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BACKGROUND
The present invention concerns improvements in a silicone catheter, a
tubular medical device for insertion into the body for the purpose of
injecting or withdrawing fluids. Although the invention is described in
the following specification with particular reference to a Foley catheter
for insertion in the urethra, the invention is equally applicable to
similarly constructed medical devices such as endotracheal tubes, tracheal
tubes, Dennis tubes (used for decompressing the intestinal tract prior to
or after gastrointestinal surgery), rectal catheters, Trocar catheters,
hematuria balloon cateters, heart catheters, and others. In addition, the
concepts of this invention could also be applied to similarly constructed
non-medical devices.
The typical Foley catheter consists of a tube or shaft containing a primary
lumen which is the conduit for removal or insertion of the appropriate
fluid. Within this primary lumen and attached to the wall thereof is at
least one secondary, smaller lumen which is the conduit for the injection
of an appropriate gas or fluid for inflation of a balloon anchoring the
catheter inside the patient. The anchoring balloon generally consists of a
thin, elastic material extending around the exterior of the catheter near
its tip and attached at its edges or shoulders to the exterior of the
catheter. A hole in the tube wall permits the inflation gas or fluid to
enter underneath the elastic material and expand it to a balloon-like
configuration.
In use the catheter is inserted into the body cavity, and an inflation
fluid is pumped through the secondary lumen to expand the anchoring
balloon. This prevents accidental removal of the catheter from the patient
and stations the catheter in the appropriate position for efficient use.
Once the catheter is so anchored, body fluids can be drained or
therapeutic fluids can be injected into the body through the primary
lumen. When the catheter is no longer needed, the inflation balloon is
deflated by releasing the inflation fluid or gas and the catheter is
withdrawn from the body.
Although this basic design for a catheter has been used for a number of
years, serious problems remain in the design and method of construction of
these devices. In particular, there are problems with the previous methods
which have been used to provide the elastic exterior portion which forms
the anchoring bubble. For example, in U.S. Pat. No. 3,734,100 issued to
Walker et al, a catheter construction is described in which a separate
cuff portion is glued at its shoulders to the surface of the catheter.
Although the patent illustrates the exterior surface of the catheter as
being smooth and regular, unfortunately, in practice the thickness of the
cuff portion and the effects of the glue cause surface irregularities at
the cuff's shoulders. Thus, in a typical catheter produced by the process
of Walker et al. the shoulders of the cuff will protrude beyond the
remaining exterior surface of the catheter. This situation is undesirable
because such irregularities impede the insertion or withdrawal of the
catheter and increase the discomfort to the patient. In addition, such a
catheter construction is generally not amenable to mechanized production,
and manufacture of the catheters by hand causes additional imperfections
and defects.
As an alternative, it has been suggested to cover the exterior of the
catheter tube in the area underlying the bubble with a masking (release)
material, coating the tip of the catheter including the bubble region with
a flexible, inflatable material, and subsequently removing the underlying
masking material. For example, in the patents to Harautuneian (U.S. Pat.
No. 3,292,627 and 3,304,353) the use of a water soluble masking material
is suggested. However, there are several problems with this design. First,
in simultaneously removing the water soluble release material and
inflating the anchoring bubble, difficulties in the dissolution of the
release material frequently cause particles to become lodged in the
inflation lumen preventing further expansion of the anchoring bubble.
Similarly, as discussed in Harautuneian's U.S. Pat. No. 3,452,756, the
masking layer may only dissolve in the localized area adjacent the hole to
the inflation lumen. Thus, the anchoring bubble may expand preferentially
in that area and cause localized pressure injurious to the body tissue.
The same general type of system is shown in British Pat. No.
1,234,037--Steer et al.
Alternatively, U.S. Pat. No. 3,544,668 issued to Dereniuk illustrates the
use of a gel as a masking layer beneath the anchoring balloon. Following
the formation of the outer skin on the catheter, the gel layer is
volatilized by heat. This method is difficult to practice because the
outer covering of the catheter must be applied before the release coating
has dried or is touched.
A further difficulty encountered with the prior art processes for
manufacturing a catheter is that they are not applicable to the most
useful catheter material, silicone. It has been found that silicone is
more compatible with human tissue than previously used rubber and plastic
materials and that the incidence of infection in the body tissues is
appreciably reduced with silicone catheters. Although attempts have been
made to apply silicone coatings over conventional catheter tube materials,
silicone bonds poorly with the majority of these other materials.
Accordingly, it is preferable to construct the entire catheter of
silicone. Because of silicone's hydrophobic nature, however, it is
difficult to apply coatings such as those in the Harautuneian and Dereniuk
patents to silicone. Thus, in addition to the problems generally
encountered with water soluble or volatile release coatings, a further
defect is that these coatings are not useful in preparing silicone
catheters.
In view of the foregoing, it is an object of the present invention to
provide an improved catheter construction.
It is also an object of this invention to provide an improved silicone
catheter and made entirely of silicone.
It is a further object of this invention to provide a catheter which does
not suffer the defects of localized swelling of the anchoring bubble or
the plugging of the inflation lumen caused by the particles from the
release coating.
SUMMARY OF THE INVENTION
The invention comprises an improved catheter design.
The catheter comprises a tubular body comprising a larger primary lumen and
a smaller inflation lumen; a smooth tip at the distal end of the catheter
body which seals the primary and inflation lumens; an outer covering
enclosing the tubular catheter body and the tip; the tubular catheter body
containing a first hole to permit communication of fluids through the
outer covering between the exterior of the tube and the primary lumen and
a second hole permitting communication between the exterior of the tube
and the inflation lumen but not through the outer covering; and a thin
thermoplastic tape surrounding the tube underneath the outer covering and
overlapping the second hole, the width of the tape corresponding to the
desired base of the anchoring bubble formed by inflation of a portion of
the outer layer, the tape containing a hole adjacent said second hole to
permit the inflation of the anchoring bubble and the tape comprising a
material to which said outer layer does not adhere.
DESCRIPTION OF THE DRAWINGS
The invention is illustrated by the following drawings in which FIG. 1 is a
"top view" of the uninflated catheter.
FIG. 2 is a partially cutaway "side view" of the uninflated catheter.
FIG. 3 is a cross section of the uninflated catheter taken along line 3--3
of FIG. 2.
FIG. 4 is a cutaway "side view" of the catheter illustrating the inflated
anchoring bubble.
FIG. 5 is a cross section of the uninflated catheter taken along line 4--4
of FIG. 2.
Referring to the drawings, the first step in the preparation of the
catheter is the formation of tube 10 comprising primary lumen 12 and
inflation lumen 14. The cross-sectional shape and orientation of these two
channels may take various forms other than that shown in FIGS. 3 and 5.
However, to provide the minimum amount of resistance for insertion into
the patient, it is desirable that the exterior cross section of the tube
be essentially round. Formation of this tube can be accomplished by an
ordinary procedure, such as extrusion, wherein a continuous length of
tubing is formed. The tubing is then cut to the appropriate length desired
for the catheter.
After the tubular body of the catheter is formed, an inflation hole 18 is
formed in the exterior of the tube to provide a passage to the inflation
lumen. This hole is located at an appropriate distance from the distal end
of the catheter tube where the anchoring bubble is to be located. The
distal end of the catheter refers to the end which is inserted into the
patient. The hole can be formed by any standard procedure, such as cutting
or punching the exterior of the catheter tube.
Following the formation of the inflation hole a layer of thermoplastic
material 20 is wrapped around the tube so that it covers the inflation
hole. An appropriate thermoplastic material is a thermoplastic tape whose
adhesive side adheres to the exterior of tube 10 but whose opposite side
does not adhere to the subsequently applied outer layer of the catheter.
The wrapping of the thermoplastic tape can be accomplished by hand or by
mechanical means in such a manner that the ends of the tape overlap very
slightly. This will insure that the outer covering which forms the
anchoring bubble will not adhere to any portion of the underlying tube. In
addition, the thermoplastic tape should be thin, for example less than
0.05 inch and, preferably, approximately 0.004 inch in thickness. This
should avoid any impairment to the insertion of the catheter into the body
caused by an enlargement of the outside diameter of the catheter tube in
the region adjacent the thermoplastic tape.
Following the application of the thermoplastic tape a tip 16 is formed on
the distal end of the catheter. Although the tip shown in the drawing is
semi-spherical in shape, the actual configuration of the tip can take any
form which facilitates the insertion of the catheter into the body.
Preferably the tip is molded to the tubular body, for example, by placing
the end of the tube into an open metal form filled with liquid material.
This method of forming the tip is preferred, since it provides a secure
seal of the inflation lumen at 22. Without such a seal it would be
difficult, if not impossible, to inflate and maintain the anchoring
bubble.
After these essential elements of the catheter are formed and assembled, an
outer layer of material is applied over the length of the catheter by
dipping the catheter into a solution of the covering material. The coating
is dried and cured at room temperature and humidity over night or more
rapidly at elevated temperatures. Additional coatings may be required to
achieve the desired thickness of the outer layer. The procedures for dip
coating a catheter with silicone are known to those skilled in the art as
shown, for example, in U.S. Pat. No. 3,434,869, issued to Davidson. As
illustrated in the drawings, the completed coated catheter has a smooth
outer surface which does not present any protrusions which might impede
the insertion or extraction of the catheter from the body.
Following the formation of the coating, hole 26 is punched in the catheter
wall to provide a channel between the primary lumen and the exterior of
the catheter. Although this hole can take any shape and size, as shown in
the drawings, identical oval holes are punched on opposite sides of the
catheter to facilitate injection or withdrawal of the fluids from the
body.
Finally, an inflation hole 28 is formed in the thermoplastic tape. This
hole is adjacent the inflation hole 18 previously cut in the wall of the
catheter tube. The thermoplastic tape is pierced by touching the silicone
coating over the opening 18 with a hot probe the temperature of which is
adjusted to melt the thermoplastic tape but not to affect the silicone
outer coating.
The advantages of the present invention are readily apparent from FIG. 4
which shows the anchoring bubble of the catheter in its inflated position.
When the inflation fluid or gas is forced through inflation lumen 14, it
inflates the bubble as shown in FIG. 4. Since the masking material does
not have to be dissolved during inflation, the inflation occurs steadily
and uniformly and the inflation lumen will not become plugged by particles
of partially dissolved masking material.
As mentioned previously, it is desirable to build the entire catheter of
silicone because of its compatibility with human tissues. Typically the
tubular catheter body and the tip are made of much harder silicones than
that used for the outer covering.
The thermoplastic masking tape may be made of any thin material whose
adhesive side adheres to the tubular body of the catheter, whose opposite
side does not adhere to the outer coating of the catheter, and which melts
at a temperature significantly lower than that of the outer coating. Where
the tubular body and outer coating are both made from silicone, a suitable
release material consists of unreinforced polyethylene or polypropylene
tape having a thickness of approximately 0.005 inch.
Typical properties of materials presently available which are suitable for
use in the present invention are designated in Table I.
TABLE I
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Tubular Body Tip Outer Coating
Shore A
Durometer
50-70 30-70 30-55
Tensile
Strength
(p.s.i.)
900-1400 850-1200 300-900
Elongation
(%) 200-500 125-900 350-800
Tear
Resistance
(lbs./in.)
60-200 20-100 40-130
Type of
Silicone
Rubber Heat Cured* 2 Part Room**
1 Part Room**
Temperature Temperature
Vulcanizing Vulcanizing
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*Cured at elevated temperature and pressure in the presence of peroxide.
**Mixed with a catalyst, normally a metallic soap, and cured at room
temperature.
***Cured by exposure to moist air at room temperature.
Following the formation of the catheter illustrated in the drawings it is
also desirable to mold to the external end of the catheter suitable
apparatus containing a provision for injecting gas or fluid into the
inflation lumen and for connecting the primary lumen to suitable drainage
or injection means. A typical device is shown, for example, in U.S. Pat.
No. 3,769,981 to McWhorter et al.
EXAMPLE
A silicone catheter was prepared according to the following procedure.
First, a silicone mixture was prepared using the heat cured silicone
rubber described in Table II and 1.0 percent of a catalyst comprising bis
2-4 dichlorobenzoyl peroxide (Cadox TS-50). This mixture was extruded into
a tube comprising a primary lumen and an inflation lumen arranged as shown
in FIGS. 1 through 5. This tube is extruded through a tunnel maintained at
about 900.degree.F and has a residence time within the tunnel of
approximately 1-4 minutes. As the tube leaves the tunnel it is cut to the
size appropriate for the particular application of the catheter.
Subsequently, a hole is punched at the distal end of the tubular body of
the catheter at the location of the anchoring balloon, the hole providing
a passage to the inflation lumen.
At this point the tip is formed on the distal end of the catheter. A liquid
two-part silicone rubber (as described in Table II) is poured into a steel
mold, and the distal end of the catheter is immediately inserted into the
mold. The catheter is permitted to remain in the mold while the silicone
tip is cured. The catheter is removed from the mold and cured for 4 hours
at 350.degree.F. This process drives off any remaining catalyst and
completes the cure of the catheter body.
The catheter is prepared for dip coating by applying a strip of
polypropylene tape around the catheter tube so that it overlaps the
previously punched hole. The catheter is then dipped into a 40 percent
solvent solution one-part room temperature vulcanized silicone rubber (as
described in Table II) and cyclohexane, withdrawn at a slow, steady rate,
inverted and dried for 40 minutes at 120.degree.F. This dipping and drying
procedure is repeated two additional times. On the final (third) coating
blue dye is added to the silicone solution to add color to the catheter.
Following the final drying step, two elliptical drainage holes are punched
in the distal end of the catheter to provide communication of the drainage
lumen with the exterior of the catheter. A probe heated to 500.degree.F is
then applied for about 30 seconds to the catheter skin adjacent the hole
to the inflation lumen to form a hole in the polypropylene tape.
The catheter is tested by inflating and inspecting the anchoring balloon.
TABLE II
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Tubular Body Tip Outer Coating
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Shore A
Durometer
60.+-.5 35.+-.5 40.+-.5
Tensile
Strength
(p.s.i.)
1100 925 800
Elongation
(%) 350 150 700
Tear
Resistance
(lbs./in.)
90 25 100
Type of
Silicone
Rubber Heat Cured 2 Part Room 1 Part Room
Temperature Temperature
Vulcanizing Vulcanizing
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It should be mentioned that the order of the foregoing steps in the process
of producing the catheter of this invention may be varied from that
described without any effect on the catheter. For example, the tip may be
molded to the body before application of the masking tape and the order of
forming holes 26 and 28 may be reversed.
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