WO2010149984A1

WO2010149984A1 - Hydration monitoring apparatus and method

Info

Publication number: WO2010149984A1
Application number: PCT/GB2010/001260
Authority: WO
Inventors: Richard A. Dudley; Mira Naftaly
Original assignee: The Secretary Of State For Business, Innovation & Skills
Priority date: 2009-06-26
Filing date: 2010-06-25
Publication date: 2010-12-29
Also published as: GB0911143D0

Abstract

A method of measuring the hydration of blood of a human or animal comprises emitting microwave radiation into a body part (28) such as a human earlobe, detecting the microwave radiation leaving the body part (28), calculating the absorption of the microwave radiation by the body part (28) and determining the hydration of blood from the absorption of the microwave radiation.

Description

HYDRATION MONITORING APPARATUS AND METHOD

The present invention relates to hydration monitoring apparatus and to a method of monitoring hydration, in the preferred embodiment able to provide a real time indication of the hydration level of the blood of a human or animal.

Monitoring hydration levels is important in many instances, in particular to seek to prevent loss of human or animal efficiency. A low level of hydration in a human, for instance, can result in reduced mental faculty and reduced physical energy.

Many known methods of determining whether the hydration level of a person or an animal is below optimum are performed by observing signs of dehydration, that some time after a person's hydration has fallen below optimum levels. Any such method will generally result in too slow a response in regenerating a correct hydration balance. This can be particularly problematic in situations where correct hydration has a great effect on performance or can change rapidly. Examples include when the person or animal is undergoing extreme physical exertion, such as athletes or people carrying out heavy exercise who are unused to doing so. In the case of exercise, the situation is made more complex by the fact that a person's hydration is affected by a variety of personal metabolistic factors, resulting in the amount and frequency of fluid replenishment required varying with the individual. This makes it difficult to predict how much and how often any given individual should drink during exercise. By the time it is clear that a person's hydration is too low it is often too late to prevent a deterioration in performance. Even minor dehydration can cause a rapid fall-off in an athlete's performance.

Another example in which the level of hydration is important to ensure optimal performance is in the military. Military service personnel can often be exposed to extreme conditions for extended periods of time, which can result in a rapid decrease in hydration. If this is not noticed quickly enough, it can result in the reactions and capability of the serviceman being impaired. In extreme situations, such as on a battlefield, this can expose the servicemen to potentially life-threatening dangers. Another example is in hospital environments. For many patients in hospital, the usual signs of dehydration can be masked by other symptoms which the patient is presenting. Moreover, the patient himself may be unaware of the signs of dehydration, unable to communicate, or may have a reduced instinct to drink. It is thus important for healthcare personnel to be able actively to monitor a patient's hydration levels. This is particularly important in an operating theatre, where a patient's hydration level needs to be monitored throughout the procedure.

Much of the prior art requires monitoring urine or blood tests; osometers and refractometers typically being used. These are inconvenient, invasive, time consuming, expensive, require laboratory equipment and cannot be carried out on a continuous basis or in real time.

Impedance measurement devices exist but have the disadvantage that they measure the total water content of the body. Ultrasound devices can measure blood flow of the body non-invasively and, as disclosed in US-7,291,109, can be used to monitor hydration levels in soft tissue by changes in the ultrasound velocity, but do not have the sensitivity to measure the hydration of human or animal circulating blood. In addition, ultrasound is surface contact critical, requiring coupling gels for a successful measurement to be made and adding complexity and error to any measurement scenario. Several techniques have been developed to try to resolve these problems to monitor hydration levels in real time. One such technique is illustrated in US2008/0,234,600, which discloses an ear mounted temperature sensor. However, the use of a temperature sensor requires the hydration level of the body to be inferred from an expectation of the increase in core body temperature with respect to the loss of body weight owing to fluid loss. Such a method therefore does not measure hydration directly. WO2008/0, 112,136 and US2008/0,220,512 disclose measuring body water content by detecting the absorption of electromagnetic radiation in the near-infrared region of the spectrum. The near- infrared region of the spectrum is employed since it has a distinctive absorption band for water. However, penetration of near-infrared radiation into a human or animal body part is restricted by high levels of absorption and scattering resulting in measurements being limited to near to a subjects skin surface, which can limit the accuracy of any blood hydration measurement estimation.

The present invention seeks to provide an improved method and apparatus for measuring hydration level of a human or animal. According to an aspect of the invention, there is provided a method of measuring the hydration of the blood of a human or animal, comprising the steps of: transmitting microwave radiation into a body part; detecting the microwave radiation leaving the body part; calculating the absorption of the microwave radiation by the body part; and determining the hydration of the blood from the absorption of the microwave radiation.

It has been found that the transmission function of microwave radiation through a medium is related to the moisture content of the medium and that microwave radiation can thus be used to provide a reliable estimate of the content of water in a body part under examination. This measure in related directly to the hydration of the person or animal.

Preferably, the step of calculating the absorption of the microwave radiation by the body part comprises calculating the absorption of the microwave radiation by blood in the body part. By determining the hydration of the blood itself rather than, for example, muscle or bone, this preferred embodiment is able to reduce or eliminate errors in the calculation of the hydration level of the blood of the person or animal under observation caused by other body elements affecting the transmission of microwave radiation. Owing to the role of blood in the human body, the hydration level of blood provides a most effective measure of hydration of the whole body. In addition, water will appear in, or be lost from, blood before other parts of the body. Accordingly, this preferred embodiment is able to monitor hydration directly rather than infer a hydration level, as well as being able to provide a very rapid detection of changes in a person's or animal's hydration level.

Preferably, the majority of the chosen body part consists substantially of blood. The preferred body part is the earlobe, which it has been found provides access to blood.

While it has been known that a microwave transmission function can be indicative of water content, microwaves have been avoided for the measurement of the hydration level of humans and animals given the potential risks of microwaves and the power levels expected to be necessary. The inventors have found, however, that by selecting a body part which consists substantially of blood, such as extremities like the earlobe, the microwave power can be maintained at a very low and safe level, preferably less than 1 W and more preferably of the order of milliWatts. Therefore, the risk of the microwave heating the body is substantially reduced, enabling this method to be employed safely on living creatures.

According to another aspect of the present invention, there is provided hydration monitoring apparatus for measuring the hydration level of blood of a human or animal, comprising a probe element provided with microwave emitter element and a microwave sensor element facing the emitter element and spaced therefrom by a gap; the probe including a fixing element designed to fix the probe to a body of a human or animal part such that a selected body part fits within the gap between the emitter and sensor.

According to another aspect of the present invention, there is provided hydration monitoring apparatus for measuring the hydration level of blood of a human or animal, comprising a probe element provided with microwave emitter element, a microwave sensor element and a microwave reflection element, the reflection element being spaced from the emitter element and sensor element by a gap; the probe including a fixing element designed to fix the probe to a body of a human or animal part such that a selected body part fits within the gap.

In one embodiment, the emitter and sensor elements are located adjacent one another and both facing the reflection element. The apparatus provides, in the preferred embodiments, a probe element which can fit to a patient's ear so as to measure the water levels in blood passing through the earlobe, and an indicator unit coupled to the probe element which can indicate the hydration level of the person. The indicator is usefully a unit which can be held or worn by a person and can provide a visual indication of hydration and/or an acoustic or tactile indication of hydration level, preferably including one or more warning indicators when hydration is determined to fall below, or exceed, predetermined thresholds. Embodiments of the present invention are described below, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a schematic diagram of an embodiment of hydration monitoring device; Figure 2 is a side sectional view of a wave guide according to an embodiment of the invention;

Figure 3 is a front sectional view of the wave guide of Figure 2;

Figure 4 is a side sectional view of a wave guide according to an embodiment of the invention; Figure 5 is a front sectional view of the wave guide of Figure 4;

Figure 6 to is an exploded view of a microwave emitter in accordance with an embodiment of the invention;

Figure 7 is a perspective view of the assembly of Figure 6 showing how the components thereof fit together; Figure 8 is a perspective view of the assembly of Figures 6 and 7 in completed form;

Figure 9 is a schematic diagram of another embodiment of probe assembly;

Figure 10 is a perspective view of a preferred embodiment of probe unit of the monitor assembly, fitted to a person's ear; Figures 11 and 12 are perspective views of two embodiments of monitor assembly, one using a wired connection to a control unit and the other a wireless coupling to these components;

Figure 25 is a schematic system diagram of the principal components forming the hydration monitoring device of the preferred embodiments; Figure 14 is a graph showing the microwave absorption in liquid water;

Figures 15 to 26 are graphs of preliminary experiments showing the transmission loss as a function of weight of microwave radiation passing through a cellulose sponge saturated with pharmaceutical grade saline solution at frequencies of 5GHz, 6GHz, 12.4GHz, 13GHz, 14GHz, 15GHz, 16GHz, 18GHz, 20GHz, 22GHz, 24GHz and 26GHz respectively; Figure 27 is a graph showing the relationship between the absorption coefficient of liquid water and the observed slope of the transmission loss for the preliminary experiments of Figures 15 to 26;

Figures 28 to 33 show the results for a prototype, showing transmission loss as a function of weight of a sponge saturated with controlled amounts of pharmaceutical grade saline for a microwave signal at frequencies of 13GHz, 14GHz, 15GHz, 16GHz, 17GHz and 18GHz respectively;

Figure 34 is a graph showing the relationship between the absorption coefficient of liquid water and the observed slope of the transmission loss for a prototype;

Figure 35 is a graph showing transmission loss of an athlete exercising and losing weight as a result of loss of hydration.

Referring to Figure 1 , the embodiment of hydration monitoring device shown comprises a microwave emitter 12 and a microwave receiver 14. Both the microwave emitter 12 and the microwave receiver 14 are provided as free space coupling elements such as a horn antenna, waveguide launch or antenna structure. The microwave emitter 14 preferably comprises a microwave source 18 and a shielding element 16. The microwave receiver 14 preferably comprises a shielding element 20 and a microwave detector 22. The microwave emitter 12 and microwave receiver 14 can also be provided as unshielded elements, such as patch antennas. The microwave emitter and receiver can also be provided as a coax cavity or a waveguide.

Referring to Figures 2 to 5, there is shown an embodiment provided with shielding elements 16, 20 which are waveguides arranged to produce a parallel waveform. The waveguides comprise a metallic casing 36, preferably aluminium or copper, which partially encloses an area containing an antenna 40 which emits or detects a microwave signal. The casing 36 has an opening 38 on the axis along which the microwaves are intended to travel. The casing 36 can be cylindrical, as shown in these Figures, with an axis corresponding to the axis along which the microwaves are intended to travel. In another embodiment, the casing may be cuboidal such that its cross-section taken through a plane perpendicular to the axis of microwave travel is square. In embodiments with a cuboidal casing, the opening 38 can be circular, but is preferably square and formed by the cubical casing 36 being provided with only five sides so that one side is open. A cuboidal casing 36 enables the waveguide to be made smaller than a cylindrical casing. Within the casing 36, surrounding the antenna 40, is provided a dielectric medium 42. The higher the value of the dielectric constant of the dielectric medium 42, the smaller the device, so a higher value is preferable. This is because the value of the dielectric constant is used in calculating the dimensions of the waveguide. A high dielectric constant therefore enables the size of the waveguide to be reduced in comparison to an air-filled cavity. In one embodiment, the dielectric medium 42 is PTFE with a dielectric constant of 2.5.

In one embodiment, the waveguides can be manufactured as blocks of PTFE to five sides of which are attached sections of copper foil which are soldered together to provide the casing 36. The blocks of PTFE can then be drilled to suit the core diameter of a coaxial cable to provide the electrical signal and the braided exterior of the cable can be soldered to the copper foil.

By providing horn antennas which are shielded, there is less leakage of the electrical signal since the microwave radiation is better restricted to a single path. This additionally reduces multiple path reflections and can also minimise the adverse effect of the proximity of other conductors such as human tissue. Figures 6 to 8 show a series of views of an early prototype of probe useful in the description of the components use din the preferred embodiments. The structure of this example is similar for both the emitter 12 and the receiver 14, although the description below focuses solely upon the emitter 12 for the sake of conciseness. The cover comprises a front part 50 and a rear part 52, in this example made of a plastics material, and typically shaped to be comfortable against the skin. The front cover part 50 and the rear cover part 52 can be secured together to contain the waveguide. The front cover part 50 comprises an opening 54 in the centre of its front face through which the microwave signal can pass. Around the inside of the opening 54 is a flange. An internal cover 56 is provided to sit within the opening 54 and comprises a lip 58 to abut against the inside surface of the flange of the front cover part 50, to prevent the internal cover 56 from falling out through the opening 54. The internal cover 56 can be made of the same dielectric material 42 which fills the casing 36 of the waveguide, which is preferably PTFE. Resilient elements such as springs 60 are provided between the rear cover part 52 and the internal cover 56 to ensure that the lip 58 is held in abutment with the flange of the front cover part 50.

The waveguide comprises the dielectric material 42 and casing 36 as described above. These are contained within a mid-assembly 62, which may be made of aluminium. The mid-assembly 62 is provided with protrusions 64 which abut against the cover 50, 52 and serves to hold the waveguide securely in a central position within the emitter 12. The mid-assembly also comprises a channel 66 for the insertion of a coaxial cable comprising the antenna 40. The coaxial cable and antenna 40 are held securely in position in the channel by a clamping plate 68. The coaxial cable is preferably soldered to the clamping plate 68, which in turn is soldered to the mid-assembly 62. The microwave emitter 12 and microwave receiver 14, of similar or compatible design, are attached to a frame or other housing (not shown) able to be secured to a human or animal such that the microwave emitter 12 and receiver 14 are placed either side of a human or animal body part. The emitter and receiver are arranged in use such that a primary microwave signal 11 emitted from the microwave emitter 12 passes through the body part to receiver 14. In a preferred embodiment, this is achieved by having the microwave emitter 12 and microwave receiver 14 arranged such that they face each other but are separated by a small gap into which the human or animal body part can be placed. This arrangement is shown in Figure 1. Another embodiment is shown in schematic form in Figure 9. In this embodiment, in stead of having the emitter 12 and receiver 14 facing one another, these are arranged on the same side of the human or animal body part 28 when they are in position. On the other side of the human or animal body part 28 and across a gap 13 is a reflector plate 30. The emitter 12 and the receiver 14 are configured to be at an angle with respect to each other so that a primary microwave signal 11 emitted from the microwave emitter 12 and reflected from the reflector plate 30 will reach the microwave detector 14. This embodiment has the advantage that the primary microwave signal 11 passes through the human or animal body part 28 twice, in slightly different locations. It is therefore able to average out anomalies that may occur in an arrangement in which the microwave signal 11 passes only once through the human or animal body part 28.

In all the embodiments disclosed herein, it is preferred to minimise the path that the microwave radiation has to travel from the emitter to receiver while maintaining sufficient separation between the components of the device 10 to enable the human or animal body part 28 to be inserted into the gap 13. This gap 13 should preferably be no more than around 8mm across, more preferably no more than around 6 mm and most preferably no less than 3mm across. In one embodiment, the gap is 5.4mm across. The gap between the components can be preset, and is preferably so, but it is not excluded that the probe could be adjustable so as to give a gap of varying size to accommodate different physiologies.

The probe can be fitted to any part of the human or animal body which is composed substantially of blood. Preferably, the body part 28 is a physiologically simple part of the body which is substantially devoid of muscle, bone or major organs. The preferred body part 28 is the ear and, in the case of humans, most preferably the earlobe. However, the body part 28 can be other extremities of the body that are devoid of muscle, bone or major organs, such as the webbing between the fingers or toes.

The probe can be secured to the body part by any of a variety of methods temporary adhesive, with a clip holding the emitter and receiver units or the emitter/receiver units and reflector element. Of course, a clip should not impart a force which might cause causes pain or restrict blood flow to the body part being tested. The probe can additionally be secured, for example in the case of a human ear, by an appropriate ear fitting one which a preferred embodiment is shown in Figure 10. Referring to Figure 10, the probe has an ergonomic shape designed to hold onto the ear 28, in such a manner that it remains securely located and yet is not uncomfortable. A hook element 26 locates in to the bottom of the outer ear with the emitter 12 and receiver 14 positioned either side of the earlobe. The probe housing 32 extends to below the earlobe and includes, in this example, a cable which is coupled to a control and indicator unit, described below. The probe housing 32 has rounded surfaces able to conform to the lower ear shape and which do not present any sharp surfaces which might cause discomfort or injury. In this embodiment, the hook element 26, in conjunction with the positioning of the emitter 12 and receiver 14 units close to the ear lobe are sufficient to hold the probe in position even during sport, exercise or work.

As explained above, in place of a fixed structure probe of the type shown in Figure 10, the probe could be in the form of a sprung arm arrangement in which the emitter 12 and probe 14 are located on respective sprung arms and thus held to the earlobe by the force of the spring, in a manner akin to some pinless earrings. In another embodiment, the emitter 12 and receiver 14 units could include magnetic elements to cause these to be attracted to one another. These are not, however, preferred implementations.

The microwave emitter 12 is configured to emit microwave radiation within a range of 0.1GHz to 110GHz, more preferably in the range of 3GHz to 110GHz, even more preferably in the range of 10GHz to 26GHz. The preferred embodiments us a range of 13GHz to 18GHz or of 14GHz to 20GHz. This range, it has been found, is able to balance error (which is high at low frequency) and absorption (which is too high at high frequency).

The emitter 12 preferably emits the microwave signal at a power in the range from 200μW to 1W. The device 10 can be configured to alter the specific power in response to the level of signal received at the receiver 14. More preferably, the power output is in the order of milliwatts.

Referring again to Figure 1 as well as to Figures 11 and 12, the microwave emitter 12 and the microwave receiver 14 are preferably coupled to a separate processing and control unit 24. The processing and control unit 24 provides closed loop control of the microwave power, phase, frequency and modulation. In a preferred embodiment, the processing and control unit comprises some of the functionality of a Vector or Scalar Network Analyser (VNA and SNA) to make transmission and/or reflection absorption measurements similar to S₂i and Sn methodologies. The processing and control unit preferably includes conventional electronic processing components programmed to provide calibration and normalisation of the system, and to determine the hydration of the blood of the human or animal body part 28. The hydration is preferably calculated from a calibration curve of hydration against microwave loss. Such a calibration curve can be determined from a plot of microwave losses against corresponding readings from blood tests.

The processing and control unit 24 can be provided in different forms. In one embodiment, it is integral with the probe. In other embodiments, the unit 24 is separate from the probe. Figure 11 shows the processing unit 24 linked to the probe by a wire or cable, while Figure 12 shows the unit 24 being coupled to the probe by a wireless connection. In both embodiments there is preferably also a coupling, advantageously wireless, to a display of a wrist carried device such as a multi function watch (the latter may also provide other physiological measures such as heart rate and so on).

More specifically, in some embodiments, such as those shown in Figures 11 and 12, the control unit 24 is coupled to a remote processing unit. The remote processing unit can be, for example, a mobile telephone, a Personal Digital Assistant (PDA), a watch, a computer, or other means employing electronic processing. The means of coupling the control unit to the remote processing unit can be any conventional means of communication such as a wired connection such as USB, Ethernet, or a wireless connection such as Wi-Fi or Bluetooth. In embodiments with a remote processing unit, the remote processing unit receives data from the control unit and on the basis of the data, it performs calibration and normalisation calculations, and transmits instructions to the control unit. In response to the received instructions, the control unit adjusts if necessary the microwave power, phase, frequency and modulation. It also analyses the data to determine the level of hydration. This can then be presented to the user in a number of ways, some of which are described below. Close loop feedback for the control unit 24 is only an option in the preferred embodiments as other calibration methodologies could be used. Preferably, only the minimum required components are included on the probe in order to keep this as small and light as possible for convenient fixation to the body. The minimal components typically comprise a power source such as a battery; circuitry to control the operation of the microwave emitter 12 and microwave receiver 14; and preferably a wireless communication device for receiving instructions from and transmitting data to a remote processing unit such as those described above. However, in some embodiments, data storage is provided on the probe such that a substantial amount of data can be accumulated before it is transferred to a remote processing unit. This can conserve power which might otherwise be used in constant use of the communication means.

The processing unit can offer a variety of functions such as those shown in Figure 13. In one embodiment, the processing unit comprises a simple set of warning sounds and/or lights which are activated to warn the user in response to the processing unit determining a low level of hydration. Other embodiments include a liquid crystal display for providing the user with a percentage hydration, and data storage to keep a record of hydration levels over time.

In one embodiment, the device 10 also includes a heart rate monitor, which can be mounted adjacent to the microwave emitter 12 or the microwave receiver 14. Since the device 10 is designed to be placed at a body part which is substantially composed of blood and preferably one with an active flow of blood such as near the head, it also serves as an effective place to measure the pulse rate of the subject. This is particularly true where the body part 28 is a human earlobe. The heart rate monitor preferably is located against the skin at the body part 28 and provides an electrical signal in response to detection of a pulse. The heart rate monitor can be coupled to the processing and control unit 24 in an analogous manner to that for the microwave emitter 12 and receiver 14. The processing and control unit 24 can thereby provide power to the heart rate monitor, and the heart rate monitor can provide a signal indicative of the heart rate to the processing and control unit 24. The microwave emitter 12 is preferably configured to provide a microwave reference signal 34 to the microwave receiver 14 in addition to the primary signal 11. This can be achieved by direct coupling between the microwave emitter 12 and the microwave receiver 14. However, preferably, this is achieved by arranging the microwave emitter 12 such that it can emit a second microwave signal to the microwave receiver 14 which does not pass through the body part 28. This is implemented in the embodiment depicted in Figure 10, 11 and 12 by providing either an emitter 12 and a receiver 14 which extend below the lobe of the ear 28 or by providing a reference emitter and a reference receiver located below the end of the earlobe. In the first embodiment, the emitter 12 is arranged to emit two microwave signals, the primary signal 11 through the lobe of the ear 28 and the reference signal 34 across the empty gap 13. Advantageously, the primary 11 and reference 34 signals are emitted simultaneously so that their comparison allows all effects other than absorption in the body part 28 to be removed from the hydration calculations.

The microwave signal may be modulated or pulsed. Modulation of the microwave emitter 12 by amplitude, phase, frequency or other common modulation scheme may be used to enable the microwave receiver 14 and the processing and control until 24 to improve the dynamic range of detection and thus sensitivity of the device 10. Modulation is commonly used for the purpose of enhancing the dynamic range and sensitivity of many industrial instruments, an example is the lock-in amplifier. In addition the microwave signal may be pulsed to reduced the average power emitted by the device 10 and reduce exposure of the body part 28 and in addition increase the battery life of the device 10. In one embodiment which is particularly suitable for sports, the time between readings is more than 30 seconds; preferably a set of readings is not taken more frequently than every minute. When readings are not being taken, the device can advantageously be switched off to conserve power. The device is preferably waterproof.

The device 10 works in the following way in the preferred embodiment. For the purposes of the following description, it is assumed that the user is a sports person using the device 10 to monitor their hydration during exercise. However, methods analogous to that described below can be employed for other uses for the monitoring of the level of hydration of a human or an animal. The user attaches the device 10 to his ear and secures it in place such that the lobe of his ear 28 is correctly aligned in the gap 13 between the microwave emitter 12 and the microwave receiver 14 (or between the microwave emitter 12 and the reflector plate 30 for embodiments such as that depicted in Figure 9). The user then switches the device 10 on by operation of the control unit.

The control unit causes the microwave emitter 12 to emit a primary microwave signal 11 through the earlobe 28 within the preferred power and frequency ranges described above. In some embodiments, the microwave emitter 12 emits a signal at a single frequency, whereas in other embodiments the microwave emitter 12 emits a signal at multiple frequencies or sweeps the frequency between two limits. However, preferably, the microwave emitter 12 does not sweep frequencies but uses at most 5 distinct frequencies, most preferably 2 or 3 distinct frequencies. Recording absorption through the body part at multiple frequencies allows comparison with the expected water absorption with frequency data, for example as shown in Figure 14, for calibration and improved resolution calculation. The availability of redundancy in frequency selection may be needed if a subject's body part 28 has higher than expected levels of absorption at one frequency which has operational benefits. However, the use of multiple or swept frequencies within a final device increases both the components complexity and cost of manufacture, which may be prohibitive for some applications but not others.

Preferably, the microwave emitter 12 simultaneously emits a reference signal 34 corresponding in power, phase, frequency, and modulation to the primary microwave signal 11 across the gap 13 but not through the earlobe 28, as described above.

The microwave signals 11, 34 are detected by the microwave receiver 14 and the magnitude and phase change of the signals are calculated. An advantage of the embodiments in which a reference signal 34 is provided separately to the primary signal 11 is that the reference signal 34 provides a useful calibration signal. A comparison of the received reference signal and the received primary signal can show how much of the signal that has passed through the body part 28 has been absorbed by the water in the body part 28, and how much is a result of, for example, attenuation, scattering or interference from the device 10 electronics and coupling to the body part. As stated above, a comparison of the primary 11 and reference 34 signals can effectively remove effects other than those caused by the absorption in the body part 28. In a practical embodiment, the control unit acquires only raw and unprocessed data, and transmits them via one of the media described above to the remote processing unit for analysis. As described above, in one embodiment, the remote processing unit is in the form of a watch, and provides a percentage hydration. In embodiments in which the processing unit comprises data storage, the absolute hydration levels over time can be recorded and the processing unit can perform statistical analysis on them to determine a maximum, minimum and optimal hydration level for that particular user. This can be converted into a percentage score, and can be supplemented with a light unit, a sound unit, and/or a vibratory unit to warn the user when his hydration level is falling significantly below the optimum or other threshold level. It can thereby warn the user of a decreasing hydration level in time for him to be able to rectify it by appropriate intake of fluids. Owing to the inherent delay between the intake of fluids and a consequent increase in hydration, an early warning signal of a non-optimal hydration level such as this can help an athlete maintain peak performance. In addition, it can enable an athlete to become aware of how his body responds to exercise and the intake of fluids such that he can become used to the quantity and frequency with which he needs to drink in order to maintain peak performance. This can be particularly beneficial in training for competitive situations in which the device would not be worn during the competition itself. To assist in this personal analysis, in some embodiments, the data from the data storage of the processing unit can be downloaded onto a computer via a conventional communication link for more in depth analysis by appropriate software.

As described above, in some embodiments, the hydration monitor is supplemented by a heart rate monitor. The heart rate monitor provides data which can be transmitted and/or stored together with the hydration data in the manner described above. Heart rate can be indicative of physical exertion. In some embodiments, the heart rate data can be downloaded for example to a computer together with the hydration data. The computer can determine a correlation to show how the hydration level of a user responds to physical exertion of different intensities and/or can present the data in a graphical format for the user to study to develop a personal plan for the optimum intake of fluids.

The data storage can also be used to assist in calibration. For example, since hydration levels do not change suddenly, the processing unit can determine based on the data stored in the data storage if there has been a sudden change in detected absorption. It can then attribute this to a movement of the device 10 such that the microwave signal now passes through a slightly different part of the body. This can be due to, for example, the device having fallen off and then been put back in a slightly different position. Having determined this, the processing unit can recalibrate the processing of the data by deeming the new measured absorption to correspond to the same level of hydration as for the previous data reading.

By comparing a user's hydration and heart rate data with the user's normal response pattern from the stored data, the device can predict when a user's hydration is likely to drop and communicate this to the user by one of the means described above. Such prediction enables a user to take on more fluid in sufficient time to prevent the drop occurring. Such a prediction can also be made by calculating the rate of change of the hydration level and thereby calculating the expected time until the hydration level falls below a preferred range.

An advantage of the embodiments taught herein is that they can provide a real time, non-invasive hydration monitor providing a measurement comparable to a blood test. By selecting a body part which is thin, hydration can be detected by the absorption of microwaves in transmission without requiring the microwave signal to be of sufficient power to carry any health risks for the subject. By measuring the hydration over a body part which is composed substantially of blood, absorption of the microwaves by that body part will be substantially as a result of absorption by the water in the blood. Particularly in the case of a human ear lobe, since it is near to the head, it has a very active flow of blood and most other surrounding tissue is fat which has a constant very low moisture content which can be accounted for in the calibration settings. Accordingly, the error level owing to absorption by nearby elements is minimised and the hydration calculation is accurate and reliable.

A portable, cheap, robust, easily used hydration monitor has wide applications in professional and amateur sports, in healthcare, in care for the elderly and disabled, and in veterinary care.

Such a device can provide significant advantages for the situations highlighted above in which the hydration level of a person or animal can change rapidly, such as in the performance of rigorous exercise, for example for an athlete or a racehorse, or in extreme conditions, such as for military service personnel. In addition, the device is beneficial for people or animals, for example patients in hospitals or animals in veterinary procedures, who are either unable to communicate their dehydration or who are simply unaware of it.

Features of the above embodiments and modifications can be combined and interchanged from one embodiment to another.

Theory and Experimental Background

The preliminary experiments of the project were devoted to measurements of transmission loss in a phantom with the aim of determining the optimum operating frequency of the proposed device. The chosen phantom was a cellulose sponge saturated with pharmaceutical-grade saline solution so as to simulate the fluid content of living tissues (comprising blood and intercellular serum).

Measurements were carried out by varying the saline content in the sponge while recording the transmission loss as a function of the sponge weight. The loss of a dry sponge was <0.2 dB at all frequencies.

Patch antennas were used for experiments at 5 and 6 GHz; while at high frequencies horn antennas were employed. The transmission loss through the phantom was measured by a VNA.

Microwave absorption in water increases steeply with frequency and is directly related to the number of water molecules, making it possible to obtain the water content by measuring the transmission loss. Low absorption coefficients and thus loss, at frequencies below 3 GHz make the measurement of water content in a body part 28 with a thickness less than 8 mm more challenging than at frequencies above 3 GHz, it is therefore preferential to operate the device 10 above 3 GHz. However, operation below 3 GHz is possible and may be utilised.

The microwave absorption in liquid water is shown in Figure 14 (Segelstein, David J. "The complex refractive index of water", Thesis (M.S.)— Department of Physics. University of Missouri-Kansas, City, 1981.)

Transmission through an absorbing material is described by the Beer-Lambert law:

Wherein IT and I₀ are respectively the transmitted and incident intensity, u is the absorption coefficient, C is the concentration of the absorbing species, and t is the sample thickness. The coefficient T represents the transmission efficiency (or the loss factor) between the emitter and the receiver.

Loss = -1Og(I₁. /I₀) = -logT + α x (Ct) ₍₂₎ The measured loss in the absorbing material is expected to be linear with water concentration, and therefore with the weight of the material. The slope will be proportional to the absorption coefficient.

A large absorption coefficient will result in a greater differentiation, and may therefore give a better resolution in the measurement of water concentration. On the other hand, if the transmission loss is very high, the system will require high sensitivity and a low noise floor in order to provide an accurate measurement.

Figures 15 to 26 plot the transmission loss results as a function of weight at different frequencies for the preliminary experimental set-up described. The scatter in the data is due largely to the difficulty of ensuring a uniform distribution of water across the area of the sponge. Different symbols represent separate experimental runs. Solid lines are linear fits to the data.

It is seen that at low frequencies of 5 and 6GHz there is a strong deviation from the expected linear relationship. At higher frequencies, typically above 10GHz, the results indicate that water content may be reliably measured as a function of microwave transmission loss. The slopes of the linear relationship suggest that a resolution of 0.1dB in the measurement of loss are sufficient to detect variations in the water content of 1% or less.

Equation 2 implies that a linear relationship is expected between the slopes of the linear fits in Figures 15 to 26 and the absorption coefficient shown in Figure 14. This is indeed demonstrated by the data in Figure 27, showing the relationship between the absorption coefficient of liquid water and the observed slope of the transmission loss, and confirms the reliability of microwave hydration method measurements. The slope of the linear relationship in Figure 27 depends on the sample parameters such as thickness and area. A prototype devices for measuring microwave transmissions through a human earlobe was designed and tested. The device was symmetrical with respect to transmitter/receiver and used purpose-built microwave launch cavities. The device was tested using a VNA and was operated at 13-18 GHz, which has been identified as a preferred frequency range for microwave hydration measurements. The prototype produced a stable, highly reproducible near-flat reference signal across its bandwidth (the reference signal was recorded in the absence of any absorber in the air gap between the transmitter and the receiver). Tests on human volunteers revealed that transmission loss measured by the prototype was affected by device positioning on the ear. The conclusions from the test on human volunteers were twofold:

At frequencies of 13-18 GHz microwave absorption loss in the human ear is in the range of values which can be measured sufficiently accurately with easily available instrumentation.

Secure, stable, reproducible positioning of the transmitter and receiver on the ear is beneficial for reliable readings.

It was decided to verify the performance of the prototype using tissue phantoms, similarly to the earlier experiments. As previously, the phantoms were cellulose sponges saturated with controlled amounts of pharmaceutical-grade saline. The results of transmission loss as a function of weight are shown in Figures 28-33. Different symbols represent separate experimental runs.

The scatter of data in Figures 28-33 are due to variations in the positioning of the phantom and perhaps even more significantly to the inhomogeneous distribution of water in the phantom. It was observed that different areas in the sponge often produced slightly different loss data.

As was done for the earlier experiments (see Figure 27), Figure 34 plots the relationship between the absorption coefficient of water and the slopes at different frequencies as obtained in Figures 28-33. Deviations from linearity in Figure 24 of the data at 15 and 16 GHz indicate effects of transmitter-receiver coupling at these frequencies, which relate to the properties of the microwave cavity launcher.

In testing prototype devices on five volunteers, it was found that compression of the earlobe did not appear to affect materially the readings. For four of the volunteers, readings between 33 to 34 deb were recorded. For the fifth volunteer, who had a physical condition that causes dehydration, the reading was 29 deb. This reading rose after a 200ml drink and an assimilation period of 15 minutes to 30.6 deb, and following correct water intake and a further 15 minute assimilation period, to 33.4 deb. In addition, the device could be removed and replaced on the earlobe without a noticeable change in reading.

Dehydration test

Another prototype device was tested on a volunteer athlete during exercise.

The weight of the athlete was monitored at intervals during the exercise. Concurrently, the loss readings given by the device were recorded throughout the exercise period.

It was expected that as the weight of the athlete decreased due to water loss, the transmission loss should also decrease. The data recorded in the test is shown in Figure 35. Although the transmission loss data are very noisy (lower graph), there is a clear trend of loss decrease during the exercise, as could be expected.

The noise in the data is caused primarily by the poor fit of the prototype on the ear. As the athlete moved, the position of the device on the earlobe shifted frequently and randomly, resulting in different areas of the earlobe being measured. Moreover, the angle between the earlobe and the device contacts also varied uncontrollably. A design of probe as that shown in Figures 10 to 12 is expected to overcome such difficulties, by in effect clipping the probe to the ear and holding the earlobe within the two sides of the device which hold the emitter and receiver or emitter/receiver and reflector. If necessary, the two sides of the device could be slightly sprung to hold these against the opposing sides of a user's earlobe, even as the user moves on exercising, for instance.

Although the preferred form of probe housing is shown in Figures 10 to 12, other embodiments are envisaged, as disclosed above. It may also be preferable in some instances to have a probe housing with a hook which hooks over and behind the outer ear. The ultimate choice of probe design may be dependent upon application and user preference.

Claims

1. A method of measuring the hydration of blood of a human or animal, comprising the steps of: emitting microwave radiation into a body part; detecting the microwave radiation leaving the body part; calculating the absorption of the microwave radiation by the body part; and determining of the hydration of blood from the absorption of the microwave radiation.

2. A method of measuring the hydration of blood of a human or animal according to claim 1 wherein the step of calculating the absorption of the microwave radiation by the body part comprises calculating the absorption of the microwave radiation by the blood in the body part.

3. A method of measuring the hydration of blood of a human or animal according to claim 1 or 2, wherein the body part consists substantially of blood.

4. A method of measuring the hydration of blood of a human or animal according to any preceding claim, wherein the body part is a human earlobe.

5. A method of measuring the hydration of blood of a human or animal according to any preceding claim, wherein the microwave radiation is emitted at a frequency of between 0.1 GHz and 110GHz.

6. A method of measuring the hydration of blood of a human or animal according to any preceding claim, wherein the microwave radiation is emitted at a frequency of between 3 GHz and 26GHz.

7. A method of measuring the hydration of blood of a human or animal according to any preceding claim, wherein the microwave radiation is emitted of a frequency of between 13GHz and 18GHz.

8. A method of measuring the hydration of blood of a human or animal according to any preceding claim, wherein the microwave radiation is emitted at no more than 5 distinct frequencies.

9. A method of measuring the hydration of blood of a human or animal according to claim 8, wherein the microwave radiation is emitted at 2 or 3 distinct frequencies.

10. Hydration monitoring apparatus for measuring the hydration level of blood of a human or animal, comprising a probe element provided with microwave emitter element and a microwave sensor element facing the emitter element and spaced therefrom by a gap; the probe including a fixing element designed to fix the probe to a body of a human or animal part such that a selected body part fits within the gap between the emitter and sensor.

11. Apparatus according to claim 10, wherein the gap is not more than 8mm across.

12. Apparatus according to claim 11 , wherein the gap is not less than

3mm across.

13. Apparatus according to any one of claims 10 to 12, wherein the gap is sized to receive a part of a human earlobe.

14. Apparatus according to any one of claims 10 to 13, including a measuring unit coupled to the microwave sensor element and operable to measure the level of microwave radiation received by the sensor element.

15. Apparatus according to claim 14, wherein the measuring unit is operable to compared the level of microwave radiation received by the sensor element with the level of microwave radiation emitted by the emitter element.

16. Apparatus according to any one of claims 10 to 15, including a control unit operable to determine a hydration level from the measured microwave radiation received by the sensor.

17. Apparatus according to claim 16, wherein the control unit is operable to compare the determined hydration level with at least one hydration threshold level.

18. Apparatus according to claim 16 or 17, including an indicator unit coupled to the control unit and operable to provide an indication of the determined hydration level.

19. Apparatus according to claim 18, wherein the indicator unit is operable to provide at least one warning when the determined hydration level is determined to be below a predetermined threshold.

20. Apparatus according to claim 18 or 19, wherein the indicator unit is a separate unit from the probe element, the apparatus including a wired or wireless coupling between the probe element and the indicator unit.

21. Apparatus according to claim 20, wherein the control unit is located in the indicator unit.