fig1 a and 1b show examples of arrangements of sense electrodes 12 for sensing flow rates , chemical concentration , or another charge - based parameter of an ionic fluid in accordance with embodiments of this invention . in each of fig1 a and 1b , a pair of sense electrodes 12 ( designated as 12 a and 12 b ) is shown on an outer surface 14 of a sensor 10 . the electrodes 12 a - b form a sensing element adapted to sense charged - based parameters of an ionic fluid using impedance measurements that are based on known principles , for example , as described in h . ayliffe and r . rabbit , “ an electric impedance based mems flow sensor for ionic solutions ,” meas . sci . technol . 14 , pp . 1321 - 1327 ( 2003 .). an electric current , typically about 0 . 1 milliamps or less alternating current at a frequency of about 100 hz to about 100 khz , is applied across the paired electrodes 12 a - b . the electrodes 12 a - b are sufficiently close together to insure that charged ions remain separated , enabling an impedance or charge measurement of the ionic fluid to take place with the electrodes 12 a - b . in fig1 a , the sensor 10 is oriented so that fluid flow is in a direction through a gap between the paired electrodes 12 a - b . in the embodiment of fig1 b , the flow direction is aligned with the sense electrodes 12 a - b , which include a third sense electrode 12 c . similar to the paired electrodes 12 a - b , an ac current of up to about 0 . 1 milliamps at a frequency of about 100 hz to about 100 khz is applied across the electrodes 12 a and 12 c , enabling the electrodes 12 a and 12 c to sense charged - based parameters of an ionic fluid using impedance measurements . all three electrodes 12 a - c are aligned , with the third electrode 12 c separated from the electrodes 12 a - b by a pair of biasing electrodes 18 held at a potential , for example , about 10 mv to about 10 vdc . the biasing electrodes 18 apply a biasing charge to ionize the ionic fluid being sensed , thereby achieving better separation of positive and negative ions within a transient ion cloud that travels across the sense electrodes 12 a and 12 b and the third electrode 12 c located downstream and upstream , respectively , from the biasing electrodes 18 . by comparing upstream measurements between the electrodes 12 a and 12 c to downstream measurements between the electrodes 12 a and 12 b , flow versus ion concentration measurements can be made and a zero - flowrate value can be estimated . other configurations can be also be utilized , for example , other three - electrode schemes , and the invention encompasses such variations in electrode configurations . fig2 shows how the impedance ratio of flow to no flow as a function of frequency can be used with the electrode arrangements of fig1 a and 1b to measure flow rates . the phase as a function of frequency can also be used to monitor flow rate . fig3 illustrates that the charge ion concentration may vary over distance from the bias electrodes 18 , similar to how thermal flow sensing elements measure flow rate using a hot wire element . by using photolithography methods , patterned metal electrodes 12 and 18 can be formed to define small gaps therebetween , for example , less than one hundred micrometers between the paired electrodes 12 a - b , preferably a few micrometers up to tens of micrometers , which is sufficient to insure that a significant percentage of separated positive and negative ions will be present at and between the electrodes 12 a - b . the electrodes 12 and 18 should be resistant to corrosion in the ionic solution being assessed . for this reason , platinum , palladium , silver , titanium , and iridium alloys and silver oxide are believed to be well suited as materials for the electrodes 12 and 18 . for ac and charge measurements , the electrodes 12 and 18 can be passivated with a thin dielectric corrosion resistant layer . the electrodes 12 , which may be in any suitable form , including two - dimensional structures ( for example , flat pads or contacts ) and three - dimensional structures ( for example , probes , etc .) that protrude from the sensor surface 14 , and are connected to electronic components as well as an inductor coil that form part of the sensor 10 , as discussed below . the sensor 10 incorporating the sensing element formed by the electrodes 12 and 18 of fig1 a and 1b can have a wide variety of configurations , including cylindrical and disk - shaped exteriors . fig4 a - 4g and 5 represent some of the possible configurations , and in these figures consistent reference numbers are used to identify functionally similar structures . in fig4 a , the sensor 10 is represented as a sealed cylindrical - shaped capsule . a hermetic housing 20 encloses a core and coil assembly 22 and electronics 24 ( for example , printed circuit boards ( pcbs ) and / or application - specific integrated circuits ( asics )), with only the electrodes 12 and 18 ( of which only 12 a and 12 b are shown ) exposed at the external surface 14 of the housing 20 . the electrodes 12 a - b are shown as located at the end of the cylindrical - shaped housing , though it is foreseeable that the electrodes 12 and 18 could be located elsewhere , for example , along the sides of the housing 20 . the sense electrodes 12 and possibly some of the electronics 24 may be mounted on a substrate , connected together using various methods known in the art , for example , wirebonding , flexible connectors , etc ., or potted together using a biocompatible epoxy or any other suitable potting material . the surface 14 carrying the sense electrodes 12 can be either rigid or flexible substrate material , or a combination ( for example , a rigid - flex substrate , where part of the substrate is rigid and another part is flexible ). in the case of flexible substrates , various polymers , parylene , silicone , or other biocompatible flexible material may be used . in the case of rigid substrates , glass , silicon , ceramics , carbides , alloys , metals , hard polymers , teflon , are some examples , although other types of materials can also be used . in the case of rigid - flex substrates , the rigid and flexible parts may be made from dissimilar material . the electrodes 12 themselves , especially three - dimensional electrodes 12 , can also either be rigid , flexible , or a combination . for example the electrodes 12 can be formed by a flexible three - dimensional element made from polymers , parylene , silicone , etc ., which is partially or fully metallized . the electrodes 12 may also have a flexible tip portion connected to a rigid portion connected to the substrate surface 14 , or alternatively may have a rigid tip portion connected to the substrate surface 14 via a flexible portion . the rigid and flexible portions may be made from similar or dissimilar material . fig4 b and 4c are similar to fig4 a , but show the electrodes 12 and 18 protruding from the housing 20 ( fig4 b ) and the inclusion of a second element 26 ( fig4 c ), which may be another sensing element adapted to sense a different parameter or an actuating element adapted to physically induce , stimulate , or respond to conditions within the ionic fluid being evaluated by the sensor 10 . as nonlimiting examples , the sensor 10 can further incorporate various other miniature sensing elements adapted to detect and / or monitor various physiological parameters of a patient , such as pressure , strain , temperature , and / or velocity sensors , and / or measure specific chemistries such as gas content ( for example , o2 and co2 ) and glucose levels , or incorporate various miniature actuators , including but not limited to thermal generators , voltage sources , current sources , probes , electrodes , drug delivery pumps , valves , meters , microtools for localized surgical procedures , radiation emitting sources , defibrillators , muscle stimulators , and pacing stimulators . various specific examples of these types of miniature sensors and actuators are known to those skilled in the art , and any one or more of these can be utilized in the sensor 10 of the present invention if capable of sufficiently small size to permit placement of the sensor 10 within a catheter for delivery and implantation , or otherwise permit surgically implanted . a particular example is to incorporate a pressure sensor into the sensor 10 for patients with traumatic brain injury to monitor brain pressure to allow for tailoring of the sensor operation . by measuring different physiologic parameters , the sensor 10 can use the measured physiologic parameter ( s ) to control , adjust or manipulate and the stimulating function ( for example , patter , frequency , location , amplitude , etc .). this approach allows dynamic and smart stimulation and allows the implementation of a closed - loop system . for example , if the implanted sensor 10 is adapted to sense flow , the sensor 10 can operate with other implanted or non - implanted devices ( such as sensors , actuators , valves , etc .) as part of a closed loop control system which can stimulate , monitor , measure one or more physiological parameter , and perform additional actions all based on feedback from one or more of the units in the system . fig4 d and 4e depict further embodiments of cylindrical - shaped sensors 10 , in which the electrodes 12 and / or 18 protrude from the housing 20 to define anchoring features . in fig4 d , the electrodes 12 / 18 are in the form of wire loops that are connected to and arcuately extend between the opposing ends of the housing 20 . in fig4 e , the electrodes 12 / 18 are in the form of wire arms that are attached at an intermediate region of the housing 20 and arcuately extend toward the opposing ends of the housing 20 . in each case , the electrodes 12 / 18 permit the sensor 10 to be placed and anchored within a tubular - shaped passage , such as an artery . fig4 f and 4g represent sensors 10 configured for use in intracranial applications to monitor the cerebral spinal fluid of a patient . for this purpose , the housing 20 containing the electrodes 12 , electronics 24 , etc ., is adapted for deep implantation within the brain , whereas the antenna 22 is fabricated on a flexible or rigid film 28 that can be located remove from the sensor 10 , as represented in fig1 . a cable 30 interconnects the implantable housing 20 with the antenna 22 . fig4 f differs from fig4 g by the inclusion of a second element 26 , similar to fig4 c . fig1 represents a similar implantation , but with a cylindrical antenna 22 similar to the core and coil assembly 22 of fig4 a through 4e . the antenna 22 ( and any packaging thereof ) can be attached to the skull and placed under the scalp , or attached under the skull above the dura mater such that it does not pierce the dura , or placed outside the body ( for example , attached on top of the scalp ). an advantage of the sensor configurations of fig4 f , 4 g , 10 and 11 is the ability for a very small footprint for the implanted portion of the sensor 10 within the brain ( or another location ), while bulkier portions of the sensor 10 are placed at a separate and possibly more favorable location . fig5 shows yet another alternative for implementing the electrodes 12 and 18 of fig1 a and 1b . in fig5 , the electrodes 12 and 18 are disposed outside the sensor housing 20 at the end of a cable 32 , and therefore somewhat remote from the antenna 22 and electronics 24 sealed within the housing 20 . the implantable sensor 10 and its components may be physically realized with a combination of any of several technologies , including those using microfabrication technology such as microelectromechanical systems ( mems ). the housing 20 can be made from a variety of materials , such as glass , ceramics , polymers , silicone , parylene , etc . the hermetic sensor housing 20 may also be formed from anodically bonded layers of glass and silicon ( doped or undoped ). alternatively , the internal components of the sensor 10 may be potted together using known biocompatible materials to effectively form the exterior of the housing 20 . in some cases , it may be desirable to apply additional materials ( organic , metal , or any biocompatible material ) to the exterior of the housing 20 to protect certain regions of the sensor 10 . for example , a coating may be applied to all but the electrodes 12 and 18 , or the electrodes 12 and 18 may be coated with a material that differs from the material applied to the remainder of the sensor housing 20 . examples of suitable coating materials include polymers , parylene , silicone , hydrogels , titanium , nitrides , oxides , carbides , silicides , etc . the sensors 10 represented in fig4 a - 4g and 5 can be integrated into other systems , such as shunts , valves , pumps , tubing , defibrillators , drug pumps , pacemakers , neural stimulators , deep brain stimulators , and other implantable devices . fig6 and 7 represent two types of wireless telemetry schemes that can be utilized with the invention to noninvasively monitor the sensor 10 and evaluate the health of the patient as well as the operation of the sensor 10 . these schemes are disclosed in commonly - assigned u . s . pat . nos . 6 , 926 , 670 and 6 , 968 , 734 to rich et al ., whose contents are incorporated herein by reference . in fig6 , the sensor 10 is shown as operating in combination with a non - implanted external reader unit 40 , between which a wireless telemetry link is established using a resonant scheme . the sensor 10 contains the sense electrodes 12 ( and optionally the biasing electrodes 18 ), the antenna 22 , and a capacitor 32 . the antenna 22 serves as an inductor coil , and together the antenna 22 and capacitor 32 form an lc ( inductor - capacitor ) tank resonator circuit that has a specific resonant frequency , expressed as 1 /( lc ) 1 / 2 , which can be detected from the impedance of the circuit . notably , the impedance across the sense electrodes 12 influences the impedance of the tank circuit . at the resonant frequency , changes in the impedance sensed with the electrodes 12 present measurable changes in magnetically - coupled impedance load to an external coil 42 associated with the reader unit 40 . based on the antenna 22 and capacitor 32 being fixed and therefore having fixed inductance and capacitance values , the reader unit 40 is able to determine the impedance sensed by the electrodes 12 by monitoring the resonant frequency of the circuit . the reader unit 40 is then able to determine the charged - based parameter being sensed by the electrodes 12 , for example , flow rate , by monitoring the frequency at which the impedance of the antenna 22 or ionic charge changes . fig7 shows the sensor 10 modified to operate in combination with a modified reader unit 40 . a wireless telemetry link is established between the sensor 10 and reader unit 40 using a passive , magnetically - coupled scheme , in which on - board circuitry of the sensor 10 receives power from the reader unit 40 . in the absence of the reader unit 40 , the sensor 10 lays passive and without any internal means to power itself . when a reading is desired , the reader unit 40 must be brought within range of the sensor 10 . in fig7 , the sensor 10 again contains the sense electrodes 12 ( and optionally the biasing electrodes 18 ) and the antenna 22 , which serves as an inductor coil , and the reader unit 40 again has a coil 42 by which an alternating electromagnetic field is transmitted to the antenna 22 of the sensor 10 to induce a voltage in the sensor 10 . when sufficient voltage has been induced in the sensor 10 , a supply regulator ( rectification ) circuitry 35 converts the alternating voltage on the antenna 22 into a direct voltage that can be used by the electronics 24 as a power supply for signal conversion and communication . at this point the sensor 10 can be considered alert and ready for commands from the reader unit 40 . the sensor 10 may employ the antenna 22 as an antenna for both reception and transmission , or it may utilize the antenna 22 solely for receiving power from the reader unit 40 and employ a second coil 38 for transmitting signals to the reader unit 40 . signal transmission circuitry 36 receives an encoded signal generated by signal conditioning circuitry 37 derived from the output of the electrodes 12 , and then generates an alternating electromagnetic field that is propagated to the reader unit 40 with the antenna 22 or coil 38 . the reader unit 40 may interface to a user interface , which may include a display , computer , or other data logging devices . a large number of possible geometries and structures are known and available for the antennas / coils 22 , 38 and 42 of fig6 and 7 . the coil conductors may be wound around a ferrite core to enhance magnetic properties , deposited on a flexible substrate , or formed into a cylindrical - shaped cover . the coil conductors are preferably made at least in part with a metal of high conductivity , such as copper , platinum , silver , gold , etc . the coil conductors may alternately be fabricated on implantable substrates . methods of fabrication of coils on substrates include but not limited to one or more or any combination of the following techniques : sputtering , electroplating , lift - off , screen printing , and other suitable methods known to those skilled in the art . as previously noted , the supply regulator ( rectification ) circuitry 35 converts the alternating voltage on the antenna 22 into a direct voltage that can be used by the electronics 24 as a power supply for signal conversion and communication . efficient realization of such a circuit can employ standard electronic techniques and may include full - bridge or half - bridge diode rectifiers . the rectification circuitry 35 may include a capacitor for transient energy storage to reduce the noise ripple on the output supply voltage . the rectification circuitry 35 may be implemented on the same integrated circuit die with other components of the electronics 24 . many different circuits for the signal transmission circuitry 36 and signal conditioning circuitry 37 are also known to those skilled in the art . impedance - and capacitance - to - frequency conversion , sigma delta , and other analog - to - digital conversion techniques are all possible conditioning circuits that may be used . in a preferred embodiment of the invention , the readout unit 40 receives data from the sensor 10 using the 13 . 56 mhz ism band . two modes of operation can be employed : ( 1 ) a data - logging measurement mode with optional data rates of , for example , 1 hz and below , and ( 2 ) a real - time dynamic measurement mode with data rates of , for example , 100 to 500 hz , for compliance and impulse tests . the readout unit 40 is represented in fig6 and 7 as comprising analog rf front end electronics providing processing and user interface capabilities . a graphical user interface program can be used to control information ( e . g ., icp monitor ) and created in , for example , the labview and c visual programming languages . the external readout unit 40 can be adapted to perform one or more of the following : remote monitoring of patients , including but not limited to home monitoring ; monitoring of patients with telephone - based ( or similar method ) data and information delivery ; monitoring of patients with wireless telephone - based ( or similar method ) data and information delivery ; monitoring of patients with web - based ( or similar method ) data and information delivery ; closed - loop drug delivery to treat diseases ; warning systems for critical worsening of diseases and related conditions ; portable or ambulatory monitoring or diagnostic systems ; battery - operation capability ; data storage ; reporting global positioning coordinates for emergency applications ; communication with other medical devices including but not limited to pacemakers , defibrillator , implantable cardioverter defibrillator , implantable drug delivery systems , non - implantable drug delivery systems , and wireless medical management systems . the sensor 10 is shown in fig7 without a battery , and therefore its operation does not require occasional replacement or charging of a battery . instead , the energy required to perform the sensing operation is entirely derived from the reader unit 40 . however , the sensor 10 could be modified to use an additional capacitor , battery , rechargeable battery , or other power storage device to power the sensor 10 when the reader unit 40 is not sufficiently close to induce a voltage in the sensor 10 . such an active scheme option is particularly useful for ( but not limited to ) situations in which long - term data acquisition without continuous use of the readout unit is desirable . with an on - board power source , data may be stored in the sensor 10 and downloaded intermittently using the readout unit 40 as required . as those skilled in magnetic telemetry are aware , a number of modulation schemes are available for transmitting data via magnetic coupling . particularly suitable schemes include but are not limited to amplitude modulation , frequency modulation , frequency shift keying , phase shift keying , and also spread spectrum techniques . a preferred modulation scheme may be determined by the specifications of an individual application , and is not intended to be limited under this invention . in addition to the many available modulation techniques , there are many technologies developed that allow the sensor 10 to communicate the charge - based parameter signal to the reader unit 40 . it is understood that the reader unit 40 may transmit either a continuous level of rf power to supply the sensor 10 with needed energy , or the reader unit 40 may pulse the power allowing temporary storage in a battery or capacitor device . similarly , the sensor 10 of fig7 may signal back to the reader unit 40 at any interval in time , delayed or instantaneous , during reader rf transmission or alternately in the absence of reader transmission . if periodic functioning of the sensor 10 is employed , a charging phase may be employed during which a storage device ( such as a capacitor , rechargeable battery , etc .) within the sensor 10 is charged , while flow measurements are made during another phase of operation that may be entirely separate or overlap the charging phase . the sensor 10 and its companion reader unit 40 permit a physician , caregiver or patient to monitor the sensor 10 at any time , and can be employed for home care and monitoring as well as in a hospital or physician &# 39 ; s office . the sensor 10 and reader unit 40 may be implemented as a remote monitoring system , including but not limited to home monitoring , which may include telephone - based , wireless communication - based , web - based , etc ., delivery of information received from the sensor 10 by the reader unit 40 and then presented to a physician or caregiver . information such as the patient &# 39 ; s name , current weight , prior weight ( for example , prior to surgery ), body temperature , blood pressure and posture can all be entered into the reader unit 40 before a measurement is taken to assist in obtaining an accurate reading and arriving at appropriate decisions about the integrity of the sensor 10 . the sensor 10 or its reader unit 40 can also perform algorithms to account for pressure or strain changes and its affect on flow rate due to the position of the patient , weight gain , body temperature . the sensor 10 can be implanted using a surgical procedure or a minimally - invasive outpatient technique . the insertion and placement of the sensor 10 into the brain ( and various other locations ) can be a relatively simple procedure and done by a trained technician rather than a highly specialized surgeon . this aspect of the invention is an important advantage over existing flow sensing devices . if configured similar to that shown in fig4 a through 4e , the sensor 10 can be implanted using catheter insertion method . for example , the sensors 10 shown in fig4 d and 4e can be attached or lodged in a duct or other tubular - shaped passage ( e . g ., an artery or iv tube ) for measurement of a fluid flowing through the duct . alternatively , fig8 and 9 depict a method by which the sensor 10 can be delivered surgically or via a catheter and anchored within an opening in a wall 46 so that only the electrodes 12 protrude into a flow path within a cavity defined by the wall 46 . in the embodiment of fig8 and 9 , the sensor 10 is placed within an anchor 44 disclosed in commonly - assigned u . s . patent application ser . no . 11 / 684 , 910 , whose contents are incorporated herein by reference . alternatively , anchoring provisions may be incorporated directly into the housing 20 , or incorporated directly into the housing 20 by additional assembly steps ( for example , fig4 d and 4e ). for example , various anchoring features and fasteners known in the art could be used , including those adapted to attach to the skull or scalp using wires , screws ( helical or otherwise ), bolts , mesh , stents , springs , stitches , expandable tines , etc . suitable anchoring mechanisms can also form part of another device with which the sensor 10 is implanted . for example , in patients with hydrocephalus , the anchoring mechanism can be part of the shunt used for draining the excess fluid . suitable materials for anchors used with the sensor 10 include , but not limited to , nitinol , teflon , parylene , polymers and metals . in view of the foregoing , it should be apparent that blood flow measurement is a notable medical application of the sensor 10 , as well as its expansion to cardiac output measurement . another important medical application is hydrocephalus shunts that can measure csf flow discharge , sense whether the shunt is clogged , provide advance warning for changing the shunt , detect false emergencies , and timely detect actual emergencies . in all such applications , multiple sensors 10 may be used , either in close proximity , or in separate locations . the multiple sensors 10 may each be a completely separate unit and not share any common elements , or share a common coil or other sensor component . in some cases , the sensor 10 may include or be used with multiple stimulating electrodes on either the same or multiple different substrates . the sensor 10 can be implanted in a variety of internal organs , glands , ducts , and vessels , including but not limited to the heart , brain , kidneys , lungs , bladder , ureter , urethra , spinal cord , arteries , veins , lymph ducts , reproductive systems , and abdomen . the sensor 10 and its systems can be used in the treatment of many different disease , including but not limited to cardiovascular disease , depression , parkinson &# 39 ; s disease , multiple sclerosis , amyotrophic lateral sclerosis ( als , often referred to as “ lou gehrig &# 39 ; s disease ”) alzheimer &# 39 ; s , borderline personality , compulsive disorders , addictions , stroke , brain trauma , brain injury , inflammation in the brain , tumors , hydrocephalus , cerebral palsy , essential tremor , coma , mental retardation , dystonia , and tremor due to multiple sclerosis . the sensor 10 and its system can significantly improve the tailored treatment of many severe diseases as a result of offering an easy to use and a relatively low - cost option for performing non - invasive , realtime , detailed and chronic monitoring / stimulation at home , in the doctor &# 39 ; s office , or in the hospital . while the invention has been described in terms of specific embodiments , it is apparent that other forms could be adopted by one skilled in the art . for example , the physical configuration of the sensor 10 could differ from that shown , and materials and processes other than those noted could be used . therefore , the scope of the invention is to be limited only by the following claims .
a system for monitoring a charge - based physiological parameter within an internal organ of a living body , and a sensor adapted to be implanted in the living body and an organ therein . the sensor includes sensing elements adapted to sense the charge - based physiological parameter within the organ , and the sensing elements include at least first and second sensing elements that are electrically conductive , aligned , spaced apart and exposed at the exterior of the sensor . the sensor further includes a device for passing an alternating current from the first to the second sensing elements through an ionic solution contacting the sensing elements . the sensor also includes a device for generating a signal corresponding to the impedance of the ionic solution based on the alternating current .