Abstract:
System for monitoring the hydration status of a living being by exploiting the modulation of backscattered RF electromagnetic waves from a wearable patched placed on the skin, said system comprising:
       a RF-tag adapted to be directly or indirectly fixed to the skin and containing at least one passive integrated circuit coupled to an antenna;   an electromagnetic RF power source for supplying different signal frequencies to said RF-tag;   a reader adapted to receive and to process the backscattered electromagnetic waves generated by said electromagnetic power source.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of PCT application No. PCT/IB2014/064158, filed Aug. 29, 2014, the entire content of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to the monitoring of the hydration status of a living being. It more precisely relates to the topical monitoring of physiological parameters reflecting the hydration status. 
       BACKGROUND OF THE INVENTION 
       [0003]    Dehydration (hypohydration) is a condition that occurs when the loss of body fluids, mostly water, exceeds the amount that is taken in. More water is moving out of the cells and bodies than what an individual takes in through drinking With dehydration, the excessive loss of body water is accompanied by disruption of metabolic processes 
         [0004]    The term dehydration can refer to the following conditions:
       hypernatremia which is defined by an elevated sodium level in the blood (loss/deficit of free water and the attendant “excess” concentration of salt). It is related to a disruption of the body&#39;s electrolyte-water balance (osmolarity).   hypovolemia which is a state of decreased blood volume; more specifically, decrease in volume of blood plasma (loss of blood volume, particularly plasma). The volume loss can be isotonic and preserve the electrolyte-water balance or not (for instance hypotonic state: salt depletion). Hypovolemia is reported to be the most common dehydration form.       
 
         [0007]    Hypernatremia and hypovolemia can co-exist or occur independently; therefore it is important that the measurement principle of the hydration can cover both of them. Particularly, the method used for hydration estimation should be able to quantify the body water loss with an order of magnitude of a few percents variations that reflect different levels of dehydration and different symptoms (see Table below). 
         [0000]    
       
         
               
               
               
             
           
               
                   
               
               
                 Body water loss 
                 Symptoms 
                 Level of dehydration 
               
               
                   
               
             
             
               
                   &gt;2% 
                 Threshold: dehydration 
                 Minimal 
               
               
                   
                 starting 
               
               
                 &gt;3-4% 
                 Tolerable dehydration, no 
                 Mild 
               
               
                   
                 critical symptoms 
               
               
                 &gt;5-8% 
                 Fatigue &amp; dizziness 
                 Moderate 
               
               
                  &gt;10% 
                 Physical and mental 
                 Severe 
               
               
                   
                 deterioration, severe thirst 
               
               
                 &gt;15-25%  
                 Lethal 
                 Very severe 
               
               
                   
               
             
          
         
       
     
         [0008]    According to the French National Institute of Health, in 2003 the heat wave in France caused 14′802 heat-related deaths, mostly among the elderly. The number of annual deaths in the UK resulting from the heat is expected to rise by 257% by 2050. The main cause of these worrying figures is acute dehydration. 
         [0009]    In sports, dehydration during intense effort and under hot environment conditions leads to reduced performance. 
         [0010]    Body mass is often used to assess the rapid changes of hydration in both laboratory and field environments. The level of dehydration is expressed as a percentage of starting body mass. There is evidence that body mass may be a sufficiently stable physiological marker for monitoring daily fluid balance, even over longer periods (1-2 weeks). Over longer periods, changes in body composition (fat and lean mass) are also reflected grossly as changes in body mass, thus limiting this technique for assessment of hydration. This technique is not 100% robust and cannot be used for a real-time assessment of dehydration. Experts acknowledge the difficulty in attempting to assess this true hydration and promote the use of a number of the more established methods to ensure the best representation of hydration status. 
         [0011]    There is a stringent demand in terms of wearable systems able to non-invasively evaluate in real time and autonomously the hydration level and the related critical thresholds of a person in various life scenarios. 
       GENERAL DESCRIPTION OF THE INVENTION 
       [0012]    The present invention relates to a wearable system for a topical monitoring of at least one physiological parameter reflecting the hydration status such as skin conductivity, ion concentration in sweat or red blood cell characteristics, said system comprising:
       a RF-tag adapted to be directly or indirectly fixed to the skin and containing at least one passive integrated circuit coupled to an antenna;   an electromagnetic power source for supplying different signal frequencies to said RF-tag;   a reader adapted to receive and to process the backscattered electromagnetic waves generated by said electromagnetic power source.       
 
         [0016]    The present invention advantageously offers a remote type of measurement that exploits the influence of the hydration level in the body tissues on some typical characteristics (amplitude and phase) of backscattered electromagnetic waves generated by the electromagnetic source, via the induced impedance change in the antenna when the RF-tag is fixed to the skin. 
         [0017]    The RF-tag-antenna sensing works on the principle of mirroring a change in some physical parameter of interest to a controlled change in RF tag electrical properties. 
         [0018]    In the present invention sensing is obtained via a tag backscatter power, i.e. for considered transmitted power, an induced mismatch will cause less power to be transmitted to the RF-tag IC for its operations and this appears as a reduced backscatter power response of the tag. This differential backscatter power is used as a sensing mechanism. 
         [0019]    The system according to the invention may be advantageously used to non-invasively evaluate in real time the hydration level of a person by a multi-parameter extraction from the backscattering, in the form of radiating electromagnetic waves, of biological tissue. 
         [0020]    The incident electromagnetic waves may be supplied externally by a smart hub (smart phone, smart watch or any other personal mobile device) and, after interaction with the RF-tag, the same device detects the backscattered signal and evaluates, based on dedicated signal processing algorithm parameters characterizing the hydration level. The invention can be extended to other applications such as the detection of ions (e.g. sodium, chloride, etc.) or molecules (e.g. glucose, etc.) in skin and sweat, which are known to influence the electrical properties of the sweat and of the skin tissues, therefore being detectable by a changes in the backscattered signals. 
         [0021]    The data measured by the passive system are preferably wirelessly collected by the smart hub that can perform more complex signal processing and may serve as interactive display interface with an end-user. 
         [0022]    The system according to the invention is capable to detect and perform measurements of electromagnetic waves after they have interacted with the constituents of a biological tissue (typically the skin) that are in close contact with the RF-tag (all acting as an equivalent meta-material with an equivalent impedance depending on the characteristics of the tissues and body fluids) and after they have been backscattered in order to extract multiple parameters. These parameters enable the characterization and extraction of the dehydration level related to both hypernatremia and hypovolemia. 
         [0023]    The system according to the invention permits full autonomy and wireless connectivity. It is externally and remotely powered by an electromagnetic radiation emitting device and does not need any energy supply source, which constitutes a considerable advantage compared to any other hydration sensing techniques. 
         [0024]    The system according to the invention can be used in combination with existing smart phones or smart watches operating in the range of GHz frequencies but can be extended to other type of remote sensing such as the THz range. 
     
    
     
       DETAILED DESCRIPTION OF THE INVENTION 
         [0025]    The invention will be better understood below with some examples. It should be understood that they are presented to exemplify certain forms of the invention which are not intended to limit the scope of the invention. 
           [0026]      FIG. 1  represents the concept of the invention for determining hydration with a single-site backscattering measurement, multi-parameter extraction and wireless communication to a smart hub (called here mobile device) exploiting: an RF tag placed on the skin, an electromagnetic RF source such as the ones available in mobile phones, an RF reader to detect and evaluate the backscattered signals. 
           [0027]      FIG. 2  shows an equivalent circuit of the sensing meta-material composed of a tag flexible substrate, antenna and the skin. The variable impedance of the skin modulates the backscattered wave characteristics which thus carry information on the skin hydration level. 
           [0028]      FIG. 3  depicts the RF Reader that includes both the transmitter for the electromagnetic RF sources and the receiver, based on a conventional architecture but with ability to vary the frequency of the electromagnetic wave that is backscattered, according to the method described in this invention. 
       
    
    
       [0029]    In one example of the invention, the passive monitoring tag is powered by an incident wave emitted by a smart hub and comprises a thin flexible substrate (e.g. polyethylene naphthalate, polyethylene terephthalate, polyurethane, polytetrafluoroethylene, paper, polyimide, silicone, etc.) and protective insulating material, one or more antennas to sense signals, in a way adapted to contact human body (skin), and integrated circuit to send backscattered signals to the same mobile hub, and two temperature sensors for sensing both the core-body temperature and the environment temperature. The system is primarily intended to monitor body hydration level but may encompass many other uses. 
         [0030]    In the example illustrated on  FIG. 1  an external smart hub is configured to emit electromagnetic waves at a plurality of frequencies in the low GHz range (implemented in smartphones) at the passive tag in contact with an individual&#39;s skin. The backscattered waves are wirelessly sent back to a data acquisition module of the same smart hub. This mobile hub is equipped with dedicated signal processing algorithms to treat the signals, extract physiological parameters such as the water fraction of the tissue, the cardiac output, the red blood cell content and determine the hydration level. 
         [0031]    The extraction of the physiological parameters is made possible by the fact that the backscattered waves sent back by the tag antenna are modulated by the varying electrical properties (e.g. impedance, permittivity) of the skin tissue in contact with the tag. That is to say, the meta-material composed of the tag flexible substrate, antenna and the skin exhibits an equivalent impedance which varies as function of the skin properties that are particularly dependent on its level of hydration ( FIG. 2 ). The loss between the incident and backscattered waves are mainly dominated by the properties of the tissue and they modify the apparent impedance of the passive tag antenna. Permittivity changes, due to the skin tissue dehydration for instance, generate variation of the phase velocity and wavelength of the backscattered radiations. These alterations enable to extract the red blood cell content as well as the cardiac output by Doppler flowmetry for instance. Similarly, at GHz frequencies, the polarization of water molecules in the skin tissue dominates the losses by adsorption of the waves and enables to determine the tissue water content. 
         [0032]    A significant part of the invention concerns the use of different signal frequencies that enables to probe at different depth of the skin tissue. Such method scanning different frequencies in the investigation, not only provides access to different tissue layers related losses with a good control of the analyzed depth but also permits to discriminate the most relevant signals from perturbations and noise. 
         [0033]    The system according to the invention exploits a backscattering high frequency (MHz, GHz or higher) wave measurement which mirrors the impedance characteristics of a combined flexible patched antenna and of a biological tissue, depending on its hydration level. Externally supplied electromagnetic waves with different signal frequencies in the MHz to GHz range are used and their backscattering characteristics detected and measured. Different modulation schemes available for state-of-the-art tag-to-reader wireless communication can be used. From this measurement, multiple parameters reflecting both hypernatremia and hypovolemia, are extracted therefore levels of any type of body (de)hydration can be quantified. 
         [0034]    Such a system can be directly applied on the skin of a person and/or inside some types of garments. 
         [0035]    The system is non-invasive and preferably includes a remote RF-tag temperature sensing for accurate calibration on the same substrate. 
         [0036]    The system can be made in disposable (such as based on organic electronic or dissolvable materials) and non-disposable (using established and industrially available RF technologies) embodiments. The integration of the antenna can be made on a bio-compatible thin flexible material (such as polyimide) substrate on which metal lines using any type of thin metal layers define the antennas (the use of highly conductive metals or of noble metals such as platinum and gold is preferred for best performance). 
         [0037]    The RF-tag is autonomous and does not require any integrated power source. It is powered by the energy coming from the RF waves. 
         [0038]    The RF-tag is designed to wirelessly communicate (backscatter radiation from), preferably with a RF Reader integrated in a personal portable hub serving as display interface and offering full local analog-to-digital conversion and digital signal processing capability (running specific algorithms) to extract the information for hydration level but also for other complementary biological parameters characterizing body fluids and skin features. 
         [0039]    The invention may be used in different fields, in particular: 
         [0000]    First Responder (e.g. Firefighters): 
         [0040]    First responders in general and firefighters in particular, are highly exposed to dehydration: the high temperature around them, the physical effort, and the heavy clothing they wear, make them prone to high sweat rates. 
         [0041]    Studies have shown that dehydration leads to a loss of alertness, concentration and increased fatigue. This is particularly damaging for professionals who need to react swiftly and remain highly focused. 
       Athletes: 
       [0042]    Studies have shown that as the level of dehydration increases during exercise, various physiological functions are progressively impacted: heart rate and core temperature continually increase over time, while blood volume, stroke volume, cardiac output, and skin blood flow all decrease. For athletes, dehydration is the consequence of the thermal regulation of the body through sweating, which is a mix of water and minerals. In the most intensive effort, the quantity of water lost by the human body can overpass 3.5 litres. In case this loss of water and minerals is not compensated, dehydration can lead to hypovolemia (drop in blood pressure), hypokalemia (paralysis of limbs) and hyponatremia (breathing disorder, disorientation). 
         [0043]    These cases are extreme, but what is sure to be a consequence of dehydration is underperforming. Glucose is the gas of the body and glycogen is made of three molecules of water. Thus, without water, an athlete will not be able to use its reserves of glucose. It is estimated that a water loss of 1% to 2% leads to a drop in performance of 10%. 
       Elderly and Infants: 
       [0044]    Fragile segments of the population, like the elderly and infants, are also at risk of dehydration, particularly during extreme heat. The summer 2003 heat wave in France for example caused 14′802 heat-related deaths, mostly among the elderly, according to the French National Institute of Health. 9  Furthermore a recent study predicts that the number of annual deaths in the UK that occur as a result of the heat will rise by 257% by 2050. 10    
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