FR2668017A1 - Electrical stimulation circuit for a hearing aid - Google Patents

Abstract

This circuit, which delivers, to a cochlear electrode, implanted in the inner ear of a subject, an alternating electrical signal as a function of a microphone signal picked up and amplified, comprises means for amplifying the microphone signal consisting of an amplifier (10, 20) operating in class D, receiving, as input, the said microphone signal and delivering, as output, the said coded alternating electrical signal (Vs). The output signal of the amplifier can be delivered to the electrode by means of a voltage step-up transformer (81) coupled to means (82) forming a low-pass filter; it can also be coupled by the use of means forming a low-pass filter, means being furthermore provided for transposing the power supply voltage of the output stage of the amplifier in such a way as to increase the voltage excursion of the said alternating electrical signal applied to the electrode.

Description

Electrical stimulation circuit for cochlear prosthesis
The present invention relates to the treatment of deafness, in particular that of total deafness, by electrical stimulation of the inner ear.

 In this technique, one or more electrodes are implanted in the inner ear in the cochlea or on it (the implantation can be both intra-cochlear and extracochlear), electrodes which, under the effect of application of an appropriate signal, will exert on the cochlea an electric action, possibly doubled by a mechanical action, thus exciting the fibers of the auditory nerve and giving artificially to the subject, through this "hearing sensation.

 It should be noted that this technique is opposed to that of "hearing aids" or "acoustic prostheses" proper, where a simple audio amplifier circuit is used receiving the signal from a microphone and restoring it to an electro- transducer. acoustic (earpiece) located in the ear; such a technique can only be applied to the treatment of hearing impairment, and not to that of total deafness; moreover, it is based on an entirely different principle.

 In cochlear stimulation, the electrodes are excited by biphasic signals, i.e. signals having alternately positive and negative polarity and such that the amounts of current applied to the electrodes, and therefore passing through the tissues, are respectively the same for one polarity and for the other, so that, after applying a charge, to cancel this charge by reversing the current and applying the immediately consecutive phase of opposite polarity.

 As the tissues do not charge exactly symmetrically on application of a positive charge and on application of a negative charge, it may be necessary to obtain an effective total cancellation of the charge by reversing the current. , to generate quasi-biphasic signals whose amounts of current, which are not exactly similar in one phase and in the other, compensate for this asymmetry of tissue behavior. In the following, however, such quasi-biphasic signals will be assimilated to exactly biphasic signals, the appropriate adjustment of the shape of the signals applied by the device being within the competence of the specialist in this technique.

 Depending on the techniques used, it is possible to provide a single electrode (so-called "single-channel" technique) or a plurality of electrodes implanted at different points (technique called "multi-channel").

 In the case of a multichannel device, each electrode always receives a signal of identical duration and shape, but differing from one electrode to another. The acoustic excitation device then comprises, in addition to a microphone sensor, a battery of filters for analyzing the microphone signal received, these filters being in number equal to that of the electrodes. A microprocessor then selects the channel on which the signal level is the most important and then excites the corresponding electrode; for example, a long pulse is delivered if the dominant frequency is a low frequency, or a short pulse if the dominant frequency is an acute frequency.

 The present invention belongs to the second category, namely that of single-channel prostheses, therefore using only one implanted electrode.

In any event, the excitation devices currently used, whether they are for single-channel or multi-channel type devices, all have the quadruple drawback of being both:
specific: their operating principle is unrelated
with that of hearing aids, which leads to cir
cooked specially developed for this technique,
~ expensive, due to their specificity and small series (these
techniques are still very little developed),
~ bulky :: these devices are generally brought to the
belt and connected to the ear by a wire transmitting the signals
to the electrode subcutaneously or percutaneously, and they're
therefore relatively handicapping for those who wish them
use all day long, and
~ not very economical from the point of view of their consumption: this,
relatively high, requires a relatively large capacity stack
important, therefore bulky and cannot therefore be
housed only in a housing of non-negligible dimensions,
typically the case worn on the belt mentioned above.

 The object of the present invention is to remedy these various drawbacks by proposing a single-channel type device produced in a very simple manner by adapting a hearing aid of the type used to remedy common hearing impairments, that is to say - say including a microphone, an amplifier and a headset.

 We will see in particular that it is possible to use, after an appropriate adaptation, a prosthesis of the “behind the ear” type, therefore light, which overcomes the drawbacks of the current cases worn on the belt.

 Furthermore, as will also be seen, the design of the excitation device of the present invention makes it possible to have an excellent efficiency from the electrical point of view, therefore to be satisfied with the small batteries incorporated in the conventional housings of the behind-the-ear type. ", unlike current cochlear stimulation devices which, due to their low efficiency, all require the use of a battery of relatively large volume, making the case worn on the belt even more necessary.

 In addition, the fact of making the cochlear excitation device of the present invention from a conventional hearing aid, appropriately modified, makes it possible to considerably lower the cost of this device.

 Essentially, as will be explained below, in order to produce such a cochlear stimulation device from elements existing on the market in the neighboring field of hearing aids, it will be necessary to modify the existing device in order to deliver a level of high voltage, compatible with cochlear excitation (of the order of 0.5 to 2 V depending on the individual).

 These various aims are achieved, according to the invention thanks to an electrical stimulation circuit of the aforementioned type, that is to say delivering to a cochlear electrode, implanted in the inner ear of a subject, an alternating electrical signal function of a microphonic signal picked up and amplified, characterized in that it comprises amplifying means of the microphonic signal consisting of an amplifier operating in class D, receiving as input said microphonic signal and delivering as output said alternating electrical signal.

 In particular, it will be possible to produce such a cochlear stimulation device in the form of a housing having the same appearance as that of a conventional hearing aid (but ensuring a completely different function, as indicated), therefore capable of '' be very well accepted by the subject: the psychological handicap of learning to hear by a deaf person will not be coupled with an aesthetic and functional handicap linked to the obligation to take with you a large box worn on the belt and connected by a thread to a subcutaneous or percutaneous coupling.

 In a first embodiment, adapted in particular to extra-cochlear stimulation, the amplifier output signal is delivered to the electrode via a step-up transformer coupled to means forming a low-pass filter .

 In a second embodiment, suitable in particular for intra-cochlear stimulation, the output signal of the amplifier is delivered to the electrode by means forming a low-pass filter, means being further provided for transposing the supply voltage of the output stage of the amplifier so as to increase the voltage excursion of said AC electrical signal applied to the electrode.

0
Other characteristics and advantages of the invention will appear on reading the detailed description below of two embodiments of the invention, made with reference to the accompanying drawings.

 FIG. 1 illustrates a first embodiment of the invention.

 Figure 2 shows the shape of the output signals for different microphone signal levels.

 FIG. 3 illustrates an example of the layout of the circuit of FIG. 1.

 Figure 4 shows a partial modification of the circuit of Figure 1 corresponding to a second embodiment of the invention.

0
FIG. 1 represents the general diagram of the cochlear excitation device according to the invention.

 As indicated above, this electronic device can be produced by modifying and adapting a commercially available product for a different application, which is, in the example shown, the circuit of the hearing aid marketed under the references P4 or P4x by the company BERNAFON. We will therefore only briefly describe the parts of this circuit that have not been modified, more details can be found in the technical manual for this device.

 The excitation circuit of the invention essentially comprises, as said above, an amplifier of the so-called "class D" type, therefore operating in switching mode.

 In addition to its particular operating mode, the particularisms of which will be used to ensure the coding of the cochlear excitation signal, this type of amplifier has a very high efficiency, of the order of 80%, which makes it particularly advantageous in the 'envisaged application, and allows to use it with an integrated power supply provided by a simple mercury battery type "button". There is therefore no need for a bulky external power supply, as is currently the case for the cochlear excitation devices worn on the belt and containing a relatively bulky battery necessary according to the low efficiency of the circuits used until now.

 This amplifier essentially comprises a control stage 10, a switching power stage 20, a power adjustment device ~ 30, a voltage source 40 (here a 1.5 V mercury battery), a voltage tripler 50 making it possible to deliver a no-load voltage of the order of 3.8 V and a load voltage of the order of 3 V, as well as a clock generator 60 ensuring the timing of the control circuit 10.

 In the case of the acoustic prosthesis (that is to say of a simple amplifier reproducing a microphonic signal applied in the conduit of the external ear of the subject), the amplifier supplies a headphone 70, represented in dashed lines on the drawing, which has been omitted in the case of the present invention: since it involves directly and electrically exciting the cochlea, acoustic stimulation of the external ear is of no help in overcoming total deafness) , and replaced by a set 80, which will be described later.

 The control circuit 10 essentially comprises an integrated assembly 11, with essentially a generator of input signals îîa and a sawtooth generator llb. The circuit îîa receives the signals Ue picked up by a microphone, and delivers, as a function of these signals, voltages U1 and U2, respectively positive and negative, of constant level, and a differential voltage Ud, varying as the microphone signal Ue.

 For its part, the sawtooth generator, supplied by the clock signal CK, delivers a sawtooth Uc whose extreme positive and negative amplitudes are symmetrical with respect to the reference voltage, at a recurrence frequency of l 'order of 32 kHz, corresponding to a periodicity of 30 Rs.

 The different signals U1, U2, Ud and Uc are then combined by three comparators 12, 13, 14 and by logic gates 15, 16, 17. The resulting signals obtained at the output of gates 16 and 17 are applied to the stage of power 20, which consists of four bridge-mounted transistors, namely two p-channel MOS type transistors 21, 23 and two n-channel MOS type transistors 22, 24.

 Between the respective common points of the transistors 21, 22 and 23, 24, a voltage Vo is obtained having the shape illustrated in FIG. 2.

 In FIG. 2a, the signal Vo is shown in the absence of modulation: it is then a recurrent signal of amplitude + (Vx -Vp), Vx and Vp being the potentials applied to the two other common points of the bridge (that is to say, respectively, between the transistors 21, 23 and 22, 24). This signal is in the form of a brief recurrent bipolar pulse, with a periodicity of the order of 30 Cls (frequency of the order of 32 kHz).

 When a modulating signal Ue appears, the signal changes in the manner illustrated in FIGS. 2b (for a modulating signal of moderate amplitude) and 2c (for a modulating signal of maximum amplitude): we can see that the pulse, which always keeps the same frequency of recurrence, sees its width increasing, as much for its positive alternation as for its negative alternation, in proportion to the amplitude of the modulating signal.

 The device further includes a power adjustment circuit making it possible to adjust the potential Vx, and therefore the maximum amplitude (both positive and negative) of the pulse delivered by stage 20, this so as to respect the dynamic range. hearing of the subject (hearing threshold and pain threshold). It will be noted that this characteristic is particularly useful in the case of electrical stimulation of the inner ear, because of the weak dynamics of the auditory nerve of the subjects to whom this technique is prescribed.

 This stage 30 comprises an operational amplifier 31 operating as a comparator, one of the inputs of which is connected at a point of a divider network 32 selected by the switch shown diagrammatically at 33; in practice, these are solid state switches controlled by appropriate programming of an integrated circuit.

This part of the device having no interest in the description of the invention, it will not be described in more detail.

 The inverting input of the operational amplifier 31 receives the potential Vx making it possible to define the amplitude of the pulse delivered by the amplifier. Finally, the output of this amplifier controls a p-channel MOS transistor 34. With such a configuration, the voltage Vx is clipped as soon as it exceeds a given threshold, determined by the programming of the switch 33, thus allowing compliance with the pain threshold specific to the subject.

 In the acoustic prosthesis of known type mentioned above, the signal Vo feeds an earphone 70 which acoustically reproduces a signal which is the image of the microphone signal Ue picked up and amplified, the inductance of the earphone acting as a filter for removing the 32 kHz component from the signal carrier.

 To be able to use such signals in another application than that for which the amplifier was designed - that is to say to use them in cochlear excitation instead of using them in simple supply of a hearing aid earpiece ~ , the circuit has been modified as illustrated, by removing the pre-existing earpiece 70 and replacing it with an exciter assembly 80.

 In the first embodiment illustrated in FIG. 1, this exciter assembly consists of a step-up transformer 81 whose primary is connected between the midpoints of the transistors 21, 22 and 23, 24, respectively, and whose secondary is connected to the two wires of the electrode implanted in the cochlea. A capacitor 82 is provided to filter the carrier at 32 kHz, which is undesirable for stimulation of the auditory nerve. This capacitor can for example have a value of the order of 68 to 100 nF.

 The electrodes used can be, for example, those used for the cochlear implant system sold under the name INERMD by the company SYMBION, Inc., or any other comparable cochlear stimulation electrode.

 It will be noted that, in addition to its subsidiary role of filtering the carrier at 32 kHz (in combination with the capacitor 82), the step-up transformer # of voltage 80 is essential to obtain at the output voltage levels compatible with a cochlea arous excitation, typically between 0.2 and 4 V depending on the subject.

 The device used only delivering at the output of the switching stage a voltage of + 1.3 V peak to peak (voltage which is suitable for the excitation of an acoustic transducer such as the earpiece 70), this voltage is insufficient for electrical stimulation. It is for this reason that the step-up transformer 80 is provided, the transformation ratio of which can be of the order of 1: 2.

The device thus produced has the following characteristics:
~ maximum output voltage Vs: 4 V peak to peak, adjustable from
200 mV to 4 V by programming switch 33 in operation
tion of the subject's own sensitivity,
~ maximum current output at the electrode: 2 mA,
nominal load impedance (equivalent impedance of
the electrode): 1.2 to 15 kQ,
- bandwidth: 200 to 3000 Hz,
- autonomy with a 1.5 V mercury battery: 240 hours
approximately (average value).

FIG. 3 shows the physical layout of the various elements of the circuit, produced here in the form of an adaptation of the prosthesis
BERNAFON P4 or P4X.

 As can be seen in this figure, the assembly 90 of the elements has been completely integrated into the body of the modified pre-existing prosthesis, so that it is thus possible, for the first time, to have a device for excitation of cochlear prosthesis in the form of a simple case of type "behind the ear".

 This embodiment in the form of an apparatus of the behind-the-ear type "is however not limiting, and other embodiments can be envisaged on the same model, for example identical type of intra-auricular or intra-conch apparatus as for their external appearance to conventional hearing aids, but no longer delivering an acoustic signal but an electrical signal.

 We can recognize in Figure 3 the various conventional elements of this assembly 90, namely the housing 91 of the battery 40, the printed circuit 92 carrying the various components, the potentiometer 93 for adjusting the power level, the flap 94 allowing d '' access the various programming parameters of the input signal and sawtooth generator, the conduit 95 forming an acoustic conduit (in the case of the acoustic prosthesis), the microphone 96, the integrated circuit 97 and the earphone housing 98.

 In the case of the invention, the earpiece which was in the housing 98 is removed and the space thus freed is used to house the step-up transformer 81 and the filtering capacitor 82.

 The wires connected to the electrodes can then be slid through the conduit 95, which previously served as an acoustic conduit and connected to the electrodes proper by percutaneous or transcutaneous route, in itself known manner.

FIG. 4 represents another possible embodiment of the invention, that is to say another possible adaptation of the prosthesis
BERNAFON P4 or P4X to transform it into a generator of electrical excitation signals for the cochlea. This second embodiment is more particularly suitable for stimulation of the intra-cochlear type, while the first is more suitable for extra-cochlear stimulation, taking into account the voltage and current levels respectively delivered in both cases. .

 In this second embodiment, no more transformer is used to raise the voltage, and the electrodes (excitation signal Vs) are then directly connected to the common points of the transistors 21, 22 and 23, 24, respectively, with simple placing in parallel a series LC cell formed by an inductor 83 and a capacitor 84 intended to filter the residual carrier at 32 kHz.

 To have output voltage levels compatible with cochlear excitation (that is to say up to 3.8 V) while output stage 20 normally delivers a voltage of the order of 1, 5 V, the voltage tripler circuit 50 already existing in the mentioned prosthesis is used, and intended initially to supply negative voltage to the various circuits of the control stage 10 (it will be noted that, in this example, the mass of the circuit is connected to the positive potential Vdd, the reference potential not being this potential but the potential Vp connected to the negative pole of the battery).

 To have a negative potential with respect to Vp, it is therefore necessary to provide an additional circuit, which will produce a voltage -Vss of the order of -3 V (- 3.8 V unladen) with respect to Vdd.

 However, this circuit alone is not sufficient to supply the electrodes because it is only dimensioned for a maximum output current of the order of 1 to 1.5 mA maximum, while the excitation of the electrodes requires a current of the order of 2.5 to 3 mA, values for which the voltage at the output of the voltage tripler 50 would collapse.

 To overcome this drawback, the voltage produced by the voltage tripler 50 is combined with that produced by the battery 40 (1.3 V relative to the ground). This is achieved by means of two diodes 85 and 86: the diode 85 is interposed on the supply line Vp to prevent the voltage tripler 50, which supplies the highest voltage, from coming into the cell 40; the diode 86 applies the voltage from this tripler 50 to the output stage 20, which makes it possible to obtain at the terminals of the electrodes a peak-to-peak voltage of 3.8 V, with a typical maximum current of 2 mA.

 For the practical realization of this circuit, it is advantageous to reuse for inductance 83 the coil 99 (FIG. 3) forming a telephone sensor present in all acoustic prostheses, and whose high inductance makes it possible to have an efficient LC cell. A value is then chosen for the capacitor 84 making it possible to obtain a cutoff frequency of the order of 4 kHz, with a slope of 6 dB per octave.

Claims (3)

 1. An electrical stimulation circuit for a cochlear prosthesis, delivering to a cochlear electrode, implanted in the inner ear of a subject, an alternating electrical signal depending on a microphonic signal picked up and amplified,  circuit characterized in that it comprises amplifiers of the microphone signal consisting of an amplifier (10, 20) operating in class D, receiving as input said microphone signal and outputting said alternating electrical signal (Vs).  2. The electrical stimulation circuit of claim 1, in particular for extra-cochlear stimulation, characterized in that the output signal of the amplifier is delivered to the electrode via a step-up transformer ( 81) coupled to means (82) forming a low-pass filter.  3. The electrical stimulation circuit of claim 1, in particular for intra-cochlear stimulation, characterized in that the output signal from the amplifier is delivered to the electrode by means (83, 84) forming low-pass filter, means (50, 85, 86) being further provided for transposing the supply voltage of the output stage (20) of the amplifier so as to increase the voltage excursion of said electrical signal alternative applied to the electrode.