U.S. patent application number 10/347899 was filed with the patent office on 2003-06-12 for implantable nickel-free stainless steel stents and method of making the same.
Invention is credited to Anderson, David, Jalisi, Marc M., Mukherjee, Avijit.
Application Number | 20030106218 10/347899 |
Document ID | / |
Family ID | 23818038 |
Filed Date | 2003-06-12 |
United States Patent
Application |
20030106218 |
Kind Code |
A1 |
Jalisi, Marc M. ; et
al. |
June 12, 2003 |
Implantable nickel-free stainless steel stents and method of making
the same
Abstract
The present invention is directed to a stainless steel stent
which is substantially nickel-free and possesses improved
elongation and mechanical properties, including resistance to
corrosion. The stent can be embodied in a substrate with one or
more metallic claddings overlaying the substrate. The substrate and
claddings can include radiopaque materials and stainless steel.
Inventors: |
Jalisi, Marc M.; (Temecula,
CA) ; Anderson, David; (Temecula, CA) ;
Mukherjee, Avijit; (Union City, CA) |
Correspondence
Address: |
FULWIDER PATTON LEE & UTECHT, LLP
HOWARD HUGHES CENTER
6060 CENTER DRIVE
TENTH FLOOR
LOS ANGELES
CA
90045
US
|
Family ID: |
23818038 |
Appl. No.: |
10/347899 |
Filed: |
January 20, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10347899 |
Jan 20, 2003 |
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09457774 |
Dec 9, 1999 |
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6508832 |
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Current U.S.
Class: |
29/896.6 ;
623/1.15; 623/1.34 |
Current CPC
Class: |
A61L 31/18 20130101;
A61L 31/022 20130101; Y10T 29/496 20150115 |
Class at
Publication: |
29/896.6 ;
623/1.15; 623/1.34 |
International
Class: |
B23P 015/16; A61F
002/06 |
Claims
What is claimed:
1. A stent for implantation in a body lumen of a patient,
comprising: a pattern of struts interconnected to form a structure
that contacts a body lumen wall to maintain the patency of the body
lumen, wherein the struts are made from stainless steel that is
virtually free of any trace of nickel.
2. The stent of claim 1, wherein the structure has a substrate and
a first metallic cladding.
3. The stent of claim 2, wherein the substrate is made from
stainless steel that is virtually free of any trace of nickel and
the first metallic cladding is selected from the group of
radiopaque materials including platinum-10% iridium, platinum,
gold, palladium, tantalum, tungsten, and other radiopaque
materials.
4. The stent of claim 2, wherein the structure has a second
metallic cladding.
5. The stent of claim 4, wherein the second metallic cladding
includes a metal selected from the group of metals including
stainless steel.
6. The stent of claim 1, wherein the amount of nickel in the
structure is less than or equal to 0.3 percent by weight.
7. A stent for implantation in a body lumen of a patient,
comprising: a pattern of struts interconnected to form a structure
that contacts a body lumen wall to maintain the patency of the body
lumen, wherein the structure includes a substrate made from
nickel-titanium alloy, a first metallic cladding made from a
stainless steel that is virtually free of any trace of nickel, and
a second metallic cladding made from a radiopaque material.
8. The stent of claim 7, wherein the amount of nickel in the
structure is less than or equal to 0.3 percent by weight.
9. The stent of claim 7, wherein the structure is constructed from
BioDur.RTM. 108 Alloy.
10. The stent of claim 7, wherein the second metallic cladding is
selected from the group of radiopaque materials including
platinum-10% iridium, platinum, gold, palladium, tantalum,
tungsten, and other radiopaque materials.
11. A method of fabricating a stent for implantation within a body
lumen, comprising the steps of: providing a substrate tube having
an outside surface and an inside surface, wherein the substrate
tube is formed from stainless steel that is virtually free of any
trace of nickel; disposing a first cladding tube overlaying the
substrate tube, wherein the first cladding tube includes a
radiopaque material selected from the group of radiopaque materials
including platinum-10% iridium, platinum, gold, palladium,
tantalum, tungsten, and other radiopaque materials; joining the
first cladding tube to the outside surface of the substrate tube to
form a laminated tube; and forming stent struts in the laminated
tube.
12. The method of claim 11, further comprising the steps of:
providing a second cladding tube including a metal selected from
the group of metals including stainless steel; disposing the second
cladding tube overlaying the first cladding tube; and joining the
second cladding tube to the first cladding tube.
13. The method of claim 11, wherein the step of joining the first
cladding tube further includes deep drawing the laminated tube.
14. The method of claim 11, wherein the step of joining the first
cladding tube further includes rolling and drawing the laminated
tube to mechanically interlock and to generate heat to fuse the
first cladding tube to the substrate tube.
15. The method of claim 11, wherein the step of joining the first
cladding tube further includes passing the substrate tube through a
series of dies to reduce the outside surface of the substrate tube
by about 25 percent.
16. The method of claim 11, wherein the step of disposing the first
cladding tube further includes providing an outside diameter of the
first cladding tube that has an interference fit with the outside
surface of the substrate tube.
17. The method of claim 11, wherein the step of forming stent
struts further includes chemically etching the laminated tube.
18. The method of claim 11, wherein the step of forming stent
struts further includes laser cutting the laminated tube.
19. The method of claim 11, further comprising the step of cold
working the laminated tube.
20. The method of claim 11, wherein the first cladding tube has a
wall thickness that is less than a wall thickness of the substrate
tube.
21. A method of fabricating a stent for implantation within a body
lumen, comprising the steps of: providing a substrate tube having
an outside diameter, wherein the substrate tube is formed from
stainless steel that is virtually free of any trace of nickel;
providing a first cladding tube having an inside diameter, wherein
the inside diameter has an interference fit with the outside
diameter of the substrate tube, and wherein the first cladding tube
includes a radiopaque material selected from the group of
radiopaque materials including platinum-10% iridium, platinum,
gold, palladium, tantalum, tungsten, and other radiopaque
materials; disposing the first cladding tube overlaying the
substrate tube; joining the first cladding tube to the substrate
tube to form a laminated tube; and forming a pattern of stent
struts in the laminated tube to obtain the stent.
22. The method of claim 21, wherein the step of joining the first
cladding tube further includes deep drawing the laminated tube.
23. The method of claim 21, further comprising the steps of:
providing a second cladding tube including a metal selected from
the group of metals including stainless steel, wherein the second
cladding tube includes an inside diameter having an interference
fit with an outside diameter of the first cladding tube; disposing
the second cladding tube overlaying the first cladding tube; and
joining the first cladding tube, the second cladding tube, and the
substrate tube to form a laminated tube.
24. The method of claim 21, wherein the step of joining the first
cladding tube further includes deep drawing the laminated tube.
25. The method of claim 24, wherein the step of deep drawing
further includes passing the laminated tube through a series of
dies.
26. The method of claim 21, wherein the method further comprises
annealing the laminated tube.
27. A stent for implantation within a body lumen, comprising: a
substrate tube having an exterior, wherein the substrate tube is
formed from stainless steel that is virtually free of any trace of
nickel; a metallic cladding mechanically interlocked under pressure
over the xterior of the substrate tube; and a pattern of stent
struts formed in the substrate and metallic cladding.
28. The stent of claim 27, wherein the cladding includes a
radiopaque material selected from the group of radiopaque materials
including platinum-10% iridium, platinum, gold, palladium,
tantalum, tungsten, and other radiopaque materials.
29. The stent of claim 27, wherein a wall thickness of the cladding
is less than a wall thickness of the substrate tube.
30. The stent of claim 27, wherein the stent includes a second
layer of metallic cladding mechanically interlocked under pressure
to an exterior of the cladding over the substrate tube.
31. A stent for implantation within a body lumen, comprising: a
substrate tube having an exterior, wherein the substrate tube is
formed from stainless steel that is virtually free of any trace of
nickel; a metallic cladding mechanically interlocked under pressure
over the exterior of the substrate tube; and a pattern of stent
struts formed in the substrate and metallic cladding.
32. The stent of claim 31, wherein the metallic cladding is formed
from a stainless steel selected from the group of stainless steels
including Type 316L stainless steel.
33. A stent for implantation within a body lumen, comprising: a
substrate tube having an exterior, wherein the substrate tube is
formed from stainless steel that is virtually free of any trace of
nickel; a first cladding mechanically interlocked under pressure
over the exterior of the substrate tube, wherein the first cladding
is formed from a radiopaque material; a second cladding
mechanically interlocked under pressure over the exterior of the
first cladding, wherein the second cladding is formed stainless
steel that is virtually free of any trace of nickel; and a pattern
of stent struts formed in the substrate and metallic cladding.
34. A method of fabricating a stent for implantation within a body
lumen, comprising the steps of: providing a substrate sheet having
an outside surface and an inside surface, wherein the substrate
sheet is formed from stainless steel that is virtually free of any
trace of nickel; disposing a first cladding sheet overlaying the
substrate sheet, wherein the first cladding sheet includes a
radiopaque material selected from the group of radiopaque materials
including platinum-10% iridium, platinum, gold, palladium,
tantalum, tungsten, and other radiopaque materials; joining the
first cladding sheet to the outside surface of the substrate sheet
to form a laminated sheet; rolling the laminated sheet into a
laminated tube; welding the laminated tube; and forming stent
struts in the laminated tube.
35. The method of claim 34, further comprising the steps of:
providing a second cladding sheet including a metal selected from
the group of metals including stainless steel; disposing the second
cladding sheet overlaying the first cladding sheet; and joining the
second cladding sheet to the first cladding sheet.
36. The method of claim 34, wherein the step of joining the first
cladding sheet further includes deep drawing the laminated
sheet.
37. The method of claim 34, wherein the step of forming stent
struts further includes chemically etching the laminated tube.
38. The method of claim 34, wherein the step of forming stent
struts further includes laser cutting the laminated tube.
39. The method of claim 34, wherein the method further comprises
the step of cold working the laminated tube.
40. The method of claim 34, wherein the method further comprises
the steps of: disposing a second cladding sheet overlaying the
laminated sheet, wherein the second cladding sheet includes a metal
selected from the group of metals including stainless steel; and
joining the second cladding sheet to the laminated sheet.
41. The method of claim 34, wherein the first cladding tube has a
wall thickness that is less than a wall thickness of the substrate
tube.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to expandable intraluminal
vascular grafts, generally referred to as stents. More precisely,
the present invention is directed to a stent constructed from
stainless steel that is virtually free of any traces of nickel and
can have a metallic cladding.
[0002] Stents are used to maintain patency of vessels in the body.
A variety of delivery systems have been devised that facilitate the
placement and deployment of stents. The stent is initially
manipulated while in its contracted or unexpanded state, wherein
its reduced diameter more readily allows it to be introduced into
the body lumen and maneuvered into the target site where a lesion
has been dilated. Once at the target site, the stent is expanded
into the vessel wall to allow fluid to more freely flow through the
stent, thus performing a scaffolding function. Such stents usually
are mounted on balloon catheters and are expanded by inflating the
balloon on which the stent is mounted. Deflation of the balloon and
removal of the catheter leave the stent implanted in the vessel in
an expanded state.
[0003] Stents are typically formed from biocompatible metals such
as stainless steel, nickel-titanium, tantalum, and the like, which
provide sufficient hoop strength to perform the scaffolding
function. Furthermore, stents have minimal wall thicknesses in
order to minimize blood flow blockage. However, stents can
sometimes cause complications including thrombosis and neointimal
hyperplasia, such as by inducement of smooth muscle cell
proliferation at the site of implantation of the stent. Such stents
typically also do not provide for delivery of localized therapeutic
pharmacological treatment of a blood vessel at the location being
treated with the stent, which can be useful for overcoming such
problems.
[0004] In the evolution of stents, there have been developments in
the field of stents coated with a layer of polymers. The polymeric
materials are typically capable of absorbing and releasing
therapeutic drugs. Examples of such stents are disclosed in U.S.
Pat. No. 5,443,358 to Eury; U.S. Pat. No. 5,632,840 to Campbell;
U.S. Pat. No. 5,843,172 to Yan; and U.S. Serial No. 08/837,993,
filed April 15, 1997, by Yan.
[0005] Aside from coated stents, there have been developments in
the field of multilayer grafts. An example of a multilayer graft is
disclosed in U.S. Pat. No. 4,743,252 to Martin, Jr. et al. Martin
et al. shows a composite graft having a porous wall structure to
permit ingrowth, which graft includes a generally non-porous,
polymeric membrane in the wall to prevent substantial fluid passage
therethrough so as to provide an implantable porous graft that does
not require preclotting prior to implantation.
[0006] Grafts sometimes have multiple layers for strength
reinforcement. For example, U.S. Pat. No.5,282,860 to Matsuno et
al. discloses a stent tube comprising an inner tube and an outer
polyethylene tube with a reinforcing braided member fitted between
the inner tube and the outer tube. The inner tube is made of a
fluorine-based resin.
[0007] U.S. Pat. No. 5,084,065 to Weldon et al. discloses a
reinforced graft assembly made from a vascular graft component and
a reinforcing sleeve component. The reinforcing sleeve component
may include one or more layers. The second component of the two
component system includes the reinforcing sleeve component. Like
the graft component, the reinforcing component includes a porous
surface and a porous subsurface. Specifically, the reinforcing
sleeve component includes multiple layers formed from synthetic,
biologic, or biosynthetic and generally biocompatible materials.
These materials are typically biocompatible polyurethane or similar
polymers.
[0008] Notably, it has been observed that some prior art stents
implanted in a body lumen have been prone to corrosion over time.
This corrosion can reduce the yield strength of the stent and
produce a danger to the patient. Therefore, it would be desirable
to produce a stent that has a relatively high resistance to
corrosion over the life of the patient.
[0009] What has been needed and heretofore unavailable is a
substantially nickel-free stent that possesses improved elongation
and mechanical properties, including resistance to corrosion. It is
also desirable that such a stent have relatively good ductility,
yet maintain a high yield strength. The present invention satisfies
these and other needs.
SUMMARY OF THE INVENTION
[0010] The present invention is directed to an expandable
intraluminal vascular graft, generally referred to as a stent. More
precisely, the present invention is directed to a stent constructed
from stainless steel that is virtually free of any traces of nickel
and has a metallic cladding.
[0011] In one aspect of the invention, there is provided a stent
constructed from stainless steel that is virtually free of any
trace of nickel. "Nickel-free" stainless steel has some particular
advantages over 316L stainless steel which is a widely used
material for manufacturing stents, particularly coronary stents.
These advantages include the virtual elimination of nickel which
can otherwise cause allergic reactions in some patients when
implanted in an artery. Nickel-free stainless steel also can have
greater strength and ductility as compared to 316L stainless
steel.
[0012] In another aspect of the present invention, there is
provided a substrate tube having an exterior, wherein the substrate
tube is formed from stainless steel that is virtually free of any
trace of nickel. A metallic cladding can be mechanically
interlocked under pressure over the exterior of the substrate tube.
The metallic cladding can include a radiopaque material. A pattern
of stent struts can then be formed in the substrate and metallic
cladding.
[0013] In another aspect, a pattern of stent struts are
interconnected to form a structure that contacts a body lumen wall
to maintain the patency of the body lumen. The structure includes a
substrate made from nickel-titanium, a first metallic cladding made
from a stainless steel that is virtually free of any trace of
nickel, and a second metallic cladding made from a radiopaque
material.
[0014] In another aspect of the invention, there is a first
cladding mechanically interlocked under pressure over the exterior
of the substrate tube. This first cladding can be formed from a
radiopaque material. A second cladding then can be mechanically
interlocked under pressure over the exterior of the first cladding.
The second cladding can he formed from stainless steel that can be
virtually free of any trace of nickel. A pattern of stent struts
can be formed in the substrate and metallic claddings.
[0015] The present invention is also directed to a method of
fabricating a stent for implantation within a body lumen including
the step of providing a substrate tube having an outside surface
and an inside surface, wherein the substrate tube is formed from
stainless steel that is virtually free of any trace of nickel. A
first cladding tube is disposed over the substrate tube, wherein
the first cladding tube includes a radiopaque material selected
from the group of radiopaque materials including platinum-10%
iridium, platinum, gold, palladium, tantalum, tungsten, and other
radiopaque materials. The first cladding tube is joined to the
outside surface of the substrate tube to form a laminated tube.
Stent struts are then formed in the laminated tube by selectively
removing portions of the laminated tube to form a strut
pattern.
[0016] In another aspect of the invention, a method of fabricating
a stent for implantation within a body lumen includes the step of
providing a substrate tube having an outside diameter, wherein the
substrate tube is formed from stainless steel that is virtually
free of any trace of nickel. A first cladding tube is provided
having an inside diameter which has an interference fit with the
outside diameter of the substrate tube. The first cladding tube may
be a radiopaque material selected from a group of radiopaque
materials including platinum-10% iridium, platinum, gold,
palladium, tantalum, tungsten, and other radiopaque materials. The
first cladding tube is disposed over the substrate tube and is
joined to the substrate tube to form a laminated tube. A pattern of
stent struts are then formed in the laminated tube to create the
stent.
[0017] The present invention also is directed to a method of
fabricating a stent for implantation within a body lumen including
the step of providing a substrate sheet having an outside surface
and an inside surface, wherein the substrate sheet is formed from
stainless steel that is virtually free of any trace of nickel. A
first cladding sheet is disposed over the substrate sheet and is
joined to the outside surface of the substrate sheet to form a
laminated sheet. The first cladding sheet may include a radiopaque
material selected from a group of radiopaque materials including
platinum-10% iridium, platinum, gold, palladium, tantalum,
tungsten, and other radiopaque materials. The laminated sheet is
then rolled into a laminated tube. The laminated tube is welded and
stent struts are formed in the laminated tube. As a result, the
finished stent has a cladding layer laminated onto the nickel-free
tubular substrate.
[0018] Other features and advantages of the present invention will
become more apparent from the following detailed description of the
invention, when taken in conjunction with the accompanying
exemplary drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a perspective view of a stent of the present
invention.
[0020] FIG. 2 is an enlarged cross-sectional view taken along lines
2-2 of FIG. 1 illustrating one embodiment of the present
invention.
[0021] FIG. 3 is an enlarged cross-sectional view similar to that
shown in FIG. 2 but of an alternative embodiment of the present
invention.
[0022] FIG. 4 is a perspective view of one embodiment of a
laminated tube mounted on a mandrel and undergoing compression
applied by an external roller.
[0023] FIG. 5 is a perspective view of a deep drawing operation
showing a laminated tube prior to passing through a die.
[0024] FIG. 6 is a perspective view of another embodiment of a
laminated tube.
[0025] FIG. 7 is a perspective view of a step in the process of
manufacturing a stent out of sheets of metal.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0026] As shown in the exemplary drawings, the present invention is
embodied in a metal cladded expandable intraluminal vascular graft,
generally referred to as a stent. Like reference numerals indicate
like or corresponding elements among the figures.
[0027] As explained above, it would be desirable to be able to
produce a stent that possesses improved elongation and mechanical
properties, including resistance to corrosion. It is also desirable
that such a stent have relatively good ductility, yet maintain a
high yield strength.
[0028] In accordance with the present invention, FIG. 1 illustrates
one possible embodiment of a stent 4 for implanting in a body
lumen. It is contemplated that many different stent designs can be
produced. An intricate pattern of interconnecting members 6 and
cylindrical elements (or "rings") 8 can be produced that enable the
stent to expand radially when subjected to the appropriate radially
directed forces such as are exerted by the inflation of an
underlying balloon. These interconnecting members and cylindrical
elements are often referred to as "struts." A myriad of strut
patterns are known for achieving various design goals such as
enhancing strength, maximizing the expansion ratio or coverage
area, enhancing longitudinal flexibility or longitudinal stability
upon expansion, etc. One pattern may be selected over another in an
effort to optimize those parameters that are of particular
importance for a particular application.
[0029] FIG. 2 is a cross-sectional view of one embodiment of stent
4 of the present invention and more specifically, is the
cross-section of the strut which forms the cylindrical element 8.
The stent includes substrate layer 14 and cladding layer 12.
Referring to FIG. 3, an alternative embodiment of the stent
includes substrate layer 30, first cladding layer 32, and second
cladding layer 34.
[0030] FIG. 4 provides a perspective view of one embodiment of the
present invention. As seen in this simplified view, the present
invention contemplates creation of a metallic clad stent by joining
metal cladding tube 12 to an exterior surface of substrate tube 14
to form a laminated tube 10. Fundamental to this joining process is
first defining the initial diameters of metal cladding tube 12,
which should already be in a tubular configuration as seen in FIG.
4, and of substrate tube 14. Tubes 12, 14 can be made by
conventional fabrication processes, such as drawing, rolling sheet
stock and welding the seam, etc. During these preliminary steps,
the diameters and wall thicknesses of tubes 12, 14 are selected and
set.
[0031] Preferably, there should be an interference fit between the
outside diameter of substrate tube 14 and the inside diameter of
metal cladding tube 12. The interference fit prevents unwanted,
relative shifting between substrate tube 14 and the concentrically
disposed cladding tube 12.
[0032] It is further preferable that the wall thickness of
substrate tube 14 be greater than the wall thickness of metal
cladding tube 12. The initial wall thicknesses are important,
because the present invention processes encompass a deep drawing
operation that reduces the diameter and wall thickness of each tube
while increasing its length. In order to maintain continuous
contact at the interface between the interior of metal cladding
tube 12 and the exterior of substrate tube 14, it is preferable
that the initial wall thicknesses of the respective parts be as
described above.
[0033] In concept, the volumes of metal cladding tube 12 and
substrate tube 14 are conserved throughout the various cold drawing
stages of diameter reduction. Because the outer tube has a larger
surface area than the inner tube, its initial wall thickness must
be thinner than the initial wall thickness of the inner tube in
order to obtain a decrease in diameter proportionate to the inner
tube. In this way, the respective diameters of tubes 12, 14 and
their wall thicknesses are reduced proportionately and their
lengths increased identically thus minimizing the chance of
delamination.
[0034] In one embodiment, the metal cladding tube 12 is made from a
radiopaque material such as platinum-10% iridium, platinum, gold,
palladium, tantalum, tungsten, or other radiopaque materials.
Accordingly, the present invention can lend improved visibility for
the physician. It is also contemplated that the metal cladding tube
12 can also be stainless steel, such as 316L stainless steel.
Moreover, the stent can be formed from nickel-free stainless steel
only, without any claddings.
[0035] Substrate tube 14 is preferably made from stainless steel
that is virtually free of any trace of nickel. BioDur.RTM. 108
Alloy is one such material that may be used; however, it is
contemplated that other suitable materials may be used with the
present invention in place of or in addition to BioDur.RTM. 108
Alloy.
[0036] BioDur.RTM. 108 Alloy is available from Carpenter Technology
Corporation, 101 West Bern Street, Reading, Pa. 19601, U.S.A. The
alloy is a substantially nickel-free austenitic stainless alloy.
The alloy contains a high nitrogen content to maintain its
austenitic structure. Consequently, the alloy has improved levels
of tensile and fatigue strength when compared to such
nickel-containing alloys as Type 316L (ASTM F138), 22Cr-13Ni-5Mn
Alloy (ASTM F1314), and 734 Alloy (ASTM F1586). Additionally,
ductility is not compromised when BioDur.RTM. 108 Alloy is used to
fabricate a stent instead of 316L stainless steel.
[0037] Furthermore, the resistance of BioDur.RTM. 108 Alloy to
pitting and crevice corrosion is equal to or greater than that of
the Type 316L Alloy and equivalent to the 22Cr-13Ni-5Mn and 734
Alloys. BioDur.RTM. 108 Alloy is non-magnetic and essentially free
of ferrite phase. The following table provides a list of elements
present in the alloy along with their percentages present by
weight. These are typical or average values.
1 TYPE ANALYSIS WEIGHT PERCENTAGE Carbon 0.08 Max. Manganese 23
Silicon 0.75 Max. Phosphorus 0.03 Max. Sulfur 0.01 Max. Chromium 21
Nickel 0.3 Max. Molybdenum 0.7 Copper 0.25 Max. Nitrogen 0.97 Iron
Balance
[0038] Notably, BioDur.RTM. 108 Alloy is useful in applications
requiring high levels of strength and corrosion resistance. With
respect to some materials, it should be noted that allergic
reactions in a small percentage of patients have been observed due
to the presence of nickel. The fact that BioDur.RTM. 108 Alloy is
virtually nickel-free makes it a potential candidate for use in
devices that contact the human body in service.
[0039] BioDur.RTM. 108 Alloy possesses a high resistance to
corrosion owed to its high levels of chromium, nitrogen, and
molybdenum. BioDur.RTM. 108 Alloy exhibits corrosion resistance
equivalent to or greater than the nickel-containing alloys,
22Cr-13Ni-5Mn and 734 Alloy. Furthermore, the corrosion resistance
of BioDur.RTM. 108 Alloy is superior to Type 316L Alloy.
BioDur.RTM. 108 Alloy possesses a density of 7630 Kg/M.sup.3 (0.276
lb/in.sup.3).
[0040] Notably, BioDur.RTM. 108 Alloy is less ferromagnetic than
316L stainless steel. This is important due to the modem medical
trend toward noninvasive treatment modality. BioDur.RTM. 108 Alloy
is compatible with procedures such as magnetic resonance imaging
(MRI), CT scanning, and computer tomography.
[0041] Annealing of BioDur.RTM. 108 Alloy is preferably
accomplished in the range of 1040.degree. C. to 1150.degree. C.
(1900.degree. F. to 2100.degree. F.). The alloy is typically
annealed in the lower part of this range in order to preserve a
fine grain size. The alloy is preferably rapidly cooled from the
annealing temperature. The reason for this is that slow cooling
through the range of 980.degree. C. to 810.degree. C. (1800.degree.
F. to 1500.degree. F.) can, under some circumstances, cause
precipitation of a chromium nitride phase (Cr.sub.2N) that could
adversely affect corrosion resistance and toughness.
[0042] The choice of materials for metal cladding tube 12 and
substrate tube 14 are important and are preferably chosen to assure
consistent tube diameter and wall thickness reduction while
minimizing the chance of delamination of the concentric tubes. Of
course, reversing the material selection for the substrate tube and
the cladding tube in specific applications is also contemplated, as
are other combinations of materials.
[0043] Laminated tube 10 including metal cladding tube 12 over
substrate tube 14 is optionally mounted on mandrel 16 and rolled by
application of external pinching pressure through roller 18. This
is represented in the perspective view of FIG. 4.
[0044] Along with the rolling operation depicted in FIG. 4, the
present invention contemplates a deep drawing operation shown in
the perspective view of FIG. 5. Here, laminated tube 10 with metal
cladding tube 12 surrounding substrate tube 14 is shown prior to
passing through opening 20 of die 22. By passing through a series
of dies 22 with sequentially decreasing opening diameters, it is
possible to deep draw laminated tube 10 to the final desired
diameter. As the name suggests, this cold drawing process is
preferably carried out at room temperature, below the
recrystallization temperatures of the tube materials.
[0045] By repeating the operations shown in FIGS. 4 and 5, it is
possible to reduce laminated tube 10 from a starting outside
diameter of, for example, 1/2 inch down to {fraction (1/16)} inch.
The starting wall thickness for the outside tube is approximately
{fraction (1/16)} or {fraction (1/32)} inch and is reduced down to
about 0.003 inch. Of course, the present invention process allows
for wall thicknesses ranging from 0.0022 to 0.06 inch. A finished
stent might have an outside diameter on the order of about 0.06
inch in the unexpanded condition. The stent can be expanded to an
outside diameter of 0.1 inch or more when deployed in a body
lumen.
[0046] In one embodiment process, the rolling and cold drawing
operations are repeated to achieve a maximum of 25 percent in
reduction of surface area. Each sequence of operations slowly
reduces the diameter of laminated tube 10 while proportionately
increasing its length.
[0047] Although the present invention rolling and deep drawing
processes are conducted at room temperature, the pressures involved
cause the temperature between metal cladding tube 12 and substrate
tube 14 to elevate sufficiently to create heat fusion between the
two materials. Also due to the tremendous pressures involved, a
mechanical interlock bond is created between the two adjacent
layers. In this manner, metal cladding tube 12 is permanently
attached to substrate tube 14 and delamination of the two materials
is minimized under normal operating conditions for the stent.
[0048] As a result of empirical observations, it is preferred that
the material with a smaller coefficient of thermal expansion be
used as substrate tube 14. Conversely, the material with a greater
coefficient of thermal expansion should be used in metal cladding
tube 12. Again, this assures that during the rolling and cold
drawing processes the surfaces in common between the two tubes 12,
14 remain in contact and do not delaminate.
[0049] In one method, laminated tube 10 undergoes about a
twenty-five percent (25%) diameter reduction from the rolling and
cold drawing operations. This is accomplished by passing laminated
tube 10 through a series of dies 22 with each die reducing the
diameter by preferably one percent (1%). With a series of
twenty-five dies 22, it is possible to achieve the twenty-five
percent (25%) diameter reduction.
[0050] Laminated tube 10 then undergoes another twenty-five percent
(25%) diameter reduction by cold drawing and rolling. This cycle is
repeated until the desired diameter of laminated tube 10 is
reached. Throughout the present invention process, laminated tube
10 may optionally undergo anneal cycles in order to impart desired
material properties such as ductility, strength, etc. It should be
noted that processes for hardening BioDur.RTM. 108 Alloy by heat
treatment are not currently known but it may be hardened by cold
working.
[0051] Through the present invention process, it has been observed
that the finished composite stent has a straightness of 0.02 inch
per inch for a six to twelve inch length tube. In other words,
there is virtually no curvature or bend in the finished stent. When
the final diameter is reached, the tubes are cut to specific
lengths. Laminated tube 10, which has now been cladded with metal
tube 12, is further processed to form stent struts. Stent struts
may be formed by a laser cutting process. Such a process is shown
and disclosed in, for example, U.S. Pat. No. 5,759,192 to Saunders,
entitled "Method and Apparatus for Direct Laser Cutting of Metal
Stents," whose entire contents are hereby incorporated by
reference. Another method for forming such stent struts is by
chemical etching. Such a process is disclosed in, for example, U.S.
Pat. No. 5,735,893 to Lau et al., entitled "Expandable Stents and
Method for Making Same," whose contents are incorporated herein by
reference. The laminated tube 10 will end up looking like the stent
of FIG. 1 or possess another desired design.
[0052] FIG. 6 provides a perspective view of an alternative
embodiment showing laminated tube 28 including substrate tube 30
that is laminated with first metallic cladding tube 32. The first
cladding tube can include a radiopaque material selected from the
group of radiopaque materials including platinum-10% iridium,
platinum, gold, palladium, tantalum, tungsten, and other radiopaque
materials. The substrate tube 30 can be formed from stainless steel
that is virtually free of any trace of nickel, such as BioDur.RTM.
108 Alloy. Second metallic cladding tube 34 is laminated to the
outer surface of first metallic cladding tube 32. The second
cladding tube can include a metal selected from the group of metals
including stainless steel. The stainless steel can be chosen to be
virtually nickel-free.
[0053] It is also contemplated that substrate 30 can be made from
nickel-titanium, first metallic cladding 32 can be made from a
stainless steel that is virtually free of any trace of nickel, and
second metallic cladding 34 can be made from a radiopaque
material.
[0054] The multiple layers of cladding of composite stent 28 are
created as previously described in connection with FIGS. 4 and 5,
except that second metallic cladding tube 34 is added to the
outside surface of first metallic cladding tube 32. The three tubes
30, 32, 34 then undergo the rolling an cold drawing operations as
described previously. When the final diameter is reached, composite
stent 28 is cut to the desired length and processed to form a stent
resembling the stent of FIG. 1 or possess another desired
design.
[0055] Referring to FIG. 7, it is also contemplated that stent 4
can be manufactured in a similar manner using sheets instead of
tubes. A substrate sheet 40 is provided having an outside surface
and an inside surface, wherein the substrate sheet is formed from
stainless steel that is virtually free of any trace of nickel, such
as BioDur.RTM. 108 Alloy. A first cladding sheet 42 is disposed
overlaying the substrate sheet, wherein the first cladding sheet
includes a radiopaque material selected from the group of
radiopaque materials including platinum-10% iridium, platinum,
gold, palladium, tantalum, tungsten, and other radiopaque
materials. The first cladding sheet is joined to the outside
surface of the substrate sheet to form a laminated sheet. The
laminated sheet is rolled into a laminated tube. The laminated tube
is welded and stent struts are formed in the laminated tube by
chemical etching or laser cutting.
[0056] It is contemplated that second cladding sheet can be
provided including a metal selected from the group of metals
including stainless steel, such as Type 316L. The second cladding
sheet is disposed overlaying first cladding sheet 42 and then
joined to the first cladding sheet. Alternatively, a second
cladding sheet can be joined to the laminated sheet. Preferably,
the first cladding sheet has a wall thickness that is less than
that of the substrate sheet. As described above, the steps of deep
drawing and cold working can be used in the manufacturing
process.
[0057] Consequently, a stent may be produced exhibiting improved
elongation and mechanical properties, including resistance to
corrosion. The stent further has relatively good ductility, yet
maintains a high yield strength.
[0058] While the invention has been illustrated and described
herein in terms of its use a stainless steel stent formed from
stainless steel that is virtually free of any trace of nickel, such
as BioDur.RTM. 108 Alloy, it will be apparent to those skilled in
the art that the invention can be used in other instances. For
example, the dimensions and materials referenced herein are by way
of example only and not intended to be limiting. Thus, certain
stent dimensions may vary to suit a particular application.
Additionally, any of a variety of stent designs and applications
can benefit from the present invention. Furthermore, stainless
steel that is virtually free of any trace of nickel, such as
BioDur.RTM. 108 Alloy, cay be used in other medical components that
contact the human body in service. Other modifications and
improvements may be made without departing from the scope of the
invention.
* * * * *