THERMISTOR ASSEMBLIES AND METHODS FOR MAKING SAME

Abstract: New thermistor assemblies and methods for making such thermistor assemblies are disclosed. In one embodiment, the present thermistor assembly (10) comprises a thermistor (14) including a thermistor element (20), a protective coating (22) substantially surrounding the thermistor element (20) and electrically conductive lead wires (24, 26) in electrical communication with the thermistor element (20) and extending beyond the protective coating (22); an elongated member (16) including electrically conductive extension wires (28, 30) each of which is bonded to a different one of the lead wires (24, 26) in a bonding zone (36), and an electrically insulating covering (32) located on said extension wires (28, 30) proximal of the bonding zone (36); and an electrically insulating thermally conductive overcoating (18) located over said protective coating (22), lead wires (24, 26) and the bonding zone (36), and being derived from a mixture comprising at least one polymerizable component and at least one thixotropic component.

Description: THERMISTOR ASSEMBLIES AND METHODS FOR MAKING SAME Background of the Invention The present invention relates to sensor assemblies, for example, to thermistor assemblies, and to methods for making such sensor assemblies. More particularly, the invention relates to sensor assemblies which are electrically insulated with overcoatings which allow for rapid sensor response to changes in the value of the parameter being sensed, and which are preferably biocompatible so that the sensor assemblies can be used in vivo for the treatment/diagnosis of medical patients. Sensing elements which provide an electrical signal in response to the value of the parameter being sensed often are provided in a disassembled state. For example, thermistors are often sold as basic units which include the usual thermistor element, for example, a conventional thermistor chip, wafer, bead or the like; a protective coating, often made of glass, hermetically sealed over the thermistor element; and relatively short length, on the order of about 1 inch, electrically conductive lead wires in electrical communication with the thermistor element and which extend beyond the protective coating. In order to effectively use such thermistor units, it is often necessary to bond these lead wires to electrically conductive extension wires in an elongated cord, which extension wires are ultimately in electrical communication with a remotely located temperature monitoring unit, which may be of conventional design. In assembling the thermistor/elongated cord combination, it is often important that the connection between the lead wires and the extension wires be electrically insulated, for example, for reasons of safety and to protect the integrity of the electrical signals being transmitted by these wires. In addition, it is often important that the thermistor assembly be such that the included thermistor have a fast or rapid response time, that is, that the included thermistor be rapidly responsive to changes in the temperature being sensed. Further factors become important when the thermistor assembly is to be used in vivo, for example, in the presence of bodily fluids, such as blood. In such applications, the thermistor assembly should preferably be biocompatible, that is, should preferably be such as to cause no substantial harm to the individual in whose body it is placed and to remain substantially intact (maintain substantial structural integrity) while in such individual's body. The thermistor assembly production operation itself should be such as to provide reliable products, which meet relatively tight or high quality control requirements. Thermistor assemblies have been produced by bonding the thermistor's lead wires to the extension wires of the elongated cord and then applying a precursor of a polyvinyl formal or phenol-formaldehyde-type thermosetting resin to the thermistor and bonded wires. The applied precursor is then cured at elevated temperatures, for example, on the order of about 350°C to 450°C, to produce a polyvinyl formal or phenol-formaldehyde-type resin coating. While this coating provides adequate electrical insulation and thermal conductivity, some problems do exist. For example, the thickness of the overcoating may vary substantially from one assembly to another, making mass production of such assemblies very difficult and/or causing the production of a substantial amount of off-spec product. The use of highly elevated curing temperatures may be detrimental to the thermistor element and result in the production of off- spec product. With regard to in vivo applications, the polyvinyl formal or phenol-formaldehyde-type resins are very stable in bodily fluids, but can be toxic. Thus, such overcoated thermistor assemblies are often further overcoated with relatively thick biocompatible material for use in in vivo applications. Such relatively thick further overcoatings increase the response time of the thermistor assemblies. New sensor assemblies, and in particular thermistor assemblies, and methods for making such assemblies would clearly be advantageous. Hudock U.S. Patent 4,623,559 discloses a thermistor which includes a protective coating of an ultraviolet radiation curable composition containing polyester and monoacrylate covering the thermistor wafer. An anti-sag or thixotropic agent may be included in the coating. Portions of the thermistor leads proximal of the wafer are not covered by this coating. Also, no other protective coating, e.g., glass bead, surrounds the wafer. The polyester-containing coating may tend to be hydrolitically unstable and/or water absorbing, making such materials of questionable value in jLn vivo applications and in other applications in which the sensor assembly is exposed to aqueous media. Dankert U.S. Patent 3,839,783 discloses bonding the distal ends of lead wires to a thermistor wafer by coating the combination with an electrically insulating, thermally conductive material, such as an aluminum oxide-loaded epoxy material. Portions of the thermistor leads proximal of the wafer are not coated, and no other protective coating surrounds the wafer. Webler U.S. Patent 4,796,640 discloses a thermistor for use in measuring the temperature of a fluid flowing within an individual's body. The bead of a bead-type thermistor is disclosed as being covered with a thin layer of electrically insulating material, such as one or more thin coatings of vinyl and/or urethane to provide saline production. Portions of the thermistor leads proximal of the bead are not disclosed as being covered with this material. Also, no thixotropic agent is disclosed as a component of this coating. Lucey U.S. Patent 4,282,269 discloses coating electronic components, e.g., capacitors, with ultraviolet radiation curable protective coating compositions including a borate. The borate is disclosed as acting to decrease the cure time while increasing the thickness of the films that are cured. The preferred materials are ultraviolet light-curable acrylate compositions that are substituted with either epoxy or urethane groups and which contain a borate. A thixotrope can be added, if needed. This patent does not disclose coating thermistors or thermistor leads, and is not concerned with the use of sensor assemblies, such as thermistor assemblies, in in vivo applications. Summary of the Invention New sensor assemblies, for example, thermistor assemblies, and methods for making the same have been discovered. The present assemblies are very effectively provided with electrically insulating overcoatings of controlled thickness which can be mass produced. In addition, particularly with regard to temperature sensor assemblies, such as thermistor assemblies, the present assemblies are overcoated with materials which are effectively thermally conductive, thus permitting such assemblies to be rapidly or quickly responsive to changes in the temperature being sensed. The overcoatings on the present assemblies are preferably biocompatible, making such assemblies useful for in vivo applications. Further, such overcoatings are preferably substantially non-water absorbing and have a high degree of hydrolytic stability. The present sensor assemblies provide accurate, reliable and reproducible measurements, and preferably maintain structural integrity even after being exposed to aqueous media, for example, bodily fluids, for prolonged periods of time. The present sensor assemblies can be very effectively and efficiently produced, for example, on a mass scale, using the methods of the present invention. Such methods reduce the number of steps needed to produce sensor assemblies, as well as produce reliable and reproducible product at a relatively high rate. Further, the present methods produce assembled sensors which meet high quality control standards, for example, in terms of overcoating thickness and sensor responsiveness (speed of response) , without the production of excessive amounts of out of specification or off-spec product. In one broad aspect of the present invention, sensor assemblies are provided which comprise a sensor including a sensor element adapted to sense the value of a parameter and to provide an electrical signal which varies with variations in the parameter value. Electrically conductive lead wires in electrical communication with the sensing element are included. An elongated member or cord is provided which includes electrically conductive extension wires each of which is bonded to a different one of the lead wires of the sensor in a bonding zone. An electrically insulating covering, which may be considered a part of the elongated member, is located on the extension wires proximal of the bonding zone. An electrically insulating overcoating is located over the sensing element, the lead wires and the bonding zone, and preferably a portion of the electrically insulating covering, and is derived from a mixture comprising at least one polymerizable component and at least one thixotropic component. The thixotropic component is preferably present in the overcoating precursor mixture in an amount effective to at least facilitate controlling the thickness of the overcoating. A particularly useful embodiment of the present invention relates to thermistor assemblies. In this embodiment, the thermistor assembly comprises a thermistor including a thermistor element, such as the usual thermistor chip, wafer, bead or the like, a protective coating substantially surrounding the thermistor element, preferably hermetically sealed around the thermistor element, and electrically conductive lead wires in electrical communication with the thermistor element and extending beyond the protective coating. An elongated member is provided and includes electrically conductive extension wires each of which is bonded to a different one of the lead wires of the thermistor in a bonding zone, and an electrically insulating covering located on the extension wires proximal of the bonding zone. An electrically insulating, thermally conductive overcoating is provided and is located over the protective coating, the lead wires and the bonding zone, and preferably over a portion of the electrically insulating covering, and is derived from a mixture comprising at least one polymerizable component and at least one thixotropic component. The thixotropic component is preferably present in the overcoating in an amount effective to enhance the thermal conductivity of the overcoating relative to a substantially identical overcoating without the thixotropic component. This enhanced thermal conductivity reduces the time required for the thermistor assembly to respond to variations in the value of the temperature being sensed. The present methods for producing sensor assemblies comprise bonding the electrically conductive lead wires of a sensor, for example, a thermistor, to the electrically conductive extension wires of an elongated member so as to form composite wires including a bonding zone. A mixture comprising at least one polymerizable component and at least one thixotropic component is applied onto the sensor and the bonding zone, and preferably onto a portion of the elongated member having an electrically insulating covering proximal of the bonding zone. The polymerizable component or components in the applied mixture are cured. This applied mixture is preferably exposed to ultraviolet radiation, gamma ray radiation and/or electron beam radiation at conditions effective to at least partially cure the polymerizable component or components in the applied mixture. The partially cured applied mixture may be subjected to conditions, for example, elevated temperatures, for a time sufficient to complete the curing and/or to more securely adhere the cured applied mixture to the other components of the assembly.. In any event, an electrically insulating, preferably thermally conductive, overcoating is formed from the applied mixture. Using a polymerizable component or mixture of polymerizable components which forms a polymer in the overcoating in response to being exposed to one or more forms of radiation noted above provides substantial advantages. This feature allows the configuration of the overcoating to be set and at least partially cured without resort to elevated temperatures, and excessive transporting and processing of the partially finished assembly. The use of such forms of radiation to at least partially cure the polymerizable component or components very effectively, efficiently and quickly sets the final configuration, in particular the thickness, of the overcoating. This method of at least partially curing the overcoating facilitates the mass production of sensor assemblies with substantially reproducible sensing, for example, response time, characteristics. If necessary or desired, after being exposed to such radiation, the overcoating can be subjected to elevated temperature for a time sufficient to further cure or completely cure the polymer/polymerizable component or components present in the at least partially cured overcoating and/or to more securely adhere the overcoating to one or more other components of the thermistor assembly. The present overcoatings are effective to provide electrical insulation, for example, for safe use. Such overcoatings preferably have a dielectric strength of at least about 500 volts/mil, that is, at least about 500 volts/0.001 inch. The overcoating should be sufficiently thick to provide effective electrical insulation. However, excessively thick overcoatings are to be avoided as wasteful and, particularly with regard to temperature sensors, as being detrimental to the operation of the sensor assembly. In one useful embodiment, the overcoating preferably has an average thickness in the range of about 0.0005 inch to about 0.05 inch, more preferably about 0.001 inch to about 0.02 inch. Preferably, the present overcoatings have at least one of the following characteristics: is biocompatible, is hydrolytically stable and is substantially non-water absorbing. As used herein, the overcoatings are biocompatible if they can be exposed to bodily (human) tissue or fluids for a period of time sufficient to achieve the desired result, for example, a desired medical diagnosis and/or treatment procedure, with no significant detrimental effect on the individual whose bodily tissue or fluids are so exposed. As used herein, the overcoatings are hydrolytically stable if they remain structurally intact after being exposed to an aqueous medium, such as blood or a conventional saline solution, for 72 hours at 37°C. As used herein, the overcoatings are substantially non-water absorbing if they absorb no more than 5%, preferably no more than 1%, by weight of water after being exposed to an aqueous medium, such as blood or a conventional saline solution, for 10 hours at 37°C. Such characteristics of the present overcoating are beneficial, particularly when the sensor assemblies are to be used in in vivo medical applications, for example, in conjunction with a catheter such as a multi-lumen catheter. The present overcoatings include at least one polymer derived at least in part from a polymerizable component or a mixture of polymerizable components. Such polymerizable component or components are preferably such as to be capable of being polymerized by exposure to ultraviolet radiation, gamma ray radiation, and/or electron beam radiation. Any suitable polymer or mixture of polymers may be included in such overcoatings provided that the resulting overcoatings function as described herein. In one embodiment, the polymer is a thermosetting polymer. Examples of useful polymers for inclusion in the present overcoatings include epoxy polymers, polymers derived from at least one of methacrylic acid, methacrylic acid esters, acrylic acid, acrylic acid esters, and mixtures thereof. Specific examples of polymerizable formulations from which such polymers can be obtained include the methacrylic acid ester adhesive formulation sold under the trademark DYMAX Multi-care 20159 by Dymax Engineering Adhesives, and the epoxide/polyol formulation sold under the trademark ENVIBAR UV1244X3 by Union Carbide Chemicals and Plastics Company Inc. The mixture from which the overcoating is derived may include at least one initiator, e.g., at least one photoinitiator, in an amount effective to initiate the polymerization of the polymerizable component or components, particularly if the polymerization is to be initiated by ultraviolet radiation. Any suitable initiator or mixture of initiators, such as those conventionally used to initiate polymerization in the presence of ultraviolet radiation, may be included. Care should be exercised to avoid initiators or amounts of initiators which may have a detrimental effect on the overcoating or its properties, for example, its biocompatibility. Examples of useful initiators include benzophenone, diethyoxyacetophenone, 2,3-dimethyoxy-2-phenylacetophenone, benzoin methyl ether, benzoin ethyl ether, benzoin isopropyl ether, benzoin isobutyl ether, chlorothio-xanthanone, azo-bis- isobutylronitrile, N-methyl diethanolaminebenzophenone and the like. Such initiator or initiators are preferably present in an amount in the range of about 1% to about 10%, more preferably about 2% to about 5%, by weight based on the total weight of the polymerizable component or components present in the mixture. In addition, catalysts, such as amines, for example, phenylenediamine; benzyldimethyl a ine; methylbenzyldimethylamine; dimethylaminomethylphenol, and the like may be used in amounts effective to promote the initiation mechanism, generally about 0.05 part to about 1.0 part per 100 parts of the mixture. One or more thixotropic components are included in the present overcoatings and mixtures, for example, to obtain the result or results noted elsewhere herein. Any suitable thixotropic component or components may be employed provided that such component or components function as described herein. Care should be exercised in choosing the thixotropic component or components to avoid those components which may adversely effect the desired properties, e.g., biocompatability, of the sensor assembly. The thixotropic component is preferably inorganic. Examples of useful thixotropic components include silica, such as fumed silica; colloidial aluminum silicate clays; carbonates, silicates and sulfates of alkaline earth metals; metal oxides, such as magnesia; and the like. The thixotropic component or components are preferably present in the mixtures and overcoatings in an amount in the range of about 0.1% to about 15%, more preferably about 0.2% to about 7%, by weight based on the total weight of the mixtures and overcoatings, respectively. The present methods for producing sensor assemblies, such as the present sensor assemblies, involve the application of a mixture comprising at least one polymerizable component and at least one thixotropic component, such as the mixtures described elsewhere herein. These methods comprise bonding the electrically conductive lead wires of a sensor, such as described herein, to the electrically conductive extension wires of an elongated member so as to form composite wires including a bonding zone in which the lead wires and extension wires are bonded together. A mixture, as described above, is applied onto the sensor and the bonding zone. This applied mixture is then subjected to conditions effective to cure (polymerize) the polymerizable component or components therein. The applied mixture is preferably exposed to ultraviolet radiation, gamma ray radiation and/or electron beam radiation at conditions effective to at least partially cure the at least one polymerizable component in the applied mixture. Elevated temperatures, for example, in the range of about 100°C to about 150 C, may be employed to further cure the overcoating and/or to enhance the adhesion of the overcoating to one or more other components of the assembly. An electrically insulating, preferably thermally conductive, overcoating is formed from the applied mixture. The bonding step preferably includes spot welding the lead wires to the extension wires. The applying step preferably includes dipping the sensor and the bonding zone into a quantity of the mixture. In one embodiment the elongated member includes an electrically insulating covering located on the extension wires proximal of the bonding zone. The applying step acts to apply the mixture onto a portion of this covering. In this manner, the entire sensor assembly is electrically insulating. The invention, together with additional features and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying illustrative drawings. Brief Description of the Drawings Fig. 1 is a schematic view partly in cross section showing an embodiment of the present thermistor assembly. Fig. 2 is a partial cross sectional view taken generally along line 2-2 of Fig. 1. Fig. 3 is a cross-sectional view taken generally along line 3-3 of Fig. 1. Fig. 4 is a block flow diagram illustrating one embodiment of the present method of producing sensor assemblies. Fig. 5 is a side view of a multi-lumen medical catheter employing a thermistor assembly in accordance with the present invention. Fig. 6 is a view, partially in cross-section, taken generally along line 6-6 of Fig. 5. Detailed Description of the Drawings Fig. 1 illustrates a thermistor assembly, shown generally at 10, in electrical communication with a conventional temperature monitor unit 12. Thermistor assembly 10 includes a thermistor, shown generally at 14, an elongated extension cord, shown generally at 16, and an overcoating 18. Thermistor 14 includes a thermistor chip 20, of conventional design, a hermetically sealed glass bead 22 which forms a protective coating around chip 20, and two lead wires 24 and 26 which are electrically connected to chip 20 and extend beyond glass bead 22. Lead wires 24 and 26 act to transmit electrical signals to and from chip 20, and may be made of any suitable electrically conductive, preferably metallic, material. In a particularly useful embodiment, lead wires 24 and 26 are made of a platinum- containing alloy. Thermistor 14 is often provided as a basic unit from which thermistor assembly 10 is produced or manufactured. Elongated extension cord 16 includes extension wires 28 and 30, and an electrically insulating covering 32. Extension wires 28 and 30 extend distally beyond the distal end 34 of cord 16. However, these extension wires 28 and 30 are ultimately in electrical communication with temperature monitor unit 12, so that together with lead wires 24 and 26 they provide for electrical communication between chip 20 and unit 12. Extension wires 28 and 30 may be made of any suitable electrically conductive, preferably, metallic, material. The materials from which lead wires 24 and 26, and extension wires 28 and 30 are made should be chosen from materials which can be effectively bonded together, for example, so as to maintain the structural integrity of the bond during use of the assembly 10 and to maintain the integrity of the electrical signals to be transmitted through such wires. In one embodiment, the extension wires 28 and 30 are made of a nickel alloy, such as a nickel magnet wiring material. Each of the lead wires 24 and 26 is bonded to extension wires 28 and 30, respectively, for example, by conventional spot welding techniques. This bonding occurs in a bonding zone 36 which includes the proximal end portions of lead wires 24 and 26 and the distal end portions of extension wires 28 and 30. In any event, wires 24 and 28 and wires 26 and 30 are effectively bonded together so that such wires are in electrical communication with each other. Overcoating 18 includes an epoxy polymer, such as that derived from the epoxide/polyol formulation sold under the trademark ENVIBAR UV1244X3 by Union Carbide Chemicals and Plastics Company, Inc. Overcoating 18 also includes solid particles 38 of a thixotropic component, such as fumed silica, for example, that sold under the trademark Cab-O- Sil by Cabot Corp. Particles 38 act to enhance the thermal conductivity of overcoating 18 and, during the assembly operation, facilitate controlling the thickness of the overcoating. Overcoating 18 is preferably substantially free of borate components, in particular effective amounts of one or more borate components. Overcoating 18 has an average thickness, particularly around the distal half of the glass bead 22, in the range of about 0.005 inch to about 0.02 inch. Overcoating 18 extends around glass bead 22, lead wires 24 and 26, bonding zone 36, the exposed distal portions of extension wires 28 and 30 and the distal end portion 39 of covering 32. The dielectric strength of overcoating 18 is at least about 500 volts/mil, for example, about 1800 volts/mil, and, together with covering 32, provides assembly 10 with effective electrical insulation. In addition, overcoating 18 is biocompatible, has substantial hydrolytic stability and is substantially non- water absorbing. This combination of features makes thermistor assembly 10 very effective for use in in vivo medical applications. Figs. 5 and 6 illustrate one very useful application for the thermistor assembly 10. A multi-lumen catheter, shown generally at 40, includes an elongated, flexible catheter body or tube 42, for example, sized to be received within a vein or artery and moved into the heart of a human medical patient. The catheter 40 has a proximal end 44 and a distal end 46, and includes a balloon 48 adjacent the distal end. It should be noted that thermistor assembly 10 can be used in other applications, including other catheter applications. The catheter 40 has a plurality of lumens, including a central lumen 50 and a thermistor lumen 52, as well as other lumens, such as a balloon inflation lumen, a through lumen, an injectate lumen and an electrical wires lumen. All of the lumens extend longitudinally within the tube 42 from the proximal end 44 to the distal end 46. However, the balloon inflation lumen is plugged by a plug at the distal end 46. The thermistor lumen 52, the injectate lumen and the electrical wires lumen are similarly plugged at the distal end 46. In addition, the central lumen 50 carries an illuminating optical fiber 54 and an imaging optical fiber 56 and is fully closed or plugged by these fibers and an adhesive which retains these fibers in the central lumen 50. The optical fibers 54 and 56 extend completely through the central lumen 50 from the proximal end 44 to the distal end 46. Only the through lumen is capable of conducting a fluid completely through the catheter 40 from the proximal end 44 to. the distal end 46. The through lumen terminates in a distal port at the distal end 46 of the catheter. For additional details on catheters of this type and methods for manufacturing such catheters, see commonly assigned U.S. Patent Application Serial No. , filed (Attorney's Docket No. D-2344) , which is hereby incorporated in its entirety herein by reference. The thermistor assembly 10 is mounted within the thermistor lumen 52 by a thermistor mounting body 58 as shown in Fig. 6. The mounting body 58 has a somewhat concave outer surface 60 which cooperates with the tube 42 to define a cavity 62 which opens radially outwardly at the opening 64 of the tube 42. Mounting body 58 is adhesively held in place in thermistor lumen 52. The thermistor assembly 10 is partially embedded in the mounting body 58 and projects from the mounting body into the cavity 62. The thermistor assembly 10 projects into the cavity 62 so that fluid can enter and flow through the cavity, and the thermistor assembly 10 is in good heat- transfer relationship to the fluid passing over the catheter 40 at that location. A portion of the volume of the cavity 62 is occupied by the thermistor assembly 10. The opening 64 and the cavity 62 are sufficiently large so that fluid flowing along the tube 42 can readily flow over the portion of the thermistor assembly 10 which projects into the cavity 62. This places the thermistor assembly 10 in good heat-transfer relationship to any fluid flowing along the tube 42 at that location. The thermistor assembly 10 is elongated and is oriented so that its longitudinal dimension extends generally longitudinally of the thermistor lumen 52. Thus, the long axis of the thermistor assembly 10 extends generally parallel to the direction of any fluid flowing along the longitudinal axis of the catheter 40. The mounting body 58 can be constructed in different ways, but preferably, it includes a base 66 of electrical insulating material located between a wall 68 which separates the lumens 50 and 52 and the thermistor assembly 10 and a thermally conductive layer 70. The thermally conductive layer 70 comprises a matrix of electrically insulating material and a filler carried by the matrix. Both the base 66 and the matrix are preferably constructed of a material which is adherent to the tube 42 and which is an electrical insulator. The material of the base 66 is also preferably a good thermal insulator to insulate the thermistor assembly 10 from any fluid flowing in the through lumen of the catheter 40. Generally, polymeric materials can be used for the base and the matrix with urethane and epoxy being preferred. The filler is constructed of a material which is more thermally conductive than the electrical insulating material. For example, the filler may be ceramic, carbon, graphite, or a metal, such as silver, nickel, gold, platinum and aluminum. Examples of suitable ceramics are aluminum oxide, aluminum nitride, boron oxide, boron nitride, silicon oxide and silicon nitride. For example, the filler may be in the form of strands, chopped fibers or particles, with dentritic-shaped particles being preferred for improved thermal conductivity. By way of example, a preferred ceramic-filled epoxy is EP21TDCLV-2AN obtainable from Master Bond Inc. Ceramic is the preferred material for the filler because it is not electrically conductive. The base 66 may be of various different configurations and, in this embodiment, has a concave outer surface 72 facing outwardly toward the opening 64. Preferably, the outer surface 72 is spaced from the thermistor assembly 10 to provide space between the base 66 and the thermistor assembly 10 for the thermally conductive layer 70. The thermally conductive layer 70 preferably is sandwiched between the thermistor assembly 10 and the base 66 and extends part way around the sides of the thermistor assembly 10. Thus, the thermally conductive layer is located to facilitate heat transfer from the fluid outside the tube 44 to the thermistor assembly 10. The thermally conductive layer 70 has sufficient electrical insulating or dielectric properties so as to be safely usable. If desired, a very thin layer 74 of the base material, such as urethane or epoxy, may be placed over the exposed regions of the thermistor assembly 10. The layer 74 is really not part of the mounting body in the sense that it serves any mounting function although it is adhered to the thermistor assembly 10 and the thermally conductive layer 83. The layer 74 is very thin because it is only used for electrical isolation, and it may or may not be considered part of the mounting body 63. If desired, the thermally conductive layer 70 can be omitted, and the entire mounting body can be comprised of the base 66. In this event, the conductive layer 70 is replaced with the base 66 so that the mounting body 58 has the same configuration as the combined base 66 and conductive layer 70 shown in Fig. 6. In use of the catheter 40, the catheter tube 42 is introduced through a vein or artery of a patient and into the heart using known techniques. The balloon 48 is inflated through the balloon inflation lumen, and the inflated balloon is used to carry the distal end 46 of the catheter to the desired location. For example, the balloon 19 is carried into the pulmonary artery. The location of the tube 42 within the heart will depend upon the procedure to be carried out. For example, to calculate ejection fraction, the tube 42 is inserted into the heart so as to place the injectate port in the right atrium, the thermistor assembly 10 into the pulmonary artery and the distal end 46 into the pulmonary artery. A bolus of cold fluid is then injected into the right atrium through the injectate port and allowed to mix with the bloodstream in the right ventricle. The blood and cold fluid mixture flow along the catheter tube and over the thermistor assembly 10 in the pulmonary artery. The temperature of the mixture changes with each heartbeat, and the thermistor assembly 10 can track each temperature change so as to provide a stepped temperature chart. This information can then be processed in accordance with known techniques to provide ejection fraction. Pressure can be monitored, if desired, through the through lumen of catheter 40. Referring now to Fig. 4, the manufacture of thermistor assembly 10 is illustrated. It should be understood that other sensor assemblies can be produced using substantially similar or analogous manufacturing methods. Thus, such sensor assemblies and methods for making such other sensor assemblies are within the scope of the present invention. The thermistor 14 is positioned to be parallel to the edge of the elongated extension cord 16, which is conveniently wound onto a spool. During the bonding step, generally shown at 80, the lead wires 24 and 26 are bonded to the extension wires 28 and 30, respectively. This bonding can be done in any suitable manner, provided that firm (strong) wire-to-wire attachment is obtained, and the resulting composite wire is effectively electrically conductive. The area of the wires at which the wires are actually bonded together is referred to as the bonding zone 36. The wires can be very effectively bonded together using conventional spot welding techniques. After the bonding, the thermistor 14 and bonding zone 36 is dipped in acetone or other suitable liquid medium for a period of time, on the order of about 2 seconds to about 1 minute, for example, about 5 seconds, sufficient to clean and/or rinse contamination from these components. As much of the exposed extension wires 28 and 30 as possible are preferably subjected to this dipping step. The bonded thermistor 14, bonding zone 36 and exposed extension wires 28 and 30 are then dipped in acetic acid, or other suitable etching medium, for a period of time, on the order of about 10 seconds to about 10 minutes, for example, about 2 minutes, sufficient to etch or otherwise condition the lead wires, bonding zone and extension wires so that such conditioned wires and zone are more strongly adhered to the overcoating 18 to be placed thereon relative to the unconditioned wires and zone. After this conditioning, the thermistor bonding zone and extension wires are dipped in a liquid medium, e.g., acetone, for about 2 seconds to about 1 minute, for example, about 10 seconds, to rinse off the conditioning medium. The conditioned thermistor, bonding zone, extension wires and distal end portion of the covering of the extension cord are then dipped into a quantity of a mixture comprising polymerizable components and about 5% by weight of fumed silica particles. This mixture is curable by exposure to ultraviolet radiation. A particularly useful formulation of polymerizable component for use in the present invention is the epoxide/polyol-containing mixture sold under the trademark ENVIBAR UV1244X3, described previously. This applying step, shown generally at 82 in Fig. 4 can be performed using techniques other than dipping. For example, the mixture may be sprayed on, painted on and the like. In any event, the applying step produces a coating of a curable monomer mixture including a thixotropic agent on the thermistor 14, bonding zone 36, exposed extension wires 28 and 30 and the distal end portion 39 of covering 32. In the dipping embodiment, the thickness of the mixture coating depends at least partially on the rate at which the bonded thermistor is removed from the coating mixture. For example, with a thermistor having a glass protective coating with a diameter of about 0.015 inch to about 0.017 inch, a coated diameter of about 0.020 inch to about 0.024 inch is obtained by withdrawing the bonded thermistor from the coating mixture at the rate of about 3 to about 5 inches/minute. After this applying step 80, the coated thermistor is examined, for example, under a microscope, for coating defects. If any such defects exist, the applying step is repeated. After the coated thermistor passes the defect examination, it is exposed in a exposing step, shown generally at 84 in Fig. 4, to ultraviolet radiation, for example, from a ultraviolet lamp, at conditions effective to partially cure the polymerizable components in the applied mixture. This exposing step 84 occurs for a period of time, for example, in the range of about 10 seconds to about 10 minutes or more. In one embodiment, the coated thermistor is exposed to ultraviolet radiation for about 1 minute, then rotated 180° and exposed to ultraviolet radiation for another minute. The partially cured coated thermistor together with the entire spool of extension cord is then subjected to elevated temperatures in a temperature treatment step, shown generally at 86 in Fig. 4, for a period of time to completely cure the coating. For example, the partially cured coated thermistor/spool combination can be placed in a conventional circulation oven and left undisturbed at about 100° C to about 150°C, e.g., about 120° C, for about 5 minutes to about 3 hours, e.g., about 30 minutes. The completely cured thermistor assembly which has a configuration as shown in Fig. 1, is cooled to room temperature prior to being incorporated into a temperature sensing system, such as the catheter system noted previously. While this invention has been described with respect to various specific examples and embodiments, it is to be understood that the invention is not limited thereto and that it can be variously practiced within the scope of the following claims.

Claims: WHAT IS CLAIMED IS: 1. A thermistor assembly comprising: a thermistor including a thermistor element, a protective coating substantially surrounding said thermistor element and electrically conductive lead wires in electrical communication with said thermistor element and extending beyond said protective coating; an elongated member including electrically conductive extension wires each of which is bonded to a different one of said lead wires in a bonding zone, and an electrically insulating covering located on said extension wires proximal of said bonding zone; and an electrically insulating, thermally conductive overcoating located over said protective coating, said lead wires and said bonding zone, and being derived from a mixture comprising at least one polymerizable component and at least one thixotropic component. 2. The assembly of claim 1 wherein said overcoating is located over a portion of said covering. 3. The assembly of claim 1 wherein said overcoating is biocompatible. 4. The assembly of claim 1 wherein said overcoating is hydrolytically stable and is substantially non-water absorbing. 5. The assembly of claim 1 wherein said at least one polymerizable component forms a polymer in said overcoating in response to being exposed to at least one of ultraviolet radiation, gamma ray radiation and electron beam radiation. 6. The assembly of claim 1 wherein said at least one thixotropic component is present in said mixture in an amount effective to at least facilitate controlling the thickness of said overcoating. 7. The assembly of claim 1 wherein said at least one thixotropic component is present in said overcoating in an amount effective to enhance the thermal conductivity of said overcoating relative to a substantially identical overcoating without said at least one thixotropic component. 8. The assembly of claim 1 wherein said overcoating includes at least one polymer selected from the group consisting of epoxy polymers, polymers derived from at least one of methacrylic acid, methacrylic acid esters, acrylic acid, acrylic acid esters and mixtures thereof. 9. A sensor assembly comprising: a sensor including a sensing element adapted to sense the value of a parameter and to provide an electrical signal in response to said parameter value, and electrically conductive lead wires in electrical communication with said sensing element; an elongated member including electrically conductive extension wires each of which is bonded to a different one of said lead wires in a bonding zone, and an electrically insulating covering located on said extension wires proximal of said bonding zone; and an electrically insulating overcoating located over said sensing element, said lead wires and said bonding zone, and being derived from a mixture comprising at least one polymerizable component and at least one thixotropic component. 10. The assembly of claim 9 wherein said overcoating is located over a portion of said covering. 11. The assembly of claim 9 wherein said overcoating is biocompatible. 12. The assembly of claim 9 wherein said overcoating is hydrolytically stable and is substantially non-water absorbing. 13. The assembly of claim 9 wherein said at least one polymerizable component forms a polymer in said overcoating in response to being exposed to at least one of ultraviolet radiation, gamma ray radiation and electron beam radiation. 14. The assembly of claim 9 wherein said at least one thixotropic component is present in said mixture in an amount effective to at least facilitate controlling the thickness of said overcoating. 15. The assembly of claim 9 wherein said overcoating includes at least one polymer selected from the group consisting of epoxy polymers, polymers derived from at least one of methacrylic acid, methacrylic acid esters, acrylic acid acrylic acid esters and mixtures thereof. 16. The assembly of claim 9 wherein said parameter is temperature and said overcoating is thermally conductive. 17. The assembly of claim 16 wherein said at least one thixotropic component is present in said overcoating in an amount effective to enhance the thermal conductivity of said overcoating relative to a substantially identical overcoating without said at least one thixotropic component. 18. The assembly of claim 9 wherein said overcoating has an average thickness in the range of about 0.0005 inch to about 0.05 inch. 19. The assembly of claim 9 wherein said sensor further includes a protective coating other than said overcoating which substantially completely surrounds said sensing element, and said overcoating is located over said protective coating. 20. A method for producing a thermistor assembly comprising: bonding the electrically conductive lead wires of a thermistor to the electrically conductive extension wires of an elongated member so as to form composite wires including a bonding zone in which said lead wires and said extension wires are bonded together; applying a mixture comprising at least one polymerizable component and at least one thixotropic component onto said thermistor and said bonding zone to form a coated component; and subjecting said coated component to conditions effective to cure said at least one polymerizable component in said coated component and form an overcoating from said applied mixture. 21. The method of claim 20 wherein said subjecting step comprises exposing said coated component to at least one of ultraviolet radiation, gamma ray radiation and electron beam radiation at conditions effective to at least partially cure said at least one polymerizable component in said coated component. 22. The method of claim 21 wherein said subjecting step exposing said coated component to ultraviolet radiation. 23. The method of claim 21 wherein said exposing is effective to partially cure said at least one polymerizable component, and said partially cured at least one polymerizable component is subjected to elevated temperature conditions effective to further cure said partially cured at least one polymerizable component. 24. The method of claim 20 wherein said elongated member includes an electrically insulating covering located on said extension wires proximal of said bonding zone, and said applying step acts to apply said mixture onto a portion of said covering. 25. The method of claim 20 wherein said thermistor includes a thermistor element and a protective coating substantially surrounding said thermistor element, and said applying step acts to apply said mixture onto said protective coating. 26. The method of claim 20 wherein said overcoating is biocompatible, is hydrolytically stable and is substantially non-water absorbing. 27. The method of claim 20 wherein said at least one thixotropic component is present in said mixture in an amount effective to at least facilitate controlling the thickness of said overcoating. 28. The method of claim 20 wherein said at least one thixotropic component is present in said overcoating in an amount effective to enhance the thermal conductivity of said overcoating relative to a substantially identical overcoating without said at least one thixotropic component. 29. The method of claim 20 wherein said overcoating includes at least one polymer selected from the group consisting of epoxy polymers, polymers derived from at least one of methacrylic acid, methacrylic acid esters, acrylic acid acrylic acid esters and mixtures thereof. 30. A method for producing a sensor assembly comprising; bonding the electrically conductive lead wires of a sensor adapted to sense the value of a parameter and to provide an electrical signal in response to said parameter value to the electrically conductive extension wires of an elongated member so as to form composite wires including a bonding zone in which said lead wires and said extension wires are bonded together; applying a mixture comprising at least one polymerizable component and at least one thixotropic component onto said sensor and said bonding zone and forming a coated component; and subjecting said coated component conditions effective to cure said at least one polymerizable component in said coated component and form an overcoating from said applied mixture. 31. The method of claim 30 wherein said subjecting step comprises exposing said coated component to at least one of ultraviolet radiation, gamma ray radiation and electron beam radiation at conditions effective to at least partially cure said at least one polymerizable component in said coated component. 32. The method of claim 31 wherein said subjecting step comprises exposing said coated component to ultraviolet radiation. 33. The method of claim 31 wherein said exposing is effective to partially cure said at least one polymerizable component, and said partially cured at least one polymerizable component is subjected to elevated temperature conditions effective to further cure said partially cured at least one polymerizable component. 34. The method of claim 30 wherein said elongated member includes an electrically insulating covering located on said extension wires proximal of said bonding zone, and said applying step acts to apply said mixture onto a portion of said covering. 35. The method of claim 30 wherein said overcoating is biocompatible, is hydrolytically stable and is substantially non-water absorbing. 36. The method of claim 30 wherein said at least one thixotropic component is present in said mixture in an amount effective to at least facilitate controlling the thickness of said overcoating. 37. The method of claim 30 wherein said overcoating includes at least one polymer selected from the group consisting of epoxy polymers, polymers derived from at least one of methacrylic acid, methacrylic acid esters, acrylic acid acrylic acid esters and mixtures thereof. 38. The assembly of claim 30 wherein said parameter is temperature and said overcoating is thermally conductive. 39. The assembly of claim 38 wherein said at least one thixotropic component is present in said overcoating in an amount effective to enhance the thermal conductivity of said overcoating relative to a substantially identical overcoating without said at least one thixotropic component. 40. A catheter for measuring the temperature of a fluid within a living body, said catheter comprising: an elongated tube sized to be received within a vein or an artery and having proximal and distal ends, a peripheral wall, at least one lumen extending longitudinally within said tube and an opening in said peripheral wall which extends from the lumen to the exterior of said tube; and a thermistor assembly located at least partially in said lumen in or near said opening, said thermistor assembly comprising: a thermistor including a thermistor element, a protective coating substantially surrounding said thermistor element and electrically conductive lead wires in electrical communication with said thermistor element and extending beyond said protective coating; an elongated member including electrically conductive extension wires each of which is bonded to a different one of said lead wires in a bonding zone, and an electrically insulating covering located on said extension wires proximal of said bonding zone; and an electrically insulating, thermally conductive overcoating located over said protective coating, said lead wires and said bonding zone, and being derived from a mixture comprising at least one polymerizable component and at least one thixotropic component. 41. The catheter of claim 40 wherein said overcoating is biocompatible, hydrolitically stable and substantially non-water absorbing.

Inventors: SOONG-WU SU SYIN, SOONG-WU, SU, SYIN
Applicants: BAXTER INT, BAXTER INTERNATIONAL INC.
Application year: 1992
Main classification: G01K7
IPC codes: A61B5, G01K7, H01C7, H01C17