A FEEDBACK SYSTEM TO CONTROL ELECTRODE VOLTAGES IN A COCHLEAR STIMULATOR AND THE LIKE

Abstract: An implanted device such as cochlear implant having electrodes is provided with a pulse generator for generating stimulating pulses. The pulses are adjusted to insure that during the idle periods between the pulses, the voltage on the electrodes is at zero or at a preselected level. In one embodiment, the pulse durations or amplitudes are adjusted. In another embodiment the voltage on the electrodes is measured before and during each pulse to determine IR drops from the electrodes to a reference point, and to compensate for the same.

Description: A FEEDBACK SYSTEM TO CONTROL ELECTRODE VOLTAGES IN A COCHLEAR STIMULATOR AND THE LIKE BACKGROUND OF THE INVENTION A. Field of Invention This invention pertains to a cochlear implant systems using a plurality of electrodes to apply stim¬ ulation related, for example, to ambient sounds, to a patient's tissues or nerves and more particularly to a system adapted to reduce and/or control the voltages across the electrode/tissue interfaces when the elec¬ trodes are inactivate. B. Description of the Prior Art The subject invention pertains primarily to tissue stimulating systems. For example, electrode systems are known for providing therapy to a patient suffering loss of hearing. All of these systems typically require two sections: an internal or im¬ planted section, and an external section. The exter¬ nal section includes a microphone for receiving am¬ bient sounds and converting them into corresponding electrical signals. These electrical signals are pro- cessed and sent to the implanted section. The im¬ planted section then generates stimulation signals used to excite the auditory nerve of the patient between an intra-cochlear electrode array and one or more extra-cochlear electrodes. These electrodes are normally metallic and the interface impedance between the electrode and the tissue is largely capacitive. Therefore, when current is passed through these electrodes the voltage between the electrodes and the tissue (i.e. , the electrode voltage) changes, as it does when current is passed through any capacitor. One disadvantage of this phenomenon is that the rate at which electrochemical reactions at the metal/tissue interface proceed is a function of the voltage across that interface. Reactions that at resting potential (i.e., the potential of the electrode metal with respect to the tissue with no applied current) are so slow as to be insignificant can become significant when the poten¬ tial across the metal/tissue interface changes. Manufacturers of cochlear implant systems there- fore have to be careful to control electrode voltages to keep them in a region where any electrochemical reactions occur at a rate too slow to cause damage. Different electrochemical reactions occur at different voltage levels and their rate is not a necessarily monotonic relationship with the electrode voltage. Therefore, careful control of the electrode voltages can minimize harmful electrochemical reac¬ tions resulting from stimulation. One method normally used to control the electrode voltages consists of connecting all the electrodes together, through low impedance switches, (i.e., shorting the electrodes) after every stimulation pulse. Another method consists of providing decoupling capacitors in series with the electrodes. However, these capacitors are bulky and take up valuable space in the implant. Both methods described above are capable of main¬ taining a substantially minimal DC current between the electrodes during idle periods, however, they cannot be used to control the DC voltage level between the electrodes and the tissue. OBJECTIVES AND SUMMARY OF THE INVENTION In view of the above-mentioned disadvantages of the prior art, it is an objective of the present invention to provide a cochlear implant having, during idle periods, a preselected DC voltage between different stimulation electrodes or between the stimulating electrodes and the tissue. A further objective is to provide a cochlear implant wherein stimulation pulses are applied by electrodes to a patient's aural nerve, said stimula¬ tion pulses being arranged to provide a preselected DC voltage between said electrodes during the idle periods between the pulses. A further objective is to provide a cochlear implant with means for substantially eliminating DC electrode voltages between different electrodes during idle periods, if required. Other objectives and advantages of the invention shall become apparent from the following description. Briefly, a cochlear implant system constructed in accordance with this invention includes a housing arranged and constructed for implantation subcutane- ously into a patient, an electrode array coupled to said housing and extending into the cochlea for apply¬ ing stimulation pulses to the patient's aural nerve, and means disposed in said housing for selectively controlling said pulses in a manner selected to result in a preselected voltage between said electrodes dur¬ ing idle periods between said pulses. The preselected voltage may be substantially zero or at a small level selected to minimise electrochemical reactions at the electrodes. Alternatively, the implant may be provided with a zero-crossing sensing network for obtaining said preselected voltage. BRIEF DESCRIPTION OF THE FIGURES Figure 1 shows a block diagram of a cochlear implant constructed in accordance with the present invention; Figure 2 shows a somewhat diagrammatic view of the electrode array used in the implant of Figure 1; Figures 3A and 3B show time-dependent voltage and current pulses generated between the electrodes of Figure 2; Figure 4 shows a zero-crossing network for an alternate embodiment of the invention; and Figure 5 shows voltage and current pulses generated in the embodiment of Figure 4. DETAILED DESCRIPTION OF THE INVENTION Referring now to Figures 1 and 2, a cochlear implant 10 constructed in accordance with this inven¬ tion includes an implantable housing 12 and an elec- trode carrier 14 terminating in an electrode array 16. Array 16 is preferably composed of two sets of elec¬ trodes: a set of inter-cochlear electrodes, such as electrodes 16A, 16B... arranged and constructed to be disposed in the cochlea 17 of the patient and a set of extra-cochlear electrodes such as electrodes 16C, and 16D. More particularly, the intra-cochlear elec¬ trodes 16A, 16B... of electrode array 16 are disposed adjacent to the basilar membrane 21 and the: associated auditory nerve (not shown) . Optionally, additional extra-electrodes 19 may be provided extending from the housing 12 or a portion of the housing 12 may be made conductive to form an extra-cochlear electrode (not shown) . Housing 12 includes various circuitry, such as a current generator 18, control logic 20, and a switch¬ ing network 22. In response to external signals from a receiver (not shown) , the control logic 20 sets the current generator to generate cochlear stimulation current pulses on lines 24A and 24B to switching net- work 22. The switching network 22, in response to control signals from logic 20 couples these cochlear stimulation signals sequentially to one of the elec¬ trodes of array 16 through conductors 26. Biphasic pulses are normally generated between the said electrodes . Importantly, in accordance with this invention, the housing 10 further includes a plurality of elec¬ trode selector switches 28, a sample-and-hold (S/H) circuit 30, a variable voltage offset source 31, and a differential amplifier 32. The electrode switches 28 are used by the control logic 20 to connect two of the electrodes of array 16 (or any other electrodes pre¬ sent) via the appropriate conductors 26 to the S/H circuit 30. The voltages sensed by this circuit 30 are fed to the differential amplifier, and after amplification therein, to the scaling network 34. The scaling network generates in response on offset or error signal on line 36. The current generator 18 uses this signal to generate the current stimulation pulses as discussed below. The stimulation pulses generated by the system of Figure 1 are illustrated in Figures 3A and 3B. In a first embodiment of the invention shown in Figure 3A, a biphasic current IS is applied between two elec¬ trodes 16A and 16B. This current consists of a plur- ality of pulses Pl, P2 separated by idle periods TI . Each current pulse consists of a negative phase phi followed by a positive phase ph2 for substantially neutralizing residual charge or voltage build up in the tissues. The voltage between the electrode 16A and an extra-cochlear electrode such as 19 is shown as VA in Figure 3A. As shown in Figure 3A, prior to pulse Pl , elec¬ trode 16A is at a static residual voltage of -Vax, while the voltage on the electrode VB is Vbx. The purpose of the S/H circuit 30, amplifier 32 and scal¬ ing network 34 is to eliminate the voltage differen- tial between electrodes 16A and 16B or alternatively to set this voltage differential to a value defined by the variable voltage offset source 31. During the idle period, prior to pulse Pl, the respective conductors 26 corresponding to electrodes 16A and 16B are connected to S/H circuit 30. This circuit measures and stores the voltages on the elec¬ trodes on capacitors 30A, 3OB during the idle period prior to pulse Pl, i.e., voltages -Vax and +Vbx. The voltage Vax is offset by a constant voltage offset source 31. The resulting signal and the voltage +Vbx are fed to differential amplifier which generates an output signal indicative of the difference between its inputs. This output signal is fed to a scaling net- work which, in response, generates a scaled feedback difference signal to the current generator 18. The time for the onset of the next stimulation current pulse Pl is determined by the control logic 20. The current generator 18 generates the current pulses Pl, P2... so that their phases have a nominal duration TO. However, the current generator regulates the duration of phase ph2 in response to the signal 38 from scaling network 36 to compensate for the resid¬ ual voltages on the electrodes during idle periods. For example, if the potential difference between the two electrodes 16A, 16B (i.e. Vax-VBx) is negative, then the current generator extends the duration of ph2 by an incremental amount dTl. This extended duration causes the voltage VA to rise by an amount +ΔVA1. When the pulse Pl terminates, the voltage VA drops to +VA1 as shown. Similarly the Voltage VB drops to - VB1. However, the differences between the two vol¬ tages is VA, VB, i.e., ΔVA1-ΔVB1 is smaller than it was before pulse Pl . Since this difference is now positive, during the next pulse P2, the second phase ph2 is shortened by a small amount dT2. In this man- ner after several pulses, the residual voltages of the electrodes 16A, 16B are stabilized during the idle periods either near zero, or at a value set by the offset differential voltage source 31. The relation- ship between the change in the voltage in VA and VB and the amount of time dt depends on the slope of the respective voltage curves. This slope is dependent on the capacitance between the electrodes and the scabbing tissue, and since this parameter is highly variable, a deliberately small value of constant is chosen to avoid instability from positive feed back. The time required for the residual voltages to settle can therefore extend over several idle periods, as discussed above. Figure 3B shows a different embodiment of the invention. In this embodiment, the period of each phase phi, ph2 remains constant (i.e., TO) for various pulses. However, their nominal amplitudes 10 is in¬ creased or decreased by Δl by the current generator, to increase the maximum amplitudes of voltages VA, VB by ΔVA and ΔVB respectively based on the initial val¬ ues VAX, VBX. The net result is that the residual electrode voltages are stabilized over several pulses, in a manner similar to the embodiment of Figure 3A. Figures 4 and 5 provide an alternate embodiment in which the relatively long time period required for the system to stabilize has been eliminated. Referr¬ ing to Figure 4, implant 110 includes a housing 112 connected to intra-cochlear electrodes 116A and 116B and an extra-cochlear electrode 119. Electrodes 116A and 116B are part of an electrode array (not shown) similar to array 16 in Figure 1 while electrode 119 is a separate electrode similar to 19 or, alternatively, is found in housing 112. Housing 112 includes a pair of voltage buses Vdd and Vss, a switch bridge consisting of, as an example, switches 124A, 124B, 124C and 124D and a current sup¬ ply 118 generating a constant current I in the direc¬ tion indicated. The housing further includes switches 126A, 126B, 126C and 126D used to monitor the voltages on the electrodes as discussed more fully below, as well as a voltage offset source 131 and a differential amplifier 132. Finally associated with switches 126A- D and amplifier 132 are two capacitors Cl and C2. In general terms, the implant 112 applies a biphasic stimulation current pulse having two phases phi, ph2 between electrodes 116A and 116B, similar to the phases shown in Figure 3A. However, during ph2 the voltage on the electrodes 116A and 116B is care¬ fully monitored and when the voltages are equal, ph2 is turned off thereby insuring that the there is no voltage drop across these electrodes. Optionally, the current phase ph2 is turned off when the voltage drop across the these electrodes reaches a preselected level. Importantly, because of the impedance Z of the tissue between the electrodes 116A, 116B, there is an IR drop during the application of the stimulation current. This IR drop is also measured and taken into consideration. The various signals generated by the circuitry of Figure 4 during a typical biphasic stimulation current are shown in Figure 5. In this Figure the biphasic current pulse is shown as signal I, having phases phi and ph2. Next, going down from the top of the Figure, are shown the corresponding positions of switches 126A, 126B, 126C and 126D. As indicated in Figure 4, voltage signals VI, and V2 indicate the voltages be¬ tween the electrode 116A and electrode 116B and the positive bus Vdd, respectively. VC1, VC2 are the vol¬ tages across the capacitors VC1 and VC2. V(C1+C2) is the algebraic sum of the voltages across capacitors Cl and C2. Finally, voltage VZ is the voltage between node Z( the inverting input to the amplifier 132) and the bus Vdd. The operation of the circuitry of Figure 4 shall now be explained in conjunction with Figure 5. All the switches 124A-D, 126A-D are controlled by control logic 120. The two phases phi and ph2 are generated by selective closing of switches 124A-D. More specifically, phi is generated by closing switches 124B and 124C, and ph2 is generated by closing 124A and 124D. While these switches are closed, current from source 118 flows between the electrodes 116A and 116B. Switches 126A-D are used for two purposes. First, to measure the IR drop across the electrodes 116A, 116B during the application of the biphasic current pulse, and second, to provide compensation for this IR drop for the termination of ph2. More speci¬ fically, as shown in Figure 5, switches 126A and 126B are closed for a short period just prior to phi. Dur- ing this time, there is a residual voltage VX between electrodes 116A and 116B. Closing switches 126A and 126B causes the capacitor Cl to charge to this voltage VX. When phi is initiated, switch 126A is opened and switch 126C is closed. During phi current I flows between the electrodes 116A and 116B, so that the voltage across these electrodes is VX +IR, where R represents the tissue impedance. Closing switch 126C while switch 126B is also closed causes the capacitor C2 to charge to VX-IR) . The voltage reversal is caused by the manner in which the switches 126B and 126C are arranged across the capacitor C2. Because of this reversal, the net voltage across the two capaci¬ tors Cl and C2, i.e., V(C1+C2) , is -IR, as shown. Shortly after phi is initiated, switches 126B and 126C are opened and the capacitors maintain their charge to act as a standard sample and hold circuits. Mean- while, during phi, the voltage VI is slightly below Vdd (due to the voltage drop across switch 124B, since all these switches are solid state switches) and the voltage V2 has the shape of a negative ramp as shown. At the beginning of phase ph2 switch 126D is closed. At this point amplifier 132 receives at its non-inverting input the signal VI and the signal VZ (i.e., V2-IR) at its inverting input. Thus, during phase ph2 the amplifier compares the voltages at the electrodes 116A and 116B with automatic compensation for the IR drop therebetween. During this time, the voltage VI drops linearly as shown in Figure 5. This process continues until the signal VI drops below VZ. At this moment the output of amplifier 132 forces the control logic 120 to turn switches 124A and 124D so as to end phase ph2. Thus the duration of phase ph2 can be shorter or longer than TO, the duration of phase phi. At the end of ph2, the voltage on electrodes 116A and 116B should be the same thereby eliminating, at least theoretically any residual voltage there¬ between during the idle periods. The process described above results, ideally, in no residual voltage between the electrodes.. If a preselected voltage is desired between these elec- trodes, the offset voltage source 131 may be set for the desired voltage value. Otherwise the source 131 is set to zero. In the embodiments shown in Figures 3A and 4 , the voltages between the electrodes of the intra-cochlear electrodes during idle period are controlled by changing the periods of the phase ph2 of the current pulses. However these changes in duration are relatively small so that they have no effect on the ability of the patient to perceive this stimulation and to translate them into ambient sounds. The description of the embodiment in Figure 4 describes how the voltage between electrodes 116A and 116B, (i.e., the two electrodes through which stimula¬ tion current is passing) is controlled. An alterna¬ tive arrangement is to control the voltage between electrode 116A and a third electrode 119. This is achieved by using selector switches (not shown) which connect electrode 119 rather than 116B to switches 126B and 126D at the aforementioned times. Since no current flows through electrode 119, it is therefore possible to use this electrode 119 as a stable refer¬ ence voltage whose level with respect to the tissue voltage will not change. Other stimulation electrodes such as electrode 116A can therefore have the resting (idle period) level of their electrode/tissue voltage accurately controlled. The invention can also be used to maintain zero or close to zero DC current through electrode 116A provided its electrode/tissue interface capacitance is large enough such that its voltage during a stimula- tion pulse is not disturbed by more than a few milli¬ volts. A typical extracochlear electrode easily meets this requirement. Under these circumstances the elec¬ trode/tissue interface capacitance of electrode 116A acts as a non-leaky integrator of the current passed through it. If the system is configured to maintain the voltage on electrode 116A at a constant value (by using a non-stimulated electrode 119 as a constant reference voltage) then the DC current through elec¬ trode 116A must be zero since the integrated current through it is maintained at zero by the system. Although the invention has been described with reference to several particular embodiments, it is to be understood that these embodiments are merely illus¬ trative of the application of the principles of the invention. Accordingly, the embodiments described in particular should be considered exemplary, not limit- ing, with respect to the following claims

Claims: Claims: 1. An implantable device comprising: pulse generating means for generating stimulation a sequence of stimulation pulses separated by idle periods; a first and second electrode for receiving said stimulating pulses, with a residual voltage being generated intrinsically on said electrodes during said idle periods; means for measuring said residual voltage; and means for controlling said pulses to change said residual voltage to a preselected value. 2. The device of claim 1 wherein said preselected value is substantially zero. 3. The device of claim 1 wherein said pulses have pulse periods and wherein said control means adjusts said pulse periods. 4. The device of claim 3 wherein said measuring means measures the residual voltages from each of said electrodes to a reference to obtain corresponding mea¬ sured voltages and wherein said control means termin¬ ates said pulse periods when said measured voltages have a preselected difference. 5. An implantable device for applying thera¬ peutic stimulation to body tissues, said device comprising: a housing, including pulse generator means for generating pulses having pulse periods and being separated by idle periods; a plurality of electrodes for receiving said pulses; measuring means for measuring a residual voltage between said electrodes and another reference point having a constant voltage with respect to the tissue voltage, to obtain corresponding measured vol- tages; control means for selectively applying said pulses to said electrodes, said control means receiving said measured voltages and controlling said pulses to set said electrode voltages to a preselected level . 6. The device of claim 5 wherein said control means includes a first set of switches for selectively connecting said pulses to said electrodes and a second set of switches for connecting said electrodes to said measuring means during said idle periods. 7. The device of claim 5 wherein said pulses are biphasic pulses . 8. The device of claim 5 wherein said control means changes said pulse periods in accordance with said measured voltages. 9. The device of claim 5 wherein said pulses have pulse amplitudes, and said control means changes said pulse amplitudes in response to said measured voltages. 10. The device of claim 5 wherein said means for measuring includes means for determining said elec¬ trode voltages prior to each pulse to obtain initial voltages, and wherein said control means terminates each pulse after a time period dependent on said initial voltages. 11. The device of claim 5 wherein said means for measuring includes means for measuring an IR drop from said electrodes to a reference point during said pulses, and wherein said control means controls said pulses to adjust for said IR drop. 12. The device of claim 5 wherein said electrodes are cochlear implant electrodes.

Inventors: CARTER PAUL MICHAEL, MONEY DAVID KERRY, CARTER, PAUL, MICHAEL, MONEY, DAVID, KERRY
Applicants: COCHLEAR LTD, COCHLEAR LIMITED
Application year: 1995
Main classification: H04R25
IPC codes: A61F11, A61N1, H04R25