Abstract:
A moisture sensor includes a pair of electrode plates separated by a moisture absorbent material that forms the dielectric of a capacitive sensor. As the absorbent dielectric material absorbs moisture, such as perspiration, the capacitance of the sensor changes reflecting a quantitative measure of perspiration absorbed. The sensor can be stabilized by capacitively coupling the dielectric material to the skin of the user to improve sensor stability and noise rejection. The sensor can include a capacitive sensing integrated circuit that measures the capacitance of the sensor in close proximity to the electrodes to limit the introduction of noise.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims any and all benefits as provided by law including benefit under 35 U.S.C. §119(e) of the U.S. Provisional Application No. 62/127124, filed Mar. 2, 1015, the contents of which are incorporated herein by reference in its entirety. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
       [0002]    Not Applicable 
       REFERENCE TO MICROFICHE APPENDIX 
       [0003]    Not Applicable 
       BACKGROUND 
       [0004]    1. Technical Field of the Invention 
         [0005]    The present invention is directed to a moisture sensor and more specifically, to a perspiration sensor for quantitatively measuring levels of perspiration. In accordance with some embodiments, the perspiration sensor includes a capacitive sensor that can be coupled to the skin and utilize capacitance to measure perspiration. The perspiration sensor can be shielded and stabilized to reduce noise by electrically coupling the capacitor dielectric material to the skin. 
         [0006]    2. Description of the Prior Art 
         [0007]    The prior art perspiration sensors are generally related to devices for determining galvanic skin response. These devices involve the measurement of the electrical resistance of the skin, but do not provide a quantitative indication of the volume or level of perspiration over time. 
       SUMMARY 
       [0008]    The present invention is directed to a method and system for obtaining a quantitative measurement of moisture, and more specifically, perspiration. Various embodiments of the invention utilize the dielectric properties of perspiration in a capacitive sensor that includes a pair of ground shielded parallel electrodes sandwiching a moisture absorbent dielectric material (e.g., a microfiber cloth). In accordance with some embodiments, the dielectric material can be electrically coupled (e.g., capacitively coupled) to the skin of the user to provide a more stable signal because the skin is capacitively coupled to earth ground which is weakly capacitively coupled to the ground of signal measurement circuit (e.g., the capacitance to digital signal converter integrated circuit). The method includes a providing a capacitor that includes a dielectric material that absorbs moisture (e.g., perspiration) in contact with the skin, wherein the capacitor includes one or more inlets that enable perspiration released from the skin to become absorbed by the absorbent dielectric material and electrically (e.g., capacitively) coupling the dielectric material to the body. 
         [0009]    In accordance with the invention, the perspiration sensor can be constructed having three layers, a first or top layer, a second or middle layer and a third or bottom layer. The bottom layer can include an adhesive or other skin contacting material that maintains contact with and capacitively couples the sensor to the skin. The bottom layer and the top layer include the first and second electrode plates that form the capacitor with dielectric material surrounded by the middle layer sandwiched in between. The bottom layer, middle layer and top layer form a central chamber that encloses the dielectric material and positions it in a dielectric space between the first and second electrodes. The bottom layer can also include one or more pores, inlets or vents that enable perspiration released from the skin to enter central chamber and be absorbed by the dielectric material in the dielectric space. The bottom layer can also include a skin coupling electrode that becomes electrically coupled to the skin when the bottom surface of the bottom layer is adhered to or placed in contact with the skin. The skin coupling electrode can be electrically connected to the central chamber and the dielectric material. When the bottom surface includes an adhesive material, the skin coupling electrode capacitively couples the dielectric material in the central chamber to the skin to provide ground shielding adjacent to the electrodes to minimize environmental noise. In accordance with some embodiments of the invention, the dielectric material can be weakly but consistently (capacitively) coupled to earth ground through the skin. 
         [0010]    Each of the layers can be formed from a rigid printed circuit board (PCB) or a flexible PCB, and each of the layers can be laminated together as is well known in the art. The dielectric material can be an absorbent material that rapidly replaces air with perspiration, such as a tufted microfiber cloth. The thickness of the middle layer can be selected to define and control the distance between the sensing electrode plates of the capacitor. In accordance with some embodiments, the middle layer includes a rigid material having a predefined thickness to tightly control the electrode plate spacing and slightly thinner than the thickness of the microfiber cloth causing portions of the cloth to extrude through the pores or inlets of the bottom layer facilitate moisture absorption. 
         [0011]    In accordance with some embodiments of the invention, the device can further include a capacitance measurement integrated circuit (e.g., IC chip) mounted to the top layer that enables close proximity measurement of the change in capacitance of the sensor to minimize the introduction of noise. In this configuration, the measured capacitance can be transmitted (e.g., by wire or wirelessly) to a remote system for storing and/or analyzing the capacitance data and determining perspiration rates (e.g., volume and volume over time). 
         [0012]    In accordance with some embodiments of the invention, the device can be fully or partially enclosed or encapsulated in polymer or elastomeric material (e.g. PDMS, or silicone) that protects the sensor from the environment. One or more of the layers can include one or more partially or fully enclosed anchor rings projecting from the perimeter of the device such that the polymer material passes through the opening in the anchor rings to more securely anchor the encapsulating material to the outside of the device. 
         [0013]    In operation, the bottom of the device is coupled to the skin by an adhesive material and perspiration released by the skin passes through the inlets and become absorbed by the microfiber dielectric material. The capacitance of the first and second electrodes changes as the dielectric material absorbs perspiration. In addition, the bottom of the device includes a skin electrode that is electrically connected to the dielectric material and the central chamber. The skin electrode can be electrically connected to the middle layer by plated through holes or vias in the bottom layer. The vias in the middle layer can include electrodes that directly contact the dielectric material, enabling the dielectric material to be coupled (e.g., capacitively coupled) to the skin through the adhesive. The integrated circuit on the top layer includes a circuit that determines a measure of the capacitance between the first and second electrodes while the skin electrode serves to stabilize the capacitance measurement against noise. 
         [0014]    These and other capabilities of the invention, along with the invention itself, will be more fully understood after a review of the following figures, detailed description, and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0015]    The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee. 
           [0016]    The accompanying drawings, which are incorporated into this specification, illustrate one or more exemplary embodiments of the inventions and, together with the detailed description, serve to explain the principles and applications of these inventions. The drawings and detailed description are illustrative, and are intended to facilitate an understanding of the inventions and their application without limiting the scope of the invention. The illustrative embodiments can be modified and adapted without departing from the spirit and scope of the inventions. 
           [0017]      FIG. 1  is a photograph of a perspiration sensor according to an embodiment of the invention. 
           [0018]      FIGS. 2A and 2B  show diagrammatic views of a silicon encapsulated perspiration sensor according to an embodiment of the invention. 
           [0019]      FIGS. 3A and 3B  show diagrammatic views of a perspiration sensor according to some embodiments of the invention. 
           [0020]      FIG. 4  shows an exploded isometric top view of a perspiration sensor according to an embodiment of the invention. 
           [0021]      FIG. 5  shows an exploded isometric bottom view of a perspiration sensor according to an embodiment of the invention. 
           [0022]      FIG. 6A  shows an exploded cross-section view of a perspiration sensor according to an embodiment of the invention. 
           [0023]      FIG. 6B  shows a cross-section view of an assembled perspiration sensor according to an embodiment of the invention shown in  FIG. 6A . 
           [0024]      FIG. 7A  shows a diagram of a perspiration sensor according to some embodiments of the invention,  FIG. 7B  shows a diagrammatic view of the middle layer according to an embodiment of the invention, and  FIG. 7C  shows a cutaway view of a perspiration sensor according to some embodiments of the invention with a portion of the encapsulation material removed. 
           [0025]      FIGS. 8A and 8B  show diagrams of dielectric material according to some embodiments of the invention. 
           [0026]      FIGS. 9A and 9B  show diagrams of electronic circuits for measuring the change in capacitance between the electrodes of a perspiration sensor according to some embodiments of the invention. 
           [0027]      FIGS. 10A, 10B, 10C and 10D  show graphs of performance data for 5 a perspiration sensor according to some embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0028]    The present invention is directed to methods and systems for obtaining a quantitative measurement of moisture. One specific application for the invention includes the detection and measurement of perspiration. For purposes of illustration, the invention is described herein in the context of measuring perspiration, however, embodiments of the invention can be used to measure other sources of moisture. 
         [0029]    In accordance with some embodiments of the invention, the perspiration sensor includes a pair of ground shielded parallel electrodes sandwiching a moisture absorbent dielectric material (e.g., a microfiber cloth) that forms a capacitor. The sensor allows perspiration to become absorbed by moisture absorbent dielectric material which changes the dielectric constant the dielectric material and is reflected in the measured capacitance of the electrode plates of the perspiration sensor. In accordance with some embodiments of the invention, the dielectric material can be electrically coupled (e.g., capacitively coupled) to the skin of the user to provide for more stable signal measurement. In accordance with some embodiments of the invention, the absorbent dielectric material can be weakly (e.g., 10 pF or less) but consistently (e.g., up to 10% variation) coupled through the skin to earth ground which is weakly coupled to signal ground of capacitance measuring circuit. The method includes providing a capacitor that includes an exposed dielectric material that can absorb moisture (e.g., perspiration) produced by the skin. As perspiration is released from the skin and becomes absorbed by the absorbent dielectric material, the dielectric properties of the material between the electrode plates change resulting in a change in the capacitance of the perspiration sensor. An internal or external measurement component can be connected to the electrodes of the capacitor to measure the changes in capacitance of the perspiration sensor. 
         [0030]    The dielectric properties of perspiration closely resemble that of saline which in turn closely resemble that of water. The dielectric constant of water is about 80 times that of air. An air capacitor formed by two parallel plates will increase capacitance significantly when air is replaced with water, saline, or perspiration. The approximate capacitance C of the parallel plates can be determined by 
         [0000]    
       
         
           
             C 
             = 
             
               
                 ɛ 
                 0 
               
                
               
                 ɛ 
                 r 
               
                
               
                 A 
                 d 
               
             
           
         
       
     
         [0000]    where A is the area of the electrode plate, d is the distance between the plates; ε 0  is the dielectric constant of free space and ε r  is the relative dielectric constant of the material between the plates. 
         [0031]    In accordance with the invention, the dielectric material can be selected to rapidly absorb the perspiration. In accordance with some embodiments of the invention, the dielectric material can have predefined wicking or absorbency properties that provide a desired level of perspiration absorption. In operation, the dielectric material is initially in a dry state and has dielectric properties similar to air and produces an initial capacitance level. As the dielectric material is exposed to moisture (e.g., perspiration) the moisture rapidly replaces the air, changing the dielectric properties of the dielectric material and the measured capacitance level of the sensor. 
         [0032]    In accordance with some embodiments of the invention, the absorbent dielectric material can include a tufted microfiber cloth. This material has been found to have improved capillary suction and to diffuse perspiration faster and more uniformly than other materials as well as provides higher absorption densities. Other absorbent materials, such as cellulose paper, foamy elastomers, cotton, wool, air, and moisture wicking materials, can be used. The absorbent dielectric material can be configured to have a large surface area to volume ratio of the material that results in capillary suction causing the air filled space to become filled with environmental moisture or perspiration. A capacitance measuring circuit can be provided onboard or in close proximity to the capacitor sensor to minimize noise and convert the capacitance to a digital signal for transmission to a connected device. 
         [0033]      FIGS. 1, 3A and 3B  shows a moisture or perspiration sensor  100  according to various embodiments of the invention with the encapsulating polymer removed to provide a better view of the structure of the device.  FIGS. 2A and 2B  show diagrammatic views of a perspiration sensor  200  according to embodiments of the invention encapsulated in an encapsulating material, such as polymer material (e.g., silicone, PDMS, polyimide, TPE, PET, PVC, and MMA). The perspiration sensor  200  can include channels  210  that serve to guide moisture, such as perspiration, toward inlets in the perspiration sensor  200 . 
         [0034]    As shown in  FIGS. 3A and 3B , the perspiration sensor  100  can be constructed from two or more layers of insulating or dielectric material (e.g., a first layer  110 , a second layer  120  and the third layer  130 ). In accordance with some embodiments of the invention, each layer can include a non-conductive substrate (e.g. FR4 epoxy fiberglass, PDMS, or polyimide) having a conductive layer (e.g., copper and/or tin) on one or both surfaces of the substrate, such as a printed circuit board (PCB) or flexible PCB. In accordance with some embodiments of the invention, each layer can include a protective and/or insulating coating (e.g., solder mask coating) covering a portion or all of each surface of each layer. Each of the layers that make up the perspiration sensor  100  can be bonded together using well known adhesives (e.g., epoxy, polyimide, and/or silicone based adhesives). The surfaces of at least some of the layers can include exposed pads enabling electronic components such as integrated circuits, discrete components (e.g., resistors, capacitors, diodes and other passive devices) to soldered in place. The layers can also include vias or plated through holes that allow circuit traces to extend through the layer can make contact with circuit traces of the other layers. In accordance with some embodiments of the invention, some or all of the layers can include castellated vias on or extending from the external edges that provide for mechanical alignment and enable low temperature fabrication—the castellated vias are positioned away from central chamber and can be soldered without fear of melting or otherwise damaging the absorbent dielectric material. 
         [0035]      FIG. 4  shows an exploded view of a perspiration sensor  100  according to some embodiments of the invention. In this embodiment, the perspiration sensor  100  includes a first or top layer  110 , a second or middle layer  120  and the third or bottom layer  130 . The first layer  110  includes a first electrode  112  on the underside of the first layer (hidden from view in  FIG. 4 , but shown in  FIG. 5 ). The first layer  110  can also include circuit traces that enable a sensing integrated circuit  140  to soldered in place and electrically connected to the first electrode  112  and second electrode  134  and wires (not shown) that connect the perspiration sensor  100  to other devices. The third layer  130  includes one or more inlets  132  and the second electrode  134 . The third layer  130  can also include a skin electrode  136  (hidden from view in  FIG. 4 , but shown in  FIG. 5 ) and an adhesive material covering the skin electrode  136  to adhere the perspiration sensor  100  to a surface such as a skin surface. The second layer  120  forms a ring  122  that at least partially surrounds or encloses the dielectric material  124  in the dielectric space defined by the central chamber  26 . When the three layers are bonded together, they form a central chamber  126  which encloses the dielectric material  124  (e.g., the moisture absorbent material). In accordance with some embodiments of the invention, the thickness of each layer can be selected to minimize the overall thickness of the sensor to improve user comfort. Thus, the first layer  110  and the third layer  130  can be thicker or thinner than the middle layer  120 . Alternatively, each of the layers can have the same or different thicknesses. In accordance with some embodiments, the sensor detection area, defined by the inlets  132  can cover an area of 1 cubic centimeter and provide an average pore density of 50 pores (e.g., in the arm pit). 
         [0036]      FIG. 5  shows an exploded bottom view of the perspiration sensor  100  (e.g., showing the third layer  130  on top). As shown in  FIG. 5 , the third layer  130  can include a skin electrode  136  on the outer surface thereof. The skin electrode  136  can be provided in the form of a ring, as shown, or as a set of interconnected contact points over the outer surface of the third layer  130 . In accordance with some embodiments, the skin electrode  136  can be electrically connected to the dielectric material  124  in the central chamber  126  of the perspiration sensor  100  by extending circuit traces around or vias through the third layer  130  to the middle layer  120  and in contact with the dielectric material  124 . In accordance with other embodiments, the skin electrode  136  can be electrically connected to the dielectric material  124  by circuit traces or wires that extend from the inner surface of the third layer  130  into the central chamber  126 . The skin electrode  136  can be covered with a solder mask or other insulating material (e.g., skin adhesive tape). When the perspiration sensor  100  is adhered or placed in contact with the skin, the skin electrode  136  capacitively couples the dielectric material  124  to the skin. 
         [0037]      FIG. 6A  shows an exploded cross-section view and  FIG. 6B  shows an assembled cross-section view of a perspiration sensor  100  according to some embodiments of the invention. In this embodiment, an adhesive layer  150  adheres the third layer  130  of the perspiration sensor  100  to the surface of the skin  160  enabling the device to measure perspiration. The perspiration sensor  100  includes a first layer  110 , second layer  120  and a third layer  130 . The first electrode  112  is formed on the inner surface of the first layer  110  and the second electrode  134  is formed on the inner surface of the third layer  130 . A mask  114  on the first layer and a mask  138  on third layer can be included to electrically insulate the first electrode  112  and the second electrode  134  from the dielectric material  124  while enabling the dielectric material  124  to be in intimate contact with the first electrode  112  and the second electrode  134  while preventing the moistened dielectric material from shorting the first electrode to the second electrode. The insulating masks  114  and  138  can be formed from any solder mask insulating material (e.g., a layer or film of epoxy or UV cured polymer or resin). The second layer  120  is bonded between the first layer  110  and the third layer  130  supporting the first electrode  112  at predefined distance with respect to the second electrode  134  and forming the central chamber  126  that encloses the moisture absorbent dielectric material  124 . The third layer  130  includes one or more inlets  132  that allow the moisture (e.g., perspiration) to enter the central chamber  126  and become absorbed by the moisture absorbent dielectric material  124  as well as to allow air initially contained within the moisture absorbent dielectric material  124  to escape. As shown in  FIG. 6B , when the layers are bonded together, the moisture absorbent dielectric material  124  can become partially or fully extruded through the inlets  132  to facilitate moisture absorption. In accordance with some embodiments, an outlet can be provided through the middle layer  120  or the first layer  110  to enable air initially contained within the moisture absorbent dielectric material  124  to escape. The perspiration from skin  160  enters the inlets  132  and is absorbed by the moisture absorbent dielectric material  124 . 
         [0038]    As shown in  FIGS. 6A and 6B , the third layer  130  includes one or more vias or plated through holes that electrically connect the skin electrode  136  to the inner surface of the third layer  130  and after assembly, make electrical contact with an inner trace or electrode  122  on the middle layer  120 . The inner electrode  122  can extend into the central chamber  126  and make contact with the moisture absorbent dielectric material  124 . In some embodiments of the invention, all or a portion of the inner surface  128  of the middle layer  120  can include a conductive material (e.g., copper or tin) that makes contact with the moisture absorbent dielectric material  124 . 
         [0039]      FIG. 7A  shows a perspective view of a perspiration sensor  100  according to some embodiments of the invention and  FIG. 7B  shows a diagrammatic view of the middle layer  120 . In these embodiments, the first layer  110 , the middle layer  120  and the third layer  130  each include pads  122 ,  236  and plated through holes or partial holes or vias  228  that enable circuit traces to extend between layers. Solder can be applied to the plated through holes or vias  228  to create a physical connection between the layers and an electrical connection between the pads  122 ,  236  on the outside surfaces of the sensor  100  and the dielectric space defined by the central chamber  126 . In some embodiments, the third layer  130  can include pads (not shown) that serve as the skin electrode  136  and can be electrically connected to circuit traces on the middle layer  120  and the first layer  110  soldering together the vias  228 . Similar vias  226  can be provided on the inner surface of the middle layer  120  and connected by circuit traces  222  to one or more of the vias  228  to provide an electrical connection between the skin electrode  136  and the moisture absorbent dielectric material  124 . 
         [0040]    In accordance with some embodiments of the invention, the perspiration sensor  100  can also include one or more anchor rings  220  that project from the peripheral edge of the device as shown in  FIGS. 7A and 7B . The anchor rings  220  serve to provide features in the peripheral structure of the sensor device to aid in anchoring the device in the encapsulating material such as silicone, PDMS, polyimide during assembly.  FIG. 7C  shows a partially cut away view of a perspiration sensor  100  according to some embodiments of the invention. In this embodiment, the vias  228  are shown along the outer surface of the sensor  100  and the anchor rings  220  are shown extending from the outer peripheral surface of the sensor  100 . In some embodiments, the encapsulating material  205  can at least partially extend into the openings of the anchor rings  220 . 
         [0041]      FIGS. 8A and 8B  show photos and diagrams of microfibers. Microfiber based materials provide for improved absorption of moisture. In some embodiments, the microfibers can be split microfibers which have an X or asterisk shaped cross-section as shown in  FIG. 8A . This structure results in the formation of microchannels in the fibers that help absorb moisture (e.g., by capillary action) better than regular solid fibers. As shown in  FIG. 8B , these microfibers can be loosely woven into a tufted cloth that provides good absorption of moisture. 
         [0042]    In accordance with some embodiments, the perspiration sensor can include a capacitive sensor signal measuring integrated circuit that accurately measures the capacitance or capacitive signal and converts it to a digital signal for transmission to a remote device. In accordance with some embodiments of the invention, the capacitive sensor signal measuring integrated circuit can include a ZSSC3123 integrated circuit (ZMDI, Dresden, Germany and Milpitas, Calif.).  FIG. 9A  shows a block diagram of the integrated circuit.  FIG. 9B  shows a diagram of a charge balancing circuit for converting the analog capacitance signal to a digital signal. The circuit includes a 1st order charge-balancing capacitance-to-digital converter. Capacitor CB can be a fixed reference capacitor internal to the IC itself. The measurement determines the amount of time it takes each cycle to charge and discharge the reference capacitor. The capacitor CA is driven by a square wave voltage with excitation frequency in the 100 kHz range to prevent aging effects that occur when driven by a DC signal. The output signal generated by this circuit is a ratio of sensor capacitance to reference capacitance. 
         [0043]      FIG. 10A  shows a sensitivity graph for a capacitive perspiration sensor according to the invention. As shown in  FIG. 10A , the sensitivity is higher (e.g., 2.5 pF/μL) at lower moisture levels and decreases (e.g., to 1.2 pF/μL) as the level of moisture increases. 
         [0044]      FIG. 10B  shows a responsivity graph for a capacitive perspiration sensor according to the invention. The graph in  FIG. 10B  shows the response of the capacitive perspiration sensor according to the invention over time at 5 different moisture levels (e.g., 0.1 μL, 0.2 μL, 0.5 μL, 1.0 μL, and 2.0 μL). 
         [0045]      FIG. 10C  shows the effect of shielding (e.g., capacitive coupling the dielectric to the skin) on a capacitive perspiration sensor according to the invention. As shown in  FIG. 10C , an unshielded capacitive perspiration sensor exhibits signal spikes upon contact whereas the shielded capacitive perspiration sensor according to the invention does not. 
         [0046]      FIG. 10D  shows a comparison of the range and sensitivity of shielded and unshielded capacitive perspiration sensors according to the invention. As shown in  FIG. 10D , the shielded sensor has approximately the same range and sensitivity as an unshielded sensor. 
         [0047]    In accordance with some embodiments of the invention, the perspiration sensor can be part of system that quantitatively measures perspiration of a user in real time. The sensor can be connected to a data-logging hub (e.g. BioStamp TM by MC10 Inc., a smartphone or data recorder). The perspiration sensor can measure a change in capacitance over time and calculate perspiration moisture volume using a predetermined calibrated curve. The data logging hub can include a computer processor and associated memory that can communicate with the perspiration sensor to receive sensor data. The data logging hub can include additional wired or wireless communication components to enable the sensor data to be stored in a remote database or processed by a remote data processing system. 
         [0048]    While some embodiments of the present invention are described in the context of a perspiration sensor, the invention can be used for measuring moisture in other applications. In some embodiments, the moisture sensor can be used to measure perspiration to test the efficacy of anti-perspirant products. In other applications, the moisture sensor can be installed in a helmet worn by an athlete, a soldier or a fighter pilot as well as other areas of the body to provide continuous physiological monitoring, for example, for health, wellness, hydration and/or stress monitoring. In accordance with some embodiments, the central chamber or an adjacent collection chamber can include analyte sensors and/or assays to detecting the presence and/or quantity of components of the absorbed perspiration. For example, a sodium sensor can be included for diagnosis of cystic fibrosis. 
         [0049]    Other embodiments are within the scope and spirit of the invention. For example, due to the nature of hardware and software, functions described above can be implemented using software, hardware, firmware, hardwiring, or combinations of any of these. Features implementing functions may also be physically located at various positions, including being distributed such that portions of functions are implemented at different physical locations. 
         [0050]    Further, while the description above refers to the invention, the description may include more than one invention.