U.S. patent application number 12/540617 was filed with the patent office on 2010-04-22 for mixtures for forming porous constructs.
Invention is credited to Richard King, Hengda D. Liu, Bryan Smith.
Application Number | 20100098574 12/540617 |
Document ID | / |
Family ID | 41559467 |
Filed Date | 2010-04-22 |
United States Patent
Application |
20100098574 |
Kind Code |
A1 |
Liu; Hengda D. ; et
al. |
April 22, 2010 |
Mixtures For Forming Porous Constructs
Abstract
Provided are methods comprising at least one metal powder with
an extractable material and a composition comprising a polyol, a
hydrophilic polymer, or both in order to form a mixture in which
the metal powder and the extractable material assume respective
positions. The composition functions as a homogenizing agent that
allows the mixture to remain well-mixed for extended periods of
time under ambient conditions. Also provided are green bodies and
porous constructs, including implants, that are made in accordance
with the disclosed methods. The green bodies and porous constructs
have a substantially uniform porosity that is at least partially
attributable to the ability of the composition to maintain the
metal powder and the extractable material in their respective
positions prior to sintering.
Inventors: |
Liu; Hengda D.; (Warsaw,
IN) ; Smith; Bryan; (US) ; King; Richard;
(Warsaw, IN) |
Correspondence
Address: |
WOODCOCK WASHBURN LLP
CIRA CENTRE, 12TH FLOOR, 2929 ARCH STREET
PHILADELPHIA
PA
19104-2891
US
|
Family ID: |
41559467 |
Appl. No.: |
12/540617 |
Filed: |
August 13, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61092199 |
Aug 27, 2008 |
|
|
|
Current U.S.
Class: |
419/10 ; 419/62;
75/230; 75/252 |
Current CPC
Class: |
A61L 2400/18 20130101;
A61L 27/56 20130101; C22C 14/00 20130101; A61L 27/04 20130101; B22F
3/1121 20130101 |
Class at
Publication: |
419/10 ; 419/62;
75/230; 75/252 |
International
Class: |
B22F 3/12 20060101
B22F003/12; B22F 1/00 20060101 B22F001/00; B22F 3/02 20060101
B22F003/02 |
Claims
1. A method comprising combining at least one metal powder with an
extractable material and a composition comprising a polyol, a
hydrophilic polymer, or both, thereby forming a mixture in which
said metal powder and said extractable material assume respective
positions.
2. The method according to claim 1 wherein a subcombination of said
metal powder and said extractable material is formed prior to
combining said metal powder and said extractable material with said
composition.
3. The method according to claim 2 wherein said subcombination
comprises metal powder in an amount that is about 5 percent by
volume to about 45 percent by volume of said subcombination.
4. The method according to claim 2 wherein said subcombination is
formed as a substantially homogeneous blend of said metal powder
and said extractable material.
5. The method according to claim 1 further comprising subjecting
said mixture to one or more of blending, shaking, and stirring.
6. The method according to claim 1 wherein said composition
comprises a polyol in an amount that constitutes about 10 percent
by volume to about 70 percent by volume of said composition.
7. The method according to claim 1 wherein said polyol is glycerol,
sorbitol, mannitol, xylitol, inositol, polyol sucrose, lactitol,
maltitol, ethylene glycol, propane-1,2-diol, propane-1,3-diol,
butane-1,4-diol, butane-1,3-diol, butane-2,3-diol, butene-1,4-diol,
butine-1,4-diol, pentane-1,5-diol, an isomeric pentanediol,
pentenediol, or pentinediol, hexane-1,6-diol or an isomeric
hexanediol, hexenediol or hexinediol, heptane-1,7-diol, an isomeric
heptanediol, heptenediol or heptinediol, octane-1,8-diol, an
isomeric octanediol, octenediol, or octinediol, trimethylol
propane, pentaerythritol, or any combination thereof.
8. The method according to claim 1 wherein said polyol is
glycerol.
9. The method according to claim 1 wherein said composition
comprises a hydrophilic polymer in an amount that constitutes no
more than about 5 percent by volume of said composition.
10. The method according to claim 1 wherein said hydrophilic
polymer is a polyvinyl alcohol, polyvinyl pyrrolidone, polyethylene
oxide, a sodium salt of polyacrylic acid, a sodium salt of carboxyl
methyl cellulose, a sodium salt of alginic acid, a sodium salt of
hyaluronic acid, polyether polyol, polyether polyol modified with
hydrophilic vinyl monomer, polytetramethylene polyol, or any
combination thereof.
11. The method according to claim 1 wherein said metal powder
comprises titanium, a titanium alloy, a cobalt-chromium alloy,
aluminum, molybdenum, tantalum, magnesium, niobium, zirconium,
stainless steel, nickel, tungsten, or any combination thereof.
12. The method according to claim 1 wherein said extractable
material comprises ammonium bicarbonate, urea, biuret, melamine,
ammonium carbonate, naphthalene, sodium bicarbonate, sodium
chloride, ammonium chloride, calcium chloride, magnesium chloride,
aluminum chloride, potassium chloride, nickel chloride, zinc
chloride, ammonium bicarbonate, sodium hydrogen phosphate, sodium
dihydrogen phosphate, potassium dihydrogen phosphate, potassium
hydrogen phosphate, potassium hydrogen phosphite, potassium
phosphate, magnesium sulfate, potassium sulfate, alkaline earth
metal halides, crystalline carbohydrates, polyvinyl alcohol,
polyethylene oxide, polypropylene wax, sodium carboxymethyl
cellulose, or any combination thereof.
13. The method according to claim 1 further comprising: shaping
said mixture into a shaped object; and, compacting said shaped
object to form a green body.
14. A green body prepared in accordance with the method of claim
13.
15. The method according to claim 13 further comprising processing
said green body.
16. The method according to claim 13 further comprising heating
said green body for a time and under conditions effective to
evaporate at least some of said extractable material yet
substantially maintain said metal powder in its position in said
green body.
17. The method according to claim 13 further comprising exposing
said green body to a solvent in which said extractable material is
soluble.
18. The method according to claim 17 wherein said extractable
material is soluble in an aqueous solvent, an organic solvent, or
both.
19. The method according to claim 17 wherein said green body is
immersed in said solvent.
20. The method according to claim 17 wherein said composition is
soluble in said solvent.
21. The method according to claim 17 wherein the green body has a
total porosity of about 50% to about 95%.
22. The method according to claim 21 wherein the green body has a
substantially uniform porosity.
23. The method according to claim 22 wherein the porosity of said
green body varies by a standard deviation of less than about 3.
24. The method according to claim 22 wherein the porosity of said
green body varies by a standard deviation of less than about 2.
25. The method according to claim 16 further comprising sintering
said green body.
26. An implant made in accordance with the method of claim 25.
27. A mixture made in accordance with the method of claim 1.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims priority to U.S. Provisional
App. No. 61/092,199, filed Aug. 27, 2008, the entire contents of
which are hereby incorporated in their entirety.
TECHNICAL FIELD
[0002] The present invention pertains to, among other things, the
preparation of mixtures that can be used for the production of
porous metal constructs.
BACKGROUND
[0003] Porous metal constructs are widely used as, among other
things, orthopedic implants, supports for catalysts, bone growth
substrates, and filters. The "space holder" method is a well known
process for making metallic foam structures and employs dissolvable
or otherwise removable space-holding materials that are combined
with metallic powders and subsequently removed from the combination
by various methods, including heat evaporation or liquid
dissolution, leaving behind a porous matrix formed from the
metallic powder. The porous matrix material is then sintered to
further strengthen the matrix structure. Numerous variations on the
space holder concept are known in the art. See, e.g., U.S. Pat.
Nos. 3,852,045; 6,849,230; U.S. Pub. Nos. 2005/0249625;
2006/0002810.
[0004] Preferably, the mixture that results from the combination of
space-holding materials and metallic powder features an even
spatial distribution of the respective particle types. The
positioning of the space-holding materials determines where,
following processing to remove the space holder, pores will be
present; therefore, a homogeneous distribution of space-holding
particles will yield a construct having an even distribution of
pores. Structural uniformity can provide numerous benefits. For
example, orthopedic implants having uniform porosity are more
likely to possess mechanical stability and to permit homogeneous
ingrowth of bone and tissue, thereby leading to improved biological
fixation.
[0005] Water or organic solvents may be used in small, controlled
quantities to ensure a well-mixed combination of metallic powder
and space holder material. However, water and organic solvents can
gradually evaporate if the combination is stored for several days
without commencing additional processing steps, and such
evaporation can result in the segregation of the respective
constituents of the combination. Other methods are said to involve
the mixture of space holder material with liquid organics such as
alcohols, isoparafinic solvents, acetone, or polyethylene glycol,
followed by addition of metal powder in order to form mixtures that
may be used for the preparation of porous metal articles. See,
e.g., U.S. Pub. Nos. 2006/0002810, 2006/0228247. However, organic
solvents may be undesirably toxic, expensive, combustible, and/or
volatile, while the high viscosity of polyethylene glycol may make
mixing difficult. There exist other drawbacks to such methods that
justify a need for additional techniques for the preparation of
mixtures that can be used to form constructs having uniform or
otherwise controllable porosity.
SUMMARY OF THE INVENTION
[0006] One aspect of the present invention provides methods
comprising combining at least one metal powder with an extractable
material and a composition comprising a polyol, a hydrophilic
polymer, or both, thereby forming a mixture in which the metal
powder and the extractable material assume respective positions.
The present methods may further comprise shaping the mixture into a
shaped object and compacting the shaped object to form a green
body. The extractable material may be removed from the green body,
and the green body may be sintered to form a porous metal construct
for use as, for example, an implant. Also disclosed are green
bodies and implants that are formed in accordance with such
methods.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIGS. 1-3 illustrate the tendency of a mixture comprising
metal powder, space holder material, and reverse osmosis (RO) water
to segregate after storage for four days following initial
preparation of such mixture, as compared with the ability of
mixtures that are prepared in accordance with the present invention
to remain well-mixed up to and exceeding the same period of
time.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0008] The present invention may be understood more readily by
reference to the following detailed description taken in connection
with the accompanying figures and examples, which form a part of
this disclosure. It is to be understood that this invention is not
limited to the specific products, methods, conditions or parameters
described and/or shown herein, and that the terminology used herein
is for the purpose of describing particular embodiments by way of
example only and is not intended to be limiting of the claimed
invention.
[0009] In the present disclosure the singular forms "a," "an," and
"the" include the plural reference, and reference to a particular
numerical value includes at least that particular value, unless the
context clearly indicates otherwise. Thus, for example, a reference
to "a polyol" is a reference to one or more of such polyols and
equivalents thereof known to those skilled in the art, and so
forth. When values are expressed as approximations, by use of the
antecedent "about," it will be understood that the particular value
forms another embodiment. As used herein, "about X" (where X is a
numerical value) preferably refers to .+-.10% of the recited value,
inclusive. For example, the phrase "about 8" preferably refers to a
value of 7.2 to 8.8, inclusive; as another example, the phrase
"about 8%" preferably refers to a value of 7.2% to 8.8%, inclusive.
Where present, all ranges are inclusive and combinable. For
example, when a range of "1 to 5" is recited, the recited range
should be construed as including ranges "1 to 4", "1 to 3", "1-2",
"1-2 & 4-5", "1-3 & 5", and the like. In addition, when a
list of alternatives is positively provided, such listing can be
interpreted to mean that any of the alternatives may be excluded,
e.g., by a negative limitation in the claims. For example, when a
range of "1 to 5" is recited, the recited range may be construed as
including situations whereby any of 1, 2, 3, 4, or 5 are negatively
excluded; thus, a recitation of "1 to 5" may be construed as "1 and
3-5, but not 2", or simply "wherein 2 is not included."
[0010] The disclosures of each patent, patent application, and
publication cited or described in this document are hereby
incorporated herein by reference, in their entirety.
[0011] The present invention pertains to, among other things,
methods for forming mixtures that have and can retain a uniform
distribution of metallic particles relative to particles of
extractable material, as well as green bodies and sintered porous
metal constructs (including implants) that are produced in
accordance with such methods. In contrast with previously-existing
processes, the present methods may employ safe, biocompatible,
odorless, inexpensive, low volatility homogenizing agents and do
not otherwise involve potentially harmful or inefficient processing
steps. For example, the methods disclosed in U.S. Pub. Nos.
2006/0228247 involve the use of liquid organics such as alcohols,
isoparafinic solvents, acetone, or polyethylene glycol as
homogenizing agents; these agents are used to coat particles of
space holder material, to which metal powder is subsequently added.
The prescribed homogenizing agents are undesirably toxic,
combustible, volatile, and/or high viscosity, which can present
dangers or processing challenges during large-quantity product
manufacture. Certain organic agents are also likely to evaporate
relatively quickly from particle mixtures, thereby preventing
long-term storage prior to subsequent processing steps.
Furthermore, the step of addition of metal powder to the coated
particles of space holder material can present safety issues, as
the likelihood is high that metal particles will become airborne
during the addition step. Also, the combination of metal powder and
coated space holder material is said to be accomplished through the
use of a v-blender, on which residual deposits are likely to form;
the removal of such deposits may be difficult and will lower the
efficiency of the process. As described herein, the present
invention suffers from none of these drawbacks.
[0012] Provided are methods comprising combining at least one metal
powder with an extractable material and a composition comprising a
polyol, a hydrophilic polymer, or both, thereby forming a mixture
in which the metal powder and the extractable material assume
respective positions. The use of polyols and/or hydrophilic
polymers to prepare the present mixtures confers numerous
advantages. Such compounds are typically safe, biocompatible,
odorless, inexpensive, and are characterized by low volatility. As
demonstrated herein, the homogenizing agents of the present
invention allow the metal powder and the extractable material to
maintain their respective positions within the mixture for extended
periods of time prior to any additional processing steps.
Furthermore, unlike the prior art methods, the present methods need
not (but nonetheless may, if desired) comprise the step of first
combining a homogenizing agent with an extractable material,
followed by adding a metal powder to the combination of the
homogenizing agent and the extractable material, thereby reducing
the probability that fine metal powder will become airborne and
present a safety hazard during the manufacturing process. These and
other advantages will become readily apparent in the present
disclosure.
[0013] In one embodiment of the present methods, each of the metal
powder, extractable material, and composition comprising a polyol,
a hydrophilic polymer, or both may be mixed together in a single
step. In another embodiment, a subcombination of the metal powder
and extractable material may be formed prior to combining the metal
powder and the extractable material with the composition. The
subcombination may be formed as a substantially homogeneous blend
of the metal powder and the extractable material. As used herein, a
"substantially homogenous" mixture or blend of two particulate
materials is characterized by a mostly even spatial distribution of
the particles of the two materials relative to one another; when
the subcombination comprises a major component and a minor
component, the mixture may be a substantially uniform dispersion of
the particles comprising the minor component among the particles
comprising the major component. Those skilled in the art may
readily appreciate various methods for forming a substantially
homogenous mixture of particles, including one or more of blending,
shaking, and stirring. Other techniques for forming a mixture of
the metal powder and an extractable material will also be readily
appreciated. See, e.g., U.S. Pat. Nos. 3,852,045, 6,849,230; U.S.
Pub. Nos. 2005/0249625, 2006/0002810. The mixture of the metal
powder, extractable material, and composition comprising a polyol,
a hydrophilic polymer, or both may also be formed using blending,
shaking, stirring, or any other suitable technique. Preferably,
formation of the mixture results in a substantially homogenous
blend of the respective constituents. In one embodiment,
preparation of the subcombination and/or the mixture of the metal
powder, extractable material, and composition may be accomplished
by agitating the ingredients together in a closed container using a
conventional shaker. Unlike the prior art methods, which made use
of a v-blender, the use of a shaker in accordance with the present
invention is convenient and safe: a shaker is readily maintained
and cleaned in between batches, and a closed container precludes
the potentially dangerous dispersion of metal powder particles into
the ambient atmosphere.
[0014] The subcombination may comprise metal powder in an amount
that is about 5 percent by volume to about 45 percent by volume,
preferably about 15 percent by volume to about 40 percent by
volume, the balance of the subcombination comprising the
extractable material. Once the extractable material is removed from
the green body that is formed from the mixture of the metal powder,
extractable material, and composition in later stages of the
present methods, the resulting porosity of the green body may be
about 55% to about 95%, preferably about 60% to about 85%. The
removal of the extractable material is described more fully
infra.
[0015] The composition with which the metal powder and extractable
material are combined may comprise a polyol. The polyol may be
present in the composition in an amount that constitutes about 5
percent by volume to about 80 percent by volume, about 10 percent
by volume to about 70 percent by volume, about 20 percent by volume
to about 60 percent by volume, or about 25 percent by volume to
about 50 percent by volume. The balance of the composition may
comprise water. When the composition comprises one or more polyols,
the solution viscosity of the composition may be about 2 to about
20 centistokes at 20.degree. C. Polyols are well known among those
skilled in the art and constitute alcohols comprising two or more
hydroxyl groups. In accordance with the present invention, polyols
comprising three or more hydroxyl groups are preferred; for
example, glycerol represents a highly preferred polyol for use in
forming the present compositions. Other, nonlimiting examples of
polyols include sorbitol, mannitol, xylitol, inositol, polyol
sucrose, lactitol, maltitol, ethylene glycol, propane-1,2-diol,
propane-1,3-diol, butane-1,4-diol, butane-1,3-diol,
butane-2,3-diol, butene-1,4-diol, a pentanediol isomer,
pentenediol, pentinediol, a hexanediol isomer, hexenediol,
hexinediol, a heptanediol diol isomer, heptenediol, heptinediol, an
octanediol isomer, octenediol, octinediol, trimethylol propane,
pentaerythritol, or any combination thereof. Other suitable polyols
may be readily identified by those skilled in the art.
[0016] The composition with which the metal powder and extractable
material are combined may also or alternatively comprise a
hydrophilic polymer. The hydrophilic polymer may be present in the
composition in an amount that constitutes no more than about 5
percent by volume, no more than about 3 percent by volume, or no
more than about 2 percent by volume of said composition. The
balance of the composition may comprise water. When the composition
comprises one or more hydrophilic polymers, the solution viscosity
of may be about 5 to about 25 centistokes at 20.degree. C.
Hydrophilic polymers are well known among those skilled in the art.
Nonlimiting examples of suitable hydrophilic polymers include
polyvinyl alcohols, polyvinyl pyrrolidone, polyethylene oxide,
sodium salts of polyacrylic acid, sodium salts of carboxyl methyl
cellulose, sodium salts of alginic acid, sodium salts of hyaluronic
acid, polyether polyol, polyether polyol modified with hydrophilic
vinyl monomer, polytetramethylene polyol, or any combination
thereof.
[0017] When the composition comprising a polyol, a hydrophilic
polymer, or both comprises both one or more polyols and one or more
hydrophilic polymers, the total complement of polyol and the total
complement of hydrophilic polymer are preferably present in amounts
that correspond to the respective ranges recited above. The balance
of such compositions comprising both one or more polyols and one or
more hydrophilic polymers may comprise water.
[0018] The amount of composition comprising a polyol, a hydrophilic
polymer, or both in the mixture may be about 0.3% to about 1.5% by
volume. In preferred embodiments, the amount of composition
comprising a polyol, a hydrophilic polymer, or both in the mixture
may be about 0.6% to about 0.75% by volume, the balance of the
mixture comprising the metal powder and extractable material.
[0019] The metal powder may comprise any biocompatible metal,
nonlimiting examples of which include titanium, a titanium alloy
(e.g., Ti-6Al-4V), a cobalt-chromium alloy, aluminum, molybdenum,
tantalum, magnesium, niobium, zirconium, stainless steel, nickel,
tungsten, or any combination thereof. In accordance with known
methods for forming porous constructs using metal powders, it will
be readily appreciated that the metal powder particles may be
substantially uniform or may constitute a variety of shapes and
sizes, e.g., may vary in terms of their three-dimensional
configuration and/or may vary in terms of their respective major
dimension. Measured with respect to a given particle's major
dimension, particle size may be from about 20 .mu.m to about 100
.mu.m, from about 25 .mu.m to about 50 .mu.m, and from about 50
.mu.m to about 80 .mu.m. The metal powder particles may be
spheroids, roughly cylindrical, platonic solids, polyhedrons,
plate- or tile-shaped, irregularly shaped, or any combination
thereof. In preferred embodiments, the metal powder comprises
particles that are substantially similarly shaped and substantially
similarly sized.
[0020] The extractable material may be a material that is soluble
in an aqueous solvent, an organic solvent, or both, and may include
a salt, a sugar, a solid hydrocarbon, a urea derivative, a polymer,
or any combination thereof. Nonlimiting examples include ammonium
bicarbonate, urea, biuret, melamine, ammonium carbonate,
naphthalene, sodium bicarbonate, sodium chloride, ammonium
chloride, calcium chloride, magnesium chloride, aluminum chloride,
potassium chloride, nickel chloride, zinc chloride, ammonium
bicarbonate, sodium hydrogen phosphate, sodium dihydrogen
phosphate, potassium dihydrogen phosphate, potassium hydrogen
phosphate, potassium hydrogen phosphite, potassium phosphate,
magnesium sulfate, potassium sulfate, alkaline earth metal halides,
crystalline carbohydrates (including sucrose and lactose or other
materials classified as monosaccharides, disaccharides, or
trisaccharides), polyvinyl alcohol, polyethylene oxide, a
polypropylene wax (such those available from Micro Powders, Inc.,
Tarrytown, N.Y., under the PROPYLTEX.RTM. trademark), sodium
carboxymethyl cellulose (SCMC), or any combination thereof.
[0021] The particles constituting the extractable material may be
substantially uniform or may constitute a variety of shapes and
sizes, e.g., may vary in terms of their three-dimensional
configuration and/or may vary in terms of their respective major
dimension.
[0022] The extractable material can be present in a wide variety of
particle sizes and particle size distributions suitable to produce
a desired pore size and pore size distribution. Certain preferred
particle size ranges are from about 200 .mu.m to about 600 .mu.m,
from about 200 .mu.m to about 300 .mu.m, and from about 425 .mu.m
to about 600 .mu.m. The extractable material particles may be
spheroids, roughly cylindrical, platonic solids, polyhedrons,
plate- or tile-shaped, irregularly shaped, or any combination
thereof. In preferred embodiments, the extractable material
comprises particles that are substantially similarly shaped and
substantially similarly sized. Because the size and shape of the
pores of the porous body that is produced from the mixture of the
metal powder, the extractable material, and the composition roughly
correspond to the size and shape of the particles of the
extractable material, one skilled in the art will readily
appreciate that the characteristics of the particles of the
extractable material may be selected according to the desired
configuration of the pores of the resulting porous product.
[0023] In accordance with the present invention, when the
extractable material comprises particles that are substantially
similarly shaped and substantially similarly sized, the porosity of
the porous body that is formed using the extractable material of
this type will be substantially uniform. As demonstrated in
Examples 1 and 2, infra, the use of an inventive composition
comprising a polyol, a hydrophilic polymer, or both permits the
formation of a stable mixture of the composition, the metal powder,
and the extractable material and more homogeneity of structure.
Prior techniques did not produce porous constructs having the
degree of uniformity that can be achieved in accordance with the
present methods, even when the extractable materials were selected
for the uniformity of particle size and shape. Thus, not only does
the present invention provide for stable mixtures of metal powder
and extractable materials that do not segregate during prolonged
periods of storage (for example, for at least four days, at least
one week, at least ten days, or at least two weeks) following the
combination of the metal powder and extractable material with the
composition comprising a polyol, a hydrophilic polymer, or both,
but the present invention also provides porous bodies (including
both green bodies and sintered bodies) that possess substantially
uniform porosity when such porous bodies are produced in accordance
with the disclosed methods. "Substantially uniform porosity" refers
to the characteristic of a green body, or a porous body that is
produced from a green body, whereby the porosity does not
significantly vary across the body. For example, in some
embodiments the porosity of the body varies by a standard deviation
of no more than about 3, where porosity is measured in terms of
percentage. In yet other embodiments, the porosity of the body
varies by a standard deviation of no more than about 2.
[0024] The present methods may further comprise shaping the mixture
of the at least one metal powder, the extractable material, and the
composition comprising a polyol, a hydrophilic polymer, or both, in
order to form a shaped object, and compacting the shaped object to
form a green body. The present invention is also directed to green
bodies that are produced in accordance with any process described
herein. The shaping process can comprise filling a mold with the
mixture, the mold having at least roughly the three-dimensional
parameters of the desired final porous product, allowing for
subsequent processing steps such as machining. In other
embodiments, the mold need not be designed to produce near-net
shape parts or parts whose molded form resembles the desired final,
sintered part; molds may produce generic shapes, such as bars,
rods, plates, or blocks, that may be subsequently machined in the
green state to produce a part that after sintering-induced
shrinkage closely approximates the desired shape of the final
product, with optional machining of the sintered part. Molds and
mold assemblies for such purposes are well known among those
skilled the art and may allow for the preparation of bodies that
are, for example, spheroid, ovoid, hemispherical, cuboid,
cylindrical, toroid, conical, concave hemispherical (i.e.,
cup-shaped), plate- or tile-shaped, irregular, or that adopt any
other desired three-dimensional conformation. Once formed from the
mixture in accordance with any aspect(s) of the preceding
description, the resulting shaped object may be compacted to form
the green body. The shaped object is compacted while contained
within a mold assembly. Compacting may be uniaxial, multi-axial, or
isostatic. In preferred embodiments, a cold isostatic press is used
to compact the shaped object into the green body. Following the
compacting procedure, the resulting green body may be removed from
the mold and processed. Processing may include machining or
otherwise refining the shape of the green body.
[0025] Whether or not machining is performed after compacting, the
green body may then be exposed to a solvent in which the
extractable material is soluble. As indicated above, the
extractable material may be soluble in an aqueous solvent, an
organic solvent, or both. The exposure of the green body to the
solvent may comprise immersing the green body in the solvent, for
example, by immersing the green body in a bath comprising the
solvent for a time sufficient to remove at least some of the
extractable material. Depending on various factors such as the type
of solvent chosen relative to the identity of the extractable
material, the temperature of the solvent, and the time of exposure
to the solvent, the removal of extractable material from the green
body can range from partial to complete. The exposure of the green
body to the solvent in which the extractable material is soluble
preferably removes the extractable material from at least the
surface of the green body to a depth at least about 1 mm, at least
about 3 mm, at least about 5 mm, at least about 7 mm, or at least
about 10 mm from any given surface of the green body.
[0026] The composition comprising the polyol, hydrophilic polymer,
or both may also be soluble in an aqueous solvent, an organic
solvent, or both. When such is the case, the composition may be
removed from the green body during the process of exposing the
green body to a solvent in which the extractable material is
soluble. For example, if the composition comprises glycerol, the
composition can be removed from the green body using water, and if
the extractable material is soluble in an aqueous solvent, then the
extractable material and the composition may be removed from the
green body during a single processing step. The solubility of the
composition may permit the effectively complete removal thereof
from the green body, and under such circumstances no contamination
of the green body (or any sintered porous body produced from the
green body) by the composition will occur.
[0027] In another embodiment, the extractable material is insoluble
in aqueous or organic solvent, and is removable under heating
conditions. In such circumstances, after the formation of the green
body via compacting (and whether or not machining is performed
following compacting), the green body may be heated for a time and
under conditions effective to evaporate at least some of the
extractable material yet substantially maintain the metal powder in
its position in the green body. Depending on various factors such
as the identity of the extractable material, the temperature of the
heating environment, and the time of heating, the removal of
extractable material from the green body can range from partial to
complete, and the heating of the green body preferably removes the
extractable material from at least the surface of the green body,
down to at least about 5% of the total depth of the green body.
Preferably, the thermal removal of extractable material is
performed at temperatures much lower than sintering temperature, in
order to avoid contamination of the green body material with C, N,
O, or H from organic space holders. For example, the thermal
removal of extractable material may occur at less than about
100.degree. C., which is sufficient to cause the decomposition of
some extractable materials, such as ammonium bicarbonate.
[0028] The removal of the extractable material under heating
conditions may occur before, after, or contemporaneously with the
removal of the composition. The composition may be removable under
the same heating conditions as are effective for the removal of the
extractable material. In other instances, the composition is
soluble in an aqueous solvent, organic solvent, or both, and the
removal of the composition, for example, by immersion of the green
body in a bath comprising the appropriate solvent or solvents, may
be performed prior to or following the removal of the extractable
material by heating.
[0029] As indicated above, once the extractable material is removed
from the green body, the resulting porosity of the green body may
be about 50% to about 95%, preferably about 60% to about 85%.
[0030] Following the removal of the composition and the extractable
material from the green body, the present methods may further
comprise sintering the green body. Sintering is typically performed
in a vacuum furnace and those skilled in the art will readily
appreciate the appropriate conditions for sintering a green body
comprising a metal powder. Sintering may be followed by additional
processing steps, including machining to refine the shape
characteristics of the sintered body. The present invention is also
directed to sintered porous bodies that are made in accordance with
the described methods. The sintered porous bodies may comprise
implants, such as orthopedic implants used to replace or repair any
damaged or otherwise compromised element or portion of an element
of a skeletal system. The present implants are characterized by
substantially uniform porosity and therefore provide numerous
benefits associated with this property, such as mechanical
stability and homogeneous ingrowth of bone and tissue, thereby
leading to improved biological fixation.
Example 1
Stability of Inventive Mixtures
[0031] A first mixture comprising titanium metal powder, NaCl
extractable material, and glycerol solution was prepared. A second
mixture comprising titanium metal powder, NaCl and reverse osmosis
(RO) water was also prepared, and both mixtures were placed under a
fume hood in separate open containers. Both mixtures were left
undisturbed under the fume hood for a period of four days. As shown
in FIG. 1, the mixture that was prepared using RO water did not
remain well-mixed following the four-day period; the circled
portion of FIG. 1 clearly shows the segregation of NaCl particles
from the particles of titanium. In contrast, the mixture that was
prepared using glycerol solution remained well-mixed following the
same four-day period, with no visible segregation of the salt
particles from the metal powder (circled portion of FIG. 2). A
third mixture, comprising titanium metal powder, NaCl extractable
material, and glycerol solution, was prepared and placed under a
fume hood in an open container and left undisturbed for a period of
one week. The mixture was subsequently compacted using a cold
isostatic press before the image provided in FIG. 3 was taken. As
may be easily observed from FIG. 3 (circled portion), the salt
portion of the mixture remained mixed with Ti powder; as will be
readily appreciated by one of ordinary in the art, a homogeneous,
compacted mixture can only result if the uncompacted mixture was
also homogenous.
[0032] Therefore, mixtures of metal powder, extractable material,
and a composition comprising a polyol, a hydrophilic polymer, or
both remain well-mixed following the combination of the respective
ingredients, even when stored in a container that is exposed to the
ambient atmosphere for at least as long as seven days. In contrast,
mixtures that are prepared using traditional methods, such as
combining the metal powder and extractable material with water, do
not remain as well-mixed and evince segregation between the metal
particles and the particles of extractable materials,
respectively.
Example 2
Porosity of Sintered Bodies
[0033] Three separate mixtures were prepared. The first mixture
comprised a combination of titanium powder, NaCl, and 25% by volume
glycerol solution as a homogenizing agent. The second mixture
included the same ingredients but used a 50% glycerol solution
instead of a 25% solution, and the third mixture also included
titanium powder and NaCl, but made use of reverse osmosis (RO)
water as the homogenizing agent rather than glycerol. Each of the
three mixtures were subsequently compacted, exposed to aqueous
solvent to remove the NaCl particles, and sintered to form a porous
body. The three samples were otherwise alike with respect to
titanium powder particle size, salt particle size, green porosity,
the total quantity of homogenizing agent combined with the titanium
powder and NaCl, compaction pressure, sintering temperature, and
sintering time.
[0034] The porosity of each of the three samples was measured by an
imaging process using Image-Pro.RTM.Plus software (Media
Cybernetics, Inc., Silver Spring, Md.). Each sample was sectioned
into four pieces and each piece was cold mounted and then grinded
and polished to reveal the original porous structure. Two optical
microscope images (50.times. magnification) were taken from each
section, providing a total of eight images from each sample. In the
acquired images, metal scaffold material appears dark and pores
appear white, which permits measurement of the metal foam porosity
based on the areas respectively occupied by the metal and pores. As
shown in Table 1, below, the samples prepared using glycerol
solution each feature porosities that have a standard deviation
that is only about one half of the standard deviation of the
porosity of the sample that is prepared using RO water.
TABLE-US-00001 TABLE 1 Homogenizing Agent Porosity (%) 25% Glycerol
50% Glycerol RO Water Maximum 73.63 74.38 74.94 Minimum 68.75 68.51
66.39 Range 4.88 5.87 8.55 Mean 71.69 72.08 71.64 Standard
Deviation .+-.1.80 .+-.1.79 .+-.3.48
The sample that was prepared using 25% glycerol solution possessed
the narrowest range for measured porosity, indicating that among
the tested samples, the highest degree of uniformity was achieved
when 25% glycerol solution was combined with titanium powder and
NaCl particles in forming the porous body.
* * * * *