GB2065430A - A tubular heating device - Google Patents

Abstract

A heating device in tubular form has an inner tube 1 made of a fluorocarbon resin, an intermediate, heat-generating layer 21 positioned around the inner tube, the intermediate layer comprising an electroconductive layer comprising a filled resin, and an outer layer 3 made of a fluorocarbon resin surrounding the intermediate layer, a plurality of conductors 23 being embedded in or positioned in contact with the intermediate layer along the length of the device. When a voltage is applied across the conductors, heat is generated within the tubular device, by passage between the conductors via the electroconductive material of the intermediate layer. The device may be used to heat e.g. blood or as a catheter. <IMAGE>

Description

SPECIFICATION A tubular heating device The present invention relates to a heater in tube form which is applicable in a variety of tubing or piping situations wherein a heat generating ability is required, for example, medical tubes such as catheters and the like.

Tubes through which medical liquids, such as blood, artificial blood, salt solution and dialyzing solution are injected or withdrawn from the body sometimes need to be heat generative. Such medical tubes should be chemically inert, at least on their interior surface, non-toxic, free from exudation, able to generate heat evenly along their length and be heat insulative across the tube walls. Those tubes should also be chemically inert on their outside surfaces when they are inserted into the human body. An example of such an application is a tubular heater which is inserted into the lung to collect breath samples without condensation of moisture on the inside tube wall in order to analyze it by gaschromatography. Such tubes should be flexible, resistant to kinking, and smooth on the surfaces, so as not to damage body tissues when inserted into and extracted from body cavities.These tubes should have a wall which will not allow the leakage of inside gas.

The conventional tubes used as mentioned above are usually made of a suitable plastic in which 2 nichrome wires are placed such that one takes the electric current one way and the other brings the current back in the opposite direction. These nichrome wires must be connected to form a closed circuit when the tube is cut to a proper length for use. However, such connections are rather difficult, and the connected portion can be dangerous due to electrical leakage. Any electrical leakage must be strictly avoided in medical applications.

Since a voltage drop is created between the input end and the top of the tube in heated tubes using nichrome wire, a relatively high voltage must be applied. However, the higher the voltage, the less preferable it becomes for medical purposes. The stiffness of the nichrome wire can impair the overall flexibility of the tube.

According to the present invention there is provided a heating device of tubular form comprising: (a) an inner tube made of a fluorocarbon resin; (b) an intermediate heat-generating layer positioned around said inner tube said intermediate heat-generating layer comprising a resin layer containing electroconductive mate rial; (c) an outer layyer made of a fluorocarbon resin surrounding said intermediate layer, and (d) a plurality of conductors embedded in or positioned in contact with said intermediate layer along the length of said device and separated from each other, and electrically interconnected by said electroconductive material, whereby, when a voltage is applied across said conductors, heat is generated within said device.

The invention will now be particularly described by way of example with reference to the accompanying drawings in which: Figure 1 is a partially cut away view of one embodiment of tube heater according to the present invention; Figure 2 is an enlarged perspective view with partially disassembled elements of heat generating tape used in the tube heater of Fig. 1; Figure 3 shows a partially cut away view of another embodiment of the present invention, and Figure 4 shows an example of the insulative sealing of a cut end of the tube heater of this invention.

As shown in the drawing, a heater in tube form comprises an inside tube layer 1 made of a fluorocarbon resin, a heat generating layer wrapped around this inside tube layer, the heat generating layer being comprised of a filled, heat generating resin layer, and a plurality of conductors 23 and 23' which are embedded in or arranged in adjacent contact with the heat generating resin layer along the length of the tube and which are in parallel or helical relationship along the length of the tube and are separated from each other, and an outside layer 3 made of fluorocarbon resin surrounding the heat generating layer.

The fluorocarbon resins used as the material constituting the inside and outside layers 1 and 3 include polytetrafluorethylene (PTFE), a copolymer of tetrafluoroethylene and hexafluoropropylene (FEP), a copolymer of tetrafluoroethylene and perfluoroalkoxyalkylvinylether (PFA), a copolymer of ethylene and tetrafluoroethylene (ETFE) and similar polymers. These fluorocarbon resins are generally more chemically stable than other plastics. They are more difficult to dissolve, decompose, and change naturally or by other chemicals, and they have little or no toxicity or adverse effects, and excel in such areas as their anti-thrombic property, water repellency, surface smoothness, heat resistance, insulation resistance, and so on. Of the suitable fluorocarbon resins, PTFE is the best in all the properties mentioned above.

A sintered, solid PTFE tube obtained by sintering a conventional paste-extruded PTFE tube can be used as the inside tube 1. Also, a sintered, expanded PTFE tube can be used having a microstructure of nodes interconnected by fibrils and obtained by means of the expansion process disclosed in Japanese Patent Publication JPP Sho 51-18991. A melt-extruded fluorocarbon resin rube other than PTFE can be used and a tube produced by wrapping fluorocarbon tapes around a metal mandrel, heating to fuse the tapes and then pulling out the mandrel can also be employed.

As a means of forming the heat-generating layer 2 over the inside tube 1, Fig. 1 shows an example in which the heat generator is in tape form 21 and is applied helically around the outside surface of the inner tube 1. The tape form heat generator 21 is comprised of an electrically heat generating filled layer 22 made of a thin narrow plastic, two pieces of thin, narrow metal tape conductors 23 and 23' being iengthwisely arranged on both sides of layer 22, and two sheets of fluorocarbon resin jacket tape 24 and 24' being used to sandwich the layer 22 and conductors together. These elements are bonded together by heat fusion or glue adhesion.The electrically heat generating layer 22 is made up of a mixture of a resin such as polyvinylchloride, polyolefin, fluorocarbon resins and the like, with electroconductive powders, such as graphite powder, carbon black powder, carbon fiber, metal powder, and the like, in the mixing proportion yielding a desired electroconductivity. One typical example is a carbon filled PTFE shaped article, which can be produced by blending electroconductive carbon black in an amount of 10 wt% with PTFE fine powder by means of the coagulation method, mixing the carbon-blended PTFE with a liquid lubricant (e.g., naphtha) in an amount of ca.

40 wt%, extruding the lubricated PTFE mixture through a ram extruder, rolling down the extruded sheet to a thickness of about 4 mils, and removing the lubricant from the sheet by evaporation. When electrical voltage is applied to conductors 23 and 23', the electric current flows between the conductors across the heat generating layer 22 generating heat. The conductors 23 and 23' may be either in contact with or embedded in the heat generating layer 22. For more effective heat generation, the conductors 23 and 23' may be more than two in number and arranged as stated in prior Japanese Utility Model Application JUMA Sho 52-9524, in which the conductors are held in a spaced, parallel relationship and electrical voltage is applied selectively or alternately in the fashion of reversed voltage.The heating element in tape form may be produced by placing an electrically heat generating film 22 and spaced, parallel conductors 23 and 23' between PTFE jacketing tapes 24 and 24' and bonding these elements into one body. The electrically conducting, heat generating film 22 may either run the entire length or over a portion solely or intermittently of the heater tube in this invention. In the case where film 22 is placed intermittently along the length, there are formed portions where heat is generated and portions where heat is not generated.

Fig. 3 shows another example of forming a heat generating layer 2 in which a resin paste or solution with the inclusion of electro-conductive powder is coated and dried to form the electrically heat generating resin layer 2.

Onto the surface of layer 2, two thin, narrow tape conductors 23 and 23' are placed longitudinally in a parallel, spaced relationship.

Conductors 23 and 23', spaced apart from each other, may be wrapped spirally around the tube formed by layer 2. The order of fabricating conductors 23 and 23' and the paste may be reversed; i.e. laying or wrapping conductors 23 and 23' first, then applying the electroconductive paste or pastes and then drying, thus covering the conductors. Rather than coating and drying the resin paste (containing conductive powder) on the outside of inner tube layer 1 to form electrically heat generating layer 2, a preformed electroconductive resin tape may be applied by helical or roll wrapping around inner tube 1. Conductors 23 and 23' may be placed in desired portions with intervals in between. In such a case, electrical voltage is applied to the conductors 23 and 23' of each portion.

The outside layer of fluorocarbon resin is formed over heat generating layer 2 by means of either: (1) paste-extrustion of a PTFE tube jacket followed by sintering; (2) spiral or parallel wrapping of an unsintered or partially sintered, micro-porous PTFE followed by sintering; or (3) melt-extrusion of fluorocarbon resins other than PTFE over the heat generating layer 2.

Fig. 4 shows an insulation tape or insulation resin 4 attached to the cut end section of a cut tube.

According to the heater tube of the present invention as produced by the methods mentioned above, the following advantages can be achieved: (1) Since the material of the inside and outside tubes 1 and 3 is fluorocarbon resin, both the inside and outside surfaces of the tube are chemically inert, and hence non-toxic and free of exudate.

(2) Since the heat generating layer comprises electrically heat generating layer 22 (or 2) and parallel, spaced conductors 23 and 23' which are placed in intimate contact with or fully embedded in layer 22 (or 2), conductors 23 and 23' need not be connected at the end if the tube is cut to a length, as is done in the conventional heating tubes using nichrome wires. The section should only be sealed, for example, by applying an insulating tape 4 (e.g., unsintered PTFE tape) or coating with an insulating resin solution to provide an insulation treatment 4 as shown in Fig. 4. The inside and outside layers provide an excellent electrical insulation due to the fluorocarbon material and are electrically safe because of no leakage.Unlike the conventional tube heaters using nichrome wires, the electrical voltage applied to the heat generating layer 22 (or 2) is held constant (i.e. no voltage gradient) throughout the entire length of the tube; thus it can be kept low and, therefore, a uniform heat generation is accomplished throughout the tube.

(3) The heating tube of the present invention is more flexible than the conventional tubes utilizing nichrome wires. By using microporous materials in inside and outside layers 1 and/or 3, the flexibility of the tube can be increased even further, meanwhile retaining its anti-kinking property and heat insulative property. When the micro-porous material is a fluorocarbon having a fibrillated microstructure (1-1,000 microns fibril length), it is difficult for liquid such as water to permeate through the wall due to the excellent water repellency of the fluorocarbon. However, this micro-porous material allows gases to permeate. Thus, if gas permeation is not desired, the pores should be closed, for example, by coating them with a fluoro-elastomer solution on the outside surface of inner tube 1 and/or the outside surface of outside layer 3.

(4) The inside and outside surfaces of the tube are very smooth and slippery due to the nature of fluorocarbon resins. If the inside and outside layers are porous, the inside and outside surfaces are soft, and cause no wrinkles at the inside of the bent portion when the tube is bent. Thus, the liquid put into and out of the interior of the tube flows quite well, and the tube, with its soft texture coupled with its flexibility as stated in item (3) above, can be inserted into body cavities without causing damage to the human tissues.

(5) Due to the excellent heat and chemical resistance of fluorocarbon resins, the tube can be sterilized at a high temperature and with strong chemicals, thus allowing versatile choices of sterilization and cleaning methods.

(6) At least the inner layer 1 can be made of micro-porous fluorocarbon material having fibril length of 1-1,000 microns. Thus, the tube heater of the present invention itself can be a vascular prosthetic having anti-thrombic as well as heat-maintaining or generating properties.

The tube heater of the present invention having various advantages mentioned above can be used in a variety of applications, particularly as catheters, vascular grafts, liquid pipings and the like.

The heater in tube form of the present invention is further explained with reference to the following examples.

Example 1.

A melt-extruded PFA tube, 2.0 mm l.D. and 2.6 mm O.D., was used as the inside tube.

Over the surface of the tube 1 was spirally wrapped a fluorinated resin heat generating element in tape form 21 of the construction of Fig. 2 (width 5 mm, thickness 0.3 mm, spacing between conductors 1.9 mm, and electrical resistance 30 ohm/m) with a center to center pitch wrapping of 10 mm. Over the resultant tube was then helically wrapped an unsintered, stretched PTFE tape, 0. 1 mm thick, providing 3 overlapping layers. The tube, covered with the unsintered PTFE tape, was put in a molten salt oven for two seconds to sinter the tape, and then cooled by immersion in water to produce a tube heater of the present invention.

The tube thus obtained, 4.0 mm O.D., was flexible, with a soft outer surface, and was smooth, thus being suitable as a catheter tube to be inserted into the body organs such as esophagus, lung, etc.

The tube was cut to 2 m in length. When an electrical voltage of 7 V was applied to the exposed conductors 23 and 23', the interior of the tube became heated to 37"C (at 26"C room temperature), and when 10 V was applied, it became heated to 42"C. From one end of the tube at 37"C and 42"C, air was introduced into the interior at a flow rate of 0.1 m/min. Out of the other end was obtained heated air at 35"C and 38"C, respectively.

Example 2.

An expanded PTFE tube (2.0 mm l.D., 2.6 mm O.D., 1.5X stretch ratio), having an outside coating of a fluorocarbon elastomer resin solution to a thickness of 0.1 mm, was used as the inner tube 1. With other conditions being the same as conducted in Example 1, heat generating layer 2 and outside layer 3 were formed to produce the heater tube of this invention. In order to prevent the collapse of inner tube 1 and to facilitate the forming of layers 2 and 3, a copper wire, 1.9 mm O.D., was inserted inside the inner tube 1 during processing and extracted after processing.

The tube heater thus obtained had an antithrombic inner surface and a soft, smooth outer surface, and thus was well suited for a blood vessel equipped with heating means and also as a catheter for body cavities.

Application of 1 5 V between the ends of conductors 23 and 23' of a 2 m long heater tube sample increased the temperature inside the tube to 55"C (room temperature 26"C).

Water flow (21"C, Flow rate 0.1 m/min) was heated to 36"C through the tube.

Tube temperature can be controlled optionally by setting a temperature sensor such as a thermocouple at the desired position of the tube and connecting it to temperature control equipment.

Claims (9)

1. A heating device of tubular form comprising: (a) an inner tube made of a fluorocarbon resin; (b) an intermediate heat-generating layer positioned around said inner tube; said inter mediate heat-generating layer comprising a resin layer containing electroconductive material; (c) an outer layer made of a fluorocarbon resin surrounding said intermediate layer, and (d) a plurality of conductors embedded in or positioned in contact with said intermediate layer along the length of said device and separated from each other, and electrically interconnected by said electroconductive material whereby, when a voltage is applied across said conductors, heat is generated within said device.

2. A device according to claim 1 wherein said conductors are in substantially parallel relationship along the length of said device.

3. A device according to claim 1 wherein said conductors are in helical configuration along the length of said device.

4. A device according to any preceding claim wherein said inner tube is of porous expanded polytetrafluoroethylene.

5. A device according to any preceding claim wherein said outer layer is of porous, expanded polytetrafluoroethylene.

6. A device according to claim 4 wherein a sealing layer is used to close the pores of said tube.

7. A device according to claim 5 having a sealing layer closing the pores of said outer layer.

8. A device according to claim 1 wherein said intermediate heat-generating layer is a composite structure having an inner, heatgenerating electroconductive layer comprising a filled resin having a plurality of conductors embedded in or positioned in contact with said electro-conductive layer in spaced, substantially parallel relationship along the length of said layer, said electroconductive layer and conductors being positioned between two fluorocarbon protective layers, thereby forming said composite structure, said structure being helically wrapped around said inner tube to form said intermediate layer in said heating device.

9. A heating device substantially as herein described with reference to the accompanying drawings.