WO2006083955A2

WO2006083955A2 - Method for the texturing of surfaces by aqueous plasma electrolysis

Info

Publication number: WO2006083955A2
Application number: PCT/US2006/003541
Authority: WO
Inventors: Louis F. Toth; Charles E. Smith, Jr.; Kevin R. Williams
Original assignee: Tokusen U.S.A., Inc.
Priority date: 2005-02-04
Filing date: 2006-01-31
Publication date: 2006-08-10
Also published as: WO2006083955A3

Abstract

A method for producing a roughened or textured layer on a surface such as a metal or an alloy using aqueous plasma electrolysis. In this process continuous arc-discharges are established and maintained in an electrolyte flowing between two electrodes, one of which is the workpiece to be treated. The treatment process can be performed discretely (one piece at a time) or continuously (reel to reel). The texturing is produced by the action of the arc-discharge processes resulting in localized melted areas on the workpiece's surface. The melted areas subsequently solidify creating a new topological configuration on the workpiece. By altering the electrolysis parameters, e.g. the voltage, current, electrolyte composition, etc., surface texturing characteristics such as the dimensional scale and topological morphology can be varied. Surface texturing as described herein can be successfully utilized to markedly improve soft and hard tissue biological integration of in-vivo medical implants. A cellular type texture structure can also be used as a pharmaceutical depository for subsequent drug elution after in-vivo device implantation.

Description

Method for the Texturing of Surfaces by Aqueous Plasma Electrolysis

Technical Field

The present invention relates to an aqueous plasma electrolysis method for forming texture on surfaces, particularly biomedical metals or alloys. The textured surfaces are useful on devices intended for in vivo implantation, such as within a human body.

Background of the Invention

Biomedical research has established that connective tissue does not readily attach to implants with very smooth surfaces. Rather, the body will form a tissue capsule surrounding the implant subsequently sealing it off from the rest of the body. The implant's function can be seriously compromised by this encapsulation. For example, weakly attached connective tissue will constantly shift around the implant causing inflammation. Ultimately, this can lead to a growing capsule of dead tissue. By adding surface roughness achieved by texturing, the implant's biological attachment with connective tissue cells, called fibroblasts, increases considerably. These cell-implant device surface interactions derive at least partially from how protein layers are composed and organized on the implant's surface. These interactions have been found to be influenced by the implant's topography and micromechanical properties, which can affect how well the implant's surface binds with water, ions and biomolecules.

Surface roughness can be imparted to substrates by various means. For instance, a substrate can be held in a suitable vise and cut or turned with appropriate cutting tools to produce gouges, craters, steps, etc., that in effect roughen the substrate's surface. Typically these "cut" surfaces exhibit a macro scale relief.

Another texturing technique blasts the substrate with grit entrained in a high-pressure gas or liquid jet. The impinging grit gouges or compresses the workpiece's surface. By varying the grit size and type, blasting pressure etc., the coverage and dimensional scale of the surface texturing can be varied. Mixing two sizes of grit can yield a duplex surface texture.

Chemical treatments are also used for surface texturing. These are dissolving processes where the substrate is immersed in a suitable liquid media, typically strong acids or alkalis, for a fixed period of time so that a predetermined volume of surface is removed. In the chemical process there is no control over the resultant surface topology, it typically mimics the material's microstructural morphology.

Electrochemical texturing processes are similar to chemical treatments except that relatively low voltages and currents are impressed on the substrate to affect anodic surface dissolution. By varying the current and voltage levels the reaction speed can be moderated to control the extent of texturing. Some control of surface topology is possible by impressing complex electrical waveforms on the substrate. Another technique, thermal spraying, is somewhat like grit blasting except thermal energy is imparted to the grit raising its temperature to melting or near melting in a gas jet. As the heated grit impinges onto the substrate it splats, solidifies and adheres. Control of the grit temperature, spraying pressure/velocity, grit type etc., lends control to the resultant surface texture. All of the above processes, although capable of producing surface texture, have some limitations. None are quite capable of producing texture features into the nanometer range. Also, these techniques are basically incapable of independently manipulating the morphology of the resultant surface texture. Compared to these aforementioned treatment processes, aqueous plasma electrolysis's most marked advantages over them are its abilities to: produce metastable phases and non-equilibrium solid solutions and nanoscopic, microscopic and amorphous surface structures/topology, create different surface topologies, control the texturing dimensional scale, and form a texturing topology independently of the substrate's microstructure. These abilities could all possibly result in better biological assimilation of the implant device.

U.S. Pat. No. 6,454,569 presents a dental implant device that contains ordered microgeometric repetitive surfaces to promote maxillofacial bone or tissue growth with said implant surface. The patent teaches that surface textures may be effected by such treatments as laser cutting, acid etching, photolithography, abrasion/roughening, plasma spraying, calcium sulfate, biocompatible glass, collagen, hydroxapatite, growth factor compounds, and combinations thereof. U.S. Pat. No. 6,127,596 describes an implantable orthopedic prosthesis that has a macro/micro textured bone and soft tissue attachment surface. This attachment surface is a regular grid of depressions separated by orthogonal ridges formed by electro-discharge machining (EDM), an electrical erosion technique. In U.S. Pat. No. 6,008,432 a metallic texture coating is produced on a hip stem implant by arc-deposition of the texture coating onto an intermediate bonding layer.

U.S. Pat. No. 5,645,740 discloses a system and assemblage for producing microtextured systems on implants using a radiated beam. With this system it is purported that the geometric configuration and the dimensions of the microtextured surfaces can be predetermined, thereby optimizing implant acceptance and durability in the host tissue.

U.S. Pat. No. 6,585,875 and WO 01/09410 describe a process and apparatus for cleaning and coating metal surfaces using electro-plasma technology. Basically, an electrolytic cell operating at >30 volts is established at a workpiece by suitable adjustment of electrolysis parameters and using an electrically conductive foam electrolyte.

U.S. Pat. No. 6,368,467 describes an electro-plating plasma arc deposition process where an electric arc is maintained between the anode and cathode during the deposition of the metal coating onto the surface of the cathode. None of these patents recognize nor claim the use of plasma electrolysis for texturing biomedical devices for -enhancing soft or hard tissue integration nor for the creation of cellular depositories for pharmaceuticals.

References mentioned in the description are not admitted to be prior art with respect to the present invention.

Disclosure of the Invention

Accordingly in a first aspect the present invention provides a process for rearranging or texturizing the surface of a substrate so that if it is used in- vivo as an implant or prosthetic device, the textured surface will markedly improve the biological integration of the implant device. The original surface is rearranged by local melting and resolidification. This is accomplished by a subjecting the substrate to a continuous arc-discharge process within an aqueous plasma environment. Depending upon the gap between the anode and cathode (substrate) and the composition of the electrolyte, the electrolysis electrical parameters are adjusted to suitable values to ensure a stable, continuous plasma arc envelope surrounding the cathode. There are no upper or lower limits placed on the electrolysis voltage or current, they are adjusted to whatever values are required to generate stable plasma. Furthermore the substrate to be textured can be held stationary in the plasma envelope or apparatus can be utilized which continuously moves the substrate through the plasma envelope.

In a second aspect of this invention the electrolysis and process parameters can be varied to adjust or change the topology or morphology of the substrate's textured surface. For example, the electrolysis and process conditions could be adjusted to yield a "furrow" type texture - a continuous structure of valleys and ridges. Another adjustment in the electrolysis and process conditions can yield "cellular or crater" structures. These could act as pharmaceutical depositories, the pharmaceutical being dispensed or eluted after the device is implanted. A coating on the pharmaceutical-containing cell could regulate the dispensing speed of the pharmaceutical. Brief Description of the Drawings

Fig. 1 is a micrograph of a substrate textured by the process of the present invention using the parameters of Example 1.

Fig. 2 is a micrograph of a substrate textured by the process of the present invention using the parameters of Example 2.

Fig. 3 is a micrograph of a substrate textured by the process of the present invention using the parameters of Example 3.

Fig. 4 is a micrograph of a substrate textured by the process of the present invention using the parameters of Example 4. Fig. 5 is a micrograph of a substrate textured by the process of the present invention using the parameters of Example 5.

Fig. 6 is a micrograph of a substrate textured by the process of the present invention using the parameters of Example 6.

Fig. 7 is a micrograph of a substrate textured by the process of the present invention using the parameters of Example 7.

Fig. 8 is a micrograph of a substrate textured by the process of the present invention using the parameters of Example 8.

Fig. 9 is a micrograph of a substrate textured by the process of the present invention using the parameters of Example 9. Fig. 10 is a micrograph of a substrate textured by the process of the present invention using the parameters of Example 10.

Fig. 11 is a micrograph of a substrate textured by the process of the present invention using the parameters of Example 11.

Best Mode for Carrying Out the Invention

The electrolysis cell consists of two electrodes, the anode and the cathode, the latter being the substrate that will be textured. The anode architecture can take various geometrical forms: coaxial, tubular, planar, conformal etc. The cathode is maintained within a predetermined, fixed gap from the anode. The annular space between the anode and cathode contains the electrolyte, a combination of water and one or more dissolved inorganic salts. The electrolyte's ingress and egress within the gap is maintained by whatever plumbing arrangement is necessary to maintain a stable plasma envelope during processing.

The inorganic salt dissolved in the electrolyte serves two basic functions. It increases the electrolyte's ionic conductivity and it influences the resultant texture morphology by chemical interaction during the micro-arcing process. Certain distinguishing electrolyte characteristics are evident, for example, sulfate salts appear to be more effective at influencing texturing morphology than carbonate or bicarbonate salts. Also, an alkaline pH appears to be much more effective than an acidic pH. The operating parameters for the plasma texturing process are adjusted to provide the requisite conditions for establishing a stable and continuous plasma envelope within the electrodes' gap. The actual voltage and current values depend upon the cell design as well as the electrodes' geometry. Sufficient current must be passed to heat the electrolyte to near boiling and sufficient voltage must be applied to sustain the arc discharges. These values can range up to 4.0 amps and 200 volts.

The electrolyte must be circulated through the cell to prevent salt precipitation, to maintain a constant electrolyte temperature and to prevent changes in the electrolyte's salt concentration. The present invention represents an improvement over the prior art methods of texturization in that different topologies, at various dimensional scales, can be created by adjusting the process parameters and/or electrolyte chemistry. In addition, by virtue of the micro-arc melting and resolidification, the composition, phase structure and crystalline nature of the substrate's surface can be modified or changed. This could have a significant beneficial impact on the biological behavior of the textured device. For instance, an amorphous surface layer can improve the device's corrosion resistance. Incorporation of a metallic substance such as silver into the textured surface could render the textured surface antimicrobial. The present invention also improves the texturing process because the resultant textured morphology is not related to or dependent upon the substrate's original or parent microstructure. In the chemical or electrochemical texturing processes, the texturing process is a metal removal process that follows the contours of the parent microstructure. Thus the resultant textured surface exhibits the morphology of the parent microstructure. Because the plasma texturing process is a melting and resolidification process the resolidified surface is independent of the parent microstructure, thus different types of topologies or morphologies can be created.

The present invention is also not limited by the workpiece's geometry. It can be wire, tubular, sheet or a complex geometrical architecture. In the complex geometry instance, the anode should be a conformal reverse replica of the workpiece thereby maintaining a constant gap distance between the two electrodes. In this case the anode would be stationary. In another embodiment, a smaller anode could be made to traverse the surface of the workpiece, at a constant gap, while maintaining a constant the plasma envelope during the anode traverse.

EXAMPLES

The present invention will be further described with reference to the following examples. These examples will show the different texturing morphologies that can be obtained by varying the aqueous plasma processing conditions, e.g. electrolyte, residence time or processing speed, voltage and substrate.

Example 1

With reference to Fig. 1. 5000X (2-2X5)

Texture: Nodular

Substrate: 0.635 mm Titanium

Electrolyte: NaOH, 10 wt. %

Electrolyte Temperature: 80°C Voltage: 86V • -

Current: 0.63-0.91 A

Speed: -0.5 m-min^"1 Example 2

With reference to Fig. 2. 5000X (24-2B)

Texture: Nodular - Slight Cellular Substrate: 0.279 mm Titanium Electrolyte: Na₂SO₄, 10 wt. % Electrolyte Temperature: 80°C Voltage: 93 V Current: 0.45-0.66 A Speed: -6.5 m-min^'1

Example 3

With reference to Fig. 3.

10.000X (25-1 D) Texture: Feathery

Substrate: 0.279 mm Titanium

Electrolyte: K₂SO₄, 10 wt. %

Electrolyte Temperature: 8O⁰C

Voltage: 92V Current: 0.57-0.80 A

Speed: -6.5 m-min^"1

Example 4

With reference to Fig. 4. 5000X (25-2D)

Texture: Feathery

Substrate: 0.279 mm Titanium

Electrolyte: Na₂S₂O₃, 10 wt. %

Electrolyte Temperature: 80⁰C Voltage: 92V

Current: 0.46-0.74 A

Speed: -6.5 m-min^'1 Example 5

With reference to Fig. 5. 5000X (26-2B) Texture: Nodular

Substrate: 0.279 mm Titanium Electrolyte: NaCI, 10 wt. % Electrolyte Temperature: 80°C Voltage: 86V Current: 0.43-0.74 A Speed: -6.5 m-min^"1

Example 6

With reference to Fig. 6. (33-1 AB)

Texture: Cellular

Substrate: 0.279 mm Titanium

Electrolyte: Na₂SO₄, 10 wt. %

Electrolyte Temperature: 80°C Voltage: 95V

Current: 0.40 to 0.80 A

Residence Time: 10 sec

Example 7 With reference to Fig. 7.

1500X (33-5AB)

Texture: Cellular

Substrate: 0.279 mm Titanium

Electrolyte: Na₂SO₄, 10 wt. % Electrolyte Temperature: 8O⁰C

Voltage: 95V

Current: 0.40 to 0.80 A

Residence Time: 45 sec Example 8

With reference to Fig. 8. 1500X (33-8AB) Texture: Large Cellular

Substrate: 0.279 mm Titanium Electrolyte: Na₂SO₄, 10 wt. % Electrolyte Temperature: 80⁰C Voltage: 100V Current: 0.40 to 0.80 A Residence Time: 30 sec

Example 9

With reference to Fig. 9. 1500X (38-9C)

Texture: Nodular-Slight Cellular

Substrate: 0.279 mm Titanium

Electrolyte: Na₂SO₄, 10 wt. %

Electrolyte Temperature: 80°C Voltage: 95V

Current: 0.40 to 0.80 A

Speed: -0.26 m-min^"1

Example 10 With reference to Fig. 10.

1500X (39-3B)

Texture: Nodular

Substrate: 0.250 mm 316L Stainless Steel

Electrolyte: Na₂SO₄, 10 wt. % Electrolyte Temperature: 80°C

Voltage: 25V

Current: 0.40 to 0.80 A

Residence Time: 30 sec Example 11

With reference to Fig. 11. 1500X (39-6B) Texture: Large Island

Substrate: 0.250 mm 316L Stainless Steel Electrolyte: Na₂SO₄, 10 wt. % Electrolyte Temperature: 80⁰C Voltage: 95V Current: 0.40 to 0.80 A Speed: -0.18 m-min^'1

REFERENCES

U.S. PATENT DOCUMENTS us 6,585,875 B1 7/2003 us 6,454,569 9/2002 us 6,368,467 B1 3/2002 us 6,127,596 10/2000 us 6,008,432 12/1999 us 5,981 ,084 11/1999 us 5,958,604 9/1999 us 5,700,366 12/1997 us 5,645,740 7/1997

FOREIGN PATENT DOCUMENTS WO 01/09410 A1 2/2001

OTHER PUBLICATIONS

Zinger et. al., Scale-Resolved Electrochemical Surface Structuring of Titanium for Biological Applications, Journal of the Electrochemical Society, Vol. 150, No.11 , 2003, pp. B495-B503. Ronold et. al., Analysing the Optimal Value for Titanium Implant Roughness in Bone Attachment Using a Tensile Test, Biomaterials, Vol. 24, 2003, pp. 4559-4564.

Lu and Leng, Quantitative Analysis of Osteoblast Behavior on Microgrooved Hydroxyapatite and Titanium Substrata, Journal of Biomedical Materials Research Part A, Vol. 66A, No. 3, 2003, 677-687.

Industrial Applicability

Claims

WHAT IS CLAIMED IS:

1. A process for texturing the surface of a substrate, comprising the step of exposing the surface of the substrate to a continuous arc discharge within an aqueous plasma environment.

2. The process of claim 1 where the substrate is held stationary in the plasma environment.

3. The process of claim 1 where the substrate is moved through the plasma environment.

4. The process of claim 1 where the electrical parameters of the continuous arc discharge are adjusted to achieve a desired morphology of the textured surface.

5. The process of claim 1 where the morphology of the textured surface is a "furrow" type structure of valleys and ridges.

6. The process of claim 1 where the morphology of the textured surface comprises crater structures.

7. The process of claim 1 where the aqueous plasma environment comprises an electrolyte.

8. The process of claim 7 where the electrolyte comprises water and at least one dissolved inorganic salt.

9. The process of claim 8 where the dissolved inorganic salt is selected from the group comprising sulfate salts, carbonate salts and bicarbonate salts.

10. The process of claim 8 where the dissolved inorganic salt is a sulfate salt.

11. The process of claim 8 where the electrolyte has an alkaline pH.

12. The process of claim 8 where the electrolyte has an acidic pH.

13. The process of claim 7 where the electrolyte is maintained at a temperature near the boiling point.

14. The process of claim 7 where the electrolyte is continuously circulated.

15. The process of claim 1 where the substrate is electrically connected as a cathode and together with an anode -comprises an electrolytic cell.

16. The process of claim 15 where the cathode is maintained at a fixed distance from the anode.

17. The process of claim 15 where the cathode is selected from the group of structures comprising a wire, a tube or a sheet.

18. The process of claim 15 where the anode conforms to the structural shape of the cathode.

19. The process of claim 15 where the anode traverses the surface of the cathode at a fixed distance.