Patent Publication Number: US-8527062-B2

Title: Power scheme for implant stimulators on the human or animal body

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application of, and claims priority to, U.S. application Ser. No. 11/598,965, filed Nov. 14, 2006, for Power Scheme for Implant Stimulators on the Human or Animal Body. 
    
    
     FEDERALLY SPONSORED RESEARCH 
     This invention was made with government support under grant No. R24EY12893-01, awarded by the National Institutes of Health. The U.S. Government may have certain rights in the invention. 
    
    
     BACKGROUND 
     1. Field 
     The present disclosure relates to implants for humans or animals. In particular, it relates to a power scheme for implant stimulators (also referred to as implants in the present application) on the human or animal body. 
     2. Related Art 
     The following paragraphs will introduce some art possibly related to the present application. 
     In 1755 LeRoy passed the discharge of a Leyden jar through the orbit of a man who was blind from cataract and the patient saw “flames passing rapidly downwards.” Ever since, there has been a fascination with electrically elicited visual perception. The general concept of electrical stimulation of retinal cells to produce these flashes of light or phosphenes has been known for quite some time. Based on these general principles, some early attempts at devising a prosthesis for aiding the visually impaired have included attaching electrodes to the head or eyelids of patients. While some of these early attempts met with some limited success, these early prosthetic devices were large, bulky and could not produce adequate simulated vision to truly aid the visually impaired. 
     In the early 1930&#39;s, Foerster investigated the effect of electrically stimulating the exposed occipital pole of one cerebral hemisphere. He found that, when a point at the extreme occipital pole was stimulated, the patient perceived a small spot of light directly in front and motionless (a phosphene). Subsequently, Brindley and Lewin (1968) thoroughly studied electrical stimulation of the human occipital (visual) cortex. By varying the stimulation parameters, these investigators described in detail the location of the phosphenes produced relative to the specific region of the occipital cortex stimulated. These experiments demonstrated: (1) the consistent shape and position of phosphenes; (2) that increased stimulation pulse duration made phosphenes brighter; and (3) that there was no detectable interaction between neighboring electrodes which were as close as 2.4 mm apart. 
     As intraocular surgical techniques have advanced, it has become possible to apply stimulation on small groups and even on individual retinal cells to generate focused phosphenes through devices implanted within the eye itself. This has sparked renewed interest in developing methods and apparati to aid the visually impaired. Specifically, great effort has been expended in the area of intraocular retinal prosthesis devices in an effort to restore vision in cases where blindness is caused by photoreceptor degenerative retinal diseases such as retinitis pigmentosa and age related macular degeneration which affect millions of people worldwide. 
     Neural tissue can be artificially stimulated and activated by prosthetic devices that pass pulses of electrical current through electrodes on such a device. The passage of current causes changes in electrical potentials across retinal neuronal cell membranes, which can initiate retinal neuronal action potentials, which are the means of information transfer in the nervous system. 
     Based on this mechanism, it is possible to input information into the nervous system by coding the sensory information as a sequence of electrical pulses which are relayed to the nervous system via the prosthetic device. In this way, it is possible to provide artificial sensations including vision. 
     Some forms of blindness involve selective loss of the light sensitive transducers of the retina. Other retinal neurons remain viable, however, and may be activated in the manner described above by placement of a prosthetic electrode device on the inner (toward the vitreous) retinal surface (epiretinal). This placement must be mechanically stable, minimize the distance between the device electrodes and the retinal neurons, and avoid undue compression of the retinal neurons. 
     In 1986, Bullara (U.S. Pat. No. 4,573,481) patented an electrode assembly for surgical implantation on a nerve. The matrix was silicone with embedded iridium electrodes. The assembly fit around a nerve to stimulate it. 
     Dawson and Radtke stimulated a cat&#39;s retina by direct electrical stimulation of the retinal ganglion cell layer. These experimenters placed nine and then fourteen electrodes upon the inner retinal layer (i.e., primarily the ganglion cell layer) of two cats. Their experiments suggested that electrical stimulation of the retina with 30 to 100 uA current resulted in visual cortical responses. These experiments were carried out with needle-shaped electrodes that penetrated the surface of the retina (see also U.S. Pat. No. 4,628,933 to Michelson). 
     The Michelson &#39;933 apparatus includes an array of photosensitive devices on its surface that are connected to a plurality of electrodes positioned on the opposite surface of the device to stimulate the retina. These electrodes are disposed to form an array similar to a “bed of nails” having conductors which impinge directly on the retina to stimulate the retinal cells. U.S. Pat. No. 4,837,049 to Byers describes spike electrodes for neural stimulation. Each spike electrode pierces neural tissue for better electrical contact. U.S. Pat. No. 5,215,088 to Norman describes an array of spike electrodes for cortical stimulation. Each spike pierces cortical tissue for better electrical contact. 
     The art of implanting an intraocular prosthetic device to electrically stimulate the retina was advanced with the introduction of retinal tacks in retinal surgery. De Juan, et al. at Duke University Eye Center inserted retinal tacks into retinas in an effort to reattach retinas that had detached from the underlying choroid, which is the source of blood supply for the outer retina and thus the photoreceptors. See, e.g., E. de Juan, et al., 99 Am. J. Opthalmol. 272 (1985). These retinal tacks have proved to be biocompatible and remain embedded in the retina, and choroid/sclera, effectively pinning the retina against the choroid and the posterior aspects of the globe. Retinal tacks are one way to attach a retinal electrode array to the retina. U.S. Pat. No. 5,109,844 to de Juan describes a flat electrode array placed against the retina for visual stimulation. U.S. Pat. No. 5,935,155 to Humayun describes a retinal prosthesis for use with the flat retinal array described in de Juan. 
     Retinal implants receiving power from an external unit through an inductive power link coupled through coils are known. When the coil sizes and positioning are limited by the physical conditions, the power delivering efficiency can be reduced dramatically, in which case the maximum power to the implant may be limited. On the other hand, a higher amount of power used by the implant also means a worse condition in terms of thermal dissipation. 
     Both of the situations above require a reduced power demand by the implant stimulator. For applications that need a large number of stimulation channels, such as in the case retinal prosthesis, the efficiency between the output current and the input power becomes critical. 
     SUMMARY 
     According to a first aspect of the present disclosure, a power control system for an implant on a human or animal body is shown, comprising: a charging circuit to provide power to deliver controlled stimulation currents to a tissue of the human or animal body; a capacitive storage arrangement connected with the charging circuit and charged by the charging circuit; a shunting arrangement to limit voltage on the capacitive storage arrangement; a driver array configured to transfer charges from the capacitive storage arrangement to the tissue; and an electrode array connected with the driver array and the tissue. 
     According to a second aspect of the present disclosure, a shunting circuit to regulate capacitor voltage in an implant for a human or animal body is shown, the implant comprising a capacitive storage arrangement, the shunting circuit comprising: a current sensor comprising terminals connected in parallel with the capacitive storage arrangement, the current sensor sinking current from the capacitive storage arrangement when the voltage of the capacitive storage arrangement reaches a voltage value. 
     According to a third aspect of the present disclosure, a constant current type electrode driver for an implant for a human or animal body is shown, the electrode driver comprising: a driver controller to generate anodic and cathodic stimulation switching signals and generate an output current defining a current amplitude for stimulation; a conversion circuit to convert the output current into an anodic current or a cathodic current; and a switching arrangement to allow selection between the anodic current or the cathodic current. 
     According to a fourth aspect of the present disclosure, a compliance monitoring circuit to monitor and control compliance voltage of an electrode contacting a tissue of a human or animal body is shown, the electrode being connected with an electrode driver comprising an output MOSFET transistor having a drain-source voltage Vds, a gate-source voltage Vgs and a threshold voltage Vt, the compliance monitoring circuit monitoring a condition Vgs−Vds&gt;Vt and generates an alert signal when such condition is met. 
     According to a fifth aspect of the present disclosure, a power monitor to monitor charging and draining conditions of i) a capacitive storage arrangement and ii) shunting circuits comprised in an implant for a human or animal body is shown, the power monitor comprising: a current monitor, the current monitor comprising an analog to digital converter digitizing analog inputs from the shunting circuits and outputting an output current level; and a capacitor monitoring circuit, to monitor whether implant power falls below a power value. 
     According to a sixth aspect of the present disclosure, a method to control power in an implant for a human or animal body is shown, the implant comprising electrodes contacting a tissue of the body, the method comprising: capacitively storing electric charges; providing the electric charges to the electrodes; monitoring when the electric charges are above a high value or below a low value; and controlling the electric charges when above the high value or below the low value. 
     According to a seventh aspect of the present disclosure, a circuit is shown, comprising: an electrode driver array; storage capacitors connected with the electrode driver array, wherein charge stored on the storage capacitors is adapted to be transferred to an array of electrodes connected with the electrode driver array; a charging circuit charging the storage capacitors; and a monitoring circuit to continuously monitor voltage on the storage capacitors and to control at least one between the electrode driver array and the charging circuit based on the voltage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1-1 ,  1 - 2  and  1 - 3 , to be seen as connected side by side, show a general diagram of the implant power control scheme in accordance with the present disclosure. 
         FIG. 2  shows a circuital scheme of an electrode driver. 
         FIG. 3  is a timing diagram showing current and voltage signals related to the present disclosure. 
         FIG. 4  shows a circuital scheme of one of the shunt regulator circuits of  FIG. 1 . 
         FIG. 5  shows a circuital scheme of the compliance monitor circuit of  FIG. 1 . 
         FIG. 6  is a timing diagram showing signals related to the circuit of  FIG. 5 . 
         FIG. 7  shows a circuital scheme for the power monitor circuit of  FIG. 1 . 
         FIGS. 8-1  and  8 - 2 , to be seen as connected side by side, are a flow chart showing the power control flow of the scheme shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1-1 ,  1 - 2  and  1 - 3  are a diagram of an implant power control scheme in accordance with an embodiment of the present disclosure. 
     A retinal implant  10  receives power from an external unit  20  through an inductive power link  30  coupled through coils. Control and status information are exchanged between retinal implant  10  and external unit  20  through data link  40 . While the embodiment of  FIGS. 1-1 ,  1 - 2 ,  1 - 3  and the following figures is concerned with a retinal implant, the person skilled in the art will appreciate that the same scheme can be used also for other types of implants on the human or animal body, such as cochlear implants or implants to restore neuronal functions impaired due to injuries or diseases. 
     Power at the implant side is received by implant coil  50 . Implant coil  50  is tunable with capacitor C 3  to the power carrier frequency. The received AC power is converted into DC power by a rectifier circuit  60 . Rectifier circuits are known per se to the person skilled in the art. In the case at issue, the rectifier circuit  60  can comprise, by way of example, diode bridges or MOSFET circuits. 
     The output V+, VM, V− of rectifier circuit  60  provides the power to deliver controlled stimulation currents to the tissue  70 . A small portion of the output of rectifier  60  can be tapped out or diverted to supply circuits for other operations through a regulator circuit  75 . Such operations can include RF data receiving and transmitting, logic control, signal measurements and so on. 
     The output of rectifier circuit  60  continuously charges two capacitive storage arrangements which are shown as C 1  and C 2  in  FIG. 1-2  provided to supply all electrode drivers for bi-phase or multi-phase stimulation. Those capacitive arrangements could also be represented as arrays of capacitors or storage devices based on a capacitive behavior to boost output power. 
     The voltages of C 1  and C 2  are limited by shunt regulator circuits  80  and  90 , respectively. The charge stored on C 1  and C 2  is transferred to the tissue  70  through a plurality of electrode drivers  100 - i  forming a driver array  100  comprised of electrode drivers  100 - 1 ,  100 - 2 , . . . ,  100 - i , . . .  100 - n . The drivers act as a controlled energy transporter to allow stimulation of the tissue  70  in form of bursts of biphasic (anodic and cathodic) current pulses. Each driver can comprise, for example, a constant current source or sink circuit, as later shown in  FIG. 2 . Each driver is connected to a respective stimulation electrode  245 - i  (see also  FIG. 2 ). The plurality of stimulation electrodes  245 - i  forms a stimulating electrode array  245  in direct contact with the tissue  70 , as shown in  FIG. 1-2 . As also explained above, while the present embodiment makes reference to a retinal tissue, other types of human or animal tissue can be envisaged by the person skilled in the art. 
     The present disclosure provides for monitoring and control of excessively low capacitor voltage. In particular, when charges are transferred from capacitors C 1 , C 2  to the tissue  70 , the voltage on the capacitors drops. A minimum value for this voltage is defined by a so-called compliance voltage. The compliance voltage is defined by the electrode-tissue impedance at the interface  245 - 70  and the stimulating current  225  flowing between a driver  100 - i  and a respective electrode  245 - i . If the voltage on C 1  and C 2  falls too close to the compliance voltage, the respective driver  100 - i  will not be able to maintain the required amplitude of the stimulation current  255 . 
     In order to prevent this from happening, a compliance monitor circuit  110  is provided. Circuit  110  monitors occurrence of the above situation and notifies the external unit  20  through the back telemetry  160 - 180  to lower the stimulation current amplitude  255  by way of external controller  700  and the transceiver  710  and forward data link  40 , or increase the capacitor voltage accordingly by way of the external controller  700  and coil driver  720 . 
     The implanted device  10  is continuously powered and controlled by the external unit  20  through the inductive power link  30  and the data link  40 , respectively. The external unit  20  comprises an external controller  700 , a coil driver  720  and a data transceiver  710 . The external controller  700  can include an information collector such as a camera in the case of retinal prosthesis, a microphone in the case of cochlear prosthesis, or some other form of sensory devices such as pressure, position or touch sensors for various other neuronal-stimulation applications. The external controller  700  can include a Digital Signal Processing Unit or a similar operation processor to synthesize the sensed information from the sensors and the feedback information from the implant  10  and generate controls accordingly to command the implanted device to deliver appropriate stimulation (amplitude and timing) to the tissue through the data transceiver  710 . The data transceiver  710  ensures that the commands from the external controller  700  are delivered to the implant  10  reliably and the feedback from the implant  10  is received correctly. The data transceiver  710  communicates with the implant  10  in predefined communication protocols through its data antenna  730 . In the meantime, the coil driver  720  ensures that adequate but not excessive power is delivered to the implant  10  for the intended stimulation intensity. 
     Also excessively high capacitor voltage is monitored, to avoid use of unnecessary high power to deliver the same charge. In particular, shunt regulators  80  and  90  include a circuital arrangement to program the level of the nominal capacitor voltage to a required value, for further power saving, as later discussed with reference to  FIG. 4 . 
     A power monitor circuit  120  is further provided, to monitor the charging and draining conditions of the capacitors, so that the external unit  20  can optimize the RF powering condition (see coil driver/monitor  720 ) and also stop stimulation when the implant  10  cannot be adequately powered. The power monitor circuit  120  will be explained in greater detail in  FIG. 7 . 
     The implant  10  further comprises an implant controller  130  and an implant transceiver  140 . Implant controller  130  comprises a main controller  150  and a back telemetry (BT) controller  160 . Implant transceiver  140  comprises a data receiver  170  and a back telemetry (BT) transmitter  180 . 
     Implant controller  130  and implant transceiver  140  allow the stimulation control and power control to operate on a system level. In other words, they allow the external unit  20  to appropriately and accurately control the implant powering and stimulation. 
     The data receiver  170  receives forward telemetry (FT) data from the external unit  20  through data link  40 . FT data is decoded by the main controller  150  to control the output of the electrode drivers  100 , the rail voltages (as later shown in  FIG. 4 ) and other operations, such as monitoring of the electrodes and the device status for safety reasons or for conducting system tests, and so on. 
     BT controller  160  collects and encodes implant information from the compliance monitor  110  and the power monitor  120 , such as implant powering, stimulation, electrode condition and other safety information of the implant, e.g. device failure, electrode failure, excessive power condition etc. BT controller  160  sends the implant information back to the external unit  20  through BT transmitter  180 , so that the system can act on such information. 
     For example, the output of the power monitor  120  is fed to the BT controller  160 , which will include this information in the back telemetry stream. If the external unit  20 , upon receiving the back telemetry data, determines that the currents flowing through the shunt regulators  80 ,  90  are too high, it will adjust the input to the coil driver  720  to lower the level of power driving the coil by an amount that is predetermined by the power control protocol. After the coil power is lowered, the shunt regulator currents will decrease, thus forming a closed control loop. 
     A possible type of electrode driver  100 - i  to be used in the present disclosure is a constant current type electrode driver. Current type electrode drivers output current pulses whose timing, duration and amplitude can be controlled through input commands. See, for example, “A Neuro-Stimulus Chip With Telemetry Unit For Retinal Prosthetic Device” by Liu, W., Vichienchom, K., Clements, M., DeMarco, S. C., Hughes, C., McGucken, E., Humayun, M. S., De Juan, E., Weiland, J. D., Greenberg, R., Solid-State Circuits, IEEE Journal of, Volume 35, Issue 10, October 2000, pages 1487-1497. 
     In this respect,  FIG. 2  shows a more detailed circuital diagram of one of the electrode drivers  100 - i  of  FIG. 1 , the electrode driver being a constant current electrode driver. 
     Electrode driver  100 - i  comprises a driver controller  190 . The driver controller  190  takes commands from the ‘StimControls’ signals  195  coming from the main controller  150  and translates those signals into anodic and cathodic stimulation switching signals  200  and  210  that control pulse duration and timing. Driver controller  190  also provides an output current  220  that defines the current amplitude of the stimulation phases. MOSFET transistors M 16  and M 18  convert current  220  into anodic current  230 . MOSFET transistors M 18 , M 15 , M 11  and M 12  convert current  220  into cathodic current  240 . MOSFET transistors M 13 , M 14  and M 17  act as output switches. 
     As already explained with reference to previously discussed  FIG. 1-1  through  1 - 3 , electrode driver  100 - i  receives power from storage capacitors C 1  and C 2  (see voltage signals V+, V− shown in  FIG. 2 ). Storage capacitor C 1  provides power for anodic pulses. Storage capacitor C 2  provides power for cathodic pulses. 
     Similarly to  FIG. 1-2 ,  FIG. 2  shows a return electrode  250 . The return or common electrode  250  is connected to the junction Vce between C 1  and C 2 . The cathodic stimulation currents  240 , taken from C 1 , flow from the return electrode  250 , through the tissue  70 , to the V− rail of C 1 . The anodic stimulation currents  230 , taken from C 2 , flow from the V+ rail of C 2 , through the tissue  70 , to the return electrode  250 . The electrical properties of the electrode-tissue interface can be modeled to a certain degree of accuracy by the simplified RC network shown in the shaded tissue circle. 
       FIG. 3  is a timing diagram showing the relationship between the stimulation timing, stimulation currents, surplus currents and voltages on capacitors C 1  and C 2 . 
     During the cathodic stimulation phase  260 , the cathodic stimulation switching signal  210  (first graph on  FIG. 3  from the top) is ON, thus turning ON both MOSFETs M 13  and M 14  (see also  FIG. 2 ). On the other hand, the anodic stimulation switching signal  220  (second graph on  FIG. 3  from the top) is OFF during that time interval, thus keeping the MOSFET M 17  and the anodic current  230  OFF. Meanwhile, output current signal  255  (third graph on  FIG. 3  from the top) carries the programmed cathodic amplitude control current  270  ( FIG. 3 ), so that a current  240  ( FIG. 2 ) proportional to signal  220  ( FIGS. 2 and 3 ) will flow from the return electrode  250  to V− through MOSFETs M 14 , M 12  and the tissue  70  ( FIG. 2 ), causing a stimulation. When cathodic current  240  flows, charge is drawn from C 1 , causing a dip ΔV  280  on the capacitor voltage VC 1 , as also shown in the VC 1  graph of  FIG. 3 . 
     The charge Q delivered during a stimulation phase is
 
 Q= Cathodic Current 240×Duration Switching Signal 210 ON
 
     To the capacitor C 1 , a loss in charge Q means a drop in voltage ΔV
 
Δ V=Q/C 1=(Cathodic Current 240×Duration Switching Signal 210 ON)/ C 1
 
     If the capacitor C 1  is being constantly charged with a current Icharge, the voltage on C 1  at the end of the stimulation will be
 
Δ V= (Cathodic current 240 −I charge)×Duration Switching Signal 210 ON/ C 1
 
     During the anodic stimulation phase  290 , the anodic stimulation switching signal  200  (second graph on  FIG. 3  from the top) is ON, thus turning ON MOSFET M 17 , while stimulation switching signal  210  keeps the cathodic circuit OFF during that time interval. Output current signal  255  carries the programmed anodic amplitude control current  300  ( FIG. 3 ), so that a current  230  ( FIG. 2 ) proportional to signal  255  ( FIGS. 2 and 3 ) for the anodic phase will flow from V+ to the return electrode  250  through MOSFETs M 16  and M 17  and the tissue  70  ( FIG. 2 ), releasing the charge collected on the stimulating electrode  245  during the cathodic phase  260  and achieving electrical chemistry balance. When anodic current  230  flows, charge is drawn from C 2 , causing a dip ΔV  310  on the capacitor voltage VC 2 , as also shown in the VC 2  graph of  FIG. 3 . Similarly to what discussed above, the voltage drop ΔV on capacitor C 2  is
 
Δ V=Q/C 2=(Anodic Current 230×Duration Switching Signal 200 ON)/ C 2
 
     It should be noted that when power is supplied to the drivers  100 , the charges on capacitors C 1  and C 2  are drawn only during the stimulation pulses (see time intervals  260  and  290  of  FIG. 3 ), but are being injected by the power charging circuit  60  all the time. Therefore, the charging current can be controlled to inject sufficient charges to the capacitors as needed by the stimulation, to minimize the value of surplus currents IshL and IshH to achieve good power efficiency. The charging current is controlled through control of the power driving the external coil and the shunt regulator current value included in the back telemetry data as the output of the control closes the power control loop. 
     As also shown in  FIGS. 1-2  and  2 , the capacitor voltages V+, V− are the supply rails to the electrode drivers  100 - i . According to a further embodiment of the present disclosure, the capacitor voltages can be limited. Limitation of those voltages can be applied for two reasons. A first reason is that the electronic chip on which the implant  10  is operated should operate under a specific voltage limit for safety reasons. A second reason is that the power needed to inject a certain amount of charge in the capacitor is proportional to the capacitor voltage itself. Therefore, limiting the capacitor voltage to a level just satisfying the need of electrode compliance voltages will also allow power to be saved. 
     As also explained with reference to  FIGS. 1-1 ,  1 - 2  and  1 - 3 , the voltages of C 1  and C 2  are limited by shunt regulator circuits  80  and  90 , respectively. In particular, shunt regulator circuits  80  and  90  can limit the capacitor voltages to a preprogrammed value during charging. After the voltage on capacitors C 1  and C 2  reaches the safety voltage limit, circuits  80  and  90  will bypass (shunt) the surplus charging current and the capacitor voltages will not rise further. The shunting circuits  80  and  90  may also comprise a current tap out, which provides a quantitative indication of the surplus current. Such indication can be used to estimate the power condition of the implant  10 . Shunting circuits  80  and  90  can also comprise a rail control mechanism, which allows the external unit  20  to set the capacitor voltage limit through binary control bits. 
       FIG. 4  shows in greater detail the internal structure shunt regulator circuits  80 ,  90 , in accordance with a further embodiment of the present disclosure. The shunting circuit comprises a current sensor and a current sink. In the embodiment of  FIG. 4 , the current sensor is comprised of a diode stack D 1 -D 11  in series with a sampling resistor R 1 , and the current sinking circuit is comprised of an npn transistor amplifier Q 1 . During operation, the first terminal VH of the current sensor is connected with either V+ or VM (see  FIGS. 1-1 ,  1 - 2  and  1 - 3 ), while the second terminal VL of the current sensor is connected with VL either VM or V− (see  FIGS. 1-1 ,  1 - 2  and  1 - 3 ). Therefore, during operation, the two terminals of the current sensor are connected in parallel with the capacitors C 1 , C 2 . When the voltage across each capacitor reaches a value that is greater than the sum of the “knee” voltages of PN junction diodes D 1 -D 11 , the current through the diodes increases rapidly, so as the voltage across sampling resistor R 1  and the base current of transistor Q 1 . The common emitter transistor Q 1  will then sink a larger current through its collector that is determined by the current gain of the transistor. The sensitivity of the current sensor is controlled by resistor R 1 . The person skilled in the art will understand that transistor Q 1  can be replaced by any other arrangement suitable for the same purpose. For example, if CMOS technology is used, the current gain of a single transistor may be limited, and Q 1  could be replaced with a Darlington pair and/or a transistor array to increase the current gain for a better shunt effect. The person skilled in the art will also understand that the number of diodes D 1 -D 11  can be any number, depending on the highest rail voltage specified. 
     The shunting circuit of  FIG. 4  also comprises a transistor Q 2  and a current mirror MOSFET circuit M 1 -M 4 . In particular, Q 2  and M 1 -M 4  provide a current tap out Ishunt for quantitative measurement of the surplus current flowing through the shunting circuit of  FIG. 4 . The output signal Ishunt is sent to power monitor  120 , as already shown in  FIGS. 1-1 ,  1 - 2  and  1 - 3  and also later shown in  FIG. 7 . 
     The embodiment of  FIG. 4  also provides for the presence of a rail control mechanism along connection RC between main controller  150  and shunting circuits  80 ,  90  (see  FIGS. 1-1 ,  1 - 2  and  1 - 3 ). In particular, control of a rail voltage can be obtained by selectively shorting a certain number of diodes of the diode chain in the shunting circuit using MOSFET switches (see switches S 1 -S 3  in  FIG. 4 ). As shown in  FIG. 4 , the connection of the switches S 1 -S 3  with the diodes D 1 -D 11  provides a direct binary encoding, allowing the rail voltage limit to be conveniently programmed without the need of an encoding interface. It should be noted that the resolution of the rail voltage control is one diode voltage drop which is logarithmically proportional to the diode current. In the embodiment shown in  FIG. 4 , only a small portion of the surplus current will flow through the diode chain D 1 -D 11 . Therefore, the diode voltage drop is close to the turn-on voltage for the operating diodes, and the error caused by the ON resistance of the switches for shorted diodes is insignificant. 
     When the electrode voltage reaches the compliance limit due to increased electrode impedance, the output current  255  of the electrode driver cell (see, e.g.,  FIG. 2 ) will deliver less amount of current than what is set to be. This situation will affect the output accuracy of the implant  10  and is also likely to result in unbalanced current pulses that are not favorable to the electrodes  245  or the tissue  70 . The electrode voltages, however, are determined by the in vivo electrode-tissue impedances which can vary greatly over time or among different electrodes, thus making any preset limit inefficient. 
     A further embodiment of the present disclosure addresses the above issue by providing a compliance monitoring circuit as already mentioned with reference to circuit  110  of  FIG. 1-3 , which circuit is shown in  FIG. 5  in greater detail. Although the compliance monitoring circuit  110  of  FIG. 1-3  has been shown separately from the driver array  100 , such circuit, portions of which are represented in  FIG. 5  with reference numerals  320 - i  and  325 , can be embedded into each current driver cell  100 - i . Therefore, according to a further embodiment of the present disclosure, there will be one compliance monitoring circuit per driver. Generally speaking, the compliance monitor  320 - i / 325  monitors the voltage of electrode  245  to see if such voltage has reached the compliance limit. When such limit is reached, the compliance monitor generates a signal CompAlert  340  ( FIGS. 1-3  and  5 ) to alert the BT controller  160  and/or the external unit  20 , so that a decision as to whether the rail voltage should be increased or the stimulation current decreased can be made. 
     Box  330 - i  of  FIG. 5  represents a portion of driver cell  100 - i  of  FIG. 2 . Such portion  330 - i  comprises cathodic or sink current MOSFETs M 11 , M 12 , M 13  and M 14 . 
     Generally speaking, ignoring the short channel effect of the MOSFET, the drain output current of the electrode driver is set by a gate-source voltage Vgs but not a drain-source voltage Vds as long as the output MOSFET is in the saturation region, i.e., when the drain-source voltage Vds is larger than the gate-source voltage Vgs subtracting the threshold voltage Vt of the device. The drain current falls off rapidly as the drain-source voltage decreases further. This condition to hold the drain output current constant can be written as:
 
 Vds&gt;=Vgs−Vt   (1)
 
     In the electrode driver circuit, the above condition of drain-source voltage Vds of the output MOSFETs, together with the power supply rail voltage Vp, defines the output compliance limit of the electrode stimulator. For example, if the power supply voltage is 10 volts, the threshold voltage Vt is 1 volt, and the gate-source voltage Vgs is set at 1.5 volts, then the compliance limit can be defined as Vcomp=Vp−(Vgs−Vt)=9.5 volts. The compliance monitor circuit according to the present disclosure detects the condition of failing to meet (1), i.e., it detects the condition that fulfils:
 
 Vds&lt;Vgs−Vt (2), or  Vgs−Vds&gt;Vt   (3)
 
     The circuit shown in  FIG. 5  is a direct realization of the condition of equation (3). In a cathodic phase first stimulation protocol, the voltage on the electrode  245  tends to shift towards a negative value relative to the return electrode  250  because of the capacitive component in the electrode impedance. Therefore, the sink part (MOSFETS M 11 -M 14 ) of the current driver is more susceptible to reaching the compliance limit. 
     In the circuit of  FIG. 5 , MOSFET M 12  is the output transistor that is responsible for maintaining a stable current output. Vgs represents the gate-source voltage difference of M 12 , while Vds represents the drain-source voltage difference of M 12 . The compliance monitor circuit  320  monitors the difference Vgs−Vds of M 12  by way of a subtracting circuit  350  whose output is compared with the threshold voltage Vt reference  360  through comparator  370 . The output of comparator  370  drives an open drain nMOS transistor M 21  whose output CC is fed to node Vc, where the outputs of the compliance monitors of all drivers  100 - 1 ,  100 - 2  . . . ,  100 - i , . . .  100 - n  are line OR-ed through MOSFET M 100 . 
     In normal operation, equation (3) is not fulfilled, and the output of comparator  370  is LOW, shutting off M 21  so that its output CC does not affect node  380 . However, if the output voltage of the electrode  245  reaches the compliance limit, equation (3) is satisfied and the output of comparator  370  becomes HIGH, turning on M 21 , so that its output CC pulls down the voltage on node  380  to LOW. The voltage change on node  380  will be sensed by a compliance logic circuit  390  to generate the CompAlert signal  340  discussed above. 
       FIG. 6  is a timing diagram for signal  210 , output current  255 , Vds, Vgs-Vds, CC and CompAlert, showing the behavior of those signals during operation of the compliance monitoring circuit discussed with reference to  FIG. 5 . 
     According to a further embodiment, the present disclosure also provides for power monitoring circuits  120  to monitor the charging and draining conditions of the capacitors, as already explained with reference to  FIGS. 1-1 ,  1 - 2  and  1 - 3 . In particular, the power monitoring circuits monitor the surplus currents flowing through the shunting circuits  80 ,  90  and the voltages on the capacitors C 1 , C 2 . Monitoring the surplus current allows a flexible, quantitative control of the inductive power loop, i.e. the loop including the coil driver, the coil pair, and the power storage capacitors. 
     For example, the power level can be predefined (by way of a feedforward method) or adaptive to the load requirement (by way of feedback information). In addition, monitoring of the storage capacitor voltages prevents the implant device  10  from operating with an insufficient power condition that may compromise safety. 
     With reference to  FIG. 4 , applicants have already discussed how the surplus current is tapped out as Ishunt through Q 1  and M 1 -M 4  in the shunting circuits  80 ,  90 .  FIG. 7  shows a current monitor  480  comprising an analog-to-digital converter (ADC)  490  that digitizes the analog Ishunt inputs IShH and IshL received from shunting circuits  80  and  90  (see also  FIG. 1-2 ). The converted digital output I_Level is sent to BT controller  160  and eventually reported to the external unit  20 . 
     Reference can also be made to the flow chart shown in the following  FIGS. 8-1  and  8 - 2 , where this current data is checked against a preset power level (threshold) (see steps S 4  and S 20  of  FIG. 8-1 ) and a decision is made as to whether increase or decrease the power output of the RF coil driver (see the left portion of  FIG. 8-1 ). 
     Meanwhile, the voltages on C 1  and C 2  are also monitored by a capacitor monitoring circuit  500  comprising a subtracting circuit  510 , comparators  520 ,  530  and NOR logic  540 . In normal power conditions, the output CapLevel of the capacitor monitoring circuit is HIGH. If either one of the capacitor voltages VC 1 , VC 2  falls below a preset threshold Vref (depending on the implementation of the circuit), the output CapLevel becomes LOW. The LOW value of CapLevel indicates that implant power has fallen below a critical low level required for safe operation. Upon receipt of a CapLevel LOW signal, the external control will have to stop the stimulating operation of the implant and raise the RF power output level until CapLevel reaches again and stays at a HIGH value. 
       FIGS. 8-1  and  8 - 2  are a flow chart showing the system power control flow. During start-up step S 1  the PowerLevel limit (high and low threshold values of the digital output of ADC  490  in  FIG. 7  to be checked against) and the rail control bits (see  FIG. 4 ) are set. Step S 2  checks the value of the CapLevel signal discussed in  FIG. 7 . If the value is HIGH, the implant is under normal operating conditions and the flow proceeds to step S 3  where the value of the CompAlert signal  340  of  FIG. 5  is checked, to see whether the voltage on electrode  245  has reached the compliance limit. In case such limit has not been reached the flow proceeds to step S 4  to check whether the power (represented by the surplus current from ADC  490 ) has exceeded the high threshold of PowerLevel limit set in step S 1 . Should this not be the case, the flow proceeds to step S 20  to check whether the power has fallen below the low threshold of the PowerLevel limit. If not (meaning that the implant power is within the expected range), the flow goes back to step S 2  for continuous monitoring. However, if the answer to the determination made in step S 20  is affirmative, the flow first checks if the power driving the external coil (RF Power) has reached its maximum level (an external unit specification) in a step S 21 . If it has not reached the maximum level, the coil drive is increased by an amount predefined in the control protocol in a step S 22  and the flow goes back to step S 2  to continue a next round of monitoring of the implant power. If, on the other hand, the power driving the external coil has reached the maximum level so that no further increase of driving power is allowed, a low power alert of the implant is asserted in step S 23 . 
     On the other hand, if the answer to the question of step S 4  is positive, which means that the implant power is excessive, the flow moves to step S 5  where it is checked if the power driving the external coil is at its minimum level. If the power is not at its minimum, such power is lowered in step S 6  (lower coil drive) and the flow goes back to step S 2 . On the contrary, if the RF driving power level is already at its minimum, indicating that the PowerLevel overshoot of step S 4  could be due to malfunctioning of the external unit, safety measures are activated in step S 7 . Turning back to step S 2 , if the value of the CapLevel signal of  FIG. 7  is LOW, it indicates a critical low power level that could be due to poor coupling of the coils, insufficient rail voltages, excessive stimulation current, or malfunction in the powering loop that includes the external coil driver, the coils and the implant circuits. When this happens, the flow first proceeds to step S 8  where it is checked if the power driving the external coil has been at its maximum. If that condition is satisfied, the flow proceeds to step S 9  where the rail voltage is checked. If also that condition is satisfied (i.e. rail voltage at its maximum value), safety measures are activated in step S 10  to stop the stimulation operation. If, on the other hand, the rail voltage is not at its maximum value, such voltage is increased at step S 11  and the flow goes back to step S 2 . Similarly, if the coil driving power checked in step S 8  is not at its maximum, such power is increased in step S 12  and the flow goes back to step S 2 . Turning to step S 3 , in case the compliance limit has been reached, the flow moves to step S 13  where it is checked if the rail control has set the rail voltage to its maximum value. If yes, the electrode impedance is checked at step S 14  and the maximum stimulation current is recalculated accordingly at step S 15 . Once this has been done, the flow moves to step S 4 , already discussed above, to check the power level. Turning to step S 13 , if the rail voltage is not at its maximum value, such value is increased at step S 16  and the flow moves to step S 4  already discussed above. 
     While several illustrative embodiments of the invention have been shown and described in the above description, numerous variations and alternative embodiments will occur to those skilled in the art. Such variations and alternative embodiments are contemplated, and can be made without departing from the scope of the invention as defined in the appended claims.