GB2352816A

GB2352816A - A thermistor probe or catheter comprising a temperature dependent material placed between concentric conductors

Info

Publication number: GB2352816A
Application number: GB9918219A
Authority: GB
Inventors: Robert Twiney; Roger Whatmore
Original assignee: Bowthorpe Components Ltd
Current assignee: Bowthorpe Components Ltd
Priority date: 1999-08-04
Filing date: 1999-08-04
Publication date: 2001-02-07
Also published as: GB9918219D0

Abstract

A catheter thermistor comprises an elongate central conductor 10, which is coated at one end thereof with a layer 11 of a ceramics material having a negative temperature coefficient. The exposed remainder of the inner conductor is coated with a layer 12 of a glass insulating material, which co-extends with the ceramics layer 11. A conductive layer 13 of silver ink is then applied over the ceramics and insulating layers 11,12. An outer insulating layer 14 of glass or epoxy is applied over the length of the outer conductive layer 13 and over the ends of the internal layers 10,11,13, so as to form a capped end to the thermistor. The overall diameter of the catheter thermistor is 100žm or less and is thus far less traumatic to insert into a patient, compared with conventional catheter thermistors, which are typically 500žm in diameter. Also, the reduced size of the catheter thermistor means that multiple sensors can be mounted in the same physical volume as a conventional catheter thermistor.

Description

2352816 Thermistor Probe This invention relates to a thermistor probe, for

example for use as a catheter sensor for measuring temperature inside the body of humans and animals.

Catheter sensors for measuring temperature inside the body are well known. one such sensor comprises a ceramics bead or disc having a negative temperature coefficient (NTC) of electrical resistance. Contact is made with the bead or disc via a pair of electrodes respectively attached to metallised terminals applied to its opposed faces. The bead or disc is then encapsulated in an insulating material such as glass.

A disadvantage of the above-mentioned sensor is that it is bulky in construction, thereby making it difficult and traumatic to insert into a patient.

There is an increasing demand to insert further sensors into the body for sensing further parameters. However a further disadvantage of known catheter temperature sensors is that their size leaves very little room for such further sensors.

Small children, infants and babies etc. have small veins. Thus, conventional catheter temperature sensors cannot be used, owing to their large physical size.

We have now devised a thermistor probe which alleviates the abovementioned problems.

In accordance with this invention, there is provided a thermistor probe comprising an elongate central conductor and an elongate outer tubular conductor surrounding said inner conductor, the conductors being interconnected at or adjacent an outer free end of the probe by a temperature dependent resistive material disposed between the conductors.

In use, electrical current f lows along one of the conductors, through the resistive material and back along the outer conductor. The resistance of the material varies with temperature and thus the current f lowing through the probe is proportional to the temperature at or adjacent its outer free end.

The construction of the probe is such that its diameter is significantly less than conventional catheter thermistors.

Preferably an outer layer of insulating material extends over the longitudinal outer surface of the outer 2 conductor.

Preferably the insulating material also extends over the free outer end of the probe.

Preferably said resistive material extends only over a portion of the inner conductor, an inner layer of insulating material being applied over the inner conductor where said resistive material is not applied.

Preferably the insulating material is glass or a plastics material, such as a polymer.

Preferably said resistive material extends completely around said inner conductor at or adjacent said outer free end of the probe.

Preferably the resistive material comprises a ceramics material having a negative coefficient of temperature.

However, any suitable material having a positive or negative coefficient of temperature could be used instead.

Preferably, the inner conductor is circular in section.

Also in accordance with this invention, there is provided a method of forming a thermistor probe, the method comprising applying a temperature dependent resistive material to an elongate conductor, at or adjacent a free outer end of the probe, and applying an elongate outer conductor over said inner conductor and resistive material.

Preferably the inner conductor is coated with said resistive material by electrophoresis, sputtering or dip- coating, such that particles of the material are deposited on the conductor. Preferably the conductor is coated with a ceramics material. 30 Preferably an inner insulating layer is applied to regions of the inner conductor to which no resistive material is applied. Preferably the outer conductive layer is applied by coating the longitudinal external surfaces of the inner insulating layer and the resistive material with a conductive material.

Preferably the conductive material comprises an ink or paint, which is deposited on the longitudinal external surfaces of the inner insulating layer and the resistive material.

3 Alternatively, the conductive material comprises a metal which is applied by sputtering.

Preferably the conductive material is fired after it has been applied to the probe.

Preferably an outer insulating layer is applied over the outer conductor and to the end of the probe.

An embodiment of this invention will now be described by way of example only and with reference to the accompanying drawings, in which:

FIGURE I is a longitudinal section through a catheter thermistor in accordance with this invention; FIGURE 2 is a sectional view along the line II-II of Figure 1; and FIGURE 3 is a sectional view along the line III-III of Figure 1.

Referring to the drawings, there is shown a catheter thermistor comprising an elongate central conductor 10 formed of 504m platinum wire, which is coated at one end thereof with a 1-10gm layer 11 of a ceramics material having a negative temperature coefficient. The conductor 10 is coated by immersing its end in a solution containing ceramics particles. The particles are then deposited on the conductor 10 by a process known as electrophoresis, in which an electrical current is passed through the solution, between the conductor 10 and another electrode: the electrical current causes the ceramics particles to be deposited on the conductor 10. The ceramic is then f ired in place onto the platinum wire 10 a high temperature.

The exposed remainder of the inner conductor is coated with a 5-10gm layer 12 of insulating material, which co-extends with the ceramics layer 11.

A 5gm conductive layer 13 of silver ink is then applied over the ceramics and insulating layers 11, 12. The ink is fixed in position by firing the assembly to a temperature of 6000C. Thus, the material of the insulating layer 12 is preferably glass, so that it can withstand the firing temperature.

Finally, a 5-104m outer insulating layer 14 of glass or epoxy is applied over the length of the outer conductive layer 4 13 and over the ends of the internal layers 10, 11, 13, so as to f orm a capped end to the thermistor.

The opposite end of the thermistor is attached via flexible wires 15 to a temperature sensing instrument (not 5 shown).

It will be appreciated that the overall diameter of the catheter thermistor is 100gm or less and is thus far less traumatic to insert into a patient, compared with conventional catheter thermistors, which are typically 500gm in diameter.

Also, the reduced size of the catheter thermistor means that multiple sensors can be mounted in the same physical volume as a conventional catheter thermistor.

Claims

1. A thermistor probe comprising an elongate central conductor and an elongate outer tubular conductor surrounding said inner conductor, the conductors being interconnected at or adjacent an outer free end of the probe by a temperature dependent resistive material disposed between the conductors.

2. A thermistor probe as claimed in claim 1, in which an outer layer of insulating material extends over the longitudinal outer surface of the outer conductor.

3. A thermistor probe as claimed in claim 2, in which the insulating material also extends over the free outer end of the probe.

4. A thermistor probe as claimed in any preceding claim, in which said resistive material extends only over a portion of the inner conductor, an inner layer of insulating material being applied over the inner conductor where said resistive material is not applied.

5. A thermistor probe as claimed in any of claims 2 to 4, in which the insulating material is glass.

6. A thermistor probe as claimed in any of claims 2 to 4, in which the insulating material is a plastics material.

7. A thermistor probe as claimed in any preceding claim, in which said resistive material extends completely around said inner conductor at or adjacent said outer free end of the probe.

8. A thermistor probe as claimed in any preceding claim, in which the resistive material comprises a ceramics material having a negative coefficient of temperature.

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9. A thermistor probe as claimed in any preceding claim, in which the inner conductor is circular in section.

10. A thermistor probe substantially as described herein with reference to the accompanying drawings.

11. A method of forming a thermistor probe, the method comprising applying a temperature dependent resistive material to an elongate conductor, at or adjacent a free outer end of the probe, and applying an elongate outer conductor over said inner conductor and resistive material.

12. A method as claimed in claim 11, in which the inner conductor is coated with said resistive material by electrophoresis.

13. A method as claimed in claim 11, in which the inner conductor is coated with said resistive material by sputtering.

14. A method as claimed in claim 11, in which the inner conductor is coated with said resistive material by dipcoating.

15. A method as claimed in any of claims 11 to 14, in which the conductor is coated with a ceramics material.

16. A method as claimed in any of claims 11 to 15, in which an inner insulating layer is applied to regions of the inner conductor to which no resistive material is applied.

17. A method as claimed in claim 16, in which the outer conductive layer is applied by coating the longitudinal external surfaces of the inner insulating layer and the resistive material with a conductive material.

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18. A method as claimed in claim 17, in which the conductive material comprises an ink, which is deposited on the longitudinal external surfaces of the inner insulating layer and the resistive material.

19. A method as claimed in claim 17, in which the conductive material comprises paint, which is deposited on the longitudinal external surfaces of the inner insulating layer and the resistive material.

20. A method as claimed in claim 17, in which the conductive material comprises a metal, which is deposited on the longitudinal external surfaces of the inner insulating layer and the resistive material by sputtering.

21. A method as claimed in any of claims 17 to 20, in which the conductive material is fired after it has been applied to the probe.

22. A method as claimed in any of claims 11 to 21, in which an outer insulating layer is applied over the outer conductor and to the end of the probe.

23. A method of forming a thermistor probe, the method being substantially as herein described with reference to the accompanying drawings.