WO2012110816A1

WO2012110816A1 - Coating method

Info

Publication number: WO2012110816A1
Application number: PCT/GB2012/050345
Authority: WO
Inventors: Thomas Campbell Prentice; Andrew Robert Mccabe
Original assignee: Accentus Medical Plc
Priority date: 2011-02-16
Filing date: 2012-02-15
Publication date: 2012-08-23
Also published as: GB201102683D0

Abstract

A method of applying a coating to a substrate material (30) for use as a composite body wherein the material is a polymer or a ceramic, which method comprises plasma spraying a powder such as zirconia or titania onto the substrate material to form an initial layer (32). Coatings of other materials such as titanium metal (34) or hydroxyapatite (36) can subsequently be deposited directly or indirectly onto the initial layer (32). The initial layer (32) avoids problems of poor adhesion.

Description

Coating Method

The present invention relates to the provision of a coating on a material so as to form a composite body. The invention relates in particular to the deposition of a coating onto a ceramic or polymer. The invention also relates to a composite body that includes such a coating deposited onto a ceramic or polymer material.

The present invention particularly relates to composite bodies for use as implants. Various surgical procedures require the use of implants. A joint in the human body, such as a hip joint, may degenerate with age or illness, and it may become necessary to replace the natural joint with a prosthetic joint. Such a prosthetic joint is formed from a high-strength material which is not only able to accommodate the loads that the artificial joint may encounter, but is also

biocompatible with the human body. For example materials suitable for making prosthetic joints include metal alloys such as those of titanium (for example ΤΪ-6ΑΙ- 4V), or cobalt chrome alloys. Such alloys are of sufficient strength to withstand the loading conditions after implantation, but are stiffer than bone. The subsequent bonding of such metal implants to adjacent bones can be enhanced by coating the metal implants with a porous coating that assists bone in-growth. For example porous coatings of titanium metal or of hydroxyapatite may be deposited by plasma spraying of powders.

For some implants it would be desirable to be able to use materials that are less stiff, for example polymeric materials, but it is not easy to ensure that implanted polymeric materials bond securely to bone. Similarly it is sometimes desirable to use materials that are lighter, such as ceramics, for example alumina. Depositing metal or hydroxyapatite coatings onto polymeric materials and onto ceramic materials, and achieving a satisfactory bond, has been found to be difficult to achieve. Accordingly, the present invention provides a method of applying a coating to a substrate material for use as a composite body which method comprises plasma spraying powder of glass, silica, zirconia or titania onto the substrate material to form an initial coating, and then applying a further coating of metal and/or hydroxyapatite onto the initial coating, wherein the substrate material is a polymer or a ceramic.

The present invention also provides a composite body comprising a substrate material with an initial coating of glass, silica, zirconia or titania on the substrate material, and a further coating of metal and/or hydro xyapatite on the initial coating, wherein the substrate material is a polymer or a ceramic.

In a preferred embodiment the composite body is an implant. The preferred material to provide this initial coating is titania (i.e. titanium dioxide), which is found to adhere well to the underlying polymer or ceramic. There are numerous suitable commercially-available titania powders that are suitable for forming such coatings, the powders varying in particle size for example in the range 5-150 μηι such as 8-88 μηι or 38-106 μπι.

As an alternative to the use of titania powder, a fine powder (< 100 μηι), for example 8 - 88 μηι or 5 - 50 μηι, of titanium sponge particles might be plasma sprayed in such a way as to form a coating with a high proportion of titanium oxide. The polymer is preferably a heat-resistant polymer, as the polymer surface will be heated during the plasma spraying step. For example the polymer is preferably undamaged at temperatures up to 200 °C, for example (in the case of a thermoplastic polymer) having a melting point above 200 °C, and preferably above 250 °C. The polymer may be a polyaryletherketone (PAEK) as such polymers can have high melting points. For example it may be polyetheretherketone (PEEK), which has a melting point of about 343 'Ό. For use in an implant, the polymer should also be biocompatible. Furthermore the polymer may be loaded with another material, for example with a biocompatible fibrous material such as carbon fibre, to modify its mechanical properties. The process may also be applicable to other polymers such as polyamide (with a melting point of about 216°C), or polyamide (suitable for use up to 230 ^<€).

The ceramic of the substrate may be, for example an oxide substance such as, alumina or zirconia, or a non-oxide substance such as silicon nitride, silicon carbide or boron carbide. In a preferred embodiment of the present invention the ceramic is alumina or zirconia, more preferably alumina, more preferably fully dense alumina, which may be referred to as alpha alumina or recrystallised alumina.

Another suitable ceramic would comprise a mixed ceramic containing alumina and zirconia, for example in proportions 75% and 25% respectively.

It will be understood that the initial coating, for example of titanium dioxide, may fully or partially cover the surface of the material. For example, where the composite body is to be used as an implant it is preferable to cover all parts of the material that will come into contact with bone with the initial coating. Preferably the outer surface of the final composite body is coated. The initial coating, for example of titania, may vary in thickness according to the specification requirements for the substrate or medical device. Examples of initial coating thickness may be, but are not limited to a range of 10 to 50 μηι, for example 20 to 40 μπι. In one embodiment of the present invention, the further coating is a metal coating, and is of titanium. The present invention also provides a composite body comprising a substrate material and an initial coating of titanium dioxide on the substrate material wherein the substrate material is a polymer or a ceramic and further comprises a further coating of titanium and/or hydroxyapatite on the initial coating. The initial coating may be of titanium dioxide.

The further coating, which may for example be of titanium, titanium alloy or cobalt chrome alloy, may vary in thickness according to the particular initial coating, and the requirements of the type of substrate or medical device. Examples of further coating thickness may be, but are not limited to a range of 10 to 150 μηι, for example 80 to 100 μηι or 20 to 35 μηι.

There are many commercially available powders, for example of titanium, titanium alloy or cobalt chrome alloy, that are suitable for making the further coating. Examples of powder particle sizes may be, but are not limited to a range 10 to 500 μηι, for example 200 to 350 μηι or 90-25 μηι.

Where the substrate material is a ceramic and the coatings are deposited by plasma spraying of powders, the coatings are preferably applied quickly in sequence, so the substrate material is not allowed to cool significantly between application of the first and second coatings. Preferably each coating is applied no more than 10 minutes after the previous coating, more preferably not more than 5 minutes after the previous coating. Indeed preferably the successive coatings are applied immediately after each other, with no time interval.

In a further embodiment of the present invention, the method further comprises applying a coating of a biological-fixation-enhancing phosphate, such as hydroxyapatite. Hydroxyapatite (sometimes referred to as hydroxylapatite or as calcium hydroxyapatite) has the formula Cai₀(PO₄)6(OH)2 and so is chemically similar to the mineral component of bones. It is known that hydroxyapatite may support bone ingrowth and osseointegration, and so is suitable for example in orthopaedic, dental, spinal, craniofacial and maxillofacial applications. As an alternative to depositing hydroxy-apatite, a coating of a substituted hydroxyapatite, such as silicon- substituted hydroxyapatite might be deposited; or a coating of a chemically-similar compound such as tricalcium phosphate (formula Ca₃(P0₄)2) or brushite (formula CaHP0₄.2H₂0). These materials are also believed to enhance osseointegration. The biological-fixation-enhancing phosphate coating may be deposited by plasma spraying, or by another deposition technique such as electro-deposition.

The coating of biological-fixation-enhancing phosphate may be deposited directly onto the coating of metal, such as titanium. The present invention also provides a composite body comprising a substrate material and an initial coating (e.g. of titanium dioxide) on the substrate material wherein the substrate material is a polymer or a ceramic, and further comprises a coating of titanium on the titanium dioxide and a coating of a biological-fixation-enhancing phosphate on the titanium. Alternatively the biological-fixation-enhancing phosphate may be applied directly onto the initial coating.

The coating of biological-fixation-enhancing phosphate, for example hydroxyapatite, may vary in thickness according to the particular initial coating and/or further coating such as titanium, and the requirements of the type of substrate or medical device. Examples of biological-fixation-enhancing phosphate coating thickness may be, but are not limited to a range 10 to 200 μηι, for example 40 to 85 μηι.

There are many examples of hydroxyapatite powder that are suitable for forming the biological-fixation-enhancing coating. The powder particle sizes may be, but are not limited to a range 10 to 200 μηι, for example 45 to 135 μηι.

Preferably the biological-fixation-enhancing coating, such as hydroxyapatite has a calcium to phosphate atomic ratio of 1 .67 to 1 .76, and a coating crystallinity of greater than or equal to 45%.

The initial coating and/or further coating(s) may incorporate an active therapeutic agent. Examples include a biocide to reduce the risk of infection, an agent to reduce the risk of disease, or an agent to promote bone or tissue growth.

This sequence of coatings is of particular use in forming a body for use as an implant. The resulting composite body may be for example an implant for use in a surgical procedure, that is to say a prosthetic or orthopaedic implant, e.g. for dental, spinal, craniofacial or maxillofacial applications. However, such composite bodies may be used in other applications, for example as a component of a machine. The composite body may also be subjected to a surface roughening and/or cleaning prior to coating. Roughening may be achieved by many techniques, for example grit blasting using materials such as alumina or sintered hydroxyapatite.

The initial coating is deposited on the material by plasma spraying. The parameters for operating the plasma torch are typically taken from those parameters already known for operating a plasma torch for deposition onto metal and ceramic.

Any further coatings can be deposited on the initial coating by suitable methods. Typically plasma spraying is a suitable technique for depositing titanium and/or hydroxyapatite. Alternatively electro-deposition may be used for depositing coatings such as titanium or hydroxyapatite, or to increase the thickness of a layer deposited by a different process.

The invention will now be further and more particularly described, with reference to the following examples and with reference to the accompanying drawings in which:

Figure 1 shows a perspective view of a prosthetic hip joint implant;

Figures 2A, 2B, 2C and 2D show a sectional view through a ceramic object at successive stage during a production process.

Referring to figure 1 , this shows a perspective view of a prosthetic hip joint 10, including an acetabular cup implant 12 and a femoral prosthesis 14. The femoral prosthesis 14 consists of a stem 15 that would be fitted into the top of a patient's femur 17, and a projecting head in the form of a ball 16 with a polished surface. The acetabular cup implant 12 is intended to be fixed into the patient's pelvis 20, and it consists of a bone-compatible coating 21 on the surface of a socket insert 22, the socket insert being made from ceramic, such as alumina; in an alternative the socket insert 22 is made of PEEK. The socket insert 22 defines a part-spherical recess 23 into which the ball 16 of the femoral prosthesis 14 fits, providing a low friction joint. The coating 21 includes on its outer surface a layer of hydroxyapatite, providing a porous microstructure which enhances bone re-growth from the pelvis 20. In this example the outer surface of the cup implant 12 is generally hemispherical, but with a number of ridges 25.

The process of making such a coating on a ceramic or PEEK substrate will now be described, in relation to figures 2A to 2D in which the substrate 30 is shown schematically as being of cylindrical cross-section. It will be appreciated that the process is applicable to substrates of any desired shape. The process uses plasma spraying of appropriate powders. In each of the following Examples the plasma spraying was carried out using a Plasma Technik M1000 system with F4 Vb torch. The powder was supplied at the rate specified, by a Twin-10 powder feeder, and the plasma spray torch was held by an ASEA robot arm.

Figure 2A shows the substrate 30 of fully dense alpha alumina prior to the coating process. It will be appreciated that it is difficult to apply a well-adhered coating to this form of ceramic because of its susceptibility to thermal shocks, and because of its surface smoothness. Although titanium can be deposited by plasma spraying of titanium powder, it is found that it does not adhere to the ceramic substrate.

Example 1 - Application of a thin titanium dioxide (titania) coating to an alumina ceramic cup.

Titanium dioxide was applied to an alumina ceramic cup 54 mm in diameter. Titanium dioxide was plasma sprayed onto the ceramic cup at a spray distance of 250 mm, the plasma gas flow was 60 slpm (standard litres per minute) argon and 5 slpm hydrogen, the current was 550 amps and the spray rate 20 g/min. A coating of mass 1 .5 g was deposited.

The titanium dioxide powder was Metco 102 (particle size 8 to 88 μηι) from Sulzer Metco.

The coating obtained was coloured black, was approximately 1 10 μηι thick, and was well adhered to the cup as demonstrated by attempts to scrape it off with a metal scalpel. The coating was also found to be electrically conducting (140 Ω over the ceramic cup).

Figure 2B shows the substrate 30 with a coating 32 of titanium dioxide.

Example 2 - Application of a titanium coating onto the thin titanium dioxide coating of Example 1

The well adhered titanium dioxide coating was used as a bond coat onto which to spray a titanium coating. Without the titanium dioxide bond coating, attempts to spray titanium directly onto the ceramic had failed.

Titanium was applied to a titanium dioxide coated alumina ceramic cup of Example 1 . Titanium powder was plasma sprayed onto the coated ceramic cup at a spray distance of 250 mm, the plasma gas flow was 60 slpm argon and 5 slpm hydrogen, the current was 450 amps and the spray rate 20 g/min. A coating of mass approximately 1 .0 g was deposited and had a thickness of 80 μηι.

The titanium powder was 99840/65 (particle size range 150 to 230 μηι) from Metallisation Ltd.

In order to prevent spalling, the titanium coating was applied as soon as possible after the titanium dioxide so that the cup did not cool down too much.

Preferably the time interval between coating with titanium dioxide and coating with titanium metal is no more than 5 minutes, more preferably no more than 2 minutes. A fine titanium powder was used in order to reduce the possibility of grit blasting the surface. Leaving too a long a time interval between applying the coatings typically results in cracks and spallation of the Ti0₂ coating. Figure 2C shows the substrate 30 with the coating 32 of titanium dioxide and also the coating 34 of titanium metal.

Example 3 - Application of hydroxyapatite onto the titanium coating of Example 2 Hydroxyapatite was applied to the titanium coated alumina ceramic cup of example 2. Hydroxyapatite was plasma sprayed onto the coated ceramic cup at a spray distance of 150 mm, the plasma gas flow was 60 slpm argon and 3 slpm hydrogen, the current was 450 amps and the spray rate 18 g/min. A coating of mass approximately 1 .5 g was deposited and had a thickness of 120 μηι.

The hydroxyapatite powder was Medipure 20-15 (particle size 45 to 130 μηι) from Medicoat.

The combination of coatings deposited in Examples 1 to 3 was titanium dioxide, then titanium and then hydroxyapatite. These coatings were successfully deposited onto an alumina ceramic orthopaedic cup to provide a total coating thickness of 310 μηι.

Figure 2D shows the substrate 30 with the titanium dioxide coating 32, the titanium metal coating 34 and the hydroxyapatite coating 36. The following Examples relate to deposition of a coating onto a polymeric substrate. It is difficult to obtain satisfactory adhesion if titanium metal is deposited directly onto a polymeric substrate.

Example 4 - Application of a thin titanium dioxide coating to a PEEK disc.

Titanium dioxide was applied to a PEEK disc of diameter 25 mm, whose surface had been roughened by grit blasting with alumina prior to coating, and then mounted on a metal tensile pull-off test specimen. Titanium dioxide was plasma sprayed onto the PEEK disc at a spray distance of 250 mm, the plasma gas flow was 60 slpm argon and 5 slpm hydrogen, the current was 550 amps and the spray rate 20 g/min. A coating of mass 0.1 g was deposited and had a thickness of approx 50 μηι.

Care was taken not to melt the PEEK. The coating was well adhered, when tested with a scalpel. The coating was also found to be electrically conducting (140 Ω over the PEEK disc).

Example 5 - Application of a titanium coating onto the thin titanium dioxide coating of Example 4 The well adhered titanium dioxide coating was used as a bond coat onto which to spray a titanium coating. Without the titanium dioxide bond coating, attempts to spray titanium directly onto the polymer had failed, and the titanium had the effect of grit-blasting the surface of the PEEK.

Titanium was applied to the titanium dioxide coated PEEK disc of Example 4. Titanium powder was plasma sprayed onto the coated disc at a spray distance of 250 mm, the plasma gas flow was 60 sipm argon and 5 sipm hydrogen, the current was 450 amps and the spray rate 20 g/min. A coating of mass approximately 0.1 g was deposited and had a thickness of 40 μηι.

The titanium powder was 99840/65 (particle size range 150 to 230 μηι) from Metallisation Ltd. The coating was well adhered, with a tensile strength measured to be about

23 MPa using the ASTM F1 147-05 standard test method.

Example 6 - Application of hydroxyapatite onto the titanium coating of Example 5 Hydroxyapatite was applied to the titanium coated PEEK disc of Example 5.

Hydroxyapatite was plasma sprayed onto the coated disc at a spray distance of 150 mm, the plasma gas flow was 60 sipm argon and 3 sipm hydrogen, the current was 450 amps and the spray rate 18 g/min. A coating of mass approximately 0.1 g was deposited and had a thickness of 60 μηι.

The combination of coatings deposited in Examples 4 to 6 was titanium dioxide, then titanium and then hydroxyapatite. These coatings were successfully deposited onto a PEEK disc configured for tensile testing. The total coating thickness was 160 μηι.

Example 7 - Application of a hydroxyapatite coating onto the thin titanium dioxide coating of Example 4.

The well adhered titanium dioxide coating was used as a bond coat onto which to spray a hydroxyapatite coating.

Hydroxyapatite was applied to the titanium dioxide coated PEEK disc of Example 4. Hydroxyapatite was plasma sprayed onto the coated disc at a spray distance of 150 mm, the plasma gas flow was 60 slpm argon and 3 slpm hydrogen, the current was 450 amps and the spray rate 18 g/min. A coating of mass

approximately 0.10 to 0.13 g was deposited and had a range of thicknesses of 60 to 80 μπι. The hydroxyapatite powder was Medipure 20-15 (particle size 45 to 130 μηι) from Medicoat.

The coating was well adhered, with a tensile strength measured to be about 15-18 MPa using the ASTM F1 147-05 standard test method.

The Examples 1 to 3 show how the coating 21 can be deposited onto the ceramic of the socket insert 22; the coating 21 corresponds to the combination of the coatings 32, 34 and 36 shown in figure 2D. The outermost coating 36 of

hydroxyapatite enhances bone regrowth, and hence the bonding of bone to the acetabular cup implant 12 after it has been implanted into the pelvis 20.

The Examples 4 to 7 show how a substantially identical coating can be deposited onto a substrate of PEEK. This might again be an acetabular cup implant 12 as shown in figure 1 , and again the outermost layer of hydroxyapatite would enhance bone regrowth and hence the bonding of bone to the acetabular cup implant. In another application the PEEK substrate might comprise an intra-vertebral cage, and in this context too the coating of hydroxyapatite would enhance bone regrowth and hence the bonding of the cage to the adjacent vertebrae. It will be appreciated that the PEEK substrate, when coated in this way, may be for use in other implant applications, as mentioned previously, for example for orthopaedic, dental, spinal, craniofacial and maxillofacial applications.

Claims

1 . A method of applying a coating to a substrate material for use as a composite body which method comprises plasma spraying powder of glass, silica, zirconia or titania onto the substrate material to form an initial coating, and then applying a further coating of metal and/or hydroxyapatite onto the initial coating, wherein the substrate material is a polymer or a ceramic.

2. A method according to claim 1 wherein the substrate material is a ceramic comprising alumina.

3. A method according to claim 1 wherein the substrate material is a polymer comprising polyetheretherketone (PEEK) or a polyimide.

4. A method according to any one of the preceding claims which further comprises plasma spraying the further coating onto the initial coating.

5. A method according to claim 4 wherein the substrate material is a ceramic and wherein the further coating is applied no more than 10 minutes after plasma spraying the initial coating onto the substrate material.

6. A method according to claim 4 or 5 wherein the further coating is of titanium.

7. A method according to any one of claims 1 to 3 wherein the initial coating is of titanium dioxide, and which comprises applying the further coating onto the titanium dioxide by electro-deposition.

8. A method according to any one of the preceding claims wherein the further coating is of metal, which comprises applying a coating of a biological-fixation- enhancing phosphate onto the metal coating.

9. A method according to any one of the preceding claims wherein the composite body is an implant.

10. A composite body comprising a substrate material with an initial coating of glass, silica, zirconia or titania on the substrate material and a further coating of metal and/or hydroxyapatite on the initial coating, wherein the substrate material is a polymer or a ceramic.

1 1 . A composite body according to claim 10 wherein the further coating is of titanium.

12. A composite body according to claim 10 or claim 1 1 further comprising a coating of a biological-fixation-enhancing phosphate.

13. A composite body according to any one of claims 10 to 12 wherein the composite body is an implant for use in surgery.