FIELD OF THE INVENTION

The invention relates to a heat-flow sensor, a method of measuring the temperature of a subject using a heat-flow sensor, a temperature sensing arrangement and a heat-flow sensor system.

BACKGROUND OF THE INVENTION

Core body temperature (CBT) is an important vital sign in medical environments. A patient under anesthetic is unable to regulate his/her body temperature, and operating rooms are generally cooled to a low level. Hypothermia occurs when the body core temperature drops below 36°C to a potentially dangerous level. Surgical patients are often hypothermic upon leaving the operating room. Hypothermic patients run the risk of heart complications, especially during the first 24 hours after surgery, since hypothermia acts as a shock to the system. Other problems associated with hypothermia are increased risk of infection and bleeding. For these reasons, the CBT is generally closely monitored during medical procedures, or during long-term monitoring of a recovering patient. Conventional methods can involve intrusive probes (esophageal, rectal, urethral).

There are active heat-flow sensors that require heating elements and control loops to control the heating elements and passive heat-flow sensors that do not require heating elements and thus have the advantage that they are less hazardous to the patient and consume relatively little power.

US 2005/0043631 Al discloses a medical thermometer with a miniature sensor that touches the patient's skin by a spring-loaded probe containing a first sensor, a second sensor and a thermal insulator. The device computes a deep body temperature by using data of probe housing temperature and accounting for multiple responses of skin contact temperature sensor before and after touching the skin. In an embodiment, a heater is provided for heating a plate at the tip that touches the skin and that is in thermal coupling with a first sensor in order to pre-warm and speed up the first sensor.

DE 35 27 942 Al discloses a method and a device for measuring the core body temperature of biological measurement subjects by undertaking the measurement on the surface of the skin by means of an electronic thermometer which has an electronic evaluation circuit, two temperature sensors being arranged in the entire sensor housing in series with a heating resistor and the measurement subject. The heating resistor can be preheated. SUMMARY OF THE INVENTION

It is an object of the invention to provide an improved passive heat-flow sensor for measuring the temperature of a subject. It is a further object to provide a corresponding method of measuring the temperature of a subject using a heat-flow sensor, a temperature sensing arrangement and a heat-flow sensor system.

In a first aspect of the present invention a heat-flow sensor is presented comprising

a contact face for placement on a subject during a temperature monitoring procedure;

a thermistor arrangement including an inner thermistor, an outer thermistor arranged relative to the inner thermistor to measure a heat flow outward from the subject and an insulating material arranged between the inner thermistor and the outer thermistor and a pre-heating arrangement for pre-heating the insulating material of the thermistor arrangement before performing a measurement.

In a further aspect of the present invention a temperature sensing arrangement for monitoring the temperature of a subject is presented comprising

a heat-flow sensor as disclosed herein; and

an evaluation unit arranged to receive temperature measurement values from at least one combined thermistor arrangement of the heat-flow sensor and to calculate the temperature of the subject on the basis of the received temperature measurement values.

In a still further aspect of the present invention a method of measuring the temperature of a subject using a heat-flow sensor as disclosed herein is presented comprising the steps of

triggering pre-heating of the insulating material of the thermistor arrangement by the pre-heating arrangement,

- placing the contact face of the pre-heated heat-flow sensor on the subject during a temperature monitoring procedure;

receiving temperature measurement values collected by the thermistor arrangement of the heat-flow sensor; and calculating the temperature of the subject on the basis of the received temperature measurement values.

Preferred embodiments of the invention are defined in the dependent claims. It shall be understood that the claimed method, temperature sensing arrangement and heat- flow sensor system have similar and/or identical preferred embodiments as the claimed sensor, in particular as defined in the dependent claims and as disclosed herein.

The present invention is based on the idea to pre-heat the insulating material of the thermistor arrangement before a measurement is actually performed, e.g. before the sensor is applied on the patient to reduce the warm-up time needed with passive heat-flow sensors, which conventionally require a rather long warm-up time before the sensor shows an accurate reading. For instance, with embodiments of a conventional sensor the user, e.g. a caregiver or medical staff, only gets a proper reading after about 10 minutes, which is too long in daily clinical practice. This time is thus drastically shortened, e.g. to only one minute or a few minutes, by use of the present invention. Additionally, the pre-warmed sensor is more comfortable to the patient when attached to the skin. In other applications, the heating may only be started when the sensor is applied, which would also be beneficial, i.e. reduce the time to start a proper measurement.

With such a heat-flow sensors, during a temperature monitoring interval, the temperatures of the two thermistors are observed. The deep body temperature Tab (core body temperature) can be expressed as:

where Tl is the temperature at the inner thermistor; T2 is the temperature at the upper thermistor; RV is the thermal resistivity between inner thermistor and outer thermistor; and RB is the thermal resistivity of the body to which the sensor is applied, for example skin thermal resistivity. The thermal resistivity of a patient's skin can be estimated, or an already established value can be used by default.

In the context of the invention, the term "subject" can relate to any living being or any non-living object, i.e. the term "subject" is not limited to persons or any other living beings. Critical thermal conditions generally arise in the context of operative situations, emergency medical situations, etc. in which a human patient may enter a state of hypothermia or hyperthermia. Therefore, without restricting the invention in any way, the term "patient", when used hereinafter, it shall be generally understood as "subject" with the above mentioned meaning including living beings and non-living objects. While the principle of the invention can be used in an active sensor (which uses a heating element controlled to bring the sensor to a zero heat-flux state), it is particularly suited to use in a passive heat-flow sensor. Therefore, without restricting the invention in any way, the term "heat-flow sensor" as used in the following in the context of the invention may be assumed to refer to a passive heat-flow sensor.

A thermistor is a device whose electrical resistivity changes in response to a change in temperature, and can be embedded in the material of a heat-flow sensor. A thermistor can be realized as a component with two electrical connectors so that it can be included in an appropriate circuit. A temperature change is registered as a change in current or voltage, depending on the circuit realization. A thermistor component can also be realized as a compact integrated circuit (IC) device.

In a preferred embodiment the pre-heating arrangement is configured to preheat the insulating material of the thermistor arrangement to a temperature in the range of 30° to 38°C, in particular in the range of 33° to 37°C. This provides that the sensor can be used without any or without any substantial delay after application to the patient for making measurements.

In another embodiment the pre-heating arrangement comprises a chemical reaction unit configured to generate heat through chemical reaction, in particular through exothermic reaction, in response to a trigger. Such a chemical reaction is e.g. used in hand warmers which are held in the hand and produce heat on demand to warm cold hands. The technology and the means used in such hand warmers can be used in the sensor for providing the pre-heating. A trigger may hereby be a user interaction, such as pressing a button, actuating a switch, breaking a seal, breaking a plate, opening a packaging of the sensor, taking a sensor out of the shelf or storage, etc.

Preferably, the exothermic reaction unit is configured to generate heat through exothermic oxygenation or crystallization in response to a trigger. For generating heat through exothermic crystallization supersaturated solutions (typically sodium acetate) may be used. The release of heat may be triggered by flexing a small metal disk, which generates nucleation centers that initiate crystallization. Heat is required to dissolve the salt in its own water of crystallization and it is this heat that is released when crystallization is initiated. For generating heat through exothermic oxygenation cellulose, iron, water, activated carbon, vermiculite (water reservoir) and salt (catalyst) may be used. The release of heat through the exothermic oxidation of iron may be triggered by exposing the iron to air, e.g. when the sensor is taken out of a shelf or is unwrapped or its package is removed. Further, the pre-heating arrangement may comprise an electric heating unit, e.g. including an electrical resistor and a battery for providing a current through the resistor upon actuating a button or switch.

In another embodiment the device further comprises a trigger unit allowing a user to trigger the pre-heating by the pre-heating arrangement. As mentioned above, said trigger unit may comprise a button, a switch, a breakable element like a seal, stick or disk, which is integrated into the sensor or arranged on the outer surface.

The sensor may further comprise a lateral thermistor arranged relative to the inner thermistor to measure a horizontal heat flow along the contact face. The inner thermistor and the outer thermistor preferably measure a vertical heat flow outward from the subject, i.e. heat flow between the inner thermistor and the outer thermistor. The inner thermistor and the lateral thermistor measure a horizontal heat flow along the contact face, i.e. heat flow between the inner thermistor and the lateral thermistor.

An advantage of this embodiment of the heat-flow sensor is that the combination of a lateral heat flow monitor with the usual vertical heat flow monitor allows a much more precise temperature measurement, particularly since the lateral heat flow is explicitly measured, instead of only being estimated (as is the case for conventional heat-flow sensors). This can be very advantageous in situations for which a precise temperature monitoring is required, for example to provide medical personnel with precise information regarding a patient's core body temperature, for example in an emergency situation requiring rapid decision-making. Further, the core body temperature of the subject - for example a patient during and after surgery - can be determined to a much greater degree of precision, so that critical situations such as hypothermia can be detected and dealt with in a timely manner.

In this context, it shall be noted that the geometrical terms used herein relate to an assumed horizontal plane representing the outer surface of the subject and serve only to define a reference space. A "horizontal heat flow" is therefore any heat flow along the surface of the subject, and a "vertical heat flow" is any heat flow outward from the surface of the subject. It shall be understood that, in reality, a heat-flow sensor can assume any orientation when attached for example to the skin of a patient.

In the context of the invention, the arrangement of one thermistor relative to another is to be understood to mean that these two thermistors are essentially aligned in the direction along which a heat flow is to be measured. Here also, the geometrical terms "upper", "inner" and "outer" are used in the context of the reference space. Therefore, it will be understood that the inner thermistor is arranged towards a central region of the heat-flow sensor, and the outer (or upper) thermistor is arranged towards an outer region of the sensor and in line with the inner thermistor to measure a heat flow outward from the subject. The outward heat flow is in a direction from the inner thermistor to the outer thermistor when the subject is warmer than the sensor; when the subject is cooler than the sensor the heat flow direction is in reverse. Similarly, it will be understood that a lateral thermistor is arranged towards a side of the sensor and in line with the inner thermistor to measure a heat flow along the surface of the subject, for example along the patient's skin. The lateral heat flow is in a direction between the inner thermistor and the lateral thermistor and serves to detect any difference in temperature between the inner sensor region and the side of the sensor containing the lateral thermistor.

In a further embodiment the pre-heating arrangement comprises a current input for receiving and/or a current supply for supplying an electric current for supply of at least one of the inner thermistor and the outer thermistor to pre-heat the insulating material of the thermistor arrangement, said at least one of the inner thermistor and the outer thermistor representing the pre-heating arrangement. The inner thermistor and/or the outer thermistor are thus used not only for making measurements, but also to pre-heat the thermistor arrangement in advance of making measurements. The current may be supplied from an external current source, e.g. as provided in a storage or shelf, or from an internal current supply, such as a small battery or an arrangement that generates sufficient current in response to a trigger, e.g. through a chemical reaction.

The sensor may further comprise a data output for providing measurement data acquired by the thermistor arrangement. The data output may be a wireless or wired interface for providing the measurement data of the thermistor to another entity including an evaluation unit for further processing, such as a patient monitor, laptop, workstation, smartphone, etc., which may be located close to the patient (e.g. next to the patient bed) or at a remote distance (e.g. in the nurse room, at the doctor's office, etc.). The sensor and the evaluation unit are thus part of a temperature sensing arrangement, which may include further elements.

Preferably, a common interface is used as current input and data output, which saves hardware equipment.

The pre-heating arrangement is preferably arranged within the insulating material between the inner thermistor and the outer thermistor. This improves the pre-heating effect and particularly ensures that the insulating material between the thermistors is preheated, which contributes to a faster operational readiness of the sensor. Preferably, material with a low specific heat is used as insulating material, i.e. a material for which a low amount of energy is needed to heat up a certain mass of the material by 1°C. Generally, the insulating material is higher in volume and mass than the small thermistors. When the insulating material is warmed up, the thermistors will follow quickly. To get the sensor at an

equilibrium phase, both the sensors and partly the insulating material will have a higher temperature compared to the environmental temperature. The insulating material may e.g. be a foam, which is inexpensive and has a high thermal resistance (so that the sensor can be thin). Other materials are possible, such as silicone.

The heat-flow sensor is preferably realized as a wearable dispensable device, e.g. in the form of a sticker that can be applied to the patient's skin by a sticky film provided on the contact surface of the sensor. A patient can thus wear the heat-flow sensor for a long- term temperature monitoring interval.

In a still further aspect of the present invention a heat-flow sensor system is presented comprising

- a plurality of heat-flow sensors each including a contact face for placement on a subject during a temperature monitoring procedure and a thermistor arrangement including an inner thermistor, an outer thermistor arranged relative to the inner thermistor to measure a heat flow outward from the subject and an insulating material arranged between the inner thermistor and the outer thermistor and

- a dispenser including a storage for storing a plurality of the heat-flow sensors and a heating arrangement for heating the heat-flow sensors stored in the storage.

In such a system, the pre-heating of the sensor, in particular of the thermistor arrangement, is particularly provided by a heating arrangement, e.g. an electrical heater, provided in the dispenser in which the sensors are stored before use. This provides basically the same effect as explained above for the sensor including the pre-heating arrangement.

The thermal resistivity of the intervening path between two thermistors (inner thermistor and outer thermistor; inner thermistor and lateral thermistor) is determined by structural properties of the thermistor arrangement such as the material of the sensor and the thickness of the sensor. The thermal resistivity can be measured and can be a known quantity.

For accurate temperature sensing, any inner thermistor of the heat-flow sensor is preferably close to or coincident with the contact face of the sensor. Similarly, any lateral thermistor of an enhanced thermistor configuration is preferably positioned towards an outer region of the sensor and also close to or coincident with the contact face of the sensor. Any outer thermistor is preferably close to the "uppermost" surface of the sensor, i.e. its outside surface when attached to the subject.

In a preferred embodiment, the thermistors can be connected via wire connections to an evaluation unit. For example, temperature measurement values can be received by an evaluation unit connected to the sensor by a cable connection. In another preferred embodiment, the sensor can be equipped with an interface for transmitting the temperature measurement values wirelessly to the evaluation module. The sensor may also incorporate an analog-to-digital converter to convert analogue measurement values into digital values for data transmission.

An evaluation unit of the temperature sensing arrangement can preferably be realized as a portable device. For example, the patient or any medical personnel can use a hand-held device with a display such as a tablet computer or smartphone to observe temperature development. In a wearable realization, results of temperature monitoring can be shown on the display of a smart watch or similar device.

In a further preferred embodiment, the temperature sensing arrangement can be incorporated in a patient support device such as an operating table in a surgical operating theatre, a mattress of a hospital bed, an infant sleeping bag or incubator of a neonatal ward, etc.

In another embodiment, a single inner thermistor is used, and this is connected to an outer thermistor and also to a lateral thermistor to achieve the favorable side compensation for accurate estimation of deep body temperature. During a temperature monitoring interval, the temperatures of the three thermistors are observed. The deep body temperature Tdb (core body temperature) can be expressed as:

T1 - T2 T1 - T3

T _db = n + RV RH

(1) where Tl is the temperature at the inner thermistor; T2 is the temperature at the outer thermistor; T3 is the temperature at the lateral thermistor; RV is the "vertical" thermal resistivity between inner thermistor and outer thermistor; and RH is the "horizontal" thermal resistivity of the electrical connection between inner thermistor and lateral thermistor. RB is the thermal resistivity of the body to which the sensor is applied, for example skin thermal resistivity. The thermal resistivity of a patient's skin can be estimated, or an already established value can be used by default. An inner thermistor, common to both vertical and lateral thermistor pairs of an enhanced thermistor configuration, can be located near the center of the sensor, preferably as close as possible to the contact surface. This arrangement may be preferred for a practical realization of the heat-flow sensor that comprises only a single enhanced thermistor configuration. Such an "enhanced single heat-flow sensor" can provide temperature measurements relating to the outward heat flow from the patient, enhanced or augmented by temperature measurements in one lateral direction along the patient's skin. This configuration already enables a relatively accurate estimation of the patient's core body temperature.

The heat-flow sensor may further comprise only such enhanced thermistor arrangements. These can be separate and distinct from each other. However, in a preferred embodiment, the enhanced thermistor arrangements share a single inner thermistor and a single outer thermistor. This enhanced single heat-flow sensor measures heat flow in one vertical direction through the inner and outer thermistors, and augments the vertical heat flow information by additional information obtained by measuring heat flow in several sideways or lateral directions, whereby each lateral direction effectively passes through the inner thermistor and one lateral thermistor. By incorporating more than one lateral thermistor, it is possible to monitor heat flow in more than one lateral direction, allowing a better estimation of the thermal behavior of the area under the sensor contact surface.

In another preferred embodiment, in addition to its enhanced thermistor arrangement(s), the proposed heat-flow sensor may comprise a separate vertical thermistor arrangement with a further inner thermistor and a further outer thermistor arranged relative to that inner thermistor to measure a further vertical heat flow outward from the subject. This additional vertical thermistor arrangement is functionally independent of any combined thermistor arrangement, and such an embodiment may be referred to as an enhanced dual heat-flow sensor. Preferably, the vertical thermistor arrangement is positioned centrally in the heat-flow sensor. A centrally positioned and independent vertical thermistor arrangement can be flanked by a plurality of equidistantly arranged enhanced thermistor configurations, for example.

In an embodiment comprising a vertical thermistor arrangement and one enhanced thermistor arrangement, it is not necessary to know the thermal resistivity of the skin, since this term cancels out of the equation for deep body temperature Tdb, which is now expressed as:

_ T\(TV\ - TV2) + K ^■ TV\(T2 - T\) + L - TV\(T2> - T\)

d ^{b ~} TVl - TV2 + K(T2 - Tl) + L(T3 - Tl) where TV1 is the temperature at the inner thermistor of the vertical thermistor arrangement and TV2 is the temperature at the outer thermistor of the vertical thermistor arrangement; Tl is the temperature at the inner thermistor of the enhanced heat-flow thermistor arrangement,

T2 is the temperature at the outer thermistor of the enhanced thermistor arrangement, and T3 is the temperature at the outer or lateral thermistor of the enhanced thermistor arrangement. K and L are scalar values. The value K is expressed as:

_{K =} (TV\ -TV2)(T\ -T _db )

( - 37Ί + T2 + 2T3 )(TVl - T _db )

The scalar value L is a ratio and can be expressed as:

L =≡l (4)

RH

where RV2 is the thermal resistivity between inner and outer thermistors of the vertical thermistor arrangement; and RH is the thermal resistivity between inner and lateral thermistors of the enhanced thermistor arrangement.

An advantage of measuring heat flow in more than one lateral direction is that it allows a precise temperature measurement even if the heat-flow sensor is not ideally or optimally in position. It can often be difficult to exactly determine a correct or ideal sensor placement, for example when a sensor is to be placed over the carotid artery. A slightly "off- center" placement could result in significant errors in temperature measurements when a conventional heat-flow sensor is used. A proposed heat-flow sensor with several enhanced thermistor configurations provides several candidate temperature measurement values, from which a more precise core body temperature can be deduced. For example, in a preferred embodiment, temperature measurements are received from a plurality of combined thermistor arrangements, and the temperature measurements are averaged before calculating the temperature of the subject. Equally, the maximum temperature value reported by a thermistor may be used for the calculation of lateral and vertical flows.

Another advantage of using several enhanced thermistor configurations is given by the ability to identify an enhanced thermistor configuration that is sub-optimally placed to measure temperature. The proposed method preferably comprises the steps of comparing temperature measurements from a plurality of enhanced thermistor

configurations; identifying an enhanced thermistor configuration that is providing unreliable temperature measurement values; and discarding those temperature measurement values.

For example, a situation might arise in which the sensor is not ideally attached to the skin of the patient. In a heat-flow sensor with three or more equidistantly arranged enhanced thermistor configurations, any significant difference between the values delivered by the enhanced thermistor configurations can be identified. If one of the enhanced thermistor configurations delivers vales that are significantly different from the values delivered by the other enhanced thermistor configurations, and if the values delivered by the other enhanced thermistor configurations are relatively similar, this would indicate that the sensor is not adequately attached to the skin. Any thermistor delivering outlier or non- conformant values is preferably disregarded so that its data does not falsify computation of the core body temperature.

In both sensors when using the side thermometer the estimated CBT is less sensitive to environmental changes and gives good results, whereas the basic single and dual heat-flow CBT sensors without a side thermometer are sensitive to the environmental changes. Including a side thermistor improves the estimated CBT during a variation of the core body temperature as well as during a variation of environmental changes.

Other objects and features of the present invention will become apparent from the following detailed descriptions considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for the purposes of illustration and not as a definition of the limits of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic representation of a first embodiment of a temperature sensing arrangement including a first embodiment heat-flow sensor according to the present invention;

Fig. 2 is a schematic representation of a second embodiment of a temperature sensing arrangement including a second embodiment heat-flow sensor according to the present invention;

Fig. 3 is a schematic representation of a third embodiment of a temperature sensing arrangement including a third embodiment heat-flow sensor according to the present invention;

Fig. 4 is a schematic representation of a fourth embodiment of a temperature sensing arrangement including a fourth embodiment heat-flow sensor according to the present invention;

Fig. 5 is a schematic representation of a fourth embodiment of a temperature sensing arrangement including a fifth embodiment heat-flow sensor according to the present invention; Fig. 6 is a schematic representation of a heat-flow sensor system according to the present invention; and

Fig. 7 is a schematic representation of an embodiment heat-flow sensor for use in a heat-flow sensor system shown in Fig. 6.

In the drawings, like numbers refer to like objects throughout. Objects in the diagrams are not necessarily drawn to scale.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Fig. 1 shows a schematic representation of a first embodiment of a heat-flow sensor 1 according to the present invention in a first embodiment of a temperature monitoring arrangement 10. The heat-flow sensor 1 comprises a contact face 11 for placement on a subject 8, for example for securely fixing it to the skin of a patient 8 during a temperature monitoring procedure. Further, a thermistor arrangement 20 is provided including an inner thermistor 21, an outer thermistor 22 arranged relative to the inner thermistor 21 to measure a heat flow outward from the subject 8 and an insulating material 24 arranged between the inner thermistor 21 and the outer thermistor 22 to thermally (and also electrically) insulate them from each other. For pre-heating the insulating material 24 of the thermistor arrangement 20 before performing a measurement a pre-heating arrangement 30 is provided.

For triggering the pre-heating operation directly at the sensor 1 an (option) trigger unit 40 is provided, e.g. a button that can be pressed to switch on the pre-heating arrangement. Further, for outputting the measured signals an output unit 50, e.g. a wireless or wired data interface, is provided.

The inner thermistor 21 is preferably arranged at an inner face of the sensor 1, and will lie in close contact to the patient's skin. The outer thermistor 22 is preferably arranged at the upper surface of the sensor 1. The thermal resistivity of the sensor 1 in the

"vertical" direction may be indicated by RV. RB indicates the thermal resistivity of the body 8 to which the sensor 1 is attached.

Obtaining a temperature measurement at any one point in time using the sensor 1 involves collecting the temperature measurement values Tl, T2 from the thermistors 21, 22 respectively (i.e. thermistor 21 delivers temperature measurement value Tl and thermistor 22 delivers temperature measurement value T2) and calculating a sensed temperature using knowledge of the heat flux through the sensor 1. To compute the sensed temperature using the heat-flow sensor 1 , it is also necessary to determine or estimate the thermal resistivity R _B of the skin, which may vary from patient to patient. The sensed body temperature Tdb may be calculated using equation (0) as already described above. To this end, the measurement values collected by the thermistors 21 , 22 are provided (e.g. sent or retrieved) to an evaluation unit 2 of the temperature monitoring arrangement 10, for example over a cable connection or wirelessly. A microprocessor 3 of the evaluation unit 2 performs the necessary computations to arrive at the body temperature. A display 4 can show core body temperature development as time progresses. While the diagram only indicates one outer thermistor, any number of outer thermistors can be implemented by such a heat-flow sensor.

Before actually performing a measurement, in particular before applying the sensor 1 at the patient's skin, the thermistor arrangement 20 is pre-heated by use of the preheating unit 30, which is in this embodiment started by actuating the trigger unit 40, e.g. by pressing a button. This provides that the thermistor arrangement 20, in particular the insulating material 24, is pre-heated so that the temperature profile required for making actual measurements with the sensor is reached quickly. A considerable waiting time (e.g. up to 10 minutes) as required with conventional sensors after fixation of the sensor to the patient's skin until the required temperature profile has been reached is thus no longer needed, but the sensor can be used directly after a short pre-heating period and subsequent application to the patient's skin. Further, this is more comfortable to the patient if the sensor is warm rather than cold as is the case with known sensors when they are fixed to the patient's skin.

Fig. 2 shows a schematic representation of a second embodiment of a heat- flow sensor la according to the present invention in a second embodiment of a temperature monitoring arrangement 10a. In this embodiment the heat-flow sensor la comprises a chemical reaction unit 31 that generates heat through chemical reaction in response to actuation of the trigger 40. Generally, the same principle as used in hand warmers may be applied for this purpose. For instance, supersaturated solutions which crystallize and produce heat after a specific trigger may be used. Hence, when the user, e.g. clinical personnel, obtains a passive heat flux sensor from the shelf, they need to trigger the exothermic crystallization by, for example, pressing a button or flexing a small metal disk (serving as trigger unit 40). The sensor material is then quickly heated to an approximate temperature of preferably 37° C, in particular to a temperature in the range of 30° to 37°C. The amount of heat produced by the crystallization and the thermal properties of the sensor material, in particular the insulating material 24, are preferably tuned on each other, i.e. the heat production has to be tuned due to heat sink of the temperature sensor in order to make sure the pre-warmed phase does preferably not exceed 37°C. Since the sensor is quickly warmed up, an accurate reading of the core body temperature can be swiftly obtained by use of the sensor la.

Generally, the exothermic reaction unit 31 is configured to generate heat through exothermic oxygenation or exothermic crystallization in response to the trigger. Hence, various principles to generate heat through chemical reaction may be applied.

In this embodiment, the output unit 50 comprises a wireless interface for wireless transmission of the temperature measurement values Tl, T2 to the evaluation unit 2, which can be realized in a hand-held device such as a smartphone or tablet computer with a display 4. A microprocessor 3 of the hand-held device can compute the core body

temperature as described above.

Fig. 3 shows a schematic representation of a third embodiment of a heat-flow sensor lb according to the present invention in a third embodiment of a temperature monitoring arrangement 10b. In this embodiment the sensor lb comprises a current input 51 for receiving an electric current I from an external current supply 5, e.g. a battery or the external device 2. The pre-heating arrangement 30b comprises an electric heating unit 32, to which the externally provided current is supplied to pre-heat the thermistor arrangement 20. To start pre-heating the user may simply switch on the external current supply 5.

Fig. 4 shows a schematic representation of a fourth embodiment of a heat-flow sensor lc according to the present invention in a fourth embodiment of a temperature monitoring arrangement 10c. In this embodiment the sensor lc comprises a common interface 53 which is alternately used as current input (during the pre-heating phase) and data output (during the measurement phase). In the pre-heating phase an external current, e.g. provided from a hand-held device, is supplied to at least one of the inner thermistor 21 and the outer thermistor 22 to pre-heat the thermistor arrangement 20. In other words, the thermistors 21, 22 can have a second function besides measuring the temperature. The electronic evaluation unit 2 can also be used to apply a current through the thermistors 21, 22. In this way heat will be produced in the thermistors (for a predetermined time) and will act as a heating element so that the sensor temperature will increase. After applying the sensor lc at the patient the externally provided current can be switched off automatically and a reliable read-out of the core body temperature can be obtained.

Fig. 5 shows a schematic representation of a fifth embodiment of a heat-flow sensor Id according to the present invention in a fifth embodiment of a temperature monitoring arrangement lOd. In this embodiment thermistor arrangement 20 of the sensor Id comprises an additional lateral thermistor 23 arranged relative to the inner thermistor 21 to measure a horizontal heat flow along the contact face 11. Thus, a third temperature measurement value T3 is obtained from the lateral thermistor S3 which is additionally used for calculating a sensed temperature using knowledge of the heat flux through the sensor Id. using e.g. equation (1) as already described above. Such a combination of a lateral heat flow monitor with the usual vertical heat flow monitor allows a much more precise temperature measurement, particularly since the lateral heat flow is explicitly measured, instead of only being estimated. The core body temperature of the subject can thus be determined to a much greater degree of precision, so that critical situations such as hypothermia can be detected and dealt with in a timely manner.

The heat flow sensor Id further comprises an internal current supply 52, e.g. a small battery, for supplying electrical current to the pre-heating unit 30b, which may be an electric heating element, or (not shown in this figure) to the thermistors 21, 22, 23 to use the thermistors as pre-heating unit as explained above. The current supply 52 may be controlled through a trigger 40 by the user, e.g. a button or switch.

The pre-heating unit 30d is arranged inside the insulating material 24, which provides the advantage of an even further improved heating effect, particularly of the insulating material, which is advantageous to make the sensor Id ready for use quickly.

Fig. 6 is a schematic representation of a heat-flow sensor system 100 according to the present invention. The heat-flow sensor system 100 comprises a plurality of heat-flow sensors 110 and a dispenser 120 including a storage 121 for storing a plurality of the heat-flow sensors 110 and a heating arrangement 122 for heating the heat-flow sensors 110 stored in the storage 121. The heat flow sensors 110 thus do generally not need to include (but may include) a pre-heating unit as provided in the above explained

embodiments, but may be constructed in a conventional manner.

A simple heat flow sensor 110 is shown in Fig. 7. It includes a contact face 11 for placement on a subject 8 during a temperature monitoring procedure and a thermistor arrangement 20 including an inner thermistor 21 and an outer thermistor 22 arranged relative to the inner thermistor 21 to measure a heat flow outward from the subject 8. Further, an optional lateral thermistor 23 is also shown. Further elements, such as further thermistors and/or an insulating layer, may additionally be provided. For illustration, the above mentioned thermal resistivities are also shown including RV, RH and RB.

Thus, the pre-heating of the heat-flow sensors 110 stored in the storage 121 of the dispenser 120 is provided by the heating arrangement 122, e.g. an electrical heating unit. A heat-flow sensor 110 taken out of the dispenser 120 through the opening 123 is thus pre- heated to the right temperature and is ready to use without any delay. A dispenser does hence not only hygienically store the sensors and allow easy and quick access to the sensors e.g. for the busy clinical staff, but also keeps them at an appropriate temperature of e.g. 37° C.

Whenever the clinical personnel need a passive heat flux sensor, they get one from the dispenser. As the sensor comes out, it warmed at a convenient 37° C. Quickly after applying the sensor at the patient a reliable read-out of the core body temperature can be obtained.

The final estimated core body temperature depends to a large extent on the geometry and thermal conductivity of the sensor. Experimental results have shown that even during sub-optimal conditions, embodiments of the proposed sensor perform very well. This improves the ability to identify a tendency towards hypothermia or hyperthermia so that preventive measures can be taken to avoid a critical situation. Further, the improvement in accuracy of the proposed embodiments of the heat-flow sensor having a lateral thermistor is because it considers lateral heat flow also, and is therefore significantly less sensitive to variations in ambient temperature.

One application area for the proposed sensor is CBT measurement in living beings, in particular humans, but also animals. This could be done by integrating a sensor in a surgical table and performing measurements of core body temperature while the patient is under anesthesia. This can also be done where the sensor is used in a wearable patch. The low need for power makes it an attractive option for wearable computing.

Further, the present invention may also be used for measuring temperature of non-living objects, such as liquids in pipes when there is no easy way to access the liquid. This can be used in oil industry for measuring oil temperature. It could also be used in water supply structures in order to determine the temperature of water or in liquid heating and cooling systems in order to determine the temperature of the liquids.

The sensor may further be used in a variety of situations where temperature information of areas that are not easily accessible is required. Still other application scenarios are e.g. temperature monitoring at the ICU or general ward.

Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention. For example, any suitable sensor shape may be used and the various elements of the different embodiment may also be used in other combinations. Further, different numbers of vertical and lateral thermistors can be incorporated in various embodiments of the proposed heat-flow sensor. As described above, calculation of core temperature can be performed on the sensor or can be performed remotely. Results can be displayed locally (on a screen) or remotely on a smart watch, mobile phone or the display of any other suitable device.

For the sake of clarity, it is to be understood that the use of "a" or "an" throughout this application does not exclude a plurality, and "comprising" does not exclude other steps or elements. The mention of a "unit" or a "module" does not preclude the use of more than one unit or module.