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 . 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 . 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 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 . 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 . 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 . fig1 a 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 . fig2 a 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 . as shown in fig3 a 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 . fig4 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 fig4 , but shown in fig5 ). 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 fig4 , but shown in fig5 ) 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 ). fig5 shows an exploded bottom view of the perspiration sensor 100 ( e . g ., showing the third layer 130 on top ). as shown in fig5 , 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 . fig6 a shows an exploded cross - section view and fig6 b 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 fig6 b , 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 . as shown in fig6 a 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 . fig7 a shows a perspective view of a perspiration sensor 100 according to some embodiments of the invention and fig7 b 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 . 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 fig7 a 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 . fig7 c 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 . fig8 a 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 fig8 a . 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 fig8 b , these microfibers can be loosely woven into a tufted cloth that provides good absorption of moisture . 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 .). fig9 a shows a block diagram of the integrated circuit . fig9 b 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 . fig1 a shows a sensitivity graph for a capacitive perspiration sensor according to the invention . as shown in fig1 a , 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 . fig1 b shows a responsivity graph for a capacitive perspiration sensor according to the invention . the graph in fig1 b 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 ). fig1 c 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 fig1 c , an unshielded capacitive perspiration sensor exhibits signal spikes upon contact whereas the shielded capacitive perspiration sensor according to the invention does not . fig1 d shows a comparison of the range and sensitivity of shielded and unshielded capacitive perspiration sensors according to the invention . as shown in fig1 d , the shielded sensor has approximately the same range and sensitivity as an unshielded sensor . 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 . 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 . 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 . further , while the description above refers to the invention , the description may include more than one invention .
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 .