Abstract:
A method and apparatus for proximity sensing and feedback regulation between a bodily implanted secondary coil and an external primary coil applied for generating and propagating a well-defined set of excitation signals used for nerve or muscle stimulation. The complete device is a combination of external and implantable or internal components. The internal component contains a relatively small magnet composed of materials that exhibit Giant Magneto-resistor characteristics, passive circuitry and a secondary coil. The external coil contains proximity sensor circuitry that is rigidly connected in a convenient enclosure mounted supercutaneously. Feedback information of the position of the implanted coil is provided to the pulse generation circuit and the parameters are adjusted to compensate for any variation in the position of the internal coil in order to maintain the necessary signal pattern.

Description:
FIELD OF THE INVENTION 
     The present invention relates to communication between a subcutaneously implanted lead-receiver and external transmitter. More specifically, the present invention relates to a proximity sensing and feedback regulation system for the reliable generation and propagation of programmed pulse excitation signals to an implant from an external controller by use of inductive fields. These excitation signals are used for stimulating excitable tissue, such as a nerve bundle or muscle. 
     BACKGROUND OF THE INVENTION 
     There is mounting scientific evidence that pulsed electrical stimulation of peripheral or cranial nerves has beneficial effects as adjunct therapy for clinical states such as partial complex epilepsy, generalized epilepsy, urinary urge incontinence, Alzheimer&#39;s disease, inappropriate sinus tacycardia, neurogenic pain, depression, and refractory angina etc. 
     Nerve and muscle cells have membranes that are composed of lipids and proteins, and have unique properties of excitability such that an adequate disturbance of the cell&#39;s resting potential can trigger a sudden change in the membrane conductance. Under resting conditions, the inside of the nerve cell (neuron) is approximately −90 mV relative to the outside. As shown in FIG. 1, a neuronal process can be divided into unit lengths, which can be represented in an electrical equivalent circuit. Each unit length of the process is a circuit with its own membrane resistance (r m ), membrane capacitance (c m ), and axonal resistance (r a ). 
     A nerve cell can be excited by increasing the electrical charge within the nerve, thus increasing the membrane potential inside the nerve with respect to the surrounding extracellular fluid. This fundamental feature of the nervous system i.e., its ability to generate and conduct electrical impulses, can take the form of action potentials, which are defined as a single electrical impulse passing down an axon. This action potential (nerve impulse or spike) is an “all or nothing” phenomenon, (shown schematically in FIG.  2 ), that is to say once the threshold stimulus intensity is reached, an action potential will be generated. As shown in the Figure, stimuli  1  and  2  are subthreshold, and do not induce a response. Stimulus  3  exceeds a threshold value and induces an action potential (AP) which will be propagated. The information in the nervous system is coded by frequency of firing rather than the size of the action potential. The threshold stimulus intensity is defined as that value at which the net inward current (which is largely determined by Sodium ions) is just greater than the net outward current (which is largely carried by Potassium ions), and is typically around −55 mV inside the nerve cell relative to the outside (critical firing threshold). If, however, the threshold is not reached, the graded depolarization will not generate an action potential and the signal will not be propagated along the axon. 
     As shown in FIG. 3, the resulting membrane voltage change will affect adjacent portions of the membrane, and in a nerve, that will propagate as a nerve impulse. In FIG. 3 the impulse is traveling from right to left. Immediately after the spike of the action potential there is a refractory period when the neuron is either unexcitable (absolute refractory period) or only activated to sub-maximal responses by supra-threshold stimuli (relative refractory period). The absolute refractory period occurs at the time of maximal Sodium channel inactivation while the relative refractory period occurs at a later time when most of the Sodium channels have returned to their resting state by the voltage activated Potassium current. The refractory period has two important implications for action potential generation and conduction. First, action potentials can be conducted only in one direction, away from the site of its generation, and secondly, they can be generated only up to certain limiting frequencies. 
     Most nerves in the human body are composed of thousands of fibers, of different sizes designated by groups A, B and C, which carry signals to and from the brain. For example, a major nerve such as the Vagus nerve may have approximately 100,000 fibers of the three different types each carrying signals. Each axon (fiber) of that nerve conducts only in one direction, in normal circumstances. The A and B fibers are myelinated (i.e., have a myelin sheath, constituting a substance largely composed of fat), whereas the C fibers are unmyelinated. 
     Components of nerve stimulation systems include electrodes next to the nerve bundle or wrapped around the nerve and a lead with conductor connected to a pulse generator. Functional electrical stimulation realizes the excitation of the nerve by directly injecting charges into the nerve via the electrodes. The electrical field, necessary for the charge transfer, is simply impressed via the wires of the electrodes. 
     The power supply can be implanted, as in a cardiac pacemaker, or alternatively, there may be an implanted lead-receiver and an external transmitter, with their respective coils being inductively coupled. The implantable lead-receiver can be miniaturized utilizing the currently available electronic technology. Alignment of the subcutaneous coil-receiver and external coil-transmitter is critical for effective electromagnetic coupling. Since the receiver and transmitter coils are coupled, the degree of coupling depends, in part, upon the physical spacing between the coils and how they are placed with respect to each other (orientation). Such issues do not normally occur when a direct electrical connection is made between the pulse source and the stimulating electrodes. 
     The system described in this invention provides improved means for proximity sensing to aid in the optimal placement of the external-transmitting coil. Furthermore, it provides the means for continuous feedback regulation of output pulses, as the two coils shift relative to each other, in the course of activities during the day or night during sleep. 
     It should be obvious to one skilled in the art that various sensor modalities such as Hall effect, ultrasonic, inductive, capacitive etc. can be considered for proximity sensing. The Hall effect sensor opens and closes a circuit electronically based on changes in magnetic flux between the sensor and a target. This sensor can provide rotation speed or position measurement. The ultrasonic sensor emits an ultrasonic pulse that reflects back from an object entering the sonic cone. The time of reflection of the signal (dependent in part on the ultrasonic reflectivity of materials) provides a measure of the distance of the sensor from an object. An inductive proximity sensor consists of a coil and ferrite arrangement, an oscillator and detector circuit and a solid-state output. The oscillator of the inductive proximity sensor creates a high frequency field radiating from a coil in front of the sensor. A metallic object can enter the field and have eddy currents induced and detected from its surface. The capacitive sensor detects the approach of a target near its leading surface and this results in an increase in the capacitance. The increased capacitance results in the increase of amplitude of an oscillatory signal which can be detected. A GMR sensor was used in this embodiment as it affords large separation between the two coils across various mediums. This sensor uses magnetic field strength and direction of the magnetic field to provide relatively accurate location measurement of a magnetic material or target. The size of the sensor and associated circuitry are also quite favorable in this case. 
     The prior art addresses proximity sensing but not with the methodology disclosed here, and not with an implanted device with passive circuitry. The Mann U.S. Pat. No. 5,545,191 discloses a transcutaneous coupling device having an implanted unit and an external unit using Velcro®) for attaching the external unit to the skin in a proper location for optimal electromagnetic coupling between the units. 
     U.S. Pat. No. 5,314,453 (Jeufter) is directed to position sensitive power transfer between an implanted power supply and a magnetic field sensing device. In this patent, a magnetically operated reed switch opens when the field from the magnet is remote or misarranged to prevent transmission of high frequency power by the transmitter antenna coil. Whereas the object of this patent is to either deliver or not deliver power to the implanted power source, the primary objective of the present invention is to regulate the output to the passive lead-receiver, thereby regulating the amount of current delivered to nerve for stimulation. 
     U.S. Pat. No. 5,811,893 (Soyck) applies a second sensor to react to the interference fields in order to maintain the signal through the feed-forward loop and preserving the functional state of the approximation switch even in the case of magnetic field interference. 
     U.S. Pat. No. 4,352,960 (Dormer) and U.S. Pat. No. 4,736,747 (Drake) merely disclose transcutaneous coupling by magnetically securing the subcutaneous and supercutaneous members. 
     U.S. Pat. No. 5,117,825 (Grevious) is directed to radio frequency coupling between an external programmable component and an implanted cardiac pacemaker for the purpose of programming the implanted device. 
     SUMMARY OF THE INVENTION 
     The present invention comprises a transcutaneous signal coupling method and apparatus for detecting the position of a passive, implanted electrode and transferring an electric excitation signal to the electrode from an external source. The apparatus of the invention includes an external unit containing a power source, a pulse generation circuit, a proximity sensing circuit, an inductive coil, and an implanted or internal unit. The internal unit contains a small magnet composed of materials, such as Samarium-Cobalt, that exhibit Giant Magneto-resistor characteristics, a passive receiving circuitry and an inductive coil. These Magneto-resistor sensors overcome a weakness in conventional resistor and Hall effect sensors with good sensing under relatively large separation between the sensor and the target magnet. The sensor of the invention measures the direction of the field applied from the magnet to the sensor within a specific range of field strength magnitude. 
     In accordance with a feature of this invention, the condition where the supercutaneous primary coil comes in optimal proximity and is located in parallel to the secondary subcutaneous coil, along its axis, can be recorded. An indicator provides measure of the proximity distance. The external unit has indicators for low battery, no output state, failure condition etc. These indicators are mounted in the remote external package. The intent of the present invention is to provide the user with detailed information concerning the state of the system while in use. 
     In another feature of this invention, as the axis or distance between the primary and secondary coils changes, the output of the external circuit provides a signal for correction, thereby providing feedback regulation of the current that is delivered to the implanted passive lead-receiver. 
     Various other features, objects and advantages of the invention will be made apparent from the following description taken together with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings illustrate the best mode presently contemplated of carrying out the invention. 
     In the drawings: 
     FIG. 1 is a schematic of the electrical equivalent circuit of the membrane of excitable cells. 
     FIG. 2 shows schematic of an action potential. 
     FIG. 3 shows propagation of a nerve impulse. 
     FIG. 4 is a generalized block diagram of the invention, including the main components of the invention such as the pulse generator, programmable circuit, coils, proximity sensor, indicators and battery unit. 
     FIG. 5 is a block diagram of the phase locked loop based pulse generator. 
     FIG. 6 provides greater details of the diagram shown in FIG. 5, such as the electronic components used in the high frequency signal generation. 
     FIG. 7 shows the electronic components used in the frequency selection and initialization stage. 
     FIG. 8 shows the passive subcutaneous (implanted) circuit, which will couple with the external coil placed supercutaneously. 
     FIG. 9 shows the circuit used to drive the proximity sensors and also provides temperature compensation. 
     FIG. 10 shows the typical placement of the device in a clinical application. 
    
    
     The following are reference numbers in the drawings: 
       1 . subthreshold nerve stimulus 
       2 . subthreshold nerve stimulus 
       3 . suprathresthold nerve stimulus 
       38 . skin 
       40 . control logic for magnitude, frequency and pulse-width 
       42 . pulse generation logic 
       44 . differential amplifier 
       45 . feed-forward signal from pulse generation circuit to external coil 
       46 . proximity sensing circuit 
       47 . temperature compensation circuit 
       48 . proximity sensing unit 
       49 . communication link between external coil and remote external circuitry 
       50 . primary inductive coil (external) 
       51 . cable to the external device 
       52 . secondary coil that is subcutaneous (embedded) 
       53 . magneto-resistor or GMR magnet 
       54 . electrode wire 
       55 . electrode (cathode) 
       56 . programmable logic 
       57 . remote external circuitry and power supply unit package (external stimulator) 
       58 . battery unit (pack) 
       59 . body part to be stimulated 
       62 . indicator unit 
       63 . audio alarm 
       64 . programming unit 
       70 . voltage controlled oscillator (VCO) block 
       72 . output buffer block 
       76 . initialization circuit block 
       78 . phase locked loop (PLL) phase comparator block 
       80 . PLL comparison frequency divider 
       82 . PLL reference frequency divider 
       83 . PLL synthesis oscillator block 
       84 . PLL crystal oscillator block 
       86 . PLL control unit block 
       94 . oscillator capacitor 1, 69 pF 
       96 . reference crystal, 2 MHz 
       98 . oscillator capacitor 2, 30 pF 
       100 . oscillator variable capacitor, 60 pF 
       102 . transistor base resistor, 100K 
       104 . transistor, 2SC1906 
       106 . variable inductor coil,  7 S144 
       108 . transistor emitter capacitor, 10 pF 
       110 . transistor base capacitor, 1 μF 
       112 . transistor base capacitor, 1 μF 
       114 . transistor emitter capacitor, 1 μF 
       116 . transistor emitter resistor, 1 KΩ 
       118 . transistor emitter capacitor, 100 pF 
       120 . variable capacitance diode, 1SV123 
       122 . transistor emitter resistor, 47 KΩ 
       124 . variable capacitance diode, 1SV123 
       126 . frequency to voltage resistor, 47 KΩ 
       128 . frequency to voltage resistor, 2.2 KΩ 
       130 . frequency to voltage capacitor, 10 μF 
       132 . frequency to voltage resistor, 2.2 KΩ 
       134 . feedback capacitor, 0.01 μF 
       136 . output buffer capacitor, 0.01 μF 
       138 . output buffer resistor 10 KΩ 
       140 . field effect transistor, 2SK439 
       142 . output capacitor, 0.001 μF 
       144 . variable inductor coil, 7S144 
       145 . complete PLL circuit with regulation 
       152 . variable capacitor, approx. 500 pF 
       154 . bridge diode rectifier 
       156 . bridge resistor, 100Ω 
       158 . capacitor, 1000 pF 
       160 . switch 
       162 . current limiting diode 
       164 . output resistor, 10 KΩ 
       165 . electrode body connection (cathode) 
       166 . output capacitor, 0.22 μF 
       167 . implant (secondary) circuit 
       169 . retum electrode connection (Anode) 
       170 . resistor, 1 KΩ 
       172 . zener diode 
       174 . resistor, 12 KΩ 
       176 . resistor, 1 KΩ 
       178 . variable resistor, 5 KΩ 
       180 . resistor, 1 KΩ 
       182 . resistor, 12 KΩ 
       184 . resistor, 1 KΩ 
       185 . amplifier output connector 
       186 . operation amplifier 
       188 . operational amplifier 
       190 . operational amplifier 
       192 . resistor, 68 KΩ 
       194 . operational amplifier 
       195 . resistor, 3.9 KΩ 
       196 . resistor, 98 KΩ 
       198 . sensor 1, Siemens GMR C6 
       200 . resistor, 1 KΩ 
       202 . sensor 2, Siemens GMR C6 
       204 . resistor, 1 KΩ 
       206 . resistor, 3.3 KΩ 
       208 . resistor, 10 KΩ 
       210 . operation amplifier 
       220 . resistor, 120 KΩ 
       222 . capacitor, 3.3 μF 
       224 . transistor, 2SC1815 
       226 . resistor, 8.2 KΩ 
       227 . program reset 
       228 . NAND gate, 74LS00 
       230 . NAND gate, 74LS00 
       232 . NAND gate, 74LS00 
       234 . JK Flip Flop, 74LS73 
       236 . NAND gate, 74LD00 
       238 . diode, 1S1588 
       240 . resistor, 1 KΩ 
       241 . capacitor, 0.47 μF 
     capacitor, 0.47 μF 
       244 . NAND gate, 74LS00 
       245 . resistor, 1 KΩ 
       246 . diode, 1S1588 
       247 . multivibrator 
       248 . NAND gate, 74LS00 
       250 . NAND gate, 74LS00 
       252 . inverter, 74LS04 
       253 . clock output of initialization circuit 
       254 . NAND gate, 74LS00 
       256 . inverter, 74LS04 
       257 . load enable from initialization circuit 
       258 . inverter, 74LS04 
       260 . NAND gate, 74LS00 
       262 . inverter, 74LS04 
       264 . NAND gate, 74LS00 
       266 . NAND, 3 input gate, 74LS10 
       267 . programmable data port 
       268 . NAND, 3 input gate, 74LS10 
       270 . NAND, 3 input gate, 74LS10 
       271 . data output from initialization circuit 
       272 . multiplexer/switch, CD4067B 
       274 . multiplexer/switch, CD4067B 
       276 . NAND gate, 74LS00 
       278 . 4 bit binary counter, 74LS93 (programmable counter) 
       280 . JK flip flop, 74LS73 
       282 . inverter, 74LS04 
       284 . NAND gate, 74LS00 
       286 . JK flip flop, 74LS73 
       288 . JK flip flop, 74LS73 
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the current embodiment for carrying out the invention. This description is not to be taken in a limiting sense, but is made merely for the purpose of describing the general principles of the invention. The scope of the invention should be determined with reference to the claims. 
     FIG. 4 shows an overall block diagram of the invention with an external coil  50 , a proximity sensing unit  48 , and a subcutaneous secondary coil  52  with a GMR magnet  53  associated with the proximity sensing unit  48 . A proximity sensing circuit  46  provides feedback of the position of the secondary embedded coil  52 . The signal output from proximity sensing circuit  46  is derived from the relative location of the coils  50  and  52 . The coil sub-assemblies consist of the coil and the associated electronic components that are rigidly connected to the respective coil. This signal from the proximity sensing circuit  46  is provided to the pulse generation logic  42  through amplifier  44  and this signal varies to compensate for any variation of the nominal signal from proximity sensing circuit  46 . The programmable parameters are stored in a programmable logic  56 . These parameters are provided to the control logic  40 , for magnitude, frequency and pulse-width control. The battery (chargeable or disposable)  58  is necessary to provide power to all other devices such as the control logic for magnitude, frequency and pulse width  40 , the pulse generation logic  42 , the differential amplifier  44 , the proximity sensing circuit  46 , the programmable logic  56 , the proximity sensing unit  48  and the indicator unit  62 . The output of the pulse generation logic  42  is provided to the external coil  50  as indicated by arrow  45 . The sensor and associated circuit present in the proximity sensing unit  48  and the external coil  50  are placed in a supercutaneous manner. The set of external components shown in FIG. 4 are installed in the remote external circuitry package  57 . 
     A voltage controlled oscillator (VCO)  70  (shown in FIG. 5) is utilized to generate an electrical excitation pulse of a pre-determined (programmable) frequency. The frequency can be changed by a phase locked loop (PLL) control signal applied to VCO block  70 . The theoretical range of the pulse frequency is quit large with values from 10 to 120 MHz. This excitation pulse signal from the primary coil  50  is inductively coupled with the secondary coil  52  for approximately up to 25 mm separation between coils. The pulse signal is induced in the secondary coil  52 , and then the waveform is shaped for an adequate signal delivered to excitable tissue  59 , such as a nerve bundle, by the implanted electrode  55  positioned in contact with the tissue. The separation between the coils  50  and  52  can vary and the signal produced at the secondary coil  52  is maintained consistent within design parameters. The proximity sensors  198 ,  202  detect the presence of a GMR (Giant Magneto-Resistor) magnet  53 , composed of Samarium Cobalt and is rigidly attached to the secondary subcutaneous coil  52 . The proximity sensors  198 ,  202  are mounted externally as a rigid assembly and sense the actual separation between the coils, also known as the proximity distance. Adjustment of the parameters of the VCO  70  is possible in order to compensate for variations in the primary and secondary coil separation. In the event the distance exceeds the theoretical limit, an alarm will sound to indicate the failure to produce an adequate signal in the secondary implant circuit  167 , as applied in the present embodiment of the device. 
     Referring to FIGS. 5 and 6, a phase locked loop synthesizing oscillation circuit  83  is used to produce a pulse of accurate frequency, the value of which is selectable. The circuit  83  compares frequency of the reference frequency source with the frequency of the signal actually produced by the VCO  70 . The oscillator circuit  83  adjusts for the change in actual frequency and controls the actual frequency to match the reference frequency. The change in the frequency is affected by changing the value of a frequency divider constant of the reference frequency and the comparison frequency components respectively. The oscillator frequency is affected by adjusting values of the variable capacitance diodes  120 ,  124  and transistor emitter capacitor  118 . The combination of resistor and capacitor components  126 ,  128 ,  130 ,  132  is used to convert the pulse signal originating from the charge pump of the synthesizing oscillator  83 , to a voltage value that adjusts for the frequency generated by complete PLL circuit  145 , by varying the capacitance of variable capacitance diodes  120 ,  124 . It will be obvious to one skilled in the art that, alternately, a direct voltage signal can be applied to the VCO  70  to bypass the above components to thus provide the appropriate frequency control. In another version of this circuit, the signal obtained from the proximity sensor circuit  46  (FIG. 4) can be applied to the VCO  70  to control the frequency of the signal. A Field Effect Transistor device  140  is used as an output buffer to isolate the external devices and to adjust the level of output signal. 
     A highly stable crystal oscillator  96  is used as the frequency reference, as shown in FIG.  6 . The crystal oscillator  96  provides a signal with a stable frequency for comparison with the frequency of comparison signal, obtained by feedback of the output of capacitor  134 . A phase comparator  78  is used to compare the actual signal frequency produced with the reference frequency, and adjusts the signal from the control unit  86  to reduce the difference of both frequencies to zero. A programmable counter  278  (shown in FIG. 7) is used to reduce the output frequency by a fraction. The phase comparison  78  function is provided by the PLL synthesis oscillator (PSO)  83 . In this version of the circuit, this device is chip # MB87014A 83 available from Motorola Corp., alternatively, a functionally equivalent version of this chip can be utilized. The oscillator  83  detects the difference of the phase comparison frequency and reference frequency signals, after appropriate division, and a phase difference signal is output to the VCO  70 , as a pulse signal of appropriate frequency. The programming of the frequency division circuit is provided by serial 16 bit data at the data port of the PSO  267 , also shown as Programmable Data in FIG.  7 . This data is read on the rising edge of the clock signal from a 2 MHz crystal oscillator  96 , for this embodiment. A high-value at the load enable  257  at its connector, allows either the comparison or reference side of the divisor to be set as per the control bit on the data line. When the 16 bits of data are all at logic low, the PLL circuit output  271  stops normal operation and no signal is provided. This state is enabled for “no pulse signal generation” condition. The clock signal for frequency divider values setup is obtained from clock output connection of initialization circuit  253 . The initialization circuit is utilized for setting the frequency dividers at power-up, but a serial update can be made to the programmable data port  267  at run time. The output voltage level of the pulse signal is of the range of 0 V to the power input supply voltage, (typically 5V). The advantage of the PLL circuit  83  is that a signal of stable high frequency is possible by referencing it at lower frequency levels using appropriate division of the high frequency VCO  70  signal and reference signal. 
     The initialization circuit  76 , shown in FIG. 7, depicts a circuit whose function is to provide dividing ratio setting data to the PLL  83 , after turning on or after a program reset signal is applied. The capacitor  222  charges to a threshold value at the base of transistor  224  and enables the pulse signal generated by the multi-vibrator  247  to be conducted by enabling the Flip Flop  234 . The multi-vibrator  247  (including logic components  236 ,  238 ,  240 ,  241 ,  242 ,  244 ,  245 ,  246 ), provide a signal of approximately 600 Hz frequency for control of other signals in this circuit. The data is present at the data lines multiplexed at the NAND gate  270  and a data output  271  from initialization circuit  76 . The load enable from initialization circuit  257  provides the signal to load this data and the control signal to the PLL  83  through its initialization circuit block  76 . This step allows for the loading of the comparison frequency divider data, followed by the reference frequency divider data, during initialization and programming setup. A binary counter Device  278  is a 16 bit counter for multiplexing the divider data to output of NAND gate  270 . The 16 bit divider data can be provided by pre-programmed storage of patterns in devices such as Field Programmable Logic Arrays  56 . Other storage devices can be applied for the same operation. The circuit shown in FIG. 6 provides a pulse waveform of the appropriate characteristics, to the external (primary) inductive coil  50 . This coil  50  is inductively coupled with secondary coil  52 , implanted subcutaneously. The secondary coil  52  propagates this signal and the implanted circuit  167  converts it to an appropriate waveform that is delivered to the electrode  55 . 
     The circuitry of the implantable lead-receiver is shown in FIG.  8 . This version of the circuit uses all passive components. In this embodiment, a 25 turn copper wire of 30 gauge thickness was used for both the primary coil  50  and secondary coil  52 . This wire was concentrically wound with the windings all in one plane. A variable capacitor  152  provides flexibility in tuning to the actual frequency received by coil  52  from the primary coil  50 . The frequency of the pulse-waveform delivered to embedded coil  52  can vary and so a variable capacitor  152  provides ability to tune the secondary implanted circuit  167  to the signal from the primary coil  50 . The pulse signal from embedded coil  52  is rectified by the diode bridge  154  and frequency reduction obtained by capacitor  158  and resistor  164 . The last component in line is capacitor  166 , used for isolating the output signal from the electrode wire  54 . The return path of signal from cathode  165  will be through the tissue for “Bipolar” stimulation. Alternatively, anode  169  can be connected to the remote ground connection of implantable circuit  167 , providing for much larger intermediate tissue for “Unipolar” stimulation. The “Bipolar” stimulation offers localized stimulation of tissue compared to “Unipolar” stimulation, where skeletal muscles can be stimulated. The implanted circuit in this embodiment is passive, so a battery does not have to be implanted. It is however possible, in a future version, to implant a battery source for use of active component logic in the implant. 
     FIG. 9 shows the circuit used to drive the proximity sensors. The two sensors  198 ,  202  obtain their feedback from the GMR magnet  53  that is implanted. This circuit also provides temperature compensation, shown in the temperature compensation circuit  47  of FIG.  9 . The sensors  198 ,  202  are ‘Giant Magneto Resistor’ (GMR) type sensors packaged as proximity sensing unit  48 . The resistance effect depends on the combination of the soft magnetic layer of magnet  53 , where the change of direction of magnetization from external source can be large, and the hard magnetic layer, where the direction of magnetization remains unchanged. The typical change in resistance due to the GMR effect can be about 5% of the nominal resistance. The resistance of this sensor varies along a straight motion through the curvature of the magnetic field. A bridge differential voltage is suitably amplified and used as feedback for appropriate compensation. 
     The Siemens GMR B6 (Siemens corp. Special components Inc. New Jersey) can be used for this function. The maximum value of the peak-to-peak signal is observed as the external magnetic field becomes strong enough, at which point the resistance increases, resulting in the increase of the field-angle between the soft magnetic and hard magnetic material. The bridge voltage also increases. In this application, the two sensors  198 ,  202  will be oriented orthogonal to each other. 
     The distance between magnet  53  and sensor is not relevant as long as the magnetic field is between 5 and 15 KA/m, and provides a range of distances between the sensors  198 ,  202  and the magnetic material. The GMR sensor registers the direction of the external magnetic field. A typical magnet to induce permanent magnetic field can be approximately 15 by 8 by 5 mm 3 , for this application. However, this sensor is sensitive to temperature, such that the corresponding resistance drops as temperature increases. This effect is quite minimal until about 100° C. A full bridge circuit can be used for such compensation as shown in temperature compensation circuit  47  of FIG.  9 . The sensors  198 ,  202  and a pair of resistors  200 ,  204  are shown as part of the bridge network for temperature compensation. It is also possible to use a full bridge network of two additional sensors in place of the resistors  200 ,  204 . The bridge output is further amplified for this application. 
     The signal from either proximity sensor  198  or  202  is rectangular if the surface of the magnetic material is normal to the sensor and is radial to the axis of a circular GMR device. This indicates a shearing motion between the sensor and the magnetic device. When the sensor is parallel to the vertical axis of this device, there is a fall off of the relatively constant signal at about 25 mm. separation. The GMR sensor combination varies its resistance according to the direction of the external magnetic field, thereby providing an absolute angle sensor. The position of the GMR magnet can be registered at any angle from 0 to 360 degrees. The characteristics of the feedback signal from the proximity sensor will be developed separately. 
     The remote circuit package is shown in FIG.  10  and an indicator unit  62  (FIG. 4) is provided for: Low Battery (if external battery is low), program number in use, proximity distance too large or proximity failure (in case the patch containing the external coil  50 , has been removed, or is twisted abnormally etc.) Indication is also provided to assist the placement of the patch. In case of general failure, a red light with audible signal is provided when the signal is not reaching the subcutaneous circuit. The information on the low battery, normal and out of power conditions will forewarn the user of the requirements of any corrective actions. Other information like the parameters of the program being applied and the normal operation of the device can be displayed as components of remote external circuitry and power supply unit  57 . The battery  58  present in the remote external circuitry  57  can be completely recharged or replaced. 
     The methodology and apparatus described here, aids in the optimal positioning of the external coil and once the external coil is secured at the appropriate position, this invention ensures continuous feedback regulation of the signal pattern output to the body part to be stimulated. 
     The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. It is therefore desired that the present embodiment be considered in all aspects as illustrative and not restrictive, reference being made to the appended claims rather than to the foregoing description to indicate the scope of the invention. 
     Various alternatives and embodiments are contemplated as being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter regarded as the invention.