U.S. patent number 3,783,868 [Application Number 05/140,869] was granted by the patent office on 1974-01-08 for percutaneous implant.
This patent grant is currently assigned to Gulf Oil Corporation. Invention is credited to Jack C. Bokros.
United States Patent |
3,783,868 |
Bokros |
January 8, 1974 |
**Please see images for:
( Certificate of Correction ) ** |
PERCUTANEOUS IMPLANT
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; Jack C. (San Diego,
CA) |
Assignee: |
Gulf Oil Corporation
(Pittsburgh, PA)
|
Family
ID: |
22493161 |
Appl.
No.: |
05/140,869 |
Filed: |
May 6, 1971 |
Current U.S.
Class: |
604/891.1;
424/448; 623/1.42 |
Current CPC
Class: |
A61N
1/05 (20130101); A61M 39/0247 (20130101); A61M
2039/0285 (20130101); A61M 2039/0261 (20130101) |
Current International
Class: |
A61M
1/00 (20060101); A61N 1/05 (20060101); A61m
031/00 (); A61f 001/00 () |
Field of
Search: |
;3/1
;128/2R,2F,260,348-351,214R,215,216,213,1R,334 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
1,161,436 |
|
Aug 1969 |
|
GB |
|
141,591 |
|
Mar 1961 |
|
SU |
|
245,277 |
|
Nov 1969 |
|
SU |
|
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/cm.sup.3 and about 2.2 grams/cm.sup.3. 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 L.sub.c and can be obtained using
an X-ray diffractometer. In this respect
L.sub.c = 0.89 .lambda./.beta. cos .theta.
wherein:
.lambda. is the wavelength in Angstroms
.beta. is the half-height (002) line width, and
.theta. 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.degree. 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.degree. to 2,300.degree. 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.degree. C. and
preferably by even higher temperatures. The pyrolytic carbons
deposited either with or without silicon at temperatures below
about 1,500.degree. 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 .times. 10.sup.-.sup.6
/.degree. C. measured between 20.degree. C. and 1,000.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, 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 (L.sub.c) 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.degree. 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.degree. C in dry O.sub.2 to form quinone
groups, or such formation of quinone groups followed by steam
autoclaving to form hydroquinone groups,
2. Oxidation at about 300.degree. C in dry oxygen to form COO
groups, and similarly followed by steam autoclaving to form
carboxyl groups,
3. Oxidation at about 500.degree. 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.
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