U.S. patent application number 10/672891 was filed with the patent office on 2005-03-31 for medical devices and methods of making same.
Invention is credited to Stinson, Jonathan S..
Application Number | 20050070990 10/672891 |
Document ID | / |
Family ID | 34376494 |
Filed Date | 2005-03-31 |
United States Patent
Application |
20050070990 |
Kind Code |
A1 |
Stinson, Jonathan S. |
March 31, 2005 |
Medical devices and methods of making same
Abstract
Medical devices, such as stents, and methods of the devices are
described.
Inventors: |
Stinson, Jonathan S.;
(Minneapolis, MN) |
Correspondence
Address: |
FISH & RICHARDSON PC
225 FRANKLIN ST
BOSTON
MA
02110
US
|
Family ID: |
34376494 |
Appl. No.: |
10/672891 |
Filed: |
September 26, 2003 |
Current U.S.
Class: |
623/1.11 ;
623/1.15 |
Current CPC
Class: |
C22C 14/00 20130101;
A61F 2310/00095 20130101; A61F 2240/001 20130101; A61L 31/022
20130101; A61F 2310/00089 20130101; A61F 2310/00101 20130101; A61F
2310/00023 20130101; A61F 2310/00131 20130101 |
Class at
Publication: |
623/001.11 ;
623/001.15 |
International
Class: |
A61F 002/06 |
Claims
1. A balloon-expandable medical stent, comprising: a generally
tubular body including an alloy having Ti at about 20 weight
percent or more and at least one of Zr, Ta, or Mo, the alloy having
a yield strength of about 45 ksi or more, a magnetic susceptibility
of about +1 or less, and a mass absorption coefficient of about 1.9
cm.sup.2/g or more.
2. The stent of claim 1 wherein the alloy has a UTS of about 90 ksi
or more and the percent tensile elongation is about 40 or more.
3. The stent of claim 1 wherein the yield strength is about 50 ksi
or greater, the percent strength to peak load is about 30 or
greater, the UTS is about 90 ksi or greater, and the percent
strength to fracture is about 40 or greater.
4. The stent of claim 1 wherein the magnetic susceptibility is
about 3.5.times.10.sup.-3 or less.
5. The stent of claim 1 wherein the mass absorption coefficient is
about 2.9 cm.sup.2/g or less.
6. The stent of claim 1 wherein the alloy includes about 50 weight
percent Ti or greater.
7. The stent of claim 1 wherein the alloy includes 20 weight
percent or greater of Zr, Ta or Mo or a combination thereof.
8. The stent of claim 1 wherein the alloy includes 80 weight
percent or less of Zr, Ta or Mo or a combination thereof.
9. The stent of claim 1 wherein the alloy includes 10 weight
percent or more of Zr.
10. The stent of claim 1 wherein the alloy includes about 50 weight
percent of Zr.
11. The stent of claim 1 wherein the alloy includes about 40 weight
percent or more of Ta.
12. The stent of claim 1 wherein the alloy includes about 75 weight
percent or less of Ta.
13. The stent of claim 1 wherein the alloy includes about 3 weight
percent or more of Mo.
14. The stent of claim 1 wherein the alloy includes about 20 weight
percent or less of Mo.
15. The stent of claim 1 wherein the alloy is Ti-Ta, Ti-Mo, Ti-Zr,
Ti-Ta-Mo, Ti-Ta-Zr, Ti-Ta-Zr-Mo, Ti-Zr-Mo, Ti 6A1-4V-Ta, Ti
6A1-4V-Mo, Ti 6A1-4V-Zr, Ti 6A1-4V-Ta-Mo, Ti 6A1-4V-Ta-Zr, Ti
6A1-4V-Ta-Zr-Mo, Ti 6A1-4V-Zr-Mo, Ti-13Nb-13Zr, Ti-13Nb-13Zr-Mo,
Ti-13Nb-13Zr-Ta, Ti-8A1-1Mo-1V, Ti-8A1-1Mo-1V-Zr, Ti-8A1-1Mo-1V-Ta,
Ti-6A1-2Nb-1Ta-0.8Mo, or Ti-6A1-2Nb-0.8Mo-Zr.
16. The stent of claim 1 wherein the alloy of CP titanium,
Ti-6A1-4V, or Ti-6A1-4V ELI alloyed with 40 to 70 weight percent of
Ta or 25 to 50 weight percent of Zr.
17. The stent of claim 16 where the alloy includes 5 to 20 weight
percent of Mo.
18. The stent of claim 1 wherein the alloy is selected from:
12 CP Titanium alloyed with: Ti--6Al--4V ELI alloyed with: 43
weight % Ta 43 weight % Ta 69 weight % Ta 69 weight % Ta 25 weight
% Ta 25 weight % Ta 49 weight % Zr 49 weight % Zr 43 weight % Ta +
5% Mo 43 weight % Ta + 5% Mo 69 weight % Ta + 5% Mo 69 weight % Ta
+ 5% Mo 25 weight % Zr + 5% Mo 25 weight % Zr + 5% Mo 49 weight %
Zr + 5% Mo 49 weight % Zr + 5% Mo 43 weight % Ta + 10% Mo 43 weight
% Ta + 10% Mo 69 weight % Ta + 10% Mo 69 weight % Ta + 10% Mo 25
weight % Zr + 10% Mo 25 weight % Zr + 10% Mo 49 weight % Zr + 10%
Mo 49 weight % Zr + 10% Mo 22 weight % Ta + 13% Mo 22 weight % Ta +
13% Mo 35 weight % Ta + 25% Mo 35 weight % Ta + 25% Mo
19. The stent of claim 1 wherein the tubular body includes wall
portions having a thickness of about 0.0015 inch to about 0.0150
inch.
20. The stent of claim 1 wherein the tubular body includes a
therapeutic agent.
21. A system including a catheter for delivery into a body lumen,
the catheter including an expandable member and a stent as
described in claim 1 disposable over the expandable member, the
expandable member expandable to a maximum diameter of about 1.5 mm
to about 14 mm.
22. An implantable medical device, comprising: an alloy having Ti
at about 20 weight percent or more and at least one of Zr, Ta, or
Mo, the alloy having a yield strength of about 45 ksi or more, a
magnetic susceptibility of about +1 or less, and a mass absorption
coefficient of about 1.9 cm.sup.2/g or more, the medical device
selected from a filter, a guidewire, a catheter, a needle, a biopsy
needle, a staple, and a cannula.
23. A method of forming a stent, comprising: providing an alloy
including Ti of about 20 weight percent or more and at least one
additive selected from the group consisting of Zr, Ta and Mo by:
contacting solid aliquots of a titanium component selected from Ti
or a Ti-containing alloy, and the additive, heating the aliquot
after the contacting, mechanically working the aliquots after
contacting by forging, extrusion, drawing or rolling, melting the
aliquots, forming a first mass, forming a tube including the alloy,
and incorporating the tube into a stent.
24. The method of claim 23 wherein the contacting includes
providing a body composed of the titanium component or the additive
including voids and inserting into the voids the additive or
titanium component.
25. The method of claim 24 wherein the body is a rod and the voids
are lumens in the rod.
26. The method of claim 25 wherein the lumens are elongate lumens
substantially arranged along the axis of the rod.
27. The method of any one of claims 24 and 26 wherein the body is
formed of the titanium component.
28. The method of claim 27 wherein the additive is the form of a
particulate or a solid wire.
29. The method of claim 23 wherein the heating includes causing
diffusion between the titanium component and the additive.
30. The method of claim 29 comprising heating to a temperature
within .+-.10% of the melting point of the titanium component.
31. The method of claim 23 comprising heating after the mechanical
working.
32. The method of claim 23 comprising: after forming the first
mass, contacting the first mass with further additive, melting the
first mass in contact with the further aliquot, and forming a
second mass having a greater amount of additive.
33. The method of claim 32 comprising mechanically working or
heating the first mass in contact with the further aliquot, prior
to melting.
34. The method of claims 23 comprising: melting by vacuum arc
remelting, electron beam, plasma or vacuum induction melting.
35. The method of claim 23 comprising forming the first mass having
a volume of about 6.5 in.sup.3 or less.
36. The method of claim 35 where the first mass is in the form of a
cylinder.
37. The method of claim 23 wherein forming the tube includes
forming a tube from the first mass by drawing or sheet-rolling.
38. The method of claim 23 wherein incorporating the tube into a
stent includes machining the tube to include apertures in the wall
of the tube.
39. The method of claim 23 wherein the stent is a vascular,
balloon-expandable stent.
40. A method of forming a medical device, comprising: providing a
metal alloy of multiple components of elements or alloys, including
a first component and a second component having a melting point
difference of about 150.degree. C. or more by contacting solid
aliquots of the first component and the second component, heating
and/or mechanically working the aliquots after contacting to form a
first mass, melting the first mass, forming a second mass from the
first mass, and incorporating the alloy into a medical device.
Description
TECHNICAL FIELD
[0001] The invention relates to medical devices, such as, for
example, stents and stent-grafts, and methods of making the
devices.
BACKGROUND
[0002] The body includes various passageways such as arteries,
other blood vessels, and other body lumens. These passageways
sometimes become occluded or weakened. For example, the passageways
can be occluded by a tumor, restricted by plaque, or weakened by an
aneurysm. When this occurs, the passageway can be reopened or
reinforced, or even replaced, with a medical endoprosthesis. An
endoprosthesis is typically a tubular member that is placed in a
lumen in the body. Examples of endoprostheses include stents and
covered stents, sometimes called "stent-grafts".
[0003] An endoprosthesis can be delivered inside the body by a
catheter that supports the endoprosthesis in a compacted or
reduced-size form as the endoprosthesis is transported to a desired
site. Upon reaching the site, the endoprosthesis is expanded, for
example, so that it can contact the walls of the lumen.
[0004] When the endoprosthesis is advanced through the body, its
progress can be monitored, e.g., tracked, so that the
endoprosthesis can be delivered properly to a target site. After
the endoprosthesis is delivered to the target site, the
endoprosthesis can be monitored to determine whether it has been
placed properly and/or is functioning properly.
[0005] Monitoring of the position of the endoprosthesis during
implantation is typically performed by a radiographic technique
such as fluoroscopy. The radiographic density of the metal
endoprosthesis is different from bone and tissue, and the device is
observed in the fluoroscopic image from the visible difference in
contrast and grey scale relative to the surrounding biological
material. The disadvantage of fluoroscopy is that the physician,
staff, and patient are exposed to ionizing radiation which can be
harmful in strong or repeated doses.
[0006] Another method of monitoring a medical device is magnetic
resonance imaging (MRI). MRI uses a magnetic field and radio waves
to image the body. In some MRI procedures, the patient is exposed
to a magnetic field, which interacts with certain atoms, e.g.,
hydrogen atoms, in the patient's body. Incident radio waves are
then directed at the patient. The incident radio waves interact
with atoms in the patient's body, and produce characteristic return
radio waves. The return radio waves are detected by a scanner and
processed by a computer to generate an image of the body.
SUMMARY
[0007] In an aspect, the invention features a balloon-expandable
medical stent. The stent includes a generally tubular body
including an alloy having Ti at about 20 weight percent or more and
at least one of Zr, Ta, or Mo. The alloy has a yield strength of
about 45 ksi or more, a magnetic susceptibility of about +1 or
less, and a mass absorption coefficient of about 1.9 cm.sup.2/g or
more.
[0008] In another aspect, the invention features a system including
a catheter for delivery into a body lumen. The catheter includes an
expandable member and a stent as described herein disposable over
the expandable member. The expandable member is expandable to a
maximum diameter of about 1.55 mm to about 14 mm.
[0009] In another aspect, the invention features an implantable
medical device including an alloy having Ti at about 20 weight
percent or more and at least one of Zr, Ta, or Mo, a yield strength
of about 45 ksi or more, a magnetic susceptibility of about +1 or
less, and a mass absorption coefficient of about 1.9 cm.sup.2/g or
more. The medical device can be a filter, a guidewire, a catheter,
a needle, a biopsy needle, a staple, or a cannula.
[0010] In another aspect, the invention features a method of
forming a stent. The method includes providing an alloy including
Ti of about 20 weight percent or more and at least one of an
additive selected from Zr, Ta or Mo. The method includes contacting
solid aliquots of a titanium component selected from Ti or a
Ti-containing alloy, and the additive heating the aliquot after the
contacting, and mechanically working the aliquots after contacting
by forging, extrusion, drawing or rolling, melting the aliquots,
forming an ingot, forming a tube including the alloy, and
incorporating the tube into a stent.
[0011] In an aspect, the invention features a method of forming a
medical device. The method includes providing a metal alloy of
multiple components of elements or alloys, including a first
component and a second component having a melting point difference
of about 150.degree. C. or more. Solid aliquots of the first
component and the second component are contacted, heated and/or
mechanically worked, then the worked components are melted. The
alloy is incorporated into a medical device.
[0012] In another aspect, the invention features a medical device
including an alloy that exhibits one or more (e.g., two, three, or
four) properties selected from radiopacity, MRI capability,
mechanical properties, and/or biocompatibility properties as
described herein, in any combination. In other aspects, the
invention features particular alloys and techniques for making the
alloys.
[0013] In yet another aspect, the invention features a medical
device including a titanium alloy having at least one of zirconium,
tantalum, molybdenum, or niobium. The alloy exhibits radiopacity,
MRI capability, mechanical properties, and/or biocompatibility
properties, and combinations of the properties as described herein.
In other aspects, the invention features particular alloys and
techniques for making the alloys.
[0014] Embodiments may include one or more of the following
advantages. A stent or other medical device is provided that
includes desirable magnetic imaging radiopacity, biocompatibility
and/or mechanical characteristics. For example, the stent is less
susceptible to magnetic resonance image degradation (e.g., less
than stainless steel) Implant movement or heating can be reduced.
The stent alloy has sufficient radiopacity that the stent is
visible by fluoroscopy. The mechanical characteristics of the alloy
enable a stent of conventional design that can be delivered into
the body in a reduced diameter configuration and then expanded at a
treatment site, e.g., by a balloon catheter. The titanium alloys
generally can exhibit enhanced strength, stiffness and radiopacity,
while maintaining low magnetic susceptibility.
[0015] Still further aspects, features, and advantages follow.
DESCRIPTION OF DRAWINGS
[0016] FIGS. 1A and 1B are perspective views of a stent in a
compressed and expanded condition, respectively.
[0017] FIGS. 2A-2C illustrate delivery of a balloon expandable
stent.
[0018] FIG. 3 is a flow diagram of a stent manufacturing
process.
[0019] FIGS. 4A-4F illustrate a process for making a medical
device.
[0020] FIGS. 5-8 are photo micrographs.
DETAILED DESCRIPTION
[0021] Structure and Alloy Formulation
[0022] Referring to FIGS. 1A and 1B, a stent 10 includes a metal
body 12 in the shape of a tube. The metal body includes aperture
regions 14 provided in a pattern to facilitate stent functions,
such as radial expansion, and lateral flexibility. Between aperture
regions are strut regions 16. Referring particularly to FIG 1A, for
delivery into the body, the stent 10 is provided or maintained in a
relatively small diameter condition corresponding to a diameter
D.sub.c. Referring to FIG 1B, upon placement at the treatment site,
the stent 10 is expanded to a larger diameter, D.sub.exp, so that
the stent is in contact with the lumen wall. The stent may be
expanded by a mechanical expander, such as an inflatable balloon,
or it may be self-expanding. The metal body of the stent may be
formed by a generally continuous sheet or by filaments that are
wrapped, braided, knitted or otherwise configured to generally
define a stent.
[0023] Referring now to FIGS. 2A-2C, the delivery of a
balloon-expandable stent is illustrated. The stent 300 is carried
on a catheter 302 over a balloon 304. When the treatment site is
reached, the balloon is expanded to expand the stent into contact
with the lumen wall. The stent may be used in the vascular system
(e.g., in the coronary or peripheral arteries), or in other body
lumens.
[0024] The stent body is formed of a metal alloy that has desirable
magnetic resonance, radiopacity, biocompatibility, and/or
mechanical characteristics. In embodiments, the alloy is a
titanium-containing alloy that includes one or more of Zr, Ta or
Mo. In particular embodiments, the alloy is formed from
commercially pure (CP) titanium or Ti-6A1-4V ELI, which has been
alloyed with one or more of Zr, Ta, or Mo by processes that include
mechanical or diffusion alloying followed by melting, as will be
described below.
[0025] The alloy is formulated to provide desired characteristics.
For MRI compatibility, the alloy is formulated to reduce signal
distortion, electrical current (e.g., eddy current) generation,
heating, movement within the body or nerve simulation, by
controlling the magnetic susceptibility and solubility of the alloy
constituents. The magnetic susceptibilities of Ti, Zr, Ta, and Mo
and other materials are provided in Table I.
1TABLE I Magnetic Susceptibilities Material: Magnetic
Susceptibility: Water at 37.degree. C. -9.05 .times. 10.sup.-6
Human tissues -11.0 .times. 10.sup.-6 to -7.0 .times. 10.sup.-6
copper -9.63 .times. 10.sup.-6 ferromagnetic iron +10.sup.5
magnetic stainless steel (martensitic) +10.sup.3 stainless steel
(austenitic) +3.5 .times. 10.sup.-3 to +6.7 .times. 10.sup.-3
heavily cold worked stainless steel +1 to +10 (austenitic) Nitinol
(Ni--Ti) +0.245 .times. 10.sup.-3 zirconium +0.109 .times.
10.sup.-3 titanium +0.182 .times. 10.sup.-3 niobium +0.237 .times.
10.sup.-3 platinum +0.279 .times. 10.sup.-3 molybdenum +0.123
.times. 10.sup.-3 tantalum +0.178 .times. 10.sup.-3
[0026] In embodiments, the magnetic susceptibility of the alloy is
less than the magnetic susceptibility of austenitic stainless
steel, e.g. about +1 or less or about 3.5.times.10.sup.-3 or less.
Solubility of the constituents can be determined by binary phase
diagrams. Suitable solubility is indicated by a single phase (alpha
or beta) or by a two phase solution (alpha and beta) at room
temperature. Examples of suitable phase diagrams are available in
the ASM Handbook, volume 3, ASM International, 1992, the entire
contents of which is hereby incorporated by reference.
[0027] For radiopacity, the alloy is formulated to a desired mass
absorption coefficient. Preferably, the stent is readily visible by
fluoroscopy, but does not appear so bright that detail in the
fluoroscopic image is distorted. In some embodiments, the alloy or
the device has a radiopacity of from about 1.10 to about 3.50 times
(e.g., greater than or equal to about 1.1, 1.5, 2.0, 2.5, or 3.0
times; and/or less than or equal to about 3.5, 3.0, 2.5, 2.0, or
1.5 times) that of 316L grade stainless steel, as measured by ASTM
F640 (Standard Test Methods for Radiopacity of Plastics for Medical
Use). Mass absorption coefficients and densities or Ti, Ta, Zr and
Mo are compared to 316L stainless steel in Table II.
2TABLE II Mass Absorption Coefficients Alloy 316L SS Ti Ta Zr Mo
Mass absorption 1.96 (Fe) 1.21 5.72 6.17 7.04 coefficient,
cm.sup.2/g Density, g/cc 8.0 4.5 16.7 6.5 10.2
[0028] In embodiments, the mass absorption coefficient of the alloy
is about 1.96 cm.sup.2/g (corresponding substantially to the mass
absorption coefficient of Fe) to about 2.61 cm.sup.2/g
(corresponding to about 0.5 the mass absorption coefficient of Ta).
Mass absorption coefficient can be calculated from the results of
radiopacity tests, as described in The Physics of Radiology, H. E.
Johns, J. R. Cunningham, Charles C. Thomas Publisher, 1983,
Springfield, Ill., pp. 133-143. A calculation of alloy mass
absorption coefficient is provided in the examples, infra.
[0029] For desirable mechanical properties, the alloy is formulated
based on solubility and phase structure. In particular embodiments,
the alloy exhibits certain mechanical properties within about
.+-.20% (e.g., within about .+-.10%, about .+-.5%, or about .+-.1%)
of the corresponding value for stainless steel. Mechanical
properties for select materials are provided in Table III.
3TABLE III Mean Tensile Test Data (Annealed Condition) 0.2% offset
% strain UTS, % strain to E, Tubing Ys, ksi to peak load ksi
fracture msi 316L SS 50 36 94 45 29 Tantalum 24 No data 35-70 40 27
CP Titanium 25-70 No data 35-80 15-25 15 Ti--6Al--4V 120 No data
130 15 17 ELI
[0030] Yield strength (YS) relates to the applied pressure needed
to flow the alloy to expand the stent. The percent strain to peak
load indicates how far the material can strain before necking
occurs. The ultimate tensile strength (UTS) is the stress value
that corresponds with strain to peak load. The percent strain to
fracture is a measure of how far the material can be stretched
prior to break, and includes uniform deformation plus location
deformation in the necked down region. This property relates to
stent strut fracture from over-expansion of the stent. Suitable
test methods for determining these parameters are described in ASTM
E8 (Standard Test Methods for Tension Testing of Metallic
Materials). In Table III, the 316L SS properties were measured from
annealed stent tubing. The other material properties were taken
from handbooks, such as American Society for Metals Handbook Desk
Edition, H. E. Boyer, T. L. Gall, 1985.
[0031] The solubility of the constituents and phase structure of
the alloy is indicated by phase diagrams. Suitable solubilities are
indicated by alpha and/or beta microstructures without substantial
amounts of more brittle phases such as alpha prime, alpha double
prime or omega phases. Active rapid cooling after melting can be
utilized to reduce precipitation of these phases. In embodiments,
the presence of brittle phases is less than about 10% (e.g., less
than about 7%, 5%, or 3%) as measured by X-ray diffraction
analysis. The presence of two phases is preferably equal to or less
than the amount in commercially available Ti-6A1-4V (available from
Allegheny Technologies Allvac (Monroe, N.C.) or Metalmen Sales
(Long Island City, N.Y.). Alloying Ti with Ta and Mo increases
modulus of elasticity. Alloying Ti with Ta, Mo, and/or Zr increases
tensile strength. In embodiments, tensile properties are balanced
by annealing the alloy. For example, annealing time and temperature
can be selected to produce a maximum level of ductility while
meeting minimum design requirements for yield strength and grain
size. Alternatively or in addition, the stent design can be
modified to accommodate less favorable mechanical properties. For
example, for a lower tensile elongation (% strain to fracture) the
stent is designed to lower the strain on the struts during
expansion, such as by increasing the number of deformation "hinge"
points in the stent so that the total stent deformation is
distributed in smaller amounts to the areas where deformation
occurs.
[0032] Biocompatibility of the stent is provided by alloying
biocompatible constituents or coating the sent with a biocompatible
material. Biocompatibility can be tested by using industry standard
ISO 10992 in-vitro and in vivo test methods, which can provide a
qualitative pass or fail indication. In embodiments, the stent has
a biocompatibility similar to or equivalent to pure titanium or
pure tantalum, as measured by ISO 10992 test methods.
[0033] In embodiments, the alloy constituents are provided in
combinations and amounts recited in the Summary and Examples. In
particular embodiments, the alloy is Ti-Ta, Ti-Mo, Ti-Zr, Ti-Ta-Mo,
Ti-Ta-Zr, Ti-Ta-Zr-Mo, Ti-Zr-Mo or Ti 6A1-4V-Ta, Ti 6A1-4V-Mo, Ti
6A1-4V-Zr, Ti 6A1-4V-Ta-Mo, Ti 6A1-4V-Ta-Zr, Ti 6A1-4V-Ta-Zr-Mo, or
Ti 6A1-4V-Zr-Mo alloy. In other embodiments, Ti-13Nb-13Zr,
Ti-8A1-1Mo-1V, Ti-6A1-2Nb-1 Ta-0.8Mo and Ti-7A1-4Mo one alloyed
with Ta, Mo, and/or Zr. In particular embodiments, the alloy is
annealed. In particular embodiments, the alloy is formed by
alloying CP titanium or Ti-6A1-4V ELI with Ta, Zr and/or Mo. In
embodiments, the alloy includes 40 to 70 weight percent tantalum or
25 to 50 weight percent zirconium with CP titanium or Ti-6A1-4V
ELI. In embodiments, 5 to 20 weight percent molybdenum is added in
place of some of the titanium for added tensile strength without
sacrificing MRI compatibility. Suitable alloys include the
following:
4 CP Titanium alloyed with: Ti--6Al--4V ELI alloyed with: 43 weight
% Ta 43 weight % Ta 69 weight % Ta 69 weight % Ta 25 weight % Ta 25
weight % Ta 49 weight % Zr 49 weight % Zr 43 weight % Ta + 5% Mo 43
weight % Ta + 5% Mo 69 weight % Ta + 5% Mo 69 weight % Ta + 5% Mo
25 weight % Zr + 5% Mo 25 weight % Zr + 5% Mo 49 weight % Zr + 5%
Mo 49 weight % Zr + 5% Mo 43 weight % Ta + 10% Mo 43 weight % Ta +
10% Mo 69 weight % Ta + 10% Mo 69 weight % Ta + 10% Mo 25 weight %
Zr + 10% Mo 25 weight % Zr + 10% Mo 49 weight % Zr + 10% Mo 49
weight % Zr + 10% Mo 22 weight % Ta + 13% Zr 22 weight % Ta + 13%
Zr 35 weight % Ta + 25% Zr 35 weight % Ta + 25% Zr
[0034] Manufacture
[0035] Referring to FIG. 3, a stent is constructed by forming an
alloy, forming a tube from the alloy, and then forming the tube
into a stent.
[0036] Referring to FIGS. 4A to 4E, an alloying process is
illustrated for forming an ingot or billet of a size and form
suitable for stent construction.
[0037] Referring to FIG. 4A, a base rod 60 and one or more additive
rods 62 are provided. For example, the base rod is Ti or a
Ti-containing alloy and the additive rod(s) are Ti, Ta, Zr, and/or
Mo. The weight of the rods are in proportion to the desired alloy
formulation.
[0038] Referring to FIG. 4B, the base rod is drilled to provide
voids 64.
[0039] Referring to FIG. 4C, additive rods 62 are inserted into the
voids 64 of the base rod 60.
[0040] Referring to FIG. 4D, this assembly is prealloyed by heating
and/or mechanically working to cause diffusion alloying between
constituents.
[0041] Referring to FIG. 4E, the assembly is provided with end caps
to prevent additive rods 62 from falling out of base rod 60. The
assembly is melted and cast once or multiple times in a vacuum are
remelt (VAR) furnace, EB melting furnace, VIM furnace, or
levitation melting furnace to allow liquid-phase alloying to
occur.
[0042] Referring to FIG. 4F, the alloy (e.g., the alloyed billet is
suitable for further processing. The billet can be drawn into
tubing or rolled into a sheet for stock stent tubing production.
For example an ingot or billet 2.5 inches in diameter by 4 inches
long can typically yield at least 1000 feet of coronary stent
tubing.
[0043] The alloying process is particularly advantageous for
alloying constituents with large melting temperature differences.
In Table IV, the melting temperatures of Ti, Ta, Zr, and Mo are
provided.
5TABLE IV Melting Temperatures Element Melting Temperature,
.degree. C. Ti 1668 Ta 2996 Zr 1852 Mo 2610
[0044] The melting temperature difference between Ti and Zr, and Ta
and Mo is over 500.degree. C. The difference between Ti and Zr is
over 150.degree. C. In the method of FIGS. 4A et seq., aliquots of
constituents of the alloy are intimately contacted, mechanically
and/or diffusion alloyed and then melted and cast into ingot. By
diffusion or mechanical alloying the aliquots, less overall mixing
is required in the melting and casting furnace.
[0045] In the prealloying step, heating is performed in an inert
gas or vacuum, or the outer surfaces of the billet could be coated
or canned with a protective metal, such as iron, that could later
be chemically dissolved. After drilling and filling or after the
diffusion heat treatment, the billet can also be extruded, drawn,
or rolled to further consolidate the assembly. The heat treatment
or working serves to hold additive material in place within the
billet during melting. Also, constituents with high melting points
can be essentially encapsulated within, e.g. titanium, minimizing
the exposure to any residual air in the casting furnace. For
diffusion heating, the assembly can be heated near the melting
temperature of the lowest melting temperature constituent and/or
the melting temperature of the material of the base rod. For
example, for a Ti base rod, the temperature is about 1600.degree.
C. or less.
[0046] In embodiments, additives to the base are made in
incremental steps in each of multiple melting and ingot casting
operations. For example, to alloy Ti 6A1-4V with 43 weight percent
tantalum, in the first melting operation the Ti-6A1 -4V bar holes
may be filled with 22 weight percent tantalum. After the first
ingot is cast, holes are drilled again and filled with another 22
weight percent tantalum and the melting is repeated. Other
sequences and magnitudes of Ta adds are made to reach the final
alloy with 43 weight percent Ta. This approach is Ta elemental
segregation in the ingot if it is added in smaller amounts in
multiple melting and ingot casting steps. In addition,
homogenization heat treatments between melts can reduce the amount
of elemental diffusion needed. Other difficult to melt alloys can
be produced by this method such as Ta-Nb, Nb-Zr, Ti-Nb, and Fe-Pt
alloy systems. In other embodiments, the additive can be provided
in the form of powder or chips rather than a solid wire or rod. The
alloying that occurs in the melting and ingot casting process can
be further improved by performing a homogenization (elemental
diffusion) heat treatment to the ingots between melting operations.
Mechanical alloying melting, casting, and heat treating operations
can be performed at commercial sources such as Pittsburgh Materials
Technology Inc. (Pittsburgh, Pa.), Applegate Group (Woodcliff Lake,
N.J.) or Albany Research Center (Albany, Ore.).
[0047] The alloy tubing is formed into stent. For example, selected
portions can be removed to define bands and struts. The portions
can be removed by laser cutting, as described, for example, in U.S.
Pat. No. 5,780,807. In certain embodiments, during laser cutting, a
liquid carrier, such as a solvent, gas, or an oil, is flowed
through the tube. The carrier can prevent drops formed on one
portion from re-depositing on another portion, and/or reduce
formation of recast material on the tubular member. Other methods
of removing portions of tubular member include mechanical machining
(e.g., micro-machining), electrical discharge machining (EDM),
photoetching (e.g., acid photoetching), and/or chemical
etching.
[0048] The stent can further be finished, e.g., electropolished to
a smooth finish, according to conventional methods. In some
embodiments, about 0.0001 inch of material can be removed from the
interior and/or exterior surfaces by chemical milling and/or
electropolishing. The stent can be annealed to refine the
mechanical and physical properties of the stent.
[0049] In use, the stent can be used, e.g., delivered and expanded,
using a catheter. Suitable catheter systems are described in, for
example, Wang U.S. Pat. No. 5,195,969, and Hamlin U.S. Pat. No.
5,270,086. Suitable stents and stent delivery are also exemplified
by the Express, Radius.RTM. or Symbio.RTM. systems, available from
Boston Scientific Scimed, Maple Grove, Minn.
[0050] The stent can be of any desired shape and size (e.g.,
coronary stents, aortic stents, peripheral vascular stents,
gastrointestinal stents, urology stents, and neurology stents).
Depending on the application, the stent can have a diameter of
between, for example, 1 mm to 46 mm. In certain embodiments, a
coronary stent can have an expanded diameter of from about 2 mm to
about 6 mm. In some embodiments, a peripheral stent can have an
expanded diameter of from about 5 mm to about 24 mm. In certain
embodiments, a gastrointestinal and/or urology stent can have an
expanded diameter of from about 6 mm to about 30 mm. In some
embodiments, a neurology stent can have an expanded diameter of
from about 1 mm to about 12 mm. An abdominal aortic aneurysm (AAA)
stent and a thoracic aortic aneurysm (TAA) stent can have a
diameter from about 20 mm to about 46 mm. Stent 100 can be
balloon-expandable, self-expandable, or a combination of both
(e.g., U.S. Pat. No. 5,366,504).
[0051] The stent can also be a part of a stent-graft. In other
embodiments, the stent includes and/or be attached to a
biocompatible, non-porous or semi-porous polymer matrix made of
polytetrafluoroethylene (PTFE), expanded PTFE, polyethylene,
urethane, or polypropylene. The endoprosthesis can include a
releasable therapeutic agent, drug, or a pharmaceutically active
compound, such as described in U.S. Pat. No. 5,674,242, U.S. Ser.
No. 09/895,415, filed Jul. 2, 2001 and U.S. Ser. No. 10/232,265,
filed Aug. 30, 2002. The therapeutic agents, drugs, or
pharmaceutically active compounds can include, for example,
anti-thrombogenic agents, antioxidants, anti-inflammatory agents,
anesthetic agents, anti-coagulants, and antibiotics.
[0052] The methods and the embodiments described above can be used
to form medical devices other than stents and stent-grafts. For
example, the methods and/or materials can be used to form filters,
such as removable thrombus filters described in Kim et al., U.S.
Pat. No. 6,146,404; in intravascular filters such as those
described in Daniel et al., U.S. Pat. No. 6,171,327; and in vena
cava filters such as those described in Soon et al., U.S. Pat. No.
6,342,062. The methods and/or materials can be used to form
guidewires, such as a Meier steerable guidewire. The methods and/or
materials can be used to form vaso-occlusive devices, e.g., coils,
used to treat intravascular aneurysms, as described, e.g., in
Bashiri et al., U.S. Pat. No. 6,468,266, and Wallace et al., U.S.
Pat, No. 6,280,457. The methods and/or materials can be used to
form wire to make catheter reinforcement braid. The methods and/or
materials can also be used in surgical instruments, such as
forceps, needles, clamps, and scalpels.
[0053] Further embodiments are provided in the following
examples.
EXAMPLES
Example 1
[0054] A titanium-tantalum alloy with a mass absorption coefficient
of at least 1.96 cm.sup.2/g (iron) and as high as 2.86 cm.sup.2/g
(half of tantalum) is formulated as follows. The atomic mass
coefficient for titanium is 1.21 and for tantalum is 5.72.
[0055] The following equation is used to provide desired
radiopacity.
[Atomic % Ti.times.1.21]+[atomic % Ta.times.5.72]=1.96 to 2.86
cm2/g. solving far x: (x)(1.21)+(1-x)(5.72)=1.96 cm2/g or 2.86
cm2/g x=0.83 (83 atomic percent Ti) or 0.63 (63 atomic percent TI)
or conversely 17 atomic percent Ta or 37 atomic percent Ta.
[0056] Conversion of atomic percent to weight percent for the 17
Ta-83 Ti alloy is as follows:
[0057] In 1023 atoms of Ti-Ta alloy, there are 0.17.times.10.sup.23
atoms of Ta and 0.83.times.10.sup.23 atoms of Ti.
[0058] 0.17.times.10.sup.23 atoms of Ta/6.02.times.10.sup.23
atoms/mole=0.028 moles of Ta
[0059] 0.83.times.10.sup.23 atoms of Ti/6.02.times.10.sup.23
atoms/mole=0.138 moles of Ti
[0060] (0.028 moles Ta)(180.95 grams/mole atomic weight)=5.07 grams
of Ta
[0061] (0.138 moles Ti)(47.88 grams/mole atomic weight)=6.61 grams
of Ti
[0062] 5.07 grams Ta+6.61 grams Ti=11.68 grams of alloy
[0063] 6.61g Ti/11.68 g=57 weight percent Ti in alloy.
[0064] 5.07g Ta/11.68 g=43 weight percent Ta in alloy.
[0065] An alloy of 83 atomic percent Ti and 17 atomic percent Ta
(57 weight percent Ti and 43 weight percent Ta) has a calculated
mass absorption coefficient equivalent to iron and a radiopacity
similar to 316L stainless steel. An alloy of 63 atomic percent Ti
and 37 atomic percent Ta (31 weight percent Ti and 69 weight
percent Ta) has a calculated mass absorption coefficient equivalent
to one-half of tantalum. The alloy constituents have magnetic
susceptibility less than 3.5.times.10.sup.-3 and are soluble in
each other. The tantalum-titanium binary phase diagram (ASM
Handbook, Volume 3 Alloy Phase Diagrams, ASM International, 1992,
p. 2.374) indicates a 43 to 69 weight percent tantalum to be
soluble in titanium as a solid solution two-phase (alpha and beta)
material at room temperature. The tantalum-titanium binary phase
diagram also indicates that the alloys with 43 to 69 percent
tantalum concentration have alpha and beta phase microstructures.
No brittle phases are evident in the phase diagram.
Example 2
[0066] A titanium-molybdenum alloy with a mass absorption
coefficient of at least 1.96 cm.sup.2/g (iron) and as high as 2.86
cm.sup.2/g (halfoftantalum) is formulated as follows.
[0067] The following equation is used to determine desired
radiopacity.
[Atomic % Ti.times.1.21]+[atomic % Mo.times.7.04]=1.96 to 2.86
cm2/g. (x) (1.21)+(1-x)(7.04)=1.96 cm2/g or 2.86 cm2/g x=0.87 (87
atomic percent Ti) or 0.72 (72 atomic percent Ti) or conversely 13
atomic percent Mo or 28 atomic percent Mo.
[0068] Conversion of atomic percent to weight percent for the 13
Mo-87 Ti alloy:
[0069] In 10.sup.23 atoms of Ti-Mo alloy, there are
0.13.times.10.sup.23 atoms of Mo and 0.87.times.10.sup.23 atoms of
Ti.
[0070] 0.13.times.10.sup.23 atoms of Mo/6.02.times.10.sup.23
atoms/mole=0.022 moles of Mo
[0071] 0.87.times.10.sup.23 atoms of Ti/6.02.times.10.sup.23
atoms/mole=0.145 moles of Ti
[0072] (0.022 moles Mo) (95.94 grams/mole atomic weight)=2.11 grams
of Mo
[0073] (0.145 moles Ti) 47.88 grams/mole atomic weight)=6.94 grams
of Ti
[0074] 2.11 grams Mo+6.94 grams Ti=9.05 grams of alloy
[0075] 6.94g Ti/9.05 g=77 weight percent Ti in alloy.
[0076] 2.11 g Mo/9.05 g=23 weight percent Mo in alloy.
[0077] An alloy of 87 atomic percent Ti and 13 atomic percent mo
(77 weight percent Ti and 23 weight percent Mo) has a calculated
mass absorption coefficient equivalent to iron and a radiopacity
similar to 316L stainless steel. An alloy of 72 atomic percent Ti
and 28 atomic percent Mo (56 weight percent Ti and 44 weight
percent Mo) has a calculated mass absorption coefficient equivalent
to one-half of tantalum and therefore has half the radiopacity of
tantalum. The alloy constituents have magnetic susceptibility less
than 3.5.times.10.sup.-3 and that are soluble in each other. The
molybdenum titanium binary phase diagram indicates (ASM Handbook,
Volume 3 Alloy Phase Diagrams, ASM International, 1992, p.2.296) 23
to 44 weight percent molybdenum to be soluble in titanium as a
solid solution single (beta) or two-phase (alpha and beta) material
at room temperature. The molybdenum-titanium binary phase diagram
also indicates that alloys with 23 to 44 percent molybdenum
concentration will have beta or beta plus alpha phase
microstructures which are common in commercialized titanium
engineering alloys such as Ti-6A1-4V. Cooling through the
temperature range of about 850 to 695.degree. C. can be performed
rapidly (e.g., by argon gas, air cool, or liquid quenchant) to
avoid precipitation of significant amounts of alpha-prime,
alpha-double prime, or omega phases.
Example 3
[0078] A method for making an alloy of Ti-6A1-4V ELI with 43 weight
percent Ta follows.
[0079] Procure a 3" diameter round bar of Ti-6A1-4V ELI (such as
form Titanium Industries, Inc. in Morristown, N.J.) and cut to 5.5
inches long. Procure 0.5" diameter tantalum rod (such as from
Rembar, Dobbs Ferry, N.Y.) and cut into lengths of 3.25". Drill
eight holes into the titanium bar that are 0.55/0.6" diameter and
4.5" deep. Put the eight 3.25" long pieces of 0.5" diameter
tantalum rod into the holes. Heat the assembly in a vacuum furnace
at 1400.degree. C. for 8 hours and vacuum cool. Gas tungsten arc
weld (GTAW or TIG) the assembly with the hole-end up to the vacuum
arc remelt (VAR) electrode holder. Vacuum arc remelt the assembly
and cast an ingot. Heat the ingot in a vacuum furnace at
1400.degree. C. for 8 hours and vacuum cool. Repeat the VAR and
heat treatment once ore or multiple times. Machine the ingot into a
2.5" diameter.times.4" long billet. Convert billet to annealed
seamless tent tubing.
Example 4
[0080] Arc melted Ti-Ta alloy button ingots were prepared. Two
ingots were melted from a 50-50 mixture (by weight) of Ti-6A1-4V
and tantalum rods. One ingot was melted from a 50-50 mixture (by
weight) of pure titanium and tantalum rods. Cold rolling and
annealing of the ingots were used to form strips for mechanical and
physical property testing.
[0081] The ingots were prepared from the following rods and charge
materials procured from Goodfellow Corporation, Berwyn, Pa.
6TABLE V Rods Material Traceability Ti--6Al--4V Goodfellow
LS251817JV; TI017910/1, 5 mm dia .times. rods 200 mm long rods, 10
pcs, 174 g, annealed Ta rods Goodfellow LS251817JV, TA007920/8,
99.9% pure, 2 mm dia .times. 200 mm long rods, 5 pcs, 53.2 g,
annealed Ti rods Goodfellow LS251817JV, TI007910/12, 2 mm dia
.times. 100 mm long rods, 20 pcs, 28.5 g, 99.6% pure, annealed
[0082]
7TABLE VI Arc Melter Charge Materials Ti--6Al--4V, # of Ingot mass,
Ingot # grams Ti, g Ta, g melt cycles g 2&3 26.0 25.8 3 51.7 1
27.6 26.5 3 53.9
[0083] The rods were cut into lengths of 1-2", cleaned in acetone,
and weighed on a digital scale. The rods were divided up by weight
into two groups for melting. The raw materials were melted in an
arc melter (Model MRF ABJ-900, Materials Research Furnaces, Inc.,
Suncook, N.H.). The arc melter was operated at 350-400 amps. Three
melt cycles were performed for each alloy.
[0084] Referring to FIG. 5, a photomacrograph shows the three
ingots after arc melting. The ingot on the left is the Ti-50Ta
alloy. The other two ingots are the 50 (Ti-6A1-4V)-50Ta alloy.
After melting, each ingot was struck ten times with a hammer to see
if it would crack or fracture. All three of the ingots withstood
the hammer test without cracking or fracturing and the ingot
deformed when hit. This test was performed as an assessment of the
formability of the material. Cracking can indicate that the alloy
is too brittle for cold rolling.
[0085] Three 0.20-0.25" thick bars were used as a starting stock
for cold rolling. The machined dimensions of the rolling blanks are
listed in the following table.
8TABLE VII Dimensions of Rolling Blanks Bar # Length, inches Width,
inches Thickness, inches 1 (Ti--Ta) 3.06 0.57 0.23 2 (Ti64--Ta)
2.17 0.57 0.23 3 (Ti64--Ta) 1.12. 0.49 0.24
[0086] The machined bars were cold rolled to a total reduction in
thickness of 50%. The dimensions after cold rolling are listed in
the following table.
9TABLE VIII Dimensions after 1st Cold Rolling Bar # Length, inches
Thickness, inches 1 (Ti--Ta) 4.7 0.10 2 (Ti64--Ta) 3.2 0.10 3
(Ti64--Ta) 1.7 0.10
[0087] The cold rolled strips were annealed in the a vacuum heat
treat furnace at 1200.degree. C. for 60 minutes in vacuum followed
by a vacuum cool. The purpose of this heat treatment was to
continue to homogenize the alloy, recrystallize the cold worked
microstructure, and soften the material to allow for further cold
rolling. Referring to FIG. 6, fine fissures were observed on the
surface of the strips. Strip #3 had small edge cracks along the
length. None of these flaws were judged to be severe enough to
impair further cold rolling.
[0088] The three strips were cold rolled to a total reduction in
thickness of -50%. The dimensions of the rolled strips are listed
in Table IX.
10TABLE IX Dimension of Strips After Second Cold Rolling Bar #
Length, inches Width, inches Thickness, inches 1 (Ti--Ta) 7.75 0.75
0.058 2 (Ti64--Ta) 4.87 0.81 0.058 3 (Ti64--Ta) 3.00 0.62 0.058
[0089] Referring to FIG. 7, the surface and edges of the strips
were examined without magnification. Strip #1 had fine edge cracks.
Strip #2 had no cracks. Strip #3 had edge cracks.
[0090] The cold rolled strips were annealed in the vacuum heat
treat furnace at 1000.degree. C. for 30 minutes in vacuum followed
by a vacuum cool. The purpose of this heat treatment was to
recrystallize the cold worked microstructure and soften the
material to allow further cold rolling. The strips were cold rolled
to 0.025" thickness. The dimensions are given in Table X.
11TABLE X Dimension of Strips After Third Cold Rolling Campaign Bar
# Length, inches Width, inches Thickness, inches 1 (Ti--Ta) 9 and 8
079 0.025 2 (Ti64--Ta) 10 0.88 0.025 3 (Ti64--Ta) 6 0.65 0.025
[0091] Referring to FIG. 8, Strips #1 and #3 had many small edge
cracks. Strip #2 did not have edge cracks.
[0092] The strips were beta solution treated in a vacuum heat treat
furnace at 850.degree. C. for 30 minutes and cooled in vacuum. The
strips were submitted for metallography. The strips were subjected
to tensile specimen machining and testing (Metcut Research
Associates, Inc. (Cincinnati, Ohio)). The tensile results were
85-115 ksi UTS, 65-105 YS, and 5-25% elongation.
[0093] Ti-6A1-4V, pure titanium, and tantalum materials had been
melted in powder metal form. Sometimes the ingots did not have
sufficient formability to allow cold rolling to a final reduction
in thickness of 50%. The large surface area of fine powder metal
may allow for significant contamination to be carried into the
ingot thereby reducing the ductility of the alloy. In this
experiment, solid rods were used instead of powder metal for the
furnace charges. The smaller surface area of the rods (relative to
the powder) should result in better ingot ductility.
[0094] All publications, applications, references, patents referred
to in this application are herein incorporated by reference in
their entirety.
[0095] Other embodiments are within the claims.
* * * * *