Abstract:
An apparatus ( 500 ) for stimulating axon growth of the nerve cells in the spinal cord of mammals to stimulate regeneration of the nerve cells in the spinal cord. Such an apparatus includes a variable current DC stimulus generator ( 420 ), and data transfer circuitry ( 410 ) in communication with the DC stimulus generator ( 420 ) and an external module ( 430 ), the data transfer circuitry ( 410 ) operable to transmit signals between the DC stimulus generator ( 420 ) and the external module ( 430 ). The DC stimulus generator ( 420 ) and data transfer circuitry ( 410 ) may be within a biocompatible container ( 510 ).

Description:
BACKGROUND 
       [0001]    Injury to the spinal cord or central nervous system can be one of the most devastating and disabling injuries possible. Depending upon the severity of the injury, paralysis of varying degrees can result. Paraplegia and quadriplegia often result from severe injury to the spinal cord. The resulting effect on the sufferer, be it man or animal, is severe. The sufferer can be reduced to a state of near immobility or worse. For humans, the mental trauma induced by such severe physical disability can be even more devastating than the physical disability itself. 
         [0002]    When the spinal cord of a mammal is injured, connections between nerves in the spinal cord are broken. The injured portion of the spinal cord is termed a “lesion.” Such lesions block the flow of nerve impulses for the nerve tracts affected by the lesion with resulting impairment to both sensory and motor function. 
         [0003]    To restore the lost sensory and motor functions, the affected motor and sensory axons of the injured nerves must regenerate, that is, grow back. Unfortunately, any spontaneous regeneration of injured nerves in the central nervous system of mammals has been found to occur, if at all, only within a very short period immediately after the injury occurs. After this short period expires, such nerves have not been found to regenerate further spontaneously. 
         [0004]    Studies have shown, however, that the application of a DC electrical field across a lesion in the spinal cord of mammals, can promote axon growth, and the axons will grow back around the lesion. Since the spinal cord is rarely severed completely when injured, the axons need not actually grow across the lesion but can circumnavigate the lesion through remaining spinal cord parenchyma. 
         [0005]    For optimal results in a human patient, a uniform electrical field of a desired strength is imposed over about 10 cm to 20 cm of damaged spinal cord for a beneficial clinical outcome. Ideally, this uniform field is imposed across the entire cross section of the spinal cord over this longitudinal extent, because of the general segregation of descending (motor) tracts to the ventral (anterior) cord, and the segregation of important (largely sensory) tracts to the posterior (dorsal) spinal cord. In paraplegic canines, this electrical field has been directly measured (Richard B. Borgens, James P. Toombs, Andrew R. Blight, Michael E. McGinnis, Michael S. Bauer, William R. Widmer, and James R. Cook Jr.,  Effects of Applied Electric Fields on Clinical Cases of Complete Paraplegia in Dogs , J. Restorative Neurology and Neurosci., 1993, pp. 5:305-322). In man however, the cross sectional area of the spinal cord is approximately two to four times that of the small to medium sized dogs treated in clinical trials, and actual invasive measurement of the imposed electrical fields in response is not feasible on human patients. 
         [0006]    Based on the responses of human paraplegics and quadriplegics to prior art therapies involving the application of an oscillating DC electrical field across a lesion in the spinal cord using three pairs of electrodes, it appears that the dorsal (posterior) location of three pairs of electrodes did not produce a uniform field over the entire unit area of the patient&#39;s spinal cord. This was revealed by the domination of sensory recovery in these patients (&lt;thirty fold over historical controls) compared to motor recovery (˜twofold greater than historical controls) using the ASIA scoring system. Thus, the voltage gradient was highest nearest to the actual placement of two pairs of electrodes on either side (two tethered to the right and left lateral facets) and the third pair sutured to the paravertebral muscle and fascia of the dorsal (posterior) facet-rostra and caudal of the spinal cord lesion (Shapiro, et al.,  Oscillating Field Stimulation for Complete Spinal Cord Injury in Humans: a Phase  1  Trial , Journal of Neurosurg. Spine 2, 2005, pp. 3-10). 
         [0007]    It would be desirable to provide a device to generate a stronger DC electrical field across the spinal cord lesion of a human in order to facilitate the creation of a uniform electrical field over the affected area. It would be further desirable to provide a method for implanting electrodes that facilitates the creation of a uniform electrical field over the affected area of the injured spinal cord. 
       SUMMARY 
       [0008]    According to at least one aspect of the disclosure, an apparatus for stimulating axon growth of the nerve cells in the spinal cord of mammals to stimulate regeneration of the nerve cells in the spinal cord comprises a variable current DC stimulus generator, polarity reversing lo circuitry, and data transfer circuitry. Such a variable current DC stimulus generator has first and second groups of oppositely polarized output electrodes, wherein one group of electrodes comprises at least three electrodes acting as a cathode of the generator, and the other group of output electrodes comprises at least three electrodes acting as an anode of the generator. Such polarity reversing circuitry is configured to reverse the polarity of the DC stimulus each time a predetermined period of time elapses, wherein each time the polarity of the DC stimulus is reversed the output electrodes which comprised the cathode before the polarity reversal comprises the anode after the polarity reversal and the output electrodes which comprised the anode before the polarity reversal comprises the cathode after the polarity reversal. Such data transfer circuitry is in communication with the DC stimulus generator, and is operable to transmit signals to and from the DC stimulus generator. 
         [0009]    According to at least one aspect of the disclosure, an apparatus for stimulating axon growth of the nerve cells in the spinal cord of mammals comprises a DC stimulus controller that controls the duty cycle of a DC stimulus generator to provide an on-cycle wherein the generator provides a DC output and an off cycle wherein the generator does not provide a DC output, the duty cycle being generated during each polarity reversal. 
         [0010]    According to at least one aspect of the disclosure, an apparatus for stimulating axon growth of the nerve cells in the spinal cord of mammals comprises a DC stimulus controller that controls the amplitude of the DC stimulus generator to provide an on-cycle wherein the generator provides a DC output and an off cycle wherein the generator does not provide a DC output, the duty cycle being generated during each polarity reversal. 
         [0011]    According to at least one aspect of the disclosure, an apparatus for stimulating axon growth of the nerve cells in the spinal cord of mammals comprises a DC stimulus controller that controls the frequency of the DC stimulus generator to provide an on-cycle wherein the generator provides a DC output and an off cycle wherein the generator does not provide a DC output, the duty cycle being generated during each polarity reversal. 
         [0012]    According to at least one aspect of the disclosure, an apparatus for stimulating axon growth of the nerve cells in the spinal cord of mammals comprising a variable current DC stimulus generator, first and second groups of electrodes, and data transfer circuitry are each components configured to be implanted in the body of a patient suffering nerve cell damage. 
         [0013]    According to at least one aspect of the disclosure, an apparatus for stimulating axon growth of the nerve cells in the spinal cord of mammals comprises an external controller for controlling the output of a DC stimulus generator. Such an external controller may be communicatively coupled with data transfer circuitry in an apparatus for stimulating axon growth of the nerve cells in the spinal cord of mammals. Such an external controller and data transfer circuitry may be capable of bi-directional communication, which may be accomplished via radio frequency transmission. 
         [0014]    According to at least one aspect of the disclosure, data transfer circuitry may comprise at least one low-pass filter, at least one transceiver, at least one voltage controlled oscillator, and at least one antenna. 
         [0015]    According to at least one aspect of the disclosure, an apparatus for stimulating axon growth of the nerve cells in the spinal cord of mammals to stimulate regeneration of the nerve cells in the spinal cord comprises a variable current DC stimulus generator, polarity reversing circuitry, data transfer circuitry, and at least one has sensor, wherein at least one sensor is capable of monitoring the electrical environment surrounding the apparatus. Such data transfer circuitry may be capable of telemetering information about the electrical environment surrounding the apparatus to an external device. Such an external device may, in response to the information about the electrical environment surrounding the apparatus, generate configuration information and transmit the configuration information to the data transfer circuitry, where the configuration information comprises parameters for controlling the output of the DC stimulus generator. Such an apparatus may comprise first and second groups of electrodes, wherein at least one electrode is configured as at least one sensor. 
         [0016]    According to at least one aspect of the disclosure, an apparatus for stimulating axon growth of the nerve cells in the spinal cord of mammals to stimulate regeneration of the nerve cells in the spinal cord comprises a variable current DC stimulus generator, polarity reversing circuitry, data transfer circuitry, and at least one has sensor, wherein at least one sensor is capable of monitoring the biological environment surrounding the apparatus. Such data transfer circuitry may be capable of telemetering information about the biological environment surrounding the apparatus to an external device. Such an external device may, in response to the information about the biological environment surrounding the apparatus, generate configuration information and transmit the configuration information to the data transfer circuitry, where the configuration information comprises parameters for controlling the output of the DC stimulus generator. Such an apparatus may comprise first and second groups of electrodes, wherein at least one electrode is configured as at least one sensor. 
         [0017]    Additional features and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of a preferred embodiment exemplifying the best mode of carrying out the invention as presently perceived. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0018]    The features and advantages of this disclosure, and the methods of obtaining them, will be more apparent and better understood by reference to the following descriptions of disclosed embodiments, taken in conjunction with the accompanying drawings, wherein: 
           [0019]      FIG. 1  shows a graph that portrays the effect of an applied steady DC field over time on the growth of cathodal and anodal facing axons; 
           [0020]      FIG. 2  shows a graph that portrays the effect of an applied oscillating field over time on the growth of cathodal and anodal facing axons; 
           [0021]      FIG. 3A  shows a first portion of a schematic of a circuit for generating an oscillating electrical field for stimulating nerve regeneration; 
           [0022]      FIG. 3B  shows a second portion of a schematic of a circuit for generating an oscillating electrical field for stimulating nerve regeneration; 
           [0023]      FIG. 4A  shows a block diagram of a neural injury treatment device; 
           [0024]      FIG. 4B  shows a schematic of a second circuit for generating an oscillating electrical field for stimulating nerve regeneration; 
           [0025]      FIG. 5  shows a schematic of a current source of the circuit of  FIG. 4B ; 
           [0026]      FIG. 6  shows a schematic of a voltage controlled oscillator of the circuit of  FIG. 4B ; 
           [0027]      FIG. 7  shows a schematic of an electromagnetic power coupling portion of the circuit of  FIG. 4B ; and 
           [0028]      FIG. 8A  shows a first portion of a schematic of a biphasic pulse generator that may serve as the pulse generator of the circuit of  FIG. 4B ; 
           [0029]      FIG. 8B  shows a second portion of a schematic of a biphasic pulse generator that may serve as the pulse generator of the circuit of  FIG. 4B ; 
           [0030]      FIG. 9  is a wave diagram of a triphasic pulse; 
           [0031]      FIG. 10  is a block diagram of a triphasic pulse generator that may serve as the pulse generator of the circuit of  FIG. 4B ; 
           [0032]      FIG. 11  shows a graph that portrays the effect of an applied pulse wave modulated oscillating field over time on the growth of cathodal and anodal facing axons. 
       
    
    
     DESCRIPTION 
       [0033]    For the purposes of promoting an understanding of the principles of the disclosure, reference will now be made to the embodiments illustrated in the drawings and described in the following written specification. It is understood that no limitation to the scope of the disclosure is thereby intended. It is further understood that the present invention includes any alterations and modifications to the illustrated embodiments and includes further applications of the principles of the disclosure as would normally occur to one skilled in the art to which this invention pertains. 
         [0034]    The application of an oscillating DC electrical field across a lesion in the spinal cord of a mammal can stimulate axon growth in both directions, i.e., caudally and rostrally. That is, growth of caudally facing axons will be promoted as will growth of rostrally facing axons. The DC electrical field is a stimulus which is first applied in one direction for a predetermined period of time and then applied in the opposite direction for the predetermined period of time. The polarity of the DC stimulus is reversed after each predetermined period of time. 
         [0035]      FIGS. 1 and 2  show the effects on axon growth by an applied steady state DC electrical field ( FIG. 1 ) and by an applied oscillating electrical field ( FIG. 2 ). Referring to  FIG. 1 , a nerve cell  10  is shown at the left-hand side of  FIG. 1  having a cell body or soma  12  from which an axon  14  extends upwardly and an axon  16  extends downwardly. At time  0 , a DC stimulus having a first polarity is applied to the nerve cell  10  such that axon  14  will be extending toward the cathode or negative pole of a DC stimulus signal and axon  16  will be extending toward the anode or positive pole of the DC stimulus. Axon  14  begins to grow almost immediately. However, after a period of time, i.e., the “die back period” (D T ), significant reabsorption of axon  16  into the cell body  12  begins and eventually axon  16  is completely reabsorbed into cell body  12 . At the right-hand side of  FIG. 1 , for illustration purposes, nerve cell  10  is shown wherein axon  14  has grown substantially longer but axon  16  has been reabsorbed into cell body  12 . 
         [0036]    In  FIG. 2 , the same reference numbers will be used to identify the elements of  FIG. 2  which correspond to elements of  FIG. 1 . Nerve cell  10  is shown at the left-hand side of  FIG. 2  having a cell body  12 , an upwardly extending axon  14  and a downwardly extending axon  16 . At time  0 , a DC stimulus having a first polarity is applied to nerve cell  10  such that axon  14  is extending toward the cathode and axon  16  is extending toward the anode of the DC stimulus. After a predetermined period of time, the polarity of the DC stimulus is reversed. Axon  14  will now be extending toward the anode and axon  16  will be extending toward the cathode of the DC stimulus. The predetermined period of time is selected to be less than the die back period (D T ) of the anodal facing axon. Significant die back of anodal facing axons begins to occur about one hour after the DC stimulus is applied. Therefore, the predetermined period should not exceed one hour. As shown in  FIG. 2 , an oscillating DC field stimulates growth of the axons facing both directions. This is due to the fact that growth of cathodal facing axons is stimulated almost immediately after the DC stimulus is applied but die back of the anodal facing axons does not become significant until after the die back period elapses. Since the polarity of the DC stimulus is switched before the die back period elapses, growth of axons in both directions is stimulated with the result that axons  14 ,  16  of nerve cell  12  both grow significantly longer as shown, for example, at the right-hand side of  FIG. 2 . 
         [0037]    In accordance with the present disclosure, the nerves in the central nervous system of a mammal are stimulated to regenerate by applying an oscillating electrical field to the central nervous system. The oscillating electrical field is a DC stimulus which is first applied in one direction, i.e. having a first polarity, for a predetermined period of time, and then applied in the opposite direction, i.e. having a second polarity opposite to the first polarity, for the predetermined period of time. In other words, the polarity of the DC stimulus is reversed after each predetermined period of time. The predetermined period of time is selected to be less than the die back period of anodal facing axons, but long enough to stimulate growth of cathodal facing axons. This pre-determined period of time will also be termed the “polarity reversal period” of the oscillating electrical field. In one disclosed embodiment, the polarity reversal period is between about thirty seconds and about sixty minutes. According to at least one embodiment of the present disclosure, there may be a period between each polarity reversal period where no voltage potential stimulus is applied (an “off cycle”). According to at least one embodiment of the present disclosure, two or more consecutive polarity reversal periods may be followed by an off cycle. 
         [0038]    Prior art technology generates a DC voltage of about 600 μV/mm that is imposed along the long axis of an injured spinal cord, and the polarity of the voltage is reversed about every 15 minutes to induce regrowth and reconnection of both ascending (towards the brain) and descending (towards the body) white matter tracts (containing only nerve fibers). In the prior art technology, therapy usually had to be discontinued within 14 to 16 weeks because of the capacity of the voltage source. Moreover, the waveform, or shape of the electrical signal based on electrical-field magnitude, the duty cycle, and the continuous DC, could not be altered. 
         [0039]    While prior art application of DC voltages have proved useful, patients who do not respond optimally may benefit from second or third regimens of therapy. This can be carried out at the discretion of the clinician with other unrelated therapies to affect an improved clinical outcome. For example, use of injections of soluble “neurotrophic” factors (BDNF, Interleukins, Inosine, etc) that can be administered for short times clinically may boost the growth response of nerve fibers experiencing DC voltages, and may extend the time post injury during which DC voltage applications are effective. 
         [0040]    Additionally, new research has revealed that this prior art method may not produce an optimum stimulation to achieve optimum results in clinical recovery. For example, in one new method of therapy, pulsatile DC fields in duce nerve regeneration. Where the DC Voltage of one polarity is “chopped” (turned on and off rapidly), no loss in its functional properties appears to occur. Intermittent DC fields also guide and induce growth. In this case, an “off” time relatively shorter than the “on time” increases the growth response to a level equal to or greater than that achieved by a steady DC field. Moreover, a substantially decreased power consumption may be possible by this mode of stimulation. For example, during a fifteen minute long imposition of the DC field on the spinal cord injury of a single polarity before duty cycle reversal, a forty-five second “on” time followed by a 15 second “off” time provides a saving in power consumption equivalent to at least about twenty-five percent of that used in the nominal duty cycle. 
         [0041]    While DC voltages induce growth towards the cathode in physiological milieu, they also may significantly reduce or eliminate retrograde degeneration of nerve fibers that have undergone secondary axotomy and have broken in two. The proximal segment (that in connection with the nerve cell body) usually survives while the distal segment (that disenfranchised from the cell body) usually dies and is lost, a process that in mammals known as Wallerian Degeneration. This is the fate of most mechanically damaged fibers following any form of nerve injury in the mammal. As distally negative extracellular voltage reduces the endogenous calcium current entering the cut end of the fiber, the concentration of calcium ions in the terminal axoplasm, and thus the “dieback” of the process is dependent on this. This distance produced by dieback, which can typically vary between tenths of a millimeter to several centimeters of the terminal nerve fiber, must be “made up” to permit extension of the fiber past the region of local injury. 
         [0042]    Another approach to therapy is to use a steady DC stimulus application in the early stages of the injury (e.g., 96 hours up to about post-injury), when most secondary axotomy occurs, reducing the “dieback” of nerve fibers, followed by a change in the waveform carried out by the clinician to improve regeneration and functional outcome, for example, back to a pusitile or time varying steady DC Field. In short, varying the field parameters may allow a more direct attack on retrograde degeneration of nerve fibers producing a better overall growth response dependent on the extent of linear growth. The devices disclosed herein may implement these new approaches to therapy. 
         [0043]      FIGS. 3A and 3B  (which together make up  FIG. 3 ) show a schematic of a circuit  300  for generating an oscillating electrical field for stimulating nerve regeneration. The circuit  300  comprises electronic components electrically interconnected as shown in  FIG. 3 . Conventional symbols are used to denote the components. The circuit  300  as shown in  FIG. 3  comprises electrodes  340 ,  342 ,  344 ,  346 ,  348 ,  350 ,  384  and  386 ; processor supervisory circuit  352 ; adjustable current sources  354 ,  356 ,  358 ,  360 ,  362 ,  364 ,  366 ,  368 ,  370 ,  372 ,  374 ,  376 ,  378 ,  388 ,  390 ,  392  and  394 ; switch  380 ; and timer  382 . The circuit  300  as shown in  FIG. 3  also comprises an optional beacon circuit  320 , which is electrically interconnected between nodes  325  and  327 . Electrodes  340 ,  342 ,  344  and  384  comprise Electrode Group A. Electrodes  346 ,  348 ,  350  and  386  comprise Electrode Group B. 
         [0044]    Electrode  340  is coupled to the output terminal  341  of the back-to-back adjustable current sources  356  and  358  which constitute a portion of the DC stimulus generator. Electrode  342  is coupled to the output terminal  343  of the back-to-back adjustable current sources  360  and  362  which constitute a portion of the DC stimulus generator. Electrode  344  is coupled to the output terminal  345  of the back-to-back adjustable current sources  364  and  366  which constitute a portion of the DC stimulus generator. Electrode  384  is coupled to the output terminal  385  of back-to-back adjustable current sources  388  and  390  which constitute a portion of the DC stimulus generator. Electrodes  340 , 342 , 344  and  384  comprise Electrode Group A and thus output terminals  341 ,  343 ,  345  and  385  constitute one group of output terminals. 
         [0045]    Electrode  346  is coupled to the output terminal  347  of the back-to-back adjustable current sources  368  and  370  which constitute a portion of the DC stimulus generator. Electrode  348  is coupled to the output terminal  349  of the back-to-back adjustable current sources  372  and  374  which constitute a portion of the DC stimulus generator. Electrode  350  is coupled to the output terminal  351  of the back-to-back adjustable current sources  376  and  378  which constitute a portion of the DC stimulus generator. Electrode  386  is coupled to the output terminal  387  of back-to-back adjustable current sources  392  and  394  which constitute a portion of the DC stimulus generator. Electrodes  346 ,  348 ,  350  and  386  comprise Electrode Group B and thus output terminals  347 ,  349 ,  351  and  387  constitute another group of output terminals. 
         [0046]    Circuit  300  includes a power supply and supervisory section  304 , and a secondary watchdog section  306 . The power supply and supervisory section  304  produces a  3 . 6  volt supply for powering the remaining devices of circuit  300 , including secondary watchdog section  306  and the optional beacon circuit  320  and the main oscillator of timer  382 . Additionally, the power supply and supervisory section  306  supervises the oscillator circuitry of the timer  382  to determine if there is failure of the oscillator circuit. 
         [0047]    The power supply and supervisory circuit  304  includes a battery  302 , processor supervisor circuit  352 , a resistor  301 , a first capacitor  303 , a second capacitor  305 , a switch  307 , a first transistor  308 , and a second transistor  309  configured as shown in  FIG. 3  to provide a 3.6 volt potential between a ground terminal  310  and a positive voltage terminal  311  for so long as the oscillator circuitry of the timer  382  is operating within desired parameters as explained in greater detail below. In one illustrated embodiment, the battery  302  may be a 3.6v Tadiran TL-5903 battery although other charge storage devices, including, but not limited to, rechargeable charge storage devices, e.g. charge storage device  429 , may be used within the scope of the disclosure. 
         [0048]    In one illustrated embodiment, the switch  307  may be an HSR-502RT reed switch available from Hermetic Switch, Inc., Chickasha, Okla. However, other switches may be used within the scope of the disclosure. The HSR-502 reed switch is a single pole-double throw (SPDT) switch enclosed in a glass capsule. 
         [0049]    In one illustrated embodiment, transistors  308  and  309  may be BSS138 transistors available from Fairchild Semiconductor Corporation, South Portland, Me., although other transistors and appropriate components can be used within the scope of the disclosure. In one illustrated embodiment, the transistors  308 ,  309  are N-Channel Logic Level Enhancement Mode Field Effect Transistors. The values of the resistor  301  and capacitors  303 ,  305  are chosen as required to meet design parameters. In the illustrated embodiment, resistor  301  is a 1 Mohm resistor and capacitors  303 ,  305  are 0.047 microfarad capacitors. 
         [0050]    The processor supervisor circuit  352  receives a clock pulse signal from the oscillator section of timer  382 . In one illustrated embodiment, the processor supervisor circuit  352  is a TPS 3823 Processor supervisor circuit with watchdog timer input (W) and Manual Reset Input (/MR) available from Texas Instruments, Dallas Tex. The illustrated processor supervisor circuit  352  includes a Power-On Reset Generator With Fixed Delay Time of 200 ms. The illustrated processor supervisor circuit  352  provides circuit initialization and timing supervision for the timer  382 . During power-on, /RESET (/RS) is asserted when supply voltage (V+) becomes higher than 1.1 V. Thereafter, the supply voltage supervisor monitors the supply voltage and keeps /RESET active as long as the supply voltage remains below the threshold voltage. An internal timer delays the return of the output to the inactive state (high) to ensure proper system reset. The delay time, td, starts after supply voltage has risen above the threshold voltage. When the supply voltage drops below the threshold voltage, the output becomes active (low) again. The illustrated processor supervisory circuit  352  has a fixed-sense threshold voltage set by an internal voltage divider. The illustrated processor supervisor circuit  352  incorporates a manual reset input, (/MR). A low level at the manual reset input (/MR) causes /RESET to become active. The illustrated processor supervisor circuit  352  includes a high-level output at /RESET (/RS). 
         [0051]    The arrangement illustrated in  FIG. 3  is configured so that when a low level is received on the /RESET pin of the processor supervisor circuit  352 , the gate of the transistor  308  receives no current effectively shutting down transistor  309 . When transistor  309  is shut down, the power supply is effectively shut down causing the remaining components of the circuit  300  to be without power. Once transistor  309  is shut down, transistor  308  asserts a low signal on the /MR pin of the supervisor circuit  352  effectively locking down the circuit until the power is cycled utilizing switch  307 . This configuration of timer  382 , supervisory circuit  352  and transistors  308 ,  309  acts as a failsafe device to shut down the oscillating field circuit whenever there is an apparent failure of the oscillator of the timer  382  so that the axons facing anodes will not be subjected to a disadvantageously oriented electrical field beyond the beginning of the die back period. 
         [0052]    The illustrated processor supervisor circuit  352  includes watchdog timer that is periodically triggered by a positive or negative transition at the watchdog timer input (W). The watchdog timer receives the clock pulse from the timer  382  of the secondary watchdog section  306 . When the supervising system fails to retrigger the watchdog circuit within the time-out interval, t tout , /RESET becomes active which, as described above shuts down transistor  309  and causes transistor  308  to assert a low signal on the /MR pin of the process supervisor circuit. This event also locks down and removes power from all of the other components of the circuit  300  (except battery  302 ) until power is cycled via switch  307 . 
         [0053]    The positive terminal of the battery  302  is electrically connected to the supply voltage input (V+) of the processor supervisory circuit  352 , one terminal of resistor  301 , the positive electrode of the second capacitor  305  and to the positive output terminal  311 . The second terminal of the resistor  301  is electrically connected to a node electrically connected to one terminal of the switch  307 , the positive electrode of the first capacitor  303  and the gate of the reset transistor  308  of the above described power-on/reset delay network. The second terminal of the switch  307  is electrically connected to the negative terminal of the battery  302 . The pole of the switch  307  is electrically connected to a node electrically connected to the negative electrode of the first capacitor  303 , the ground pin (GND) of the processor supervisor circuit  352 , the negative electrode of the second capacitor  305  and the source of the second transistor  309 . The gate of the second transistor  309  is coupled to a node coupled to the /RESET pin (/RS) and the source of the first transistor  308 . The drain of the second transistor  309  is coupled to the ground terminal  310 . The drain of the first transistor is coupled to the manual reset pin (/MR) of the processor supervisor circuit  352 . The watchdog timer input (W) of the processor supervisor circuit  352  is coupled to the PO pin of the timer  382 . 
         [0054]    The secondary watchdog section  306  includes adjustable current supply  354 , switch  380 , op amp  396 , resistors  312 - 315  and capacitors  321 . While the illustrated secondary watchdog section  306  is configured in accordance with the schematic shown in  FIG. 3 , it is within the scope of the disclosure for the secondary watchdog section  306  to be configured using other or additional components or for the section to be implemented on a single or multiple integrated circuits or a portion of a single or multiple integrated circuits implementing circuit  300 . 
         [0055]    In one illustrated embodiment, op amp 396 is an Analog Devices OP90GS Precision, Low Voltage Micropower Operational Amplifier, available from One Technology Way, Norwood, Mass. Other operational amplifiers or amplifier circuitry may be utilized within the scope of the disclosure. 
         [0056]    In one illustrated embodiment, the switch  380  is a MAX4544CSA Low-Voltage, Single-Supply Dual SPDT Analog Switch available from Maxim Integrated Products, Sunnyvale, Calif. The MAX4544 is a dual analog switch designed to operate from a single voltage supply, which because of its low power consumption (5 μW) is particularly well adapted for battery-powered equipment. The disclosed switch  380  offers low leakage currents (100 pA max) and fast switching speeds (tON=150 ns max, tOFF=100 ns max). The MAX4544 switch  380  is a single pole/double-throw (SPDT) device. 
         [0057]    In one illustrated embodiment, the timer  382  is a CD4060B type CMOS 14-stage ripple-carry binary counter/divider and oscillator, available from Texas Instruments, Dallas, Tex. The illustrated CD4060B timer  382  consists of an oscillator section and  14  ripple-carry binary counter stages. A RESET input is provided which resets the counter to the all-0&#39;s state and disables the oscillator. A high level on the RESET line accomplishes the reset function. All counter stages are master-slave flip-flops. The state of the counter is advanced one step in binary order on the negative transition of PI (and PO). All inputs and outputs are fully buffered. Schmitt trigger action on the input-pulse line permits unlimited input-pulse rise and fall times. 
         [0058]    In the illustrated embodiment, the watchdog timer input to the processor supervisor circuit  352  is coupled to the PO output of the timer  382  to provide a pulsed clock signal to indicate proper operation of the timer  382  which controls the polarity reversal period. Absence of this signal causes the supervisor circuit  352  to shut down power to the entire system. The /PO pin of the timer  382  is coupled through resistors  316  and  317  to the PI pin of the timer  382 . The positive electrode of capacitor  323  is coupled to a node coupling the terminals of resistors  316  and  317 , while the negative electrode of the capacitor  323  is coupled to a node coupled to the PO pin of the timer  382  thereby forming a free running oscillator. The period of the free-running oscillator is determined by the values of the resistors  316  and  317  and the capacitor  323 . In the illustrated embodiment, the resistors  316  and  317  each have a resistance of 1 Mohm and the capacitor  323  has a 0.047 micro-farad capacitance so that the oscillator runs at a frequency to generate the desired reversal period. The values of the resistors  316  and  317  and capacitor  323  can be varied to obtain reversal periods of different values within the scope of the disclosure. 
         [0059]    The Q 6  pin of the counter of the timer  382  is coupled to node  327  to provide a pulse to activate the optional beacon circuit  320 . The Q 14  pin of the timer  382  is coupled to a group B node  330 , i.e. a node providing power to the adjustable current sources  368 ,  370 ,  372 ,  374 ,  376 ,  378 ,  392  and  394  driving the Group B electrodes  346 ,  348 ,  350  and  386 . The reset pin of the timer  382  is coupled to a node that is coupled through the capacitor  322  to the positive voltage terminal  311  and coupled through resistor  318  to a node coupled to both the ground terminal  310  and the ground pin of the timer  382 . The power supply pin of the timer  382  is coupled to the positive voltage terminal  311 . 
         [0060]    The adjustable current source  354  of the secondary watchdog section  306  has its positive supply pin (V+) coupled to a node coupled to the positive voltage terminal  311 . This adjustable current source  354  provides a reference current that is utilized by op amp  396  to generate a signal to turn off the output power when the voltage drops below a specified value (illustratively 2.8V). In the illustrated embodiment, the adjustable current source  354  was selected to generate a second reference voltage instead of selecting a zenor diode to avoid the power loss associated with zenor diodes when utilized as reference voltage generators. The output power is interrupted in the illustrated circuit  300  by adjustable current source  354  and op amp  396  cooperating to lift the ground of switch  380  to interrupt current outflow to the group A electrodes. 
         [0061]    The negative pin (V−) of the adjustable current source  354  is coupled to the central node of a first voltage divider formed by resistors  312  and  313 . The central node of the first voltage divider is coupled through the resistor  313  to the ground terminal  310  and is also coupled through a node to the non-inverting input of op amp  396 . The capacitor  321  is in parallel with the resistor  313  between the central node of the first voltage divider and the ground terminal  310 . The resistors  314  and  315  form a second voltage divider having a central node coupled to the inverting input of the op amp  396 . The second voltage divider is coupled between the positive voltage terminal  311  and the ground terminal  310 . The positive voltage terminal  311  is also coupled to the voltage supply pin of the op amp  396  and the ground terminal  310  is coupled to the ground pin of the op amp  396 . The output of the op amp is coupled to the Ground-Negative Supply Input pin of the switch  380 . 
         [0062]    The Positive Supply Voltage Input pin of the switch  380  is coupled to the positive voltage terminal  311 . The Ground-Negative Supply Input pin of the switch  380  is coupled to the output of the op amp  396 . The Normally Open pin of the switch  380  is coupled to the ground terminal  310 . The Common pin of the switch  380  is coupled to the Group A node, i.e. the node  328  for providing the power to the adjustable current supplies  356 ,  358 ,  360 ,  362 ,  364 ,  366 ,  388  and  390  powering the Group A electrodes  340 ,  342 ,  344  and  384 . The Normally Closed pin of the switch  380  is coupled to the positive voltage terminal  311 . The Digital Control Input pin of the switch  380  is coupled to the Group B node  330  which, as mentioned above, is also coupled to the Q 14  pin of the timer  382 . Thus, the timer  382  is configured to cause the Group A electrodes and Group B electrodes to switch between anodes and cathodes to generate a waveform such as that shown in  FIG. 2 . 
         [0063]    In operation, a device comprising circuit  300  is implanted into an injured mammal shortly after the time of central nervous system injury. The device comprising circuit  300  remains implanted for a period of time post-injury. For example, the device comprising circuit  300  may remain implanted for up to fourteen weeks or longer in humans. 
         [0064]    Power may be applied to the device comprising circuit  300  for a period of time while the device is implanted. When power is applied, the circuit  300  generates an oscillating electrical field between at Electrode Group A and Electrode Group B. That is, the circuit  300  may generate a current DC stimulus the polarity of which is reversed periodically after the expiration of a predetermined period of time. The predetermined period of time may be determined by the operation of the timer  382 . Electrode Group A and Electrode Group B alternately comprise cathode and anode terminals, depending upon the polarity of the DC stimulus. 
         [0065]    The voltage between Electrode Group A and Electrode Group B is selected so as to provide sufficient field strength in the section of the spinal cord in which nerve regeneration is to be stimulated. A field strength of 200 μV/mm in the spinal cord will stimulate regeneration. The current needed to achieve this field strength is determined by the geometry of the animal in which a device comprising the circuit  300  is used. 
         [0066]    Illustratively, electrodes  340 ,  342 ,  344 ,  346 ,  348 ,  350 ,  384 , and  386  may comprise silastic insulated platinum electrodes. Electrode Groups A and B are implanted on opposite sides of a lesion in the spinal cord. It is sufficient to implant Electrode Groups A and B in a laminectomy adjacent the spinal cord but not actually in the spinal cord. Further, moving the anode from within the laminectomy to a site on the muscle dorsal to the same area results in only about a ten percent drop in field strength as does the converse of moving the cathode to a more superficial position while leaving the anode in the laminectomy. 
         [0067]    Significant recovery of sensory function (ascending functions) has resulted from prior art technology, however, motor recovery has not been as robust. This has been documented in treatment of clinical paraplegia in dog and paraplegia/quadriplegia in man. The major sensory columns in mammals (Dorsal Coolum /Medial Lenmiscus system) are located bilaterally in the dorsal or posterior spinal cord, while long tract motor columns are located in the anterior (ventral) cord, as are the location of the major, bilaterally located, alpha motorneuron plexus in the upper (pectoral) and lower (lumbosacral) intumescences of the spinal cord. 
         [0068]    Applicants have also found that the field strength within the spinal cord at the site of the lesion depends upon the location of the current delivery electrodes. The convergence of current to an electrode produces high current density and hence higher field strength near each electrode. The closer one electrode is to the lesion site, the less critical is the placement on the other to maintain high field strengths. However, as a current delivery electrode approaches the lesion, current direction becomes less uniform. 
         [0069]    At a lesion exactly half-way between two electrodes placed on the midline, the current will all be oriented along the long axis of the subject animal. As one of the electrodes is moved closer to the lesion, there will be a larger vertical (dorsal-ventrical) component of the current at the lesion (assuming that the electrodes remain a few millimeters dorsal to the target tissue). As a compromise between uniform current direction and maximum field strength, applicants have chosen to position the electrodes two vertebral segments on either side of the lesion in their spinal cord studies. In the guinea pig studies applicants have conducted, it appears that the critical distance to be within one convergence zone of an electrode (that area in which the current convergence to the electrode so dominates the field strength that the position of the other electrode is relatively inconsequential) is approximately 1 cm. Therefore, by placing one electrode within 1 cm of the lesion, the position of the other becomes relatively inconsequential and becomes a matter of convenience. It should be noted, however, that the electrodes can be located further from the lesion. If they are, the field strength of the electrical field at the lesion for a given magnitude of current will be reduced. Therefore, the magnitude of the current would have to be increased to yield the same electrical field strength at the lesion. 
         [0070]    For optimal results in a human patient, uniform electrical field of the desired strength is imposed over about 10 cm to 20 cm of damaged spinal cord for a beneficial clinical outcome. Ideally, this uniform field is imposed across the entire cross section of the spinal cord over this longitudinal extent, because of the general segregation of descending (motor) tracts to the ventral (anterior) cord, and the segregation of important (largely sensory) tracts to the posterior (dorsal) spinal cord. This uniform electrical field of the desired strength may be generated by placing two pairs of electrodes, for example electrodes  340 ,  346 ,  342 , and  348 , on either side (two tethered to the right and left lateral facets) and a third pair, for example electrodes  344  and  350 , sutured to the paravertebral muscle and fascia of the dorsal (posterior) facet-rostra and caudal of the spinal cord lesion. Additionally, a fourth pair of electrodes, for example  384  and  386 , are sutured to paravertebral musculature at the extreme mediolateral/ventral (anterior) vertebral column. The placement of this fourth pair of electrodes  384  and  386  should alleviate the reduction of the voltage gradient imposed over motor columns in the anterior (ventral) spinal cord. 
         [0071]    Once inside a patient, it is difficult to verify the operation of a device comprising circuit  300 . Visible verification is virtually impossible while the device is within a patient. Operation of the device within the patient could be determined by attaching an electrocardiogram (EKG) system to the patient and waiting to observe a small transient on the EKG record associated with the reversal of the electrical field imposed over the spinal cord, but this is a time consuming procedure. 
         [0072]    Optional beacon circuit  320  can be used with circuit  300  to enable rapid verification of device operation. Beacon circuit  320  can be any circuit that enables visible and/or audible verification of device operation. Beacon circuit  320  also can transmit data regarding device operation, such as, for example, using RF telemetry. In an embodiment, a small LED “beacon” is inserted into circuit  300 . A periodic visible burst of light such as, for example, every 7 seconds, reveals nominal unit operation prior to implantation. 
         [0073]      FIG. 4A  shows a neural injury treatment device  500 . The neural injury treatment device  500  includes a skin  510 . The skin  510  may comprise a ceramic and/or titanium or other bio-compatible material, making the neural injury treatment device  500 , in theory, surgically implantable for the life of the patient. The skin  510  may provide a container for the electronics of the neural injury treatment device  500 . In one embodiment, the skin  510  may comprise medically approved ceramic available from the Sigma-Aldrich Corporation. In another embodiment, the skin  510  may comprise a Titanium cases. In yet another embodiment skin  510  may comprise a titanium portion, a bio-compatible material portion and a ceramic portion. 
         [0074]    One advantage of a skin  510  comprising ceramic is that lifetime implantable ceramic cases provide the ability to mold the container into a desired shape. Additionally, because ceramic is transparent to electromagnetic radiation, it may be desirable to fabricate at least a “window” of the skin  510  from ceramic in order to facilitate the transmission of electromagnetic waves carrying power and data. The ceramic material used to fabricate the skin  510  may be obtained as a powder to facilitate the custom molding of shapes. For example, one useful shape for the skin  510  may be to mold the container into the form of an intervertebral disc or vertebral facet for certain applications in the spinal/orthopaedic management of fracture/dislocation associated spinal cord injuries. Because ceramic is transparent to electromagnetic waves, such a skin  510  facilitates the functionality of telemetry, antennae  418 , fail-safe off, and other capabilities associated with telemetry. 
         [0075]    The neural injury treatment device  500  may also include a wireless data module  410 , a stimulator module  420 , a charge storage device  429 , a first electrode group  442  (Electrode Group A) and a second electrode group  444  (Electrode Group B). The first and second electrode groups  442  and  444  may comprise silastic insulated platinum electrodes, similar to the electrodes described above. The neural injury treatment device  500  may also include an external module  430 . The external module  430  may include data acquisition  446 , device programming  448 , and inductive power-coupling hardware  444  configured to interface with the wireless data module  410  and the stimulator module  420 , as shown, for example, in  FIG. 4B . 
         [0076]      FIG. 4B  shows a schematic of the circuit  400  for generating an oscillating electrical field for stimulating nerve regeneration. The circuit  400  provides a means to treat spinal cord injury, as well as other nerve cell injuries. The circuit  400  facilitates these treatments by providing imposed gradients of DC voltage between about 200 to about 900 μV/mm. These voltage gradients may induce functional regeneration and reconnection of mechanically injured neural axons in vertebrates. 
         [0077]    The circuit  400  may include the wireless data module  410 , the stimulator module  420 , the external module  430 , and the electrodes  440 . The wireless data module  410  may include a low-pass filter  412 , a transceiver  414 , a voltage controlled oscillator  416 , and antenna  418 . The low-pass filter  412  may be an active amplifier with low-frequency cutoff. The low-pass filter  412  may also include or be comprised of on-chip or off-chip passive resistive and capacitive devices. The transceiver  414  may be a mixer. The voltage controlled oscillator  416  may be a cross-coupled high-frequency oscillator. The antenna  418  may be a planar microstip antenna or a monolithic microwave integrated-circuit (MMIC) radiating structure integrated with or bonded to an application specific integrated circuit. The components of the circuit  400  may be CMOS or BiCMOS. 
         [0078]    The stimulator module  420  may include a current source  422 , a charge balance sensor  424 , a pulse generator  426 , an inductor  427 , a field-to-current converter  428 , and the charge storage device  429  described above in relation to  FIG. 4A . The current source  422  is shown in more detail in  FIG. 5 . A biphasic embodiment  460  of the pulse generator  426  is shown in more detail in  FIGS. 8A-B . A triphasic embodiment  470  of the pulse generator  426  is shown in more detail in  FIG. 10 .  FIG. 9  shows a sample wave form generated by the triphasic embodiment  470  of the pulse generator  426 . 
         [0079]    The inductors  427  and  434  and other power coupling components are shown in more detail in  FIG. 7 . The field-to-current converter  428  may be a radio frequency field-to-current converter. The stimulator module  420  may communicate via the wireless data module  410  with the external module  430  via antennas  418  and  432  respectively. The external module  430  may also include a subcutaneous charging device  444  for inductively charging the charge storage device  429  via field converter  428 . The electrodes  440  comprises the Electrode Group A  442  and the Electrode Group B  444 . 
         [0080]    In operation, the wireless data module  410  receives power from stimulation module  420  that receives power from the external module  430 , stores the power for a time in charge storage device  429 , and uses the stored power to generate a field between the electrode Group A  442  and the Electrode Group B  444 . The electromagnetic power coupling circuit  700 , shown in  FIG. 7 , shows the field-to-current converter  428  in more detail. Additionally, the external portion  720  of the power coupling circuit  700  is also shown in  FIG. 7 . A voltage source  702  of the external portion  720  is coupled to an R-L-C circuit comprising first and second capacitors  704  and  706 , a resistor  708 , and an inductor  434 . The external portion  720  generates an electromagnetic field, which may be induced into the inductor  427  of the field-to-current converter  428  when the inductors  434  and  427  are in proximity to one another. When that occurs, the inductor  427  provides an AC voltage to the simple rectifier circuit comprising first and second capacitors  710  and  714 , and diode  712 . In this manner, the field-to-current converter  428  may operate to transform coupled fields to direct current fields through charge-rectifying and/or signal conditioning. The field-to-current converter  428  may also regulate coupled power delivery for appropriate charging of the charge storage device  429 . 
         [0081]    Transcutaneous recharging of the charge storage device  429  can be accomplished using medically approved voltage sources such as the Quallion QL100E (weight 4 grams; capacity, 100 mAh; Operating Voltage 2.7-4.2 V; size 14.5 mm by 15.6 mm). The largest component of the circuit  400  determining its overall size is the size of the charge storage device  429 . Thus, decreasing the size of the device by using a rechargeable unit for the charge storage device  429  may reduce the size of the circuit  400  to sixty percent (or a smaller percentage) of prior art devices. This decrease in size may simplify surgical implantation, and the time of implantation. Other medical issues, such as contact necrosis, also vary with the size of the circuit  400 . The timing of recharging cycles will depend on the programmed stimulation parameters. However, charging could be accomplished at night while the patient is asleep, or for shorter periods during the day. 
         [0082]    Since the circuit  400  may be located rather superficially in back musculature beneath the back skin, an additional pair of redundant recharging electrodes may be left in situ next to the circuit  400 . These redundant recharging electrodes may be externalized simply by use of a local anesthetic and simple approach through the skin. Under special or unforeseen situations, the circuit  400  can be recharged directly by attachment of these two electrodes to a hardwired recharging unit. 
         [0083]    Returning to  FIG. 4 , the charge storage device  429  may store power received from the field-to-current converter  428  up to its maximum capacity, which is monitored by the field-to-current converter  428  to avoid over-charging of the charge storage device  429 . Upon reaching maximum capacity, the charge storage device  429  may contain enough power to power the circuit  400  for the appropriate length of time, and charging may cease. 
         [0084]    As shown, for example, in  FIG. 10  a triphasic pulse generator  470  includes a counter block  472 , a multiplexer block  474  an output  476 , a first amplitude input  478 , a second amplitude input  480 , a third amplitude input  482 , a first duration input  484 , a second duration input,  486 , a third duration input  488  and a clock input. In one embodiment of the triphasic pulse generator  470  the counting block  472  comprises three counters and the data present at the amplitude inputs  478 ,  480 ,  482  and duration inputs  484 ,  486 ,  448  comprise six words of data stored in a form of memory (not shown). The three data words present at the duration inputs  484 ,  486 ,  488  are illustratively n bits long and represent the duration of each pulse, for example, duration t 0   491 , duration t 1   492  and duration t 2   493 , as shown, in  FIG. 9 . The three data words present on the amplitude inputs  478 ,  480  ,  482  are illustratively m bits long and represent the amplitudes of each pulse, for example, amplitude AO  494 , amplitude Al  495  and amplitude A 2   496 , as shown, in  FIG. 9 . In the illustrated embodiment, the three counters in the counting block  472  reset with a value between a value between zero and 2n−1, where n is the number of bits contained in the counter. This number will represent a time until the counter rolls over. Upon rollover the counter send a flag initiating the next counter. The same operation applies for the second and third counter. While each counter is counting, a multiplexer  474  selects one of the three amplitudes stored in memory determined by the counter currently in operation. Power saving is accomplished by clock gating or reducing the number of counters needed to count the duration of the pulse. 
         [0085]    In accordance with the present disclosure, the nerves in the central nervous system of a mammal are stimulated to regenerate by applying an oscillating electrical field to the central nervous system. The oscillating electrical field is a voltage potential stimulus which is first applied in one direction for a predetermined period of time, and then applied in the opposite direction for the predetermined period of time. In other words, the polarity of the voltage potential stimulus is reversed after each predetermined period of time. The predetermined period of time is selected to be less than the die back period of anodal facing axons, but long enough to stimulate growth of cathodal facing axons. This predetermined period will be termed the “polarity reversal period” of the oscillating electrical field. In one disclosed embodiment, this polarity reversal period is between about thirty seconds and about sixty minutes. 
         [0086]    Circuit  400  when implemented with a biphasic pulse generator  460  ( FIGS. 8A-B ), triphasic pulse generator  470  ( FIG. 10 ) or other multi-phasic pulse generator as the pulse generator  426  comprises a chopping circuit. The voltage potential difference and thus the electrical field between the electrode of the first and second implant  102 ,  104  is “chopped” or turn off for a short but fixed amount of time. For example, by setting jumper  620  to a 25% duty cycle and jumper  622  to a 50% duty cycle, the electrical field exhibits an on duty cycle Don  1202  of 75% (jumper  620  plus jumper  622 ) and off duty cycle Doff  1204  for 25% of the time, chopped once per minute producing a wave form as shown in  FIG. 11 . If this amount of time is small enough compared to the overall time, the nerve cell regeneration continues at the same rate as if the electrical field were held steady. However, chopping the electrical field in the manner illustrated increases battery life, or enables the battery to power other device functions while maintaining a lifespan sufficient for regeneration to be substantially completed. Additionally, punctuated, pulsatile or discontinuous oscillating electric fields are believed to work as well, if not, in some case when utilized to heal certain types of nerves, better than, constant oscillating electric fields. Thus, there is the expectation that the chopping circuit will generate a pulsatile electric field that may improve functional recovery as well as save battery life. 
         [0087]    In one disclosed embodiment, where polarity reversal period DT  1206  of the oscillating electrical field is set to 10 minutes and the duty cycle of the electrical field is set to 75%, circuit  400  produces an output wave form as shown in  FIG. 11 . It is within the scope of the disclosure for the polarity reversal period to be between about thirty seconds and about sixty minutes. It is also within the scope of the disclosure for the polarity reversal period to be between a minimal clinically effective value to stimulate nerve regeneration in the cathode-facing axon and a value less than the beginning of the die-back period in the anode-facing axon. Clinically effective results can readily be obtained when the reversal period is set between ten and twenty minutes. Highly effective clinical results may be achieved with the duty cycle set to approximately fifteen minutes. It is also within the scope of the disclosure, though not preferred because regeneration of axons induced to die back through the area of die back will be required before therapeutic growth will be induced, for the polarity reversal period to exceed the beginning of the die back period but be less than the time for die back to proceed to the point of killing the nerve cell. 
         [0088]    It is within the scope of the disclosure for the on duty cycle  1202  to be between 60% and 99%. Clinically effective results may be obtained in one embodiment when the on duty cycle  1202  is between 70% and 85%. Clinically effective results may be obtained in another embodiment when the on duty cycle  1202  is between 75% and 80%. 
         [0089]    According to at least one embodiment of the present disclosure employing a pulsatile field, there may be an off cycle between each polarity reversal period, or there may be two or more consecutive polarity reversal periods followed by an off cycle. 
         [0090]    As shown, for example, in  FIGS. 8A-B , biphasic pulse generator  460  is implemented using a binary counter  461 , a magnitude comparator  462 , a buffer  463 , a BCD-decimal decoder  464 , a second buffer  465 , a plurality of two input or gates  465 , a NAND gate  466  and a D Flip Flop  467 . Such off the shelf integrated circuits are available from many electronic device manufactures. Exemplary part numbers are shown in the drawings. The biphasic pulse generator  460  is configured to receive a plurality of high and low inputs and a fed back clock signal at the binary counter  461 . Pulsed signals output by the binary counter  461  are input to the comparator  462  along with various hi and low inputs in the manner illustrated. The biphasic pulse generator  460  outputs a signal such as that shown for example in  FIG. 11 . 
         [0091]    The charge storage device  429  provides power to the current source  422 , the charge balance sensor  424 , and the pulse generator  426 . The pulse generator  426 , shown in more detail in  FIGS. 8A-B  and/or  FIG. 10 , may generate a therapeutic waveform, as will understood by those of skill in the art of digital electronic design. For example, the pulse generator  426  may generate a pulsatile DC or intermittent DC waveform (as described above). The current source current source  422  may comprise a plurality of current sources  450 , one illustrative embodiment which is shown in  FIG. 5 . The waveform generated by the pulse generator  426  may be provided to the plurality of current sources  450 , as shown in  FIG. 4 , so as to generate a pulsatile DC or intermittent DC field of the desired nature between the electrode Group A  442  and the Electrode Group B  444 . 
         [0092]    Turning to  FIG. 5 , the current source  450  includes a biasing current source  451 , first, second, and third field-effect transistors  452 ,  456 , and  458 , respectively, and an operational amplifier  459 . The current source  450  receives a voltage waveform from the biphasic pulse generator  426  (which is represented as V DD  in  FIG. 5 ), and provides a current I OUT  at transistors  458 , which is provided to one of the electrodes of Electrode Group A  442  or Electrode Group B  444 . 
         [0093]    The wireless data module  410  may facilitate the treatment of individual patients by allowing the clinician to vary the therapy to adapt to the anatomical and/or physiological pathology of individual patients, which varies considerable after spinal cord injury. The capability of the clinician to interrogate the implanted circuit  400  and to change its stimulation parameters via the external module  430  may facilitate custom applications of the therapy. For example, excessive scar tissue may build up about electrodes tethered to the paravertebral musculature, and reduce the strength of the imposed field (reducing current flow by increasing interstitial resistivity). This is not ideal for the success of the therapy. For example, where a reduction in voltage between the Electrode Group A  442  or Electrode Group B  444  is detected by the charge balance sensor  424 , two-way telemetry between the wireless data module  410  and the external module  430  allows for correction by increasing the voltage produced by the current source  422 . For further example, where a drop or loss in voltage be detected in only one pair of the electrodes in the Electrode Group A  442  and Electrode Group B  444 , a correction may be initiated by telemetry from the external module  430  instructing the wireless data module  410  to increase the delivery of adjacent pairs of electrodes. In essence, two-way telemetry provides for changes in stimulation parameters and the ability to correct and/or tailor the regenerative electrical stimulation. 
         [0094]    In operation, the transceiver  414  transmits and receives signals via the antenna  418 , which signals may be radio frequency signals. The transceiver  414  is coupled to a voltage controlled oscillator  416  (shown in detail in  FIG. 6 ), and a low-pass filter  412 . As shown in  FIG. 6 , the voltage controlled oscillator  416  includes transistors  610 - 615 , inductors  604  and  606 , and a current source  602 . The voltage controlled oscillator  416  receives a data signal at Vin, and generates an oscillated signal at Vout. 
         [0095]    Returning to  FIG. 4 , the wireless data module  410  transmits data between the stimulator module  420  and the external module  430 , thereby facilitating two-way telemetry. The voltage controlled oscillator  416  and external module  432  shown in  FIGS. 4A and 4B  are merely illustrative, and do are not intended to limit the claimed invention in any way. 
         [0096]    While this invention has been described as having a preferred design, the present invention can be further modified within the scope and spirit of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. For example, the methods disclosed herein and in the appended claims represent one possible sequence of performing the steps thereof. A practitioner of the present invention may determine in a particular implementation of the present invention that multiple steps of one or more of the disclosed methods may be combinable, or that a different sequence of steps may be employed to accomplish the same results. Each such implementation falls within the scope of the present invention as disclosed herein and in the appended claims. Furthermore, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.