Abstract:
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.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/999,315, filed Oct. 17, 2007, the contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention generally relates to implantable sensing devices, and more particularly to an implantable sensing device equipped with electrodes to provide a flow and/or chemical sensing capability. 
     Wireless devices such as pressure sensors have been implanted and used to monitor heart, brain, bladder and ocular function. With this technology, capacitive pressure sensors are often used, by which changes in pressure cause a corresponding change in the capacitance of an implanted capacitor (tuning capacitor). The change in capacitance can be sensed, for example, by sensing a change in the resonant frequency of a tank or other circuit coupled to the implanted capacitor. The circuit can be implanted with the capacitor in the patient, and equipped with an antenna, such as a fixed coil, that receives a radio frequency (RF) signal transmitted from outside the patient to power the circuit, and also transmits the resonant frequency as an output of the circuit that can be sensed by a reader outside the patient. This approach has been applied to monitoring joint pressure, orthopedic conditions, intracranial, and cardiovascular pressures. Capacitive sensors can also be coupled with resistive strain gauges, accelerometers and optical fibers to monitor bone integrity. The necessity for implantable sensors to operate at very low currents and power levels, as well as the desire to minimize the overall size of the implant, complicates the implementation of flow rate sensors and chemical sensors that rely on such conventional sensing devices as hot-wire anemometers, piezoresistive sensors, and other flow meters with moving parts. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The present invention provides an implantable sensor, sensing system, and sensing method suitable for sensing flow rates, chemical concentrations, conductivity, and pH of various fluids within or introduced into a human body by measuring charge-based parameters associated with ionic solutions in a manner that minimizes the power requirements of the sensor. 
     According to a first aspect of the invention, the sensor is part of a system for monitoring a charge-based physiological parameter within an internal organ of a living body, and the sensor is 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. 
     A significant advantage of this invention is the ability to measure flow rates, chemical concentrations, conductivities, pH, and other charge-based parameters of a wide variety of ionic fluids, including but not limited to blood, cerebral spinal fluid (CSF), lymph, interstitial fluids, urine, saline solution, ringers lactate, intravenous (IV) solutions, drugs, dialysates and other bodily fluids and fluids that may be injected, withdrawn or infused into a patient. CSF, blood and lymph contain ions such as Na+, Cl−, K+, Mg+, Ca++ and other ions used by the human body. 
     The sensor can be configured for placement in a fluid-carrying vessel, artery, vein, heart chamber, duct, spinal column, ventricle, shunt, ureter, urethra, tube, channel, implant or cannula, and can be integrated into shunts, smart shunts, valves, catheter, pressure sensors, and other sensor implants. For applications involving drugs and IVs, the sensor can be placed in the tubing going to and from the patient instead of being implanted directly into the patient. 
     Other objects and advantages of this invention will be better appreciated from the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  represent plan views of charge-based sense electrodes arranged on a surface of an implantable sensor for sensing flow, chemical concentration, or another charge-based parameter of an ionic fluid in accordance with embodiments of this invention. 
         FIG. 2  is a graph representing correlations between impedance, frequency, and flow rates as measured with charge-based sense electrodes of this invention. 
         FIG. 3  is a graph representing correlations between ion concentrations, flow rates and distance from charge-based sense electrodes of this invention. 
         FIGS. 4A through 4G  represent various embodiments for implantable sensors that can be equipped with charge-based sense electrodes of this invention. 
         FIG. 5  represents an embodiment of a non-implantable sensor having an implantable lead that can be equipped with charge-based sense electrodes of this invention. 
         FIGS. 6 and 7  are block diagrams of wireless pressure monitoring systems that utilize resonant and passive sensing schemes, respectively, which can be utilized by monitoring systems of this invention. 
         FIGS. 8 and 9  represent use of the anchor to place an implantable sensor of this invention in a wall for sensing flow, chemical concentration, or another charge-based parameter of an ionic fluid in accordance with embodiments of this invention. 
         FIGS. 10 and 11  represent techniques for intracranial placement of implantable sensors of this invention for sensing flow, chemical concentration, or another charge-based parameter of cerebral spinal fluid in accordance with embodiments of this invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1A 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  FIGS. 1A 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  FIG. 1A , 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  FIG. 1B , 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. 
       FIG. 2  shows how the impedance ratio of flow to no flow as a function of frequency can be used with the electrode arrangements of  FIGS. 1A and 1B  to measure flow rates. The phase as a function of frequency can also be used to monitor flow rate.  FIG. 3  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  FIGS. 1A and 1B  can have a wide variety of configurations, including cylindrical and disk-shaped exteriors.  FIGS. 4A-4G  and  5  represent some of the possible configurations, and in these figures consistent reference numbers are used to identify functionally similar structures. In  FIG. 4A , 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. 
       FIGS. 4B and 4C  are similar to  FIG. 4A , but show the electrodes  12  and  18  protruding from the housing  20  ( FIG. 4B ) and the inclusion of a second element  26  ( FIG. 4C ), 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. 
       FIGS. 4D 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  FIG. 4D , 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  FIG. 4E , 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. 
       FIGS. 4F 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  FIG. 10 . A cable  30  interconnects the implantable housing  20  with the antenna  22 .  FIG. 4F  differs from  FIG. 4G  by the inclusion of a second element  26 , similar to  FIG. 4C .  FIG. 11  represents a similar implantation, but with a cylindrical antenna  22  similar to the core and coil assembly  22  of  FIGS. 4A 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  FIGS. 4F ,  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. 
       FIG. 5  shows yet another alternative for implementing the electrodes  12  and  18  of  FIGS. 1A and 1B . In  FIG. 5 , 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  FIGS. 4A-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. 
       FIGS. 6 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  FIG. 6 , 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. 
       FIG. 7  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  FIG. 7 , 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  FIGS. 6 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  FIGS. 6 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  FIG. 7  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  FIG. 7  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  FIGS. 4A through 4E , the sensor  10  can be implanted using catheter insertion method. For example, the sensors  10  shown in  FIGS. 4D 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,  FIGS. 8 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  FIGS. 8 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,  FIGS. 4D 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.