U.S. patent application number 14/895712 was filed with the patent office on 2016-05-19 for biodegradable wire for medical devices.
The applicant listed for this patent is FORT WAYNE METALS RESEARCH PRODUCTS CORP.. Invention is credited to Adam J. Griebel, Jeremy E. Schaffer.
Application Number | 20160138148 14/895712 |
Document ID | / |
Family ID | 52008755 |
Filed Date | 2016-05-19 |
United States Patent
Application |
20160138148 |
Kind Code |
A1 |
Schaffer; Jeremy E. ; et
al. |
May 19, 2016 |
BIODEGRADABLE WIRE FOR MEDICAL DEVICES
Abstract
A bioabsorbable material composition includes magnesium (Mg),
lithium (Li) and calcium (Ca). Lithium is provided in a sufficient
amount to enhance material ductility, while also being provided in
a sufficiently low amount to maintain corrosion resistance at
suitable levels. Calcium is provided in a sufficient amount to
enhance mechanical strength and/or further influence the rate of
corrosion, while also being provided in a sufficiently low amount
to preserve material ductility. The resultant ductile base material
may be cold-worked to enhance strength, such as for medical
applications. In one application, the material may be drawn into a
fine wire, which may be used to create resorbable structures for
use in vivo such as stents.
Inventors: |
Schaffer; Jeremy E.; (Leo,
IN) ; Griebel; Adam J.; (Fort Wayne, IN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
FORT WAYNE METALS RESEARCH PRODUCTS CORP. |
Wayne |
IN |
US |
|
|
Family ID: |
52008755 |
Appl. No.: |
14/895712 |
Filed: |
June 6, 2014 |
PCT Filed: |
June 6, 2014 |
PCT NO: |
PCT/US2014/041267 |
371 Date: |
December 3, 2015 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
61831800 |
Jun 6, 2013 |
|
|
|
Current U.S.
Class: |
428/649 ;
148/667; 420/402; 428/606 |
Current CPC
Class: |
C21D 9/525 20130101;
B32B 15/013 20130101; C22C 38/02 20130101; C22C 23/06 20130101;
A61L 31/022 20130101; C22F 1/06 20130101; C22C 23/02 20130101; C22C
23/04 20130101; A61L 31/148 20130101; B32B 15/01 20130101; C22C
23/00 20130101 |
International
Class: |
C22F 1/06 20060101
C22F001/06; C21D 9/52 20060101 C21D009/52; C22C 23/00 20060101
C22C023/00 |
Claims
1. A magnesium-based alloy wire, comprising: between 3 wt. %
lithium and 7 wt. % lithium; between 0.1 wt. % calcium and 1 wt. %
calcium; and balance magnesium and trace impurities.
2. The magnesium-based alloy wire of claim 1, wherein said wire
comprises between 0.20 and 0.30 wt. % calcium.
3. The magnesium-based alloy wire of claim 2, wherein the alloy
exhibits sufficient ductility to be subjected to 98% cold work
without fracture.
4. The magnesium-based alloy wire of claim 2, wherein: the alloy is
formed as a wire product having 98% retained cold work, the wire
having a yield strength reaching 276 MPa.
5. The magnesium-based alloy wire of claim 2, wherein: the alloy is
formed as a wire product having 98% retained cold work, the wire
having an ultimate tensile strength reaching 334 MPa.
6. The magnesium-based alloy wire of claim 1, wherein said wire
comprises between 0.9 wt. % and 1 wt. % calcium.
7. The magnesium-based alloy wire of claim 6, wherein the alloy
exhibits sufficient ductility to be subjected to 88% cold work
without fracture.
8. The magnesium-based alloy wire of claim 6, wherein: the alloy is
formed as a wire product having 98% retained cold work, the wire
having a yield strength reaching 240 MPa.
9. The magnesium-based alloy wire of claim 6, wherein: the alloy is
formed as a wire product having 98% retained cold work, the wire
having an ultimate tensile strength reaching 271 MPa.
10. The magnesium-based alloy wire of claim 1, further comprising
between 0.9 wt. % and 5 wt. % aluminum.
11. The magnesium-based alloy wire of claim 1, further comprising
between 0.25 wt. % and 7 wt. % rare earth metal.
12. The magnesium-based alloy wire of claim 1, further comprising
between 0.10 wt. % and 6 wt. % zinc.
13. The magnesium-based alloy wire of claim 1, further comprising
between 0.10 wt. % and 1 wt. % manganese.
14. The magnesium-based alloy wire of claim 1, further comprising
between 0.10 wt. % and 1 wt. % zirconium.
15. The magnesium-based alloy wire of claim 1, wherein the wire
lacks any other element in addition to magnesium, lithium and
calcium in an amount above 0.05 wt. %.
16. The magnesium-based alloy wire of claim 1, wherein said wire
has a diameter up to 2.5 mm.
17. The magnesium-based alloy wire of claim 1, wherein said wire
comprises a fine wire having a diameter between 20 .mu.m and 1
mm.
18. The magnesium-based alloy wire of claim 1, wherein said wire
comprises one of a wire having a round cross section, a flat wire,
a strand, a cable, a coil and tubing.
19. A stent including the magnesium-based alloy wire of claim
1.
20. A bimetal composite wire, comprising: an outer shell formed of
a first biodegradable metallic material; and an inner core formed
of a second biodegradable metallic material, said first and second
biodegradable metallic materials being different from one another
whereby said first and second biodegradable metallic materials have
differing biodegradation rates, and one of said first and second
biodegradable materials comprising a magnesium-based alloy selected
from the group consisting of: a Mg--Li--Ca alloy having between 3.0
wt. % and 7.0 wt. % Li and between 0.10 wt. % and 1.0 wt. % Ca; a
Mg--Li--Ca-RE alloy having between 3.0 wt. % and 7.0 wt. % Li,
between 0.10 wt. % and 1.0 wt. % Ca, and between 0.25 wt. % and 7.0
wt. % RE, wherein "RE" is at least one rare earth element; a
Mg--Li--Ca--Al alloy having between 3.0 wt. % and 7.0 wt. % Li and
between 1.0 wt. % and 6.0 wt. % combined Al and Ca including 0.10
to 1.0 wt. % Ca and 0.9 wt. % to 5.0 wt. % Al; and a
Mg--Li--Al--Ca-RE alloy having between 3.0 wt. % and 7.0 wt. % Li,
between 1.0 wt. % and 6.0 wt. % combined Al and Ca including 0.10
to 1.0 wt. % Ca and 0.9 wt. % to 5.0 wt. % A, and between 0.25 wt.
% and 7.0 wt. % RE, wherein "RE" is at least one rare earth
element.
21. The bimetal composite wire of claim 20, wherein said
magnesium-based alloy has an ultimate tensile strength reaching 334
MPa.
22. The bimetal composite wire of claim 20, wherein the other of
said first and second biodegradable materials is selected from the
group consisting of pure metallic iron (Fe) and an iron-based alloy
(Fe alloy).
23. The bimetal composite wire of claim 20, wherein an outer
diameter of said outer shell is less than 1 mm.
24. The bimetal composite wire of claim 20, wherein the wire lacks
any other element in addition to magnesium, lithium, calcium,
aluminum and RE in an amount above 0.05 wt. %.
25. A stent including of the bimetal composite wire of claim
20.
26. A method of manufacturing a wire, comprising the steps of:
providing an outer shell made of a first biodegradable material;
inserting a core into the outer shell to form a wire construct, the
core formed of a second biodegradable material, the first and
second biodegradable materials being different from one another,
one of the first and second biodegradable materials comprising a
magnesium-based alloy selected from the group consisting of: a
Mg--Li--Ca alloy having between 3.0 wt. % and 7.0 wt. % Li and
between 0.10 wt. % and 1.0 wt. % Ca; a Mg--Li--Ca-RE alloy having
between 3.0 wt. % and 7.0 wt. % Li, between 0.10 wt. % and 1.0 wt.
% Ca, and between 0.25 wt. % and 7.0 wt. % RE, wherein "RE" is at
least one rare earth element; a Mg--Li--Ca--Al alloy having between
3.0 wt. % and 7.0 wt. % Li and between 1.0 wt. % and 6.0 wt. %
combined Al and Ca including 0.10 to 1.0 wt. % Ca and 0.9 wt. % to
5.0 wt. % Al; and a Mg--Li--Al--Ca-RE alloy having between 3.0 wt.
% and 7.0 wt. % Li, between 1.0 wt. % and 6.0 wt. % combined Al and
Ca including 0.10 to 1.0 wt. % Ca and 0.9 wt. % to 5.0 wt. % A, and
between 0.25 wt. % and 7.0 wt. % RE, wherein "RE" is at least one
rare earth element; and
27. The method of claim 26, further comprising imparting cold work
at room temperature to the wire construct by drawing the wire
construct from a first outer diameter to a second outer diameter
less than the first outer diameter.
28. The method of claim 27, further comprising, after said
imparting step, the additional step of annealing the wire
construct.
29. The method of claim 26, further comprising forming the wire
into a stent.
30. The method of claim 26, wherein the other of said first and
second biodegradable materials is selected from the group
consisting of pure metallic iron (Fe) and an iron-based alloy (Fe
alloy).
31. The method of claim 26, wherein the wire lacks any other
element in addition to magnesium, lithium, calcium, aluminum and RE
in an amount above 0.05 wt. %.
Description
BACKGROUND
[0001] 1. Technical Field
[0002] The present invention relates to wire used in biomedical
applications and, in particular, relates to a biodegradable
composite wire for use in medical devices such as stents.
[0003] 2. Description of the Related Art
[0004] Stents are artificial tube-like structures that are deployed
within a conduit or passage in the body to alleviate a flow
restriction or constriction. Stents are commonly used in coronary
arteries to alleviate blood flow restrictions resulting, e.g., from
cardiovascular disease. However, stents may also be used in
non-coronary vessels, the urinary tract and other areas of the
body. Non-coronary applications range broadly from compliant
pulmonary vessels of children with congenital heart disease (CHD),
to atherosclerotic popliteal arteries of older patients with
critical limb ischemia (CLI). Stented lesions may be long and
tortuous as in the case of severe infrainguinal lesions, or short
and relatively uniform as in mild pulmonary artery stenoses.
[0005] Examples of non-coronary stent applications include
arteriovenous fistulas (AVFs) or false aneurysms, which may occur
as a result of trauma due to gunshot wounds, falling accidents, or
other blunt force incident. Such phenomena often occur in the upper
limbs of the body where lack of perfusion can manifest as gangrene,
severe pain, or local cyanosis. Critical limb ischemia associated
with atherosclerosis can also result in the need for radial or
axillary artery stenting, for example, to avoid amputation or other
more serious morbidities. In contrast to most thoracoabdominal
implantation sites (such as in coronary arteries), upper and lower
limb anatomy is typically subjected to greater range of motion,
thereby potentially increasing mechanical fatigue.
[0006] Typically, stents are made of either biocompatible metal
wire(s) or polymeric fiber(s) which are formed into a generally
cylindrical, woven or braided structure of the type shown in FIGS.
1A and 1B. These types of stents are typically designed to be
either "self-expanding", in which the stent may be made of a shape
memory material, for example, and deploys automatically by
expanding upon removal of a constricting force when released from a
containment device, or "balloon-expanding", in which the stent is
forcibly expanded from within by an inflatable balloon.
[0007] When a stent is implanted, it applies a radial force against
the wall of the vessel in which it is implanted, which improves
vessel patency and reduces acute closure or increases vessel
diameter. In either case, the vessel usually achieves a new
equilibrium by biological remodeling of the vessel wall over a
period of weeks or months. After such remodeling is complete, the
stent may no longer be needed for mechanical support and could
potentially inhibit further natural positive remodeling of the
vessel or limit re-intervention, for example. However, removal of
an implanted stent may be difficult.
[0008] Many known stents are formed of corrosion-resistant and
substantially non-biodegradable or non-bioresorbable metal
materials which maintain their integrity in the body for many years
after implantation. Design efforts for creating bioabsorbable
stents have focused primarily on balloon-expandable technology for
coronary pathologies, and may include polymer biodegradable stents
using poly-L lactic acid (PLLA) and poly-L glycolic acid (PLGA),
nutrient metals of magnesium (Mg), including alloys or powder
metallurgy forms of magnesium, and iron (Fe), and iron-manganese
(Fe--Mn) alloys. Some research methods have also focused on hybrids
including layered biodegradable polymers and bioabsorbable polymer
coated nutrient metals. While such materials are resorbable, their
mechanical strength and resilience may be too low for some
applications. In addition, existing bioabsorbable stent materials
may confer inadequate control over the rate of bioabsorption for
some applications (i.e., by biodegrading too slowly or too quickly
after implantation).
[0009] Other applications for bioabsorbable materials, including
nutrient metal bioabsorbable materials, include temporary fracture
fixation devices such as bone plates. In some instances, it may be
desirable for a bone plate to provide a designated level of
mechanical strength during bone regrowth following a fracture, but
to subsequently reduce or remove the mechanical support provided by
the plate once the fracture has healed. Bioabsorbable bone plates
may provide one mechanism for such variable mechanical
strength.
[0010] What is needed is a biodegradable metallic material and wire
having mechanical properties and degradation rate appropriate for
use in biomedical applications, which represents an improvement
over the foregoing.
SUMMARY
[0011] The present invention provides a bioabsorbable material
composition including magnesium (Mg), lithium (Li) and calcium
(Ca). Lithium is provided in a sufficient amount to enhance
material ductility, while also being provided in a sufficiently low
amount to maintain corrosion resistance at suitable levels. Calcium
is provided in a sufficient amount to enhance mechanical strength
and/or further influence the rate of corrosion, while also being
provided in a sufficiently low amount to preserve material
ductility. The resultant ductile base material may be cold-worked
to enhance strength, such as for medical applications. In one
application, the material may be drawn into a fine wire, which may
be used to create resorbable structures for use in vivo such as
stents.
[0012] In one exemplary application, the Mg--Li--Ca material may be
used as one or more constituents of a composite wire including, in
cross-section, an outer shell or tube formed of a first
biodegradable material and an inner core formed of a second
biodegradable material. Both the shell and core may be adapted to
resorb or disappear after post-operative vessel healing has
occurred and vessel patency has been restored, or the shell may be
the only resorbable component.
[0013] Other materials suitable for use in the composite wire
include nutrient-metal-composites and alloys of pure iron,
manganese, magnesium, and zinc. Particular metals or metal alloys
may be selected to provide a desired biodegradation rate and
desired mechanical properties. The total rate of biodegradation of
the wire, and therefore the duration of the overall mechanical
integrity of the wire, may be controlled by the relative
cross-sectional areas (i.e., the relative thicknesses) of the outer
sheath and core material relative to the overall cross-sectional
area of the wire.
[0014] When formed into a stent, for example, the first and second
biodegradable materials of the composite wire may be different, and
may have differing biodegradation rates. The first biodegradable
material may degrade relatively slowly for retention of the
mechanical integrity of the stent during vessel remodeling, and the
second biodegradable material may degrade relatively quickly. The
biodegradation rates may be inherently controlled, such as by
selection of materials, and also may be mechanically controlled,
such as by material thicknesses and the geometric configuration of
the shell, core, or overall device.
[0015] The mechanical strength of the wire may be controlled to
impart either a self-expanding character to a braided or knit stent
device made from the wire, or may be controlled to provide a high
strength wire for use in balloon-expandable wire-based stents. The
mechanical strength and elastic resilience of the wire can be
significantly impacted through thermomechanical processing.
[0016] In one form thereof, the present invention provides a
magnesium-based alloy wire, comprising: between 3 wt. % lithium and
7 wt. % lithium; between 0.1 wt. % calcium and 1 wt. % calcium; and
balance magnesium and trace impurities.
[0017] In one aspect, the magnesium-based alloy wire may have about
0.25% calcium, which may exhibit sufficient ductility to undergo
98% cold work without fracture. For an alloy formed as a wire
product having 98% retained cold work, the wire may have a yield
strength reaching 276 MPa and/or an ultimate tensile strength
reaching 334 MPa.
[0018] In another aspect, the magnesium-based alloy wire ma have
about 1 wt. % calcium, which may exhibit sufficient ductility to
undergo 88% cold work without fracture. For an alloy formed as a
wire product having 98% retained cold work, the wire may have a
yield strength reaching 240 MPa and/or an ultimate tensile strength
reaching 271 MPa.
[0019] In other aspects, the magnesium-based alloy wire may further
include between 0.9 wt. % and 5 wt. % aluminum, between 0.25 wt. %
and 7 wt. % rare earth metal, between 0.10 wt. % and 6 wt. % zinc,
between 0.10 wt. % and 1 wt. % manganese, between 0.10 wt. % and 1
wt. % zirconium, or any combination of the foregoing, except that
zirconium is not alloyed with the with alloys which also contain
aluminum or manganese.
[0020] In yet another aspect, the magnesium-based alloy wire may
have a diameter up to 2.5 mm, or may be a fine wire having a
diameter between 20 .mu.m and 1 mm.
[0021] In still another aspect, the magnesium-based alloy wire may
be formed into or included as part of a stent structure.
[0022] In another form thereof, the present disclosure provides a
bimetal composite wire, comprising: an outer shell formed of a
first biodegradable metallic material; and an inner core formed of
a second biodegradable metallic material, said first and second
biodegradable metallic materials being different from one another
whereby said first and second biodegradable metallic materials have
differing biodegradation rates, and one of said first and second
biodegradable materials comprising a magnesium-based alloy selected
from the group consisting of: a Mg--Li--Ca alloy having between 3.0
wt. % and 7.0 wt. % Li and between 0.10 wt. % and 1.0 wt. % Ca; a
Mg--Li--Ca-RE alloy having between 3.0 wt. % and 7.0 wt. % Li,
between 0.10 wt. % and 1.0 wt. % Ca, and between 0.25 wt. % and 7.0
wt. % RE, wherein "RE" is at least one rare earth element; a
Mg--Li--Ca--Al alloy having between 3.0 wt. % and 7.0 wt. % Li and
between 1.0 wt. % and 6.0 wt. % combined Al and Ca including 0.10
to 1.0 wt. % Ca and 0.9 wt. % to 5.0 wt. % Al; and a
Mg--Li--Al--Ca-RE alloy having between 3.0 wt. % and 7.0 wt. % Li,
between 1.0 wt. % and 6.0 wt. % combined Al and Ca including 0.10
to 1.0 wt. % Ca and 0.9 wt. % to 5.0 wt. % A, and between 0.25 wt.
% and 7.0 wt. % RE, wherein "RE" is at least one rare earth
element.
[0023] In one aspect, the magnesium-based alloy the bimetal
composite wire has an ultimate tensile strength reaching 334
MPa.
[0024] In another aspect, the other of said first and second
biodegradable materials is selected from the group consisting of
pure metallic iron (Fe) and an iron-based alloy (Fe alloy).
[0025] In yet another aspect, the outer diameter of the outer shell
is less than 1 mm.
[0026] In still another aspect, the bimetal composite wire may be
formed into or included as part of a stent structure.
[0027] In yet another embodiment thereof, the present disclosure
provides a method of manufacturing a wire, including providing an
outer shell made of a first biodegradable material; inserting a
core into the outer shell to form a wire construct, the core formed
of a second biodegradable material, the first and second
biodegradable materials being different from one another, one of
the first and second biodegradable materials comprising a
magnesium-based alloy selected from the group consisting of: a
Mg--Li--Ca alloy having between 3.0 wt. % and 7.0 wt. % Li and
between 0.10 wt. % and 1.0 wt. % Ca; a Mg--Li--Ca-RE alloy having
between 3.0 wt. % and 7.0 wt. % Li, between 0.10 wt. % and 1.0 wt.
% Ca, and between 0.25 wt. % and 7.0 wt. % RE, wherein "RE" is at
least one rare earth element; a Mg--Li--Ca--Al alloy having between
3.0 wt. % and 7.0 wt. % Li and between 1.0 wt. % and 6.0 wt. %
combined Al and Ca including 0.10 to 1.0 wt. % Ca and 0.9 wt. % to
5.0 wt. % Al; and a Mg--Li--Al--Ca-RE alloy having between 3.0 wt.
% and 7.0 wt. % Li, between 1.0 wt. % and 6.0 wt. % combined Al and
Ca including 0.10 to 1.0 wt. % Ca and 0.9 wt. % to 5.0 wt. % A, and
between 0.25 wt. % and 7.0 wt. % RE, wherein "RE" is at least one
rare earth element.
[0028] In one aspect, cold work may be imparted to the wire
construct at room temperature by drawing the wire construct from a
first outer diameter to a second outer diameter less than the first
outer diameter. The wire construct may then be annealed.
[0029] In another aspect, the method may include forming the
bimetal composite wire into a stent.
[0030] In yet another aspect, the other of said first and second
biodegradable materials may be selected from the group consisting
of pure metallic iron (Fe) and an iron-based alloy (Fe alloy).
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] The above mentioned and other features and objects of this
invention, and the manner of attaining them, will become more
apparent and the invention itself will be better understood by
reference to the following description of embodiments of the
invention taken in conjunction with the accompanying drawings,
wherein:
[0032] FIG. 1A is a perspective view of a braided stent;
[0033] FIG. 1B is a perspective view of a knitted stent;
[0034] FIG. 2 is a partial cross-sectional view of a composite wire
made in accordance with the present disclosure;
[0035] FIG. 3 is a schematic view illustrating an exemplary forming
process of the composite wire of FIG. 2, using a lubricated drawing
die;
[0036] FIG. 4a is an elevation, cross-sectional view of a wire made
from a solid, monolithic material .alpha. having diameter D.sub.W
and radius R.sub.W;
[0037] FIG. 4b is an elevation, cross-sectional view of a composite
wire made in accordance with the present disclosure, in which the
wire defines diameter D.sub.W and includes a core fiber made from a
first material .beta. and a shell surrounding the core fiber and
made from a second material .alpha., in which the thickness T.sub.1
of the shell creates a surface area occupying 75% of the total
cross-sectional area of the wire (.beta.-25 v/v % .alpha.);
[0038] FIG. 4c is an elevation, cross-sectional view of a composite
wire made in accordance with the present disclosure, in which the
wire defines diameter D.sub.W and includes a core fiber made from a
first material .beta. and a shell surrounding the core fiber and
made from a second material .alpha., in which the thickness T.sub.1
of the shell creates a surface area occupying 43% of the total
cross-sectional area of the wire (.beta.-57 v/v % .alpha.);
[0039] FIG. 4d is an elevation view illustrating the geometry of a
braided stent having diameter D.sub.S, the stent comprising 24 wire
elements formed into a mesh tubular scaffold, in accordance with
the present disclosure;
[0040] FIG. 5a is a graph illustrating tensile test results for
sample materials, including engineering stress-strain plots for
four Mg--Li alloy wire materials made in accordance with the
present disclosure; and
[0041] FIG. 5b is a graph illustrating computed tensile stress,
including ultimate tensile strength and yield strength, for each of
four Mg--Li alloy wire materials made in accordance with the
present disclosure.
[0042] Corresponding reference characters indicate corresponding
parts throughout the several views. Although the exemplifications
set out herein illustrate embodiments of the invention, the
embodiments disclosed below are not intended to be exhaustive or to
be construed as limiting the scope of the invention to the precise
form disclosed.
DETAILED DESCRIPTION
[0043] The present disclosure provides bioabsorbable wires
including a magnesium-lithium-calcium (Mg--Li--Ca) alloy material
which, when used to create a wire-based stent, produce dilatational
force sufficient to promote arterial remodeling and patency, while
also being capable of fully biodegrading over a specified period of
time. This controlled biodegradation promotes endothelial
vasoreactivity, improved long term hemodynamics and wall shear
stress conditions, enablement of reintervention and accommodation
of somatic growth, and mitigates fracture risk over the long term.
In one particular exemplary material, lithium content of the
magnesium-based alloy may be as little as 3.0 wt. %, 3.5 wt. %, 4.0
wt. %, 4.5 wt. %, 5.0 wt. %, 5.2 wt. % or 5.4 wt. %, and as much as
5.6 wt. %, 5.8 wt. %, 5.9 wt. %, 6.0 wt. %, 6.2 wt. %, 6.4 wt. %,
6.6 wt. %, 6.8 wt. % or 7.0 wt. %, or may be any percentage within
any range defined by any of the foregoing values. The
magnesium-based alloy also includes calcium in an amount up to 1
wt. %, and may further include up to 4 wt. % Al, and/or up to 7 wt.
% RE, where "RE" is rare earth metals as described herein.
TERMINOLOGY
[0044] As used herein, "biodegradable," "bioabsorbable" and
"bioresorbable" all refer to a material that is able to be
chemically broken down in a physiological environment, i.e., within
the body or inside body tissue, such as by biological processes
including resorption and absorption. This process of chemical
breakdown will generally result in the complete degradation of the
material and/or appliance within a period of weeks to months, such
as 18 months or less, 24 months or less, or 36 months or less, for
example. Biodegradable metals used herein include nutrient metals,
i.e., metals such as iron, magnesium, manganese and alloys thereof,
such as alloys including lithium as described in detail below.
Nutrient metals and metal alloys are those which have biological
utility in mammalian bodies and are used by, or taken up in,
biological pathways.
[0045] By contrast, "non-biodegradable" materials are materials
which cannot be broken down and eliminated from the body by normal
biological processes. While non-biodegradable materials may
experience some corrosion in vivo, their rate of corrosion stands
in contrast to biodegradable materials discussed above.
Specifically, non-biodegradable materials are degradation resistant
and may be considered "permanent" when used for medical devices.
Example non-biodegradable materials include nickel-titanium alloys
("Ni--Ti") and stainless steel, which remain in the body,
structurally intact, for a period exceeding at least 36 months and
potentially throughout the lifespan of the recipient.
[0046] The present Mg--Li--Ca alloys primarily include
biodegradable elements, with the potential addition of aluminum
(Al) and rare-earth metals (RE) which are biocompatible but would
be non-biodegradable on their own. Al and RE are rendered into a
biodegradable form by virtue of the overall chemical structure of
the present Mg--Li--Ca--(Al)-(RE) materials discussed in detail
below. For purposes of the present disclosure, "biocompatible"
refers to materials which, in the amounts specified below, will not
cause toxicity or other adverse biological effects when implanted
within a mammalian body as part of a medical device. By contrast,
non-biocompatible materials are those materials which are known to
cause harm when introduced in mammalian bodies in larger than trace
amounts. For purposes of the present disclosure, example
non-biocompatible metals include lead (Pb) and cadmium (Cd). The
present Mg--Li--Ca alloys include only biocompatible materials, and
do not include non-biocompatible materials beyond trace impurities
as further described below.
[0047] As used herein, "wire" or "wire product" encompasses
continuous wire and wire products which may be continuously
produced and wound onto a spool for later dispensation and use,
such as wire having a round cross section and wire having a
non-round cross section, including flat wire or ribbon. "Wire" or
"wire product" also encompasses other wire-based products such as
strands, cables, coil, and tubing, which may be produced at a
particular length depending on a particular application. In some
exemplary embodiments, a wire or wire product in accordance with
the present disclosure may have a diameter up to 2.5 mm. In
addition to wire and wire products, the principles of the present
disclosure can be used to manufacture other material forms such as
rod materials having a diameter greater than 2.5 mm up to 20 mm.
Thin material sheets may also be made. Exemplary tubing structures
may be in wire form or rod form, with inside diameters ranging from
0.5 mm to 4.0 mm, and wall thicknesses ranging from 0.100 mm to
1.00 mm.
[0048] As used herein, "fine wire" is a wire having a diameter
between 20 .mu.m and 1 mm. Where the wire does not have a circular
cross-section (e.g., a flat ribbon wire construct or a wire
construct with a polygonal cross-section), the diameter of the wire
is considered to be the diameter of the smallest circle that may be
circumscribed around the wire construct.
[0049] As used herein, "fatigue strength" refers to the load level
at which the material meets or exceeds a given number of load
cycles to failure. Herein, the load level is given as alternating
strain, as is standard for displacement or strain-controlled
fatigue testing, whereby terms are in agreement with those given in
ASTM E606, the entirety of which is incorporated herein by
reference.
[0050] As used herein, a "load cycle" is one complete cycle wherein
an unloaded (neutral) material is 1) loaded in tension to a given
level of alternating stress or strain, 2) unloaded, 3) loaded again
in compression to the same level of alternating stress or strain,
and 4) returned to the neutral, externally unloaded position.
[0051] As used herein, "alternating strain" refers to the
difference between the mean strain and the minimum strain level or
the difference between the maximum strain and the mean strain in a
strain-controlled fatigue cycle, where units are non-dimensional
and given as percent engineering strain.
[0052] As used herein, "engineering strain" is given
non-dimensionally as the quotient where the differential length
associated with the load is the dividend and original length the
divisor.
[0053] As used herein, "resilience" refers to an approximate
quantification of the uniaxial elastic strain capability of a given
wire test sample, and is calculated as the quotient of yield
strength and modulus of elasticity, wherein yield strength is the
dividend and modulus the divisor. Units are non-dimensional.
[0054] As used herein, "elastic modulus" is defined as Young's
modulus of elasticity and is calculated from the linear portion of
the tensile, monotonic, stress-strain load curve using linear
extrapolation via least squares regression, in accordance with ASTM
E111, the entirety of which is incorporated herein by reference.
Units are stress, in gigapascals (GPa).
[0055] As used herein, "yield strength" or "YS" refers to the 0.2%
offset yield strength calculated from the stress-strain curve in
accordance with ASTM E8, the entirety of which is incorporated
herein by reference. Yield strength gives a quantitative indication
of the point at which a material begins to plastically deform.
Units are stress, in mega-Pascals (MPa).
[0056] As used herein, "ultimate strength" or "UTS" refers to the
maximum engineering stress required to overcome in order to rupture
the material during uniaxial, monotonic load application in
accordance with ASTM E8, the entirety of which is incorporated
herein by reference. Units are stress, in mega-Pascals (MPa).
[0057] As used herein, "elongation" is the total amount of strain
imparted to a wire during a uniaxial, monotonic tensile test, en
route to specimen rupture, and is defined herein in accordance with
ASTM E8, the entirety of which is incorporated herein by reference.
Units are non-dimensional, and are given as a percentage strain
relative to the original specimen length.
[0058] As used herein, "magnesium ZM21" refers to magnesium ZM21
alloy, otherwise known as ZM-21 or simply ZM21 alloy, which is a
medium-strength forged Magnesium alloy comprising 2 wt. % Zn, 1 wt.
% Mn and a balance of Mg.
[0059] As used herein, "RE" refers to the rare earth elements given
in the periodic table of elements and including elements such as
Scandium, Yttrium, and the fifteen lanthanides, i.e. La, Ce, Pr,
Nd, Pm, Sm, Eu, Gd, Tb, Dy, . . . , to Lu.
[0060] "Nitinol" is a trade name for a shape memory alloy
comprising approximately 50 atomic % Nickel and balance Titanium,
also known as NiTi, commonly used in the medical device industry
for highly elastic implants.
[0061] "DFT.RTM." is a registered trademark of Fort Wayne Metals
Research Products Corp. of Fort Wayne, Ind., and refers to a
bimetal or poly-metal composite wire product including two or more
concentric layers of metals or alloys, typically at least one outer
layer disposed over a core filament formed by drawing a tube or
multiple tube layers over a solid metallic wire core element.
Product Construction
[0062] Material made in accordance with the present disclosure may
be formed into wire products, such as fine-grade wire having an
overall diameter D.sub.W (FIGS. 4a-4d) of less than 1 mm. In one
embodiment, a monolithic wire 31 (FIG. 4a) made of a biodegradable
Mg--Li--Ca material in accordance with the present disclosure may
have a uniform size and cross-sectional geometry along its axial
length, such as the round cross-sectional shape having outer
diameter D.sub.W as depicted. In another embodiment, a bimetallic
composite wire 30 may be formed with separate core component 34 and
shell component 32, with at least one of the components made of a
Mg--Li--Ca alloy as further described below.
[0063] Although round cross-sectional wire forms are shown in FIGS.
4a-4d and described further below, it is contemplated that
non-round wire forms may also be produced using the materials
disclosed herein. For example, ribbon materials having rectangular
cross-sectional shapes may be produced. Other exemplary forms
include other polygonal cross-sectional shapes include square
cross-sectional shapes.
[0064] Still other products formed from the present materials may
not be in wire form. Some such products include sheets or plates
(e.g., of a size used for mending various long bones or other bony
structures, including skulls), prosthetic structures such as
intramedullary rods or stems, structural materials such as rods,
springs and the like, and other products in which a ductile and/or
high-strength magnesium alloy may be desirable.
[0065] 1. Magnesium Alloyed with Lithium and Calcium
(Mg--Li--Ca)
[0066] Regardless of the product shape or size, at least a portion
of a product made in accordance with the present disclosure
includes or is made of a biodegradable Mg--Li--Ca alloy suitable
for in vivo use. The inclusion of lithium in the present alloy
promotes the formation of the body-centered-cubic beta phase to
enhance the ductility of magnesium alloy. The addition of lithium
in amounts between 3 wt. % and 7 wt. % balances the need for
particular mechanical properties in implantable devices, while
maintaining an acceptable biodegradation rate and lithium exposure
rate within the patient. Lithium content of the present Mg--Li--Ca
alloy may be may be as little as 3.0 wt. %, 3.5 wt. %, 4.0 wt. %,
4.5 wt. %, 5.0 wt. %, 5.2 wt. % or 5.4 wt. %, and as much as 5.6
wt. %, 5.8 wt. %, 5.9 wt. %, 6.0 wt. %, 6.2 wt. %, 6.4 wt. %, 6.6
wt. %, 6.8 wt. % or 7.0 wt. %, or may be any percentage within any
range defined by any of the foregoing values.
[0067] Generally speaking, it is understood that the crystal
structure of the present Mg--Li--Ca alloy, if induced to change
from hexagonal close packed (HCP) phase to a body-centered cubic
(BCC) phase, will experience an increase in ductility. Further, it
is understood that in alloy materials containing between 6 and 10.5
wt. % Li, the material will contain both HCP and BCC phases, and in
alloy materials containing more than 10.5 wt. % Li, the material
will be entirely in the BCC phase.
[0068] On the other hand, lithium levels sufficient to cause BCC
phase to appear in Mg--Li--Ca alloys also contribute to increased
cost, reduced material strength, increased corrosion rate in vivo
and increased reactivity with surrounding elements.
[0069] In the present Mg--Li--Ca alloy, it has been found that an
increase in ductility as compared to magnesium alone can be
achieved with as little as 3 wt. % Li. Moreover, as set forth in
greater detail below, this increase is sufficient to provide
enhanced workability of the alloy, thereby increasing the potential
level of cold-work-induced strengthening (which may be accomplished
by, e.g., swaging, rolling, or wire drawing). In exemplary
embodiments of Mg--Li--Ca alloys described herein, lithium is
provided in a sufficient amount to enhance material ductility by
reducing the c/a lattice parameter ratio of the
hexagonal-close-packed crystal lattice. The c/a ratio relates to
the relative distance between atoms within the stacking plane
(basal, "a") and distance between atoms across stacking planes
(prismatic, "c"). A lower c/a ratio indicates reduced crystal
anisotropy, facilitates additional modes of slip, and contributes
to the observed increase in ductility. In some exemplary
embodiments of Mg--Li--Ca alloys described herein, the provision of
lithium between 3% and 7% may also induce a secondary
body-centered-cubic crystal structure, further enhancing
ductility.
[0070] At the same time, Mg composited with less than 7 wt. % Li
maintains a very low overall reactivity with surrounding elements,
thereby rendering the resulting material suitable for use in
long-term, in vivo, and/or harsh-environment applications.
[0071] Calcium is the third base constituent of the present
Mg--Li--Ca, and benefits the material with increased strength
and/or a slowed rate of corrosion. For example, slower in vivo
degradation rates offered by the addition of Ca to the present
Mg--Li--Ca alloys may be beneficial in stents used to treat medical
conditions with a relatively long recovery time. More generally,
the slowed corrosion rate of the present Mg--Li--Ca alloys may also
confer benefits to medical devices made from, or including, thin
wire (e.g., wire having a diameter less than 100 .mu.m). Thin
materials may require a relatively slow degradation rate to
maintain a desired in vivo service life, which can be influenced by
the Ca content. In still other applications, an increase in
mechanical strength may be gained by including Ca in the present
Mg--Li--Ca alloys to impart additional mechanical force within the
body (e.g., by allowing an expanded stent to impart a greater
radial force on an adjacent vessel wall). Moreover,
application-specific tuning of the corrosion rate may be possible
by varying Li and Ca contents of the present Mg--Li--Ca alloys.
[0072] In addition, calcium, which is a nutrient metal, can be
included in the present Mg--Li--Ca alloys to improve the overall
biocompatibility profile of the alloy.
[0073] Increasing levels of calcium in the present Mg--Li--Ca
alloy, from zero to 1.0 wt. %, contributes to corresponding
increases in strength and corrosion resistance and may be varied
within this range as needed for a particular application. In the
present Mg--Li--Ca alloys, the addition of calcium may be as little
as 0.1 wt. %, 0.2 wt. % or 0.25 wt. %, or as much as to 0.5 wt. %,
0.75 wt. % or 1 wt. %, or may be any percentage within any range
defined by any of the foregoing values.
[0074] Maintaining calcium levels at or below 1 wt. % Ca avoids
material brittleness and preserves the ability of the resulting
alloy material to be cold worked (as further described below).
Moreover, it is noted that the solubility level of Ca in Mg is 1.34
wt. %, and Mg alloys made with calcium amounts exceeding this level
may become even more brittle due to the creation of Mg.sub.2Ca
secondary phases.
[0075] In one particular exemplary embodiment, the present
Mg--Li--Ca--(Al)-(RE) alloy includes 0.25 wt. % Ca, which has been
found to provide an increase in ultimate tensile strength over the
present binary Mg--Li--Ca alloy, enhance the amount of cold work
(which may alternatively be expressed as true strain, as noted
below) that the material may undergo without fracture, and increase
the material's ductility at the high levels of cold deformation
(quantified by the material's ability to elongate, as a percentage
of original length, prior to fracture). For example, Table 1 shows
a comparison of binary Mg--Li--Ca material having 6 wt. % Li and
balance Mg (Mg-6Li), tertiary Mg--Li--Ca material having 6 wt. %
Li, 0.25 wt. % Ca and balance Mg (Mg-6Li-0.25Ca), and tertiary
Mg--Li--Ca material having 6 wt. % Li, 1 wt. % Ca and balance Mg
(Mg-6Li-1Ca). As illustrated, all three alloys showed relatively
high levels of cold workability, strength and ductility, with
Mg-6Li-0.25Ca superior to the other two materials on all counts.
Mg-6Li-1Ca demonstrated lower cold workability, strength and
ductility as compared to both the Mg-6Li-0.25Ca tertiary and Mg-6Li
binary alloys.
TABLE-US-00001 TABLE 1 Calcium addition to Mg--Li alloys influences
ductility and strength. True Ultimate Tensile Elongation Alloy (wt.
%) Strain Strength (MPa) (%) Mg--6Li 2.8 305 3 Mg--6Li--0.25Ca 3.9
333 7.9 Mg--6Li--1Ca 1.2 270 1.6
[0076] 2. Addition of Aluminum (Al) to the Present Mg--Li--Ca
Alloys
[0077] Aluminum may be a constituent of the present Mg--Li--Ca
alloy (i.e., Mg--Li--Ca--Al) for certain applications where a
substantial strength increase and/or slowed rate of corrosion are
required or desired. In particular, it is contemplated that for
some in vivo applications, a potential reduction in corrosion
resistance caused by the lithium of the present Mg--Li--Ca alloys
can be mitigated by an addition of aluminum up to about 5.0 wt.
%.
[0078] Increasing levels of aluminum in the present Mg--Li--Ca--Al
alloy, from zero to 5.0 wt. %, contributes to corresponding
increases in strength and corrosion resistance and may be varied
within this range as needed for a particular application. In the
present Mg--Li--Ca alloys, the addition of aluminum may be as
little as 0.9 wt. %, 2 wt. % or 3.5 wt. %, or as much as 4 wt. %,
4.5 wt. % or 5 wt. %, or may be any percentage within any range
defined by any of the foregoing values. Slowed corrosion rates and
increased strength are desirable in some in vivo applications as
noted above with respect to calcium. Aluminum up to about 1.0 wt. %
can be used in lieu of, or in addition to, calcium in the present
Mg--Li--Ca alloys in order to achieve a desired mechanical strength
and/or rate of corrosion for a particular in vivo application.
[0079] Maintaining aluminum levels at or below 5 wt. % in the
present Mg--Li--Ca--Al alloys alleviates biocompatibility concerns.
In particular, it is noted that excessive levels of aluminum in
implanted medical devices may be undesirable. In the present
Mg--Li--Ca alloys, aluminum is provided at a relatively low level
to avoid undesirable levels while realizing the benefits set forth
above.
[0080] 3. Addition of Rare Earth Elements (RE) to the Present
Mg--Li--Ca Alloys
[0081] Rare Earth (RE) elements, consisting of the lanthanides
groups, may be employed in the present Mg--Li--Ca alloys to impart
grain refinement and dispersion strengthening, and thereby benefit
material strength and corrosion resistance.
[0082] Increasing levels of RE in the present Mg--Li--Ca alloy,
from zero to 7.0 wt. %, contributes to corresponding increases in
strength and corrosion resistance and may be varied within this
range as needed for a particular application. In an exemplary
embodiment, additions of RE elements may be used in amounts as
little as 0.25 wt. %, 0.5 wt. % or 1.0 wt. %, and as much as 3.0
wt. %, 5.0 wt. % or 7.0 wt. %, or may be any percentage within any
range defined by any of the foregoing values.
[0083] Maintaining RE levels at or below 7.0 wt. % in the present
Mg--Li--Ca alloys alleviates biocompatibility concerns. In
particular, it is noted that excessive levels of certain RE
elements (e.g., cerium, praseodymium, and yttrium) in implanted
medical devices may be undesirable. In the present Mg--Li--Ca
alloys, RE elements are provided at a relatively low level while
realizing the benefits set forth above.
[0084] 5. Addition of Zinc (Zn) to the Present Mg--Li--Ca
Alloys
[0085] Zn may be alloyed with the present Mg--Li--Ca materials to
improve the strength of the alloy by either solid-solution
strengthening or precipitation hardening. Adding Zn also improves
the corrosion resistance of the present Mg--Li--Ca by overcoming
otherwise detrimental effects of certain impurities, including Fe
and Ni. Zn is also a nutrient metal, mitigating or eliminating any
biocompatibility concerns.
[0086] In the present Mg--Li--Ca materials, Zn content is provided
in amounts of at least 0.10 wt. % but less than 6 wt. %, which is
the maximum amount used within the limits of metallurgical value of
Zn additions, as noted above.
[0087] 6. Addition of Manganese (Mn) to the Present Mg--Li--Ca
Alloys
[0088] Mn may be alloyed with the present Mg--Li--Ca materials to
improve corrosion resistance by overcoming otherwise detrimental
effects of certain impurities, including Fe and Ni. Mn is also a
nutrient metal, mitigating or eliminating any biocompatibility
concerns.
[0089] In the present Mg--Li--Ca materials, Mn is provided in
amounts of at least 0.10 wt. % but less than 1 wt. %, which is its
solubility limit.
[0090] 7. Addition of Zirconium (Zr) to the Present Mg--Li--Ca
Alloys
[0091] Zr may be alloyed with the present Mg--Li--Ca materials to
refine the grain size of the material. The resulting refined grain
size can be used to improve strength and ductility, reduce
corrosion rate, and improve fatigue performance as required or
desired for a particular application. In exemplary embodiments, Zr
is not alloyed with the present Mg--Li--Ca materials if they also
contain Al or Mn, in order to avoid the formation of secondary
phases. However, Zr may be used in the present Mg--Li--Ca tertiary
materials, as well as those also containing Zn or RE. In the
present Mg--Li--Ca materials, Zr is provided in amounts of at least
0.10 wt. % but less than 1 wt. %.
[0092] For purposes of the present disclosure, reference to "the
present Mg--Li--Ca alloys" refers to alloys including magnesium,
lithium and calcium in the amounts discussed above, as well as
alloys including one or more of aluminum and RE in any combination,
in the respective amounts discussed above, with balance magnesium.
Thus, the present Mg--Li--Ca alloys can be expressed as
Mg--Li--Ca--(Al)-(RE), where parens indicate the optional status of
Al and RE.
[0093] Exemplary Mg--Li--Ca alloys in accordance with the present
disclosure include the following and exclude all other elements not
listed, except for trace impurities (e.g., any amount less than 500
parts per million or 0.05 wt. %). [0094] A Mg--Li--Ca alloy having
between 3.0 wt. % and 7.0 wt. % Li as described above, and between
0.10 wt. % and 1.0 wt. % Ca as described above. In one exemplary
embodiment, the Mg--Li--Ca alloy is 93.75 wt. % Mg--6 wt. %
Li--0.25 wt. % Ca or 93 wt. % Mg--6 wt. % Li--1 wt. % Ca. [0095] A
Mg--Li--Ca-RE alloy having between 3.0 wt. % and 7.0 wt. % Li as
described above, between 0.10 wt. % and 1.0 wt. % Ca as described
above, and between 0.25 wt. % and 7.0 wt. % RE as described above.
[0096] A Mg--Li--Ca--Al alloy having between 3.0 wt. % and 7.0 wt.
% Li as described above, and between 1.0 wt. % and 6.0 wt. %
combined Al and Ca, including 0.10 wt. % to 1.0 wt. % Ca and 0.9
wt. % to 5 wt. % Al as described above. [0097] A Mg--Li--Ca--Al-RE
alloy having between 3.0 wt. % and 7.0 wt. % Li as described above,
between 1.0 wt. % and 6.0 wt. % combined Al and Ca including 0.10
wt. % to 1.0 wt. % Ca and 0.9 wt. % to 5 wt. % Al as described
above, and between 0.25 wt. % and 1.0 wt. % RE as described
above.
Wire Constructs Including Mg--Li--Ca--(Al)-(RE)
[0098] An alloy in accordance with the present disclosure may first
be formed in bulk, such by casting an ingot, continuous casting, or
thixomolding of the desired material.
[0099] This bulk material is then formed into a suitable pre-form
material (e.g., a rod, plate or hollow tube) by hot-working the
bulk material into the desired pre-form size and shape. For
example, the ingot may be melted using an arc-melting, cold
crucible technique in order to cast rod stock. The rod stock may
then be subjected to one or more iterations of warm or hot working,
such as forging or hot extrusion, in order to effect a large area
reduction (e.g., 8:1) resulting in an intermediate rod stock. For
purposes of the present disclosure, warm or hot working is
accomplished by heating the material to an elevated temperature
above room temperature and performing desired shaping and forming
operations while the material is maintained at the elevated
temperature. Full annealing may optionally be performed after hot
working to achieve equiaxed microstructure.
[0100] This intermediate rod stock may then be subjected to
conventional iterative cold working and annealing, as further
described below, to create an initial coarse wire structure ready
for final processing. Each iterative cold work process imparts cold
work which is stored in the material microstructure, as further
described herein, and this stored cold work is relieved by fully
annealing the material between draws, thereby enabling the next
iterative cold working process. In full annealing, the cold-worked
material is heated to a temperature sufficient to substantially
fully relieve the internal stresses stored in the material, thereby
relieving the stored cold work and "resetting" cold work to
zero.
[0101] In addition to a coarse wire material, other forms and
shapes may be produced by similar repetitive cold-forming and
annealing cycles. Constructs of potential use in a medical device
include rods, wires, tubes, sheets or plate products.
[0102] For the present Mg--Li--Ca materials, full annealing is
accomplished at a temperature about 350.degree. C. for at least 60
minutes. Alternatively, a full anneal can be accomplished with a
higher temperature, such as between 375.degree. C. and 400.degree.
C., for a shorter time, such as between 60 seconds and 30 minutes.
Of course, a relatively higher temperature annealing process can
utilize a relatively shorter time to achieve a full anneal, while a
relatively lower temperature will typically utilize a relatively
longer time to achieve a full anneal. In addition, annealing
parameters can be expected to vary for varying wire diameters, with
smaller diameters shortening the time of anneal for a given
temperature. Whether a full anneal has been accomplished can be
verified in a number of ways as well known in the art, such as
microstructural examinations using scanning electron microscopy
(SEM), mechanical testing for ductility, strength, elasticity,
etc., and other methods.
[0103] The resulting coarse wire material may then be finally
processed into a final form, such as a fine wire suitable for
integration into a stent or other medical device. Exemplary wire
constructs are described in further detail below.
[0104] 1. Monolithic Wires
[0105] FIG. 4a illustrates a cross-section of monolithic wire 31,
made entirely of a first material .alpha. having outer
cross-sectional diameter D.sub.W. Monolithic wire material 31 is
made of the present Mg--Li--Ca alloy, and may have finished
diameter D.sub.W and an axial length as required or desired for a
particular application. The production, characteristics and use of
wire 31 is described in detail below.
[0106] Starting with a coarse, initial wire construct as described
above, the present Mg--Li--Ca material may be formed into a final
wire construct by final drawing and/or annealing to produce a
monolithic wire ready to be used in vivo. For purposes of the
present disclosure, cold-working methods effect material
deformation at or near room temperature, e.g. 20-30.degree. C. The
total cold work imparted to monolithic wire 31 during a drawing
step can be characterized by the following formula (I):
cw = 1 - ( D 2 D 1 ) 2 .times. 100 % ( I ) ##EQU00001##
[0107] wherein "cw" is cold work defined by reduction of the
original material area, "D.sub.2" is the outer cross-sectional
diameter of the wire after the draw or draws, and "D.sub.1" is the
outer cross-sectional diameter of the wire prior to the same draw
or
[0108] True strain is an alternative expression of total imparted
cold work. True strain is calculated according to the following
formula (II):
ts = ln ( ( D 1 D 2 ) 2 ) ( II ) ##EQU00002##
wherein "ts" is cold work expressed as true strain, "ln" is the
natural logarithm operator, and D.sub.1 and D.sub.2 are the
diameter prior to cold work conditioning and after cold work
conditioning respectively. Further discussion of exemplary cold
work conditioning processes are presented in U.S. Patent
Application Publication No. 2010/0075168, entitled FATIGUE DAMAGE
RESISTANT WIRE AND METHOD OF PRODUCTION THEREOF, filed Sep. 18,
2009, and assigned to the present assignee, the disclosure of which
is hereby expressly incorporated by reference herein in its
entirety.
[0109] Referring to FIG. 3A, the cold work step may be performed by
the illustrated drawing process. Wire 31 is drawn through a
lubricated die 36 having an output diameter D.sub.2, which is less
than initial diameter D.sub.1 of wire 31 prior to the drawing step.
The outer diameter of wire 31 is accordingly reduced from
pre-drawing diameter D.sub.1 to drawn diameter D.sub.2, imparting
cold work cw and true strain is as set forth in equations (I) and
(II) above. draws. If the drawing step is the final step of
imparting cold work to create a finished wire, diameter D.sub.2 at
the output of die 36 equals diameter D.sub.W of monolithic wire 31
(FIG. 4a).
[0110] Although wire drawing is the illustrated method of
introducing cold work cw into the material of wire 31, it is
contemplated that cold work may be imparted by a number of other
processes within the scope of the present disclosure. For example,
net cold work may be accumulated in wire 31 by cold-swaging,
rolling the wire (e.g., into a flat ribbon or into other shapes),
extrusion, bending, flowforming, or pilgering. Cold work may also
be imparted by any combination of techniques including the
techniques described here, for example, cold-swaging followed by
drawing through a lubricated die finished by cold rolling into a
ribbon or sheet form or other shaped wire forms. In one exemplary
embodiment, the cold work step by which the diameter of wire 31 is
reduced from D.sub.1 to D.sub.2 is performed in a single draw and,
in another embodiment, the cold work step by which the diameter of
wire 31 is reduced from D.sub.1 to D.sub.2 is performed in multiple
draws (e.g., through lubricated dies having successively smaller
output diameters) which are performed sequentially without any
annealing step therebetween.
[0111] Thermal stress relieving, otherwise known in the art as
annealing, at a nominal temperature not exceeding the melting point
of either the first or second materials, may be used to improve the
ductility of wire 31 after cold work application. The softening
point of the present Mg--Li--Ca materials, and therefore their
behavior during an annealing process of a particular time and
temperature, can be controlled by introducing a particular amount
of cold work into the wire material. As noted above, deformation
energy is stored in the cold-worked structure as accumulated cold
work, and this energy serves to reduce the amount of thermal energy
required for stress relief of the wire material.
[0112] For certain exemplary Mg--Li--Ca materials, the
above-described cold work processing facilitates annealing of the
composite structure at temperatures in the range of 60 to 80% of
the melting point of the material, e.g., between about 200.degree.
C. and 400.degree. C. The time of annealing may be between 1 second
and 60 minutes, with lower temperatures resulting in longer
annealing times and higher temperatures resulting in shorter
annealing time, as described in detail above. Moreover, the
particular annealing time and temperature will also depend on the
specific wire material and wire size, as also noted above. The
annealing parameters may be chosen on a case-by-case basis to
provide the desired ductility and strength.
[0113] Post-cold-work annealing may be used for applications in
which a wire with annealed mechanical properties (i.e. low relative
tensile strength and high ductility) may be desired. In
applications where higher tensile strength is desired and a lower
relative ductility is acceptable, annealing after the cold work
processing step(s) may be omitted.
[0114] After production, the finished wire 31 may then be braided
into the shape of a stent such as that of FIG. 1A, knitted into the
shape of a stent such as that of FIG. 1B, or otherwise formed into
a medical device such as a vascular or gastric stent, aneurysm
clotting device, or blood filter, for example. For the foregoing
applications, wire 31 will typically be drawn to a final finish
diameter between 20 .mu.m and 250 .mu.m.
[0115] 2. Composite Wires
[0116] FIG. 2 illustrates composite wire 30, including shell 32
surrounding core 34.
[0117] Bimetallic composite wire 30 has a circular cross section
and extends along a longitudinal axis and includes outer shell,
sheath, or tube 32 made of a first biodegradable material and a
core 34 made of a second biodegradable material. Outer shell 32 may
be formed as a uniform and continuous surface or jacket, such that
wire 30 may be coiled, braided, or stranded as desired. Core 34
completely fills the bore formed through outer shell 32 such that
composite wire 30 forms a solid wire construct, but with two
different materials.
[0118] In an exemplary embodiment, at least one of the two
biodegradable materials used for composite wire 30 is formed from
the present Mg--Li--Ca alloy material. It is contemplated that
outer shell 32 and core 34 may be formed from the same material or
different materials, and that either shell 32 or core 34 may be
formed from any of the present Mg--Li--Ca materials as required or
desired for a particular application.
[0119] The other of the two biodegradable materials may be any of a
number of biodegradable materials in accordance with the present
disclosure. In one embodiment, one of the shell 32 or core 34 may
be iron-based, such as pure metallic iron (Fe), an
anti-ferromagnetic iron-manganese alloy (Fe--Mn) such as Fe-30Mn or
Fe-35Mn, or another iron-based alloy (Fe alloy). In another
embodiment, one of the shell 32 or core 34 may be magnesium-based,
such as pure magnesium (Mg) or a magnesium-based alloy (Mg alloy)
such as ZM21 (Mg-2Zn-1Mn), AE21 (Mg-2Al-1RE), AE42 (Mg-4Al-2RE),
WE43 (Mg-4Y-0.6Zr-3.4RE, as in yttrium, zirconium, RE). In another
embodiment, one of the shell 32 or core 34 may be a zinc-based.
[0120] For purposes of the present disclosure, bimetal composite
wire 30 can be expressed as a first material for shell 32 and a
second material for core 34, where the second material is specified
as comprising a specified balance percentage of the total wire
cross-sectional area. "DFT" is interposed between the two materials
to indicate that the material is "drawn filled tubing," i.e.,
composite wire 30 as shown in FIG. 2. For example, one composite
wire 30 made in accordance with the present disclosure may be
defined as Fe-DFT-25% MgLi, which is 75% iron and 25% of the
present Mg--Li--Ca materials.
[0121] FIGS. 4b and 4c illustrate variable relative proportions of
core 34 and shell 32 of composite wire 30. FIG. 4a shows a
monolithic wire material 31 made entirely of a first material
.alpha. having outer cross-sectional diameter D.sub.W. In an
exemplary embodiment, monolithic wire material 31 may be a
magnesium-lithium alloy such as the alloys described in Example 2
below. FIG. 4b shows a wire 30, such as a wire for a stent, in
which shell 32 is made of a first material .alpha. occupying 75% of
the total cross-sectional area of wire 30, while core 34 is formed
from a second material .beta. occupying the balance (25%) of the
cross-sectional area of wire 30. FIG. 4c shows a wire 30, such as a
wire for a stent, in which shell 32 is made of a first material
.alpha. occupying 43% of the total cross-sectional area of wire 30,
while core 34 is formed from a second material .beta. occupying the
balance (57%) of the cross-sectional area of wire 30. For Example
1, test materials in accordance with the present disclosure and
benchmark alloys including 316L stainless steel, MP35N.RTM. and
NiTi were procured as with outer diameters D.sub.W of 125 .mu.m.
Stent 40, made from wire 30 and/or 31, has outside diameter
D.sub.S, which may be about 7 mm. Stent 40 may be a tubular mesh
stent scaffold manufactured from wires 30, 31, or a combination of
wires 30 and 31. Exemplary such stents are available from
biomedical materials supplier, Fort Wayne Metals (Fort Wayne, Ind.,
USA). An exemplary strut thickness (i.e., wire diameter D.sub.W) of
127 .mu.m and expanded tubular diameter D.sub.S of 7 mm, as per
FIG. 3(d), are selected as dimensions similar to current
self-expanding stent designs which are used in peripheral vessel
scaffolding.
[0122] In one embodiment, the present Mg--Li--Ca materials may be
particularly useful as core 34 of composite wire 30 in order to
facilitate cold-work processing and its associated control over the
overall mechanical properties of wire 30. As noted above, magnesium
and its alloys typically comprise a hexagonal-close-packed (HCP)
crystal structure which possesses low ductility at room temperature
due to intrinsically limited slip systems, primarily confined to
the basal plane. Addition of Li to the base Mg material, however,
has been found to increase ductility and, therefore, cold
workability. At the same time, where iron or iron-alloy materials
are employed in composite wire 30, it is desirable to conduct wire
processing by cold-working methods in order to maximize the
mechanical properties of the iron or iron-alloy. Where such iron or
iron-alloy is used for shell 32 and the present Mg--Li--Ca material
is used for core 34, the iron or iron-alloy serves as a sheath to
confine the present Mg--Li--Ca material, thereby inducing a
compressive stress during cold work processing (e.g., wire drawing
as described below). The ductility of the present Mg--Li--Ca alloy
enables such processing techniques and therefore promotes
maximization of mechanical properties by obviating any need for
unwanted intermediate stress-relief (e.g., by annealing or
high-temperature processing as described herein) that might
otherwise be necessary to form composite wire 30 with a magnesium
alloy.
[0123] Further discussion of bimetal composite wires made from
biodegradable constituent materials are further described in U.S.
Patent Application Publication No. 2011/0319978, filed Jun. 24,
2011 and entitled BIODEGRADABLE COMPOSITE WIRE FOR MEDICAL DEVICES,
the entire disclosure of which is hereby expressly incorporated
herein by reference.
[0124] The selection of materials for shell 32 and core 34 will
inherently determine the absolute and relative biodegradation rates
of these materials, and may be chosen by one of ordinary skill in
the art in accordance with such considerations. For example, shell
32 may be formed of a relatively slower-biodegrading material and
core 34 may be formed of a relatively faster-biodegrading material.
In this arrangement, overall degradation will occur at a slow pace
until the relatively fast-degrading core 34 begins to be exposed.
At this stage, e.g. in the case of a wire construct 30 having an
iron or iron alloy outer shell 32 and a magnesium or magnesium
alloy core, an electrochemical potential will drive the more rapid
degradation of the core 34. In some designs, this intermediate
degradation point may leave behind a thin iron or iron alloy outer
shell which will possess reduced flexibility more similar to the
vascular wall, thereby permitting more natural vessel movement and
reactivity. Further, the remaining hollow outer shell 32 of iron or
iron-alloy will present additional surface area to fluid contact in
vivo, thereby causing the material to degrade more quickly than a
comparable monolithic iron or iron alloy wire.
[0125] In other embodiments, this arrangement may be reversed,
wherein shell 32 may be formed of a relatively faster-biodegrading
material and core 34 may be formed of a relatively
slower-biodegrading material. In this arrangement, the degradation
process is expected to consume outer shell 34 and leave an
intermediate and mostly continuous core 34. Similar to the
embodiment described above, this relatively thin core element will
provide improved flexibility, an increased rate of bioabsorption,
and a concomitantly improved vessel healing response with a reduced
risk of thrombosis, particle embolization, and restenosis compared
to a monolithic bioabsorbable wire.
[0126] For composite wires 30 incorporated into stents (e.g., as
shown in FIGS. 1A and 1B) or other in vivo structures, the first
biodegradable material (i.e., outer shell 32) may be chosen to
degrade in vivo at a slower rate than the second biodegradable
material (i.e., core 34), such that overall structural integrity
and strength are substantially maintained for a period of time
after initial implantation while the slower-degrading outer shell
32 bioabsorbs or bioresorbs. After the outer shell 32 erodes enough
to expose the core 34 to biodegradation by interaction with
substances in the in vivo environment, a relatively rapid
biodegradation occurs as noted above. This construction modality is
particularly useful where approximately equal amounts of the first
and second biodegradable materials are used, but any relative
proportions of the present Mg--Li--Ca materials and a second
material may be used as noted above.
[0127] Moreover, stents made from wire produced in accordance with
the present disclosure provide well-designed control over the
mechanics and pace of the overall degradation rate of the
constituent wires (and therefore, also of the stent structure
itself), thereby facilitating therapeutic optimization.
[0128] It is also contemplated that antiferromagnetic alloys of
iron and manganese may be used in either shell 32 or core 34 of
wire 30 for magnetic resonance imaging compatibility.
[0129] Optionally, shell 32 of the wire may be partially or fully
coated with a biodegradable polymer 35 (FIG. 2) that may be
drug-eluting to further inhibit neointimal proliferation and/or
restenosis. Suitable biodegradable polymers include poly-L lactic
acid (PLLA) and poly-L glycolic acid (PLGA), for example. The wire
may be coated either before, or after being formed into a
stent.
[0130] To form wire 30 (FIG. 2), core 34 is inserted within shell
32 to form a wire construct, and an end of the wire construct is
then tapered to facilitate placement of the end into a drawing die.
The end protruding through the drawing die is then gripped and
pulled through the die to reduce the diameter of the construct and
bring the materials of core 34 and shell 32 into physical contact.
After an initial draw, the inner diameter of the shell will close
on the outer diameter of the core such that the inner diameter of
the shell will equal the outer diameter of the core whereby, when
viewed in section, the inner core completely fills the outer
shell.
[0131] For example, as shown in FIG. 3B, wire 30 is drawn through a
lubricated die 36 having an output diameter D.sub.2S, which is less
than diameter D.sub.1S of wire 30 prior to the drawing step. The
outer diameter of wire 30 is accordingly reduced from pre-drawing
diameter D.sub.1S to drawn diameter D.sub.2S, imparting cold work
cw as described in detail above with respect to monolithic wire 31.
Drawing imparts cold work to the material of both shell 32 and core
34, with concomitant reduction in the cross-sectional area of both
materials. That is, each drawing step reduces the cross section of
wire 30 proportionately, such that the ratio of the sectional area
of core 34 to the overall sectional area of wire 30 is nominally
preserved as the overall sectional area of wire 30 is reduced.
Referring to FIG. 3, the ratio of pre-drawing core outer diameter
D.sub.1C to pre-drawings shell outer diameter D.sub.1S is the same
as the corresponding ratio post-drawing. Stated another way,
D.sub.1C/D.sub.1S=D.sub.2C/D.sub.2S. Further details regarding wire
drawing of composite wires are discussed in U.S. Patent Application
Serial No. 2009/0260852, filed Feb. 27, 2009, entitled "ALTERNATING
CORE COMPOSITE WIRE," the entire disclosure of which is
incorporated by reference herein. In an exemplary embodiment, the
finished wire 30 may be a fine wire, having a finished diameter
D.sub.2S of between 20 .mu.m and 1 mm. In another embodiment, wire
30 may have a finished diameter D.sub.2S up to 2.5 mm.
[0132] The fully dense (i.e., solid cross-section) composite wire
30 may be annealed after drawing, similar to wire 31 discussed
above.
Wire Properties
[0133] 1. Strength and Ductility
[0134] The yield strength of wires 30 and/or 31, and thus its
resilience, is influenced by the amount of strain-hardening
deformation applied to wires 30 and/or 31 to achieve the final
diameter D.sub.W, and by the thermal treatment applied after
drawing the wire (if any). The ability to vary the strength and
resilience of wires 30, 31 allows use of the wire in resilient
designs, such as for self-expanding stents, or for plastic-behaving
designs, such as for balloon-expanding stents. As described herein,
the present Mg--Li--Ca alloys are highly ductile, and therefore
tolerate large amounts of cold work before fracture. This ductility
and cold workability enable flexible design parameters for finished
Mg--Li--Ca alloy products, by facilitating selection from a wide
range of strength and resilience properties depending on the amount
of applied cold work.
[0135] Generally speaking, cold work processing of the present
Mg--Li--Ca materials may be used to increase stress to fracture,
with a corresponding decrease in overall strain to fracture for
many of the present Mg--Li--Ca alloy materials. Varying levels of
cold work may be applied in order to achieve varying levels of
material strength and ductility.
[0136] Strength is positively correlated with cold work/true
strain, as demonstrated below for various exemplary alloys. For
purposes of the present disclosure, the positive correlation of
strength and true strain can be assumed to be approximately linear
between the low and high levels given below.
[0137] As further described below, the present Mg--Li--Ca materials
have the ability to undergo large amounts of cold work without
fracture. This large capacity for cold work enables a wide range of
cold work strengthening options. After performing cold work, yield
strengths (YS) are improved, and are in excess of 200 MPa. Ultimate
tensile strengths (UTS) are at least about 270 MPa.
[0138] Mg--Li--Ca alloy material made in accordance with the
present disclosure, including a relatively small amount of Ca
(e.g., 0.25%) has sufficient ductility to allow cold work up to 98%
without fracture. This low-Ca Mg--Li--Ca material with 98% cold
work exhibits yield strength YS of 276 MPa and ultimate tensile
strength UTS of 334 MPa. Thus, the addition of a relatively small
amount of Ca to the binary Mg--Li material significantly increases
ductility while strength as compared to the binary Mg--Li
alloy.
[0139] Mg--Li--Ca alloy material including a larger amount of Ca
(e.g., 1.0%) has sufficient ductility to allow cold work up to 88%
without fracture. This higher-Ca Mg--Li--Ca material with 88% cold
work exhibits yield strength YS of 240 MPa and ultimate tensile
strength UTS of 271 MPa. Thus, the addition of a relatively larger
amount of Ca to the binary Mg--Li material increases strength while
somewhat decreasing ductility, as compared to the binary Mg--Li
alloy.
[0140] Thus, the present Mg--Li--Ca alloys have the ability, in
view of their cold workability, to be produced and modified without
utilizing elevated temperatures for such production.
Room-temperature or lower-temperature production of the finished
wire product represents a significant efficiency, particularly for
large-scale production, and therefore minimizes production
cost.
[0141] 2. Fatigue Endurance
[0142] Fatigue endurance of the present Mg--Li--Ca alloys can be
enhanced by imparting cold work to the material, as described in
detail above, and then performing a controlled anneal of the
material such that a refined, substantially equiaxed grain
structure is achieved.
[0143] This enhanced fatigue endurance, bioabsorbable wires and
stents made in accordance with the present disclosure can initially
withstand flexion of mobile vessels of the extremities, give
sufficient time for vessel remodeling, and then biodegrade. Thus,
the present wire is ideally suited for use in stents implanted in
high-flexion areas (i.e., extremities) and other demanding
applications.
[0144] 3. Medical Device Applications
[0145] In view of the foregoing material properties for wires made
of the present Mg--Li--Ca alloys, it can be seen that stents and
wires made in accordance with the present disclosure offer the
ability to optimize design to account for, e.g., anatomy, blood and
cell compatibility, long term endothelial functionality, fracture
resistance, and patient-specific rates of bioabsorption. Such
design optimization can be provided by, for example, cold work
conditioning, thermomechanical processing, and material selection,
and wire size and/or geometry and discussed above
[0146] For example, wires and stents made in accordance with the
present disclosure allow a surgeon to implant a naturally reactive
stent over a predetermined term and to plan for the stent to
completely biodegrade after the predetermined term. In this way,
use of the present wires and wire constructs can reduce or
eliminate late complications such as late-stent-thrombosis,
relative vessel occlusion and lifelong anti-platelet therapy. When
used in self-expanding, biocompatible, and biodegrading stent
designs the present wire can further extend this treatment option
to the challenging vasculature of the extremities.
[0147] Still another advantage of the present wire is the
opportunity to offer controllable degradation rates of stents to
allow patient-dependent time for vessel remodeling. As noted above,
patient-specific stent degradation rates also offer long-term
benefit by allowing unimpeded reintervention and natural long term
vasoreactivity.
Example
[0148] The following non-limiting Example illustrates various
features and characteristics of the present invention, which is not
to be construed as limited thereto.
[0149] In this Example, exemplary monolithic Mg--Li--Ca alloy wires
in accordance with the present disclosure were produced, tested and
characterized, particularly with regard to material workability and
mechanical strength.
[0150] 1. Production of Mg--Li--Ca Alloy Materials
[0151] Four alloys were selected, each having 6 wt. % Li, selected
additional alloying elements of Al, Ca and/or RE, and the balance
Mg. The composition of each of these alloys is set forth below in
Table 5. Ingots of each alloy were induction melted and extruded at
300.degree. C. to a diameter of approximately 4.5 mm. The extruded
ingots were then iteratively cold-drawn using standard methods, as
described in detail above, with intervening annealing at
350.degree. C.
[0152] The iterative draw-anneal cycles were repeated as necessary
until a wire was produced at a diameter of 0.9 mm, at which point a
final anneal was performed to create a stress-relieved base wire
ready for processing in accordance with the present disclosure.
[0153] Starting with respective samples of the 0.9 mm diameter
wire, cold work was imparted to the material by drawing in
accordance with the procedure described above. The amount of cold
work tolerated by each material is shown in Table 5.
[0154] Mechanical performance was then evaluated for each cold
worked sample via a uniaxial tensile test on an Instron Model 5565
test machine available from Instron or Norwood, Mass., USA). More
specifically, destructive uniaxial tension testing of the wire
materials was used to quantify the ultimate strength, yield
strength, axial stiffness and ductility of candidate materials,
using methods described in Structure-Property Relationships in
Conventional and Nanocrystalline NiTi Intermetallic Alloy Wire,
Journal of Materials Engineering and Performance 18, 582-587 (2009)
by Jeremy E. Schaffer, the entire disclosure of which is hereby
expressly incorporated herein by reference. These tests are run
using servo-controlled Instron load frames in accordance with
industry standards for the tension testing of metallic
materials.
[0155] A 127 mm gage length and crosshead speed of 12.7 mm/min was
used for the tensile testing.
TABLE-US-00002 TABLE 5 Exemplary monolithic Mg--Li wires Alloy
Composition by wt. % Cold Work (%) 1 94Mg--6Li 94 2
89.5Mg--6Li--4Al--0.5RE 75 3 93.75Mg--6Li--0.25Ca 98 4
93Mg--6Li--1Ca 88
[0156] 2. Characterization of Mechanical Properties in Tension
[0157] a. Strength
[0158] Alloy #1 (an Mg--Li base binary alloy, Table 5) exhibited
good ductility, achieving 94% cold work without fracture. Yield
strength YS, shown in FIG. 5B, was measured at 243 MPa while
ultimate tensile strength UTS was measured at 305 MPa.
[0159] Alloy #2 (Table 5), which was an Mg--Li alloy with Al and RE
additions, reduced the attainable cold work to 75%. Referring to
FIG. 5B, however, it can be seen that alloy #2 demonstrated a
dramatically improved strength as compared to alloy #1, as shown in
FIG. 5B. More particularly, alloy #2 had a yield strength YS of 455
MPa and an ultimate tensile strength UTS of 495 MPa.
[0160] For alloy #3 (Table 5), the Mg--Li base binary alloy was
used with the addition of 0.25% Ca. Formability was improved with
respect to alloy #1, with cold work increasing to 98%. In addition,
alloy #3 achieved a moderate gain in strength with a yield strength
YS of 276 MPa and an ultimate tensile strength UTS of 334 MPa.
[0161] In alloy #4 (Table 5), a higher level of Ca was composited
with the base Mg--Li binary alloy. The resulting wire material was
able to withstand less cold work, at 88%. Yield strength YS and
ultimate tensile strength UTS also both fell, to 240 MPa and 271
MPa respectively.
[0162] In all material samples tested in the present Example,
addition of cold work was enabled by the 6 wt. % Li constituent and
resulted in strengthening of the material. This effect was most
pronounced in Alloy 2, reaching a UTS of 495 MPa as illustrated in
FIG. 5B.
[0163] FIG. 5A is a plot of stress-strain data for individual wire
samples of alloy #2 prepared in accordance with the present
Example, and cold worked to various levels as specified in the
legend of FIG. 5A.
[0164] As illustrated, increasing cold work levels were associated
with increasing stress to fracture but generally decreasing overall
strain to fracture. With no cold work, testing an as-annealed wire,
engineering strain to rupture was measured at 10.6%, with an
ultimate tensile strength UTS of 289 MPa. With 20% cold work,
engineering strain to rupture dropped to 2.9% but ultimate tensile
strength UTS increased to 367 MPa. At 50% cold work, engineering
strain to rupture measured 4.1% while ultimate tensile strength UTS
further increased to 415 MPa. At 60% cold work, engineering strain
to rupture measured 3.0% while ultimate tensile strength UTS
increased to 474 MPa. At 64% cold work, engineering strain to
rupture fell again to 2.5% while ultimate tensile strength UTS rose
again to 487 MPa. At 75% cold work, the highest tested in this
Example, engineering strain to rupture reached a low point of 1.5%
while ultimate tensile strength UTS reached a high of 495 MPa.
[0165] The present Mg--Li--Ca alloys tested in this example
demonstrate the ability, in view of their cold workability, to be
produced without utilizing elevated temperatures for such
production. Room-temperature or lower-temperature production
represents a significant efficiency, particularly for large-scale
production, and therefore minimizes production cost. Thus, the high
levels of cold work tolerated by the Mg--Li--Ca alloy wires of the
present Example are amenable to an efficient, cost-effective
production method.
[0166] Excellent strength is an added benefit of cold working for
the present Mg--Li--Ca materials using including Li and Ca as
described herein. The UTS of the present Mg--Li--Ca alloys was high
in all cases, and in particular alloy 2 demonstrated a
significantly higher YS and UTS as compared to other Mg--Li
materials of comparable ductility.
[0167] The addition of as little 6 wt. % Li has been shown to
induce a cubic structure in the Mg--Li--Ca alloys tested in this
Example, rather than the predominantly HCP crystal structure
exhibited by other magnesium alloys. This cubic structure
dramatically improves the ductility of the material, enabling high
levels of cold work and obviating any need for hot-forming
processes. Accordingly, Mg--Li--Ca alloys in accordance with the
present example facilitate a decrease in processing costs and an
increase in cold-work-induced strengthening.
[0168] While this invention has been described as having an
exemplary design, the present invention can be further modified
within the spirit and scope of this disclosure. This application is
therefore intended to cover any variations, uses, or adaptations of
the invention using its general principles. Further, this
application is intended to cover such departures from the present
disclosure as come within known or customary practice in the art to
which this invention pertains and which fall within the limits of
the appended claims.
* * * * *