Prosthetic Blood Circulation Device Having A Pyrolytic Carbon Coated Blood Contacting Surface

Abstract

A prosthetic device for implantation in or use with a living body. A substrate is coated with impermeable pyrolytic carbon which provides an inert and antithrombogenic outer surface. The conditions at which the pyrolytic carbon is deposited are controlled to match the thermal coefficient of expansion of the pyrolytic carbon to that of the substrate and to provide a strong carbon which contributes substantial structural strength to the composite prosthetic device. The carbon is preferably isotropic and may be doped with a suitable carbide-forming element, such as silicon, to provide additional structural strength and wear resistance. Devices having such coatings on the portions coming in contact with blood are valuable for extracorporeal circulation of the bloodstream of a human patient.

Parent Case Text

This application is a continuation-in-part of our earlier patent applications Ser. No. 649,811, filed June 29, 1967 now U.S. Pat. No. 3,526,005 and Ser. No. 821,080, filed May 1, 1969.

Description

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

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, including those which are used extracorporeally, are currently under development. Artificial kidneys are another class of prosthetic devices becoming more and more available.

In order to further 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 within or insertion into 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 thrombogenic and are subject to degradation. Stainless steel and tantalum are among the most popular metals used today, whereas polyethylene, Teflon and the polycarbonates are examples of plastics considered suitable. 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 nonthrombogenic 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 or inserted into a living body. Still another object is to provide a method for making improved prosthetic devices, particularly for extracorporeal use. One further object is to provide improved parts for use in extracorporeal apparatus which is exposed to the circulation of blood.

These and other objects of the invention should be clearly apparent from the following description of devices embodying various features of the invention when read in conjunction with the accompanying drawings, wherein:

FIG. 1 is section view through a circulatory assist device;

FIG. 2 is an enlarged perspective view of a valve disc used in the device shown in FIG. 1;

FIG. 3 is a sectional view of another type of circulatory assist device;

FIG. 4 is a perspective view of still another circulatory assist device;

FIG. 5 is a plan view of yet another type of circulatory assist device;

FIG. 6 is a sectional view taken along line 6--6 of FIG. 5;

FIG. 7 is a perspective view of an alternative type of impeller that is employed in the general type of circulatory assist device shown in FIGS. 5 and 6; and

FIG. 8 is a perspective view of a cannula of the type which might be inserted into the circulatory system of the human body to facilitate use of some extracorporeal type of device.

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 the use of the prosthetic devices in combination with a human body, it should also be recognized that the improved prosthetic devices may 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. Moreover, for purposes of this application, the term "prosthetic device" is intended to include parts for extracorporeal devices which will be in contact with the bloodstream of a living person.

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 thrombo-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, and for flat shapes, pyrolytic carbon having a BAF as high as about 20 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 Determining the Degree of Orientation of Graphite" which appeared in the Journal of Applied Chemistry, Volume 6, page 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, and usually the coating will be at least about 50 microns thick. 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 and wear resistance of the carbon coating. As described in more detail hereinafter, silicon in an amount up to at least about 20 weight percent can be dispersed in 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 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.sup.3, and such pyrocarbon is referred to in this application as dense.

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.sub.c and can be obtained directly using an X-ray diffractometer. In this respect

L.sub.c = 0.89 .lambda./.beta. cos.theta.

wherein:

.lambda. is the wave length in A.

.beta. is the half-height (002) line width, and

.theta. is the Bragg angle

Pyrolytic carbon coatings for use in prosthetic devices should have a crystalline size no greater than about 200A. In general, the desirable characteristics of pyrolytic carbon for use in prosthetic devices are greater when the apparent crystallite size is small, and preferably the apparent crystallite size is between about 20 and about 50A.

Because the substrate material for the prosthetic device will often be completely encased in pyrolytic carbon, or at least will have one of its surfaces covered with pyrolytic carbon at a location that will 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, 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 or circulatory assist device; and for such a part, it may be necessary to coat only the surfaces which come in contact with the blood.

It is considered very important that the substrate material be compatible with pyrolytic carbon, and more particularly suitable for use at the process conditions for coating with pyrolytic carbon. Although it is desirable that the substrate material have good structural strength to resist possible failure during its end use, materials which do not have high structural strengths may be employed by using the pyrolytic carbon deposited thereupon to supply additional structural strength for the prosthetic device.

Pyrolytic carbon is, by definition, deposited by the pyrolysis of a carbon-containing substance so the substrate will be 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 1,000.degree. C. are used. 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 U.S. Pat. No. 3,298,921. Processes illustrated and described in this U.S. Pat. employ methane as the source of carbon and utilize temperatures generally in the range from about 1500 to 2,300.degree. 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 1,000.degree. C., and preferably by even high temperatures.

Because the substrate is coated at relatively high temperatures and the prosthetic device will be employed at temperatures usually 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.S. Pat. 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 therefor 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 .times. 10.sup..sup.-6 /.degree. C. measured at 20.degree. 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, for example up to about 8 .times. 10.sup. .sup.-6 /.degree. C. 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 surfaces 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.

As discussed in detail in the aforementioned U.S. Pat., the characteristics 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 the regulation of the desired thermal coefficient 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.

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 coefficient of expansion matched to within about plus or minus 50 percent of the substrate material's 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 than when placed in tension, the thermal coefficient of expansion of the pyrolytic carbon most preferably is about equal to or less than that of the substrate. Under these condition, good adherence to the substrate is established and maintained during the life of the prosthetic devices.

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 wear resistance and 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 10 and 20 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 is 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 chemisorbed oxygen therefrom. Before use, the device may be sterilized, for example, by heating in a suitable vacuum for about 6 hours at about 130.degree. C. or by steam autoclaving.

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 heparin 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 10 mgs. of heparin per ml. 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 and heparin. It should be kept in mind, however, that impermeable pyrolytic carbon is inherently thromboresistant 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.

Illustrated in FIG. 1 of the drawings is a circulatory assist device in the form of an air operated pump 11. The pump 11 has a body 13 with an inlet 15 and an outlet 17 for blood and having an opening 19 for connection to an air line 21. A flexible bladder 23 disposed within the pump body provides a pumping chamber 25 which is closed at opposite ends by an inlet valve 27 and an outlet valve 29. Each of the valves 27, 29 include doubly convex-shaped disc 31 which is proportioned to close the valve opening therethrough and which is maintained in association with the opening by a retainer 33. Each disc valve element 31, shown in FIG. 2, is formed with two identical convex surfaces.

As one example of using this pump 11, the pump inlet 15 is connected to the left ventricle of the heart. In FIG. 1, the pump 11 is shown with the inlet valve 27 in the open position so that blood flowing from the left ventricle during systole flows into the flexible bladder 23. During this filling cycle, the air line 21 connected to the opening 19 is vented. The outlet 17 from the pump 11 is connected to the descending or thoracic aorta. During the filling cycle, the outlet valve 29 in the pump is closed (as shown) as the result of the pressure in the aorta. Subsequently, an external control system supplies air pressure through the opening 19 to the region between the body 13 and the flexible bladder 23. The application of air pressure squeezes the bladder 23 closing the inlet valve 27, opening the outlet valve 29 and ejecting the blood from the bladder into the descending aorta.

It is most important that thrombosis be avoided which might result in clotting and eventual deterioration in the performance of such a pump 11. The movable valve discs 31 are one of the locations most susceptible to thrombosis, and it has been found that by providing these valve discs with exterior coatings of pyrolytic carbon, excellent resistance to thrombosis is provided. The discs 31 may be made of graphite, machined to shape and coated with a 50 -micron thick coating of dense pyrocarbon. The disc retainers 31 are also advantageously coated with pyrocarbon. Valves using such discs continue to open and close well over long periods of use for pumping human blood.

Shown in FIG. 3 is another type of circulatory assist device or pump 41 which also utilizes compressed air or the like to power the pumping operation. The pump includes an outer body 43 having formed therein a central cylindrical section 45, an upper dish-shaped section 47 and a lower dish-shaped section 49. A movable pumping element 51 has the general shape of an inverted funnel. The tubular stem portion 53 extends through a central opening in the upper body section 47 and surmounts a concave-shaped portion 55 that is contoured similarly to the internal surface of the upper pump body section. A lower inlet 57 is provided in the pump body 43 through which flow is regulated by a pivoting valve element 59. The movable pumping element 51 carries another pivoted valve element 59 in the stem portion 53 thereof which serves as the outlet valve. A passageway 61 is provided in the upper surface portion 47 of the pump body which is adapted for connection by a suitable conduit to a control mechanism (not shown).

The pump 41 may be connected in the same manner as the pump 11 illustrated in FIG. 1. In the position shown, the lower inlet valve element 59 is in open position and blood is flowing into the pumping chamber defined generally between the lower dish-shaped section 49 of the pump body 43 and the movable pumping element 51. The blood pressure in the aorta maintains the upper valve element 59 in closed position, and the movable pumping element 51 reciprocates upward with the inflow of the blood. During the filling phase, the region between the upper surface of the movable pump element 51 and the concave undersurface of the upper pump body section 47 is vented via the passageway 61. Subsequently, air pressure is applied through the passageway 61 to drive the movable pump element 51 downward. This action closes the lower inlet valve, opens the outlet valve and discharges blood from the pumping chamber into the descending aorta.

Preferably, all of the internal surfaces of the pump 41 which come in contact with blood are coated with pyrolytic carbon. In this respect, the internal surfaces of the sections 45 and 49 of the pump body 43 would be so coated along with the inner surface of the inlet 57. The entire inner surface of the movable pumping element 51 should also be coated. Likewise, both of the pivoting valve elements 59 are completely coated with a layer of pyrolytic carbon. In addition to being thromboresistant, the pyrolytic carbon provides an excellent bearing surface and exhibits good wear characteristics in the region of the cylindrical wall section 45 where there is sliding contact with the peripheral edge of the reciprocating pumping element 51. Such a pump 41 is capable of continuous operation without the development of blood clotting.

Illustrated in FIG. 4 is another type of circulatory assist device in the form of a piston-type pump 71. The pump has an outer body or casing 73 and contains a sleeve 75 that serves as a cylinder wall that is in sliding contact with a floating piston 77 which has the shape of a right circular cylinder. The piston 77 slides freely in the sleeve 75, and its movement is controlled via an opening 79 in the lower surface of the casing 75 to which a conduit is connected, as the case of the pumps 11 and 41. The casing 73 forms a pumping chamber 81 above the upper face of the piston 77 and contains, side-by-side, an inlet 83 and an outlet 85, each of which are provided with ball valves 87 and 89, respectively. Each valve includes a movable spheroid 91 and a retaining cage 93.

The operation of the pump 71 is similar to the operation previously described, and the pump is illustrated near the end of the pumping phase, just before the filling phase begins. The floating piston 77 moves downward when blood is flowing into the pumping chamber 81 through the inlet valve opening, and the blood pressure in the aorta maintains the outlet ball 91 in the closed position. Upon completion of the filling phase, air pressure is applied to the lower opening 79, forcing the floating piston 77 upward, closing the inlet valve 87 and pumping the blood from the pumping chamber 81 through the outlet valve 89 into the descending aorta.

It has been found that this piston blood pump 71 has substantially improved resistance to blood clotting if the sleeve 75 and the piston 77 are coated with an exterior layer of pyrolytic carbon. Moreover, the movable ball valve spheroids 91 are also advantageously made from a suitable substrate, such as graphite, and coated with pyrolytic carbon. Depending upon the material from which the retaining members 93 are made, these members are also provided with an outer coating of pyrolytic carbon that prevents clotting thereadjacent over a long duration of operation.

Shown in FIGS. 5 and 6 in a centrifugal type of circulatory assist device or pump 101 having a two-piece housing wherein a rotor 103 revolves. An upper portion 105 of the housing flares outward from a central inlet opening 107 to present a smooth flaring undersurface which may be described as being generally bell-shaped. A lower housing portion 109 mates with the upper portion 105 and contains a flat circular wall 111 having an upstanding short peripheral wall 113.

The rotor 103 consists of three separate sections 115, 117 and 119 each having progressively slightly greater curvature than the underside of the upper housing portion 105 which is interconnected by pins 121. The lowermost rotor section 119 is linked by suitable struts 123 to a central shaft 125 which extends downward through a drilled hole in the circular wall 111 to facilitate connection to an electric motor 127. The lower portion 109 of the two-piece housing contains a tangentially located outlet 129 in the peripheral wall 113. Spacing between the three rotor sections is such as to provide a viscous drag on the blood and impart centrifugal motion to it which propels it outward and through outlet 129. Accordingly, revolution of the three-piece rotor 103 by the electric motor 127 causes blood to be drawn into the inlet opening 107 and centrifugally discharged through the tangential outlet 129.

It has been found that the performance of the centrifugal pump 101 is substantially improved by the avoidance of clotting as a result of coating the components that come in contact with blood with a layer of pyrolytic carbon. In this respect, the interior surface of the pumping cavity formed by the two-piece housing is coated with pyrolytic carbon. Moreover, all of the surfaces of the three-segment rotor 103 and the connecting pins 121 and struts 123 are also coated.

Shown in FIG. 7 is an alternative design of a rotor 131 which is also employed in a centrifugal circulatory assist device of the general type as that shown in FIGS. 5 and 6. The rotor 131 is affixed to a drive shaft 133 attached to it and has an upper conical portion 135 from which extend six triangular-shaped blades 137. The entire rotor and any portion of the shaft which extends into the pumping cavity are preferably coated as a unit with a layer of pyrolytic carbon in the manner hereinbefore described.

Illustrated in FIG. 8 is a cannula 141 of tee shape. The cannula 141 is designed for implantation in the body of a patient who will periodically be submitted to artificial kidney treatments. For example, the long straight run 143 of the tee may be spliced into the vein of a patient while the short stem section 145 of the tee extends upward to the surface of the skin. If pyrolytic carbon is used to completely coat the cannula 141, it can be implanted as a permanent installation inasmuch as clotting is avoided. Normally, the short stem section 145 of the tee is closed by a suitable plug, and the blood flows straight through the run of the tee. When, for example, dialysis is desired, blood is removed from an artery using a similar tee and is returned to the vein via the stem 145. The ability of a pyrolytic carbon coating to permit a permanent installation of this type is of substantial advantage to a patient who must frequently be subjected to such treatments.

The following examples illustrate several coating processes for producing prosthetic devices having pyrolytic carbon surfaces exhibiting 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 9mm., an internal diameter of 7mm. and a wall thickness of 0.5mm. The artificial graphite employed has a coefficient of thermal expansion of about 4 .times. 10.sup.-.sup.6 /.degree. C. when measured at 50.degree. 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 1,350.degree. C. A flow of helium gas sufficient 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 150 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 physical characteristics of the resultant pyrolytic carbon.

When the temperature of the articles which are levitated within the reaction tube reaches about 1350.degree. C., propane is admixed with the helium to provide an upwardly flowing gas stream having a total flow rate of about 6,000 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 5 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 tube measures about 200 microns. The density of the isotropic carbon uniformly is found to be about 2.0 grams per cm..sup.3. The BAF is found to be about 1.1. The apparent crystallite size is measured and found to be about 30 to 40A. 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 4 pounds. The crushing load of the coated tubes is about 25 pounds, about 6 times higher. Another of the coated tubes is sterilized by heating to about 1000.degree.C. in a vacuum and then is soaked for 15 minutes in a dilute solution of benzalkonium chloride (1 part by 1,000 parts water). The coated tube is then removed, rinsed and then soaked for 15 minutes in a heparin solution prepared at a level of 10 mgs. of heparin per ml. of saline. After removal, the tube is rinsed ten times with saline and is then tested with blood. After contact with blood for about 24 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 coefficient of expansion of about 6.5 .times. 10.sup.-.sup.6 /.degree. C., measured at 20.degree. 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 tantalum substrate, a coating temperature of 1,600.degree. C. is employed using a 15 percent propane -- 85 percent helium gas stream having a total flow rate of about 6,000 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..sup.3. The BAF is about 1.0. The apparent crystallite size is between about 50 to 60A. The thermal coefficient of expansion of the pyrolytic carbon measures about 5 .times. 10.sup.-.sup.6 /.degree. C. at about 20.degree. C. Mechanical testing of the coated tubes shows that the strength and wearability is acceptable and that the coating is firmly affixed 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 24 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 .times. 10.sup.-.sup.6 /.degree. C., measured at 27.degree. 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.degree.C. is employed using a 15 percent propane -- 85 percent helium gas stream having a total flow rate of about 6,000cc. 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 20 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 BAF is about 1.0. The apparent crystallite size is between about 50 to 60A. The thermal coefficient of expansion of the pyrolytic carbon measures about 5 .times. 10.sup.-.sup.6 /.degree. C. at about 20.degree. C. Mechanical testing of the coated tubes shows that the strength and wearability is acceptable and that the coating is firmly affixed 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 24 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 coefficient of expansion of about 5.3 .times. 10.sup.-.sup.6 /.degree. C., measured at 20.degree. 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 molybdenum substrate, a coating temperature of 1,350.degree. C. is employed using a 30 percent propane -- 70 percent helium gas stream having a total flow rate of about 5,500cc. 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..sup.3. The BAF is about 1.1. The apparent crystallite size is between about 30 and 40A. The thermal coefficient of expansion of the pyrolytic carbon measures about 5 .times. 10.sup.-.sup.6 /.degree. C. at about 20.degree. 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 polished, 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 24 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 100 grams of zirconium oxide 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 1,350.degree.C. When this temperature is reached, propane is admixed with the helium to provide a total gas flow of about 8,000 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 isotropic carbon-silicon carbide coating shows that it has a coefficient of thermal expansion of about 6 .times. 10.sup.-.sup.6 /.degree. C. and a density of 2 grams per cm.sup.3. The coating contains about 10 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 35A. Mechanical testing of the coated 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 polished, 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 24 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 hat the examples are provided for the purpose of illustration. Any suitably shaped elements including all of those shown in the drawings can be coated to provide prosthetic devices of the improved design. Deposition of pyrolytic carbon in a fluidized bed process is excellently suited for the smooth coating of even very complex-shaped elements. The improved prosthetic devices have excellent resistance to degradation in a living body and are likewise eminently well suited for parts in circulatory assist devices that handle the bloodstream of a living human being.

Various of the features of the invention are set forth in the claims that follow.

Claims

What is claimed is:

1. A prosthetic device designed for use in the bloodstream circulation of a human being, which device comprises a substrate having a shape and size functionally desired, a dense isotropic pyrolytic carbon coating covering substantially all of the surface of said substrate which will come in contact with blood, which pyrolytic carbon has a density of at least about 1.5 grams per cc., a BAF between 1.0 and about 1.3 and an apparent crystallite size of less than about 200A. and means for direct or indirect connection of the device to the circulatory system of a human being.

2. A prosthetic device in accordance with Claim 1 wherein said pyrolytic carbon has an apparent crystallite size of about 50A. or less.

3. A prosthetic device in accordance with Claim 2 wherein said pyrolytic carbon has an apparent crystallite size of at least about 20A.

4. A prosthetic device in accordance with claim 1 wherein said pyrolytic carbon coating is at least about 50 microns thick.

5. A prosthetic device in accordance with claim 4 wherein said substrate has no radius of curvature less than one-quarter inch and said pyrolytic carbon has a BAF between 1.0 and about 2.0.

6. A prosthetic device in accordance with claim 1 wherein said pyrolytic carbon has a thermal coefficient of expansion between about 3 and 6 .times. 10.sup.-.sup.6 /.degree. C. measured at about 20.degree. C.

7. A prosthetic device in accordance with claim 6 wherein said pyrolytic carbon has a thermal coefficient of expansion plus or minus about 50 percent of the coefficient of thermal expansion of said substrate.

8. A prosthetic device in accordance with claim 1 wherein said pyrolytic carbon contains a carbide additive dispersed therein.

9. A prosthetic device in accordance with claim 8 wherein said pyrolytic carbon contains up to about 20 weight percent silicon in the form of silicon carbide.

10. A prosthetic device in accordance with claim 8 wherein said pyrolytic carbon contains silicon in an amount between about 20 and 10 weight percent in the form of silicon carbide.

11. A prosthetic device in accordance with claim 1 wherein said substrate has the shape of an element of a blood circulatory assist device.

12. A prosthetic device in accordance with claim 1 wherein said substrate has the shape of an element of an artificial heart valve.

13. A prosthetic device in accordance with claim 1 wherein said substrate has the shape of a cannula for implantation into the body of a human being.

14. A prosthetic device in accordance with claim 1 wherein said substrate is made of graphite.