WO1999006106A1

WO1999006106A1 - Apparatus for electrical stimulation of a body

Info

Publication number: WO1999006106A1
Application number: PCT/GB1998/000470
Authority: WO
Inventors: Christoffel Swanepoel
Original assignee: Neuro Stim 2000 (Proprietary) Limited; Bannerman, David, Gardner
Priority date: 1997-07-31
Filing date: 1998-02-16
Publication date: 1999-02-11
Also published as: AU6223298A

Abstract

Apparatus for electrically stimulating the body of a subject comprises first and second waveform generators. The first waveform generator produces a pulse waveform, which is integrated to convert it to a triangular waveform and to add a DC offset related to the amplitude of pulse triangular waveform. The second waveform generator produces a lower frequency square wave output which modulates the output waveform selectively. An output circuit controls the current delivered by the apparatus to a safe value. By adjusting the magnitude of the output until the AC waveform generates a tingling sensation, the subject effectively sets the average current delivered to the site of application at a safe but effective level.

Description

APPARATUS FOR ELECTRICAL STIMULATION OF A BODY

BACKGROUND OF THE INVENTION

THIS invention relates to a method of electrically stimulating the body of a subject and to apparatus for carrying out the method.

Various devices for electrical stimulation of the human body have been proposed, whether for purposes of pain control or passive exercise. Existing devices for pain control generally operate on the principle that a bipolar waveform, having a minimal or no average DC component, is applied to the body of a subject.

It is an object of the invention to provide an alternative method and apparatus of this general kind. SUMMARY OF THE INVENTION

According to the invention there is provided a method of electrically stimulating the body of a subject comprising generating a first time- varying electrical waveform; generating a resultant waveform having a time-varying component and a DC offset from the first time-varying waveform, the magnitude of the DC offset being related to the magnitude of the first time-varying waveform; and applying the resultant waveform to the body of the subject, wherein the resultant waveform has a time-varying component with a frequency in the range 50 to 200 Hz and a peak to peak amplitude in the range 0 to 30V, wherein the DC offset is in the range 0 to 15V, and wherein the current due to the application of the resultant waveform to the body of the subject is in the range 0 to 1200μA.

The resultant waveform preferably has a frequency of about 125Hz and a peak to peak amplitude of about 8V, and the average current due to the application of the resulting waveform to the body of the subject is preferably less than 750μA.

The resultant waveform may have a time-varying component which is substantially triangular, with a relatively steep rising edge and a relatively shallow falling edge.

The method may further include generating a second time-varying waveform having a frequency in the range 0.5 to 3Hz, and utilizing the second waveform to modulate the resultant waveform. The second waveform is preferably a rectangular waveform.

Further according to the invention there is provided apparatus for electrically stimulating the body of a subject, the apparams comprising:

a first waveform generator arranged to generate a first time-varying waveform having a frequency in the range 50 to 200Hz;

voltage control means for generating a resultant waveform having a time-varying component and a DC offset from the first waveform, the DC offset having a magnitude related to the amplitude of the first time-varying waveform, and the resultant waveform having a time- varying component with a peak to peak amplitude in the range 0 to 30V and a DC component with a magnitude in the range 0 to 15V;

output means for applying the resultant waveform to the body of a subject; and

current control means for adjusting the magnitude of the current resulting from the application of the resultant waveform to the subject between 0 and 1200μA.

The apparams may include a second waveform generator arranged to generate a second waveform having a frequency in the range 0.5 to 3Hz, and switch means arranged to modulate the resultant waveform with the second waveform.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic block diagram of apparams for electrically stimulating the body of a subject according to the invention;

Figure 2 is a circuit diagram of the apparams, including an amplimde control circuit and a pulse shaper circuit of the apparatus;

Figure 3 is a schematic diagram of a test load used to calibrate the apparams;

Figure 4 is a diagram illustrating an output waveform of the amplitude control circuit of Figure 2;

Figure 5 illustrates the no-load output waveform of the apparams;

Figure 6 illustrates the output waveform of the apparams into the test load of Figure 3; and

Figure 7 is a schematic diagram illustrating a pulsed output waveform of the apparams. DESCRIPTION OF AN EMBODIMENT

The present invention provides apparams which generates a time-varying electrical waveform on which is superimposed a variable DC offset, so that a unipolar time- varying resultant waveform is obtained. The time- varying waveform can be modulated by a much lower frequency second waveform, and the current resulting from the application of the resultant waveform to the body of a subject is controlled within a predetermined range.

The apparams therefore operates similarly to known transcutaneous nerve stimulation (TENS) and micro current electrical treatment (MET) equipment, but in addition causes a degree of electrolysis at the site of application of the current (due to the DC component thereof) which has been found to be useful in the treatment of pain.

A DC current in tissue which is impregnated with undesirable chemicals (toxins etc.) causes electrolysis, breaking the chemicals down into their components. These components, comprising ions, are much smaller in size than the molecules of the original chemical substance in most cases, and migrate easily into the lymphatic system or the bloodstream, to be excreted via normal bodily functions. The DC current also improves blood circulation, speeding up the removal of the electrolysis byproducts, as well as causing enzymes, co-enzymes, hormones, antibodies and other beneficial biochemical compounds to be conveyed to the affected area of the body at a greater rate.

Excess DC current is harmful, since it may cause tissue damage due to oxidation at the site of application of the electrodes and in the tissue subjected to the excess current. On the other hand, insufficient DC current causes little or no electrolysis and the healing process takes much longer.

The apparams of the present invention determines a DC offset which is effectively superimposed on or combined with a time-varying or AC electrical waveform so that the correct level of DC current is applied to the body of the subject.

Referring now to Figure 1 , the apparams comprises a DC power supply 10 which is typically a 9V DC battery. The power supply is connected via a switch 12 to a DC-DC converter 14. This device utilises a voltage controlled, 110 kHz oscillator which operates with a 50% duty cycle and which switches an N channel MOSFET. The MOSFET switches the supply voltage on and off across a 470μH ferrite core inductor, and a medium voltage capacitor is charged with half-wave rectified inductive current surges from the inductor. A voltage feedback circuit stops the oscillator when the voltage across the capacitor reaches 80V (± 5V). Thus, the DC-DC convener generates a relatively high voltage DC supply from the low voltage DC supply from the battery or other power supply.

The output of the DC-DC convener is controlled by another electronic switch 16, which is in turn controlled by the output of a medium frequency oscillator 18 and a low frequency oscillator 20, via a second electronic switch 22. The output of the medium frequency oscillator 18, comprising a pulse train, is modulated by the output of the low frequency oscillator 20 via the electronic switch 22, and this modulated waveform is fed to the control input of the electronic switch 16, so that the relatively high output voltage of the DC-DC convener is effectively modulated by the composite waveform.

The medium frequency oscillator 18 generates a low voltage pulsed output waveform with a frequency in the range of 50 to 200Hz. A frequency of about 125 Hz is preferred. The pulse width of the waveform is in the range 0.01 to 2ms, with a pulse of with about 0.1ms being preferred.

The output of the low frequency oscillator 20 is a square wave, so that the output waveform comprises the waveform generated by the medium frequency oscillator which is switched on and off at the frequency of the square wave output of the low frequency oscillator.

The modulated output of the electronic switch 16 is fed to an amplimde control circuit 24 comprising a variable impedance NPN transistor switch which permits the amplimde of the output waveform to be controlled by a user of the apparams via a potentiometer. As shown in Figure 4, the output of the amplimde control circuit 24 is a train of relatively narrow rectangular pulses. The illustrated waveform has a frequency of 125Hz and pulse width of lOOμs. The peak output voltage of the waveform can be adjusted by the amplimde control circuit between 0 and 75V. The amplitude control circuit switches into a high impedance state when the falling edge of the output pulse reaches approximately 50% of its peak value, resulting in the shape of the lower portion of the waveform in Figure 4. The output of the amplimde control circuit 24 is fed to a pulse shaper circuit 26 which comprises an RC network which removes high frequency harmonics from the output waveform. Due to the smoothing or integrating effect of the RC circuit, the output of the pulse shaper circuit is a substantially triangular waveform on a DC offset, as shown in Figure 5. The time constant of the RC circuit is critical, as this value, together with the frequency peak amplimde, the pulse width of the pulse train at the output of the amplimde control circuit 24 and the load impedance (ie. the impedance at the site of application of the output waveform to the body of the subject) will all influence the magnitude of the DC offset in the output signal. An increase in the capacitor value, peak amplimde, frequency and pulse width all increase the DC offset for a given load impedance. Assuming that the other variables remain constant, increasing the peak amplimde of the output waveform of the amplimde control circuit 24 will cause a proponional increase in the DC offset.

The amplimde control circuit 24 and the pulse shaper circuit 26 are illustrated in more detail in the circuit diagram of Figure 2. The amplimde control circuit 24 is based on a pair of transistors Q2 and Q3 together with a potentiometer PI and a resistor R15. The pulse shaping circuit 26 essentially comprises a lμF capacitor C7 connected to the emitter of the transistor Q3, and provides the required pulse shaping and DC offset. Resistors R16 and R17 provide shon circuit protection for the circuit.

The output waveform of Figure 4 is the waveform at the emitter of the transistor Q3 without the capacitor C7 in circuit, while the waveform of Figure 5 shows the waveform at the same point with the capacitor C7 in circuit.

Effectively, the capacitor C7 integrates the waveform of Figure 4.

The output of the pulse shaper circuit is fed to an output circuit 28 via an over current protection circuit 30 and a current sensor 32 which provides a current magnitude signal to an output current control circuit

34.

The current sensor 32, which is a shunt resistor, provides a current feedback signal which is used to generate an amplimde control signal which is fed to the amplimde control circuit 24 to adjust the amplimde of the output signal in order to maintain a constant, predetermined average output current value, according to the setting of a current selector control 36. A display 38 is connected to the output current control circuit 34 and displays data relating to the status of the apparams, including the average output current, the condition of the battery 10 and the remaining time of operation of the unit as set by a timer 40 via a control unit 42.

In order to test the operation of the unit, a test load comprising a lkΩ resistor Rl, a lOkΩ resistor R2 and a lμF capacitor Cl, connected as shown in Figure 3, was utilized. This load simulates the complex load of human tissue. With the output waveform and DC offset amplimde controls set to maximum, the resultant waveform is as shown in Figure 5. It can be seen that there is some distortion of the waveform, but that its basic shape is maintained. The second, low frequency oscillator 20 is controlled by a mode switch 44, which selects "normal" or "pulsed" operation of the unit. In "normal" mode, the output waveform is constant, as shown in Figure 3. When the low frequency oscillator 20 is activated, it generates a rectangular or square wave output with a frequency in the range 0.5 to 3Hz, preferably 2Hz, and this output waveform controls the electronic switch 22, thus modulating the output of the first oscillator 18 as mentioned above. The result is that the output waveform of the apparams appears in bursts which have the frequency and duty cycle of the low frequency oscillator 20, as shown in Figure 7 (not to scale).

The output current control circuit 34 is designed to limit the current due to the DC component of the output waveform between 0 and 2500μA, with the maximum value occurring at the maximum DC offset of 15V, while the output current due to the time-varying waveform is between 0 and 3500μA, with the maximum current occurring at the maximum peak to peak amplimde of 30V. Thus, the total current applied to the test load due to the resultant waveform is in the range 0 to 6000_/χA.

It will be appreciated that the test load described above provides only an approximation of the impedance of the load due to a subject. In practice, the total current applied to the body of subject will be in the range 0 to 1200μA, and typically less than 1000μA.

It will be appreciated that the amplimde of the output waveform of the apparams can be varied according to the requirements of me individual user. The magnitude of the DC offset varies with the amplimde of the alternating waveform, so that the resultant output waveform always has a DC component. The apparams has been found to be effective in the treatment of pain, infections, strains and sprains.

In order to use the apparatus, the output circuit 28 is connected to the site of application on the body of a patient or subject using carbon based electrodes. The use of carbon as an electrode base is important, as this ensures that no metallic ions are transferred into the application site due to the electrolysis effect.

The user then adjusts the amplimde control 24 to increase the amplimde of the output waveform and the corresponding DC component thereof, until a tingling sensation is felt at the electrode application site. The peaks of the AC waveform are responsible for this sensation. Once the tingling sensation is felt, the amplimde control is backed-off slightly, and the average DC current is then established at a magnitude which will not cause harm. In practice, the average DC current level has been found to not to exceed 750μA. By pre-adjusting the relative levels of the AC and DC components of the waveform by selection of the values described above, the average current (attributable largely to the DC component or DC offset) is established at the correct value.

The following voltages and currents were measured in the output circuit of the apparams of the invention with the apparams connected to various application sites on a test subject:

Across the temples:

AC voltage: 5 Volt (peak to peak

DC offset: 3 Volt AC current: 310μA (RMS) DC current: 600μA

Across the sides (Just above the hip bone):

AC voltage: 4 Volt (peak to peak)

DC offset: 5,5 Volt

AC current: 300μA(RMS)

DC current: 600μA

From the base of the foot to the calf muscle:

AC voltage: 21 Volt (peak to peak)

DC offset: 12 Volt

AC current: 450μA (RMS)

DC current: 600μA

From the top of the hand to bottom of the arm:

AC voltage: 7,2 Volt (peak to peak)

DC offset: 11 Volt

AC current: 300μA (RMS)

DC current: 530μA

From side to side across the knee:

AC voltage: 11 Volt (peak to peak)

DC offset: 6,5 Volt

AC current: 390μA (RMS)

DC current: 700μA

From the front of the shoulder to the rear on the shoulder blade: AC voltage: 5,5V (peak to peak) DC offset: 7,5 Volt AC current: 220μA (RMS) DC current: 400μA

From side to side through the calf muscle:

AC voltage: 9 Volt (peak to peak)

DC offset: 7,4 Volt

AC current: 180μA (RMS)

DC current: 320μA

From the back of the head (under hairline) to the forehead:

AC voltage: 3 Volt (peak to peak)

DC offset: 3,2 Volt

AC current: 180μA (RMS)

DC current: 320μA

From side to side through the neck:

AC voltage: 6 Volt (peak to peak)

DC offset: 5,2 Volt

AC current: 200μA (RMS)

DC current: 520μA

Average values for the different sites above are:

AC voltage: 6 Volt (peak to peak)

DC voltage: 7 Volt

AC current: 280μA (RMS) DC current: 510μA

The AC waveform also causes the TENS effect, in which the fast- conducting nerves of the subject are stimulated. This effectively "beats" the slow-conducting pain-signal carrying nerves, preventing the transmission of pain signals and thus providing an analgesic effect.

In the pulsed mode of operation (illustrated in Figure 7) the stimulating waveform triggers the release of endorphins in the body of the subjects, enhancing the pain relief effect.

It will be appreciated that the above described arrangement provides a reliable means for a user of the apparams to adjust the output level thereof until the DC component of the output waveform is at or close to its maximum effective level, but below a dangerous threshold. This makes the apparams of the invention particularly effective in use.

Claims

1. A method of electrically stimulating the body of a subject comprising generating a first time-varying electrical waveform; generating a resultant waveform having a time-varying component and a DC offset from the first time-varying waveform, the magnimde of the DC offset being related to the magnitude of the first time-varying waveform; and applying the resultant waveform to the body of the subject, wherein the resultant waveform has a time-varying component with a frequency in the range 50 to 200 Hz and a peak to peak amplimde in the range 0 to 30V, wherein the DC offset is in the range 0 to 15V, and wherein the current due to the application of the resultant waveform to the body of the subject is in the range 0 to 1200╬╝A.

2. A method according to claim 1 wherein the resultant waveform has a frequency of about 125Hz and a peak to peak amplimde of about 8V, and wherein the average current due to the application of the resulting waveform to the body of the subject is less than 750╬╝A.

3. A method according to claim 1 or claim 2 wherein the resultant waveform has a time-varying component which is substantially triangular, with a relatively steep rising edge and a relatively shallow falling edge.

4. A method according to any one of claims 1 to 3 and further including generating a second time-varying waveform having a frequency in the range 0.5 to 3Hz, and utilizing the second waveform to modulate the resultant waveform.

5. A method according to claim 4 wherein the second time-varying waveform has a frequency of about 2Hz.

6. A method according to claim 4 or claim 5 wherein the second waveform is a rectangular waveform.

7. A method of electrically stimulating the body of a subject substantially as herein described with reference to the accompanying drawings.

8. Apparams for electrically stimulating the body of a subject, the apparams comprising:

voltage control means for generating a resultant waveform having a time-varying component and a DC offset from the first time-varying waveform, the DC offset having a magnimde related to the amplimde of the first time- varying waveform, and the resultant waveform having a time-varying component with a peak to peak amplimde in the range 0 to 30V and a DC component with a magnimde in the range 0 to 15V; output means for applying the resultant waveform to the body of a subject; and

current control means for adjusting the magnimde of the current resulting from the application of the resultant waveform to the subject between 0 and 1200╬╝A.

9. Apparams according to claim 8 including a second waveform generator arranged to generate a second waveform having a frequency in the range 0.5 to 3Hz, and switch means arranged to modulate the resultant waveform with the second waveform.

10. Apparams for electrically stimulating the body of a subject substantially as herein described with reference to the accompanying drawings.