U.S. patent application number 10/216963 was filed with the patent office on 2002-12-19 for vascular stent having increased radiopacity and method for making same.
Invention is credited to Dang, Kenny.
Application Number | 20020193869 10/216963 |
Document ID | / |
Family ID | 23887335 |
Filed Date | 2002-12-19 |
United States Patent
Application |
20020193869 |
Kind Code |
A1 |
Dang, Kenny |
December 19, 2002 |
Vascular stent having increased radiopacity and method for making
same
Abstract
A stent for use in a patient's blood vessel to maintain the
patency of the vessel contains strategically located radiopaque
material. The strategic placement of the radiopaque material in the
core structure of the stent functions to enhance the resolution of
the stent under fluoroscopy. The initial part of the process
includes forming a groove in a piece of tube stock and securing
radiopaque material into the groove by press fitting or diffusion
bonding. After the securing method, a layer of material can be
sputtered coated over the only radiopaque material or over the
entire stent. Finally, a pattern of struts and splines is cut into
the tube composite to form the stent.
Inventors: |
Dang, Kenny; (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: |
23887335 |
Appl. No.: |
10/216963 |
Filed: |
August 12, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10216963 |
Aug 12, 2002 |
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09475380 |
Dec 30, 1999 |
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6471721 |
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Current U.S.
Class: |
623/1.15 ;
623/1.34 |
Current CPC
Class: |
A61F 2002/91533
20130101; A61F 2/91 20130101; A61F 2002/91558 20130101; A61F
2250/0098 20130101; A61F 2002/825 20130101; A61F 2230/0013
20130101; A61F 2/915 20130101 |
Class at
Publication: |
623/1.15 ;
623/1.34 |
International
Class: |
A61F 002/06; A61F
002/04 |
Claims
What is claimed:
1. A method for forming a vascular stent having increased
radiopacity, comprising the steps of: selecting a tube of
structural material; forming at least one groove along the tube;
inserting radiopaque material into the groove; securing said
radiopaque material to the tube; cutting the tube into a particular
pattern to form spines and struts of a stent.
2. The method of claim 1, wherein: the tube of structural material
is formed of a biocompatible material selected from the group
consisting of stainless steel, nickel-titanium alloys, and
cobalt-based alloys.
3. The method of claim 1, wherein: the step of forming the groove
comprises forming one continuous groove.
4. The method of claim 1, wherein: the radiopaque material is
formed in a strip and the step of inserting the radiopaque material
comprises inserting the strip into the groove.
5. The method of claim 1, wherein: the method of claim 1, wherein
the step of securing the radiopaque material includes the step of
laser binding the radiopaque material into the groove.
6. The method of claim 4, wherein: the step of securing the strip
includes press-fitting the strip into the groove.
7. The method of claim 4, wherein: the step of securing the strip
includes the step of diffusion bonding the strip into the
groove.
8. The method of claim 4, wherein: the step of forming the groove
comprises forming a plurality of grooves.
9. The method of claim 8, wherein: a plurality of strips of
radiopaque material are inserted into the plurality of grooves.
10. The method of claim 1, wherein: the step of securing the
radiopaque material into the groove includes press-fitting the
material into the groove.
11. The method of claim 1, wherein: the step of securing the
radiopaque material includes the step of diffusion bonding the
material into the groove.
12. The method of claim 1 wherein: the grooves are formed as a
plurality of rings.
13. The method of claim 1, wherein: the cutting step is performed
by a laser.
14. The method of claim 1, wherein: the step of forming the grooves
includes forming them on the exterior of the tube.
15. The method of claim 1, further including the step of: sputter
coating a layer of metal over the exterior surface of the
radiopaque material.
16. The method of claim 1, wherein: the step of forming the grooves
includes forming them on the exterior of the tube in a selected
pattern.
17. The method of claim 16, wherein: the step of cutting the tube
is performed in a pattern which selectively places radiopaque
material on particular struts or spines of the finished stent.
18. A stent having enhanced radiopacity, comprising: a tubular
section constructed of at least one segment of a tube; the segment
having at least one groove formed therein; a radiopaque material
disposed within the groove; and means for securing the material
within the groove.
19. The stent of claim 18, wherein: the means for securing the
radiopaque material in the groove includes sputter coating a layer
of metal over the radiopaque material.
20. The stent of claim 18, wherein: the segment includes a target
area; and the groove is formed in selective patterns in the target
area.
21. The stent of claim 18, wherein: the segment is formed with one
groove along a longitudinal line and the radiopaque material is
located in the groove.
22. The stent of claim 18 wherein: the means for securing the
radiopaque material within the groove includes press fitting the
material into the groove.
23. The stent of claim 18, wherein: the means for securing the
radiopaque material within the groove includes diffusion bonding
the material into the groove.
24. The stent of claim 18, wherein: the segment is formed with a
plurality of grooves and the radiopaque material is formed in
strips which are placed within the grooves.
25. The stent of claim 18 wherein: the means for securing the
radiopaque material within the grooves includes press fitting the
strips of radiopaque material into the grooves.
26. The stent of claim 18, wherein: the means for securing the
radiopaque material within the grooves includes diffusion bonding
the strips of radiopaque material into the grooves.
27. The stent of claim 18, wherein: the means for securing the
radiopaque material in the grooves includes sputter coating a layer
of metal over the strips of radiopaque material.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to improvements in increasing
the radiopacity of stents and improvements in their method of
manufacture, and more particularly, to a stent and a method of
manufacture where radiopaque material is secured to strategic
location(s) on the stent to improve visibility of the stent under
fluoroscopy.
[0002] Generally, stents are expandable endoprosthesis devices
which are adapted to be implanted into a patient's body lumen to
maintain the patency of the vessel. Stents are especially
well-suited for the treatment of atherosclerotic stenosis in blood
vessels. These devices are typically implanted into blood vessels
by a delivery catheter which is inserted at an easily accessible
location and then advanced through the patient's vasculature to the
deployment site. The stent is initially maintained in a radially
compressed or collapsed state to enable it to be maneuvered through
the lumen. Once in position, the stent is usually deployed either
automatically by the removal of a restraint, or actively by the
inflation of an expandable member, such as balloon, about which the
stent is mounted on the delivery catheter.
[0003] The stent must be able to satisfy a number of mechanical
requirements. First, the stent must be capable of withstanding the
structural loads that are imposed by the vessel walls. In addition
to having adequate radial strength, the stent should be
longitudinally flexible to allow it to be maneuvered through a
vascular path and to enable it to conform to a deployment site
which may not be linear or may flex. The stent material must allow
the stent to undergo expansion which typically requires substantial
deformation of localized portions of the stent's structure. Once
expanded, the stent must maintain its size and shape throughout its
service life despite the various forces that may come to bear
thereon, including the cyclic loading induced by the beating heart.
Finally, the stent must be biocompatible so as not to trigger any
adverse vascular responses.
[0004] Fluoroscopy is typically used to facilitate the precise
placement of a stent as well as to verify the position of a stent
within the patient throughout the stent's service life. The use of
radiopaque materials in the construction of the stent allows for
its direct visualization. Accordingly, different patterns and
contents of radiopactivity have different effects on the direct
visualization. For example, when a physician views a completely
radiopaque stent under fluoroscopy, he/she will likely see an
unclear and amorphous shape that extends outside the dimensions of
the actual stent. The opposite will also be true where the stent
possesses little radiopacity. In terms of fluoroscopic visibility,
the optimal stent should be visible in a clear and detailed form
without shape blurring. To date, no single material has been
identified that simultaneously satisfies all requirements inherent
in an optimal stent application. Those materials that do satisfy
the mechanical requirements are either insufficiently or
excessively radiopaque and/or are not adequately proven to be
biocompatible in a vascular setting. Thus, simply constructing a
stent which exhibits optimal radiopacity wholly out of a single
material is not currently a viable option. A number of different
approaches have, however, been employed wherein different materials
are combined in an effort to render a mechanically sound and
biocompatible stent to be visualized by a fluoroscope.
[0005] One procedure frequently used for accomplishing fluoroscopic
visibility is through physical attachment of radiopaque markers to
the stent. Conventional radiopaque markers, however, have a number
of limitations. When attached to a stent, such markers may project
from the surface of the stent, thereby exhibiting a departure from
the ideal profile of the stent. Depending on their specific
location, the marker may either project inwardly tending to disrupt
blood flow or outwardly tending to traumatize the walls of the
blood vessel. Additionally, when the metal used for the stent
structure comes in contact with the metal used for the radiopaque
marker, galvanic corrosion may occur. This corrosion may lead to
separation of the metals and thereafter contamination of the blood
stream with radiopaque material. Additionally, the radiopaque
material may come into direct contact with living tissue which may
be problematic, particularly if the material is not
biocompatible.
[0006] Stents can also be marked by plating selected portions
thereof with radiopaque material. A number of disadvantages with
this approach are apparent. Because the radiopaque material comes
into direct contact with living tissue, there can be a sizeable
amount of tissue exposure. Additionally, when the stent is expanded
and certain portions undergo substantial deformation, there is a
risk that cracks will form in the plating which can separate from
the underlying substrate. This side effect has the potential for
creating jagged edges on the stent which may inflict trauma on the
vessel wall or cause turbulence in the blood flow thereby inducing
thrombogenesis. Moreover, once the underlying structural material
becomes exposed, interfaces between the two disparate metals become
subject to the same type of galvanic corrosion as mentioned above.
Further, should the plating pattern cover less than all of the
stent's surfaces, the margins between the plated and unplated
regions are all subject to galvanic corrosion.
[0007] As a further alternative, a stent structure can be formed
from a sandwich of structural and radiopaque materials. Tubes of
suitable materials can be cold-drawn and heat treated to create a
multi-layered tubing which can be cut into a stent. Struts and
spines are formed in the multi layered tubing by cutting an
appropriate pattern of voids into the tubing using well known
techniques in the art. While this approach does provide a stent
that has enhanced radiopacity and techniques fulfills necessary
mechanical requirements, the thin cross section of the radiopaque
material is usually exposed along the edges of all cut lines. The
biocompatiblity of the radiopaque material remains an issue and
more significantly, a sizeable area is created which is subject to
galvanic corrosion. Any cuts in the sandwich structure cause two
disparate metal interfaces, i.e. the juncture between the outer
structural layer and the central radiopaque layer, as well the
juncture between the central radiopaque layer and the inner
structural layer, to become exposed to blood and tissue along the
entire lengths of these cuts.
[0008] A stent configuration that overcomes the shortcomings
inherent in previously known devices is therefore required. More
specifically, a stent is needed that provides radiopaque properties
enabling clear visibility under fluoroscopy and mechanical
properties consistent for reliable and safe use.
[0009] A method of manufacturing the above mentioned stent
configuration is also necessary. More specifically, a method is
needed that combines the prerequisites of biocompatible materials
and fluoroscopy into an advantageous method of manufacture.
SUMMARY OF INVENTION
[0010] The present invention provides a stent and method for
manufacture which overcomes some of the shortcomings of previously
known stents and methods of manufacturing stents. Most importantly,
the stent has high resolution when viewed under fluoroscopy due to
strategic placement of radiopaque material along the stent. The
stent also fulfills the requirements of having sufficient
flexibility, structural integrity and biocompatiblity, and being
safe for deployment into a patient's vasculature.
[0011] Unique to the stent described herein is an advantageously
selected pattern of radiopaque material formed on the stent.
Compared to conventional stents, which are frequently obscured
under fluoroscopy, the pattern of radiopaque material in the stent
described herein leads to improved visibility under fluoroscopy.
Unique to the process is a method of forming selected patterns of
securely mounted radiopaque material on a stent substrate. Compared
to some conventional processes of forming stents where radiopaque
material is simply layered onto the stent structure, the process
described herein utilizes a process of placing select patterns of
grooves into the stent substrate. This grooving process allows
precise placement and strong retention of radiopaque material into
strategic locations of the stent.
[0012] The material employed for said underlying structure is
selected for its structural and mechanical properties and may be
the same general material used to make conventional stents. One
preferred material is 316L stainless steel, although other
materials such as nickel-titanium, cobalt based alloys, Nitinol and
other types of stainless steel can be used. Such materials, when
used in the 0.002" to 0.003" thickness, as is typical for stent
applications, are often difficult to visualize
fluoroscopically.
[0013] The grooving process is the first operation to be performed
on a piece of tube stock. In this process, a pattern of groove(s),
preferably either rings or lines, is cut in the tube stock. The
grooves should be strategically placed at targeted locations along
the length of the tubing to obtain the desired radiopacity. The
stent may employ one or a multiple number of grooves depending on
radiopacity requirements. In forming the groove(s), an instrument,
such as a conventional Swiss screw machine can be used. Alternative
machines can also be used to perform the same grooving
operation.
[0014] After the grooving process is performed on the tube stock,
radiopaque material is inserted into the groove(s) by either
press-fitting, diffusion bonding or laser bonding. One preferred
radiopaque material is gold, although other radiopaque materials
such as platinum, tantalum, iridium, or their alloys can also be
used. When press-fit, the shapes of the strip(s) must be in close
conformance with the shape of the groove(s) while being slightly
larger than the size of the groove(s). The radiopaque strip(s) are
combined with the groove(s) in the tube such that the difference in
sizing causes the two metals to lock together in an interference
fit. The interference fit insures a strong and long lasting bond
between the two materials. When the radiopaque strip(s) are
diffusion bonded, an entirely different process of attachment can
be employed. In the diffusion bonding procedure, a vacuum is drawn
and the entire assembly (tubing with radiopaque material inserted
into the groove(s)) is heated to near the particular diffusion
bonding temperature with the bonding surfaces still exposed to the
vacuum environment. Thereafter, the bonding surfaces are brought
into contact with very moderate pressure and maintained at a
temperature and pressure sufficient for diffusion bonding. The
assembly is then cooled, resulting in a substantially unitary
diffusion bonded structure.
[0015] While not mandatory for this process, stainless steel can be
applied over the radiopaque sections of the tube to promote
biocompatibility and structural integrity. Additionally, the
stainless steel coating can act to protect the radiopaque and
structural materials from galvanically corroding. The stainless
steel, preferably 316L can be applied by a sputtering procedure, a
method of depositing a metallic film through the use of electric
discharge. Sputter coating machines are commercially available and
capable of applying an extremely even coating of material to a
workpiece. The tubing may be rotated in front of a nozzle, the
nozzle may be rotated about the tubing or a nozzle that completely
surrounds the tubing may be employed to apply the sputter coating.
While the preferred material for the sputtering is 316L stainless
steel, other suitable material can be used also. Additionally, if a
higher degree of structural rigidity is sought, the material can be
sputtered across the entire length of the tube to a sufficient
thickness such that the structural integrity of the stent is
significantly increased.
[0016] After the tube has been processed as described above, a
procedure for cutting the tube can be initiated. In this procedure,
for example, the tubing is first placed in a rotatable fixture
inside a cutting machine, where it is positioned relative to a
laser. According to machine encoded instructions, the tubing is
rotated and moved longitudinally relative to the laser. The laser
selectively removes the material from the tubing by ablation and a
pattern is cut into the tube. The laser cut provides a desired
pattern of voids defining struts and spines which allows the stent
to expand in an even manner, in accordance with well known and well
established procedures. Thereafter, the stents are subjected to the
standard industry practices of electro-polishing and possibly
annealing. Another biocompatible outer layer could also be applied
to the stent.
[0017] The above and other objects and advantages of this invention
will be apparent from the following more detailed description when
taken in conjunction with the accompanying drawings of exemplary
embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a perspective view of the tube stock with one
particular pattern of grooves cut into the tube in a ring-shaped
pattern, constant in diameter, and spaced apart longitudinally.
[0019] FIG. 2 is a perspective view of the tube stock with
radiopaque material inserted into the grooves and annularly bonded
in a complementary fit.
[0020] FIG. 3 is a perspective view of the tube stock with
radiopaque material inserted into grooves with sputtered material
entirely covering the markers.
[0021] FIG. 4 is a perspective view of a portion of a stent which
can be cut from the composite tube stock illustrated in FIGS. 1-4
and embodying features of the present invention.
[0022] FIG. 5 is a cross sectional view of a representative strut
of a stent made in accordance with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0023] A radiopaque stent and the process of forming said stent is
described herein. Unique to the stent is an advantageously selected
pattern of radiopaque material which is affixed to the stent.
Compared to conventional stents which are frequently obscured under
flouroscopy, the pattern of radiopaque material in the stent of the
present invention allows an easily discernable view of the stent
under fluoroscopy. Unique to the process of manufacturing the stent
is a method of forming selected patterns of radiopacity within the
stent. Compared to some conventional processes whereby radiopaque
material is layered onto stent structures, the method of the
present invention includes a grooving process which allows for
precise location of radiopaque material into the core of the
stent.
[0024] Referring now to the drawings, wherein like numerals
indicate like elements, a representation stent 10 made in
accordance with the present invention is shown in FIG. 4. The stent
10 can be made in any number of different strut patterns, depending
on the particular application for the stent. The stent 10 is
representative of just one design which can be used to form the
various struts and spines. Referring now to FIG. 1, a piece of tube
stock 11 to be used for the underlying structure of the stent is
shown. The material employed for said underlying structure is
selected for its structural and mechanical properties and may be
the same material from which conventional stents are made. One
preferred material is 316L stainless steel, although other
materials such as nickel-titanium, cobalt based alloys, Nitinol and
other types of stainless steel can be used. Such materials, when
used in the 0.002" to 0.003" thickness which is typical for stent
applications, are often difficult to properly visualize
fluoroscopically.
[0025] As shown in FIG. 1, a series of grooves 12 may be formed in
tube stock 11 by a Swiss screw machine operation, or other
machinery. One preferred pattern of groove(s) is either ring(s) or
line(s), but any other pattern can be used. Such grooves can be
placed at targeted locations along the length of the tubing in
order to obtain the desired radiopactivity along the stent. In
addition, the number of grooves can vary according to the stent's
application. For instance, if high radiopacity is required, a
plurality of grooves can be formed along the tube stock. If low
radiopacity is required, as little as one groove can be formed
along the tube stock. While one preferred machine to form the
grooves is a conventional Swiss screw machine, other machines can
be used to perform the same grooving operation.
[0026] Referring now to FIG. 2, the tube stock 11 may incorporate
radiopaque material 13 inserted into the grooves 12. The radiopaque
material can be formed into strips which are placed into the
grooves. One preferred radiopaque material is gold, although other
radiopaque materials such but not limited to, platinum, tantalum,
iridium, or their alloys can also be used. The strips are
preferably inserted into the grooves by either a press-fit,
diffusion bonding or laser bonding. When the strips are press-fit
into the grooves, their shape must be in close conformance with,
but slightly larger than, the width of the groove. The press fit
ensures an interference between the radiopaque strips and the tube
material which secures both materials together in a strong,
long-lasting bond. If the strips are diffusion bonded to the tube,
a different type of process is necessary. First, a vacuum is drawn
and the assembly, which includes the tube and radiopaque strips
placed in the grooves, is heated to near diffusion bonding
temperature with the bonding surfaces still exposed to the vacuum
environment. Thereafter, the bonding surfaces are brought into
contact with very moderate pressure and are maintained at a
temperature and pressure sufficient for diffusion bonding. The
assembly is then cooled, resulting in a substantially unitary
diffusion bonded structure. The advantageously selected patterns of
radiopacity will allow precise orientation or degree of expansion
to be discerned by inspection of the fluoroscopic image when the
stent is completed.
[0027] Referring now to FIG. 3, the tubing 11 may incorporate
radiopaque material 13 in the form described above with the
addition of a thin layer of stainless steel 14 covering the
radiopaque material. The stainless steel is applied to the tubing
by sputtering, a method of depositing a metallic film through the
use of electric discharge. Sputter coating machines are
commercially available and capable of applying an extremely even
coating of material to a workpiece. In practice, the tubing may be
rotated in front of a nozzle, the nozzle may be rotated about the
tubing or a nozzle that completely surrounds the tubing may be
employed to apply the sputter coating. While one preferred material
for the sputtering is 316L stainless steel, other suitable material
can be also be used. In addition to securing the radiopaque
strip(s) to the tubing, the sputtered layer of metal can function
to prevent galvanic corrosion and strengthen the entire stent. In
this regard, the material can be sputtered to a sufficient
thickness over selected regions of the tube 11 or over the entire
tube such that the structural integrity of the stent is
significantly increased.
[0028] Referring now to FIG. 4, the composite radiopaque tubing 11
is illustrated with a substantial amount of material removed to
passing the struts and spines of the stent 10. In the material
removal procedure, the tubing is placed in a rotatable fixture of a
cutting machine where it is positioned relative to a laser. The
machine rotates and moves the tubing longitudinally relative to the
laser, in accordance with machine encoded instructions. The laser
selectively removes the material from the tubing by ablation and a
pattern 14 is cut into the tube. The laser cut provides a desired
pattern of voids defining struts and spines while leaving both the
radiopaque strips 13 and sputtered stainless steel coating 14 in
strategic locations. The tube is therefore cut into the discrete
pattern of the finished stent. Further details on how the tubing
can be cut by a laser are found in U.S. Pat. Nos. 5,759,192
(Saunders) and 5,780,807 (Saunders), which have been assigned to
Advanced Cardiovascular Systems, Inc. and are incorporated herein
by reference in their entirely.
[0029] FIG. 4 illustrates a portion of a representative stent 10
where the radiopaque strips 13 and the sputtered coating 14 are
integral to the stent 10 and accompany the cut patterns 14. FIG. 5
shows a cross sectional view of a representative strut of the stent
10 made in accordance with the present invention which has a
strategically located radiopaque material and a sputtered coating
14 affixed thereto. In a preferred embodiment, the placement of the
radiopaque material on the stock tubing can be coordinated with the
particular pattern of struts and spines which will be cut into the
tube to ensure that the radiopaque material is completely
surrounded by the tubing material once the tube is cut. Therefore,
there should be no edges on a strut or spine which exposes the
layer of radiopaque material directly to blood or tissue. The layer
of coating which is sputtered onto the radiopaque material should
complete the encapsulation of the radiopaque material. In this
manner, the layers of material should not be exposed to possible
elements which can cause galvanic corrosion or the layers to
delaminate. Thereafter the stent is subjected to the standard
industry practices of electro-polishing and possibly annealing. A
biocompatable outer layer could also be added to the stent
surface.
[0030] It should be appreciated that the radiopaque material may
not be completely surrounded by tubing material and the layer of
sputter coating in all instances. It is possible that some
radiopaque material may be exposed on the sides of the stent struts
after the tubing is cut. However, exposure of the radiopaque
material can be kept at a minimum to help prevent galvanic
corrosion from occurring and the risk of cracks forming along the
struts. It is still possible to sputter an additional layer of
coating onto the stent after it has been cut to assure that no
edges of the struts expose radiopaque material directly to blood
and tissue. In this manner, the radiopaque material on the stent
can be fully encapsulated. Alternatively, a stent manufactured in
accordance with the present invention can be made by first placing
the radiopaque material into the grooves formed on the tubing and
then cutting the struts and spines of the stent prior to any
coating of the tubing. Thereafter, once the struts and spines of
the stent have been properly formed, the thin layer of coating
could then be placed on the stent to fully encapsulate the
radiopaque material.
[0031] An advantage of the stent, and the method for manufacture
described above, lies in the resolution of the stent under
fluoroscopy. As previously mentioned, the high resolution is due to
the strategically placed radiopaque strips inserted into grooves
formed in the stent. The benefits of the stent are immediately
apparent in practice where a physician, who views the stent under
fluoroscopy, will clearly see the size and location of the stent in
the vessel of the patient. The clear view of the stent enables the
physician to perform his function efficiently and safely without
the worry of incorrectly approximating the size or location of the
stent.
[0032] While a particular form of the invention has been
illustrated and described, it will also be apparent to those
skilled in the art that various modifications can be made without
departing from the spirit and scope of the invention. More
specifically, it should be clear that the present invention is not
limited to tubular type stents nor is it limited to any particular
method of forming the underlying stent structure. Additionally, the
invention is not limited to the use of any particular materials in
either the core, radiopaque coating or encapsulating layer nor is
it intended to be limited to any particular coating or application
method. Accordingly, it is not intended that the invention be
limited except by the appended claims.
* * * * *