Abstract:
A bio metric sensor device ( 1 ) comprises: —a thin, flexible, layered sensor body ( 2 ); —a conductive sense plate ( 21 ); —a first non-conductive layer ( 41 ) between the sense plate and an outer surface ( 7 ); —a conductive shield plate ( 53 ) having a passage opening ( 54 ), overlaying the sense plate; —a second non-conductive layer ( 51 ) between the sense plate and the shield plate; —conductive circuit lines ( 73 ) on an inner surface ( 6 ); —a non-conductive separation layer ( 61, 71 ) between the shield plate and the circuit lines; —a signal processing circuit ( 100 ) mounted on the inner surface ( 6 ), the circuit ( 100 ) comprising a differential amplifier ( 110 ) having an input ( 111 ); —a conductive interconnector ( 82 ) crossing the second non-conductive layer ( 51 ) and the separation layer ( 61, 71 ), extending through the passage opening ( 54 ) of the shield plate ( 53 ), coupling the sense plate ( 21 ) and said input ( 111 ) of said amplifier ( 110 ).

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates in general to a biometric sensor for sensing bioelectrical signals. 
       BACKGROUND OF THE INVENTION 
       [0002]    It is commonly known that electrical signals are generated on various places of the human body, these signals being representative for electrical activity inside the human body. Important sources of such electrical activity are the heart, the brain, moving muscles, etc. It is already known to measure these electrical signals, and to provide a time-registration of these signals such as for instance an electrocardiogram (ECG), an electro-encephalogram (EEG), an electro-myogram (EMG), in order to obtain information regarding certain body conditions. 
         [0003]    When measuring these signals, some problems have to be overcome. A first problem relates to the fact that the human skin is a poor conductor. In this context, measuring sensors can be classified as follows. A penetrating sensor, for instance a needle, penetrates the skin and will have a good electrical contact with the conductive parts of the body below the skin, but such sensors are not suitable in practical situations. Contact electrodes, in the form of a conductive plate placed in close contact with the skin, suffer from the relatively high contact resistance between the sensor and the skin. In order to reduce this problem by improving the galvanic contact, wet electrodes are used, comprising a conducting gel (containing silver chloride) between the conductive plate and the skin; however, this gel can cause irritations or even allergic reaction. 
         [0004]    In order to overcome the above-mentioned problems and disadvantages of contact electrodes, contact less sensors have already been developed for measuring the electrical signals by a capacitive coupling. However, such capacitive sensors introduce problems of a different kind. The most important problems in this respect are related to the fact that such capacitive sensors are also sensitive to electrical signals generated by the surroundings. Important sources of disturbance signal or noise signals are the electrical mains wiring (carrying voltages in the order of 230 V or more) or moving bodies which are charged electrostatically to a high voltage (which may be in the order of 1000 V or more). 
         [0005]    A second problem relates to comfort for the user. In practice, capacitive biometric sensors have already been proposed which are rigid and relatively heavy. Although biometric sensors have several possible applications, one important field of application is implementation in and or integration with clothing. In such applications, rigid sensors are undesirable, because they are not comfortable for the user. Further, rigid sensors have the problem of providing only poor contact with the skin: for a good contact, it is required that the biometric sensor has sufficient flexibility to adapt to the curvature of the body and to follow changes in this curvature, for instance in the case of moving muscles. 
         [0006]    The same types of problems are encountered when such sensors are implemented in the surface material of a chair, or a bed, or an examination table, allowing to easily obtain body-signals of a person without having to specifically apply sensors to the skin of that person. 
         [0007]    International patent publication WO 2005/032368 discloses a flexible biometric sensor, which provides a capacitive coupling with the skin. The sensor of this publication comprises a conductive cloth, provided by incorporating conductive wires in a textile material. A disadvantage of such design is that it requires an adaptation to the textile manufacturing process. A further disadvantage is that such cloth will typically cover a relatively large surface area, so that the spatial resolution of the sensor is relatively low. Conversely, if a cloth with a relatively small surface area would be used, such sensor would contain only a low number of conductive wires, providing only a poor coupling with the signals to be detected. 
         [0008]    On the other hand, such sensor will be quite sensitive for signals from the surroundings, and it will be very difficult to discriminate between actual body signal and noise signals. In this respect it is worth noting that the noise signals may have amplitudes in the order of 100 mV or more, whereas the actual body signals may have amplitudes in the order of 1 mV or less. 
         [0009]    The present invention aims to overcome the above-mentioned problems and disadvantages. 
         [0010]    Specifically, the present invention aims to provide a biometric sensor device that has sufficient flexibility for adaptation to the curvature of the human body, is suitable for incorporation in clothing, and has reduced sensitivity for electrical signals from the surroundings. 
       SUMMARY OF THE INVENTION 
       [0011]    According to an important aspect of the present invention, a biometric sensor device comprises a stack of flexible, conductive layers, separated from each other by flexible insulating layers. A first layer comprises a sensing area. A second layer comprises a guard plate. The device further comprises integrated signal processing circuitry, and a further conductive layer, connected to a predetermined voltage level, preferably zero voltage, covering the electrical circuit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    These and other aspects, features and advantages of the present invention will be further explained by the following description of a preferred embodiment of the sensor device according to the present invention with reference to the drawings, in which same reference numerals indicate same or similar parts, and in which: 
           [0013]      FIG. 1A  schematically shows a top view of a biometric sensor device according to the present invention; 
           [0014]      FIG. 1B  schematically shows a top view of the biometric sensor device of  FIG. 1A  from the opposite direction; 
           [0015]      FIG. 2  is a schematic cross section of a part of a flexfoil; 
           [0016]      FIG. 3  is a schematic cross section of the biometric sensor device of  FIG. 1A ; 
           [0017]      FIGS. 4A-C  schematically illustrate steps in a possible manufacturing process for manufacturing the biometric sensor device of  FIG. 1A ; 
           [0018]      FIG. 5  is a block diagram of an electronic signal processing circuit of the biometric sensor device of  FIG. 1A . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]      FIG. 1A  is a schematic inside view of a preferred embodiment of a biometric sensor device  1  according to the present invention, and  FIG. 1B  is a schematic outside view of the same device. The sensor device  1  comprises a thin, flexible sensor body  2 , comprising two wing parts  3 ,  4  attached to each other at a fold portion  5 . The sensor body  2  has two opposite main surfaces, i.e. a first main surface  6  visible in the inside view of  FIG. 1A , and an opposite second main surface  7 , visible in the outside view of  FIG. 1B . In use, the two wing parts  3 ,  4  will be folded together, such that the fold portion  5  takes the shape of a loop, and the first main surfaces  6  of the two wing parts will be facing each other; for this reason, the first main surface  6  will also be indicated as “inside surface”, whereas the opposite second main surface  7 , which will be on the outside of the device when folded as mentioned, will also be indicated as “outside surface”. 
         [0020]    The shape of the contour of the two wing parts  3 ,  4  is not essential. Typically, these two wing parts will have identical contours, but even that is not essential. In the illustrated embodiments, the two wing parts have an octagonal contour, but other contours, such as a circular contour, are also possible. 
         [0021]    The first wing part  3  has a first series of through holes  8  along its perimeter; likewise, the second wing part  4  has a second series of through holes  9  along its perimeter. The first holes  8  and the second holes  9  are located such that, when the two wing parts  3  and  4  are folded together, the first holes  8  and the second holes  9  are aligned with each other. These holes facilitate the sensor device  1  being attached to clothing, for instance by stitches. 
         [0022]    As will be explained in more detail below, the first wing part  3  has, on its outside surface  7 , a substantially centrally located, electrically conductive sense plate  21  and an annular, electrically conductive guard ring  22  arranged around the sense plate  21 . The shape of the sense plate  21  is not critical, but a circular shape is preferred. Likewise, the shape of the guard ring  22  is not critical, but a circular shape is preferred for the guard ring as well. The diameter of the sense plate  21  is not critical, and typically is a trade-off between on the one hand positional accuracy and on the other hand electrical sensitivity. In a suitable embodiment, the diameter of the sense plate  21  will typically be in the range from 10 to 15 mm, and an experimental embodiment has a diameter of 12 mm. The guard ring  22  may typically have a width in the order of 1 to 2 mm, and the radial distance between the sense plate  21  and the guard ring  22  may also typically be in the range of 1 to 2 mm. 
         [0023]    As visible in the inside view of  FIG. 1A , the first wing part  3  carries contact pads  16  for attaching external lines, and electronic circuit components  17 . 
         [0024]    As indicated by dotted lines, the sensor body  2  also comprises a first ground plate  13  in the first wing part  3 , and a second ground plate  14  in the second wing part  4 . These ground plates  13 ,  14 , which are both electrically conductive yet thin enough to be mechanically flexible, are located at a distance from the inside surface  6  and at a distance from the outside surface  7 , and are therefore shown in dotted lines in  FIGS. 1A and 1B . 
         [0025]    As shown in  FIG. 1A , the first wing part  3  has, at its inside surface  6 , at least one electrically conductive contact region  11 , which is electrically connected to the first ground plate  13 . Likewise, the second wing part  4  has at least one electrically conductive contact  12 , which is electrically connected to the second ground plate  14 . In the embodiment shown, the first wing part  3  has two contacts  11  located diametrically opposite to each other, and the same applies to the second wing part  4 . The contacts  11  and  12  are located such that, when the sensor body is folded, the contacts  11  and  12  of the two wing parts  3  and  4  are aligned with each other. Thus, these contacts assure an electrical connection between the first ground plate  13  and the second ground plate  14 . The contacts may also be used for mechanically sealing the sensor body  2  in its folded condition. In a possible embodiment, the contacts  11 ,  12  may be provided with a solder tin, and a local heat treatment after folding the sensor body  2  may cause the opposite contacts  11 ,  12  to be soldered to each other. 
         [0026]    In the following, a more detailed description of the internal design of the sensor body  2  will be given. 
         [0027]    First, reference is made to  FIG. 2 , schematically showing a cross section through a flexible foil  30 , commonly known as “flex foil”, and comprising a first layer  31  and a second layer  32 . The first layer  31  is electrically substantially non-conductive, and the second layer  32  is electrically substantially conductive. Typically, the second layer  32  is a thin copper layer, having a thickness in the order of about 10 to 20 μm. In a standard available product, this thickness is about 17.5 μm, in another standard available product this thickness is about 35 μm. The second layer is an insulator with the electric properties. A typical material for the non-conductive first layer  31  is capton. A flex foil  30  of the design of  FIG. 2  is commercially available, in different sizes of the thickness of the non-conductive first layer  31 , and this product is typically used as so-called “flexible PCB”. Since this material is known per se, as will be clear to a person skilled in the art, a further description is not needed here. However, it is noted that such commonly known flex foil  30  can be used in manufacturing the sensor body  2 , more particularly by attaching multiple layers of flex foil  30  on top of each other, as will be clear from the following description. Attaching can be done by using a suitable adhesive, or by performing a heat treatment causing the capton layers to flow and adhere to neighbouring layers. 
         [0028]      FIG. 3  schematically shows, not to scale, a cross section of the biometric sensor device  1  along the line III-III in  FIG. 1B . In this embodiment, the sensor body  2  is comprised of a stack of four flex foil layers  40 ,  50 ,  60 ,  70  attached on top of each other. The first flex foil layer  40  has its non-conductive layer  41  directed to the outside of the device, such that this first non-conductive layer forms the outside surface  7  of the sensor device  1 . Using commonly known techniques, such as etching, a part of the second conductive layer  42  has been removed, leaving the conductive sense plate  21  and the conductive annular guard ring  22  around the sense plate  21 . 
         [0029]    In use, the sensor device  1  may be brought in close proximity to the skin of a human body to be examined, and may even be brought in contact with this body. Then, the non-conductive layer  41  will act as an electric insulator, providing a galvanic insulation between the body and the conductive sense plate  21 , and also acting as a dielectricum between the human body and the sense plate  21 . Thus, the sense plate  21  will pick up variations in the electrical field present in the human skin. 
         [0030]    The second flex foil  50  is attached to the first flex foil  40 , such that the second non-conductive layer  51  is in contact with the first conductive layer  42 . Effectively, this means that the sense plate  21  and the guard ring  22  are completely enclosed within two non-conductive layers  41  and  51 . It is noted that, for sake of clarity, the first conductive layer  42  is depicted over the entire extent of the sensor body  2 , even in those locations where the conductive material has been removed. Thus, where the original flex foil  40  had comprised a conductive layer  42  over its entire surface, the first flex foil  40  in the sensor body  2  only has the conductive portions  21  and  22  remaining. Outside these portions  21  and  22 , the layer  42  is actually not present anymore, so that the first non-conductive layer  41  and the second non-conductive layer  51  are actually attached directly to each other in those portion where the first conductive layer  42  has been removed. However, for sake of clarity, the drawing of  FIG. 3  shows a distance between first non-conductive layer  41  and second non-conductive layer  51 , representing the removed portions of first conductive layer  42 . The same applies, mutatis mutandis, for the other layers, as should be clear to a person skilled in the art. 
         [0031]    It is further noted that the active portions of the sensor device  1  as far as the first conductive layer  42  is concerned, are the said portions  21  and  22 . The first conductive layer  42  may have been removed entirely outside these portions  21  and  22 , but it is also possible that further portions of the first conductive layer  42  are still remaining, having no active function for the sensor device, having no disturbance on the functioning of the sensor device, and possibly even contributing to the shielding of outside fields, as long as such further portions of first conductive layer  42  are not in electrical contact with the portions  21  or  22 . 
         [0032]    In the second conductive layer  52  of the second flex foil  50 , a guard plate  53  is defined, having, in the most preferred embodiment, an extent which at least corresponds to the extent of the guard ring  22 , and may be even extending beyond the outer perimeter of the guard ring  22 . Outside the guard plate  53 , the second conductive layer  52  has been removed entirely in this embodiment. 
         [0033]    The third flex foil layer  60  is attached to the second flex foil layer  50 , such that the third non-conductive layer  61  of the third flex foil layer  60  is in contact with the guard plate  53 ; thus, the guard plate  53  is entirely embedded between non-conductive layers  51  and  61 . In the third conductive layer  62  of the third flex foil  60 , only a small portion around the perimeter of the sensor body  2  has been removed, so that in the first wing  3  a large portion  63  of the third conductive layer  62  remains, defining the first ground plate  13  of the first wing part  3 . Likewise, a large portion  66  of the third conductive layer  62  remains in the second wing part  4 , defining the second ground plate  14  of the second wing part  4 . 
         [0034]      FIG. 3  shows that the third conductive layer  62  may still be present in the fold portion  5  of the sensor body  2 . Then, it is desirable that parts of the third conductive layer  62  are etched away in this folding portion  5 , leaving a few small conductive lines  15  connecting the first ground plate  13  with the second ground plate  14 , as shown in  FIG. 1A . By removing a large part of the third conductive layer  62  in the fold portion  5 , the flexibility of this fold portion  5  is improved. As long as these conductive lines  15  are intact, the contacts  11  and  12  may even be dispensed with. However, in the case of an embodiment having contacts  11  and  12  as mentioned before, the connecting lines  15  may be dispensed with, in which case the third conductive layer  62  may be removed entirely in the fold portion  5 , further increasing the flexibility of the fold portion  5 . 
         [0035]    The fourth flex foil layer  70  has its fourth non-conductive layer  71  attached to the third conductive layer  62  of the third flex foil  60 . The fourth conductive layer  72  of the fourth flex foil  70  defines the inner surface  6  of the sensor device  1 . The fourth conductive layer  72  has been etched away over a large part, leaving the electric contacts  11  and  12 , and also leaving electric circuit lines connecting the terminals of the circuit components  17  and the contact pads  16 . Since the fourth conductive layer  72  has been removed over the major part of the surface of the fourth flex foil  70 , one may also say that the inside surface  6  of the sensor device  1  is defined by the free surface of the fourth non-conductive layer  71 , and that this inside surface  6  is provided with conductive contact portions  11  and printed circuit lines  73 ,  74 ,  75 . 
         [0036]    The guard ring  22  is electrically connected to the guard plate  53 , by at least one electrical conductor  81  which crosses the second non-conductive layer  51  and which hereinafter will be indicated as an “interconnector”. In the preferred embodiment, the sensor device  1  comprises a series of such interconnectors  81 , arranged in a circular pattern, at mutual intervals, which may be as small as 1-3 mm. The guard plate  53  acts as a shield against electrical fields, largely preventing such electrical fields from reaching the sense plate  21 . The combination of the shield ring  22  and the array of interconnecting connectors  81  further improves the shielding effect, more or less as a Faraday&#39;s cage. The ground plate  13  of the first wing part  3 , which in use will be connected to a predefined voltage level, preferably zero voltage, further helps to shield off such electrical field. It can easily be seen that, when the sensor device  1  is applied to the skin of a human body, there remains only a small gap between the ground plane  13 ,  63  and the outer surface  7  of the sensor device, this gap having a width defined by the combined thicknesses of the three non-conductive layers  41 ,  51  and  61 , which will typically be less than 100 μm. Electrical field lines which are capable of penetrating this gap are further shielded by the Faraday&#39;s cage defined by guard plate  53 , guard ring  22 , and interconnectors  81 . 
         [0037]    In order to keep the possible influence of electrical fields from the surrounding as small as possible, an electrical circuit for processing the pick up signals is placed on the inner surface  6  of the first wing part  3 , having its input terminal as close to the sense plate  21  as possible. According to an important aspect of the present invention, a small opening  54  is defined in the second conductive layer  52 , for instance by etching away a corresponding small portion of the second conductive layer  52 , and likewise a small opening  64  is arranged in the third conductive layer  62 , these two openings  54  and  64  being aligned with each other. A first circuit portion  73  of the fourth conductive layer  72  is defined in alignment with said openings  54  and  64 .  FIG. 3  shows that the first circuit portion  73  and said openings  54  and  64  are aligned with the sense plate  21 , and that a second interconnector  82 , passing the second, third and fourth non-conductive layers  51 ,  61  and  71 , connects the sense plate  21  to the first circuit portion  73 , extending through said opening  54  and  64 , such that this second interconnector  82  does not contact the guard plate  53  nor the ground plate  63 .  FIG. 3  also shows a circuit component  17  in the form of a package with terminal leads, an input terminal lead  17   a  being electrically connected to said first circuit portion  73 . In the preferred embodiment, this input terminal lead  17   a  is substantially aligned with the second interconnector  82 . The circuit component  17  shown in  FIG. 3  comprises an amplifier, as will be explained later. 
         [0038]    According to a further important aspect of the present invention, a second opening  65  is defined in the ground plate  63 , and a third interconnector  83  connects a second circuit portion  74  of the fourth conductive layer  72  with the guard plate  53 . This third interconnector  83  may even, as shown, extend to the guard ring  22 . The third interconnector  83  thus passes the second, third and fourth non-conductive layers  51 ,  61  and  71 , contacts the second conductive layer  52 , and extends through the second opening  65  of the third conductive layer  62  such as not to make electrical contact with the third conductive layer  62 . The second circuit portion  74  is connected, through a printed circuit line of the fourth conductive layer  72 , to a third circuit portion  75 , to which an output terminal lead  17   b  of the amplifier component  17  is connected. 
         [0039]    An important feature of the interconnectors  81 ,  82 ,  83  is that they do not extend through the first non-conductive layer  41 . Thus, although the first interconnector  81  makes contact with the guard ring  22  portion of first conductive layer  42 , the first interconnector  81  does not extend through the first non-conductive layer  41 . More particularly, the first non-conductive layer  41  always covers the first interconnector  81  in order to prevent the possibility of galvanic contact with the first interconnector  81  from the side of the outside surface  7 . The same applies to the second and third interconnectors  82  and  83 . 
         [0040]    In  FIG. 3 , the interconnectors  81 ,  82 ,  83  are illustrated as thin, longitudinal conductors. Although such embodiment is not impossible, it is rather impractical in view of the small thickness of the flex foil layers. In a more practical, preferred embodiment, the interconnectors  81 ,  82 ,  83  are provided as metallized via&#39;s. The art of making metallized via&#39;s to provide a through-connection between two conductive layers on opposite sides of a thin non-conductive substrate is an art known per se. Nevertheless, the following figures schematically illustrate possible steps in a manufacturing process for manufacturing a sensor device according to the present invention. 
         [0041]      FIG. 4A  schematically shows a cross section of a part of the first flex foil  40 , comprising the first non-conductive layer  41  and the first conductive layer  42  extending over the entire surface. Parts of the conductive layer  42  are removed, for instance by an etching process, so that the sense plate  21  and the guard ring  22  remain. In this condition, this flex foil will be indicated as first intermediate product  240 . 
         [0042]    In a similar manner,  FIG. 4B  illustrates the second flex foil  50  with the second non-conductive layer  51  and the second conductive layer  52  extending over the entire surface. Parts of the conductive layer  52  are removed, so that the guard plate  53  with the opening  54  remains. In a next step, via&#39;s  251  and  252  are made, extending as through hole over the entire thickness of the second flex foil  50 . A first via  251  penetrates the guard plate  53 , a second via  252  is aligned with the opening  54 . In this condition, the foil will be indicated as second intermediate product  250 . As will become clear, the first via  251  in  FIG. 4B  actually represents a series of vias in a circular pattern. 
         [0043]    In a next step, the first and second intermediate products  240  and  250  are attached onto each other, as illustrated in  FIG. 4C , in such a way that the first via&#39;s  251  are aligned with the guard ring  22 , while the second via  252  is aligned with the sense plate  21 . The resulting product will be indicated as a stacked intermediate product  280 . 
         [0044]    In a next step, the first via&#39;s  251  are metallized. Since metallization processes are known per se, such process will not be explained here. It suffices to note that the metallization  253  in the via  251  makes electrical contact with the shield ring  22  as well as with the shield plate  53 . In the left-hand side of  FIG. 4C  is illustrated that the metallization  253  may be provided as a solid filling of the via  251 , but the right-hand side of  FIG. 4C , especially the enlarged detail, illustrates that the metallization  253  may be provided as a cylindrical conductor. In both cases, the figure shows that the metallization  253  has a head portion (left hand side) or a collar portion (right hand side) extending above the free surface of the guard plate  53 , but the metallization process may also be performed in such a way that the metallization  253  is flush with the free surface of the guard plate  53 . 
         [0045]    In a similar way, the third flex foil  60  may be processed to provide a third intermediate product comprised of the third non-conductive layer  61  and the ground plate  63  with openings  64  an  65 , the fourth flex foil  70  may be processed to provide a fourth intermediate product comprised of the fourth non-conductive layer  71  with contacts  11  and  12  and with printed circuit portions  73 ,  74 ,  75 , the fourth intermediate product  270  may be processed to provide via&#39;s aligned with the contacts  11 ,  12 , the third and fourth intermediate products may be attached to each other, and via&#39;s may be provided in the stacked third and fourth intermediate products, extending through first contact portion  73  and first opening  64  and extending through second contact portion  74  and corresponding opening  75 , over the entire thickness of the two stacked intermediate products. These steps are not individually illustrated. Then, the stacked combination of third and fourth intermediate products is attached to the stacked intermediate product  280 , such that the via extending through the first contact portion  73  and corresponding opening  64  is aligned with the second via  252 , and such that the second circuit portion  74  and corresponding opening  65  are aligned with the guard plate  53 . It should be noted that it is now not necessary that this via is aligned with the metallized first via  251 . 
         [0046]    Then, in a next processing step, the via&#39;s are metallized. The metallization of a via extending through a contact  11  or  12  will make electrical contact with such contact  11 ,  12  on the one hand and with the ground plate  63  on the other hand, thus providing an interconnector  84 . The metallization of the via extending through the second circuit portion  74  and the second opening  65  will make electrical contact with the second circuit portion  74  on the one hand and the guard plate  53  on the other hand, but will not make contact with the ground plate  63  in view of the relatively large opening  65 . Similarly, the metallization of the via extending through the first circuit portion  73  and the first opening  64 , and aligned with the second via  252 , will make electrical contact with on the one hand the first circuit portion  73  and on the other hand the sense plate  21 , but will not make electrical contact with the guard plate  53  nor with the ground plate  63  in view of the dimensions of the openings  54  and  64 . 
         [0047]      FIG. 5  is a block diagram schematically illustrating an input stage of a signal processing circuit  100  attached on the inner surface  6  of the first wing portion  3 . As an important component of this processing circuit, the figure shows a differential amplifier  110 , such as an operational amplifier, with a non-inverting input  111 , an inverting input  112 , and an output  114 . This amplifier  110  is part of the component  17  shown in  FIG. 3 , and the non-inverting input  111  is connected to first terminal lead  17   a  while the output  114  is connected to second terminal lead  17   b.    
         [0048]      FIG. 5  shows that the sense plate  21  is connected to the non-inverting input  111  of the amplifier  110 , through a conductor  121  which is designed to be as short as possible, and which includes the metallized via  82  and possibly a short piece of printed circuit line  73 . The amplifier  110  is a type having a very high input impedance. The amplifier  110  is basically connected as a buffer amplifier, having its inverting input  112  connected to its output  114  through a line  124 , so that the amplifier&#39;s output  114  carries the same voltage signal as the amplifier&#39;s input  111 . The circuitry  100  may have further signal processing components, or the amplifier&#39;s output  114  may be connected straight to one of the contact pads  16 , but this is not essential and not illustrated in the figures. 
         [0049]    In use, when placed in close proximity to a person&#39;s body, the sense plate  21  has a capacitive coupling with the body, the first insulating layer  41  acting as a dielectricum. The capacitance value of this coupling is typically in the order of a few pF. The input  111  of the amplifier  110  has an input resistance which, in a suitably selected amplifier, may be approximated by infinity. However, it is desirable to provide a defined leak-resistance to zero voltage level, which is provided by the resistance  130  connected between the amplifier&#39;s input terminal  73  and ground. The combination of coupling capacity and leak-resistance forms a high-pass filter. It is desirable to have the characterizing turnover frequency of this high-pass filter as low as possibly, in the order of 0.2 Hz. This leads to a design value of 100 GΩ or higher for the resistance  130 . 
         [0050]    Apart from a capacitive coupling with the body, the sense plate  21  also has a capacitive coupling with sources of electrical voltages in the surroundings. Although this coupling has a very low capacitance value, in the order of a few fF, the voltage levels of such sources may be quite high, so that the resulting voltage induced as a result of this coupling in the sense plate  21  may typically range in the order of 100 mV. The function of the shield plate  53  located closely behind the sense plate  21 , enhanced by the preferred shield ring  22  and the series of interconnectors  81  surrounding the sense plate  21 , is to shield off such disturbing electrical fields, effectively reducing the coupling capacitance between the sense plate  21  and the surroundings. 
         [0051]    It is to be noted that the sense plate  21  also has a capacitive coupling with the shield plate  53  and the shield ring  22 . Any difference in voltage level between the sense plate  21  and the shield plate  53  will cause a disturbing current between the sense plate  21  and the shield plate  53 , affecting the measuring signal. In order to eliminate or at least reduce this problem, the shield ring  22  and the shield plate  53  are connected to the amplifier&#39;s output  114  via a line  122 , which may include a resistor  123 , which may have a value in the order of a few kilo-ohms. As a result, the voltage level of the shield ring  22  and the shield plate  53  will be substantially equal to the voltage level of the amplifier&#39;s output  114 , which in turn is substantially equal to the voltage level of the amplifier&#39;s input  111 , hence substantially equal to the voltage level of the sense plate  21 . Thus, such disturbing currents are effectively avoided. Also, disturbing currents caused by possible fouling of the interface between insulating layers  41  and  51  are likewise effectively avoided. 
         [0052]    Although the shield plate  53  shields the sense plate  21  against outside electrical fields, the interconnector  82 , the amplifier&#39;s input terminal  17   a , and the printed circuit lines  73  connected to the amplifier&#39;s input terminal  17   a , are all located “beyond” the shield plate  53 , so they still have a capacitive coupling with the surroundings. Also, a creep current may be caused by some fouling of the inside surface  6 . In order to reduce the potential problems caused by such fouling, the fourth conductive layer  72  comprises a conductive shield line  125  which surrounds all printed circuit lines  73 ,  121  connected to the sense plate  21 , as shown in dotted lines in  FIG. 5 , which shield line  125  is also connected to the amplifier&#39;s output  114 . 
         [0053]    In practice, it may be difficult to find a resistor specimen having the desired resistance value of 100 GΩ, and/or such resistors are bulky and expensive. As a consequence, it may be necessary to form the leak-resistance  130  as a combination of two (or more) resistors  131 ,  132  in series. Then, the node A between two of those resistors  131 ,  132  forms a capacitive coupling with the surroundings, which, via the resistor  131 , may still affect the signal at the amplifier&#39;s input  111 . To reduce this effect, this node A is also surrounded by a guard ring  140 , which is also connected to the amplifier&#39;s output  114 , not directly, but by connecting this guard ring  140  to a node B of a series combination of two (or more) resistors  141 ,  142 . These resistors are chosen such that the ratio of resistance values R( 141 )/R( 142 ) is substantially equal to the ratio of resistance values R( 131 )/R( 132 ). 
         [0054]    To further reduce the effect of the circuit lines and circuit components being sensitive to outside electrical fields, the sensor device  1  has the second wing part  4  with the second ground plate  14 , which is electrically connected to the first ground plate  13 , either via one or more conductive lines  15  in the third conductive layer  62 , or via the contacts  11 ,  12 , or both. In the ready-to-use condition, when the second wing  4  is folded over the first wing  3 , the second ground plate  14  extends over the circuitry  100 , i.e. actually covers the circuit components  17 ,  110 ,  123 ,  131 ,  132 ,  141 ,  142 , and interconnecting circuit lines  73 ,  74 ,  75 ,  121 ,  122 ,  124 ,  125 , thus providing a shield against external electrical field for these components and circuit lines, which are enveloped between the two ground plates  13  and  14 . In this context, it is preferred that the two ground plates  13  and  14  have their edges electrically connected together on opposite sides of the circuitry  100 . For this reason, the contacts  11  are located on opposite sides of the circuitry  100 . 
         [0055]    It should be clear to a person skilled in the art that the present invention is not limited to the exemplary preferred embodiment discussed above, but that several variations and modifications are possible within the protective scope of the invention as defined in the appending claims. 
         [0056]    For instance, instead of using two-layered flexfoils, it is possible to use a flexfoil with two conductive layers on opposite sides of a non-conductive layer, or a flexfoil with two non-conductive layers on opposite sides of a conductive layer. 
         [0057]    Further, although in the preferred embodiment the first and second ground plates  13  and  14  are implemented as portions  63  and  66  of one and the same conductive layer  62 , it is possible that the second ground plate  14  of the second wing  4  is implemented as a portion of a different conductive layer  42 ,  52 , connected to corresponding contacts  12  via corresponding interconnectors. 
         [0058]    Further, although in the preferred embodiment the device comprises two wing parts folded onto each other, it is also possible that the two wing parts are implemented as separate items stacked on top of each other. 
         [0059]    Further, although in the preferred embodiment the guard plate  53  is a “solid” plate having a contour and size corresponding to the contour and size of the guard ring  22 , it is possible that the guard plate is somewhat smaller, and/or that the guard plate has small interruptions, such as to have for instance a contour in the shape of spokes, without losing its functionality entirely.