EP0094407A1

EP0094407A1 - Apparatus and method for generating nerve or muscle stimulation signals

Info

Publication number: EP0094407A1
Application number: EP82903478A
Authority: EP
Inventors: Gerhard Leendert Van Rij
Original assignee: VERENIGING VOOR WETENSCHAPPELIJK ONDERWIJS OP GEREFORMEERDE GRONDSLAG
Current assignee: VERENIGING VOOR WETENSCHAPPELIJK ONDERWIJS OP GEREFORMEERDE GRONDSLAG
Priority date: 1981-11-23
Filing date: 1982-11-23
Publication date: 1983-11-23
Also published as: WO1983001742A1; NL8105297A

Abstract

Appareil et procédé de génération de signaux de stimulation à l'aide d'un générateur envoyant les signaux à une électrode de stimulation placée à proximité d'un nerf ou d'un muscle d'un être humain ou d'un animal pour stimuler ce nerf ou ce muscle, lequel générateur comprend des moyens générant des séries de trains d'impulsions de stimulus de polarité alternée, de sorte que des paires d'impulsions successives positives et négatives ont une largeur d'impulsion égale. Ce générateur comprend en outre des moyens de modulation de la fréquence des paires d'impulsions successives positives et négatives selon une fonction qui est au moins deux fois différenciable dans le temps.Apparatus and method for generating stimulation signals using a generator sending the signals to a stimulation electrode placed near a nerve or muscle of a human or animal to stimulate this nerve or muscle, which generator comprises means generating series of pulse train of stimuli of alternating polarity, so that pairs of successive positive and negative pulses have an equal pulse width. This generator further comprises means for modulating the frequency of the successive pairs of positive and negative pulses according to a function which is at least twice differentiable over time.

Description

Apparatus and method for generating nerve or muscle stimulation signals.

The invention relates to an apparatus and method for generating stimulation signals which are supplied from a generator to a stimulation electrode in the proximity of a nerve or muscle of a human being or an animal, to stimulate said nerve or muscle, which stimulation signals are comprising a number of impulse trains of stimulus impulses of alternating polarity, whereby the pairs of succeeding positive and negative impulses have an equal impulse width. Such a method is already described.

Methods for stimulating nerves or muscles to support said nerves or muscles if they are either insufficiently functioning or not functioning at all as a result of incomplete signal transmission or complete lack of signal transmission from the brains to the related muscles are already known. Thereby electrical impulses are supplied from a generator to a stimulation electrode positioned in the proximity of a nerve or muscle of a human being or an animal.

The stimulation impulses are transferred from the generator to the muscle or nerve by means of electrodes, which are positioned in the proximity of the nerve or muscle to be stimulated. These electrodes can be manufactured from several metals such as Au, Pt, Ti, Ta, Zn, Re, W, Ni or alloys thereof. In recent years also an alloy of platinum with 10% iridium is used very successfully.

The stimulation of a nerve or muscle through metal electrode is, when looking at it as a physical-chemical system, a very complex process. In the metal of the electrode the transport of charges is carried out by electrons, however into the tissue and in the tissue fluids the transport of charges is carried out by ions. The surface of the stimulation electrode is never homogeneous. Damages, mechanical tensions and differences in crystal structure are causing the forming of micro areas each with different characteristics. Furthennore it is possible that the surface as a whole or in part is covered with an oxidation layer and/or with a coating of organic material. The layer between said electrode and the physiological medium is in constant dynamic equilibrium; currents are running from and to the surface of the electrode; the sum of these currents is zero when no external voltage is connected. Said transition layer comprises at least two sub layers of which the inner layer (the so called Helmholtz-layer), positioned near the surface of the electrode, contains ions of the body fluid which have lost their hydration layer and which are able to become specifically bounded to said surface. The outer layer, the so called diffusion e layer, has a more diffuse character. Currents are also running into the surface self, i.e. between the several micro areas, which have a mutually different electrical potential because of their structural differences. Layers onto the surface are not permanent, they are constantly formed and distructed, which means that the micro areas are changing in character. The stimulation current has to run through this physiological chemical complex.

The reactions which can take place at a platinum stimulation electrode in a chlorine ions containing medium are known for themselves. These reactions can be subdivided into irreversible reactions and reversible reactions.

When the amount of charge supplied per stimulus impulse increases too much, then irreversible reactions will take place which can cause forming of toxic substances or substances which are detrimental to the effect of the method. To avoid that the following requirements have to be fulfilled:

1. the amount of charge per stimulus impulse should be < 3 C per real surface unit in m², 2. the current density should be ≤ 300 A per real surface unit in m².

3. Shortly after an amount of charge +Q an identical amount of charge -Q should be supplied.

4. if possible the electrode should first be used as cathode. Furthermore the stimulation electrode, viewed as electric component, can in many regards be considered as a non linear system. The relation between the voltage onto and the current through the electrode is partly non linear, and also the relation between the voltage or current and the frequency is non linear. Using- a voltage with constant frequency the relation between voltage and current, or better between voltage and current density, is only linear in a restricted area above which the relation becomes non linear. The divid ing line between both areas is frequency dependent. With a frequency of 0 Hz (direct current/voltage) there is no linear area, above approximately 10 kHz the dividing line between linear and non linear area is constant. The linear area is of great importance for stimulation purposes. In this area only reversible reactions are taking place which do not change the medium in which the electrode is positioned, even over a longer time period, and which also do not effect the electrode itself. The size of this area is determined by the choice of the material for the electrode, the surrounding medium and the frequency. If the electrode is working in the non linear area, then said electrode is behaving different for positive and negative voltages; the electrode shows the characteristics of a bad rectifier. That means that also when the electrode is controlled by a voltage or current which is symmetrical in relation to the zero level, there will be a dc component.

When stimulation has to be carried out during a longer period then the already above mentioned requirements should be fulfilled. That means shortly: a restricted amount of charge ( < 3C/m²) per stimulus impulse and equilibrium between the charges of the positive and negative stimulus impulses.

In general the charge restricting requirement can be fulfilled simply by chosing the effective surface of the stimulation electrode large enough. The complete charge equilibrium requirement however still causes problems.

Initially stimulation impulses of one and the same polarity were used generated with a predetermined frequency, amplitude and impulse width and supplied to the electrode through a dc coupling. It was noted thereby that as result of the fact that the pulses are very strongly out of balance, fatiqueness phenomena are appearing resulting in a strong decrease of the stimulation effect. After a resting period however said fatiqueness phenomena were disappeared and the stimulation could be resumed.

If the Fourier-sequence of such an impulse train is developed one will find a dc voltage term. Beside the fatiqueness phenomena also affection of the electrodes is noted, which affection is caused by the direct current. Each direct current term in the Fourrier sequence of a stimulus train is therefore disastrous.

Initially the solution seemed to be a capacitive coupling between the generator and the electrode. The charge supplied with the impulses is removed in the time periods between said impulses. In the Fourier sequence of such a stimulus train there Is no direct current term. It looks as if the balanced situation is guarantied in this way.

However, a stimulation electrode is in the frequency domain a non linear element. The electrode reacts into a different way onto impulses with equal charge but with different impulse width, even when the amplitude of said impulses is such that the electrode is operating in the amplitude linear area. In practice the width of the stimulation impulses (of for instance 100.10^-6-300.10^-6s) is much smaller than the interval period between said impulses (of for instance 10.10^-3-50.10^-3s). The frequency spectrum of the stimulation impulse differs clearly from the spectrum of the impulse which is represented by the interval period. The seeming balance is therefore not complete and gradually the electrode as well as its environment are changing. That explains the still appearing fatiqueness phenomena.

It is known that this problem can be partly solved by using biphase stimulation signals consisting of a series of pulse trains of stimulation impulses with alternating polarity, equal impulse width, equal interval time and equal restricted charge. A disadvantage of an impulse series of this type is, that at the beginning and at the end of each pulse train the speed, with which the stimulated muscle has to react changes abruptly from zero to a certain value respectively from a certain value to zero, whereas furthermore at the beginning and the end of the stimulating pulse train the acceleration theoretically becomes infinitely.

An object of the invention Is now to provide a method and an apparatus for generating nerve or muscle stimulating signals not subjected to the above mentioned disadvantages.

Said object is achieved with a method of the- type described in the first paragraph in which the charge content and the frequency of pairs of successive positive and negative stimulus impulses are modulated according to a function which Is at least twice time differ- entiable. The result thereof is that the speed as well as the acceleration of the physical system, which itself has a certain inertion, maintains a finite value within a finite time interval.

According to a preferred embodiment of the invention the raodul ation of the charge content of the impulses is realized by amplitude modulation of the pairs of impulses .

According to a further embodiment of the method according to the invention the modulation of the charge content of the Impulses is realized by pulse width modulation of the pairs of impulses. According to a further preferred embodiment of the method according to the invention the twice time differential function f(t) is of the form: f(t) = a.e^xt + b.

To have any effect a stimulus impulse of a predetermined impulse width should have a certain minimum amplitude. If the impulse trains are modulated in correspondence with the invention such that the first impulses in each impulse train have a value beneath said minimum threshold value, then these first stimulus impulses in each impulse train have no effect, and furthermore it appears in practice that the generation of said non effective stimulus impulses decreases the influence of the first stimulus impulses above said threshold value. According to a preferred embodiment of the invention the modulation is carried out such that the first stimulus impulse in each impulse train has an amplitude above a predetermined threshold value. To achieve a fluent contraction of the stimulated muscle the impulse frequency of the stimulating impulses is in a preferred embodiment of the method according to the invention at least 20 Hz.

The maintain an undisturbed charge balance preferably measures are taken to choose the number of impulses in each impulse train such that the number of positive impulses in each impulse train equals the number of negative impulses in each impulse train.

To achieve a balanced frequency spectrum it is preferred that the interval times between both impulses in each impulse pair are equal. The effect of the stimulus is, apart from the temporal addition, determined by the amplitude and the pulse width of the stimulation impulse width. There is a hyperbolic relation between the amplitude and the impulses. According to a preferred embodiment the impulse width of the stimulation impulses is determined from said hyperbolic amplitude-impulse width-characteristic such, that per impulse a minimum energy amount is supplied to the muscle or nerve. The object of the invention is furthermore achieved with an apparatus for generating stimulation signals comprising a generator supplying said signals to a stimulation electrode positioned in the proximity of a nerve or muscle of a human being or an animal, to stimulate said nerve or muscle, which generator comprises means for generating series of impulse trains of stimulus impulses of alternating polarity, whereby pairs of succeeding positieve and negative pulses have an equal Impulse width, which is according to the invention characterized in that said generator further comprises means for modulating the frequency of the pairs of succeeding positive and negative impulses according to a function which is at least twice time differentiable. Further characteristics of apparatusses employing the intensive principles are described in the subclaims and will be explained in more detail in the following description of a preferred embodiment of the Invention, whereby reference is made to the attached, drawing.

Fig. 1 illustrates a stimulation signal consisting of a number of impulse trains modulated in correspondence with the invention.

Fig. 2 shows in detail a number of impulses near the beginning and near the end of each impulse train. Fig. 3 illustrates a number of impulses near the beginning and near the end of a modulated Impulse train.

Fig. 4 illustrates a part of a coded pulse series. Fig. 5 illustrates the schematical general diagram of a generator for generating stimulation signals. Fig. 6 Illustrates a detailed circuit diagram of the main oscillator and function generator of Fig. 5.

Fig. 7 illustrates a detailed circuit diagram of the adding and sampling circuits in Fig. 5.

Fig. 8 illustrates a detailed diagram of the amplitude, frequency and pulse width modulators in Fig. 5.

The Figs. 9 and 10 show wave forms on various points in the Figures 5-8. Fig. 11 illustrates a transmitter for coding and transmitting signals.

Fig. 12 illustrates a receiver for receiving and decoding signals. Fig. 13 illustrates a number of signals on various points In Figs.11 and 12.

Fig. 1 shows a stimulation signal comprising a number of succeeding impulse trains appearing each into an interval T_I, interchanged by resting periods T_E. The stimulation is carried out in the interval T_I. After the stimulation at the end of the interval T_I the physical system is left alone during the interval T_E, so that the system is slowed down by its own inertion and turns to the resting position. The length of the intervals T_I and T_E is dependent onto the physically determined stimulation and resting periods. If the method or apparatus according to the invention is for instance used for stimulating one or more of the motoric nerves influencing the breathing muscles, for instance diaphragm stimulation or stimulation of the nerves belonging to the midriff (nn.phrenici), then the Intervals T_I and T_E will be determined by the desired breathing pattern. The inhaling movement is made during T _I and the exhaling movement as well as the rest period between the breathing movements fits into T_E.

Fig. 2 shows in more detail a number of impulses near the beginning and near the end of each impulse train. In said figure the amplitude of each impulse is denoted with A₁, A₂.... A_n, the impulse width is denoted with t_p and the impulse interval time is denoted by t_i1, t_i2... t^n. As appears from this figure the amplitude, the impulse width and the impulse interval time is equal for pairs of succeeding impulses. As furthermore appears from the figures 1 and 2 the amplitude as well as the frequency of each impulse train is modulated, maintaining the above mentioned relation between the pairs of suceeding positive and negative impulses, using a twice time differentiable modulating function. Said function is preferably of the form f(t) = a.e^xt + b. However it is also possible to use a sine function or a function of the form f( t) = a . tⁿ.

Furthermore in Fig. 1 a positive and negative threshold value d is indicated. It appears in practice that stimulating Impulses having an amplitude A< d do not have any influence. With such impulses there is no contraction in the stimulated muscle. Only if Impulses with an amplitude equal to or exceeding said threshold value d are used, then the muscle will react onto said impulses.

Preferably the first impulse A₁ in each impulse train will have a value A ₁ d so that the stimulation is in fact increasing from approximately zero according to an e power function. Because said e power function is twice time differentiable also the speed and the acceleration will gradually increase so that the whole physical system is not forced to carry out a movement which because of the inertion of the system in fact is impossible.

It will be clear that when the impulse interval time t_i becomes too large, the succeeding stimulating impulses will cause a stepped contraction because the stimulated muscle will have the opportunity after the contraction caused by each stimulus impulse to relax at least partly. Therefore the interval time in a preferred embodiment should be smaller than 50 ms , or in other words the impulse frequency should be at least 20 Hz. To supply and remove the same amount of charge preferably measures are taken to generate an equal number of positive and negative Impulses in each impulse train.

The effect of the stimulus is, apart from the temporal addition, determined by the amplitude or the pulse width of the stimulating impulses. If the response of the stimulated system is constant then there is a hyperbolic relation between the amplitude and the Impulse width A = a + b, in which A = stimulating amplitude, = Impulse width and a and b are constant factors. The supplied amount of energy for a linear system Is proportional with a² 'and one can prove that the supplied amount of energy E has a minimum when ' = a/b. Therefore the stimulating effect may be optimized by experimentally determining the hyperbolic relation between A and ~ and accounting the optimum therefrom. The optimal value of is that value of the impulse width whereby for a predetermined response the minimum amount of energy has to be supplied.

In the above mentioned an example is given In which the amplitude of the stimulating Impulses is modulated. It will be clear that instead of the amplitude also the impulse width can be modulated such, that the first impulse of each impulse train has a small width and the width of the succeeding impulses is increasing according to an e power function. The amplitude of each of the impulses is in that case equal and can be determined at a value above the in Fig. 1 illustrated threshold value d.

Fig. 3 illustrates a number of impulses near the beginning and near the end of an impulse train modulated in this way. The amplitude of each of the impulses is A ^ d, the impulse width of the im pulses is denoted with \ , ii _' " n and the lmPulse interval time is denoted with t_il, t_i2,... tin.

If the method according to the invention is applied to an electrode implanted in the body of a human being or an animal, and said electrode is connected to a wireless receiver receiving signals from the generator, then preferably the stimulating signals generated by said generator are coded in such a way that disturbances onto the transmission path between the generator and the electrode are eliminated. Furthermore one has to take into account the energy consumption of the receiver circuit. Especially when receivers have to be used during a extended period, then it is important that the energy consumption of the receiver is as low as possible.

According to a preferred embodiment the signal supplied by the generator is before transmission to the receiver coded into a time code modulated signal using a start pulse, a stop pulse and a number of clock pulses, whereby the time distance between the succeeding start pulses determines the frequency, the time distance between start and stop pulse is determining the momentaneous impulse width and between each of the further clock pulses an information pulse may be present indicating together the impulse amplitude of the stimulating impulse.

With this method of coding it is possible to switch the whole receiver circuit with the exception of the front end receiver stages into a little or no energy consuming state until the moment that a start impulse is received. Said start impulse activates the receiver circuit during a predetermined period in which the succeeding information bits can be received.

Fig. 4 illustrates a part of a coded pulse series between two start pulses, indicated by "start". After the first start pulse there is in this example a period of 30 μsec increased by the Impulse width of the coded impulse. Thereafter a stop pulse Is received determining the end of the above mentioned period. If said stop pulse is not received within a predetermined interval, then the receiver can be deactivated. The receiver is also deactivated after reception of the last information bit.

After reception of the stop pulse a number of equidistant clock pulses ell, c12,... c110 are received each of which may be followed within a predetermined period of 9 μsec by an information pulse. In this embodiment the most significant information bit msb = b10 is used as sign bit. The succeeding information bits b9... b1 are representing in binary coding the amplitude of the coded impulse. In the illustrated example said information bits represent the decimal value +339.

The interval time between the succeeding stimulus Impulses is determined by the time between two successive start pulses.

In the following part of the description a practical embodiment of a circuit for generating stimulation signals in agreement with the invention is described with reference to the further Figures 5-10 of the drawing.

Fig. 5 illustrates a general diagram of the circuit for generating stimulation impulses in agreement with the invention. The configuration comprises a main oscillator 1 and a function generator 2. The main oscillator 1 is a linear sawtooth generator, generating a linear sawtooth voltage (MO in Fig. 9) of which the frequency is adjustable. Said linear sawtooth voltage is supplied to the function generator 2, delivering a number of differently shaped output voltages at a corresponding number of outputs. These outputs voltages of the function generator 2, the voltages ± B, ± Z, ± K and ± e can be described in mathematical form as follows: B is a voltage which is constant during tg and is zero during Tg-tg:

B = ±E(H(t+nTg)-H(t+nTg-tg)) (square wave voltage) Z is a voltage which is linearly increasing during tg and is zero during Tg-tg:

Z = ± Et(H(t+nTg)-H(t+nTg-tg)) (sawtooth voltage) K Is a voltage which is increasing according to a square law during tg and is zero during Tg-tg : K = ± Et²(H( t+nTg)-H( t+nTg-tg) ) e is a voltage which Is logarithmicly increasing during tg and is zero during Tg-tg : e = ±E(I-e^-t (H(t+nTg)-H(t+nTg-tg) ) . ( e-power)

The time constant can be varied using a switch and furthermore n

Fig. 6 illustrates a more detailed diagram of the main oscillator and function generator 1 respectively 2. The sawtooth voltage MO in Fig. 9 is generated in the circuit built around icla, and the frequency of said sawtooth voltage is adjustable by means of P1. In a comparator, built around ic2a, said sawtooth voltage is compared with a dc voltage of which the amplitude is adjustable by means of P2. At the output of said comparator ic2a a square wave voltage having an on time of tg (0 ≤ tg ≤ Tg) is generated. Said square wave voltage is brought to a fixed amplitude of E Volt in the buffer amplifiers ic2b and ic3a, delivering at their outputs the square wave voltages +B and -B.

In the Integrator, built around ic3b the d.c. voltage derived from PI is converted into a linear sawtooth voltage. The integration time in this circuit is adjustable by means of P3. The integrator is maintained in the reset state during tg ≤ t ≤ Tg. Because both potentiometers P3 and P2 are coupled the normalized ultimate amplitude E of the sawtooth voltage Z is independent of the time period tg. Said normalized sawtooth voltage is supplied at the outputs of the buffer amplifiers ic4a and ic4b (+Z and -Z).

The signal -Z is furthermore supplied through an adjustable potentiometer P4 to a further integrator, built around ic5a of which the integration time is adjustable by means of P5. The potentiometer P4 is coupled to P1 and the potentiometer P5 is coupled to P2. The result thereof is that at the output of said integrator a variable voltage +K, varying according to a square law is generated of which the highest amplitude is independent of the main oscillator frequency fg and independent of tg. The integrator is maintained in the reset state during tg ≤ t ≤ Tg. By means of the buffer amplifier ic5b also the inverted voltage -K. is generated. Both signals +K and -K are normalised to a voltage of E Volt.

The square wave voltage +B Is supplied to an RC-network of which the time constant is adjustable by means of P6 and the switch SI by means of which switch- one of a number of capacitors can be selected. The potentiometer P6 is coupled to P2. The resulting voltage e, which Is varying according to an e power has, notwithstanding a varying tg, a fixed shape. During the period tg ≤ t ≤ Tg the RC-network is short circuited. The wave forms +e and -e are generated at the output of the buffer amplifiers ic6a and ic6b.

The generated voltages +3, -B, +K, -K, +Z, -Z, +e and -e are supplied to a number of summing amplifiers 3, 4, 5 and 6 in Fig. 5. The summing amplifier 3 delivers at his output the signalΣAM+, the summing amplifier 4 delivers at his output the signal ΣΑM-, the summing amplifier 5 delivers at his output the signal ∑PBM and the summing amplifier 6 delivers at his output the signal Σ.FM. Said signals are mathematically defined as follows: ΣAM+ = ± a₁B ± b₁Z ± c₁K ± d₁e ∑AM- = ± a₂3 + b₂Z ± c₂K ± d₂e ∑PBM = ± a₃B + b₃Z ± c₃K ± d₃e ΣFM = ± a₄B + b₄Z ± c₄K ± d₄e Said resulting voltage signals 2ΑM+, ΣAM-, Σ.PBM and Σ.FM can be considered as modulates of respectively the positive amplitude, the negative amplitude, the impulse width and the impulse frequency. Said four modulates are sampled using sample-and-hold amplifiers 7, 8, 9 and 10 in Fig. 5. The sampling step is necessary for generating pairs of impulses of which the amplitude (positive and negative), the impulse width and the impulse frequency are equal.

Fig. 7 illustrates a more detailed diagram of the summing- and sampling-circuits, whereby it is denoted that only one of the summing circuits is illustrated which is however representative for the further circuits. The summing amplifier in Fig. 7 is built around an operational amplifier (four of which are actually present Indicated by icl0ab, 11ab ), and the signals ± B, ± Z, ± K and ± e are supplied through a resistor network to one input of said summing amplifier through adjustable potentiometers P1-4 for adjusting the amplitude of each of said signals. At the output of said summing amplifier the respective output signal ΣAM+, ΣMA -, ΣFM and ∑PBM are generated , which eventually can be inverted in a further buffer stage. Furthermore the sampling stages for sampling ΣFM and XPBM illustrated in Fig. 7 built around sample-and-hold amplifiers, indicated by sh3,4.

The signal from the sample-and-hold circuit 10 (Fig.5) is supplied to a frequency modulator 11, embodied as a linear sawtooth oscillator of which the frequency is proportional to the Input voltage ∑FM' , the sampled frequency modulator output ΣFM, delivered by the sample-and-hold stage 10. Said frequency modulator (11) delivers at one of the outputs a voltage RES2, which is supplied to the pulse width modulator 12 in Fig. 5. At another output said frequency modulator generates the sampling signals for the sample-and-hold stages 7, 8, 9 and 10.

The pulse width modulator 12 comprises a comparator in which the output voltage PBM' of the sample-and-hold stage 9 is compared with an integrated dc voltage. The integrator used for integrating said dc voltage is periodically reset by the signal RES2. The result is that said pulse width modulator delivers at his output a voltage PB, of which the frequency is linearly proportional to ΣFM' (the sampled signal ∑FM) and of which the pulse width is linearly proportional to 1PBM' ( the sampled ∑PBM) . The signal PB is used as switching signal for the amplitude modulators 13 and 14 in Fig. 5. Especially said signal PB determines the time in which 2AM+'( the sampled ∑AM) respectively ΣAM-' (the sampled ∑AM-) are switched through said amplitude modulators. At the outputs of said amplitude modulators impulse shaped signals are generated of which the amplitude is determined by ΣAM' , the width is determined by ∑PBM' and the frequency is determined by ΣFM' . By means of analogue switches one can select in this illustrated embodiment of the apparatus between positive, negative, alternating symmetrical or alternating asymmetrical impulses. The impulse voltage is by means of a voltage to current convertor 16 converted into an impulse current forming the output of the whole system.

Fig. 8 provides more details of the amplitude-, frequency- and pulse width modulators 14, 13, 12 and 11 of Fig.5. In Fig. 8 also the sample-and-hold stages 7 and 8 are illustrated in more detail, built around the sample-and-hold amplifiers shl and sh2, which are combined into the two amplitude modulators (13, 14 in Fig. 5). In the middle and lower part of Fig. 8 the signals ΣPBM' and ∑FM' are used in the already described way to generate the signal PB. The frequency modulator 11 comprises as already said a linear sawtooth generator built around the icδa and the frequency of the generated sawtooth voltage is proportional to the input voltage ΣFM' . The output voltage of said sawtooth generator is through a buffer stage ic8b supplied to the monostable multivibrator MSlb which generates the reset signal RES2. The flip flop FFla is used for generating the sampling signal SMPL.

The pulse width modulator 12 comprises a comparator built around ic9b for comparing the voltage ∑PBM' with an Integrated d.c. voltage. The integrator integrates the d.c. voltage as his negative input. The signal PB at the output of ic9b is supplied to the amplitude modulator configuration and used therein for switching the amplitude modulator circuits. The output voltages of the summing circuits 3 and 4 of Fig.5 are supplied to the sample-and-hold stages built around shl and sh2 and the output voltages thereof ∑AM+' and∑AM-' are switched through under control of the signal PB to the output buffer stages built around ic7a and ic7b.

The Figures 9 and 10 illustrate several wave forms on various points in the detailed diagram of the Figures 6, 7 and 8. The used reference symbols in Figs. 9 and 10 are identical to the reference symbols used in the Figs. 6, 7 and 8.

As already remarked above and described with reference to Fig. 4 it is also possible to use the generator described In the Figs. 5-10 as external stimulator in combination with an internal receiver/ generator in which in fact the same or almost the same stimulator impulse as generated in the external stimulatgor are regenerated on the bases of a time coded signal received from said external stimulator. Fig. 11 illustrates the transmitter used for generating the time coded control signals based on the output voltages of the external generator PB, ΣAM+ and ΣAM-. The configuration of Fig . 11 comprises a number of monostable multivibrator circuits MS1a,b, MS2a,b, MS3a,b, MS4a,b, an analogue to digital convertor A.D.conv, a flip flop FF1 and furthermore a number of or-gates, and-gates and nand-gates as well as a number of transistors, capacitors, resistors and further elements.

The transmitter generates the time coded messages which are already discussed In more detail with reference to Fig. 4. The functioning of said transmitter is not described in detail because it is assumed that the expert in this field finds in Figs. 11 and 13 and in the following list of components enough information to build a properly functioning circuit. The same applies to the receiver/ internal stimulator illustrated in Fig. 12.

Fig. 13 shows in the upper half section a number of signals at various points in the circuit diagram of Fig. 11. This signals are 1. incoming signal PB 1 μs Q_1a

2. start pulse 1 μs Q1

3. transfer pulse 1 μs Q1b

4. PB delayed pulse 30 μs Q_3a

5. stop pulse 1 μs Q_3b (conversion instruction) 6. normalized clock pulse 1μs Q_4a

7. delayed clock pulse 9 μs Q_2a

8. sampling pulse 1 μs Q_2b

9. serial output A/D convertor

10. amplitude data pulse output and-gate la 11. complete transmitting 1 μs Q4 series

The circuit diagram of the receiver/internal stimulator is illustrated in Fig.12. In the lower left hand corner of said figure the power section is illustrated consisting of a resonance circuit receiving the HF energy transmitted by the transmittor, which HF energy is rectified and used as supply voltage for the remaining circuits of the receiver. The actual receiver comprises two monostable multivibrator circuits MS5a,b and MS6a,b, a flip flop circuit FM2, a digital to analogue convertor DAconv and a number of operational amplifiers ic1, ic2 and ic3 and further small components.

Fig. 13 illustrates In the lower section a number of signals appearing at various points in this receiver circuit. These various The digital input signal to the digital/analogue convertor is decoded and the output signals of said digital/analogue convertor are supplied to operational amplifiers which are only activated during the switch-on period of the stimulation impulse. For the remaining periods they are switched to a very low energy consumption level, so that the voltage delivered by the power circuit is sufficient to supply to whole receiver circuit.

In the abvove described practical embodiment of the system for generating stimulating impulses the following components are used:

NAND 1 SN7400 QUAD 2 INPUT NAND GATE

(TTL)

NAND 2 SN75452 DUAL 2 INPUT NAND GATE,

OPEN COLLECTOR (TTL) AND 1-2 CO4081 QUAD 2 INPUT AND GATE

( CMOS )

OR 1 CD4073 DUAL 4 INPUT OR GATE

(CMOS)

FF 1,2 CD4028 DUAL FLIPFLOP (CMOS) INV CD4049 HEX INVERTER (CMOS)

SW, a,b AD7510 QUAD SPST SWITCH A/D AD7570 A/D CONVERTER D/A AD7522 D/A CONVERTER T1-5 VN66AF VMOS POWER FET It is remarked that the above described practical circuit was in fact used during a number of tests and for that reason the complete apparatus has a number of features and can generate a number of output signals which are in a practical embodiment not all necessary. Therefore the described circuit can be simplified in many ways, for instance by leaving out superfluous circuit sections, decreasing the number of selection possibilities, etc. Furthermore specific components are mentioned above, which are used in the described embodiment, however, it will be clear to the expert in this field that other components and adapted circuits can be used.

Claims

C L A I M S .

1. Method for generating stimulation signals which are supplied from a generator to a stimulation electrode positioned In. the proximity of a nerve or muscle of a human being or an animal, to stimulate said nerve or muscle, which stimulation signals are comprising a series of impulse trains of stimulus impulses of alternating polarity, whereby pairs of succeeding positive and negative pulses have an equal impulse width, characterized in that the charge contents and the frequency of the pairs of succeeding positive and negative impulses are modulated according to a function which is at least twice time differentiable.

2. Method according the claim 1, characterized in that the charge content modulation is realized by amplitude modulation of the pairs of stimulus impulses.

3. Method according to claim 1, characterized in that the charge content modulation Is realized by pulse width modulation of the pairs of stimulus impulses.

4. Method according to one of the preceding claims, characterized in that the twice time differentiable function f(t) has the form: f(t) = a.e^xt + b.

5. Method according to one of the preceding claims, characterized in that the first stimulus impulse of each impulse train has an amplitude above a predetermined threshold value.

6. Method according to one of the preceding claims characterized in that the stimulus impulses have a minimum impulse frequency of 20 Hz.

7. Method according to one of the preceding claims, characterized in that the number of positive impulses in each impulse train equals the number of negative impulses in each impulse train.

8. Method according to one of the preceding claims, characterized in that the Impulse width or the amplitude of the stimulating impulses is determined from the amplitude-Impulse width-characteristic such, that per. impulse a minimum amount of energy is supplied to the nerve or muscle.

9. Method according to one of the preceding claims, characterized in that the interval times between both pulses of an impulse pair are equal.

10. Method according to one of the preceding claims, used with a generator and a separate receiver coupled to the electrodes, characterized in that the signal generated by said generator Is coded before transmission to said receiver into a time code modulated signal having a start pulse, a stop pulse and a number of clock pulses, whereby the time difference between the succeeding start pulses determines the frequency, the time distance between start and stop pulse dertermines the momentaneous impulse width and between each of the clock pulses an Information pulse may be inserted together determining the impulse amplitude of the stimulus impulses.

11. Apparatus for generating stimulation signals comprising a generator supplying said signals to a stimulation electrode positioned in the proximity of a nerve or muscle of a human being or an animal, to stimulate said nerve or muscle, which generator comprises means for generating series of impulse trains of stimulus impulses of alternating polarity, whereby pairs of succeeding positive and negative pulses have an equal impulse width, characterized in that said generator further comprises means for modulating the frequency of the pairs of succeeding positive and negative impulses according to a function which is at least twice time differentiable.

12. Apparatus according to claim 11, characterized in that the means for modulating the charge content are means for modulating the amplitude of the pairs of stimulus impulses.

13. Apparatus according to claim 11, characterized in that the means for modulating the charge content are means for modulating the pulse width of the pairs of stimulus impulses.

14. Apparatus according to one of the preceding claims 11-13, characterized in that the twice time differentiable function f(t) used in the modulating means has the form f(t) = a.e^xt + b.

15. Apparatus according to one of the preceding claims 11-14, characterized in that the generator comprises means for determining the amplitude of the first stimulus impulse of each impulse train above predetermined threshold value.

16. Apparatus according to one of the preceding claims 11-15, characterized in that the generator comprises means for restricting the minimum impulse frequency of said stimulus Impulses to 20 Hz.

17. Apparatus according to one of the preceding claims 11-16, characterized in that the number of positive Impulses in each impulse train equals the number of negative impulses in each Impulse train.

18. Apparatus according to one of the preceding claims 11-17, characterized in that the imnpulse width or the amplitude of the stimulating Impulses is adjusted from the amplitude-impulse width-characteristic such, that per impulse a minimum amount of energy is supplied to the nerve or muscle.

19. Apparatus according to one of the preceding claims 11-18, characterized in that the means for generating said series of impulse trains are functioning such that the interval times between both pulses of an impulse pair are equal.

20. Apparatus according to one of the preceding claims 11-19, in which the generator is cooperating with a separate receiver coupled to the electrodes, characterized in that the generator comprises coding means for coding the generated signal before transmission to said receiver into a time coded modulated signal having a start pulse, a stop pulse and a number of clock pulses, whereby the time difference between the succeeding start pulses determines the frequency, the time distance between start and stop pulse determines the momentaneous impulse width and between each of the clock pulsesan information pulse may be inserted together determining the impulse amplitude of the stimulus impulses.