 26 formed of a gold-titanium alloy, layer 26 can
be relatively gold-rich (or titanium-poor) at surface 28 adjacent
to core 24, and relatively gold-poor (or titanium-rich) at outer
surface 30. At surface 28, the concentration of the radiopaque
material can be about 100%; and at outer surface 30, the
concentration of the alloying material can be about 100%. The
concentration(s) of the radiopaque material(s)1 and/or the alloying
material(s) can vary linearly or non-linearly (e.g., exponentially)
between surfaces 28 and 30. The concentration(s), e.g., of the
alloying material, can increase or decrease from surface 28 to
surface 30. In certain embodiments, layer 26 having the
compositional gradient can be formed on a radiopaque layer, such as
radiopaque layer 18.
[0052] Methods of forming compositionally-graded layer 26 include
using physical vapor deposition while controlling the source of
materials used for deposition. In another method, layer 26 can be
formed by forming alternating layers of a radiopaque material and
an alloying material in a predetermined ratio, and annealing the
layers. For example, referring to FIG. 4, to form a concentration
gradient of titanium along layer 26, layers of titanium 27a, 27b,
and 27c can be formed alternating with layers of gold 29a, 29b, and
29c. Titanium layer 27a is thicker than layer 27b, which is thicker
than layer 27c. Gold layers 29a-29c are of equal thickness. When
the layers are subsequently annealed, they can diffuse together and
form a gold-titanium alloy in which the concentration of titanium
varies along the thickness of layer 26 (here, increasing with
increasing distance from core 24).
[0053] After layer 20 or 26 is formed, stent 10 can be passivated
by exposing the stent to an appropriate environment. For example,
stent 10 can be oxidized by heating the stent in an oxidizing
atmosphere, such as one containing oxygen and/or water, to form an
oxide layer on layer 20 or 26. Nitrides can be formed by heating
stent 10 in an atmosphere containing nitrogen, nitrogen-hydrogen,
and/or ammonia. Carburizing, e.g., increasing the surface
concentration of carbon, can be performed by exposing stent 10, at
an elevated temperature, to an atmosphere rich in a hydrocarbon
gas, such as methane. Alternatively or in addition, passivation can
be performed by electropolishing to produce an oxide-rich surface
layer. In some cases, passivation can occur relatively
spontaneously, e.g., upon exposure to air, when the oxidation
potential is relatively low.
[0054] Stent 10 can then be finished, e.g., electropolished to a
smooth finish, according to conventional methods. Stent 10 can be
finished before passivation. Alternatively, stent 10 can be formed
textured.
[0055] Stent 10 can then be used, e.g., delivered and expanded,
according to conventional methods.
[0056] Generally, stent 10 can be self-expandable,
balloon-expandable, or a combination of both. Examples of stent 10
and support 12 are described in U.S. Pat. Nos. 5,725,570 (Heath)
and 5,234,457 (Andersen), all hereby incorporated by reference.
[0057] In other embodiments, stent 10 is a part of a stent-graft.
The stent-graft can be a stent attached to a biocompatible,
non-porous or semi-porous polymer matrix made of
polytetrafluoroethylene (PTFE), expanded PTFE, polyethylene,
urethane, or polypropylene. Stent 10 can include a releasable
therapeutic agent or a pharmaceutically active compound, such as
described in U.S. Pat. No. 5,674,242, and commonly-assigned U.S.
Ser. No. 09/895,415, filed Jul. 2, 2001, all hereby incorporated by
reference. The therapeutic agents or pharmaceutically active
compounds can include, for example, anti-thrombogenic agents,
antioxidants, anti-inflammatory agents, anesthetic agents,
anti-coagulants, and antibiotics.
[0058] The following examples are illustrative and not intended to
be limiting.
EXAMPLE
[0059] The following example describes ion beam assisted deposition
(IBAD) as a method for depositing thin films on a substrate, e.g.,
a stent.
[0060] Referring to FIG. 5, an IBAD system 50 generally includes a
fixture assembly 52 configured to support a stent 54, and a
deposition assembly 56. System 50 is used in a vacuum chamber 51 at
pressures of about 1.times.10.sup.-4-3.times.10.sup.-4 Torr,
provided in part by a diffusion pump 58.
[0061] Deposition assembly 56 includes two crucibles 60 and 62,
their respective shutters 64 and 66, two electron beam evaporators
68 and 70, and an ion beam gun 72. Crucibles 60 and 62, e.g., made
of graphite, contain materials to be deposited, such as gold and
titanium. Electron beam evaporators 68 and 70 are configured to
generate a flow of electrons that can be focused (e.g., using
magnetic fields) on the materials in crucibles 60 and 62,
respectively, to melt and to evaporate the materials to form
thermally evaporated materials 76. Evaporators 68 and 70 can have
water-cooled jackets that cool crucibles 60 and 62, respectively.
Ion beam gun 72 is configured to receive a flow of argon (e.g., 2-4
sccm) and to ionize the argon to form a plasma 74. Plasma 74 is
accelerated out of ion beam gun 72 to stent 54 using magnets (not
shown). Shutters 64 and 66 can be moved, e.g., swiveled, to allow
or to block the flow of evaporated material 76 from crucibles 60
and 62, respectively.
[0062] Fixture assembly 52 is generally configured to allow stent
54 to be uniformly coated with evaporated material 76. Typically,
the thermal evaporation process can deposit a film of material 76
on a substrate that is in a line of sight of crucible 60 or 62. To
provide uniform coverage on stent 54, the stent is rotated during
deposition. In embodiments, stent 54 is placed on a rotatable
spindle. The friction between the stent and the spindle can hold
the stent in place during rotation to provide a coated stent
without contact points. Alternatively, stent 54 can be clipped to a
rotatable shaft.
[0063] A quartz crystal 78 is used to determine the thickness of
the deposited material. Crystal 78 is interfaced to a controller
(not shown) and oscillated. The controller is calibrated such that
the thickness of material deposited on crystal 78 (and thus also
stent 54) can be calculated by measuring the change in the
oscillation frequency of the crystal.
[0064] A method of coating using IBAD will now be described.
[0065] Stent 54, e.g., a Nitinol or stainless steel stent, is
thoroughly chemically cleaned. For example, stent 54 can be cleaned
in a solvent (such as isopropyl alcohol or acetone) and a
degreaser, and rinsed with deionized water. Heat and/or agitation,
e.g., using ultrasonic energy, can be used to clean stent 54. Stent
54 is then placed on fixture assembly 52, which is then placed in
vacuum chamber 51, with the stent about two feet from crucibles 60
and 62.
[0066] Stent 54 is then subjected to a sputter cleaning Chamber 51
is evacuated to a pressure of about 1.times.10.sup.-5 Torr, and ion
beam gun 72 is activated. Ion beam gun 72 ionizes argon gas to form
plasma 74, and the plasma is accelerated to stent 54 to sputter
clean/etch the surface of the stent. The angle of incidence for
plasma 74 can be about 45-90.degree., e.g., about 70.degree.. In
embodiments, stent 54 is sputter cleaned for about 20-30 minutes.
An estimated 100-300 angstroms of material can be removed.
[0067] A first material, e.g., gold in crucible 60, is then
deposited. During the final ten minutes of sputter cleaning,
electron beam evaporators 68 and 70 are slowly ramped up. Shutters
64 and 66 are over their respective crucibles 60 and 62, so no
material can deposit on stent 54. After sputter cleaning is
complete and the material to be deposited is molten, shutter 64
moves, e.g., swivels, to allow evaporated material to coat stent
54. The surface of stent 54 is simultaneously bombarded with plasma
74. It is believed that as ions of the first material deposit on
stent 54, plasma 74 transfers energy to the ions, freeing some ions
from the surface of the stent and allowing some ions to migrate on
the stent surface. As a result, it is believed that a composite
including the first material is formed with enhanced density.
[0068] A second material, e.g., titanium, tantalum, or platinum, is
then deposited. After the thickness of the first material coated on
stent 54 reaches, e.g., about 200-500 angstroms, shutter 66 is
moved to allow the second material (in crucible 62) to co-deposit
with the first material. The concentrations of each material can be
controlled by adjusting the power to evaporators 68 and 70. For
example, referring to FIG. 6, initially the concentration of the
first material is relatively high, and the second material is then
slowly introduced. In embodiments, at time t, shutter 64 is moved
to prevent the first material from depositing on stent 54, and a
pure layer of the second material is deposited over the alloy layer
(i.e., the layer having the first and second materials). Then,
stent 54 is allowed to cool, chamber 51 is returned to atmospheric
pressure, and the stent is removed from the chamber.
[0069] In embodiments, stent 54 is then annealed. Annealing can
promote diffusion between the layers of materials and/or the layers
and the stent substrate, and can strengthen bonding or adhesion
between the layers. In some cases, a Nitinol stent can be annealed
at about 300-400.degree. C., and a stainless steel stent can be
annealed at about 500-1000.degree. C. Annealing times can vary,
e.g., from a few minutes to days, depending, for example, on the
diffusion of the materials in stent 54, which can be
temperature-dependent.
[0070] FIG. 7 shows ranges for some process parameters.
[0071] A stent was coated with titanium using the procedures
described above. The process parameters are shown in FIG. 8.
[0072] A stent was coated with a platinum-gold using the procedures
described above. The process parameters are shown in FIG. 9. The
platinum-gold gradient was similar to that shown in FIG. 6.
OTHER EMBODIMENTS
[0073] In other embodiments, one or more intermediate layers can be
formed between core 14 or 24 and radiopaque layer 18 or 26, i.e.,
at least a portion of the core and the radiopaque layer do not
contact. For example, in embodiments in which there is lattice
mismatch between the core and the radiopaque layer, intermediate
layer(s) can be selected to have intermediate lattice parameters to
serve as buffer layer(s), thereby reducing (e.g., minimizing)
stress between the core and the radiopaque layer. The intermediate
layer(s) can be, for example, a mixture of the core material and
the radiopaque material.
[0074] Layer 20 may not include the radiopaque material(s) in
radiopaque layer 18. For example, a radiopaque layer may include
gold, while layer 20 includes a material that can be passivated,
such as a platinum-titanium alloy.
[0075] Radiopaque layer 18, layer 20, and/or layer 26 can cover all
or only one or more selected portions of a stent. For example,
radiopaque layer 18, layer 20, and/or layer 26 may be formed only
on one or more end portions of the stent.
[0076] In some embodiments, other types of layers can be formed on
layer 20 or 26. For example, one or more selected portions of a
stent may include a magnetopaque (i.e., visible by magnetic
resonance imaging (MRI)) material on layer 20 or 26. Suitable
magnetopaque materials include, for example, non-ferrous
metal-alloys containing paramagnetic elements (e.g., dysprosium or
gadolinium) such as terbium-dysprosium, dysprosium, and gadolinium;
non-ferrous metallic bands coated with an oxide or a carbide layer
of dysprosium or gadolinium (e.g., Dy.sub.2O.sub.3 or
Gd.sub.2O.sub.3); non-ferrous metals (e.g., copper, silver,
platinum, or gold) coated with a layer of superparamagnetic
material, such as nanocrystalline Fe.sub.3O.sub.4,
CoFe.sub.2O.sub.4, MnFe.sub.2O.sub.4, or MgFe.sub.2O.sub.4; and
nanocrystalline particles of the transition metal oxides (e.g.,
oxides of Fe, Co, Ni).
[0077] In other embodiments, radiopaque layer 18, layer 20, and/or
layer 26 may be formed on medical devices other than stents and
stent-grafts, for example, those where radiopacity is desired such
as orthopedic implants.
[0078] All publications, applications, and patents referred to
herein are incorporated by reference in their entirety.
[0079] Other embodiments are within the claims.
* * * * *