DOUBLE WALLED CATHETER AND METHOD OF MANUFACTURE

Abstract: A catheter is disclosed which has resistance to collapsing when bent. The catheter comprises a first, thermo-flowed tube having a helical coil at least partially enveloped in the material of the tube. A second tube surrounds the fist tube and is formed from heat shrunk material.

Description: DOUBLE WALLED CATHETER AND METHOD OF MANUFACTURE Technical Field This invention relates to catheters especially, but not exclusively, for intravascular sensors such as those which measure p0 2 , pC02, pH and temperature. Background of the Invention Catheters are often used to introduce sensing elements into the body, particularly into a blood vessel. Usually a hollow needle is used for making an initial entry into the blood vessel. A wire guide is then passed into the needle and the needle is then withdrawn. A catheter can then be slid over the wire guide and introduced into the blood vessel. After removing the wire guide, a sensor, e.g. including one or more optical fibres are then passed into the catheter. Often such catheters have a small diameter and thin walls and kink easily. Should the catheter kink, the sensor may give a false reading or be damaged. Replacement of a damaged sensor is relatively expensive and while a change is made to a new sensor, monitoring of a patient's condition is interrupted. Various attempts have been made to render catheters or introducer sheaths resistant to kinking. One procedure is described in European patent application No. 0617977 which describes an introducer in which a helical coil is sandwiched between an inner tube of PTFE and an outer tube of heat formable polyamide resin. The outer tube is heat formed and compressed so that material passes between turns in the coil and forms a connection with the roughened outer surface of the inner tube. It is difficult to control the manufacture to cause material from the outer tube to flow between the turns of the coil without distorting the inner tube. Also the reinforced introducers described in the above reference are relatively thick because they consist of two tubes with a coil sandwiched between them. Summary of the Invention One object of the invention is to provide a more reliable and simpler method of producing a kink-resistant catheter. Another object is to produce a kink-resistant catheter consisting of a tube having a single wall in which a reinforcing coil is embedded and a method of its production. A related object is to provide a reinforced catheter suitable for introducing or guiding a sensor into human tissue, such as a blood vessel, wl ile resisting kinking and protecting the sensor from damage should the catheter be disturbed. Such sensors are typically constructed with sensing elements each of about 0.002 inches diameter, in the case of wire and 0.007 inches in diameter in the case of optical fibres. When sensing elements for all four parameters are present, the sensor has an overall diameter of approximately 0.02 inch (0.5mm). This small size is necessary to allow the sensor to be placed in an artery through a cannula and not compromise the primary function of the cannula of monitoring blood pressure and allowing blood samples to be taken. The small size of the sensors means that they are fragile. Bending of optical fibre sensors results in a change in the signals. Progress has been made in reducing the impact of this by providing a second, reference wavelength but until the advent of this invention, sensor kinking remained a problem. According to an aspect of the invention there is provided a method of making a kink-resistant catheter, said method comprising the steps of: (i) providing a helical coil, (ii) receiving the helical coil in a first tube formed from material which is flowable at a first temperature, (iii) receiving the first tube in a second tube which is heat shrinkable at a second temperature equal to or greater than said first temperature, to form an assembly, and (iv) heating the assembly to a temperature equal to or gTeater than the second temperature. In some embodiments of the invention the first tube comprises a fluorinated ethylene propylene copolymer (FEP) and the second tube comprises polytetrafluoroethylene (PTFE). In other embodiments of the invention the first tube comprises polyethylene or poiyurethane and the second tube comprises polyester, PTFE or FEP. FEP can be the second, heat-shrinkable tube in cases where the first tube is flowable at temperatures at which FEP does not flow but is heat-shrinkable. The most preferred combination is to employ a first tube which is poiyurethane and a second tube which is FEP. Preferably the first tube is heat flowable at a first temperature in the range 100 to 300°C (212 to 572°F) and the second tube is heat shrinkable at a second temperature equal to or greater than the first temperature and preferably in the range 150 to 350°C (302 to 662°F). In the manufacture of the reinforced catheter, the helical coil can be mounted on a mandrel and then introduced into the first tube, which is received within the second tube. A release agent may be provided on the mandrel to facilitate removal of the mandrel after formation of the reinforced catheter. Suitable release agents include liquid release agents such as silicone oils. However, it is possible to use a solid release coating such as a PTFE tube fitted onto the mandrel. In some embodiments of the invention the tubes extend beyond one or both ends of the helical coil. The invention also encompasses a method of making a kink-resistant catheter which method comprises forming an assembly by the steps of:- (i) supporting a helical coil on a mandrel which extends through the coil: (ii) introducing the helical coil having a plurality of turns while supported on the mandrel into a first tube, said first tube being formed from a material which is flowable at a first elevated temperature and said first tube being received within a second tube, said second tube being heat shrinkable at a second temperature which is equal to or greater than said first temperature; and (iii) heating the assembly to a temperature equal to or greater than said second temperature, whereby shrinkage of the second tube causes material of said first tube to flow between turns of the coil and substantially encapsulate the coil. According to the invention there is further provided a cannula comprising a helical coil at least partially enveloped in a matrix, said matrix comprising a first tubular member of heat flowed material which is received in a second tubular member of heat shrunk material. Preferably the helical coil is metallic, for example, stainless steel and preferably the coils have a rounded cross-section, e.g. a circular cross-section. The first tubular member may comprise FEP, polyethylene or poiyurethane. The second tubular member may comprise PTFE, polyester or FEP. Preferably the catheter has a tapered tip, the helical coil terminating inwardly of the tip. Provision of a tapered tip greatly improves the case of introducing the catheter into a blood vessel. Advantageously, a reinforced catheter of very small wall thickness can be made by removing the second tube after the first tube has been caused to flow through turns in the helical coil. This can be readily achieved, for example, by providing a second tube which extends beyond an end of the first tube, forming a slit in the projecting end of the second tube after the heat shrinkage step and peeling the second tube away from the first. By selecting materials for the first and second tubes as described above, the first tube does not bond to the second and can easily be parted. According to a further aspect of the invention, therefore, there is provided a catheter which comprises a helical coil having a plurahty of longitudinally spaced apart turns, said coil being at least partially enveloped in a matrix comprising a tubular member of heat-flowed material which extends between turns of the coil, said tubular member having an exposed outer surface and an inner surface covering said spaced apart turns. The reinforced catheters of the invention have the same or similar inside and outside diameters as conventional un-reinforced catheters but are extremely flexible and resistant to flattening when bent around a tight curve. Brief Description of the Drawings Embodiments of the invention will be described by way of a non-limiting example by reference to the accompanying figures of which: Figure 1 is a scrap longitudinal section of a catheter of the invention; Figure 2 is a scrap section of a spring and a mandrel which carries a release layer; Figure 3 is a scrap section of a spring and release layer carried on the mandrel, the mandrel being received within a first tube; Figure 4 is a scrap section of a mandrel carrying and received within concentric first and second tubes; and Figure 5 is a scrap section of a second embodiment of a catheter of the invention. A catheter 1 of the invention is shown in section in Figure 1. It is generally cylindrical and flexible and has an axial bore 2. Axial bore 2 is defined in the illustrated embodiment by first tubular member 3. Tubular member 3 is of composite structure. It comprises a matrix 4 of heat-flowed plastics material such as fluorinated ethylene propylene (FEP), polyethylene or poiyurethane. A helical coil 5 is present within matrix 4 and is substantially completely encapsulated in the matrix. Helical coil 5 provides hoop strength to the catheter and hence kink resistance. Helical coil 5 is preferably metallic such as stainless steel for example a grade available as 304 VSS - D.D (diamond drawn). Preferably the helix wire is of circular cross section although other cross sections can be used, such as rectangular, as shown in Figures 2 to 4. The turns of the helical coil 5 are conveniently spaced apart. For example in one embodiment of the invention a cylindrical helix wire about 0.05mm (0.002") diameter can have a pitch of about 0.01mm (0.004"). The first tubular member is received in a second tubular member 10. The second tubular member 10 may be bonded to the first tubular member but this is not essential and, as mentioned below, it may desirably not be so bonded. Second tubular member 10 comprises heat shrunk material such as heat shrink PTFE (polytetrafluoroethylene), polyester or FEP. It is preferred that the tip 20 of the catheter tapers inwardly. Helical coil 5 may not extend to the tip 20 and preferably does not do so. A reason for this will be explained hereinafter. A hub 30 such as a luer hub can be fitted to the end of the catheter opposite to the tip 20. A needle (not shown) can be fitted to the tip of the catheter. The flexibility of the catheter can be varied by, for example, varying the stiffness of the helical coil. Those skilled will have little difficulty in varying the flexibility of the helical coil, for example, by varying the material of which it is made or the thickness of the wire. The flexibility of the catheter can be varied in other ways such as varying the thickness or the nature of the heat flowed plastics material. A convenient way of the manufacturing the catheter will now be described by reference to Figures 2, 3, and 4. Mandrel 100 is conveniently a rigid metallic material, e.g. a stainless steel wire. Preferably a release layer 101 such as silicone oil or a PTFE coating is present on the mandrel 100. Silicone oil is satisfactory since it is available in medical grades and thus it may not matter if slight traces remain after processing. A PTFE coating is preferred because of its chemical inertness. A PTFE coating may be deposited onto the surface of the mandrel. Alternatively, a PTFE tube may be snugly fitted onto the mandrel. A helical coil 5 as described previously is slid onto the mandrel 100 over the release layer. The internal diameter of the helical coil 5 is preferably slightly larger than the external diameter of the mandrel 100. In an example the mandrel was about 0.05mm (0.002") less in diameter than the internal diameter of the coil 5. A tube of plastics material 110 flowable at a first, elevated, temperature is slid over the helical coil. In many cases the heat flowable material 110 is the same as the heat flowed plastics of matrix 4. It is not essential however since material 110 could be a material which undergoes a chemical or physical transformation on heating so that it is not heat flowable on a further heating. Suitable materials include an FEP, polyethylene and poiyurethane. A second tube 120 of heat shrinkable plastics material is then slid over the first tube to make an assembly. PTFE, polyester or FEP are preferred materials. The second tube is heat shrinkable at a temperature greater than a temperature at which the first tube is flowable, and does not flow at the temperature at which the first tube begins to flow. It will be appreciated that the coil is a relatively snug fit on the mandrel and similarly, the first tube fits relatively closely to the helical coil, while the second tube also fits closely to the first tube. For better clarity, the distances between the various components are exaggerated in Figures 2 to 4. In a preferred embodiment of the invention the helical coil does not extend to the tip of the tubes. Where the coil does not extend to the tip of the tubes a tapered portion is formed. This taper, which may be trimmed during a finishing step, facilitates introduction of the catheter into the body. The assembly of mandrel, coil, first and second tubes is then heated, e.g. in an oven or by means of a radiant heating element or elements directing heat onto the outer surface of the second tube 120. In one embodiment, the radiant heater is annular and the assembly in advanced at an appropriate speed through the annulus of the heater. The duration of heating is sufficient to ensure that the entire assembly is heated to the same temperature, such temperature being high enough to cause the material of the first tube to flow and the material of the second to shrink. The mandrel may be removed, preferably after the heating, and the release layer or coating remains on the surface of the mandrel. As the assembly is heated, the first tube softens and becomes heat flowable. The second tube then shrinks forcing the heat flowable material into the gaps between the turns of the helical coil 5 at least partially enveloping it. Preferably the heat flowable material substantially completely envelopes the helical coil. The flowable material flows between the mandrel and the coil. A circular cross-section helix wire is preferred since there is a smaller area of wire adjacent to the mandrel than if say a square cross section wire of the same cross- sectional area were used. In positions along the length of the assembly where the coil is not present, greater shrinkage takes place and this allows formation of the preferred tapered tip 20. A hollow needle may be fitted to the tip. This step may be a part of the process of manufacturing the catheter. For example, the needle may be fitted temporarily in place of the mandrel and the end of the tube further heat shrunk onto an end of the needle. The precise temperature to which the assembly should be heated depends on the nature of the polymers. The first tube is generally flowable at a temperature in the range of 100 to 300°C. The second tube is generally selected to be heat shrinkable at a temperature about 50 to 100°C higher than the temperature at which the plastics material of the first tube becomes flowable. Typically therefore the second tube heat shrinks at a temperature in the range 150 to 350°C. Temperatures higher than 350°C are generally not preferred. The mandrel 100 if present can be removed and the catheter processed further for example in conventional manner by attaching, e.g. by welding, adhesively bonding or insert moulding the catheter to the hub 30. After the heat shrink step is completed, the second tube may be removed, leaving the coil substantially embedded in the wall of the first tube. This results in a reinforced catheter having a smaller overall diameter. The second tube is conveniently removed, e.g. by slitting and peeling it away from the first tube, while the first tube is still supported on the mandrel. The mandrel may then be removed. A reinforced catheter as produced by this method is shown in Figure 5. Catheters manufactured in accordance with this invention are particularly useful for introducing sensors, e.g. optical sensors, into a patient's blood vessel for monitoring parameters such as blood oxygen, CO2 and pH. The reinforced catheter protects the delicate sensor while retaining the necessary flexibility for passing the catheter into a blood vessel. Examples of typical optical sensors are those manufactured by Diametrics Medical Limited of Short Street. High Wycombe, England under the trade mark 'Paratrend', and described in US Patents Nos. 4889407 and 5262037, the disclosure of which is specifically incorporated by reference. As explained above, in some embodiments of the invention the outer layer is stripped away once the heat flowing has occurred. To aid this it may be desirable to provide a release agent between the two tubes or to select as the material of one of the tubes a polymer which has good release properties. For example, the selection of PTFE or FEP as the material of the second tube provides such release properties. Stripping away of the outer layer provides a more flexible and thinner catheter which may therefore be preferred. A preferred technique is to provide a stainless steel mandrel covered with a thin PTFE tube and to support the helical spring on such a mandreL, while forming the reinforced catheter comprising a first, heat-flowable tube, and a second heat-shrinkable tube. After the heat-shrinking step, the mandrel is removed and replaced with an uncoated stainless steel wire. Using this wire as a support, the heat-shrinkable second tube is slit and peeled away to leave the first tube encapsulating the helical spring. In some embodiments of the invention a further tube is provided between the helical coil and the mandrel and is chosen to bond with the further tube. This helps to ensure that the coil is encapsulated by plastics materials. Poiyurethane, polyethylene or PTFE may be used for the further tube. It may be preferable for the further tube and the first tube to be of the same or similar polymer composition. In the case of preferred materials, PTFE and FEP tubing available from Zeus, Orangebury, South Carolina, USA, the PTFE shrinks at a temperature in the range 335-343°C (635-650°F) and the FEP flows at 260-277°C (500-530°F). Occasionally, the first tube may be found to adhere to parts of the mandrel despite the presence of a release oil. This may be due to surface scratches on the mandrel. The formed catheter can be removed from the mandrel by clamping the mandrel at one end (e.g. in jaws of a vice) and gripping the catheter at the other, while twisting it so that the coil tends to open up. This action effectively breaks any bond between the inner surface of the catheter and the mandrel. As explained above, the reinforced catheters of this invention may be introduced into a blood vessel using the hollow needle and wire technique, known as the Seldinger technique. In order to reduce the trauma arising from the direct introduction of the reinforced catheter of the invention into a blood vessel, a system transition from a smaller to a larger system may be adopted, such as the system described in US Patent No. 4,650,472, the disclosure of which is specifically incorporated herein by reference.

Claims: CLAIMS:- 1. A method of making a kink-resistant catheter, said method comprising the steps of: (i) providing a helical coil, (ii) receiving the helical coil in a first tube formed from material which is flowable at a first temperature, (iii) receiving the first tube in a second tube which is heat shrinkable at a second temperature equal to or greater than said first temperature, to form an assembly, and (iv) heating the assembly to a temperature equal to or greater than the second temperature. 2. A method as claimed in claim 1 wherein the first tube comprises a fluorinated ethylene propylene polymer (FEP) and the second tube comprises polytetrafluoroethylene (PTFE) 3. A method as claimed in claim 1 wherein the first tube comprises polyethylene and the second tube comprises polyester. 4. A method as claimed in claim 1 wherein the first tube is poiyurethane and the second tube is FEP. 5. A method as claimed in any one of the preceding claims wherein the first tube is heat flowable at a first temperature in the range 100 to 300°C and the second tube is heat shrinkable at a second temperature greater than the first temperature and in the range 150 to 350°C. 6. A method as claimed in claim 5 wherein the helical coil is supported on a mandrel and introduced into the first tube. 7. A method as claimed in claim 6 wherein a release layer is provided on the mandrel. 8. A method as claimed in claim 7 wherein the release layer is a PTFE tube and the mandrel is a metal wire. 9. A method as claimed in any one of the preceding claims, wherein the tubes extend beyond one or both ends of the helical coil. 10. A method of making a kink-resistant catheter which method comprises forming an assembly by the steps of:- (i) supporting a helical coil having a plurality of turns on a mandrel which extends through the coil; (ii) introducing the helical coil while supported on the mandrel into a first tube, said first tube being formed from a material which is flowable at a first elevated temperature and said first tube being received within a second tube, said second tube being heat shrinkable at a second temperature which is equal to or greater than said first temperature; and (iii) heating the resulting assembly to a temperature equal to or greater than said second temperature, whereby shrinkage of the second tube causes material of said first tube to flow between turns of the coil and substantially encapsulate the coil. 11. A method as claimed in claim 10, which includes the further step of removing the mandrel. 12. A method as claimed in claim 11, which includes the further step of removing the second tube. 13. A catheter obtained by a method as claimed in any one of claims 1 to 12. 14. A cannula comprising a helical coil at least partially enveloped in a matrix, said matrix comprising a first tubular member of heat flowed material which is received in a second tubular member of heat shrunk material. 15. A cannula as claimed in claim 14 wherein the helical coil is metallic. 16. A cannula as claimed in claim 15 wherein the helical coil is stainless steel. 17. A cannula as claimed in any one of claims 14 to 16 wherein the helical coil is formed from wire having a generally circular cross-section. 18. A cannula as claimed in any one of claims 14 to 17 wherein the first tubular member comprises FEP, polyethylene or poiyurethane. 19. A cannula as claimed in any one of claims 14 to 18 wherein the second tubular member comprises PTFE, polyester or FEP. 20. A cannula as claimed in any one of claims 14 to 19 having a tapered tip, the helical coil terminating inwardly of the tip. 21. A catheter which comprises a helical coil having a plurality of longitudinally spaced apart turns, said coil being at least partially enveloped in a matrix comprising a tubular member of heat-flowed material which extends between turns of the coil, said tubular member having an exposed outer surface and an inner surface covering said spaced apart turns. 22. A catheter as claimed in claim 21 having a tapered tip and wherein the coil terminates inwardly of said tip. 23. A catheter as claimed in claim 21 or 22 wherein the helical coil is formed from wire having a generally circular cross-section. 24. A catheter as claimed in any one of claims 21 to 23 in combination with at least one fibre optic sensor received within the tubular member.

Inventors: MARKLE DAVID REED, MARKLE, DAVID, REED
Applicants: DIAMETRICS MEDICAL LIMITED
Application year: 1999
Main classification: A61M25
IPC codes: A61M25, F16L9, F16L11