A splittable tube, useful in medical applications as a cannula for insertion into the body, comprises an inner and an outer layer. The inner layer defines a central lumen for accepting catheters and probes. The inner layer also includes a longitudinal slot through its wall for defining a stress line in the outer layer. The outer layer is very thin, membrane-like, and tears along the longitudinal stress line when the cannula is pulled to the side of a catheter occupying the central lumen. The tube may be made of electrically insulating material and used, for example, as strippable insulation as telephone or house wire, and strippable without the use of tools.

FIELD OF THE INVENTION 
This invention relates to "peel-away" tubing which may be used for 
electrical wire insulation and, more particularly, for medical usage such 
as a "peel-away" cardiovascular access port for sheaths and dilators. 
BACKGROUND OF THE INVENTION 
Catheters are frequently inserted into the human body by means of an aid 
called a "cannula." A cannula is a flexible tube which serves as a 
passageway into a blood vessel for a probe or catheter. When a probe or 
catheter is to remain in the body for an extended period, it is desirable 
to remove the cannula. However, removal of the cannula is not always a 
simple task. Frequently, a probe or catheter has an enlarged proximal end 
which makes the removal of the cannula (sleeve) difficult. This problem 
has been alleviated by a cannula which separates into two parts as 
described, for example, in U.S. Pat. No. Re. 13,855 issued Mar. 26, 1985 
and in U.S. Pat. No. 4,166,469 issued Sept. 4, 1979. Although the 
apparatus described expedites removal of the cannula, it requires two 
hands to remove it, requires a tab at the proximal end of each side of the 
cannula, requires a sleeve within a sleeve, and requires a line of 
apertures or particular molecular properties, to ensure that the cannula 
tears along a longitudinal direction. 
BRIEF DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS OF THIS INVENTION 
In accordance with the principles of this invention, a peel-away sleeve for 
use as electrical wire insulation or for a cannula for medical usage 
includes an inner and outer sleeve. The outer sleeve is a thin membrane. 
The inner sleeve has a center opening into which, for example, a catheter 
is inserted. The inner sleeve also includes an off-center side bore in the 
wall of the inner layer, extending all the way through the wall of the 
inner layer. 
The cannula is peeled away merely by pulling the cannula to one side of the 
catheter, so that the catheter enters the side bore separating the wall of 
the inner layer of the cannula and tearing the membrane outer layer 
Removal can be accomplished by grasping the proximal end of the cannula 
between two fingers of a hand and pulling to one side. The proximal end of 
the cannula includes a marking to designate the direction in which the 
proximal end should be pulled.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS OF THIS INVENTION 
FIG. 1 depicts a peel-away tube 10 comprising an inner layer 11 and a 
relatively thin outer layer 12. The figures also show a catheter 13 
inserted within the central bore 14 of the tube. Only a portion of the 
catheter is shown. The proximal end of the catheter protruding from the 
partially peeled away tube is shown broken away The distal end 16 is shown 
extruding from the tube and, itself, including a central lumen 17. 
In medical use, the tube is a cannula inserted into the human body 
frequently with the aid of a dilator. The dilator is used to extend a 
puncture wound in the body. The cannula is disposed about the dilator and 
enters the wound through the extended opening. The dilator is then 
removed, leaving the central bore of the cannula to receive, for example, 
a catheter to measure the flow of saline solution through a blood vessel. 
Such a catheter has a balloon at its distal end to close off the flow 
through a blood vessel when inflated. The balloon is inflated by air 
pressure from a fitting at the proximal end; thus, the proximal end of the 
catheter has a saline supply fitting, an air supply fitting, and may 
include a sensor fitting. 
The tube (cannula in this case) is removed from its position merely by 
grasping the proximal end 20 and by pulling the end in a direction 
opposite to the longitudinal opening 21 in the inner wall 11. Tube 10 
includes a marking arrow 22 to indicate the direction in which to pull. 
The axial opening can be seen to be triangular or wedge-shaped in a 
cross-sectional view of the tube as shown in FIG. 1 when the tube is 
peeled away, moved into the wedge 21 and causes a longitudinal stress line 
along the tip of the wedge. This, in turn, causes the membrane-thin outer 
layer to tear, along the longitudinal line at the stress point. 
FIG. 2 also shows typical dimensions for a cannula for medical use for 
accepting a probe or catheter. The inside diameter (ID) of the inner layer 
11 is 0.060". The outside diameter (OD) of the inner layer is 0.998" and 
the outer layer is 0.001". These dimensions are only illustrative; the 
outer layer may be, for example, less than 0.001". The outer layer 
maintains the circumferential integrity of the tubing until the layer is 
split. 
FIG. 3 shows a cross-section of an alternative peel-away tube. The tube 
again has an outer layer 30 and an inner layer 31; but in this case, the 
interior bore 33 has two longitudinal wedges 34, 35 penetrating the 
interior layer. The two wedges are 180.degree. apart as can be seen from 
the figure and provide opposing stress points so that the cannula can be 
removed from either side. In the case where two stress points are 
provided, the direction marking may not be necessary because separation of 
the cannula into two parts is easily achieved by pulling in any direction. 
Other geometries for the longitudinal cut in the inner wall are possible. 
FIGS. 4 and 5 show a splittable tube 50 having inner and outer walls 51, 
52. The tube has a central lumen 54 and a longitudinal slot 56 in the 
inner wall. The longitudinal slot can be seen to have a circular or oval 
geometry defining a longitudinal stress line 57. 
Similarly, FIGS. 6 and 7 depict a splittable tube 60 having an inner layer 
61 and an outer layer 62. This embodiment includes a simple slot 63 
through the wall of the inner layer. Slot 63 defines a stress line 64. It 
has been found that a catheter inserted into the central lumen of a 
splittable cannula in accordance with the principles of this invention 
causes the splitting to occur in the tube rather quickly if the 
longitudinal slot in the inner wall is of a shape to premit the catheter 
to enter and cause a spreading motion to the sides of the slot. However, 
this shape to the slot is not necesary. The shape of the longitudinal 
slot, the materials used and the thickness of the outer wall are 
considerations which may be traded off against one another. 
The tubes of FIG. 1 can be made from any extrudable resin, but it is 
preferred, particularly for medical usage, that the inner layer be made of 
low-density (2.5 melt index, 0.925 specific gravity) polyethylene such as 
Dow 640. 
The outer layer may be made from FDA-grade 0.933 density polypropylene with 
a melt flow rating between 7 and 9, such as Himont polypropylene T7673. 
Many polypropylenes are made up of long molecular chains which tend to 
line up in the longitudinal directions when extruded. Thus, polypropylene 
has a tendency to split along the direction of extrusion and in an 
uniformly straight line. 
Many other materials may be extruded in combination, including: 
a. Union Carbide high-density polyethylene (0.945 S.G.) 
b. DuPont Surlyn ionomers (#1701) 
c. Dow medical-grade polyurethane (2363 series) 
d. B. F. Goodrich Estane polyurethane (type 5533) 
e. Atochem nylon (e.g., BESNO 11) 
f. USI Chemical ethylvinyl acetate (e.g., NA293) 
g. Unichem polyvinyl chloride (e.g., 9011A-02) 
In medical applications, such as an over-sheath for dilators for cannulas, 
materials are chosen for the inner and outer layers of the splittable tube 
which provides bio-compatibility, drug compatibility, meet FDA 
requirements for cytotoxicity, and have the most desirable physical 
properties for stiffness, surface smoothness and/or flexibility. For 
non-medical applications, a dual-layer insulation for electrical wire is 
provided which is strippable without tools. 
Splittable tubes of the type disclosed herein can be made by conventional 
extrusion equipment. Such equipment produces a melt flow through a die 
having a geometry to define the central bore and longitudinal slot in the 
inner layer of the tube. A second melt flow is provided to encompass the 
so-shaped inner layer in conventional fashion. Tubes made in this fashion 
are cut to size One end of the tube may then be heated by any appropriate 
means, such as by a heat gun, and the end may be stretched to yield a 
tapered end The tip may be cut at a point where the inside diameter 
coresponds to the outside diameter of a dilator to be used with the 
cannula. 
The cannula, when used for the insertion of a pacemaker, for example, is 
usually sterilized using ethylene oxide and packaged with a dilator in a 
hermetically sealed plastic bag.