US3677795A

US3677795A - Method of making a prosthetic device

Info

Publication number: US3677795A
Application number: US821080A
Authority: US
Inventors: Jack C Bokros; Willard H Ellis
Original assignee: Gulf Oil Corp
Current assignee: Gulf Oil Corp
Priority date: 1969-05-01
Filing date: 1969-05-01
Publication date: 1972-07-18
Anticipated expiration: 1989-07-18
Also published as: GB1282685A; FR2041786A6; DE2021320B2; DE2021320C3; JPS5014837B1; CA948352A; DE2021320A1

Abstract

THIS IS A METHOD FOR MAKING A PROSTHETIC DEVICE FOR IMPLANTATION IN A LIVING BODY. THE METHOD COMPRISES THE STEPS OF FORMING A SUBSTRATE OF A MATERIAL STABLE AT TEMPERATURE OF AT LEAST ABOUT 1350* C., HEATING SAID SUBSTRATE TOGETHER WITH PARTICULATE MATERIAL TO PROVIDE ADDITIONAL DEPOSITION SURFACE AREA TO A TEMPERATURE OF BETWEEN ABOUT 1350* C. AND ABOUT 1600* C. IN A REACTION CHAMBER, FLOWING A MIXTURE OF PROPANE OR BUTANE AND AN INERT GAS THROUGH SAID REACTION CHAMBER AT ATMOSPHERIC PRESSURE, SAID PROPANE OR BUTANE CONSTITUTING BETWEEN ABOUT 15 VOLUME PERCENT AND ABOUT 40 VOLUME PERCENT OF SAID MIXTURE, SO THAT SAID SUBSTRATE BECOMES COATED WITH ISOTROPIC PYROLYTIC CARBON HAVING A BAF BETWEEN 1.0 AND 2.0, HAVING AN APPARENT CRYSTALLITE SIZE OF ABOUT 50 A. OR LESS AND HAVING A DENSITY OF AT LEAST ABOUT 1.5 GRAMS PER CM.3, AND CONTINUING SAID GAS FLOW UNTIL THE THICKNESS OF SAID ISOTROPIC CARBON DEPOSIT IS AT LEAST ABOUT 50 MICRONS, WHEREBY A DEVICE IS PRODUCED WHICH HAS EXCELLENT COMPATIBILITY WITH BODY TISSUE AND IS NONTHROMBOGENIC. IF DESIRED, THE HYDROCARBON GAS MIXTURE MAY CONTAIN A CARBIDE-FORMING ELEMENT SELECTED FROM THE GROUP CONSISTING OF SILICON, ZIRCONIUM, TITANIUM, TANTALUM, BORON, TUNGSTEN, NIOBIUM, VANADIUM, MOLYBDENUM, ALUMINUM, OR HAFNIUM.

Description

United States Patent O 3,677,795 METHOD OF MAKING A PROSTHETIC DEVICE Jack C. Bokros, San Diego, and Willard H. Ellis, Leucadia, Calif., assignors to Gulf Oil Corporation, San Diego, Calif.

N Drawing. Continuation-impart of application Ser. No. 649,811, June 29, 1967. This application May 1, 1969, Ser. No. 821,080

Int. Cl. C0lb 31/00; C23c 11/00 US. Cl. 117-46 CG 14 Claims ABSTRACT OF THE DISCLOSURE This is a method for making a prosthetic device for implantation in a living body. The method comprises the steps of forming a substrate of a material stable at temperature of at least about 1350 C., heating said substrate together with particulate material to provide additional deposition surface area to a temperature of between about 1350 C. and about 1600 C. in a reaction chamber, flowing a mixture of propane or butane and an inert gas through said reaction chamber at atmospheric pressure, said propane or butane constituting between about 15 volume percent and about 40 volume percent of said mixture, so that said substrate becomes coated with isotropic pyrolytic carbon having a BAF between 1.0 and 2.0, having an apparent crystallite size of about 50 A. or less and having a density of at least about 1.5 grams per cmfi, and continuing said gas flow until the thickness of said isotropic carbon deposit is at least about 50 microns, whereby a device is produced which has excellent compatibility with body tissue and is nonthrombogenic. If desired, the hydrocarbon gas mixture may contain a carbide-forming element selected from the group consisting of silicon, zirconium, titanium, tantalum, boron, tungsten, niobium, vanadium, molybdenum, aluminum, or hafnium.

This application is a continuation-in-part of our pending application Ser. No. 649,811, filed June 29, 1967, now US. Pat. No. 3,526,005.

This invention relates generally to prosthetic devices and more particularly to prosthetic devices for use within a living body.

Prosthetic devices, such as intravascular prostheses, have been used for a number of years, and it is expected that usage of such devices will increase in the future as medical expertise continues to improve. One example is the artificial heart valve which is used fairly extensively today, and more complex circulatory assist devices are currently under development. Artificial kidneys are another class of prosthetic devices becoming more and more available.

In order to futher the development and utilization of prosthetic devices, the surfaces of these devices which come in contact with blood and tissue should be completely compatible therewith, whether the contact be made by implantation or insertion within the body or by passage therethrough of blood at locations exterior of the body. Two of the most common materials for intravascular prosthesis are metals, for applications where high strength and good wearability are important, and plastics for applications wherein flexibility is needed. Metals are thrombogenic and are subject to corrosion. Plastics, without some treatment, are also thromobogenic and are subject to degradation. Stainless steel and tantalum are among the most popular metals used today, whereas polyethylene, Tefion and the polycarbonates are examples of plastics considered suitable.

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None of these materials are considered to be totally satisfactory for the construction of prosthetic devices.

It is an object of the present invention to provide improved prosthetic devices by utilizing improved materials of construction. Another object is to provide prosthetic devices which are nonthromobogenic and which will retain this characteristic although implanted in the body for long periods of time. A further object is to provide improved prosthetic devices which are compatible with body tissue, do not cause irritation thereof, and have good strength and resistance to deterioration when implanted within a living body. Still another object is to provide a method for making improved prosthetic devices. One further object is to provide an improved method for repairing an intravascular defect within a living body. These and other objects of the invention should be clearly apparent from the following description relating to the fabrication and use of devices embodying various features of the invention.

It has been found that prosthetic devices having improved characteristics can be made by coating suitable substrates of the desired shape and size with dense pyrolytic carbon. Dense pyrolytic carbon has been found not only to significantly increase the strength of the substrate upon which it is coated, but also to resist wear and deterioration even if implanted within a living body for long periods of time. While reference is hereinafter generally made to use of the prosthetic devices in combination with a human body, wherein of course the primary use is considered likely to occur, it should also be recognized that the improved prosthetic devices may likewise be used in other living mammals. For example, it may be desirable to use pins which include the indicated pyrolytic carbon coatings for use in repairing or setting broken bones in horses or dogs.

For use on complex shapes and in order to obtain maximum strength, it is desirable that the pyrolytic carbon be nearly isotropic. Anisotropic carbons, though thromobo-resistant, tend to delaminate when complex shapes are cooled after coating at high temperatures. Thus, for coating complex shapes (i.e., those having radii of curvature less than one-quarter inch), the pyrolytic carbon should have a BAF (Bacon Anisotropy Factor) of not more than about 1.3. For noncomplex shapes, higher values of BAF up to about 2.0 may be used. The BAF is an accepted measure of preferred orientation of the layer planes in the carbon crystalline structure. The technique of measurement and a complete explanation of the scale of measurement is set forth in an article by G. E. Bacon entitled A Method for Determing the Degree of Orientation of Graphite which appeared in the Journal of Applied Chemistry, volume 6, p. 477 (1956). For purposes of explanation, it is noted that 1.0 (the lowest point on the Bacon scale) signifies perfectly isotropic carbon.

In general, the thickness of the outer pyrolytic carbon coating should be sufficient to impart the necessary stress and strain fracture strengths to the particular substrate being coated. For example, if a fairly weak substrate is being employed, for instance one made of artificial graphite, it may be desirable to provide a thicker coating of pyrolytic carbon to strengthen the composite prosthetic device. Moreover, although an outer coating which is substantially entirely isotropic pyrolytic carbon has adequate structural strength, the codeposition of silicon or some similar carbide-forming additive improves the strength of the carbon coating. As described in more detail hereinafter, silicon in an amount up to about 20 weight percent can be dispersed as SiC throughout the pyrolytic carbon without detracting from the desirable thrombo-resistant properties of the pyrolytic carbon.

The density of the pyrolytic carbon is considered to be an important feature in determining the additional strength which pyrolytic carbon coating will provide the substrate. The density is further important in assuring that the pyrolytic carbon surface which will be exposed to body tissue or to blood in the environment wherein it will be used is smooth and substantially impermeable. Such surface characteristics are believed to reduce the tendency of blood to coagulate on the surface of the prosthetic device. It is considered that the pyrolytic carbon. should at least have a density of about 1.5 grams per cm.

A further characteristic of the carbon which also affects the strength contribution thereof is the crystallite height or apparent crystallite size. The apparent crystallite size is herein termed L and can be obtained directly using an X-ray dilfractometer. In this respect wherein:

7\ is the wavelength in A. ,8 is the half-weight (002) line width, and is the Bragg angle It is considered that the pyrolytic carbon coatings for use in prosthetic devices should have a crystalline size no greater than about 200 A. In general, it may be said that the desirable characteristics of the pyrolytic carbon for use in prosthetic devices are greater when the apparent crystallite size is small and that preferably the apparent crystallite size isbetween about and about 50 A.

Because in general the substrate material for the prosthetic device will be completely encased in pyrolytic carbon, or at least will have its surfaces covered with pyrolytic carbon which would otherwise be in contact with either body tissue or the blood, choice of the material from which to form the substrate is not of utmost importance. For example, if the particular prosthetic device is a pin or a small tube or a portion of a valve for implantation within the human body, it is likely that the prosthetic device would be completely covered with pyrolytic carbon. However, for purposes of this application, the term prosthetic device is also used to include a part of an apparatus which is used exterior of the body, for example, as a part of an auxiliary blood pump; and for such a part it may be necessary to coat only the surfaces which come in contact with the blood.

Because the substrate material may in many instances be completely surrounded by pyrolytic carbon, it is considered very important that the substrate material be compatible with pyrolytic carbon, and more particularly suitable for the process for coating with pyrolytic carbon. Although, as previously indicated, it is desirable that the substrate material have good structural strength to resist possible failure during its end use, substrate materials which do not have high structural stengths may be employed by using the pyrolytic carbon deposited thereupon to supply the required additional structural strength for the prosthetic device.

Because pyrolytic carbon is, by definition, deposited by the pyrolysis of a carbon-containing substance, the substrate material is subjected to the fairly high temperatures necessary for pyrolysis. Generally, hydrocarbons are employed as the carbon-containing substance to be pyrolyzed, and temperatures of at least about 1000 C. are needed. Some examples of the deposition of pyrolytic carbon to produce coated articles having increased stability under high temperature and neutron irradiation conditions are set forth in US. Pat. 3,298,921. The process illustrated and described in this US. patent employ methane as the source of carbon and utilize temperatures generally in the range from about 1500 to 2300 C. Although it may be possible to deposit pyrolytic carbon having the desired properties with regard to the instant invention at somewhat lower temperatures by using other hydrocarbons, for example, propane or butane, generally it is considered that the substrate material should remain substantially unaffected by temperatures of at least about 1000 C., and preferably by even higher temperatures.

Because the substrate is coated at the aforementioned relatively high temperatures although the prosthetic device will be actually employed at temperatures which will usually be very close to ambient, the coefficients of thermal expansion of the substrate and of the pyrolytic carbon deposited thereupon should be relatively close to each other if the pyrolytic carbon is to be deposited directly upon the substrate and a firm bond therebetween is to be established. Whereas in the aforementioned U patent there is description of the deposition of an intermediate low density pyrolytic carbon layer, the employment of which might provide somewhat greater leeway in matching the coefficients of thermal expansion, it is preferable to deposit the pyrolytic carbon directly upon the substrate and therefore avoid the necessity for such an additional intermediate layer. Pyrolytic carbon having the desired characteristics can be deposited having a thermal coefficient of expansion in the range of between about 3 and about 6 l0- C. Accordingly, substrate materials are chosen which have the aforementioned stability at high temperatures and which have thermal coefficients of expansion within or slightly above this general range. Examples of suitable substrate materials include artificial graphite, boron carbide, silicon carbide, tantalum, molybdenum, tungsten, and various ceramics, such as mullite.

The pyrolytic carbon coating is applied to the substrate using a suitable apparatus for this purpose. Preferably, an apparatus is utilized which maintains the substrate in motion while the coating process is carried out to assure that the coating is uniformly distributed on the desired surface of the substrate. A rotating drum coater or a vibrating table coater may be employed. When the substrates to be coated are small enough to be levitated in an upwardly flowing gas stream, a fluidized bed coater is preferably used. By coating in this manner, the desired smoothness and uniformity of the carbon surface is obtained.

As discussed in detail in the aforementioned US. patent, the charatceristics of the carbon which are deposited may be varied by varying the conditions under which pyrolysis is carried out. For example, in a fluidized bed coating process wherein a mixture of a hydrocarbon gas, such as methane, and an inert gas, such as helium or argon, is used, variance in the volume percent of methane, the total flow rate of the fluidizing gas stream, and the temperature at which pyrolysis is carried out all affect the characteristics of the pyrolytic carbon which is deposited. Control of these various operational parameters not only allows deposition of pyrolytic carbon having the desired density, apparent crystallite size, and isotropy, but also permits regulation of the desired thermal coefiicient of expansion which the pyrolytic carbon has. This control also allows one to grade a coating in order to provide a variety of exterior surfaces. For example, a highly oriented surface coating is believed to provide enhanced thromboresistance which may be desirable for certain applications. One can deposit a strong base isotropic pyrocarbon coating, having a BAF of 1.3 or less, and near the end of the coating operation, one can gradually change the coating conditions to obtain a highly oriented outer layer. Using this technique, suitable coatings having outer surfaces which are highly anisotropic and, for example, are about 25 microns thick, can be conveniently deposited.

As indicated above, a substrate material is chosen which has a thermal coefficient expansion of between about 3 and about 6X 10 C., and the carbon deposition conditions are controlled so that the pyrolytic carbon has a coefficient within the same range. Generally, when pyrolytic carbon is deposited directly upon the surface of the substrate material, the pyrolysis conditions are controlled so that the pyrolytic carbon which is deposited has a coefiicient of expansion matched to within about plus or minus 50 percent of the substrate materials thermal coefficient of expansion, and preferably to within about plus or minus 20 percent thereof. Because pyrolytic carbon has greater strength when placed in compression that when placed in tension, the thermal coefiicient of expansion of the pyrolytic carbon most preferably is about equal to or less than that of the substrate. Under these conditions, good adherence to the substrate is established and maintained during the life of the prosthetic devices. Inasmuch as many of these devices may be employed for implantation within the human body, it is extremely important that long life of the device without degradation be assured.

As previously indicated, the coating may be substantially entirely pyrolytic carbon, or it may contain a carbide-forming additive, such as silicon, which has been found to increase the overall structural strength of the coating. Silicon in an amount of up to about 20 weight percent, based upon total weight of silicon plus carbon, may be included without detracting from the desirable properties of the pyrolytic carbon, and when silicon is used as an additive, it is generally employed in an amount between about and weight percent. Examples of other carbide-forming elements which might be used as additives in equivalent weight percents include boron, tungsten, tantalum, niobium, vanadium, molybdenum, aluminum, zirconium, titanium and hafnium. Generally, such an element would not be used in an amount greater than 10 atom percent, based upon total atoms of carbon plus the element.

The carbide-forming additive is codeposited with the pyrolytic carbon by selecting a volatile compound of the element in question and supplying this compound to the deposition region. Usually, the pyrolytic carbon is deposited from a mixture of an inert gas and a hydrocarbon or the like, and in such an instance, the inert gas may be conveniently employed to carry the volatile compound to the deposition region. For example, in a fluidized bed coating process, all or a percentage of the fluidizing gas may be bubbled through a bath of methyltrichlorosilane or some other suitable volatile liquid compound. Under the temperature whereat the pyrolysis and codeposition occurs, the particular element employed is converted to the carbide form and appears dispersed as a carbide throughout the resultant product. As previously indicated, the presence of such a carbide-forming additive does not significantly change the crystalline structure of the pyrolytic carbon deposited from that which would be deposited under the same conditions in the absence of such an additive.

Pyrolytic carbon having the physical properties mentioned hereinbefore, is considered to be particularly advantageous for constituting the surface for a prosthetic device because it is antithrombogenic and is inert to the metabolic processes, enzymes, and other juices found within living bodies. The antithrombogenic properties of pyrolytic carbon are believed to be dependent upon its sterility and the removal of all the oxygen therefrom. Before use, the device may be sterilized, for example, by heating in a suitable vacuum for about six hours at about 130 C.

As an alternative to the foregoing sterilization and degassing techniques, the prosthetic devices can be sterilized in benzalkonium chloride and then treated with a suitable anticoagulant which safeguards against the occurrence of thrombosis. An anticoagulant such as haparin can be used. Application may be simply made by soaking the prosthetic device in benzalkonium chloride and then in a heparin solution. A suitable heparin solution may be prepared by mixing 2 cc. of heparin to 30 cc. of saline, saline being a solution of sodium chloride in water. The sorption of heparin by pyrolytic carbon surfaces purposely prepared with accessible porosity at the outer surface thereof is improved by pretreatment with a cationic, surface-active agent such as an aqueous solution of benzalkonium chloride. It should be kept in mind, however, that impermeable pyrolytic carbon is inherently t-hromboresistant and prior treatment with heparin is not essential.

When the prosthetic device is ready for its intended use, for example as a part of apparatus that will function exterior of a living body, or perhaps as an implant within a living body to repair an intravascular defect, known surgical procedures or the like are employed. A pyrolytic carbon-coated device may be secured in the proper location within the body, for example, by joining with Dacron cloth and appropriately suturing using standard suturing methods.

The following examples illustrate several processes for producing prosthetic devices having pyrolytic carbon surfaces which have various advantages of the invention. Although these examples include the best modes presently contemplated by the inventors for carrying out their invention, it should be understood that these examples are only illustrative and do not constitute limitations upon the invention which is defined by the claims appearing at the end of this specification.

EXAMPLE I Short tubes are constructed of artificial graphite each having a length of 9 mm., an internal diameter of 7 mm. and a wall thickness of 0.5 mm. The artificial graphite employed has a coeflicient of thermal expansion of about 4X'10- C. when measured at 50 C. The short tubes are coated with pyrolytic carbon using a fluidized bed coating apparatus.

The fluidized bed apparatus includes a reaction tube having a diameter of about 3.8 cm. that is heated to a temperature of about 1350 C. A flow of helium gas sufiicient to levitate the relatively small tubes is maintained upward through the apparatus. The small short tubes are coated together with a charge of zirconium dioxide particles of about 50 grams, which particles have diameters in the range of about to 250 microns. The particles are added along with the short tubes to provide a deposition surface area of the desired amount relative to the size of the region of the reaction tube wherein pyrolysis occurs inasmuch as the relative amount of available surface area is another factor which influences the characteristics of the resultant pyrolytic carbon.

When the temperature of the articles which are levitated within the reaction tube reaches about 1350 C., propane is admixed with the helium to provide an upwardly flowing gas stream having a total flow rate of about 6000 cc. per minute and having a partial pressure of propane of about 0.4 (total pressure one atmosphere). The propane decomposes under these conditions and deposits a dense isotropic pyrolytic carbon coating upon all of the articles in the fluidized bed. Under these coating conditions, the carbon deposition rate is about five microns per minute. The propane gas flow is continued until an isotropic pyrolytic carbon coating about 200 microns thick is deposited on the outside of the tubes. At this time, the propane gas flow is terminated, and the coated articles are cooled fairly slowly in the helium gas and then removed from the reaction tube coating apparatus.

The short tubes are examined and tested. The thickness of the pyrolytic carbon coating on the interior of the tubes measures about 200 microns. The density of the isotropic carbon uniformly is found to be about 2.0 grams per cm. The BAF is found to be about 1.1. The apparent crystallite size is measured and found to be about 30 to 40 A. Mechanical tests of the coated short tubes are made to determine their strength in comparison to additional uncoated graphite tubes. The crushing load of the uncoated graphite tubes, loaded parallel to the diameter, is found to be about four pounds. The crushing load of the coated tubes is about twenty-five pounds, about six times higher. Another of the coated tubes is sterilized by heating to about 1000 'C. in a vacuum and then is soaked for fifteen minutes in a dilute solution of benzalkonium chloride (1 part by 1000 parts water). The coated tube is then removed, rinsed and then soaked for fifteen minutes in a heparin solution prepared by adding 2 cc. of heparin to 30 cc. of saline. After removal, the tube is rinsed ten times with distilled water and is then tested with blood. After contact with blood for about twenty-four hours, no sign of clotting is shown, and clotting normally occurs within a matter of minutes. The pyrolytic carbon-coated graphite substrate articles are considered to be excellently acceptable for use as prosthetic devices within the body of human beings.

EXAMPLE II A number of short tubes having the same dimensions as those used in Example I but made of tantalum are provided. Tantalum has a thermal coefiicient of expansion of about 6.5 Xl C., measured at 20 C. The short tubes are coated in the fluidized bed reaction tube employed in Example I. In order to match the pyrolytic carbon coefficient of thermal expansion to that of the tan talum substrate, a coating temperature of 1600 C. is employed using a 15 percent propane85 percent helium gas stream having a total flow rate of about 6000 cc. per minute. The short tubes are levitated together with a similar 50 gram charge of particles of zirconium dioxide at atmospheric pressure. Deposition of pyrolytic carbon is carried out for about 20 minutes, after which period a layer of isotropic pyrolytic carbon about 150 microns thick coats the outer surface of each of the tubes. At the end of this time the propane flow is discontinued, and the coated tubes are cooled and removed from the reaction tube.

Examination and testing shows that the density of the isotropic pyrolytic carbon deposited is about 1.6 grams per cm. The BAF is about 1.0. The apparent crystallite size is between about 50 to 60 A. The thermal coefficient of expansion of the pyrolytic carbon measures about 5x10 C. at about 20 C. Mechanical testing of the coated tubes shows that the strength and wearability is acceptable and that the coating is firmly afiixed to the substrate.

One of the coated short tubes is sterilized and treated as in Example I excepting that the treatment with benzalkonium chloride and heparin is omitted. The tube: is tested with blood, and there is no sign of clotting after contact therewith for twenty-four hours. The carbon-coated tantalum articles are considered to be excellently acceptable for use as a part of a prosthetic device for implantation within a human body.

EXAMPLE III A number of short tubes having the same dimensions as those used in Example I but made of tungsten are provided. Tungsten has a thermal coefficient of expansion of about 4.4 10- C., measured at 27 C. The short tubes are coated in the fluidized bed reaction tube employed in Example I. In order to match the pyrolytic carbon coefficient of thermal expansion to that of the tungsten substrate, a coating temperature of 1600 C. is employed using a percent propane85 percent helium gas stream having a total flow rate of about 600 cc. per minute. The short tubes are levitated together with a similar 50 gram charge of particles of zirconium dioxide. Deposition of pyrolytic carbon is continued for about minutes, at which time a layer of isotropic pyrolytic carbon about 150 microns thick coats the outer surface of each of the tubes. The propane flow is discontinued, and the coated tubes are cooled and removed from the reaction tube.

Examination and testing shows that the density of the isotropic pyrolytic carbon deposited is about 1.6 grams per cc. The BAP is about 1.0. The apparent crystallite size is between about 50 to 60 A. The thermal coefficient of expansion of the pyrolytic carbon measures about 5 10 C. at about 20 C. Mechanical testing of the coated tubes shows that the strength and wearability is acceptable and that the coating is firmly afiixed to the substrate.

One of the coated short tubes is sterilized and treated as in Example I with benzalkonium chloride and heparin and tested with blood. There is no sign of clotting after contact therewith for twenty-four hours. The carbon-coated tungsten articles are considered to be excellently acceptable for use as a part of a prosthetic device for implantation within a human body.

EXAMPLE IV A number of short tubes having the same dimensions as those used in Example I but made of molybdenum are provided. Molybdenum has a thermal coeflicient of expansion of about 5.3 10 0, measured at 20 C. The short tubes are coated in the fluidized bed reaction tube employed in Example I. In order to match the pyrolytic carbon coefiicient of thermal expansion to that of the molybdenum substrate, a coating temperature of 1350 C. is employed using a 30 percent propane-70 percent helium gas stream having a total flow rate of about 5500 cc. per minute. The short tubes are levitated together with a similar 50 gram charge of particles of zirconium dioxide. Deposition of pyrolytic carbon occurs, and after about 30 minutes a layer of isotropic pyrolytic carbon about 150 microns thick coats the outer surface of each of the tubes. At the end of this time, the propane flow is discontinued, and the coated tubes are cooled and removed from the reaction tube.

Examination and testing shows that the density of the isotropic pyrolytic carbon deposited is about 2.0 grams per cm.*. The BAF is about 1.1. The apparent crystallite size is between about 30 and 40 A. The thermal coeliicient of expansion of the pyrolytic carbon measures about 5 l0- C. at about 20 C. Mechanical testing of the coated tubes shows that the strength and wearability is acceptable and that the pyrolytic carbon coating is firmly bonded to the substrate.

One of the coated short tubes is sterilized and treated as in Example I with benzalkonium chloride and heparin and is tested with blood. There is no sign of clotting after contact therewith for twenty-four hours. The carbon-coated molybdenum short tubes are considered to be excellently acceptable for use as a part of a prosthetic device for implantation within a human body.

EXAMPLE V A number of graphite tubes having the same characteristics and dimensions as those used in Example I are introduced into a reaction tube which is about 6.3 cm. in diameter, together with an ancillary charge of grams of zirconium oxiode spheroids having an average particle size of about 400 microns. A fluidizing flow of helium is fed upward through the reaction tube as the temperature of the small tubes and particles is raised to about 1350" C. When this temperature is reached, propane is admixed with the helium to provide a total gas flow of about 8000 cc. per minute, having a partial pressure of propane of about 0.4 atm. (total pressure of 1 atm.). All of the helium is bubbled through a bath of methyltrichlorosilane at about room temperature. The propane and the methyltrichlorosilane pyrolyze to deposit a mixture of isotropic carbon and silicon carbide on the small tubes, and the coating process is continued until a coating about 12 mils (300 microns) thick is obtained, a time of about an hour.

The resultant coated tubes are allowed to cool to ambient temperature, and they are then removed from the reaction tube. Examination of the istotropic carbon-silicon carbide coating shows that it has a coefficient of thermal expansion of about 6x10- C. and a density of 2 grams per cmfi. The coating contains about weight percent silicon (based upon total weight of silicon plus carbon) in the form of silicon carbide. The isotropic carbon has a BAF of about 1.1 and an apparent crystallite size of about 35 A. Mechanical testing of the coating tubes shows that the strength and wearability are fully acceptable and that there is a firm bond between the coating and the graphite substrate.

One of the coated tubes is sterilized and treated as in Example I, using benzalkonium chloride and heparin, and it is then tested with blood. There is no sign of clotting after contact with blood for twenty-four hours. The tubes which are coated with pyrolytic carbon containing the silicon carbide additive are considered to be excellently acceptable for use as a part of a prosthetic device and suitable for implantation within a human body.

Although the examples have been particularly directed to the coating and use of short tubes, it should be understood that this is for purpose of illustration only and that it is considered that any suitably-shaped elements can be coated to provide prosthetic devices. In particular, it is to be noted that deposition of pyrolytic carbon in a fluidized bed process is excellently suited for the smooth coating of even the most complex shape of element. The foregoing shows that prosthetic devices are provided which have excellent resistance to degradation in a living body and, as such, are eminently well suited for prosthetic devices which can be implanted permanently within a living human being. Pyrolytic carbon-coated substrates containing radioactive isotopes for internally treating diseases, such as cancer or tumors, are illustrative of another form of improved prosthetic device that may be produced. Various features of the invention are set forth in the following claims.

What is claimed is:

1. A method of making a prosthetic device which method comprises forming a substrate of a material stable at temperatures of at least about 1350 C., heating said substrate together with particulate material to provide additional deposition surface area to a temperature of between about 1350 C. and about 1600 C. in a reaction chamber, flowing a mixture of propane or butane and an inert gas through said reaction chamber at atmospheric pressure, said propane or butane constituting between about volume percent and about 40 volume percent of said mixture, so that said substrate becomes coated with isotropic pyrolytic carbon having a BAF between 1.0 and 2.0, having an apparent crystallite size of about 50 A. or less and having a density of at least about 1.5 grams per cm. and continuing said gas flow until the thickness of said isotropic carbon deposit is at least about 50 microns, whereby a device is produced which has excellent compatibility with body tissue and is nonthrombogenic.

2. A method in accordance with claim 1 wherein the substrate is levitated by said gas flow which is directed upward.

3. A method in accordance with claim 1 wherein said inert gas is helium.

4. A method in accordance with claim 1 wherein said gas flow rate is between about 5500 and about 6000 cc. per minute per about 11 sq. cm. of reaction chamber cross sectional area.

5. A method in accordance with claim 1 wherein said particulate matter is about equivalent to a charge of about 50 grams of ZrO particles between about and 25-0 microns in size per 11 sq. cm. of reaction chamber cross sectional area.

6. A method in accordance with claim 1 wherein'between about 30 and about 40 volume percent of propane is employed at a temperature of about 1350 C.

7. A method in accordance with claim 1 wherein about 15 volume percent of propane is employed at a temperature of about 1600 C.

8. A method in accordance with claim 1 wherein said gas mixture includes a volatile compound containing a carbide-forming element.

9. A method in accordance with claim 8 wherein said inert gas is caused to flow through a body of a liquid compound containing said carbide-forming element.

10. A method in accordance with claim 9 wherein said liquid compound is methyltrichlorosilane.

11. A method in accordance with claim 8 wherein said element is silicon.

12. A method in accordance with claim 8 wherein said element is zirconium.

13. A method in accordance with claim 8 wherein said element is titanium.

14. A method in accordance with claim 8 wherein said element is tantalum.

References Cited UNITED STATES PATENTS 3,369,920 2/1968 Bourdeau et al. 11746 CG 3,464,843 9/ 196-9 Basche 117-46 CG 3,411,949 11/1968 Hough 117-46 CG 3,526,005 9/1970 Bokros et al. 11746 CG 3,288,629 11/1966 MoCreight 117-566 CG 3,399,969 9/1968 Bokros et a1. 11746 CG 3,298,921 11/1967 Bokros et al. 117100 I 3,547,676 12/1970 Bokros et al. 117-226 3,330,698 7/1967 Podolsky 1l746 CG WILLIAM D. MARTIN, Primary Examiner M. SOFOCLEOUS, Assistant Examiner US. Cl. X.R.

3--1; 117-106 AC, Dig 11 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,677,795 Dated Ju1 1s,1972

lnventorks) Jack c. Bokros and Willard H; Ellis It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, line-l5 for "L read "L Column 3, line 17 in the equation, for "O.80)\A", read Column 3, line 69 for "process",'rea d "processes".

Column 7, line 65 for "600cm"; read "6000 cc".

Column 8, line 57 for "oxiode", read 'oxide".

Signed and sealed this 2nd day of January 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patems FORM PO-1050(10-69) USCOMM-DC 6O376-P69 U75. GOVERNMENT PRINTING OFFICE 5 I969 O366334