Deep tissue temperature probe constructions

A disposable, zero-heat-flux, deep tissue temperature probe is constructed using a support assembly with multiple sections folded together or separated into strata during assembly of the probe. The sections support elements of the probe, including thermal sensors and a thermal resistor between the thermal sensors. Optionally, one of the sections supports a heater.

RELATED APPLICATION

This application contains material related to material disclosed, illustrated, and/or claimed in the following co-pending US patent applications:

BACKGROUND

The subject matter relates to a temperature probe—a device placed on the skin of a subject to measure temperature. More particularly, the subject matter pertains to a deep tissue temperature (DTT) probe. Deep tissue temperature measurement is a non-invasive determination of the core temperature of a human body in which a probe is located over a region of deep tissue that is representative of the body core. The probe reads the temperature of that region as the core temperature.

A system for non-invasively measuring deep tissue temperature was described by Fox and Solman in 1971 (Fox R H, Solman A J. A new technique for monitoring the deep body temperature in man from the intact skin surface. J. Physiol. January 1971:212(2): pp 8-10). The system, illustrated in the schematic diagram ofFIG. 1, estimates body core temperature by indirect means using a specially designed probe that is placed upon the skin of a subject to stop or significantly neutralize heat flow through a portion of the skin in order to measure temperature. The components of the probe10are contained in a housing11. The Fox/Solman probe10includes two thermistors20mounted on either side of a thermal resistance22, which may be constituted of a layer of insulating material capable of supporting the thermistors20. The probe10also includes a heater24disposed at the top of the probe10, over the elements20,22, and24. In use, the probe10is placed on a region of the skin of a person whose deep tissue temperature is to be measured. With the bottom surface26of the probe resting on a person's body, in contact with the skin, the thermistors20measure a temperature difference, or error signal, across the thermal resistance22. The error signal is used to drive a heater controller30, which, in turn, operates to minimize the error signal by causing the heater24to provide just enough heat to equalize the temperature on both sides of the thermal resistance22. When the temperatures sensed by the thermistors20are equal or nearly so, there is no heat flow through the probe, and the temperature measured by the lower thermistor20by way of a temperature meter circuit constituted of an amplifier36and a temperature meter38is equivalent to DTT. The probe10essentially acts as a thermal insulator that blocks heat flow through the thermal resistor22; DTT probes that operate in the same manner are termed “zero-heat-flux” (“ZHF”) probes. Since the heater24operates to guard against loss of heat along the path of measurement through the probe, it is often referred to as a “guard heater”.

Togawa improved the Fox/Solman design with a DTT probe structure that accounted for the strong multi-dimensional heat transfer of dermal blood flow through the skin. (Togawa T. Non-Invasive Deep Body Temperature Measurement. In: Rolfe P (ed) Non-Invasive Physiological Measurements. Vol. 1. 1979. Academic Press, London, pp. 261-277). The probe, illustrated inFIG. 2, encloses a ZHF sensor design40, which blocks heat flow normal to the body, in a thick aluminum housing42with a disk-like construction that also reduces or eliminates radial heat flow from the center to the periphery of the probe.

ZHF deep tissue temperature measurement were improved in several ways, principally by decreasing the size and mass of a DTT probe to improve response and equilibrium times, and also by adding guard heating around the periphery of the probe to minimize radial heat losses. Nevertheless, ZHF probes have typically been expensive and non-disposable, and have not been widely adopted for clinical use, except for cardiac surgery in Japan. The sensors cannot be effectively heat sterilized, although they can be disinfected with a cold bactericidal solution.

Presently, ZHF probes based on the original Fox and Solman design comprise both software and hardware improvements. One such ZHF probe has a stacked planar structure that consists of a number of discrete layers. An advantage of this design is a narrow width, which helps minimize radial temperature differences from heat loss through the sides of the sensor. This probe includes an optimally-damped heater controller which is operated by use of a PID (Proportional-Integral-Derivative) scheme to maintain the heater temperature just slightly higher than the temperature of the skin. The small temperature difference provides an error signal for the controller. While the hardware design is not disposable, it does provide some basic improvements to the size and mass of the Fox/Solman and Togawa designs.

Maintenance of body core temperature in a normothermic range during a perioperative cycle has been shown to reduce the incidence of surgical site infection, and so it is beneficial to monitor a patient's body core temperature before, during, and after surgery. Of course non-invasive measurement is very desirable, for both the comfort and the safety of a patient. Deep tissue temperature measurement using a probe supported on the skin provides an accurate and non-invasive means for monitoring body core temperature. However, the size and mass of the Fox/Solman and Togawa probes do not promote disposability. Consequently, they must be sterilized after each use, and stored for reuse. As a result, use of these probes to measure deep tissue temperature may raise the costs associated with DTT measurement and may increase the risk of cross contamination between patients. It is therefore useful to reduce the size and mass of a DTT probe, without sacrificing its performance, so as to promote disposability.

SUMMARY OF THE INVENTION

Disposable, zero-heat-flux, deep tissue temperature probes are constructed using a support assembly constituted of a flexible substrate that supports elements of the probe. The support assembly has multiple sections that may be folded together and/or separated to form a multi-level ZHF structure. The sections support elements of the probe, including thermal sensors separated by a layer of thermal resistance interposed between adjacent sensor-supporting sections. Preferably, at least one of the sections supports a heater.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

It is desirable that zero-heat-flux, deep tissue temperature probe (DTT probe) constructions be disposable. Thus the constructions should be easy and inexpensive to fabricate and assemble, have a low mass and a low profile, and comprise inexpensive materials and parts.

It is particularly desirable that disposable DTT constructions be assembled from low-profile, light weight, flexible assemblies that enable zero-heat-flux measurement at various locations on a human or animal body.

A thermal measurement support assembly for zero-heat-flux deep tissue temperature probe (DTT probe) constructions includes a flexible substrate with at least two thermal sensors disposed in a spaced-apart relationship and separated by one or more layers of thermally insulating material. Preferably the sensors are spaced apart vertically as inFIGS. 1 and 2, and they may further be spaced apart horizontally or radially with respect to a center of measurement of vertical heat flux. The substrate supports at least the thermal sensors and the separating thermal insulating material, and it may also support one or more heaters. Once constructed, the support assembly is ready to be incorporated into the structure of a DTT probe.

A first embodiment of a support assembly for a DTT probe is illustrated inFIG. 3. The support assembly200includes a permanent heater (not shown) with an attachment mechanism (not shown) and is designed and manufactured to be disposable. The support assembly200includes a film of material coated with copper on both sides and fashioned into two disk-shaped sections202and204that are joined at a common peripheral location206disposed between the two sections. The disk shaped sections202and204include major supporting surfaces203and205respectively. The surfaces on the opposite sides of the major supporting surfaces203,205(which are not seen in these figures) support respective thermocouples whose junctions207,209are visible at the respective centers of the major supporting surfaces203,205. Signal leads211,213are connected to the thermocouples at the junctions207,209, and a common lead215is electrically coupled to the thermocouples. Preferably, a pressure-sensitive adhesive (PSA)-backed, 0.001 inch thick piece of insulative material such as a polyimide layer (Kapton® film, for example) is disposed on one of the surfaces on the opposite side of one of the major supporting surfaces203,205, and the support assembly may be folded like a clam shell, on a crease at the common peripheral location206, as shown inFIGS. 4 and 5. An insulating material with greater thermal resistance may also be interposed between the surfaces203and205to decrease the sensitivity of the support assembly. When the support assembly is so folded, the thermocouples are disposed in a stacked configuration, with the layer of insulative material disposed therebetween to provide thermal resistance. The copper disks are electrically continuous; therefore, the junction of each thermocouple is common to both disks, which makes it possible to eliminate one wire from the pair of thermocouples. Although the probe is designed to minimize radial heat losses and radial temperature differences, the placement of the thermocouples in the center of the copper disks minimizes fin effects that tend to reduce accuracy.

The thermocouples in the first support assembly embodiment illustrated inFIGS. 3-5may be assembled with other elements of a DTT probe illustrated inFIG. 1, but the heater assembly is likely to become contaminated after use, and a disposable ZHF probe design is very desirable to avoid cross contamination between patients.

With reference toFIGS. 6-9, a second embodiment of a support assembly500for a DTT probe includes a heater integrated into the support assembly. There are no wires attached to this embodiment as both signal and power leads are available on a connector tab on the circumference of the assembly.

As best seen inFIGS. 6 and 7, the support assembly500includes a flexible substrate, preferably a sheet of flexible, thermally insulative material that is formed to include a plurality of contiguous sections. For example three contiguous paddle-shaped sections with disks502,504, and506of equal diameter are formed and aligned so that their centers lie on a straight line. Each disk transitions to a tab for supporting one or more electrical leads. The tabs are indicated by reference numerals503,505, and507, respectively. The inner periphery of each disk is continuous with each adjacent inner periphery at a point that is tangent to the perimeter of the inner circle and which intersects the line upon which the centers are aligned. Thus, the inner periphery of the outer disk502is continuous with the periphery of the inner disk504at509, and the inner periphery of the outer disk506is continuous with the periphery of the inner disk504at511, which is diametrically opposite509on the periphery of the inner disk504. Each disk has two opposite-facing, disk-shaped major surfaces. Thus, the outer disk502has major surfaces A and B, the inner disk504has major surfaces C and D, and the outer506has surfaces E and F. The major surfaces A, D, and E are on one side of the support assembly500; the major surfaces B, C, and F are on the opposite side.

As seen inFIGS. 6 and 7, a heater514is formed on the major surface A by, for example, depositing a layer of copper on the surface and then etching the copper layer. The etching includes formation of leads512for the heater on the tab503that terminate in pins513at the outer edge of the tab503. The etching also exposes a ring510of insulative material at the periphery of the major surface A. The layer515of copper on major surface C is etched to expose a ring516of insulative material at the periphery of the surface. The disk517of copper film inside the ring516is used as one element of a thermocouple518. For example, the thermocouple518is fabricated by soldering, brazing, or welding one end of an insulated chromel wire519to the disk517of copper film, preferably, but not necessarily, at or near the center of the surface C. The other end of the chromel wire519is soldered, brazed, or welded to a chromel pin520mounted to the tab505. Etching the copper on the major surface C also forms a lead521and a pin522for the copper portion of the thermocouple518on the tab505. Another thermocouple525is similarly fabricated on the major surface E. Etching removes copper from the major surfaces B, D, and F so that those surfaces have no copper thereon.

With the heater514and thermocouples518and525thus formed, the support assembly500may be Z-folded as shown inFIG. 8. Preferably, the sections502and504are folded at509by swinging the major surfaces B and C together and the sections504and506are folded at511by swinging the major surfaces D and E together. The folded support assembly500is seen in the top plan view ofFIG. 9. In this aspect, the support assembly500is preferably oriented with respect to a location on a body where a deep tissue temperature reading is to be taken by denominating the heater514as the top of the assembly, and major surface F as the bottom. In this aspect, the tabs503,505, and507are aligned by the folding place so as to align all of the leads and pins on a single side of a composite tab520. Preferably, but not necessarily, the composite tab520is oriented with the aligned pins facing in the same direction as the heater on major surface A. The table inFIG. 9sets forth the pin assignments. In the table, the lower thermocouple is on major surface E and the upper thermocouple is on major surface C. The connectors on the composite tab520provide electrical access to each of the thermal sensors and to the heater. A compression connector (not seen) may be received on the composite tab520.

Final assembly of a DTT probe construction with a support assembly500according to the second embodiment is illustrated inFIG. 10. In the unfolded assembly, there are three circular disks and six surface regions. Layers formed by folding the support assembly are labeled as shown in the figures. The layers are, as follows:

Major surface A is the electric heater

Major surface B is plastic film

Major surface C is a copper layer that supports a thermal sensor

Major surface D is a plastic film

Major surface E is a copper layer that supports a thermal sensor

Major surface F is plastic film

The assembled DTT probe may include additional layers added to the structure of the probe during assembly. For example, layers of pressure-sensitive adhesive (PSA)527may be disposed between the folded sections and on the top and bottom major surfaces, an insulating layer may be disposed on the layer of PSA above the heater, and a further layer of PSA may be disposed on the top of the insulating layer. Further, a release liner may be provided on the bottom PSA layer, and an aluminum radiation shield may be supported on the top PSA layer. The exemplary embodiment of the DTT probe shown inFIG. 10, with the second support assembly embodiment, includes sixteen separate layers. The materials and constructions described for the support assembly and representative dimensions for the layers which are shown in Table I below illustrate the achievement of a disposable DTT probe with a very low vertical profile, an inexpensive construction, and a flexible structure that can adapt to differing contours of various measurement locations on the body of a person.

The second support assembly embodiment illustrated inFIGS. 6-10may be assembled with other elements of a DTT probe system, as illustrated inFIG. 1.

FIGS. 11A,11B,12A, and12B illustrate third and fourth support assembly embodiments. Each of the third and fourth embodiments is characterized by a structure with a single substrate layer formed into a plurality of contiguous sections that are separated into strata on which the thermal sensors are disposed in a spaced-apart relationship. Preferably, the thermal sensors are disposed on two contiguous elongate support members, disposed in a spaced-apart, an opposing or an adjacent relationship, inwardly of the periphery of the support assembly. Preferably, the substrate has the shape of an annulus with a circumferential heater disposed thereon and the two sensor support members projecting inwardly thereof and separated by a thermally insulating layer separate from the substrate. It is desirable to provide a lead support tab projecting outwardly of the annulus and supporting leads for the heater and the thermal sensors.

InFIGS. 11A and 11B, a two-sided, planar sheet601of flexible substrate material is provided and one side of the sheet is coated with a layer of conductive metal such as copper. The copper sheet is etched to form a heater602, thermocouple traces, leads, and pins. Chromel traces, leads, and pins are deposited on the substrate, and a single paddle-shaped section with a disk600is formed by cutting, stamping or machining the planar sheet. The disk600transitions to a tab603for supporting the copper and chromel leads and pins. The heater605is defined along the circular circumference of the disk600by a conductive trace having a triangle wave shape. The heater605surrounds the thermocouple junctions607and608. The thermocouple junctions607and608are aligned with respect to a diameter of the disk600and disposed on either side of its center. The inner section606of the substrate is die cut and removed leaving the heater605supported on an annulus of substrate material and the thermocouple junctions607and608disposed on two thermal sensor support tabs611and613that project inwardly of the annulus, in an opposing relationship. The two tabs611and613are then separated into strata and a film or layer615of thermally insulating material that is separate from the substrate is interposed therebetween, which produces a vertical separation, and inserts a thermal resistance, between the thermocouples. Assignments of the pins inFIG. 11Aare given in Table II.

FIGS. 12A and 12Billustrate a fourth support assembly embodiment similar to the single-layer probe structure of the third embodiment seen inFIGS. 11A and 11B, but with elongated, oppositely-directed thermal sensor support tabs711and713offset in an adjacent relationship on respective sides of the diameter on which the tabs611and613are aligned. This arrangement allows a thicker layer715of thermally insulating material to be placed between the tabs. Thicker insulation decreases the sensitivity of the probe and increases the amplitude of the error signal. This is an advantage because it makes it easier to operate a control algorithm when zero-heat-flux conditions prevail. The disadvantage is that increased thermal resistance increases equilibration time. The pin assignments for the fourth embodiment correspond essentially to those of the third.

The two embodiments ofFIGS. 11A,11B,12A, and12B are for a sensor assembly. A DTT probe with either embodiment may be assembled into a construction with fewer layers than that disclosed inFIG. 10. The expected advantages of these embodiments are 1) ease of construction, 2) minimization of radial temperature differences, 3) minimization of materials, and 4) because each embodiment is based on a single layer, no folding of the substrate is required during assembly or operation, although steps of tab separation and insertion of a thermally insulating layer are necessary. Each of these designs is intended to be disposable; each is designed to use circumferential heating as opposed to full diametric heating.

A support assembly according to any of the four embodiments may be constructed using a substrate constituted of a single double-sided sheet of plastic film such as Kapton® polyimide, Kaptrex® polyimide, polyester or another film of flexible, thermally insulating material. The sheet may be coated on one or both sides with a copper film and various elements such as heaters, copper disks, and copper leads and pins may be made by photo-etching before the support assembly is folded or separated. The sheet may then be die-cut to the required shape and folded or separated as described above. Other metals with high thermal conductivities, like gold or aluminum, may also be used, although copper is preferred because it can form one half of a T-type thermocouple; however, other types of thermocouples are possible, and it may be possible to dispense with metal films altogether if other thermal sensors such as balanced RTD's, thermistors, and/or point junction thermocouples are used to measure temperature. Chromel traces and leads may be formed by deposition, or by peening.

A disposable DTT probe may be easily and inexpensively made using the support assembly construction embodiments described above. Disposability makes the commercialization of a DTT probe possible. Also, a single-use probe limits the potential for cross-contamination and makes it possible for more patients to benefit from perioperative temperature monitoring.

Thus, although the invention has been described with reference to the presently preferred embodiment, it should be understood that various modifications can be made without departing from the spirit of the invention. Accordingly, the invention is limited only by the following claims.