Title:
PERCUTANEOUS IMPLANT
United States Patent 3783868


Abstract:
A percutaneous device for drug injection in a living body which is implanted through the skin and which has a pyrolytic carbon coating. The device has a stem and a stabilizing flange, and a collar associated with the stem for preventing the progressive growth of epithelium tissue along the stem and for anchoring the device. A normally closed valve is in a passageway through the stem for administration of medication.



Inventors:
BOKROS J
Application Number:
05/140869
Publication Date:
01/08/1974
Filing Date:
05/06/1971
Assignee:
GULF OIL CORP,US
Primary Class:
Other Classes:
424/448, 623/1.42
International Classes:
A61M1/00; A61N1/05; (IPC1-7): A61M31/00; A61F1/00
Field of Search:
3/1 128
View Patent Images:



Foreign References:
GB1161436A
SU141591A11961-11-30
SU245277A1
Primary Examiner:
Gaudet, Richard A.
Assistant Examiner:
Mcgowan J. C.
Attorney, Agent or Firm:
Fitch, Even, Tabin & Luedeka
Claims:
What is claimed is

1. A percutaneous implant device for drug injection in a living body, comprising a pyrolytic carbon coated refractory stem having a passageway therethrough, a pryolytic carbon coated refractory stabilizing flange adjacent the base of said stem for stabilizing the implant device in surrounding subcutaneous tissues, a pyrolytic carbon coated refractory mesh collar located curcumferentially about and projecting outwardly from said stem for preventing encapsulation of the device by the progressive growth of epithelium tissue along said stem and for anchoring the implant device upon epithelium growth therethrough, a normally closed elastomeric plug valve in said passageway for administering medication through said passageway and to prevent entrance through said passageway of external pathogens or other undesired material, and medication reservoir and release means in communication with said passageway for retaining a reservior of medication administered through said valve and for releasing medication in the reservoir into the surrounding subcutaneous tissues in a predetermined manner, said pyrolytic carbon coating on said stem, collar and flange being an integral pyrolytic carbon coating having a thickness of at least about 10 microns, a density of at least about 1.5 grams per cubic centimeter, and a Bacon Anistrophy Factor of about 1.3 or less.

2. A percutaneous implant device in accordance with claim 1 wherein said medication reservoir and release means comprises a porous membrane which is affixed to the end of said passageway adjacent said stabilizing flange.

3. A percutaneous implant device in accordance with claim 1 wherein said elastomeric plug is provided with a pressure-operable passageway therethrough.

4. A percutaneous implant device in accordance with claim 1 wherein said collar is formed from a refractory metal screen having a wire diameter of from about 0.05 mm to about 0.1 mm and wherein the spacing between the wire of said pyrolytic carbon coated collar is about 1 mm.

5. A percutaneous implant device in accordance with claim 1 wherein said collar substrate is a perforated metal sheet.

6. A percutaneous implant device in accordance with claim 1 wherein said collar substrate is a carbon fiber mesh.

7. A percutaneous implant device in accordance with claim 1 wherein said elastomeric plug valve is an unperforated elastomer plug adapted for use with a hypodermic needle.

8. A percutaneous implant device in accordance with claim 2 wherein said elastomeric plug valve is provided with a pressure-operable latent passageway therethrough.

9. A percutaneous implant device in accordance with claim 1 wherein the surface of said pyrolytic carbon is oxidized.

10. A percutaneous implant device in accordance with claim 2 wherein said stem is provided with a second pyrolytic carbon coated refractory flange, adjacent the externally positionable end of said stem opposite its base, for protection of the implantation site.

11. A percutaneous implant device for drug injection in a living body, comprising a pyrolytic carbon coated refractory stem having a passageway therethrough, a pyrolytic carbon coated refractory stabilizing flange adjacent the base of said stem for stabilizing the implant device in surrounding subcutaneous tissues, a porous, carbonaceous physiologically inert layer into and through which epithelium tissue will grow, said layer being located circumferentially about said stem for preventing encapsulation of the device by the progressive growth of epithelium tissue along said stem and for anchoring the implant device upon epithelium growth therethrough, a normally closed elastomeric plug valve in said passageway for administering medication through said passageway and to prevent entrance through said passageway of external pathogens or other undesired material, and medication and release means in communication with said passageway for retaining a reservoir of medication administered through said valve and for releasing medication in the reservoir into the surrounding tissues in a predetermined manner, said pyrolytic carbon coacting on said stem and flange being an integral pyrolytic carbon coating having a thickness of at least about 10 microns, and a density of at least about 1.5 grams per cubic centimeter.

12. A percutaneous implant device in accordance with claim 12 wherein said elastomeric plug is provided with a pressure-operable passageway therethrough.

Description:
The present invention is related to percutaneous medical devices, and more particularly to improved percutaneous implant devices for drug injection.

There is a need for a reliable percutaneous implant for circumstances requiring prolonged subcutaneous administration of medication, and particularly for circumstances where controlled, even, and continuous release of medication is desirable.

One requirement of such a percutaneous implant is that it should be capable of providing a bacteria-tight seal in conjunction with the surrounding tissues so that the implantation of the percutaneous device does not provide a source for infection, or otherwise permit entry of pathogens or other undesired foreign material. Another important criterion is that the percutaneous implant device should be biologically compatible with the living tissues in which it is to be implanted. In this regard, the percutaneous implant should not prevent healing, irritate tissues, or stimulate a strong or prolonged rejection response. Moreover, the device should be readily anchored in the surrounding tissues, should reside comfortably in the surrounding tissues, should be physiologically inert over extended time periods, and should be mechanically strong and reliable, particularly with regard to surface properties.

Furthermore, there is a natural tendency for the epithelium tissue to progressively grow down and around a percutaneous implant and eventually to encapsulate it. Upon such epithelial encapsulation, the device is merely held in a pocket which is outside the body, and accordingly does not retain its intended percutaneous nature. An implant which is thus encapsulated tends to be gradually extruded from its encapsulated pocket in the body. Accordingly, an additional criterion is that the implant prevent epithelial encapsulation.

It is an object of the present invention to provide an improved percutaneous implant device.

It is another object of the present invention to provide a percutaneous implant device for drug injection into a living body, which will provide a bacteria-tight seal in conjunction with surrounding tissues, and which is suitable for prolonged implantation without tissue irritation or rejection. It is a further object to provide a percutaneous implant which is readily anchored in the surrounding tissues, which prevents epithelial encapsulation due to the progressive growth of the epithelium, and which provides a high degree of mechanical and physiological reliability. An additional object is the provision of a percutaneous implant which is capable of providing controlled, even and continuous release of medication into a living body.

These and other objects of the invention are more particularly set forth in the following detailed description and in the accompanying drawings of which:

FIG. 1 is a perspective view of a percutaneous implant device embodying various features of the present invention adapted for administration and controlled release of medication;

FIG. 2 is a cross-sectional view of the percutaneous device of FIG. 1 taken through line 2--2 showing the device after implantation;

FIG. 2a is an illustration of an element of the percutaneous device of FIGS. 1 and 2; and

FIGS. 3, 4, 5 and 6 are cross-sectional views of various other embodiments of the present invention.

The present invention is directed to a percutaneous implant device particularly suitable for subcutaneous drug administration to a living body. The implant device comprises a stem having a passageway therethrough, a stabilizing flange adjacent the base of the stem for stabilizing the implant device in the surrounding tissues, means associated with said stem for preventing the progressive growth of the epithelium along the stem and anchoring the implant device by epithelium growth therethrough, and normally closed valve means in the passageway for administering medication through the passageway and for preventing entrance through the passageway of external pathogens or other undesired material. It is important that at least a portion of the surface of the percutaneous implant which is to come into contact with living tissues and preferably the entire surface of the stem, stabilizing flange and epithelium stopping means should have a pyrolytic carbon coating.

In addition, the percutaneous implant may have medication reservoir and release means in communication with the passageway for retaining a reservoir of medication administered through the valve means of the passageway, and for releasing into the body in a predetermined manner the reservoir of medication thus retained.

Illustrated in FIGS. 1 and 2 is percutaneous implant device 10 which is adapted for controlled, even and continuous percutaneous administration of medication to a living body. The implant device 10 comprises a stem 12 having a passageway 14 therethrough, a stabilizing flange 26 at the base 16 of the stem 12, epithelium stopping means 18 about the stem 12, and valve means 20 in the passageway 14. In addition, medication reservoir and release means 22 is provided in communication with the passageway 14. In the embodiment depicted in FIGS. 1 and 2, the stem 12 is cylindrical in exterior shape and is provided with an upper flange 24 at one end and with the subcutaneous stabilizing flange 26 at its other end. The passageway 14 through the stem 12 is defined by the interior surface 13 of the stem 12 and is also generally cylindrical in shape, having the same axis 28 of radial symmetry as the cylindrical exterior 30 of the stem. The generally cylindrical passageway 14 itself is comprised of an upper cylindrical zone 32 adjacent the end of the stem having upper flange 24, and a lower cylindrical zone 34 adjacent the stem end having the subcutaneous stabilizing flange 26. The upper zone 32 of the passageway 14 is of larger diameter than the lower zone 34. The transition in the passage way 14 between the upper zone 32 and the lower zone 34 is discontinuously abrupt and accordingly provides a washer-shaped shoulder 36 which lies in a plane orthagonal to the axis 28 of the passageway, and which has as its inner and outer circumferences 38 and 40, the respective interior terminal ends 38 and 40 of the lower and upper zones 34 and 32 of the passageway 14.

The exterior end 42 of the upper zone 32 of the passageway 14 is provided with connecting means 44 such as the illustrated threads 46 for connecting a medication injecting device to the percutaneous implant 10.

The purpose of the connecting means 44 and the shoulder 36 in the passageway 14 will be explained more fully hereinafter.

The stem 12 having passageway 14 therethrough, the upper flange 24, and the subcutaneous stabilizing flange 26 are all formed as a single unit from a substrate 48 which is subsequently provided with a pyrolytic carbon coating 50 over the entire surface of the unit. Suitable materials for the substrate 48, and the properties and deposition of the pyrolytic carbon coating will also be described more fully hereinafter.

About the stem 12 is epithelium stopping means 18 which in the illustrated embodiment of FIGS. 1 and 2 comprises a collar 52 formed from metallic screen 54 or an equivalent perforated metal sheet, of an alloy of 50 percent Molybdenum, 50 percent Rhenium. The screen 54 of the collar, appearing in more detail in FIG. 2a, has a planar rim 56 which is blunted at its outer circumference 57 to avoid tissue damage, and an internal edge 58. The wire of the screen has a diameter of from about 0.02 mm to about 0.5 mm, and preferably from about 0.05 mm to about 0.1 mm. The spacing between wires is sufficient to prevent closure during coating with pyrolytic carbon. The spacing between the wires after coating should be sufficiently large to permit growth of the epithelium tissue therethrough, but not so large as to permit progressive growth of the epithelium tissue down the stem of the implant. The maximum spacing between wires after coating is one-eighth inch and the minimum is about 0.05 mm. Preferably the spacing is about 1 mm. The screen 54 is placed about the stem 12 portion of the substrate 48 prior to deposition of the pyrolytic carbon thereupon, so that the internal edge 58 resides in a groove 60 circumferentially located around the substrate stem.

As shown in FIG. 2a, the metal screen 54 of the collar 52 is split to facilitate placement about the substrate stem, and after such placement the screen 54 may be held in place in the groove 60 in any suitable manner such as by a wire (not shown), until the pyrolytic carbon coating 50 is deposite upon the substrate 48 including the screen 54. Of course, the deposition of the pyrolytic carbon coating serves to permanently affix the screen 54 to the percutaneous implant 10.

As noted hereinabove, medication reservoir and release members 22 is provided in communication with the passageway 14, at the end thereof adjacent the stabilizing flange 26. In the illustrated embodiment, the reservoir and release means 22 comprises a porous membrane 62 which is selected to provide the desired medicinal release characteristics for the selected course of treatment and the type of medication or drug to be administered by means of the percutaneous device 10. Generally the membrane 62 will be selected to provide controlled, even, and continuous medicinal release at a predetermined rate.

The porous membrane 62 is tube-shaped and has an outside diameter approximately equivalent to the inside diameter of the lower zone 34 of the passageway 14. The upper end 64 of the tube-shaped porous membrane 62 is open, and is affixed, after the deposition of the pyrolytic carbon coating 50, to the lower zone 34 of the passageway 14. The lower, (i.e., opposite) end 66 of the tube-shaped membrane 63 is closed, so that upon attachment of the upper end 64 of the membrane 62 to passageway 14, a reservoir 68 for drugs and/or medication is provided adjacent the interior surface 70 of the membrane, and the exterior surface 71 of the membrane 62 will be exposed to subcutaneous tissues 72 upon percutaneous implantation of the device 10. Accordingly, medication contained in the reservoir will be released through the membrane into the surrounding subcutaneous tissues 72 of the living body in the desired manner.

As the inside diameter of the lower zone 34 of the passageway 14 is approximately the same diameter as the outside diameter of the tube-shaped membrane 62, an effective method of affixing the membrane 62 in communication with the passageway 14 is to insert and adhesively bond the open end 64 of the membrane with the lower zone 34. A suitable adhesive such as silicon cement may be employed for this purpose.

The porous membrane itself may be selected and fabricated from any material having the desired medicinal release properties, and adequate properties with regard to tissue compatibility and resistance to physiological degradation. For example, thin flexible membranes of cellulose nitrate-cellulose acetate are suitable for some situations, and may be readily produced by solution-casting techniques. Porous membranes of other suitable materials may also be used.

Valve means 20 in the embodiment of FIGS. 1 and 2 is an elastomeric plug 74 which snugly resides in the passageway 14 at a location in the passageway generally above (i.e., toward the upper flange 24) the medication reservoir and release means 22, and such that the shoulder 36 and the terminal ends 38 and 40 of the upper and lower zones 32 and 34 of the passageway ar adjacent an intermediate position of the plug 74. Accordingly, the plug 74 is seated against the shoulder 36 in order to resist forces applied to the plug in a direction toward the end of the passageway 14 adjacent the stabilizing flange 26. In addition, the upper surface 76 of the plug 74 lies below threads 46 of the connecting means 44. The plug 74 may be provided in any suitable manner such as by insertion and adhesive bonding of a preformed plug (e.g., of medical grade silicone), or by in situ casting of an elastomer such as a silicone elastomer prepolymer.

The elastomeric plug 74 is provided with a "latent" or pressure operable passageway 78 therethrough generally along the axis 28 of the passageway 14. The passageway 78 is constructed so that, because of the elastomeric nature of the plug 74 and/or a state of transverse compression of the plug in the passageway 14, the passageway is normally closed, and accordingly the plug will not permit the passage of any materials through the passageway in either direction. However, when fluid medication is applied under a predetermined pressure at the upper surface 76 of the plug or by means of a suitable injection device having blunted needle tip for partial insertion in the passageway, the "latent" passageway 78 opens to permit passage of the medication through the plug and into the closed drug reservoir 68 defined by the lower surface 80 of the plug 74 and the interior surface 70 of the membrane 62.

The percutaneous device 10 is implanted by any suitable surgical procedure. Generally, a vertically incision is made through the skin at the desired location for the implant. The incision is of a length sufficient to permit the edgewise insertion of the stabilizing flange 26 and of a depth sufficient to accommodate the medication reservoir and release means 22 and to place the epithelium stopping means 18 below the surface of the skin. A horizontal incision is then made in the subcutaneous tissues to accommodate the stabilizing flange 26 and the percutaneous device is inserted with the upper flange 24 outward so that the lower surface 82 of the upper flange lies adjacent the surface of the skin 84. Alternatively, for hygienic reasons, the upper flange may extend somewhat about the skin surface. The horizontal incision is then closed. A convenient method involves advancing the percutaneous device 10 so that the stem 12 lies adjacent one end of the incision, and suturing the remaining portion of the incision at its other end.

Upon healing, the epithelium 86 grows around and down the stem 12 until it encounters the pyrolytic carbon coated screen 54 of the collar 52. The epithelium encircles the individual pyrolytic carbon coated wires of the mesh screen collar 52, forms a bacteria-tight seal, and stops its downward growth, which if continued would encapsulate the percutaneous device 10. In addition, this interaction of the collar 52 with the growth of the epithelium 86 therethrough, anchors the implant and prevents it from being torn loose. In addition, this anchoring, in conjunction with the stabilizing flange 26, stabilizes the position and location of the percutaneous device in the subcutaneous tissues 72. The upper flange 24 serves to protect the implantation site.

Because of the tissue compatibility and physiological properties of the pyrolytic carbon coating 50 of the device 10, healing is rapid, without significant rejection reaction or tissue inflammation and the percutaneous implant is relatively comfortable for the patient. After healing, a medication injector (not shown) is attached to the percutaneous device 10 through the connecting means 44. In the illustrated embodiment in FIGS. 1 and 2, a syringe or other device capable of providing a measured amount of fluid medication at a predetermined pressure sufficient to activate the "latent" passageway 78 of the plug 74, is screwed into the threads 46 and seated against the upper surface 76 of the plug 74. The medication is then forced through the passageway 78 in the plug 74 into the reservoir 68, and is released from the reservoir into the surrounding tissues 72 in the desired manner. The stabilizing flange and the anchoring at the encapsulation stopping means serve to stabilize the position of the implant, and disipate the forces associated with drug injection. After administration of the medication, the syringe or other device is unscrewed, and will normally be replaced by a cap (not shown) for protecting and keeping clean the upper surface 76 of the plug 74 between medication administration.

As noted above, the percutaneous implant devices are coated with pyrolytic carbon. The coating is provided by depositing pyrolytic carbon on a suitable substrate material. Pyrolytic carbon is capable not only of significantly increasing the strength and wear resistance of the percutaneous device, but also is compatible with the surrounding tissue over prolonged time periods when implanted through the skin of a living body.

While reference is herein generally made to the use of percutaneous devices in a living human body, it should also be recognized that the percutaneous devices may also have veterinary or scientific applications in other living animals, domestic or wild.

In general, the pyrolytic carbon coating is applied to a suitable substrate material which is shaped to form a part of the percutaneous device, such that the pyrolytic carbon covers at least a major portion of the surface thereof. The thickness of the pyrolytic carbon coating should be sufficient to provide the necessary strength for its intended use, and often it is desirable to employ the coating to impart additional strength to the particular substrate being coated. Some substrates such as certain types of graphite or refractory metals may require only relatively thin pyrolytic carbon coatings, while other substrates should employ thicker coatings. In general, the coating should be at least 10 microns thick and usually at least about 25 to 50 microns or more thick. If a fairly weak substrate is being employed, for instance, one made of bulk artiticial graphite, it may be desirable to provide a thicker coating of pyrolytic carbon to strengthen the composite percutaneous device.

Moreover, although an outer coating which is relatively pure has adequate structural strength and is generally preferred, pyrolytic carbon coatings obtained through the codeposition of silicon or some other carbide-forming additive may also be employed. For example, 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 its compatibility with the epidermal and subcutaneous tissues in which it is implanted.

For use on complex shapes and in order to obtain maximum structural strength, it is desirable that a pyrolytic carbon coating on the substrate be nearly isotropic. anisotropic carbons tend to delaminate when complex shapes are cooled after depositing the pyrolytic coating at high temperatures. Thus, for coating complex shapes (i.e., those having a radius or radii of curvature less than one-fourth inch), the pyrolytic carbon should have a BAF (Bacon Anisotrophy Factor) of not more than about 1.3. For non-complex 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 in the layer planes in the carbon crystalline structure. The techniques 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, Vol. 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, while higher values indicate increasing degrees of anisotrophy.

The density of the pyrolytic carbon is considered to be an important feature in determining the additional strength which the pyrolytic carbon coating will provide the substrate. The density is further important in assuring tissue compatibility, and mechanical reliability of the coating. It is considered that the pyrolytic carbon should at least have a density of about 1.5 grams per cubic centimeter, and may range up to a density between about 1.9 grams/cm3 and about 2.2 grams/cm3. Preferably the density will be about 1.9 grams per cubic centimeter.

Another important characteristic of the pyrolytic carbon coating is its crystallite height or apparent crystallite size. The apparent crystallite size is herein termed Lc and can be obtained using an X-ray diffractometer. In this respect

Lc = 0.89 λ/β cos θ

wherein:

λ is the wavelength in Angstroms

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

θ is the Bragg angle.

Pyrolytic carbon coatings for use in percutaneous devices should have crystallite size no greater than about 200 A, and preferably between about 20 and about 50 A.

Since the substrate material for the prosthetic device will preferably be completely encased in pyrolytic carbon, choice of the material from which to form the substrate is not of utmost importance per se. However, the substrate material should have sufficient strength and structural properties to reliably withstand the conditions of use of the particular percutaneous application for which it is going to be employed. However, portions of the substrate are to be exposed to bodily tissues, for example, as might occur from machining into final form after the basic shape has been coated with pyrolytic carbon, the substrate should be selected from materials which are relatively biologically inert, preferably artificial graphite.

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

Because pyrolytic carbon is, by definition, deposited by the pyrolysis of a carbon-containing substance, 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° C. are used. Some examples of the deposition of pyrolytic carbon to produce coated articles having increased stability under high temperature and neutron radiation conditions are set forth in U.S. Pat. No. 3,298,921. Processes illustrated and described in this U.S. patent employ methane as the source of carbon and utilize temperatures generally in the range from about 1,200° to 2,300° C. Although it is possible to deposite 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 materials should remain substantially unaffected by temperatures of at least about 1,000° C. and preferably by even higher temperatures. The pyrolytic carbons deposited either with or without silicon at temperatures below about 1,500° C. are particularly suited for use in percutaneous devices because such pyrolytic carbons have exceptional tissue compatibility and mechanical reliability.

Because the substrate is coated at relatively high temperatures and the percutaneous 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 thereon should be relatively close to each other if the pyrolytic carbon is to be deposited directly upon the substrate and a firm bond between them is to be established. While the aboveidentified U.S. patent contains a description of the deposition of an intermediate, low density pyrolytic carbon layer, the employment of which might provide greater leeway in matching the coefficients of thermal expansion, it is preferable to deposite the pyrolytic carbon directly upon the substrate or an intermediate dense carbon layer. Pyrolytic carbon having the desired characteristics can be deposited having an average thermal coefficient of expansion in the range of between about 3 and about 6 × 10-6 /° C. measured between 20° C. and 1,000° 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 × 10-6 /° C. Examples of suitable substrate materials include artificial graphite, boron carbide, silicon carbide, refractory metals (and alloys) such as tantalum, molybdenum, tungsten, and various ceramics, such as mullite. A preferred substrate material is polycrystalline graphite. An example of such a graphite is the polycrystalline graphite sold under the trade name POCO AXF Graphite, which has a density of about 1.9 grams per cubic centimeter, an average crystallite size (Lc) of about 300 A, and an isotrophy of nearly 1.0 on the Bacon scale. Ceramic and metallic substrate materials which may be readily molded or shaped are particularly desirable with regard to mass-production and cost considerations. Refractory fibers and screens, particularly of refractory metal fibers, and perforated thin metal sheets are particularly suited for substrates for the epithelium encapsulation stopping means.

The pyrolytic carbon coating is applied to the substrate using a suitable apparatus for this purpose. Preferably, an apparatus is utilized which maintains a 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. When larger substrates are employed, or where it is desired to vary the thickness or other characteristics of the pyrolytic carbon coating over different portions of the substrate, different coating methods may be employed, such as supporting the substrate on a rotating or stationary mandrel within a large fluidized bed.

As discussed in detail in the above-identified United States patent, the characteristics of the carbon which is 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 the hydrocarbon gas, 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 allow deposition of pyrolytic carbon having the desired density, apparent crystallite size, and isotropy, but it also permits regulation of the desired thermal coefficient of expansion of the deposited pyrolytic carbon. This control may also be used to "grade" a coating in order to provide a variety of exterior surfaces. 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, the coating conditions can be gradually changed 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 plus or minus 25 percent of the coefficient of expansion of the substrate material, and preferably to within about plus or minus 20 percent. Because pyrolytic carbon has greater strength when placed in compression than when placed in tension, the thermal coefficient of expansion of the pyrolytic carbon is most preferably 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, and upon cooling of the pyrolytic coating-substrate composite, the pyrolytic carbon coating is placed in compression under conditions of its intended use at about ambient temperature.

As previously indicated, the coating may be substantially pure pyrolytic carbon, or it may contain a carbide-forming additive, such as silicon, which has been found to enhance the overall mechanical properties of the coating. Silicon in an amount of up to about 20 weight percent, based on the total weight of silicon plus pyrolytic carbon, may be included without detracting from the desirable physiological properties of the pyrolytic carbon, and when silicon is used as an additive, it is generally employed in an amount between about 10 and about 20 weight percent. Other carbide-forming elements which are non-toxic, such as zirconium and titanium, may also be used as additives in equivalent weight percents. Generally, such an element would not be used in an amount greater than 10 atom percent, based on the total atoms of pyrolytic carbon plus the element.

The carbide-forming additive is co-deposited 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 at which the pyrolysis and co-deposition occurs, the particular element employed is converted to the carbide form and appears dispersed as a carbide throughout the resultant product. As previously indicated, at temperature below about 1,500° C. 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.

After deposition of the pyrolytic carbon coating on the substrate, it may be desirable to physically and/or chemically modify the pyrolytic carbon surface thus provided. For example, chemisorbed gases, such as oxygen, may be removed by a vacuum-heat treatment to provide a less reactive, more hydrophobic surface, such as may facilitate more easy removal of the implant. Generally, however, for percutaneous implants which are to be attached to tissue, it is desirable that the surface reactivity of the pyrolytic carbon surface be enhanced such as by the provision of carboxyl, hydroxyl or quinone groups at the surface of the pyrolytic carbon coating.

For example, the following procedures might be followed to increase the chemical surface reactivity of the pyrolytic carbon coatings:

1. Oxidation at about 700° C in dry O2 to form quinone groups, or such formation of quinone groups followed by steam autoclaving to form hydroquinone groups,

2. Oxidation at about 300° C in dry oxygen to form COO groups, and similarly followed by steam autoclaving to form carboxyl groups,

3. Oxidation at about 500° C to form both quinone and COO groups, and similarly followed by steam autoclaving to form both hydroxyl and carboxyl groups, and

4. Oxidation with atomic oxygen at room temperature to form a monolayer of chemisorbed oxygen, followed by steam autoclaving if desired.

Pyrolytic carbon having the physical properties mentioned hereinbefore, is considered to be particularly advantageous for constituting the surface for a percutaneous implant because of its physiological inertness and exceptional compatability with living tissues. The pyrolytic carbon coating does not tend to irritate the surrounding tissues and promotes the establishment of a barrier to external pathogens.

Having described in detail the specific embodiment of FIGS. 1 and 2, the following modiifications are described to further illustrate the invention. Illustrated in cross-sectional view in FIGS. 3, 4, 5 and 6 are percutaneous implants depicting various specific embodiments of the elements of the present invention.

In FIG. 3 is illustrated a percutaneous implant device 100 comprising a unit 101 having a stem 102 with a passageway 104 therethrough, a stabilizing flange 106, and an upper flange 108. The unit 101 is formed from a suitable substrate and has a pyrolytic carbon coating 110 thereon. Prior to deposition of the pyrolytic carbon coating, in order to provide epithelium-stopping means 111 associated with the stem 102, a folded strip of refractory metal screen or equivalently perforated metal sheet 112 is secured on the stem 102 by wire 114 and the pyrolytic carbon coating is subsequently deposited on the unit 101, screen 112 and wire 114 to weld them as a single and strong structure. An unperforated elastomer plug 116 through which medication may be administered by means of a hypodermic needle serves as the valve means in the passageway, and the flexible microporous membrane 118, secured adjacent the passageway 104 to the underside 120 of the stabilizing flange 106, provides in combination with the passageway and the plug 116 a reservoir 122 from which administered medication may be released through the membrane into the surrounding tissues.

In FIG. 4, a percutaneous implant device 150 similar to that of FIG. 3 is depicted; however, the implant device 150 has no upper flange, and the upper surface 152 of the stem 154 is implanted flush with the skin and accordingly has no projections from the body that may be caught on other objects or interfere with movement. In addition, the implant device 150 is constituted to provide a relatively large medication reservoir 156, and has epithelium stopping means 158 formed from a pyrolytic carbon coated roll of multiple layers of refractory metal screen.

FIG. 5 also depicts a similar percutaneous implant device 180. The implant device 180 has no means for slowly releasing administered medication, but rather is designed for direct injection of medication into a living body. Bayonet connecting means 182 is provided in upper flange 184, and the epithelium stopping means 186 circumferentially about the stem 188 is formed from pyrolytic carbon coated, carbon-fiber mesh tube substrate as illustrated in FIG. 3. The valve means is an elastomer plug 190 secured in the passageway 192 which has a pressure-activated passage 194 therethrough to prevent entrance of external pathogens or other undesirable material, but which permits administration of fluid medication under the proper conditions of applied pressure.

Illustrated in FIG. 6 is a percutaneous implant device 200 which is designed to facilitate fabrication and application. The stem 202 is formed from a substrate 208 having a pyrolytic carbon coating 210 thereon, and is constructed in two pieces, the main body 204 of the stem, and a cap portion 206. The main body 204 of the stem 202 is constructed so that the epithelium stopping means 212 and the stabilizing flange 211 may be sequentially assembled about the main body 204 of the stem 202 and secured in place after such assembly by means of the cap portion 206, such as by cementing or screwing on the cap, or by other suitable means such as a bayonet type fastening.

The epithelium stopping means 212 may be any porous, carbonaceous surfaced, physiologically inert aggregate into and through which epithelium tissue will grow, and which will thereby arrest the progressive growth of the epithelium tissue down and around the stem. For example, the epithelium stopping means 212 may be a pyrolytic carbon coated, washer-shaped, layer of a fibrous carbon substrate such as carbon felt or cloth or yarn or the means 212 may be a carbon fiber ring such as Carbotex manufactured by Carborundum, and preferably having at least a very thin pyrolytic carbon coating. Or the epithelium stopping means 212 may be a porous carbon ring such as produced by pyrolyzing a structure formed of sintered plastic beads, or of a porous graphite such as sold under the trade name POCO-Type AX having a density of about 1.0 grams/cc. In all cases after coating, the ring of pyrocarbon is removed by machining or grinding to provide access to the porous underlying structure.

The stabilizing flange 211 may be a rigid pyrolytic carbon coated graphite or ceramic substrate, or may be of a more flexible material such as felted carbon fibers (preferably with at least a very thin pyrolytic carbon coating), or even of a more flexible material such as medical grade silicone rubber.

An alternative method of construction is to assemble the epithelium stopping means and the stablizing flange on the main body of the stem substrate and secure them with the cap portion substrate prior to coating with pyrolytic carbon. The assembled unit is then coated with pyrolytic carbon, the pyrolytic carbon coating removed circumferentially over a portion of the stem to expose the epithelium stopping means. The valve means and if desired the medicinal reservoir and release means, are then assembled subsequent to the pyrolytic carbon coating.

It is also contemplated that the percutaneous devices of the present invention might have multiple passageways each having a normally closed valve means. For example, a percutaneous device might have two passageways through the stem, each with a separate medication and release means respectively in communication therewith for separate administration of two medicants. Or, the two passageways could be connected by a single semipermeable membrane passageway to provide a U-shaped conduit which could more readily be flushed free of medication when desired by forcing a washing fluid through one passageway and out the other.

Various embodiments in addition to those described will become apparent to those skilled in the art in view of the present disclosure.

Various of the features of the invention are set forth in the following claims.