TREATMENTS TO REDUCE THROMBOGENETICITY IN HEART VALVES MADE FROM TITANIUM AND ITS ALLOYS

Abstract: The present invention provides a method for coating a titanium based component with diamond-like carbon to reduce the thrombogeneticity of the component. In a preferred embodiment, the titanium based component is a heart valve. According to the present invention, the component is placed in a vacuum chamber and heated to about 600 DEG -650 DEG C (1112-1202 DEG F). Thereafter, silicon is then deposited onto the component, and the component is simultaneously bombarded with a beam of energetic ions to form a metal-silicide bonding layer. The component then is cooled to at least about 100 DEG C (212 DEG F), preferably about 80 DEG C (176 DEG F), and a diamond-like carbon precursor is condensed onto the metal-silicide bonding layer. The precursor is simultaneously bombarded with a beam of energetic ions to form a coating of diamond-like carbon.

Description: TREATMENTS TO REDUCE THROMBOGENETICITY IN HEART VALVES MADE FROM TITANIUM AND ITS ALLOYS GEOFFREY DEARNALEY JAMES LANKFORD, JR. The present application is a continuation-in-part of copending application Serial No. 08/220,234, filed March 29, 1994. FIELD OF THE INVENTION The present invention relates to decreasing the thrombogeneticity of heart valves and other medical implants made from titanium and its alloys. BACKGROUND OF THE INVENTION Titanium has become a popular metal for the manufacture of human implants. The FDA considers titanium and many of its alloys ("titanium based materials") to be biocompatible. Titanium based materials also are easily machined, are not overly brittle, and are durable enough for the manufacture of most medical implants. Unfortunately, titanium based materials also tend to encourage thrombogenesis. This tendency is undesirable, particularly for implants used in the circulatory system. An example of a medical implant that is used in the circulatory system is a heart valve. A method for reducing the thrombogeneticity of titanium and its alloys would be highly desirable. SUMMARY OF THE INVENTION The present invention provides a method for coating a titanium based component with amorphous diamond-like carbon to reduce the thrombogeneticity of the component. In a preferred embodiment, the titanium based component is a heart valve. According to the present invention, the component is placed in a vacuum chamber and heated to about 600°-650°C (1112-1202°F). Thereafter, silicon is deposited onto the component, and the component is simultaneously bombarded with a beam of energetic ions to form a metal-silicide bonding layer. The component then is cooled to at least below about 100°C (212°F), preferably about 80°C (176°F), and a diamond¬ like carbon precursor is condensed onto the metal-silicide bonding layer. The precursor is simultaneously bombarded with a beam of energetic ions, preferably nitrogen ions, to form a coating of diamond-like carbon. DETAILED DESCRIPTION OF THE INVENTION Amorphous carbon, or "diamond-like carbon" ("DLC"), is a biocompatible coating material that is less thrombogenic than titanium. A primary concern about using DLC for medical implants is the strength of the bond that forms between the substrate and the DLC coating. The present invention provides a method for coating titanium based components, particularly components for use in the circulatory system, with a tightly adhered coat of DLC to reduce the thrombogeneticity of the component. The present invention ensures strong adherence between the DLC and the surface of the titanium based component by forming a titanium-silicide interlayer. In order to knit the successive layers of metal-silicon-DLC together effectively, it is necessary to supply a bond-interface for the metal- silicon bond as well as for the silicon-DLC bond. Without limiting the present invention, it is believed that the present method achieves this result by forming strong interatomic bonds having a character that is intermediate to the type of bond that exists between the atoms in the metal and the type of bonds that exist in the silicon. Silicon is chosen as the substance to form the interlayer because (a) DLC is known to adhere better to silicon than to any other substrat —which is attributed to strong SiC bonds formed at the interface of the two materials—and (b) titanium is known to react with silicon to form a disilicide (TiS 2 ) when reacted with silicon. The method for coating the titanium based components uses ion beam assisted deposition of silicon, followed by deposition of DLC. After conventional cleaning of the component to remove superficial contaminants, such as grease, the component is placed in a vacuum chamber that has been evacuated to a base pressure of preferably less than about 10" 5 torr. The component then is bombarded with ions, preferably argon ions, at an energy range between about 10 - 100 keV, preferably around 10 keV. This ion bombardment provides an effective means to remove some of the remaining adsorbed atoms from the surface. The component then is heated. Titanium based components tend to have a thin oxide layer on their surface which should be removed in order to form the suicide interlayer. This is accomplished by heating the titanium component sufficiently to "dissolve" the oxide layer. The component preferably should be heated to about 600-650°C (1112-1202°F) before condensing silicon on the surface of the component. After the component has been heated and while remaining at that temperature, silicon can be deposited on the surface of the component using known means. A preferred means is to position the workpiece directly over a volatilization hearth which is maintained at a preferred temperature of about 1900°C (3450°F), until a preferred coating thickness of between about 100-200 nm has been achieved. The thickness of the coating may be monitored by standard methods, e. g. , using the frequency change of a quartz crystal oscillator. The component preferably should be simultaneously bombarded with an energetic beam of ions, preferably nitrogen ions, at an energy range between about 500 eV to 100 keV, preferably between about 10-20 keV, in order to form a layer of titanium suicide at the titanium-silicon interface. Nitrogen is preferred for the ion beams of the present invention because nitrogen ions actually will bond with the substrate/coating or interlayer. Inert ions, such as argon and/or helium ions, will not bond with the substrate/film. The use of inert ions could result in bubbling and/or a weaker coating. Although it has not been proven, it is believed that strong carbon-nitrogen bonds form in the DLC layer when the ions used to make the DLC are nitrogen ions. In any event, the use of a beam of nitrogen ions can result in DLC coatings that increase wear resistance and decrease friction up to 5-7 times more than DLC coatings formed using other types of ions. Although nitrogen ions are preferred, other ions may be used, such as argon, hydrogen, silicon, methane, helium, or neon, having an energy between 500 eV to 100 keV, preferably 10-30 keV. The ion-to-atom ratio should be sufficient, preferably at least 1 ion to 10 silicon atoms, to form a layer of metal suicide at the metal-silicon interface. Thereafter, the component should be cooled to at least below about 100°C (212°F), preferably to about 80°C (176°F). The cooling preferably should be done without removing the component from the vacuum chamber. Thereafter, a diamond-like carbon (DLC) precursor should be deposited. In a preferred embodiment, the DLC precursor is polyphenyl ether. Other suitable precursor materials include carbon-based diffusion pump materials which have a low vapor pressure and can be vaporized stably at room temperature. Preferable diffusion pump fluids include, but are not necessarily limited to: polyphenyl ether; polydimethyl siloxane; pentaphenyltrimethyl siloxane; and, elcosyl napthalene. The precursor is vaporized and condensed onto the surface of the component using known means. Generally, the precursor is placed in a reservoir, heated to between about 150 β C-170°C (302°F-338°F), and directed onto the cooled component. Substantially simultaneously, the component should be bombarded, either in a continuous or interrupted fashion, with an energetic beam of ions. A preferred ion source is nitrogen. Other suitable ions include, but are not necessarily limited to, argon, hydrogen, silicon, methane, helium, or neon. The ion beam should have an energy between about 500 eV to 100 keV, preferably between about 10-30 keV. The energy of bombardment must be sufficient to ionize the constituent molecules in the precursor film, and to rupture the bonds between hydrogen and other atoms, thereby releasing the hydrogen into the surrounding vacuum to be pumped away. The rate of arrival of the ions should be controlled in relation to the rate of arrival of the precursor molecules. This process should require about one ion for every 100 atoms in the final product coating; however, the ion-to-atom ratio will vary according to the mass and energy of the ion species. Typically, 100 eV must be deposited for each carbon atom in the coating. Persons of ordinary skill in the art will recognize how to achieve the correct linear energy of transfer in the ionizing process. The procedure should be continued until a thickness of DLC between about 100 nm-10 microns is achieved. Example 1 A DLC coating of approximately 1 micron in thickness is prepared by nitrogen ion bombardment of a polyphenyl ether precursor. A titanium based heart valve comprised of a titanium alloy containing vanadium and aluminum is cleaned in isopropyl alcohol prior to coating. Isopropyl alcohol is chosen because it leaves few, if any, residues. Wear testing reveals that, under some circumstances, there could be a loss of adhesion of the coating. Example 2 A titanium alloy heart valve of the same composition as in Example 1 is treated using a bond-coat of silicon. After conventional solvent cleaning of the component to remove superficial contaminants, such as grease, the component is placed in a vacuum chamber that has been evacuated to a base pressure of 10" 5 torr. The component then is bombarded with nitrogen ions at an energy of about 10 keV to remove some of the remaining adsorbed atoms from the surface. The component is heated to about 600°C (1112°F). Silicon then is deposited onto the outer surface of the component. The workpiece is positioned directly over the volatilization hearth which is maintained at a temperature of about 1900°C (3450°F), until a preferred coating thickness of about 100 nm has been achieved. The thickness of the coating is monitored by standard methods, e. g. , using the frequency change of a quartz crystal oscillator. The component is simultaneously bombarded with an energetic beam of nitrogen ions at an energy of about 20 keV and an ion-to-atom ratio of at least 1 ion to 10 silicon atoms for about 15 minutes to form a layer of metal suicide at the metal-silicon interface. Thereafter, the component is cooled to about 80°C without removing the component from the vacuum chamber. Polyphenyl ether is heated to at least about 150°C (302°F) and condensed onto the surface of the component. The component simultaneously is bombarded with an energetic beam nitrogen ions having an energy of about 20 keV and an ion-to-atom ratio of at least 1 ion to 100 precursor molecules. The procedure is continued until a thickness of DLC of about 100 nm is achieved. In prolonged wear tests, at a contact pressure of 6.9 MPa under serum, i.e., load and environmental conditions equivalent to those encountered in vivo by a heart valve, no decohesion or loss of DLC is observed after about 10.0 million reciprocated wear cycles. Persons of ordinary skill in the art will recognize that many modifications may be made to the present invention without departing from the spirit and scope of the present invention. The embodiment described herein is meant to be illustrative only and should not be taken as limiting the invention, which is defined in the following claims.

Claims: We claim: 1. A method for coating a titanium based component with diamond-like carbon comprising: subjecting said titanium based component to a vacuum of at least about 10" 5 torr; heating said component to at least about 600°C-650°C (1112-1202°F); condensing silicon onto said outer surface of said component in an amount sufficient to form a titanium-silicide bonding layer; substantially simultaneously bombarding said component with a first energetic beam of ions at a first energy, a first density, and for a first amount of time sufficient to form said titanium-silicide bonding layer; cooling said titanium based component to at least about 100°C (212°F); condensing a diamond-like carbon precursor onto said outer surface of said component at a temperature and for a time sufficient to form a film of precursor molecules on said titanium-silicide bonding layer; substantially simultaneously bombarding said component with a second energetic beam of ions at a second energy, a second density, and for a second amount of time sufficient to form a coating of diamond- like carbon on said component. 2. The method of claim 1 wherein said first and second ions are nitrogen ions. 3. The method of claim 1 wherein and said first and second energies of ion bombardment are between about 10-30 keV. 4. The method of claim 1 wherein said component is bombarded with argon ions at about 10 keV energy before said metal alloy is heated. 5. The method of claim 2 wherein said component is bombarded with argon ions at about 10 keV energy before said metal alloy is heated. 6. The method of claim 1 wherein said component is cooled to a temperature of about 80°C before beginning to condense said diamond-like carbon precursor onto said surface of said component. 7. The method of claim 2 wherein said component is cooled to a temperature of about 80°C before beginning to condense said diamond-like carbon precursor onto said surface of said component. 8. The method of claim 3 wherein said metal alloy component is cooled to a temperature of about 80°C before beginning to condense said diamond-like carbon precursor onto said surface of said component. 9. The method of claim 5 wherein said metal alloy component is cooled to a temperature of about 80°C before beginning to condense said diamond-like carbon precursor onto said surface of said component. 10. The method of claim 1 wherein said silicon is condensed on said outer surface of said component to a thickness of between about 100-200 nm. 11. The method of claim 2 wherein said silicon is condensed on said outer surface of said component to a thickness of between about 100-200 nm. 12. The method of claim 3 wherein said silicon is condensed on said outer surface of said component to a thickness of between about 100-200 nm. 13. The method of claim 5 wherein said silicon is condensed on said outer surface of said component to a thickness of between about 100-200 nm. 14. The method of claim 6 wherein said silicon is condensed on said outer surface of said component to a thickness of between about 100-200 nm. 15. The method of claim 7 wherein said silicon is condensed on said outer surface of said component to a thickness of between about 100-200 nm. 16. A titanium based component produce by a process comprising: subjecting said titanium based component to a vacuum of at least about 10 "5 torr; heating said component to at least about 600°C-650°C (1112-1202°F); condensing silicon onto said outer surface of said component in an amount sufficient to form a titanium-silicide bonding layer; substantially simultaneously bombarding said component with a first energetic beam of ions at a first energy, a first density, and for a first amount of time sufficient to form said titanium-silicide bonding layer; cooling said titanium based component to at least about 100°C (212°F); condensing a diamond-like carbon precursor onto said outer surface of said component at a temperature and for a time sufficient to form a film of precursor molecules on said titanium-silicide bonding layer; substantially simultaneously bombarding said component with a second energetic beam of ions at a second energy, a second density, and for a second amount of time sufficient to form a coating of diamond- like carbon on said component. 17. A medical implant comprising a titanium based component having an outer surface comprised of a silicon-based interlayer bonded to a coating of diamond-like carbon. 18. The medical implant of claim 17 wherein said silicon-based interlayer comprises a titanium silicide bonding layer at said outer surface of said titanium based component; and a carbon-silicon bonding layer at said outer surface of said silicon-based interlayer. 19. The medical implant of claim 17 comprising a heart valve. 20. The medical implant of claim 18 comprising a heart valve. 21. A method for coating a metal alloy component with diamond like carbon comprising: subjecting said metal alloy component to a vacuum of at least about 10 " 5 torr; heating said component to the highest temperature acceptable for said metal alloy; condensing silicon onto said outer surface of said component in an amount sufficient to form a metal- . silicide bonding layer; substantially simultaneously bombarding said component with a first energetic beam of nitrogen ions at a first energy, a first density, and for a first " 13 amount of time sufficient to form a metal-silicide 14 bonding layer; ψ 15 cooling said component to at least about 100°C (212°F); 16 condensing a diamond-like carbon precursor onto said 17 outer surface of said component at a temperature 18 and for a time sufficient to form a film of 19 precursor molecules on said metal-silicide bonding 20 layer; 21 substantially simultaneously bombarding said component 22 with a second energetic beam of nitrogen ions at a 23 second energy, a second density, and for a second 24 amount of time sufficient to form a coating of 25 diamond-like carbon on said component.

Inventors: DEARNALEY GEOFFREY, LANKFORD JAMES JR, DEARNALEY, GEOFFREY, LANKFORD, JAMES, JR.
Applicants: SOUTHWEST RES INST, SOUTHWEST RESEARCH INSTITUTE
Application year: 1996
Main classification: A61L33
IPC codes: A61F2, A61L33, C23C14, C23C16