U.S. patent application number 10/984062 was filed with the patent office on 2006-05-11 for quaternary cobalt-nickel-chromium-molybdenum fatigue resistant alloy for intravascular medical devices.
Invention is credited to Robert Burgermeister, Randy-David Burce Grishaber.
Application Number | 20060096672 10/984062 |
Document ID | / |
Family ID | 35517194 |
Filed Date | 2006-05-11 |
United States Patent
Application |
20060096672 |
Kind Code |
A1 |
Burgermeister; Robert ; et
al. |
May 11, 2006 |
Quaternary cobalt-nickel-chromium-molybdenum fatigue resistant
alloy for intravascular medical devices
Abstract
A biocompatible solid-solution alloy may be formed into any
number of implantable medical devices. The solid-solution alloy
comprises a combination of elements in specific ratios that improve
its fatigue resistance while retaining the characteristics required
for implantable medical devices. The biocompatible solid-solution
alloy is a quaternary cobalt-nickel-chromium-molydenum alloy having
substantially reduced titanium content.
Inventors: |
Burgermeister; Robert;
(Bridgewater, NJ) ; Grishaber; Randy-David Burce;
(Asbury, NJ) |
Correspondence
Address: |
PHILIP S. JOHNSON;JOHNSON & JOHNSON
ONE JOHNSON & JOHNSON PLAZA
NEW BRUNSWICK
NJ
08933-7003
US
|
Family ID: |
35517194 |
Appl. No.: |
10/984062 |
Filed: |
November 9, 2004 |
Current U.S.
Class: |
148/419 ;
420/585; 420/588 |
Current CPC
Class: |
C22C 19/07 20130101;
C22C 19/051 20130101; C22F 1/10 20130101 |
Class at
Publication: |
148/419 ;
420/585; 420/588 |
International
Class: |
C22C 30/00 20060101
C22C030/00 |
Claims
1. A biocompatible, load-carrying metallic structure being formed
from a solid-solution alloy comprising nickel in the range from
about 33 weight percent to about 37 weight percent, chromium in the
range from about 19 weight percent to about 21 weight percent,
molybdenum in the range from about 9 weight percent to about 11
weight percent, iron in the range from about 0 weight percent to
about 1 weight percent, manganese in the range from about 0 weight
percent to about 0.15 weight percent, silicon in the range from
about 0 weight percent to about 0.15 weight percent, carbon in the
range from about 0 to about 0.025 weight percent, phosphorous in
the range from about 0 to about 0.015 weight percent, boron in the
range from about 0 to about 0.015 weight percent, sulfur in the
range from about 0 to about 0.010 weight percent, titanium in an
amount not to exceed 0.015 weight percent and the remainder
cobalt.
2. The biocompatible, load-carrying metallic structure according to
claim 1, wherein the solid-solution alloy is constructed through
thermomechanical processing to exhibit relatively high strength and
low ductility characteristics in the fully cold-worked state.
3. The biocompatible, load-carrying metallic structure according to
claim 1, wherein the solid-solution alloy is constructed through
thermomechanical processing to exhibit relatively moderate strength
and moderate ductility characteristics in the partially cold-worked
state.
4. The biocompatible, load-carrying metallic structure according to
claim 1, wherein the solid-solution alloy is further constructed
through age hardening for a predetermined time within a gaseous
environment at a temperature less than the annealing temperature to
precipitate one or more secondary phases, including at least one of
intragranular and intergranular phases, from a substantially single
phase structure.
5. The biocompatible, load-carrying metallic structure according to
claim 4, wherein the age hardening temperature is in the range from
about 1,000 degrees Fahrenheit to about 1,950 degrees
Fahrenheit.
6. The biocompatible, load-carrying metallic structure according to
claim 4, wherein the age hardening gaseous environment comprises
hydrogen, nitrogen, argon and air.
7. The biocompatible, load-carrying metallic structure according to
claim 3, wherein the solid-solution alloy is further constructed
through stress relieving for a predetermined time within a gaseous
environment at a temperature less than the annealing temperature
while maintaining a substantially single phase to increase
toughness and ductility.
8. The biocompatible, load-carrying metallic structure according to
claim 7, wherein the stress relieving temperature is about or less
than 100 degrees Fahrenheit below the annealing temperature.
9. The biocompatible, load-carrying metallic structure according to
claim 7, wherein the stress relieving gaseous environment comprises
hydrogen, nitrogen, argon and air.
10. The biocompatible, load-carrying metallic structure according
to claim 1, wherein the solid-solution alloy is constructed through
thermomechanical processing to exhibit relatively low strength and
high ductility characteristics in the fully annealed state.
11. The biocompatible, load-carrying metallic structure according
to claim 1, wherein the medical device comprises a fixation
device.
12. The biocompatible, load-carrying metallic structure according
to claim 1, wherein the medical device comprises an artificial
joint implant.
13. The biocompatible, load-carrying metallic structure according
to claim 3, wherein the solid-solution alloy is further constructed
through stress relieving for a predetermined time with a vacuum
environment at a temperature less than the annealing temperature
while maintaining a substantially single phase to increase
toughness and ductility.
14. The biocompatible, load-carrying metallic structure according
to claim 13, wherein the stress relieving temperature is about or
less than 100 degrees Fahrenheit below the annealing temperature.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to alloys for use in
manufacturing or fabricating implantable medical devices, and more
particularly, to implantable medical devices manufactured or
fabricated from alloys that are highly fatigue resistant.
[0003] 2. Discussion of the Related Art
[0004] Percutaneous transluminal angioplasty (PTA) is a therapeutic
medical procedure used to increase blood flow through an artery. In
this procedure, the angioplasty balloon is inflated within the
stenosed vessel, or body passageway, in order to shear and disrupt
the wall components of the vessel to obtain an enlarged lumen. With
respect to arterial stenosed lesions, the relatively incompressible
plaque remains unaltered, while the more elastic medial and
adventitial layers of the body passageway stretch around the
plaque. This process produces dissection, or a splitting and
tearing, of the body passageway wall layers, wherein the intima, or
internal surface of the artery or body passageway, suffers
fissuring. This dissection forms a "flap" of underlying tissue,
which may reduce the blood flow through the lumen, or completely
block the lumen. Typically, the distending intraluminal pressure
within the body passageway can hold the disrupted layer, or flap,
in place. If the intimal flap created by the balloon dilation
procedure is not maintained in place against the expanded intima,
the intimal flap can fold down into the lumen and close off the
lumen, or may even become detached and enter the body passageway.
When the intimal flap closes off the body passageway, immediate
surgery is necessary to correct the problem.
[0005] Recently, transluminal prostheses have been widely used in
the medical arts for implantation in blood vessels, biliary ducts,
ureters, or other similar organs of the living body. These
prostheses are commonly referred to as stents and are used to
maintain, open, or dilate tubular structures. An example of a
commonly used stent is given in U.S. Pat. No. 4,733,665 to Palmaz.
Such stents are often referred to as balloon expandable stents.
Typically the stent is made from a solid tube of stainless steel.
Thereafter, a series of cuts are made in the wall of the stent. The
stent has a first smaller diameter, which permits the stent to be
delivered through the human vasculature by being crimped onto a
balloon catheter. The stent also has a second, expanded diameter,
upon application of a radially, outwardly directed force, by the
balloon catheter, from the interior of the tubular shaped
member.
[0006] However, one concern with such stents is that they are often
impractical for use in some vessels such as the carotid artery. The
carotid artery is easily accessible from the exterior of the human
body, and is close to the surface of the skin. A patient having a
balloon expandable stent made from stainless steel or the like,
placed in their carotid artery, might be susceptible to severe
injury through day-to-day activity. A sufficient force placed on
the patient's neck could cause the stent to collapse, resulting in
injury to the patient. In order to prevent this, self-expanding
stents have been proposed for use in such vessels. Self-expanding
stents act like springs and will recover to their expanded or
implanted configuration after being crushed.
[0007] The prior art makes reference to the use of alloys such as
Nitinol (Ni--Ti alloy), which have shape memory and/or superelastic
characteristics, in medical devices, which are designed to be
inserted into a patient's body, for example, self-expanding stents.
The shape memory characteristics allow the devices to be deformed
to facilitate their insertion into a body lumen or cavity and then
be heated within the body so that the device returns to its
original shape. Superelastic characteristics, on the other hand,
generally allow the metal to be deformed and restrained in the
deformed condition to facilitate the insertion of the medical
device containing the metal into a patient's body, with such
deformation causing the phase transformation. Once within the body
lumen, the restraint on the superelastic member can be removed,
thereby reducing the stress therein so that the superelastic member
can return to its original un-deformed shape by the transformation
back to the original phase.
[0008] One concern with self-expanding stents and with other
medical devices formed from superelastic materials, is that they
may exhibit reduced radiopacity under X-ray fluoroscopy. To
overcome this problem, it is common practice to attach markers,
made from highly radiopaque materials, to the stent, or to use
radiopaque materials in plating or coating processes. Those
materials typically include gold, platinum, or tantalum. The prior
art makes reference to these markers or processes in U.S. Pat. No.
5,632,771 to Boatman et al., U.S. Pat. No. 6,022,374 to Imran, U.S.
Pat. No. 5,741,327 to Frantzen, U.S. Pat. No. 5,725,572 to Lam et
al., and U.S. Pat. No. 5,800,526 to Anderson et al. However, due to
the size of the markers and the relative position of the materials
forming the markers in the galvanic series versus the position of
the base metal of the stent in the galvanic series, there is a
certain challenge to overcome; namely, that of galvanic corrosion.
Also, the size of the markers increases the overall profile of the
stent. In addition, typical markers are not integral to the stent
and thus may interfere with the overall performance of the stent as
well as become dislodged from the stent.
[0009] A concern with both balloon expandable and self-expandable
stents is magnetic resonance imaging compatibility. Currently
available metallic stents are known to cause artifacts in magnetic
resonance generated images. In general, metals having a high
magnetic permeability cause artifacts, while metals having a low
magnetic permeability cause less or substantially no artifacts. In
other words, if the stent or other medical device is fabricated
from a metal or metals having a low magnetic permeability, then
less artifacts are created during magnetic resonance imaging which
in turn allows more tissue in proximity to the stent or other
medical device to be imaged.
[0010] Artifacts created under magnetic resonance imaging are
promoted by local magnetic field inhomogeneities and eddy
currents-induced by the magnetic field generated by the magnetic
resonance imaging machine. The strength of the magnetic field
disruption is proportional to the magnetic permeability of the
metallic stent or other medical device. In addition, signal
attenuation within the stent is caused by radio frequency shielding
of the metallic stent or other medical device material.
Essentially, the radio frequency signals generated by the magnetic
resonance imaging machine may become trapped within the cage like
structure of the stent or other medical device. Induced eddy
currents in the stent may also lead to a lower nominal radio
frequency excitation angle inside the stent. This has been shown to
attenuate the signal acquired by the receiver coil of the magnetic
resonance imaging device. Artifact related signal changes may
include signal voids or local signal enhancements which in turn
degrades the diagnostic value of the tool.
[0011] Accordingly, there is a need to develop materials for
implantable medical devices, such as stents, that are magnetic
resonance imaging compatible while retaining the toughness,
durability and ductility properties required of implantable medical
devices such as stents.
[0012] Additionally, any intravascular device should preferably
exhibit certain characteristics, including maintaining vessel
patency through a chronic outward force that will help to remodel
the vessel to its intended luminal diameter, preventing excessive
radial recoil upon deployment, exhibiting sufficient fatigue
resistance and exhibiting sufficient ductility so as to provide
adequate coverage over the full range of intended expansion
diameters.
[0013] Accordingly, there is a need to develop precursory materials
and the associated processes for manufacturing intravascular stents
and other implantable medical devices that provide device designers
with the opportunity to engineer the device to specific
applications.
SUMMARY OF THE INVENTION
[0014] The present invention overcomes the limitations of applying
conventionally available materials to specific intravascular
therapeutic applications as briefly described above.
[0015] In accordance with one aspect, the present invention is
directed to a biocompatible, load-carrying metallic structure. The
metallic structure being formed from a solid-solution alloy
comprising nickel in the range from about 33 weight percent to
about 37 weight percent, chromium in the range from about 19 weight
percent to about 21 weight percent, molybdenum in the range from
about 9 weight percent to about 11 weight percent, iron in the
range from about 0 weight percent to about 1 weight percent,
manganese in the range from about 0 weight percent to about 0.15
weight percent, silicon in the range from about 0 weight percent to
about 0.15 weight percent, carbon in the range from about 0 to
about 0.025 weight percent, phosphorous in the range from about 0
to about 0.015 weight percent, boron in the range from about 0 to
about 0.015 weight percent, sulfur in the range from about 0 to
about 0.010 weight percent, titanium in an amount not to exceed
0.015 weight percent and the remainder cobalt.
[0016] The biocompatible, solid-solution alloy for implantable
medical devices of the present invention offers a number of
advantages over currently utilized alloys. The biocompatible alloy
of the present invention has improved magnetic resonance imaging
compatibility than currently utilized ferrous materials, is less
brittle than other alloys, has enhanced ductility and toughness,
and has increased fatigue durability. The biocompatible alloy also
maintains the desired or beneficial characteristics of currently
available alloys including strength and flexibility.
[0017] The biocompatible, solid-solution alloy for implantable
medical devices of the present invention may be utilized for any
number of medical applications, including vessel patency devices
such as vascular stents, biliary stents, ureter stents, vessel
occlusion devices such as atrial septal and ventricular septal
occluders, patent foramen ovale occluders and orthopedic devices
such as fixation devices.
[0018] The biocompatible, solid-solution alloy of the present
invention is simple and inexpensive to manufacture. The
biocompatible alloy may be formed into any number of structures or
devices. The biocompatible, solid-solution alloy may be
thermomechanically processed, including cold-working and heat
treating, to achieve varying degrees of strength and ductility. The
biocompatible alloy of the present invention may be age hardened to
precipitate one or more secondary phases.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] The foregoing and other features and advantages of the
invention will be apparent from the following, more particular
description of preferred embodiments of the invention, as
illustrated in the accompanying drawings.
[0020] FIG. 1 is a graphical representation of the transition of
critical mechanical properties as a function of thermomechanical
processing for quaternary cobalt-nickel-chromium-molybdenum alloys
in accordance with the present invention.
[0021] FIG. 2 is a graphical representation of the endurance limit
as a function of thermomechanical processing for a quaternary
cobalt-nickel-chromium-molybdenum alloy in accordance with the
present invention.
[0022] FIG. 3 is a flat layout diagrammatic representation of an
exemplary stent fabricated from the biocompatible alloy in
accordance with the present invention.
[0023] FIG. 4 is an enlarged view of the "M" links of the exemplary
stent of FIG. 3 in accordance with the present invention.
[0024] FIG. 5 is an enlarged view of a portion of the exemplary
stent of FIG. 3 in accordance with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0025] Biocompatible, solid-solution strengthened alloys such as
iron-based alloys, cobalt-based alloys and titanium-based alloys as
well as refractory metals and refractory-based alloys may be
utilized in the manufacture of any number of implantable medical
devices. The biocompatible, solid-solution alloy for implantable
medical devices in accordance with the present invention offers a
number of advantages over currently utilized medical grade alloys.
The advantages include the ability to engineer the underlying
microstructure in order to sufficiently perform as intended by the
designer without the limitations of currently utilized materials
and manufacturing methodologies.
[0026] For reference, a traditional Cobalt-based alloy such as
MP35N (i.e. UNS R30035) which is also broadly utilized as an
implantable, biocompatible device material may comprise a
solid-solution alloy comprising nickel in the range from about 33
weight percent to about 37 weight percent, chromium in the range
from about 19 weight percent to about 21 weight percent, molybdenum
in the range from about 9 weight percent to about 11 weight
percent, iron in the range from about 0 weight percent to about 1
weight percent, titanium in the range from about 0 weight percent
to about 1 weight percent, manganese in the range from about 0
weight percent to about 0.15 weight percent, silicon in the range
from about 0 weight percent to about 0.15 weight percent, carbon in
the range from about 0 to about 0.025 weight percent, phosphorous
in the range from about 0 to about 0.015 weight percent, boron in
the range from about 0 to about 0.015 weight percent, sulfur in the
range from about 0 to about 0.010 weight percent, and the remainder
cobalt.
[0027] In general, elemental additions such as chromium (Cr),
nickel (Ni), manganese (Mn), silicon (Si) and molybdenum (Mo) were
added to iron- and/or cobalt-based alloys, where appropriate, to
increase or enable desirable performance attributes, including
strength, machinability and corrosion resistance within clinically
relevant usage conditions.
[0028] In accordance with an exemplary embodiment, an implantable
medical device may be formed from a solid-solution alloy comprising
nickel in the range from about 33 weight percent to about 37 weight
percent, chromium in the range from about 19 weight percent to
about 21 weight percent, molybdenum in the range from about 9
weight percent to about 11 weight percent, iron in the range from
about 0 weight percent to about 1 weight percent, manganese in the
range from about 0 weight percent to about 0.15 weight percent,
silicon in the range from about 0 weight percent to about 0.15
weight percent, carbon in the range from about 0 to about 0.025
weight percent, phosphorous in the range from about 0 to about
0.015 weight percent, boron in the range from about 0 to about
0.015 weight percent, sulfur in the range from about 0 to about
0.010 weight percent, titanium in an amount not to exceed 0.015
weight percent and the remainder cobalt.
[0029] In contrast to the traditional formulation of MP35N, the
intended composition does not include any elemental titanium (Ti)
above conventional accepted trace impurity levels. Accordingly,
this exemplary embodiment will exhibit a marked improvement in
fatigue durability (i.e. cyclic endurance limit strength) due to
the minimization of secondary phase precipitates in the form of
titanium-carbides.
[0030] The preferred embodiment may be processed from the requisite
elementary raw materials, as set-forth above, by first mechanical
homogenization (i.e. mixing) and then compaction into a green state
(i.e. precursory) form. If necessary, appropriate manufacturing
aids such as hydrocarbon based lubricants and/or solvents (e.g.
mineral oil, machine oils, kerosene, isopropanol and related
alcohols) may be used to ensure complete mechanical homogenization.
Additionally, other processing steps such as ultrasonic agitation
of the mixture followed by cold compaction to remove any
unnecessary manufacturing aides and to reduce void space within the
green state may be utilized. It is preferable to ensure that any
impurities within or upon the processing equipment from prior
processing and/or system construction (e.g. mixing vessel material,
transfer containers, etc.) be sufficiently reduced in order to
ensure that the green state form is not unnecessarily contaminated.
This may be accomplished by adequate cleaning of the mixing vessel
before adding the constituent elements by use of surfactant based
cleaners to remove any loosely adherent contaminants.
[0031] Initial melting of the green state form into a ingot of
desired composition, is achieved by vacuum induction melting (VIM)
where the initial form is inductively heated to above the melting
point of the primary constituent elements within a refractory
crucible and then poured into a secondary mold within a vacuum
environment (e.g. typically less than or equal to 10.sup.-4 mmHg).
The vacuum process ensures that atmospheric contamination is
significantly minimized. Upon solidification of the molten pool,
the ingot bar is substantially single phase (i.e. compositionally
homogenous) with a definable threshold of secondary phase
impurities that are typically ceramic (e.g. carbide, oxide or
nitride) in nature. These impurities are typically inherited from
the precursor elemental raw materials.
[0032] A secondary melting process termed vacuum arc reduction
(VAR) is utilized to further reduce the concentration of the
secondary phase impurities to a conventionally accepted trace
impurity level (i.e. <1,500 ppm). Other methods may be enabled
by those skilled in the art of ingot formulation that substantially
embodies this practice of ensuring that atmospheric contamination
is minimized. In addition, the initial VAR step may be followed by
repetitive VAR processing to further homogenize the solid-solution
alloy in the ingot form. From the initial ingot configuration, the
homogenized alloy will be further reduced in product size and form
by various industrially accepted methods such as, but not limited
too, ingot peeling, grinding, cutting, forging, forming, hot
rolling and/or cold finishing processing steps so as to produce bar
stock that may be further reduced into a desired raw material
form.
[0033] In this exemplary embodiment, the initial raw material
product form that is required to initiate the thermomechanical
processing that will ultimately yield a desired small diameter,
thin-walled tube, appropriate for interventional devices, is a
modestly sized round bar (e.g. one inch in diameter round bar
stock) of predetermined length. In order to facilitate the
reduction of the initial bar stock into a much smaller tubing
configuration, an initial clearance hole must be placed into the
bar stock that runs the length of the product. These tube hollows
(i.e. heavy walled tubes) may be created by `gun-drilling` (i.e.
high depth to diameter ratio drilling) the bar stock. Other
industrially relevant methods of creating the tube hollows from
round bar stock may be utilized by those skilled-in-the-art of tube
making.
[0034] Consecutive mechanical cold-finishing operations such as
drawing through a compressive outer-diameter (OD), precision shaped
(i.e. cut), circumferentially complete, diamond die using any of
the following internally supported (i.e. inner diameter, ID)
methods, but not necessarily limited to these conventional forming
methods, such as hard mandrel (i.e. relatively long traveling ID
mandrel also referred to as rod draw), floating-plug (i.e.
relatively short ID mandrel that `floats` within the region of the
OD compressive die and fixed-plug (i.e. the ID mandrel is `fixed`
to the drawing apparatus where relatively short workpieces are
processed) drawing. These process steps are intended to reduce the
outer-diameter (OD) and the corresponding wall thickness of the
initial tube hollow to the desired dimensions of the finished
product.
[0035] When necessary, tube sinking (i.e. OD reduction of the
workpiece without inducing substantial tube wall reduction) is
accomplished by drawing the workpiece through a compressive die
without internal support (i.e. no ID mandrel). Conventionally, tube
sinking is typically utilized as a final or near-final mechanical
processing step to achieve the desired dimensional attributed of
the finished product.
[0036] Although not practically significant, if the particular
compositional formulation will support a single reduction from the
initial raw material configuration to the desired dimensions of the
finished product, in process heat-treatments will not be necessary.
Where necessary to achieve intended mechanical properties of the
finished product, a final heat-treating step is utilized.
[0037] Conventionally, all metallic alloys in accordance with the
present invention will require incremental dimensional reductions
from the initial raw material configuration to reach the desired
dimensions of the finished product. This processing constraint is
due to the material's ability to support a finite degree of induced
mechanical damage per processing step without structural failure
(e.g. strain-induced fracture, fissures, extensive void formation,
etc.).
[0038] In order to compensate for induced mechanical damage (i.e.
cold-working) during any of the aforementioned cold-finishing
steps, periodic thermal heat-treatments are utilized to
stress-relieve (i.e. minimization of deleterious internal residual
stresses that are the result of processes such as cold-working)
thereby increasing the workability (i.e. ability to support
additional mechanical damage without measurable failure) the
workpiece prior to subsequent reductions. These thermal treatments
are typically, but not necessarily limited to, conducted within a
relatively inert environment such as an inert gas furnace (e.g.
nitrogen, argon, etc.), a oxygen rarified hydrogen furnace, a
conventional vacuum furnace and under less common process
conditions, atmospheric air. When vacuum furnaces are utilized, the
level of vacuum (i.e. subatmospheric pressure), typically measured
in units of mmHg or torr (where 1 mmHg is equal to 1 unit torr),
shall be sufficient to ensure that excessive and deteriorative high
temperature oxidative processes are not functionally operative
during heat treatment. This process may usually be achieved under
vacuum conditions of 10.sup.-4 mmHg (0.0001 torr) or less (i.e.
lower magnitude).
[0039] The stress relieving heat treatment temperature is typically
held constant between 82 to 86% of the conventional melting point
(i.e. industrially accepted liquidus temperature, 0.82 to 0.86
homologous temperatures) within an adequately sized isothermal
region of the heat-treating apparatus. The workpiece undergoing
thermal treatment is held within the isothermal processing region
for a finite period of time that is adequate to ensure that the
workpiece has reached a state of thermal equilibrium and for that
sufficient time is elapsed to ensure that the reaction kinetics
(i.e. time dependent material processes) of stress-relieving and/or
process annealing, as appropriate, is adequately completed. The
finite amount of time that the workpiece is held within the
processing is dependent upon the method of bringing the workpiece
into the process chamber and then removing the working upon
completion of heat treatment. Typically, this process is
accomplished by, but not limited to, use of a conventional
conveyor-belt apparatus or other relevant mechanical assist
devices. In the case of the former, the conveyor belt speed and
appropriate finite dwell-time, as necessary, within the isothermal
region is controlled to ensure that sufficient time at temperature
is utilized so as to ensure that the process is completed as
intended.
[0040] When necessary to achieve desired mechanical attributes of
the finished product, heat-treatment temperatures and corresponding
finite processing times may be intentionally utilized that are not
within the typical range of 0.82 to 0.86 homologous temperatures.
Various age hardening (i.e. a process that induces a change in
properties at moderately elevated temperatures, relative to the
conventional melting point, that does not induce a change in
overall chemical composition change in the metallic alloy being
processed) processing steps may be carried out, as necessary, in a
manner consistent with those previously described at temperatures
substantially below 0.82 to 0.86 homologous temperature. For
Co-based alloys in accordance with the present invention, these
processing temperatures may be varied between and inclusive of
approximately 0.29 homologous temperature and the aforementioned
stress relieving temperature range. The workpiece undergoing
thermal treatment is held within the isothermal processing region
for a finite period of time that is adequate to ensure that the
workpiece has reached a state of thermal equilibrium and for that
sufficient time is elapsed to ensure that the reaction kinetics
(i.e. time dependent material processes) of age hardening, as
appropriate, is adequately completed prior to removal from the
processing equipment.
[0041] In some cases for Co-based alloys in accordance with the
present invention, the formation of secondary-phase ceramic
compounds such as carbide, nitride and/or oxides will be induced or
promoted by age hardening heat treating. These secondary-phase
compounds are typically, but not limited to, for Co-based alloys in
accordance with the present invention, carbides which precipitate
along thermodynamically favorable regions of the structural
crystallographic planes that comprise each grain (i.e.
crystallographic entity) that make-up the entire polycrystalline
alloy. These secondary-phase carbides can exist along the
intergranular boundaries as well as within each granular structure
(i.e. intragranular). Under most circumstances for Co-based alloys
in accordance with the present invention, the principal secondary
phase carbides that are stoichiometrically expected to be present
are M.sub.6C where M typically is cobalt (Co). When present, the
intermetallic M.sub.6C phase is typically expected to reside
intragranularly along thermodynamically favorable regions of the
structural crystallographic planes that comprise each grain within
the polycrystalline alloy in accordance with the present invention.
Although not practically common, the equivalent material phenomena
can exist for a single crystal (i.e. monogranular) alloy.
[0042] Additionally, another prominent secondary phase carbide can
also be induced or promoted as a result of age hardening heat
treatments. This phase, when present, is stoichiometrically
expected to be M.sub.23C.sub.6 where M typically is chromium (Cr)
but is also commonly observed to be cobalt (Co) especially in
Co-based alloys. When present, the intermetallic M.sub.23C.sub.6
phase is typically expected to reside along the intergranular
boundaries (i.e. grain boundaries) within a polycrystalline alloy
in accordance with the present invention. As previously discussed
for the intermetallic M.sub.6C phase, the equivalent presence of
the intermetallic M.sub.23C.sub.6 phase can exist for a single
crystal (i.e. monogranular) alloy, albeit not practically
common.
[0043] In the case of the intergranular M.sub.23C.sub.6 phase, this
secondary phase is conventionally considered most important, when
formed in a manner that is structurally and compositionally
compatible with the alloy matrix, to strengthening the grain
boundaries to such a degree that intrinsic strength of the grain
boundaries and the matrix are adequately balanced. By inducing this
equilibrium level of material strength at the microstructural
level, the overall mechanical properties of the finished tubular
product can be further optimized to desirable levels.
[0044] In addition to stress relieving and age hardening related
heat-treating steps, solutionizing (i.e. sufficiently high
temperature and longer processing time to thermodynamically force
one of more alloy constituents to enter into solid
solution--`singular phase`, also referred to as full annealing) of
the workpiece may be utilized. For Co-based alloys in accordance
with the present invention, the typical solutionizing temperature
can be varied between and inclusive of approximately 0.88 to 0.90
homologous temperatures. The workpiece undergoing thermal treatment
is held within the isothermal processing region for a finite period
of time that is adequate to ensure that the workpiece has reached a
state of thermal equilibrium and for that sufficient time is
elapsed to ensure that the reaction kinetics (i.e. time dependent
material processes) of solutionizing, as appropriate, is adequately
completed prior to removal from the processing equipment.
[0045] The sequential and selectively ordered combination of
thermomechanical processing steps that may comprise but not
necessarily include mechanical cold-finishing operations, stress
relieving, age hardening and solutionizing can induce and enable a
broad range of measurable mechanical properties as a result of
distinct and determinable microstructural attributes. This material
phenomena is represented by the curves illustrated in FIG. 1. In
accordance with the present invention, FIG. 1 illustrates a
relationship of change in measurable mechanical properties such as
yield strength and ductility (presented in units of percent
elongation) as a function of thermomechanical processing (TMP), for
example, cold working and in-process heat-treatments. In this
example, representative thermomechanical (TMP) groups one (1)
through five (5) were subjected to varying combinations of
cold-finishing, stress relieving and age hardening and not
necessarily in the presented sequential order. In general, the
principal isothermal age hardening heat treatment applied to each
TMP group varied between about 0.74 to 0.78 homologous temperatures
for group (1), about 0.76 to 0.80 homologous temperatures for group
(2), about 0.78 to 0.82 homologous temperatures for group (3),
about 0.80 to 0.84 homologous temperatures for group (4) and about
0.82 to 0.84 homologous temperatures for group (5). Each workpiece
undergoing thermal treatment was held within the isothermal
processing region for a finite period of time that was adequate to
ensure that the workpiece reached a state of thermal equilibrium
and to ensure that sufficient time was elapsed to ensure that the
reaction kinetics of age hardening was adequately completed.
[0046] More so, the effect of thermomechanical (TMP) on cyclic
fatigue properties on Co-based alloys, in accordance with the
present invention, is illustration in FIG. 2. Examination of curves
in FIG. 2, reveals the relationship of fatigue strength (i.e.
endurance limit) as a function of thermomechanical processing for
the previously discussed TMP groups (2) and (4). TMP group (2) from
this figure as utilized in this specific example shows a marked
increase in the fatigue strength (i.e. endurance limit, the maximum
stress below which a material can presumably endure an infinite
number of stress cycles) over and above the TMP group (4) process.
As a result of reducing the amount of metallurgically significant
amounts of elemental titanium within the alloy described in the
present invention, the overall preponderance of titanium-carbide
precipitates will be reduced thereby leading to increase in
measurable fatigue strength. In general, the role of
secondary-phase particulates tends to make the overall structure
more prone to fatigue induced damage due to the incoherent
interface between particulate and matrix.
[0047] The above-described alloy may be utilized in any number of
implantable medical devices. The alloy is particularly advantageous
in situations where magnetic resonance imaging is a useful
diagnostic tool such as determining in-stent restenosis.
Accordingly, although the alloy may be utilized for any implantable
medical device, an exemplary stent constructed from the alloy is
described below.
[0048] FIG. 3 is a flat layout of an exemplary embodiment of a
stent that may be constructed utilizing the alloy of the present
invention. The stent 10 comprises end sets of strut members 12
located at each end of the stent 10 and central sets of strut
members 14 connected each to the other by sets of flexile "M" links
16. Each end set of strut members 12 comprises alternating curved
sections 18 and diagonal sections 20 connected together to form a
closed circumferential structure. The central sets of strut members
14 located longitudinally between the end sets of strut members 14
comprise curved sections 22 and diagonal sections 24 connected
together to form a closed circumferential ring-like structure.
[0049] Referring to FIG. 4 there is illustrated an enlargement of
the flexible "M" links 16 of the stent 10. Each "M" link 16 has a
circumferential extent, i.e. length, L' above and L'' below line
11. The line 11 is drawn between the attachment points 13 where the
"M" link 16 attaches to adjacent cured sections 18 or 22. Such a
balanced design preferably diminishes any likelihood of the
flexible connecting link 16 from expanding into the lumen of artery
or other vessel.
[0050] As illustrated in FIG. 3, the diagonal sections 20 of the
end sets of strut members 12 are shorter in length than the
diagonal sections 24 of the central sets of strut members 14. The
shorter diagonal sections 20 will preferably reduce the
longitudinal length of metal at the end of the stent 10 to improve
deliverability into a vessel of the human body. In the stent 10,
the widths of the diagonal sections 20 and 24 are different from
one another.
[0051] Referring to FIG. 5, there is illustrated an expanded view
of a stent section comprising an end set of strut members 12 and a
central set of strut members 14. As illustrated, the diagonal
sections 24 of the central sets of strut members 14 have a width at
the center thereof, T.sub.c, and a width at the end thereof,
T.sub.e, wherein T.sub.c is greater than T.sub.e. This
configuration allows for increased radiopacity without affecting
the design of curved sections 22 that are the primary stent
elements involved for stent expansion. In an exemplary embodiment,
the curved sections 22 and 18 may be tapered and may have uniform
widths with respect to one another as is explained in detail
subsequently. The diagonal sections 20 of the end sets of strut
members 12 also have a tapered shape. The diagonal sections 20 have
a width in the center, T.sub.c-end, and a width at the end,
T.sub.e-end, wherein T.sub.c-end is greater than T.sub.e-end.
Because it is preferable for the end sets of strut members 12 to be
the most radiopaque part of the stent 10, the diagonal section 20
center width T.sub.c-end of the end sets of strut members 12 is
wider than the width T.sub.c of the diagonal section 24. Generally,
a wider piece of metal will be more radiopaque. Thus, the stent 10
has curved sections with a single bend connecting the diagonal
sections of its sets of strut members, and flexible connecting
links connecting the curved sections of its circumferential sets of
strut members.
[0052] The width of the curved sections 22 and 18 taper down as one
moves away from the center of the curve until a predetermined
minimum width substantially equal to that of their respective
diagonal sections 24 and 20. To achieve this taper, the inside arc
of the curved sections 22 and 18 have a center that is
longitudinally displaced from the center of the outside arc. This
tapered shape for the curved sections 22 and 18 provides a
significant reduction in metal strain with little effect on the
radial strength of the expanded stent as compared to a stent having
sets of strut members with a uniform strut width.
[0053] This reduced strain design has several advantages. First, it
can allow the exemplary design to have a much greater usable range
of radial expansion as compared to a stent with a uniform strut
width. Second, it can allow the width at the center of the curve to
be increased which increases radial strength without greatly
increasing the metal strain (i.e. one can make a stronger stent).
Finally, the taper reduces the amount of metal in the stent and
that should improve the stent thrombogenicity.
[0054] The curved sections 18 of the end sets of strut members 12
and the curved sections 22 of the central sets of strut members 14
have the same widths. As a result of this design, the end sets of
strut members 12, which have shorter diagonal sections 20, will
reach the maximum allowable diameter at a level of strain that is
greater than the level of strain experienced by the central sets of
strut members 14.
[0055] It is important to note that although a stent is described,
the alloy may be utilized for any number of implantable medical
devices.
[0056] Although shown and described is what is believed to be the
most practical and preferred embodiments, it is apparent that
departures from specific designs and methods described and shown
will suggest themselves to those skilled in the art and may be used
without departing from the spirit and scope of the invention. The
present invention is not restricted to the particular constructions
described and illustrated, but should be constructed to cohere with
all modifications that may fall within the scope for the appended
claims.
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