U.S. patent application number 10/103411 was filed with the patent office on 2002-10-10 for stainless steel alloy with improved radiopaque characteristics.
Invention is credited to Craig, Charles Horace, Radisch, Herbert R., Trozera, Thomas.
Application Number | 20020144757 10/103411 |
Document ID | / |
Family ID | 24451960 |
Filed Date | 2002-10-10 |
United States Patent
Application |
20020144757 |
Kind Code |
A1 |
Craig, Charles Horace ; et
al. |
October 10, 2002 |
Stainless steel alloy with improved radiopaque characteristics
Abstract
The present invention is directed towards an austenitic,
stainless steel series 300 alloy having improved radiopaque
characteristics. The modified stainless steel alloy consists
essentially of, in weight percent, about 1 C Mn Si P S
.ltoreq.0.030 .ltoreq.2.000 .ltoreq.0.750 .ltoreq.0.023
.ltoreq.0.010 Cr Mo Ni Fe "X" 12.000- 0.00- 10.000- 46.185- 2.000-
20.000 3.000 18.000 74.000 10.000 whereby variable "X" could be
comprised from a group consisting of Gold, Osmium, Palladium,
Platinum, Rhenium, Tantalum, Tungsten or Iridium. The alloy
provides a unique combination of strength, ductility, corrosion
resistance, and other mechanical properties which also has improved
radiopaque characteristics.
Inventors: |
Craig, Charles Horace;
(Lakeside, CA) ; Radisch, Herbert R.; (San Diego,
CA) ; Trozera, Thomas; (Del Mar, CA) |
Correspondence
Address: |
Charles Horace Craig
9354 Harritt Road
Lakeside
CA
92040
US
|
Family ID: |
24451960 |
Appl. No.: |
10/103411 |
Filed: |
March 20, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10103411 |
Mar 20, 2002 |
|
|
|
09612157 |
Jul 7, 2000 |
|
|
|
Current U.S.
Class: |
148/327 ;
420/57 |
Current CPC
Class: |
C22C 38/58 20130101;
C22C 38/44 20130101; C22C 38/002 20130101 |
Class at
Publication: |
148/327 ;
420/57 |
International
Class: |
C22C 038/38; C22C
038/58 |
Claims
We claim:
1. A modified series 300 stainless steel alloy which provides
increased radiopaque characteristics over standard 300 stainless
steel.
2. A steel alloy as recited in claim 1, wherein said alloy is used
for fabricating intravascular stents.
3. A steel alloy as recited in claim 1, wherein said steel alloy
consisting essentially of, in weight percent, about
14 C Mn Si P S Cr Mo Ni Fe "X" .ltoreq.0.030 .ltoreq.2.000
.ltoreq.0.750 .ltoreq.0.023 .ltoreq.0.010 12.000- 0.00- 10.000-
46.185- 2.000- 20.000 3.000 18.000 74.000 10.000
whereby variable "x" could be comprised from a group consisting of
Ir.
4. A steel alloy as recited in claim 3, wherein a portion of
Iridium replaces a portion of Iron.
5. A steel alloy as recited in claim 3, wherein a portion of
Iridium replaces a portion of Molybdenum.
6. A steel alloy as recited in claim 3, wherein a portion of
Iridium replaces a portion of both Iron and Molybdenum.
7. A modified series 300 stainless steel alloy which provides
increased radiopaque characteristics over standard 300 stainless
steel, said alloy consisting essentially of, in weight percent,
about
15 C Mn Si P S Cr Mo Ni Ir .ltoreq.0.030 .ltoreq.2.000
.ltoreq.0.750 .ltoreq.0.023 .ltoreq.0.010 12.000- 2.000- 10.000-
2.000- 20.000 3.000 18.000 10.000
and the balance is essentially iron.
8. A modified series 300 stainless steel alloy which provides
increased radiopaque characteristics over standard 300 stainless
steel, said alloy consisting essentially of, in weight percent,
about
16 C Mn Si P S Cr Mo Ni Ir .ltoreq.0.030 .ltoreq.2.000
.ltoreq.0.750 .ltoreq.0.023 .ltoreq.0.010 12.000- 0.00- 10.000-
2.000- 20.000 3.000 18.000 10.000
and the balance is essentially iron.
9. A modified series 300 stainless steel alloy which provides
increased radiopaque characteristics over standard 300 stainless
steel, said alloy consisting essentially of, in weight percent,
about
17 C Mn Si P S Cr Mo Ni Fe "X" .ltoreq.0.030 .ltoreq.2.000
.ltoreq.0.750 .ltoreq.0.023 .ltoreq.0.010 12.000- 0.00- 10.000-
46.185- 2.000- 20.000 3.000 18.000 74.000 10.000
whereby variable "X" could be comprised from a group consisting of
Gold, Osmium, Palladium, Platinum, Rhenium, Tantalum, Tungsten or
Iridium.
Description
PRIOR APPLICATIONS
[0001] This application is a continuation-in-part of application
Ser. No. 09/612,157 filed on Jul. 7, 2000. It was disclosed in the
application that this inventions is an austenitic steel alloy
having radiopaque characteristics.
BACKGROUND OF THE INVENTION
[0002] This invention relates to an austenitic steel alloy, and in
particular to such an alloy and an article made therefrom in which
the elements are closely controlled to provide a unique combination
of high tensile strength, ductility, good resistance to stress
cracking and corrosion, and have improved radiopaque
characteristics.
[0003] Austenite generally does not exist at room temperature in
plain-carbon and low-alloy steels, other than as small amounts of
retained austenite that did not transform during rapid cooling.
However, in certain high-alloy steels, such as the austenitic
stainless steels and Hadfield austenitic manganese steel, austenite
is the dominant microstructure. In these steels, sufficient
quantities of alloying elements that stabilize austenite at room
temperature are present (e.g., manganese and nickel). The crystal
structure of austenite is face-centered cubic (fcc) as compared to
ferrite, which has a body centered cubic (bcc) lattice. An fcc
alloy has certain desirable characteristics; for example, it has
low-temperature toughness, excellent weldability, and is
nonmagnetic. Because of their high alloy content, austenitic steels
are usually corrosion resistant. Disadvantages of the austenitic
steels are their relative high costs, their susceptibility to
stress-corrosion cracking (certain austenitic steels), the fact
that they cannot be strengthened other than by cold working, and
interstitial solid-solution strengthening.
[0004] The austenitic stainless steels (e.g., type 301, 302, 303,
304, 305, 308, 309, 310, 314, 316, 317, 321, 330, 347, 348, and
384) generally contain from 6 to 22% nickel to stabilize the
austenite microstructure at room temperature. They also contain
other alloying elements, such as chromium (16 to 26%) for corrosion
resistance, and smaller amounts of manganese and molybdenum. The
widely used type 304 stainless steel contains 18 to 20% Cr and 8 to
10.5% Ni, and is also called 18-8 stainless steel. The yield
strength of annealed type 304 stainless steel is typically 290 MPa
(40 ksi), with a tensile strength of about 580 MPa (84 ksi).
However, both yield and tensile strength can be substantially
increased by cold working. However, the increase in strength is
offset by a substantial decrease in ductility, for example, from
about 55% elongation in the annealed condition to about 25%
elongation after cold working.
[0005] Some austenitic stainless steels (type 200, 201, 202, and
205) employ interstitial solid-solution strengthening with nitrogen
addition. Austenite, like ferrite, can be strengthened by
interstitial elements such as carbon and nitrogen. However, carbon
is usually excluded because of the deleterious effect associated
with precipitation of chromium carbides on austenite grain
boundaries (a process called sensitization). These chromium
carbides deplete the grain-boundary regions of chromium, and the
denuded boundaries are extremely susceptible to corrosion. Such
steels can be desensitized by heating to high temperature to
dissolve the carbides and place the chromium back into solution in
the austenite. Nitrogen, on the other hand, is soluble in austenite
and is added for strengthening. To prevent nitrogen from forming
deleterious nitrides, manganese is added to lower the activity of
nitrogen in the austenite, as well as to stabilize the austenite.
For example, type 201 stainless steel has composition ranges of 5.5
to 7.5% Mn, 16 to 18% Cr, 3.5 to 5.5% Ni, and 0.25% N. The other
type 2xx series of steels contain from 0.25 to 0.40% N.
[0006] Another important austenitic steel is austenitic manganese
steel. Developed by Sir Robert Hadfield in the late 1890s, these
steels remain austenitic after water quenching and have
considerable strength and toughness. A typical Hadfield manganese
steel contains 10 to 14% Mn, 0.95 to 1.4% C, and 0.3 to 1% Si.
Solution annealing is necessary to suppress the formation of iron
carbides. The carbon must be in solid solution to stabilize the
austenite. When completely austenitic, these steels can be work
hardened to provide higher hardness and wear resistance. A
work-hardened Hadfield manganese steel has excellent resistance to
abrasive wear under heavy loading. Because of this characteristic,
these steels are ideal forjaw crushers and other crushing and
grinding components in the mining industry. Also, Hadfield
manganese steels have long been used for railway frogs (components
used at the junction point of two railroad lines).
[0007] AISI Types 304L, 316L, 321 and 347 stainless steels are
austenitic, chromium-nickel and chromium-nickel-molybdenum
stainless steels having the following compositions in weight
percent:
2 Type 304 L Type 316 L Type 321 Type 347 wt. % wt. % wt. % wt. % C
0.03 max 0.03 max 0.08 max 0.08 max Mn 2.00 max 2.00 max 2.00 max
2.00 max Si 1.00 max 1.00 max 1.00 max 1.00 max P 0.045 max 0.045
max 0.045 max 0.045 max S 0.03 max 0.03 max 0.03 max 0.03 max Cr
18.0-20.0 16.0-18.0 17.0-19.0 17.0-19.0 Ni 8.0-12.0 10.-14.0
9.0-12.0 9.0-13.0 N 0.10 max 0.10 max 0.10 max -- Mo -- 2.0-3.0 --
-- Fe Bal. Bal. Bal. Bal. Source: METALS HANDBOOK RTM. Desk
Edition; Chapt. 15, pages 2-3; (1985). The AMS standards for these
alloys restrict copper to not more than 0.75%.
[0008] The above-listed chromium-nickel and
chromium-nickel-molybdenum stainless steels are known to be useful
for applications which require good non-magnetic behavior, in
combination with good corrosion resistance. One disadvantage of the
series 300 stainless steels is their poor radiopacity. For example,
a stent made from standard 300 series stainless steel can not be
sufficiently radiopaque for clinical observation due to the thin
cross-section of the struts. Therefore, this present invention
alloy can be useful in clinical observations because it can be
radiopaque in these cross-sections. There continues to be a demand
for improved chromium-nickel and chromium-nickel-molybdenum
stainless steels, particularly for these alloys having increased
radiopaque characteristics.
[0009] Given the foregoing, it would be highly desirable to have an
austenitic stainless steel that provides better radiopacity than is
provided by the known austenitic stainless steels.
SUMMARY OF THE INVENTION
[0010] The invention generally relates to an austenitic 300 series
stainless steel alloy that provides better radiopacity than is
provided by the known austenitic stainless steels. One application
for the present invention is to use the austenitic stainless steel
alloy with increased radiopacity for fabricating intravascular
stents. In this clinical setting, the interventionalist uses
angiographic and fluoroscopic techniques that employ X-rays and
materials that are radiopaque to the X-rays to visualize the
location or placement of the particular device within the human
vasculature. Typically stents are fabricated from a variety of
stainless steels, with the 316 series representing a large
percentage of the stainless steel used to fabricate currently
marketed stents. The typical composition of 316 series stainless
steel is shown in Table I.
3 TABLE I Component (%) C Mn Si P S Cr Mo Ni Fe Standard 316 0.020
1.760 0.470 0.014 0.002 17.490 2.790 14.680 62.774
[0011] While the 300 series of stainless steel has several
characteristics, such as strength, flexibility, fatigue resistance,
biocompatibility, etc. rendering it a good material to make an
intravascular stent, one significant disadvantage of 316 series
stainless steel, as well as other 300 series of stainless steel, is
that they have relatively low radiopaque qualities and therefore
not readably visual under fluoroscopic observation. A need has
arisen to modify the stainless steel composition so it has
radiopaque properties while at the same time, maintaining those
characteristics which render it as a material of choice for
fabricating stents.
[0012] Modified stainless steel of the 300 series for increasing
radiopaque characteristic could be produced by creating alloys
containing varying amounts of elements that have dense mass and
radiopaque characteristics. The chemical make-up of standard series
300 stainless steel, using series 316 as an example, along with the
possible chemical ranges of various such alloys are shown on the
following Table.
4 TABLE II Component (%) C Mn Si P S Cr Mo Ni Fe X Standard 316
0.020 1.760 0.470 0.014 0.002 17.490 2.790 14.680 62.774 0.000
Modified 300A .ltoreq.0.030 .ltoreq.2.000 .ltoreq.0.750
.ltoreq.0.023 .ltoreq.0.010 12.000- 000- 10.000- 46.185- 2.000-
20.000 3.000 18.000 74.000 10.000 Variable "X" could be comprised
of or a combination of Au, Os, Pd, Pt, Re, Ta, W or Ir.
[0013] Other features and advantages of the present invention will
become more apparent from the following detailed description of the
invention.
[0014] It is an object of the present invention to provide an
austenitic 300 series stainless steel alloy that provides better
radiopacity than is provided by the known austenitic stainless
steels.
[0015] It is another object of the present invention to provide a
stent or prosthesis which can be readily delivered to, expanded and
embedded into an obstruction or vessel wall with relatively high
radiopaque characteristics for fluoroscopy during all phases of the
interventional procedure.
[0016] Another object of the present invention is to provide a
material which has superior properties, including radiopacity, for
fabricating any stent design or format.
DETAILED DESCRIPTION
[0017] The alloy according to the present invention comprises a
stainless steel series 300 compound used to fabricate a stent which
replaces a portion of the iron or molybdenum component of the 300
series with one or combination of several elements containing
radiopaque properties. Examples of such elements are gold (Au),
osmium (Os), palladium (Pd), platinum (Pt), rhenium (Re), tantalum
(Ta), tungsten (W) or iridium (Ir). This group consists of elements
with dense masses. The dense mass provides these materials with
improved absorption of X-rays thus providing improved radiopaque
characteristics. By including one or more of these elements in a
series 300 stainless steel, thereby creating the present invention
alloy, X-rays employed in angiogram procedures or cineograms allow
the visualization of certain devices, such as a stent, during all
phases of a standard clinical procedure. The alloy for fabricating
stents contains a range of 2.0 to 10.0 percent of one or more of
these radiopaque elements, with a preferred range of 4.0 to 5.0
percent. Replacing too much of the radiopaque element with the iron
or molybdenum component could possible decrease the beneficial
qualities of 300 series stainless steel for manufacturing stents
without contributing significantly improved radiopaque
characteristics. It is anticipated that various combinations of the
radiopaque elements can be used to replace the iron or molybdenum
component without adversely affecting the ability to form
austenite.
[0018] The foregoing, as well as additional objects and advantages
of the present invention, achieved in a series 300 stainless steel
alloy, is compared with standard 316 stainless steel and summarized
in Tables III through XI below, containing in weight percent,
about:
5 TABLE III Component (%) C Mn Si P S Cr Mo Ni Fe X Standard 316
0.020 1.760 0.470 0.014 0.002 17.490 2.790 14.680 62.774 0.000
Modified 300A .ltoreq.0.030 .ltoreq.2.000 .ltoreq.0.750
.ltoreq.0.023 .ltoreq.0.010 12.000- 0.00- 10.000- 46.185- 2.000-
20.000 3.000 18.000 74.000 10.000 Where variable "X" could be
comprised of or a combination of Au, Os, Pd, Pt, Re, Ta, W or
Ir.
[0019]
6 TABLE IV Component (%) C Mn Si P S Cr Mo Ni Fe Au Standard 316
0.020 1.760 0.470 0.014 0.002 17.490 2.790 14.680 62.774 0.000
Modified 316B .ltoreq.0.030 .ltoreq.2.000 .ltoreq.0.750
.ltoreq.0.023 .ltoreq.0.010 12.000- 0.00- 10.000- 46.185- 2.000-
20.000 3.000 18.000 74.000 10.000
[0020]
7 TABLE V Component (%) C Mn Si P S Cr Mo Ni Fe Os Standard 316
0.020 1.760 0.470 0.014 0.002 17.490 2.790 14.680 62.774 0.000
Modified 316B .ltoreq.0.030 .ltoreq.2.000 .ltoreq.0.750
.ltoreq.0.023 .ltoreq.0.010 12.000- 0.00- 10.000- 46.185- 2.000-
20.000 3.000 18.000 74.000 10.000
[0021]
8 TABLE VI Component (%) C Mn Si P S Cr Mo Ni Fe Pd Standard 316
0.020 1.760 0.470 0.014 0.002 17.490 2.790 14.680 62.774 0.000
Modified 316B .ltoreq.0.030 .ltoreq.2.000 .ltoreq.0.750
.ltoreq.0.023 .ltoreq.0.010 12.000- 0.00- 10.000- 46.185- 2.000-
20.000 3.000 18.000 74.000 10.000
[0022]
9 TABLE VII Component (%) C Mn Si P S Cr Mo Ni Fe Pt Standard 316
0.020 1.760 0.470 0.014 0.002 17.490 2.790 14.680 62.774 0.000
Modified 316B .ltoreq.0.030 .ltoreq.2.000 .ltoreq.0.750
.ltoreq.0.023 .ltoreq.0.010 12.000- 0.00- 10.000- 46.185- 2.000-
20.000 3.000 18.000 74.000 10.000
[0023]
10 TABLE VIII Component (%) C Mn Si P S Cr Mo Ni Fe Re Standard 316
0.020 1.760 0.470 0.014 0.002 17.490 2.790 14.680 62.774 0.000
Modified 316B .ltoreq.0.030 .ltoreq.2.000 .ltoreq.0.750
.ltoreq.0.023 .ltoreq.0.010 12.000- 0.00- 10.000- 46.185- 2.000-
20.000 3.000 18.000 74.000 10.000
[0024]
11 TABLE IX Component (%) C Mn Si P S Cr Mo Ni Fe Ta Standard 316
0.020 1.760 0.470 0.014 0.002 17.490 2.790 14.680 62.774 0.000
Modified 316B .ltoreq.0.030 .ltoreq.2.000 .ltoreq.0.750
.ltoreq.0.023 .ltoreq.0.010 12.000- 0.00- 10.000- 46.185- 2.000-
20.000 3.000 18.000 74.000 10.000
[0025]
12 TABLE X Component (%) C Mn Si P S Cr Mo Ni Fe W Standard 316
0.020 1.760 0.470 0.014 0.002 17.490 2.790 14.680 62.774 0.000
Modified 316B .ltoreq.0.030 .ltoreq.2.000 .ltoreq.0.750
.ltoreq.0.023 .ltoreq.0.010 12.000- 0.00- 10.000- 46.185- 2.000-
20.000 3.000 18.000 74.000 10.000
[0026]
13 TABLE XI Component (%) C Mn Si P S Cr Mo Ni Fe Ir Standard 316
0.020 1.760 0.470 0.014 0.002 17.490 2.790 14.680 62.774 0.000
Modified 316B .ltoreq.0.030 .ltoreq.2.000 .ltoreq.0.750
.ltoreq.0.023 .ltoreq.0.010 12.000- 0.00- 10.000- 46.185- 2.000-
20.000 3.000 18.000 74.000 10.000
[0027] The alloy for fabricating a series 300 stainless steel with
improved radiopaque properties can contain up to 0.03% of carbon.
The carbon element contributes to good hardness capability and high
tensile strength by combining with other elements such as chromium
and molybdenum to form carbides during heat treatment. However, too
much carbon adversely affects the fracture toughness of this
alloy.
[0028] Chromium contributes to the good hardenability corrosion
resistance and hardness capability of this alloy and benefits the
desired low ductile-brittle transition temperature of the alloy.
Therefore, at least about 12%, and preferably at least about 17.5%
chromium is present. Above about 20% chromium the alloy is
susceptible to rapid overaging such that the unique combination of
high tensile strength and high fracture toughness is not
attainable.
[0029] Nickel contributes to the hardenability of this alloy such
that the alloy can be hardened with or without rapid quenching
techniques. Nickel benefits the fracture toughness and stress
corrosion cracking resistance provided by this alloy and
contributes to the desired low ductile-to-brittle transition
temperature. Accordingly, at least about 10.0%, and preferably at
least about 14.7% nickel is present. Above about 18% nickel, the
fracture toughness and impact toughness of the alloy can be
adversely affected because the solubility of carbon in the alloy is
reduced which may result in carbide precipitation in the grain
boundaries when the alloy is cooled at a slow rate, such as when
air cooled following forging.
[0030] Molybdenum is present in this alloy because it benefits the
desired low ductile brittle transition temperature of the alloy.
Above about 3% molybdenum the fracture toughness of the alloy is
adversely affected. Preferably, molybdenum is limited to not more
than about 1.2%. However, the entire portion of the molybdenum can
be replaced with certain radiopaque elements such as Ta without
adversely affecting the desired characteristics of the alloy.
[0031] The alloy for fabricating a series 300 stainless steel stent
with radiopaque properties can also contain up to 2.0% manganese.
Manganese is partly depended upon to maintain the austenitic,
nonmagnetic character of the alloy. Manganese also plays a role, in
part, providing resistance to corrosive attack.
[0032] The balance of the alloy according to the present invention
is essentially iron except for the usual impurities found in
commercial grades of alloys intended for similar service or use.
The levels of such elements must be controlled so as not to
adversely affect the desired properties of this alloy. For example,
phosphorus is limited to not more than about 0.008% and sulfur is
limited to not more 0.004%. In addition, the alloy for fabricating
a series 300 stainless steel alloy with radiopaque properties can
contain up to 0.75% silicon. Furthermore, the alloy for fabricating
a series 300 stainless steel stent with radiopaque properties can
contain up to 0.023% and 0.002% phosphorus and sulfur,
respectively, without affecting the desirable properties.
[0033] No special techniques are required in melting, casting, or
working the alloy of the present invention. Arc melting followed by
argon-oxygen decarburization is the preferred method of melting and
refining, but other practices can be used. In addition, this alloy
can be made using powder metallurgy techniques, if desired. This
alloy is also suitable for continuous casting techniques.
[0034] The alloy of the present invention can be formed into a
variety of shapes for a wide variety of uses and lends itself to
the formation of billets, bars, rod, wire, strip, plate, or sheet
using conventional practices.
[0035] The alloy according to the present invention can be useful
in a variety of applications requiring high strength and radiopaque
characteristics, for example, to fabricate stents of other medical
applications.
[0036] It is apparent from the foregoing description and the
accompanying examples, that the alloy according to the present
invention provides a unique combination of tensile strength and
radiopaque characteristics not provided by known series 300
stainless steel alloys. This alloy is well suited to applications
where high strength, biocompatibility and radiopacity are
required.
[0037] The terms and expressions which have been employed herein
are used as terms of description and not of limitation. There is no
intention in the use of such terms and expressions to exclude any
equivalents of the features described or any portions thereof. It
is recognized, however, that various modifications are possible
within the scope of the invention claimed.
[0038] While the invention has been illustrated and described
herein in terms of its use as an intravascular stent, it will be
apparent to those skilled in the art that the stent can be used in
other instances such as to expand prostate urethras in cases of
prostate hyperplasia. Other modifications and improvements may be
made without departing from the scope of the invention.
[0039] Other modifications and improvements can be made to the
invention without departing from the scope thereof.
[0040] The alloy of the present invention is readily melted using
conventional and/or vacuum melting techniques. For best results, as
when additional refining is desired, a multiple melting practice is
preferred. The preferred practice is to melt a heat in a vacuum
induction furnace (VIM) and cast the heat in the form of an
electrode. The electrode is then remelted in a vacuum arc furnace
(VAR) and recast into one or more ingots.
[0041] The alloy can be prepared from heats which can be melted
under argon cover and cast as ingots. The ingots can be maintained
at a temperature range of 2100-2300 degree F. (1149-1260 degree C.)
for 2 hours and then pressed into billets. The billets may be
ground to remove surface defects and the ends cut off. The billets
can then be hot rolled to form intermediate bars with an
intermediate diameter. The intermediate bars are hot rolled to a
diameter of 0.7187 in. (1.82 cm) from a temperature range of
2100-2300.degree. F. (1 149-1260.degree. C.). The round bars are
straightened and then turned to a final diameter or alternately,
sheets are rolled to the desired diameter with optional
intermediate anneals are required. All of the bars or sheets can be
pointed, solution annealed, water quenched, and acid cleaned to
remove surface scale.
[0042] To evaluate improved radiopacity of the present invention,
stents can be fabricated from the present invention alloy and
testing in animal studies utilizing standard angiography equipment.
The stent fabricated from the alloy can be deployed in an animal
model with other FDA approved stents with know radiopacity
characteristics.
[0043] The terms and expressions that have been employed herein are
used as terms of description and not of limitation. There is no
intention in the use of such terms and expressions to exclude any
equivalents of the features described or any portions thereof. It
is recognized, however, that various modifications are possible
within the scope of the invention claimed.
* * * * *