CHÉTELAT, Olivier (Route de Champréveyers 2, Hauterive, CH-2068, CH)
NEUMAN, Victor (Route des Jopesses 4, Cormondrèche, CH-2036, CH)
BERTCHI, Mattia (Ch. Du Flonzel 39, La Conversion, CH-1093, CH)
CHÉTELAT, Olivier (Route de Champréveyers 2, Hauterive, CH-2068, CH)
NEUMAN, Victor (Route des Jopesses 4, Cormondrèche, CH-2036, CH)
| Claims 1. A sensor (1) for measuring a core body temperature of a user, comprising: a first temperature probe (2) to be placed against the skin (3) of the user for measuring a first temperature (7C); a second temperature probe (6) at a distance from the first temperature probe (2), for measuring a second temperature (Td) and; a thermal insulator (4) provided between the first and second temperature probes (2, 6) for thermally insulating first and second temperature probes (2, 6) from each other; characterized in that the thermal insulator (4) embeds the first temperature probe (2), forming a contact surface (7) extending in the plane of the first temperature probe (2) and destined to be in contact engagement with the skin (3); and in that the sensor (1) further comprises a thermally conductive shield (5) covering at least portion of the thermal insulator (4), the second temperature probe (6) being in contact with the conductive shield (5). 2. The sensor (1) according to claim 1, wherein the second temperature probe (6) is in contact with a portion (9) of the thermal shield (5) arranged to be in contact with the skin (3). 3. The sensor (1) according to the claims 1 or 2, wherein the the second temperature probe (6) is an absolute temperature sensor and the first temperature probe (2) is a thermocouple with its reference at a location at a substantially identical temperature as the one measured by the second temperature probe (6). 4. The sensor (1) according to any of the claims from 1 to 3, further comprising a perfusion probe for measuring a perfusion parameter depending on the level of blood perfusion of the skin. 5. The sensor (1) according to any of the claim from 1 to 4, wherein said skin (3) comprises an outer skin region (1 1) having an outer and surface thermal resistance (h, r2) and an inert region (12) having an inert thermal resistance (R), and wherein the thermal conductivity (MS) of thermal insulator (4) is chosen such that the resistance ratio term ^ R T1J) is constant. 6. A sensor system (20) comprising the sensor (1) characterized by any of the claims from 1 to 5 and a control device controlling the operation of the sensor (1). 7. The sensor system (20) according to claim 6, wherein said control device is located on a first PCB (21) mounted in the sensor (1). 8. A method for measuring a core body temperature (Tb) of a user using the sensor (1) characterized by any of the claims from 1 and 5, comprising: providing the sensor (1) on the user such that the first temperature probe (2) and the contact surface (7) of the thermal insulator (4) contact the skin (3); measuring the first and second temperature (T0 Td) with the first and second temperature probe (2, 6), respectively; and determining the core body temperature (Tb) from the first and second measured temperatures (T0 Td). 9. The method according to claim 8, wherein the skin (3) comprises an outer skin region (1 1) having an outer and surface thermal resistance ri) and an inert region (12) having an inert thermal resistance (R), and wherein said determining the core body temperature (Tb) is performed using Equation (5): Tb b = Tcc + (KTcc - Td a)j ^ R rJ) . 10. The method according to claim 9, wherein the resistance ratio term ( \^S+r21— R r1JJ in Eq Muation ( v5) ' is assumed to be constant. 11. The method according to the claims 9 or 10, wherein the sensor (1) further comprises a constant heat source above the thermal insulator (4), and wherein said determining the core body temperature (Tb) is performed using Equation (5) to which a term -βQ is added, where β being a constant and Q the power of the constant heat source. 12. The method according to claim 8, wherein said determining the core body temperature (Tb) is performed using Equation (7): Tb = F(s)Tc + G(s)Td + C where F(s) and G(s) are transfer functions chosen so as to compensate for dynamic change in parameters of the system formed by the sensor (1) and the skin (3), and C is a constant or a signal determined by the dissipated power on the thermally conductive shield (5) chosen so as to cancel the effect of said dissipated power on (Tb). 13. The method according to any of the claims from 9 to 12, wherein the sensor (1) further comprises a perfusion probe for measuring a perfusion parameter, and wherein the method further comprises a step of measuring the perfusion parameter and using this parameter for determining a value for the resistance ratio term in Equation (5). |
Reference data
This application claims priority to United States provisional patent application, Serial No. 61/228,797, filed on June 27, 2009. Priority to the provisional application is expressly claimed, and the disclosure of which is hereby incorporated herein by reference in its entirety and for all purposes.
Field
The present disclosure concerns the measurement and monitoring of physiological parameters and more particularly to a sensor device and a method for measuring and determining a core body
temperature of a user.
Background
When determining the internal temperature of an object, or core body temperature, heat flux through the surface layer of the object depends not only on the difference between the temperature of the object and the ambient temperature but also on the thermal conductivity of the object, the thermal conductivity being a characteristic of the material of which the object is made. For physiological temperature measurement, the object thermal conductivity is not only unknown and anisotropic but also variable in time, as a function of perfusion, for instance. Known solutions are implemented, for example, by thermometers measuring the
temperature of a human being from the surface of the skin or by
thermometers measuring the temperature of the contents of a process container from the surface of the container. Normal human body temperature varies slightly from person to person and by the time of day. Consequently, each type of measurement has a range of normal temperatures. The range for normal human body temperatures, taken orally, is 36.8±0.7°C. Temperature in the anus, vagina, or in the ear is about 37.6°C, in the mouth is about 36.8°C, and in the armpit is about 36.4°C. Core body temperature is the operating
temperature of an organism, specifically in deep structures of the body such as the pulmonary artery, in comparison to temperatures of peripheral tissues. The unobtrusive and non invasive monitoring of this physiological parameter is the object of intense research since more than 40 years but the gold standard is still the classical rectal temperature, a rather obtrusive method, especially for continuous ambulatory monitoring. A common method of measurement includes rectal temperature.
This measurement method is however invasive. Oral temperature
measurement is also often used but can be affected by eating, hot or cold drinks, and mouth-breathing. Ear temperature measurement, such as eardrum temperature measurement using infrared sensors is not accurate. Skin temperature measurement can be influenced by medication, clothing, emotion, ambient temperature, etc. Moreover this latter method is not always reliable, especially due to core body temperature variations, as will be discussed below. Table 1 summarizes the different core temperature measurement methods and ranks their respective advantages comprising robustness, comfort, accuracy, and safety, using a ranking number comprised between 1 and 4, the higher the better.
Fig. 1 schematically illustrates a known sensor 1, in a single-probe configuration, for measuring the core body temperature Tb comprising a first temperature probe 2 placed in contact with the skin 3 of a user. The first temperature 7 " i measured by the first temperature probe 2 corresponds to the temperature at the surface of the skin 3. As shown in Fig. 1, the skin 3 can be modelled by comprising an outer skin region 11 characterized by an outer thermal resistance r having a resistance value that varies with the perfusion of the outer skin region 11. The skin 3 further comprises an inert tissue region 12 showing no significant variation in perfusion and which can be characterized by a constant thermal inert resistance R and a core body region 13 being essentially at the core body temperature. The determination of the core body temperature Tb thus depends on the values of the outer thermal and thermal inert resistances r, R. Moreover, the determination of Tb further depends on a heat exchange resistance A modelling the heat exchange with the ambient air as shown by Equation 1 :
RT
T b = T 1 + (T 1 - T a ) j^ (Equation 1 ).
An ambient temperature probe (not shown) can be used to measure an ambient temperature T 3 ,, as shown in Fig. 1. The measured first temperature 7 " i can then be correlated to the core body temperature Tb assuming that the heat exchange resistance A 1 the outer thermal and inert thermal resistances r, R are constant. However, if the inert thermal resistance R can be reliably assumed to be constant, the outer thermal resistance r tends to vary significantly in time, since the skin 3 assists the human thermoregulation mainly through variable perfusion and sweat. Moreover, the heat exchange resistance A is unpredictable. Consequently, if the measuring method described above is simple, it has low reliability. Table 2 summarizes the advantages of the above method where the different advantageous characteristics are rated by a number comprised between 1 and 4, the higher the better.
Robustness Comfort Accuracy Safety
1 4 1 4
Table 2
Fig. 2 illustrates another configuration of the sensor 1, where a second temperature probe 6 for measuring a second temperature T 2 is disposed at a distance from the first temperature probe 2. In the two-probe configuration of Fig. 2, a thermal insulator 4 is disposed between the first and second temperature probe 2, 6 for thermally insulating the probes 2, 6 from each other. In this set-up, a heat flow between the two temperature probes 2, 6 can be determined from the difference between the first and second temperature T and T 2 , measured at both sides of the thermal insulator 4 having a known thermal resistance value S. A compensation means for determining the heat flow and an evaluation unit connected to the compensation means permits to take into consideration the heat flow, increasing the accuracy of the measurement of the body temperature Tb. Such a two-probe sensor is described in US Patent 7,299,090. Here, the body temperature Tb can be modelled using a thermal flux model defined by Equation 2:
T t = T 1 + (T 1 - T 2 ) j ^ (Equation 2), where R and r are defined as above. In practice, for best results, the thermal flux model of Equation 2 must be compensated either by the measurement of ambient temperature T 3 , to take into account some lateral leakage of thermal flow through the thermal insulator 4, or by using an additional lateral insulation of the sensor to minimize the leakage of the thermal flow.
An advantage of the two-probe sensor 1 according to Fig. 2 is that it takes into account the temperature exchange with the ambient environment making the measurement more robust with respect to changes of the ambient temperature T 3 ,. However, the measurement is still affected by possible changes of the perfusion, or outer thermal resistance r. The two-probe sensor 1 has an accuracy typically comprised between -0.7°C to 0.6°C when compared to rectal temperature, even under controlled conditions, such as bed rest and normal ambient temperature. However, the two-probe sensor 1 shows some correlation with the circadian rhythm approximated as a sine wave. Table 3 summarizes the advantages of the above two-probe sensor 1 where the different advantageous characteristics are rated by one of the numbers comprised between 1 and 4.
Robustness Comfort Accuracy Safety
Table 3 Fig. 3 represents yet another configuration of the sensor 1, where a heater device 14 is placed on top of the second temperature probe 6 of the sensor configuration of Fig. 2. In this modified two-probe sensor configuration, the first temperature 7 " i is measured when the heat flow is controlled to zero (zero heat flow) by using a servo loop (not shown) controlling the heater device 14. More particularly, the heater temperature Th is adjusted such that the second temperature T 2 becomes equal to the first temperature 7V When this condition is fulfilled, the core body temperature Tb, that can be determined using Equation 2, becomes equal to the heater temperature Th.
The modified two-probe sensor 1 has an accuracy of about ±0.3°C compared to rectal or ear temperature measurements, when measured at the forehead or the sternum. The measurement is completely independent of the ambient temperature 7 a , the outer thermal resistance r or perfusion in the outer skin region 1 1, the inert thermal resistance R 1 and heat exchange resistance A. However, the modified two-probe sensor 1 comprising the regulation closed-loop is complex and has high power consumption. The modified two-probe sensor 1 is therefore not suitable for use in portable systems. Moreover, there is a risk of burn if the device heater 14 is controlled to a temperature exceeding 41 °C, depending on the difference between the first and second temperatures 7 " i and T 2 . Table 4 summarizes the advantages of the above sensor with the different advantageous characteristics being rated by one of the numbers comprised between 1 and 4.
Robustness Comfort Accuracy Safety
Table 4 Summary
The present application discloses a sensor device and a method for measuring and determining a core body temperature of a user which overcome at least some limitations of the prior art. According to the embodiments, a sensor for measuring a core body temperature of a user, can comprise: a first temperature probe to be placed against the skin of the user for measuring a first temperature; a second temperature probe at a distance from the first temperature probe, for measuring a second temperature and; a thermal insulator provided between the first and second temperature probes for thermally insulating first and second temperature probes from each other; characterized in that the thermal insulator embeds the first temperature probe, forming a contact surface extending in the plane of the first temperature probe and destined to be in contact engagement with the skin; and in that the sensor further comprises a thermally conductive shield covering at least portion of the thermal insulator, the second temperature probe being in contact with the conductive shield.
In an embodiment, the second temperature probe is in contact with a portion of the thermal shield arranged to be in contact with the skin.
In another embodiment, the second temperature probe can be an absolute temperature sensor and the first temperature probe is a thermocouple with its reference at a location at a substantially identical temperature as the one measured by the second temperature probe. In yet another embodiment, the sensor can further comprise a perfusion probe for measuring a perfusion parameter depending on the level of blood perfusion of the skin.
In yet another embodiment, said skin can comprise an outer skin region having an outer and surface thermal resistance and an inert region having an inert thermal resistance, and the thermal conductivity of thermal insulator can be chosen such that the resistance ratio term
(R+r x S r 2 \■ . .
\ —S+r 2 -— R T - 1 J is constant.
In yet another embodiment, any of the claims from 1 to 5 and a control device controlling the operation of the sensor (1). In yet another embodiment, said control device can be located on a first PCB mounted in the sensor.
The present disclosure also pertains to a method for measuring a core body temperature of a user using the sensor and comprising:
providing the sensor on the user such that the first temperature probe and the contact surface of the thermal insulator contact the skin;
measuring the first and second temperature with the first and second temperature probe, respectively; and
determining the core body temperature from the first and second measured temperatures. In another embodiment, said determining the core body temperature can be performed using Equation (5):
T b b = T c c + (KT c c - T d a) j ^
R r J) .
In yet another embodiment, the resistance ratio term
can be assumed to be constant.
In yet another embodiment, the sensor can further comprise a constant heat source above the thermal insulator, and said determining the core body temperature is performed using Equation (5) to which a term -βQ is added, where β being a constant and Q the power of the constant heat source. In yet another embodiment, said determining the core body temperature can be performed using Equation (7):
T b = F(s)T c + G(s)T d + C
where F(s) and G(s) are transfer functions chosen so as to compensate for dynamic change in parameters of the system formed by the sensor and the skin, and C is a constant or a signal determined by the dissipated power on the thermally conductive shield chosen so as to cancel the effect of said dissipated power on. In yet another embodiment, the method can further comprise a step of measuring the perfusion parameter and using this parameter for determining a value for the resistance ratio term in Equation (5).
The sensor and measuring method disclosed herein allow for performing measurements that are more robust to artifacts, and to determine the core body temperature with a better accuracy than when conventional sensors are used. The present sensor allows for cancelling the effects of perfusion variations within the tissue. The claimed sensor does not require an additional heater device and a servo loop circuitry. It is thus simpler and less costly to make and has improved power consumption performance compared to conventional sensors. Other advantages of the sensor disclosed comprise a better comfort of the user, lessened
obtrusiveness. Moreover, the sensor is well adapted for long-term use and has increased safety.
Brief Description of the Drawings The preferred embodiments will be better understood with the aid of the description of an embodiment given by way of example and illustrated by the figures, in which:
Fig. 1 shows a configuration of a conventional sensor for measuring a core body temperature of a user; Fig. 2 shows another configuration of the conventional sensor;
Fig. 3 illustrates yet another configuration of the conventional sensor; Fig. 4 represents a sensor for measuring the core body
temperature of a user according to an embodiment;
Fig. 5 shows the sensor according to another embodiment;
Fig. 6 illustrates a sensor system comprising the sensor according to the embodiments and a control device;
Fig. 7 represents the mean square error between the simulated core body temperature and its true value plotted against the thermal conductivity of the insulator resistance for the sensor according to the embodiments; and Fig. 8 shows a graph where the core body temperature is plotted against the power of a constant heat source added to the sensor according to the embodiments.
Detailed Description of possible embodiments
Fig. 4 represents a sensor 1 according to an embodiment. More particularly, Fig. 4 shows a half cross-section view of the sensor 1 having a circular shape. The sensor 1 comprises a first temperature probe 2 to be placed against the surface of the skin 3 of a user for measuring a first temperature T 0 In Fig. 4, the first temperature probe 2 is disposed concentric with the symmetry axis A-A of the sensor 1. A thermal insulator 4 embeds the first temperature probe 2, forming a contact surface 7 extending in the plane of the first temperature probe 2 and destined to be in contact engagement with the skin 3 when the sensor 1 is placed on the user. The sensor 1 further comprises a thermally conductive shield 5 covering the outer surface 8 of the thermal insulator 4. In the example of Fig. 4, the shield 5 comprises a portion, here a flange 9 at its periphery, that covers a portion of the contact surface 7 and comes also in contact with the skin 3 when the sensor 1 is placed on the user. Other configurations of the conductive shield 5 are possible provided the shield 5 covers at least a portion of the thermal insulator outer surface 8. The conductive shield 5 has a thermal conductivity that is much higher compared to the one of the thermal insulator 4 such that the temperature can be considered as substantially equal on the whole conductive shield 5.
The sensor 1 further comprises a second temperature probe 6 for measuring a second temperature Td. The second temperature probe 6 is disposed at a distance from the first temperature probe 2, such as to be thermally insulated from the first temperature probe 2 by the thermal insulator 4 and in contact with the conductive shield 5. More particularly, the second temperature probe 6 is disposed on the outer surface of the conductive shield 5 such that the measured second temperature Td corresponds substantially to the temperature of the shield 5. In the example of Fig. 4, the second temperature probe 6 is disposed in contact with the portion, or flange 9, such that is also contacts the skin 3 when the sensor 1 is worn by the user. This placement of the second temperature probe 6 minimizes possible inertia due to stabilizing the temperature of the shield 5 compared to, for example, placing the second temperature probe 6 on top of the shield, opposed to the first temperature probe 2.
Also schematically represented in Fig. 4 are the underlying regions of the skin 3 and comprising an outer skin region 11 below the skin surface, an inert region 12, and a core body region 13. The heat exchange from the user inner body, or core body region 13, towards the exterior of the outer skin region 11 is also represented by an equivalent circuit representing a thermal flux model involved in the determination of the core body temperature Tb. More particularly, the outer skin region 11 has a variable perfusion mainly induced by vasoconstriction or vasodilatation of the blood vessels due to adjusting to external temperature variations, and/or varying activity intensity of the user. The outer skin region 11 can be characterized by an outer thermal resistance r^ and a surface thermal resistance r 2l each of the resistances r 2 having a value that increases with decreasing perfusion and decreases with increasing perfusion of the outer skin region 11. The inert tissue region 12 has a perfusion that is
substantially constant and can be characterized by an inert thermal resistance R of constant value. Fig. 5 represents the sensor 1 according to an exemplary embodiment. More particularly, the left hand side of the figure shows a perspective and across-section view of the sensor 1 having a circular shape, whereas a detail of the temperature probes 2, 6 and a portion of an electronic circuit are shown in the right hand side of the figure. As represented, the first temperature probe 2 is placed concentric with the conductive shield 5 and thermal insulator 4, on the contact surface 7 destined to be in contact with the skin 3. In the present embodiment, the first temperature probe 2 is formed by one end of a thermocouple 15 and the second temperature probe 6 is formed by its other end. In this configuration, the difference between the first and the second
temperatures T c - T^ is measured by using both ends of the thermocouple 15. Instead of measuring T c one can of course measure Td and use the difference to compute T 0 Other arrangements of the first and second temperature probes
2, 6 are possible. For example, the second temperature probe 6 can be an absolute temperature sensor and the first temperature probe 2 can be a thermocouple with its reference at a location that is at a substantially identical temperature as the one measured by the second temperature probe 6.. The first and second temperature probes 2, 6 and the thermally conductive shield 5 can be made of metal, such as stainless steel. The thermal insulator 4 can be a material having an appropriate thermal conductivity, for example of about 0.03 VWKm, as will be discussed below. Such a thermal insulator 4 can be a gel material or air. Air (without convection) has a thermal conductivity of about k = 0.026 W/Km between 20°C and 40°C and is a good isolating material for this application. The thermal insulator 4 can be embedded between a plastic foil, such as a polycarbonate foil, destined to contact the skin 3 and the shield 5.
Preferably, the thermal insulator 4 has a thermal conductivity comprised between 0.02 and 0.05 W/Km.
Fig. 6 shows a sensor system 20 comprising the sensor 1 and a control device controlling the operation of the sensor 1, according to an embodiment. In the example of Fig. 6, the control device can comprise circuit elements such as a signal conditioning and pre-amplification module 22, AD converters 24, power supply 28, and the chipset needed for recording the data measured by the first and second temperature probes 2, 6, such as connectors 25, and memory 27. The circuit elements of the control device can be located on a first PCB 21 mounted in the sensor 1 or also mounted on a second PCB 29, as shown in Fig. 6. In this configuration, the thermal insulator 4 can be disposed between the foil and the first PCB 21. The sensor 1 can further comprises a motion sensor 31, mounted in the second PCB 29 in the example of Fig. 6, for measuring motions of the user and determining its level of activity. The motion sensor can be a three-axis accelerometer. The sensor 1 can also comprise an ambient temperature probe (not shown) for measuring the ambient temperature T a and possibly other probes (also not shown) for measuring other physiological parameter such as SpO 2 and/or heart beat measurements. As shown in Fig. 6, the control device can be controlled by a computer 32. Since the core body temperature Tb varies relatively slowly, its sampling frequency need not be greater than about 1 Hz, a value of 0.2Hz can be appropriate. The signals generated by the motion sensor 31 can be sampled at 20Hz.
In an embodiment, a method for measuring a core body temperature of a user using the sensor 1 comprises:
providing the sensor 1 on the user such that the first temperature probe 2 and the lower surface 7 contact the skin 3;
measuring the first and second temperature T Cl T^ with the first and second temperature probe 2, 6, respectively; and
determining the core body temperature Tb from the first and second measured temperatures T Cl Td.
The core body temperature Tb can be determined form the first and second measured temperatures T Cl T^ using the thermal flux model described above and defined by Equation 4: T b D = T c c + < vJcc ~ W αJ (\—s//r 2 )J = Tc + (T c - W ( \^s+r 2 ^ R rA 1 J
(Equation 4). In the case where the change in perfusion in the outer skin region 1 1 is substantially homogeneous, or the outer and the surface thermal resistances r 2 vary proportionally, the ratio /Vr 2 can be assumed to be constant. The term comprising the ratios of the resistances in
Equation 4 can then be approximated to be constant. Furthermore, if (by an appropriate selection of the thermal conductivity k of the thermal insulator 4) the ratio RIS is set to be equal to /Vr 2 , the determination of the body temperature Tb becomes independent of the values of the outer thermal and surface resistances r^, r 2l and of the heat exchange resistance A and ambient temperature T a . In this case, Equation 4 can be rewritten as Equation 5:
T b = T c + b (T c - T d ) (Equation 5), where b is a coefficient corresponding to the term of the resistance ratios in Equation 4 being constant. Fig. 7 shows the mean square error E between the simulated measured core body temperature Tb and a simulated true value, as a function of the thermal conductivity k, or the inverse of the insulator resistance S (k = MS), of the thermal insulator 4. Here, the ambient temperature T a was defined on a horizontal isotherm located at the distance of 2.4mm from the top of the sensor 1. For each value of the thermal conductivity k, the precision of the estimation of the core body temperature Tb has been computed (quadratic error) using Equation 5 with the coefficient b (b(k)) minimizing the quadratic error. Fig. 7 shows that the lowest mean square error E is minimal for the thermal conductivity k equal to about 0.03 VWKm, and the thermal insulator 4 should be selected with its insulator resistance S around this optimal value. Given this value of the thermal conductivity k, Equation 5 can be rewritten as:
T b = T c - 1.54 (T c - T d ) (Equation 6), where the coefficient b takes the value 1.54. In yet another embodiment (not represented), the two- dimensional temperatures profiles were simulated in the case where a constant heat source is added above the thermal insulator 4, or on the face of the PCB 21 opposed to the thermal insulator 4. Such simulations are useful at assessing the influence of thermal dissipation of the electronic components placed on the sensor 1, such as the circuit elements of the control device described above. Here, the constant heat source was modelled as a flat ring, placed concentric with the sensor 1, such as to cover a surface area of about 3 cm 2 and producing a constant power over its whole surface. Moreover, the simulations were performed assuming a value of 25°C for the ambient temperature 7 a , a thermal conductivity value of k=0.04 W/Km for the thermal insulator 4, a true core body temperature of 37°C, and a body thermal conductivity for R of 1 W/Km. The value of the constant heat source was varied between 1 mW and 1 W. In the graph of Fig. 8 the determined core body temperature Tb is plotted against the power of the constant heat source Q. As shown in Fig. 8, the determined core body temperature Tb decreases proportionally with the power of the constant heat source Q and can then be related to the power intensity of the heat source by Equation 7: T bιQ = T b - βQ (Equation 7), where 7b,Q is the core body temperature estimated in the presence of the electronic heat source Q, Tb is the core body temperature estimated without the electronic heat source, and β is a constant term whose value depends on the size, intensity and location of the constant heat source Q on the sensor 1, and corresponds to 0.68 when the simulations are performed using the above parameters. Consequently, when the heat source Q is added to the sensor 1, determining the core body temperature can be performed using Equation 5 and adding the term -βQ, yielding Equation 8: T bιQ = T c + b (T c - T d ) - βQ (Equation 8). where the assumption of the resistance ratio term in Equation 5 being constant (b) has been made.
In an embodiment not represented, the sensor 1 further comprises a perfusion probe for measuring a perfusion parameter depending on the level of blood perfusion of the skin 3, and more particularly to the outer skin region 1 1. Here, the perfusion probe can be based on a photopletysmography (ppg) sensor, for example, formed from an emitter comprising a radiation source emitting at the red and/or infrared wavelengths, and a receiver, such as a photodetector, for receiving the optical radiation emitted by the emitter and transmitted through the skin tissue. The radiation source can be an LED, preferably emitting at 660 nm and/or 940 nm. The receiver can be connected to an analog-to-digital converter (ADC) module delivering corresponding measured ppg signal.
In yet another embodiment not represented, instead of the constant coefficients of the linear function of Equation 5 or of the affine function of Equation 6, transfer functions are used. Such transfer functions allow for correcting the determination of the core body temperature T b for possible rapid changes in perfusion in the outer skin region 1 1 and/or ambient temperature T a . Thus, in a more generic way, Equation 5 or 6 can be written as
T b = F(s)T c + G(s)T d + C (Equation I) 1 where Tb is the measured core body temperature, F(s) a transfer function to be applied on the first temperature signal T 0 G(s) a transfer function to be applied on the second temperature signal Td, and C a constant or a signal determined by the dissipated power on the thermally conductive shield 5. Here, the transfer functions F(s) and G(s) are chosen so as to compensate for dynamic change in parameters of the system formed by the sensor 1 and the skin 3, the parameters comprising, for example, external
temperature, perfusion and possibly other parameters. In yet another embodiment not represented, the sensor 1 is used to measure the local temperature at a measurement point corresponding to the location of the first temperature probe 2.
The sensor 1 according to the embodiments can yield good core body temperature measurement accuracy (theoretically infinite on the described model) since the influence of the ambient temperature T a and variable perfusion can be compensated. The sensor 1 is simple and has improved power consumption performance compared to the conventional sensors. Since the sensor 1 is not obtrusive, it provides a better wear comfort for the user. Preferably, the sensor 1 can be worn on locations comprising the upper sternum and the forehead. Such locations are relatively constant in structure for most users, including users suffering from obesity. Table 5 summarizes the advantages of the above sensor with the different advantageous characteristics being rated by one of the numbers comprised between 1 and 4.
Robustness Comfort Accuracy Safety
Table 5
Reference numbers
1 sensor
2 first
3 skin
4 thermal insulator
5 thermally conductive shield
6 second temperature probe
7 contact surface of the thermal insulator
8 outer surface of the thermal insulator
9 flange
1 1 outer skin region
12 inert tissue region
13 core body region
14 heater device
15 thermocouple
20 sensor system
21 first PCB
22 signal conditioning and pre-amplification module
23 second PCB
24 AD converters
25 connectors
26 transmitter/receiver
27 memory
28 power supply
29 second PCB
30 third PCB
31 motion sensor
32 computer
A heat exchange resistance
b coefficient
E mean square error
k thermal conductivitv of the thermal insulator Q power of the constant heat source
R inert thermal resistance
r outer thermal resistance
/"1 outer thermal resistance
r 2 surface thermal resistance
S insulator resistance
r a ambient temperature
Tb core body temperature
7b,Q core body temperature with an electronic heat source 7 " i, T c first temperature
T 2 , Td second temperature
Th heater temperature
y a ambient temperature distance