U.S. patent application number 10/342033 was filed with the patent office on 2003-06-12 for implantable metallic medical articles having microporous surface structure.
This patent application is currently assigned to Syntheon, LLC. Invention is credited to Bales, Thomas O., Jahrmarkt, Scott L..
Application Number | 20030108659 10/342033 |
Document ID | / |
Family ID | 25389230 |
Filed Date | 2003-06-12 |
United States Patent
Application |
20030108659 |
Kind Code |
A1 |
Bales, Thomas O. ; et
al. |
June 12, 2003 |
Implantable metallic medical articles having microporous surface
structure
Abstract
A process for creating surface microporosity on a titanium (or
other metal) medical device includes creating a surface oxide layer
on the device; placing the device, which is connected to a negative
terminal of an electrical power supply, into a calcium chloride
bath; connecting the positive terminal of the power supply to an
anode immersed in or containing calcium chloride thereby forming an
electrolytic cell; passing current through the cell; removing the
device from the bath; and cooling and rinsing the device to remove
any surface salt. If necessary, the device is etched to remove
metal oxide which may have formed during the cooling process. The
resulting device has a microporous surface structure.
Alternatively, only a designated surface portion of a medical
device is made microporous, either by applying a non-oxidizing
mask, removing a portion of the oxide layer, or subtracting a
portion of a microporous surface.
Inventors: |
Bales, Thomas O.; (Coral
Gables, FL) ; Jahrmarkt, Scott L.; (Miami Beach,
FL) |
Correspondence
Address: |
David P. Gordon, Esq.
65 Woods End Road
Stamford
CT
06905
US
|
Assignee: |
Syntheon, LLC
|
Family ID: |
25389230 |
Appl. No.: |
10/342033 |
Filed: |
January 14, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10342033 |
Jan 14, 2003 |
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09886545 |
Jun 21, 2001 |
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6527938 |
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Current U.S.
Class: |
427/2.24 ;
606/76; 623/23.5; 623/23.74 |
Current CPC
Class: |
A61F 2/32 20130101; A61B
17/866 20130101; C23F 1/26 20130101; A61F 2/01 20130101; A61F 2/28
20130101; A61B 2017/00243 20130101; A61L 27/04 20130101; C22C 14/00
20130101; A61F 2310/00071 20130101; A61B 17/12 20130101; A61B 17/72
20130101; A61F 2/82 20130101; A61L 31/146 20130101; A61C 8/0012
20130101; A61F 2002/30925 20130101; A61F 2310/00017 20130101; A61B
17/06166 20130101; A61C 8/0013 20130101; A61F 2/30767 20130101;
A61F 2310/00029 20130101; A61B 17/064 20130101; A61F 2/0077
20130101; A61F 2240/001 20130101; A61B 17/80 20130101; A61L 31/022
20130101; A61L 2400/18 20130101; A61B 17/86 20130101; Y10T
428/12479 20150115; A61C 8/0015 20130101; C23C 8/80 20130101; A61L
27/56 20130101; A61L 27/06 20130101; A61F 2310/00598 20130101; A61F
2310/00616 20130101; A61F 2/38 20130101; A61F 2/3094 20130101; A61F
2310/00023 20130101; A61L 27/50 20130101; A61L 31/14 20130101 |
Class at
Publication: |
427/2.24 ;
623/23.5; 623/23.74; 606/76 |
International
Class: |
A61F 002/02 |
Claims
1. A medical device, comprising: a metal structure at least a
portion of which has a microporous surface structure, said
microporous surface structure formed by the process of oxidizing a
surface of said medical device, and reducing said oxide layer to
form a microporosity in said surface, wherein said structure at
least partly defines one of (i) a bone implant, (ii) a bone
replacement, (iii) a structural device which expands and reinforces
arterial, vascular, or other body structures, (iv) a wire
embolization coil, (v) an enclosure for a pacemakers, a
defibrillator, or an implantable infusion pump, (vi) a pacing lead,
(vii) a wire suture or ligature, (viii) a surgical staple, (ix) a
filter to catch thrombi and emboli, and (x) an orthodontic implant
or appliance.
2. A medical device according to claim 1, wherein: said metal is
one of titanium and a titanium alloy.
3. A medical device according to claim 1, wherein: said reducing
includes electrolyzing said medical device in a bath of molten
calcium chloride for a period of time.
4. A medical device according to claim 1, wherein: said microporous
surface structure includes a plurality of non-linear tunnel-like
micropores.
5. A medical device according to claim 4, wherein: at least one of
said micropores extends into and beneath said surface of said
device, turns under said surface and interconnects with another of
said at least one of said micropores.
6. A medical device, comprising: a metal structure at least a
portion of which has a microporous surface structure including a
plurality of non-linear tunnel-like micropores, wherein said
structure at least partly defines one of (i) a bone implant, (ii) a
bone replacement, (iii) a structural device which expands and
reinforces arterial, vascular, or other body structures, (iv) a
wire embolization coil, (v) an enclosure for a pacemakers, a
defibrillator, or an implantable infusion pump, (vi) a pacing lead,
(vii) a wire suture or ligature, (viii) a surgical staple, (ix) a
filter to catch thrombi and emboli, and (x) an orthodontic implant
or appliance.
7. A medical device according to claim 6, wherein: at least one of
said micropores extends into and beneath said surface of said
device, turns under said surface and interconnects with another of
said at least one of said micropores.
8. A medical device according to claim 6, wherein: said metal is
one of titanium and a titanium alloy.
9. A medical device, comprising: a metal structure at least a
portion of which has a microporous surface structure including a
plurality of micropores extending in a non-line-of-sight manner
into said structure, wherein said structure at least partly defines
one of (i) a bone implant, (ii) a bone replacement, (iii) a
structural device which expands and reinforces arterial, vascular,
or other body structures, (iv) a wire embolization coil, (v) an
enclosure for a pacemakers, a defibrillator, or an implantable
infusion pump, (vi) a pacing lead, (vii) a wire suture or ligature,
(viii) a surgical staple, (ix) a filter to catch thrombi and
emboli, and (x) an orthodontic implant or appliance.
10. In a metal medical device, the improvement being: a surface of
said metal medical device treated by a process comprising, a)
oxidizing a surface of said medical device; and b) reduction of at
least a portion of said surface such that a microporosity is
provided on said surface of said medical device, wherein said
medical device is one of (i) a bone implant, (ii) a bone
replacement, (iii) a structural device which expands and reinforces
arterial, vascular, or other body structures, (iv) a wire
embolization coil, (v) an enclosure for a pacemakers, a
defibrillator, or an implantable infusion pump, (vi) a pacing lead,
(vii) a wire suture or ligature, (viii) a surgical staple, (ix) a
filter to catch thrombi and emboli, and (x) an orthodontic implant
or appliance.
11. The improvement of claim 10, wherein: said metal is one of
titanium and titanium alloy.
12. The improvement of claim 10, wherein: said microporosity
includes a plurality of non-linear tunnel-like micropores.
13. The improvement of claim 12, wherein: at least one of said
micropores extends into and beneath said surface of said device,
turns under said surface and interconnects with another of said at
least one of said micropores.
14. In a metal medical device, the improvement being: a surface of
said metal medical device having a microporous surface structure
including a plurality of non-linear tunnel-like micropores.
15. The improvement of claim 14, wherein: at least one of said
micropores extends into and beneath said surface of said device,
turns under said surface and interconnects with another of said at
least one of said micropores.
16. The improvement of claim 14, wherein: said medical device is
one of (i) a bone implant, (ii) a bone replacement, (iii) a
structural device which expands and reinforces arterial, vascular,
or other body structures, (iv) a wire embolization coil, (v) an
enclosure for a pacemakers, a defibrillator, or an implantable
infusion pump, (vi) a pacing lead, (vii) a wire suture or ligature,
(viii) a surgical staple, (ix) a filter to catch thrombi and
emboli, and (x) an orthodontic implant or appliance.
17. The improvement of claim 14, wherein: said metal is one of
titanium and titanium alloy.
18. In a metal medical device, the improvement being: a surface of
said metal medical device having a microporous surface structure
including a plurality of micropores which extend into the surface
in a non-line-of-sight manner.
Description
[0001] This application is a divisional of U.S. Ser. No.
09/886,545, filed Jun. 21, 2001, which is hereby incorporated by
reference herein in its entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates broadly to medical articles and
devices. More particularly, this invention relates to methods of
treating the surface of the medical article or devices to affect
the surface structure thereof, and medical articles and devices
having such modified surface structure.
[0004] 2. State of the Art
[0005] Metals and metal alloys, and particularly titanium and
titanium alloys, are used for a great variety of implantable
articles for medical applications. Among these applications are:
structural articles which are used to repair or replace or
reinforce bones or to reconstruct joints; structural articles to
expand and reinforce arterial, vascular, and other body structures
with lumens; wire embolization coils for occluding arteries;
enclosures for pacemakers, defibrillators, and implantable infusion
pumps; pacing leads; wire sutures and ligatures; staples; filters
to catch thrombi and emboli; and, so forth. All implantable
articles suffer from some degree of bio-incompatibility, which may
be manifested as tissue inflammation, necrosis, hyperplasia,
mutagenicity, toxicity, and other reactions, such as attack by
giant cells and leukocytes, and macrophages. While titanium and its
alloys are generally considered inert when implanted, some
biological and biochemical interactions still may occur, and others
have found it desirable to provide various coatings on the surface
of titanium and titanium alloy implants for certain purposes. The
same holds true for many other metals and metal alloys.
[0006] In the area of vascular stents others have coated stents
(whether made of titanium or other materials) with biological
agents (such as genetic material or cellular material) or chemical
agents (such as anti-proliferation reagents or cell-growth factors)
to reduce problems associated with hyperplasia or inflammation. In
order to attach these biological or chemical agents to the surface
of a metallic stent, the agents have been mixed with binders such
as elastomers or bio-resorbable polymers. These binders can also
create problems in that they can cause inflammation, and they can
cause the surface of the stent to have more friction, which reduces
the ease of stent delivery.
[0007] In the field of dental and orthopedic implants, there are
sometimes problems associated with acceptance of the implant by
body tissues. These problems may be ameliorated by adding
anti-inflammatory agents to the surface of the implant. Also, it
has been shown that for some implants, it is advantageous for the
surface of the implant to be microporous to allow ingrowth of
either soft tissue or hard tissue (bone) to enhance the anchoring
of the implant in the body. Such microporous surfaces are generally
created by attaching a layer of sintered spherical powders to
selected surfaces of the implant in areas where tissue ingrowth is
desired.
[0008] However, attachment of these sintered-powder layers requires
additional processing steps, and there is a practical limit to the
size of pores that can be achieved. Also, the temperature at which
the powders must be sintered approaches the melting point of the
material, and the implant is left in a fully-annealed condition,
which may be lower in strength than desired. Also, sintered-powder
coatings on titanium articles must be applied in a
high-temperature, high-vacuum furnace, which is necessarily an
expensive and labor-intensive process.
[0009] In the field of implanted electrodes, it has been found that
sintered powder coatings enhance the attachment of the electrodes
and help them to retain a low-impedance connection to the tissue.
Such electrodes are generally manufactured by machining an
electrode component, applying a multiple-layer coating of powdered
metal in an organic binder, and sintering the coated electrode in a
controlled-atmosphere (or high vacuum) furnace.
[0010] Other medical implants, such as vena-cava filters, aneurism
clips, staples, and sutures, are constructed of wire and thus have
a relatively large surface area for their size. Accordingly,
methods which allow the addition of biological and biochemical
agents to the surface of the implant may be advantageous in
minimizing the adverse reactions of body tissues with the
implant.
[0011] Another type of implant, embolization coils, are intended to
cause thrombosis so that arteries may be blocked off to mitigate
the danger of an aneurism or to deny the blood supply to a tumor.
In such devices it may be advantageous to apply biological or
chemical agents to the surface of the coils in order to enhance the
formation of thrombus.
[0012] In the field of arterial stents, coatings have been applied
to stainless steel and titanium alloys (e.g., TiNi alloys) to
retard tissue reactions such as thrombosis, inflammation, and
hyperplasia. Such coatings have been based upon stable
bio-compatible polymers (such as styrene-isobutylene-styrene
(SIBS)) and bio-resorbable polymers, such as polyglycolic acid. In
the work known to date, the active chemical or biological agent is
mixed with the polymeric coating material, and the agent then
elutes from the coating once the implant is placed in the body.
[0013] U.S. Pat. No. 5,972,027 relates to a stent formed of graded
layers of powdered metal, with some of the surface layers formed of
powder made of larger particle sizes. Once the stent has been
sintered, the major portion of the stent is consolidated to a
substantially solid form, but that portion of the surface that was
made with larger particle-size powder remains microporous. In this
way, a stent is manufactured so that at least some parts of the
surface are microporous and can be infiltrated with a biological or
chemical agent. Such a process is very difficult, since the stent
must be made from a "green" preform that is very thin. The finished
thickness of an arterial stent ranges from approximately 50 to 125
microns (or approximately 0.002 to 0.005 inches), and the
microporous surface layer would be only a fraction of that
thickness. Such a thin preform would be very fragile and difficult
to handle prior to being sintered.
[0014] Other techniques have been described for creating a
micro-microporous surface on a metallic article, and such processes
might be used for creating a microporous coating on a metallic
implant. Such processes include ion milling, photo-chemical
machining, electro-discharge machining, and micro-machining using
conventional cutting tools.
[0015] Of these methods, only the first two are suitable for
creating a large number of very small pores (micropores), in the
range of 1 to 50 microns in size. Such methods are more suitable
for application to flat substrates because they rely on optical or
quasi-optical processes. It would be difficult and expensive to
apply these processes to small non-flat articles, such as stents,
bone screws, dental implants, and clips. Moreover, ion milling and
photo-chemical machining has a porous structure limited by the
line-of-sight operation of the respective processes.
[0016] The last three methods are suitable for creating larger
pores or pockets in the surface of implants, but such larger pores
would require the chemical or biological agent to be bound to the
article by means of some binding agent, usually a polymer.
Moreover, electro-discharge machining and micro-machining also
provide a porous structure limited by the line-of-sight operation
of the respective processes.
[0017] Thus, all of the known methods require either very expensive
processes to produce a fine microporous structure, or else it is
necessary to use a binding material to attach the biological or
chemical agent to the implant article. In addition, all the
processes for fine microporous structure are line-of-sight
operations which provide straight linear pores. Such linear pores
are not optimized for anchoring of the implants via bone and tissue
ingrowth.
SUMMARY OF THE INVENTION
[0018] It is therefore an object of the invention to provide a
process for modifying the surface of a metal or metal alloy implant
to create a microporous surface layer thereon.
[0019] It is another object of the invention to provide a process
for particularly modifying the surface of a titanium or titanium
alloy implant to create a microporous surface layer thereon.
[0020] It is a further object of the invention to provide a process
for creating a microporous surface on an implant article that could
be preferentially applied to only a desired portion of the surface
of the implant.
[0021] It is also an object of the invention to provide an
efficient process which would create a fine microporous structure
on the surface of an implant article that would allow a biological
or chemical agent to be infiltrated into the surface of the article
without the need for binding agents.
[0022] It is another object of the invention to provide an implant
having a microporous surface structure in which the pores are
adapted to provide enchanced anchoring to bone and other
tissue.
[0023] In accord with these objects, which will be discussed in
detail below, a process for creating a microporous layer on the
surface of a titanium or titanium alloy medical device comprises
the following steps. The device is cleaned to ensure that it is
free of any surface contaminants that could react with and diffuse
into the metal when it is heated. A surface layer of titanium oxide
or titanium oxy nitride is then created on the surface of the
device. According to a preferred reduction process to produce a
porous layer at the location of the oxide layer, the oxidized
titanium device is placed into a bath of molten calcium chloride
and connected to the negative terminal of an electrical power
supply. The positive terminal of the electrical power supply is
connected to a suitable anode preferably immersed in or containing
the molten calcium chloride. An electrical current is then passed
through the electrolytic cell. After a time, the titanium device is
removed from the molten salt bath, allowed to cool, and rinsed with
purified water to remove any surface salt. If necessary, the
resulting titanium device may be etched to remove any thin layer of
titanium oxide which may have formed during the cooling process.
The above described process is suitable where it is desired to
modify substantially the entire surface of a medical device. The
pores provided by the process will be non-linear and tunnel through
the surface in a non-line-of-sight manner and interconnect under
the surface of the device.
[0024] According to another embodiment, only a designated portion
of the surface of a medical device is made microporous. This is
done by one of by several techniques. According to a first
technique, an area which is not to be treated is masked. The
non-masked surface is then subject to oxidation. The remainder of
the process is then as described above. According to a second
technique, the entire surface of the device is oxidized. The
oxidation layer is then selectively removed by etching. The oxide
layer is then reduced as described above. According to a third
technique, the device may be oxidized and processed through the
process as described above so that the entire surface area is made
microporous. Then, selected areas of the microporous surface layer
may be removed by any subtractive process, such as etching,
machining, grinding, etc. With the above techniques, it is possible
to produce a titanium or titanium alloy device which has only
selected areas of its surface having a microporous structure, and
the remaining areas consisting of its base material.
[0025] In addition, the processes described can be used on medical
devices made of other metal or metal alloy substrate materials.
Examples of alternative substrates include reactive and refractory
metals, cobalt alloys, nickel alloys, and stainless steel
alloys.
[0026] Further, other reduction processes can be used to reduce the
oxide layer to a metallic layer, including direct reduction by
means of an active metal, electrochemical reduction in mixed molten
salts, and electrochemical reduction in non-aqueous solvents.
[0027] Additional objects and advantages of the invention will
become apparent to those skilled in the art upon reference to the
detailed description.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0028] Any of a great variety of titanium or titanium alloy medical
articles and devices (hereinafter, collectively "devices") may
benefit from a microporous surface layer. Exemplar devices include
structural devices which are used to repair or replace or reinforce
bones or to reconstruct joints (e.g., hip joint implants, knee
joint implants, bone plates and screws, intramedullary nails,
etc.); structural devices to expand and reinforce arterial,
vascular, and other body structures with lumina (i.e., stents);
wire embolization coils for occluding arteries; enclosures for
pacemakers, defibrillators, and implantable infusion pumps; pacing
leads; wire sutures and ligatures; staples; filters to catch
thrombi and emboli; orthodontic implants, archwire, and appliances;
and so forth. Examples of such devices are described in the
following U.S. Pat. No.: 5,868,796 to Buechel et al. entitled
"Prosthesis with biologically inert wear resistant surface"; U.S.
Pat. No. 6,152,960 to Pappas entitled "Femoral component for knee
endoprosthesis"; U.S. Pat. No. 6,077,264 to Chemello entitled
"Intramedullary nail for the osteosynthesis of bone fractures";
U.S. Pat. No. 6,096,040 to Esser entitled "Upper extremity bone
plates"; U.S. Pat. No. 5,785,712 to Runciman et al. entitled
"Reconstruction bone plate"; U.S. Pat. No. 6,048,343 to Mathis et
al. entitled "Bone screw system"; U.S. Pat. No. 6,117,157 to
Tekulve entitled "Helical embolization coil"; U.S. Pat. No.
5,895,980 to Thompson entitled "Shielded pacemaker enclosure"; U.S.
Pat. No. 6,112,118 to Kroll et al. entitled "Implantable
cardioverter defibrillator with slew rate limiting"; U.S. Pat. No.
4,103,690 to Harris entitled "Self-suturing cardiac pacer lead";
U.S. Pat. No. 4,901,721 to Hakki entitled "Suturing device"; U.S.
Pat. No. 6,071,120 to Birkel entitled "Method and apparatus for
ligating orthodontic appliances"; U.S. Pat. No. 5,893,869 to
Barnhart et al. entitled "Retrievable inferior vena cava filter
system and method for use thereof"; U.S. Pat. No. 5,941,896 to Kerr
entitled "Filter and method for trapping emboli during endovascular
procedures"; and U.S. Pat. No. 5,725,554 to Simon et al. entitled
"Surgical staple and stapler". All of the above patents are hereby
incorporated by reference herein in their entireties.
[0029] According to the process of the invention, a microporous
layer may be created on the surface of such titanium. A titanium
medical device is obtained and preferably cleaned to ensure that it
is free of any surface contaminants that could react with and
diffuse into the titanium when the titanium is heated. This is
preferable, as it is well known that contaminants such as organic
materials, oxides, metals, halogens, and chalcogenide elements such
as sulfur and oxygen will react with titanium and diffuse into it,
creating additional phases in the metal and embrittling it.
Preferable methods of cleaning include of mechanical polishing,
acid etching, and/or electropolishing.
[0030] Once the device is cleaned, a surface layer of titanium
oxide or titanium oxynitride is created on the device by heating
the titanium device in an atmosphere of pure oxygen or a mixture of
oxygen and nitrogen. Inert gases such as helium or argon may be
added to dilute the oxygen or nitrogen. The preferable temperature
range for this oxidization process is 700 to 900.degree. C. By
altering the time and temperature of the oxidation process, the
thickness of the oxide layer may be controlled. This thickness
preferably ranges from a few microns to a few hundred microns. An
exemplar range includes approximately 5 to 250 microns
[0031] Alternatively, another process may be used to create the
oxidation layer. For example, the titanium device may be oxidized
by a chemical solution such as a mixture of hydrofluoric and
perchloric acids; by any of the standard vacuum-deposition
techniques, such as ion implantation, plasma etching, or chemical
vapor deposition, etc; or, by immersion in a suitable electrolyte
(such as a potassium hydroxide solution) and passing an electric
current therethrough (with the device positive) to create an
`anodized` oxide coating on the device.
[0032] The oxidation layer is then reduced (deoxidized). According
to a preferred embodiment of reduction, the oxidized titanium
device is placed into a salt bath of molten calcium chloride and
connected to the negative terminal of an electrical power supply,
thereby making the medical device a cathode. The positive terminal
of the electrical power supply is connected to a suitable anode,
preferably made from either graphite or titanium, immersed in or
containing the molten calcium chloride, thereby forming an
electrolytic cell. An electrical current is then passed through the
electrolytic cell at a current density of approximately 300
milliamperes per square centimeter of cathode area. The current
reduces the titanium oxide to titanium, providing a microporosity
on the surface structure of the device. Electrolysis preferably
occurs at a temperature of 700 to 1000.degree. C. The period of
time required to complete the reduction of titanium oxide to
titanium, and to extract the diffused oxygen in the base titanium
metal, will vary from a few minutes to a few hours. Reduction of
metal oxides in this manner is also described in PCT/GB99/01781,
entitled "Removal of oxygen from metal oxides and solid solutions
by electrolysis in a fused salt" by Fray et al., which is hereby
incorporated by reference herein in its entirety.
[0033] The titanium device is then removed from the molten salt
bath, and allowed to cool. The device is then rinsed clean of any
remaining salt, preferably using purified (distilled) water.
[0034] The titanium device may be etched, if necessary, to remove
any thin layer of titanium oxide which may have formed during the
cooling process. A preferred mixture for etching includes
hydrofluoric acid (HF) and nitric acid (HNO.sub.3). Sulfuric acid
may be added to the HF/HNO.sub.3 mixture to increase the activity
of the etching solution. Alternatively, other etchants such as
concentrated carboxylic acids (e.g., oxalic acid or citric acid)
may be used. Finally, the etchant is then rinsed from the
device.
[0035] The resulting implantable medical device has a microporous
surface structure which is different, at least on a microscopic
scale, than the porous surfaces of the prior art. The porous
surface includes non-linear tunnel-like micropores in which a
continuous individual micropore may extend into and beneath the
surface of the device and then turn under the surface and
interconnect with other micropores. This microporous surface
structure facilitates cell ingrowth and thereby aids in stabilizing
the device at its implant location within the body. In addition,
the micropores can retain genetic material, cellular material, and
biological or chemical agents (e.g., anti-inflammatory agents,
anti-proliferation reagents, cell-growth factors) without high-cost
sintered powdered layers for biological material or agent
retention, or the use (and associated negative reactions) of
binders.
[0036] According to a second embodiment of the invention, only a
designated portion of the surface of a titanium device is surface
modified. This may be done by any of several techniques.
[0037] According to a first technique, prior to performing the
oxidation step of the process, an oxidation-resistant coating that
survives the high-temperature oxidation step is applied as a mask
to areas of the device where it is desired to not have a
microporous surface. Exemplar coatings include a thermally-sprayed
coating of calcium chloride, or a plating of gold. Alternatively,
the oxidation-resistant coating is applied to the entire surface of
the device, and then selectively removed from the areas where it is
desired to create a microporous surface. For example, a gold
plating may be chemically etched away in selected areas using aqua
regia--a mixture of nitric acid (HNO3) and hydrochloric acid (HCl)
in an approximately one to three ratio. After the
oxidation-resistant coating is applied, the device is subjected to
the oxidative step, and the remainder of the process is carried out
as described above. It may be desirable to remove the
oxidation-resistant coating after the oxidative step, but before
the molten-salt bath in order to prevent contamination of the bath
or the titanium with decomposed mask material.
[0038] According to a second technique, the entire surface of the
device is oxidized. The oxidation layer is then removed from
selected areas by etching with a suitable etchant, such as a
mixture of nitric and hydrofluoric acids. The device is then
processed in the molten salt bath as described above.
[0039] According to a third technique, the device is oxidized and
processed through the process as described above so that the entire
surface area is made microporous. Then, selected areas of the
microporous surface layer are removed by any subtractive process,
such as etching, machining, grinding, etc.
[0040] Using any of the above techniques or another suitable
technique, it is possible to produce a device with only designated
areas of its surface having a microporous structure, and the
remaining areas consisting of non-microporous base material.
[0041] As it is often desirable to construct medical devices from
an alloy of titanium rather than from essentially pure titanium, it
is noted devices made of titanium alloys may be similarly
processed. Other titanium alloys include: (1) commercially-pure
titanium, consisting of titanium plus small amounts of
"interstitial" elements, such as carbon, oxygen, and nitrogen, to
modify the yield and tensile strength, (2) Ti--6Al--4V (a common
implant alloy), (3) titanium and nickel alloy (TiNi, a
superelastic/shape-memory alloy), (4) solid-solution titanium
alloys, such as Ti--Pt, Ti--Au, Ti--Pd, Ti--Hf, Ti--Nb, and (5)
other alloys of titanium, including beta-titanium alloys and
alpha-beta alloys. These alloys of titanium have a greater range of
stiffness, hardness, yield strength, ultimate tensile strength,
machinability, and other properties which may be advantageous in
some implant applications, such as orthopedic implants. In such
cases, it is possible to perform the process just as described
above, or a surface layer of pure titanium may be created on the
surface of the device by several different means: (1) the surface
may be etched with an etchant which preferably removes other
alloying elements, (2) a superficial layer of pure titanium may be
added to the device by means of any standard additive process,
e.g., plasma deposition, sputtering, physical vapor deposition,
chemical vapor deposition, thermal spray, or electroplating; or (3)
the device may be made up of a composite material which has a
substrate of the desired titanium alloy and a superficial layer of
pure titanium.
[0042] In addition, while the foregoing process has been described
with respect to the production and subsequent reduction of oxide
layers on titanium and titanium alloys, these processes are also
applicable to other substrate materials. Examples of alternative
substrates include: (1) reactive and refractory metals such as
zirconium, halfnium, and niobium; (2) cobalt alloys, such as
chromium-cobalt-molybdenum alloys (Haynes.RTM. 214 and ASTM F75
Cast Alloy) and other cobalt alloys such as MP-35N and
Elgiloy.RTM.; (3) nickel alloys such as Inconel.TM.; and (4)
stainless steel alloys including austenitic alloys such as 304,
316, 317, 321, and 347, martensitic alloys such as 440A, 440B,
440C, ferritic alloys such as 410 and 431, and
precipitation-hardening alloys such as 17-4PH, 17-7PH, Custom
455.RTM., Custom 465.TM., etc.
[0043] When using any of the above alternative substrate materials,
the thermally-formed oxide layers consist of oxides of the elements
comprising the substrate. In the case of pure metals, such as
zirconium or halfnium, the oxide layer is a mixture of various
oxides of that element. In the case of alloy substrate elements,
the oxide layer consists of combined oxides of the substrate
elements. For example, 304 stainless steel forms an oxide film of
mixed oxides of iron, chromium and nickel. When the mixed oxide
films are reduced to form a porous metallic layer, the metal layer
resulting from the reduction of the mixed oxides is an alloy of the
metallic elements forming the mixed oxide. This alloy is similar in
composition to the alloy of the substrate, though there may be a
change in composition brought about by differential resistance to
oxidation of the alloying elements and by the different reduction
potentials of the various metal oxides.
[0044] In addition, other reduction processes can be used to reduce
the metal oxide layer to a metallic layer, including: (1) direct
reduction by means of an active metal, (2) electrochemical
reduction in mixed molten salts, and (3) electrochemical reduction
in non-aqueous solvents.
[0045] With respect to direct reduction, a deoxidant (an alkali
metal or alkaline earth metal) in a molten solution with a carrier
metal (also an alkali metal or alkaline earth metal) can be used to
reduce a refractory or reactive metal such as titanium, zirconium,
halfnium, thorium, vanadium, niobium, tantalum, chromium,
molybdenum, tungsten, and alloys comprising these metals. See U.S.
Pat. No. 4,923,531 to Fisher entitled "Deoxidation of titanium and
similar metals using a deoxidant in a molten metal carrier", which
is hereby incorporated by reference herein. In Fisher's preferred
embodiment, a molten mixture of calcium (the deoxidant) in sodium
is used in direct contact with oxidized titanium (TiO.sub.2) to
reduce the oxide to metallic titanium. In practice, the mixture of
molten metals is held in an inert atmosphere and the oxidized
article is introduced to the melt and held there for a period of
time. Since calcium oxide is more stable than titanium dioxide, the
intimate contact of these materials results in a
reduction-oxidation reaction in which the titanium dioxide is
reduced to titanium metal and the calcium metal is oxidized to
calcium oxide. It is noted that Fisher describes a process intended
for use primarily with metallic articles of titanium, zirconium,
etc., which contain high levels of dissolved oxygen rather than
articles having oxide compounds of the metals on their surface.
Nevertheless, according to the present invention, if the process is
used at a sufficiently high temperature for a sufficiently long
reaction time, the oxides reduce to metals. It is also noted that
Fisher cites patents which teach the reduction of other oxides (in
the form of ores) of titanium, zirconium, etc., to metal. For
example, U.S. Pat. No. 2,834,667 to Rostron teaches direct thermal
reduction of titanium dioxide by using metallic magnesium at a
temperature not substantially less than 1000.degree. C., U.S. Pat.
No. 2,537,068 to Lilliendahl et al. teaches the reduction of
zirconium oxide or double chloride with calcium at temperatures
between 1100.degree. and 1200.degree. C., and U.S. Pat. No.
2,653,869 to Gregory et al. discusses the manufacture of vanadium
powder from vanadium trioxide mixed with calcium and calcium
chloride at a temperature between 900.degree. and 1350.degree. C.
Each of the patents to Rostron, Lilliendahl et al., and Gregory et
al. are hereby incorporated by reference herein in their
entireties. In addition, in the ancient "thermite" reaction, iron
oxide is reacted with metallic aluminum to form metallic iron.
Thus, oxygen-contaminated reactive and refractory metals (or their
oxides) can be contacted with molten active metals such as calcium
and magnesium in order to remove the oxygen and reduce the oxides
to pure metals.
[0046] Another manner of direct reduction utilizes pure molten
alkaline earth metals. U.S. Pat. No. 5,022,935 to Fisher, entitled
"Deoxidation of a refractory metal", which is hereby incorporated
by reference herein in its entirety, describes the use of pure
molten calcium to reduce refractory metals containing oxygen as a
contaminant.
[0047] Yet another manner of direction reduction uses vapor phase
alkaline earth metals. U.S. Pat. No. 5,211,775 to Fisher, entitled
"Removal of oxide layers from titanium castings using an alkaline
earth deoxidizing agent", which is hereby incorporated by reference
herein in its entirety, describes a process in which calcium is
used to remove oxygen contamination of titanium articles. In this
process the calcium is used in vapor phase. As the calcium vapor
reacts with the oxidized or oxygen-contaminated surface of the
titanium article, calcium oxide forms on the surface. This oxide is
later removed by rinsing or acid pickling.
[0048] In view of the foregoing, it will be appreciated that an
alkali metal or alkaline earth metal can be used to chemically
remove the oxygen which has either been absorbed into or combined
with a titanium article. The deoxidizing metal can be used alone as
a liquid or in vapor phase, or it may be combined with another
metal to form a liquid phase. In the process at least two
beneficial actions are achieved. First, oxygen which has been
absorbed into a refractory metal, such as titanium, is removed by
diffusing out of the refractory metal at a high temperature, and
then is chemically bound by the deoxidizing metal (e.g., calcium).
Second, non-metallic oxides of the refractory metal or alloy are
reduced by direct contact with the deoxidizing metal so that the
refractory metal oxides are reduced to the metallic form. In the
present invention, both of these actions are important because the
metal alloy article has been covered by an oxide and the metal
alloy article also has been internally contaminated by oxygen which
has diffused into it during the oxidation step. The proposed
processes in which a molten deoxidizing metal, such as calcium, is
held in contact with the oxidized titanium oxide article resolves
both of these conditions: the oxide surface layer is reduced to
metal, and the oxygen which has diffused into the titanium alloy is
removed by diffusion as it is bound up by the deoxidizing
metal.
[0049] With respect to electrochemical reduction in mixed molten
salts, such mixed molten-salt electrolytic baths may be used in
order to achieve lower temperatures than would be possible with
pure salts such as calcium chloride. In the preferred reduction
process described above, it is necessary for the metallic element
whose salt is used to have a higher electrodeposition potential
than that required to deoxidize the metal oxides in question. The
process works similarly if a sufficiently high potential is used
such that the cation of the salt (e.g., calcium) is actually
deposited onto the titanium article. In view of the fact that pure
calcium may be used to reduce the oxides and remove absorbed oxygen
from titanium, it is clear that there is no need to prevent the
calcium or other cation from plating out on the titanium article,
except that it may necessitate some further cleaning steps after
the deoxidation.
[0050] If a mixed-salt bath is used in place of pure calcium
chloride, it is preferable for the mixed-salt bath to contain at
least a substantial portion of the salts which have a high
solubility for oxygen. For example, it is preferable that at least
one of the following salts be present in the mixed molten-salt
bath: BaCl.sub.2, CaCl2, CsCl, LiCl, SrCl.sub.2, or YCl.sub.3. By
mixing two or more salts, it is possible to form mixtures which
have lower melting points than any of the constituent salts. The
mixture of two or more components which forms the minimum melting
point is known as the eutectic.
[0051] Eutectic mixtures of salts such as LiCl and KCl have been
used for this purpose, and in fact it has been shown that titanium
may be electrodeposited from such mixtures. See B. N. Popov and H.
Wendt, "Electrodeposition of Titanium from Molten Salts," in
Emerging Materials by Advanced Processing, Ed. Max-Planck Institut
fur Metalforschung, Frankfurt (1988). In addition, extensive work
has been done to characterize the thermal and electrochemical
properties of such molten-salt mixtures. See "Thermodynamic
Evaluation and Optimization of the
LiCl--NaCl--KCl--RbCl--CsCl--MgCl.sub.2--CaCl.sub.2--SrCl.sub.2
System Using the Modified Quasichemical Model" Chartrand, P. and
Pelton, A. D., Center for Research in Computational
Thermochemistry, Ecole Polytechnique de Montreal.
[0052] By using a carefully chosen eutectic mixture (e.g.,
LiCl--CaCl.sub.2 with a melting point of 475.degree. C.) rather
than pure CaCl.sub.2 (with a melting point 772.degree. C.), it is
possible to carry out the deoxidation electrolysis process at a
substantially lower temperature. The diffusion rate of oxygen
through titanium at 475.degree. C. is expected to be several orders
of magnitude slower than at 772.degree. C., as is the rate at which
the titanium crystal lattice could re-align itself as the oxygen is
removed. Thus, it is expected that at the lower temperature the
porous structure of the titanium metallic layer formed by the
reduction of the oxide is much finer than it would be if formed at
the higher temperature.
[0053] Some molten salt baths, e.g., 1-ethyl-3-methyimidazolium
chloride-aluminum trichloride molten salt, are liquid at very low
temperatures, even below room temperature. Such salt baths may be
suitable for electrochemical reduction of thermally formed oxide
layers as described here, though it is expected that the rate of
reaction will be considerably slower at low temperature. Also, very
little dissolved oxygen would be removed from the substrate metal
in such as low-temperature process, and a very fine level of
porosity is expected because of the limited ability of the titanium
or other metal atoms to rearrange at the lower temperature.
Nonetheless, such a low-temperature process may be preferable
because of its ease of execution and because of the reduced risk of
contaminating the base metal, even if only very thin layers of
oxide are reduced.
[0054] With respect to electrochemical reduction in non-aqueous
solvents, non-aqueous electrolytes have been developed for plating
of materials whose cations are not stable in aqueous solutions
(e.g., aluminum, titanium, calcium, and zirconium) and for use in
lithium batteries. Typical electrolytes used in lithium primary
cells consist of a lithium salt (usually lithium perchlorate) and
an organic solvent in which that salt is soluble, such as propylene
carbonate and various ethers. For example, U.S. Pat. No. 4,721,656
to Vance et al. entitled "Electroplating aluminum alloys from
organic solvent baths and articles coated therewith", teaches the
use of aluminum and lithium chlorides dissolved in anhydrous
toluene as an electrolyte for the plating of aluminum alloys.
Another example is provided in U.S. Pat. No. 4,525,250 to
Fahrmbacher-Lutz et al., entitled "Method for chemical removal of
oxide layers from objects of metal", which teaches the use of a
methanol-based electrolyte containing hydrogen fluoride and one or
more alkali fluorides and/or ammonium fluoride for the removal of
titanium oxides from articles of titanium. Yet another example is
provided in U.S. Pat. No. 4,465,561 to Nguyen, et al., entitled
"Electroplating film-forming metals in non-aqueous electrolyte",
which teaches the use of toluene and para-xylene as solvents for
electrolytes for plating metals which cannot be plated in an
aqueous environment. A further example is provided in U.S. Pat. No.
6,156,459 to Negoro et al., entitled "Nonaqueous-electrolytic
solution secondary battery", which teaches the use of aprotic
organic solvents, such as propylene carbonate, ethylene carbonate,
butylene carbonate, dimethyl carbonate, diethyl carbonate, methyl
ethyl carbonate, .gamma.-butyrolactone, methyl formate, methyl
acetate, 1,2-dimethoxyethane, tetrahydrofuran,
2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane,
formamide, dimethylformamide, dioxolane, dioxane, acetonitrile,
nitromethane, ethyl monoglyme, phosphoric acid triesters,
trimethoxymethane, dioxolane derivatives, sulfolane,
3-methyl-2-oxazolidinone, propylene carbonate derivatives,
tetrahydrofuran derivatives, ethyl ether, and 1,3-propanesulfone
(preferably ethylene carbonate and propylene carbonate) as
non-aqueous solvents for electrolytes in lithium batteries. Each of
the patents in the above examples is hereby incorporated by
reference herein in their entireties.
[0055] As in the foregoing applications to electroplating and
lithium battery chemistry, non-aqueous solvents which have the
power to dissolve alkali metal and alkaline-earth metal halides and
haloxides (e.g., LiCl, CaCl.sub.2, LiClO.sub.4) are suitable for
the electrochemical reduction of oxides of metals such as titanium,
zirconium, nickel, chromium, and iron. Thus, electrolytes
consisting of salts such as calcium chloride and lithium
perchlorate dissolved in solvents such as propylene carbonate are
applicable for treating thermally-formed oxide surface layers on
titanium substrates so that the oxide layers are reduced to porous
metal layers.
[0056] It should be noted that if a low-temperature process is used
to reduce the oxide layer to metal, a relatively small amount of
oxygen that might have been dissolved into the metal substrate
would be removed during the electrochemical reduction process.
Since the removal of dissolved oxygen is dependent upon the ability
of oxygen to diffuse out of the metal substrate, it is expected
that when the reduction process is carried out at low temperatures,
such as at room temperature, very little of the dissolved oxygen in
the substrate titanium would be removed. This limited elimination
of oxygen would likely result in reduced ductility in the titanium
(or other metal) article. As such, if the medical device article is
relatively thin or if the article is subject to bending after the
reduction process (or in use), the remaining dissolved oxygen would
likely be deleterious.
[0057] There have been described and illustrated herein several
embodiments of processes for surface treating metal and metal alloy
medical devices, and particularly surface treating titanium and
titanium alloy medical devices. While particular embodiments of the
invention have been described, it is not intended that the
invention be limited thereto, as it is intended that the invention
be as broad in scope as the art will allow and that the
specification be read likewise. Thus, while particular titanium
alloys have been disclosed, it will be appreciated that other
titanium alloys may be used as well. In addition, while particular
oxidation and etchant processes are disclosed, it will be
appreciated that other types of such processes can be used. Also,
while several methods for reduction have been disclosed, it will be
appreciated that yet other such methods can be used. Furthermore,
while several methods have been disclosed for reducing only
portions of a metal or metal alloy device, yet other methods can be
used. Moreover, while particular medical devices made of metal or
metal alloys have been disclosed, it will be appreciated that other
medical devices made of metal or metal alloys can be thusly
treated. It will therefore be appreciated by those skilled in the
art that yet other modifications could be made to the provided
invention without deviating from its spirit and scope as
claimed.
* * * * *