Neuromuscular stimulation using multistage current driver circuit

Abstract

Neuromuscular stimulation is widely used for rehabilitation and movement assist devices, due to its safety, efficacy, and ease of operation. For repeatable and accurate muscular contractions, a voltage controlled current sources (VCCS) with high compliance is required. Conventional VCCS design requires high-voltage rated operational amplifiers, which are expensive and consume large power. Moreover, conventional stimulators are not viable for simultaneous stimulation of muscle synergies, as they require multiple VCCS operating at the same time. This invention presents a neuromuscular stimulator with a multistage driver circuit wherein, a VCCS connected to an output driving stage comprising of folded-cascode transistor buffers and a bidirectional current mirror circuit. The multistage driver circuit uses inexpensive low-voltage rated operational amplifiers that consume 95% less power. Additionally, we disclose a stimulation method wherein only a single current source drives several output drivers connected in series or parallel to simultaneously stimulate multiple muscles or muscle synergies.

Claims

1. A neuromuscular electrical stimulator, comprising: a waveform generator having a symmetrical and symmetrical output; a bidirectional voltage controlled current source having an output voltage less than 100 volts; a bidirectional current mirror circuit connected to an output of the bidirectional voltage controlled current source, wherein a current output by the bidirectional current mirror circuit has a same value and a higher voltage as a current output by the bidirectional voltage controlled current source; a unity gain current buffer connected to the output of the bidirectional voltage controlled current source and an input to the bidirectional current mirror circuit; and a biological interface, wherein the biological interface includes at least a working electrode and a return electrode to deliver the current output by the bidirectional current mirror circuit.

2. The stimulator of claim 1, wherein the bidirectional voltage controlled current source lacks an op-amp rated at 100 volts or higher.

3. The stimulator of claim 1, wherein the stimulator includes a single and only bidirectional voltage controlled current source, the single and only bidirectional voltage controlled current source is a bidirectional Howland current pump, the current mirror circuit is a bidirectional current mirror circuit with a sourcing current portion and a sinking current portion, and both the sourcing current portion and the sinking current portion receive the output of the single and only bidirectional voltage controlled current source.

4. The stimulator of claim 1, wherein the bidirectional voltage controlled current source, the bidirectional current mirror circuit, and the unity gain current buffer comprise a multistage driver and wherein the waveform generator includes a plurality of independent channels, the stimulator further comprising: a plurality of the multistage drivers each coupled to one channel of the plurality of independent channels.

5. The stimulator of claim 1, wherein the bidirectional current mirror circuit, and the unity gain current buffer comprise an output driver, the stimulator further comprising: a single and only bidirectional voltage controlled current source; and a plurality of the output drivers each coupled in series or parallel with one another, wherein the single and only bidirectional voltage controlled current source is connected to the output drivers.

6. The stimulator of claim 5, further comprising: a network of switches and resistors to selectively attenuate or divert the electrical signals supplied by the plurality of output drivers.

7. The stimulator of claim 1, further comprising: a controller including a processor and memory, wherein the controller is configured to instruct the waveform generator to output digital waveform electrical pulses and turn on step-up DC-DC converters connected to the stimulator to provide a desired operating voltage to the stimulator prior to operating the stimulator.

8. The stimulator of claim 7, wherein the controller is configured to receive stimulation parameters input from a graphical user interface on a connected computer.

9. The stimulator of claim 7, wherein the controller includes a plurality of interrupts configured to deactivate the stimulator upon activation of an emergency off switch.

10. The stimulator of claim 7, wherein the biological interface is a skin interface including a plurality of electrodes configured to attach to human skin and deliver the current to at least one nerve or muscle beneath the skin.

11. The stimulator of claim 1, wherein the bidirectional current mirror circuit, and the unity gain current buffer comprise an output driver, the stimulator further comprising: a first output driver and a second output driver, each output driver connected to a corresponding biological interface, wherein the second output driver receives an input from the return electrode of the biological interface connected to the first output driver.

12. A multistage driver for a neuromuscular electrical stimulator, the driver comprising: a bidirectional voltage controlled current source means; a unity gain current buffer means connected to an output of the bidirectional voltage controlled current source means; and a bidirectional current mirror circuit means connected to an output of the unit gain current buffer means.

13. The multistage driver of claim 12, wherein the multistage driver lacks an op-amp rated at 100 volts or higher.

14. The multistage driver of claim 12, wherein the bidirectional current mirror circuit means, and the unity gain current buffer means comprise an output driver, the multistage driver further comprising: a plurality of the output drivers each coupled in series or parallel with one another, and wherein a network of switches and resistors are configured to selectively attenuate or divert the electrical signals supplied by the plurality of output drivers.

15. The multistage driver of claim 12, wherein the voltage controlled current source means is configured to produce a symmetrical and symmetrical biphasic electrical waveform at any voltage, and wherein the controlled current source means is configured to reproduce the biphasic electrical waveform at a threshold voltage for nerve or muscle stimulation.

16. A method of electrically stimulating nerves or muscles, the method comprising: receiving, at a bidirectional voltage controlled current source from a waveform generator, an electrical signal, wherein the bidirectional voltage controlled current source has an output voltage less than 100 volts, the bidirectional voltage controlled current source is connected to a bidirectional current mirror circuit through a unity gain current buffer; and outputting, from the bidirectional current mirror circuit, the electrical signal to a biological interface including at least a working electrode and a return electrode.

17. The method of claim 16, wherein the bidirectional voltage controlled current source is a bidirectional Howland current pump and is the single and only bidirectional voltage controlled current source that outputs voltage less than 100 volts, wherein the bidirectional current mirror circuit is a bidirectional current mirror circuit with a sourcing current portion and a sinking current portion, and both the sourcing current portion and the sinking current portion receive the output of the single and only bidirectional voltage controlled current source.

18. The method of claim 16, further comprising: attaching the biological interface to an external human skin position over a nerve or muscle to be stimulated.

19. The method of claim 18, the bidirectional voltage controlled current source is the single and only bidirectional voltage controlled current source that outputs voltage less than 100 volts, and additional nerves or muscles are simultaneously stimulated, and wherein the method further comprising: receiving, at a plurality of output drivers from the single and only bidirectional voltage controlled current source, the electrical signal, wherein each output driver include the bidirectional current mirror circuit, and the unity gain current buffer, outputting, from the plurality of output drivers, the simultaneous electrical signal to the additional nerves or muscles.

20. The method of claim 16, wherein the waveform generator is controlled by a controller including a processor and memory that instructs the waveform generator to output digital waveform electrical pulses and turn on step-up DC-DC converters connected to the stimulator to provide a desired operating voltage to the stimulator prior to operating the stimulator, wherein the waveform is biphasic and one of symmetrical and asymmetrical output.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is the schematic circuit diagram of a single channel of the neuromuscular electrical stimulator (NMES) with multistage driver circuit

(2) FIG. 2 illustrates one embodiment of the disclosed NMES for stimulating antagonist forelimb muscle pair of a patient

(3) FIG. 3A shows an example of the timing profiles for the sequential activation of two stimulation channels of the NMES device.

(4) FIG. 3B illustrates sample current waveforms generated by the NMES device when configured for symmetrical and asymmetrical output

(5) FIGS. 4A and 4B are flowcharts that diagrammatically illustrate the microcontroller's program for operating the NMES device

(6) FIG. 5 presents another embodiment of the disclosed NMES for simultaneously stimulating multiple muscles using a single VCCS

(7) FIG. 6 extends the embodiment from FIG. 5 and illustrates simultaneous stimulation of several muscles using a single VCCS

(8) FIG. 7 illustrates the multistage NMES device for simultaneously and selectively stimulating large as well as small muscles.

DETAILED DESCRIPTIONFIGS. 1-4FIRST EMBODIMENT

(9) In one embodiment shown in FIG. 1, the neuromuscular stimulator consists of a waveform generator 110, a voltage controlled current source (VCCS) 112, and output driving circuitry 114 and 116. The waveform generator 110 consists of a pair of signal generators V.sub.P and V.sub.N that connect to an operational amplifier (op-amp) U.sub.1. The periods of V.sub.P and V.sub.N correspond to periods of the positive and negative phases of a biphasic stimulation waveform, respectively. In some embodiments, the periods and shape of V.sub.P and V.sub.N are generated by a microcontroller or by a digital-to-analog converter.

(10) Op-amp U.sub.1 is configured as a difference amplifier with adjustable gain, to compute the difference between V.sub.P and V.sub.N. The amplitude of V.sub.P can also be scaled using the resistor divider network at the positive terminal of U.sub.1, such that V.sub.P is a fraction of V.sub.N. When V.sub.P is a fraction of V.sub.N, the period of V.sub.P is proportionally increased by the microcontroller, such that net charge per phase (amplitudeperiod) is equal for both positive and negative phases of the biphasic stimulation waveform. Thus, by adjusting the amplitude and periods of the positive and negative phases, either symmetrical or asymmetrical waveforms can be generated. In some embodiments, the output of U.sub.1 is scaled using a potentiometer U.sub.2 to generate a graded input voltage V.sub.IN, which is further buffered using a unity gain amplifier U.sub.3.

(11) V.sub.IN is connected to circuit 112, which is a VCCS known in the literature as the Howland current pump. The circuit of 112 consists of a low-voltage rated op-amp U.sub.4 operating at LV (e.g. 15V) and precision resistors R.sub.1-R.sub.4 and R.sub.X. In some embodiments, a low-voltage, high-output current operational amplifier (e.g. LM7321, LM7372 from Texas Instruments) can be used in place of U.sub.4. Further, it can be shown that when resistors in the positive and negative feedback paths of the op-amp in circuit 112 are selected such that

(12) $\begin{array}{cc}\frac{R_1}{R_2+R_X}=\frac{R_3}{R_4},& \left(1\right)\end{array}$
then the load current I.sub.L is proportional to V.sub.IN. Further, if R.sub.3=R.sub.4, then

(13) $\begin{array}{cc}{I}_L=-\frac{V_{\mathrm{IN}}}{R_X}.& \left(2\right)\end{array}$
Therefore by adjusting V.sub.IN or R.sub.X, we can set the desired load current I.sub.L generated by 112. However, the compliance voltage of the VCCS is limited by its supply voltage to within LV.

(14) To improve the output voltage compliance of the VCCS, the current I.sub.L is drives an output stage using a pair of folded cascode bipolar transistors (Q.sub.N0, Q.sub.P0) shown in 114. The common-base, folded cascode transistors 114 provide unity gain buffering between the VCCS 112 and a high-voltage bidirectional current mirror circuit 116. The bidirectional current mirror circuit is implemented using the modified Wilson's current mirror topology in 116. The circuit in 116 is symmetrical about the middle, with top half capable of sourcing current and bottom half capable of sinking current, and is powered by stepped-up DC-DC high voltage (HV, e.g. 150V). The current mirror topology can provide high output compliance=HV, using low-cost, high-voltage rated transistors. If we select the PNP and NPN transistors (i.e., Q.sub.P1-Q.sub.P4 and Q.sub.N1-Q.sub.N4) to have identical DC current gains, then we can show that

(15) $\begin{array}{cc}{I}_{\mathrm{OUT}}=-I_L=\frac{V_{\mathrm{IN}}}{R_X}.& \left(3\right)\end{array}$
Thus, in this embodiment, by buffering of the Howland current pump 112 and Wilson's current mirror circuits 116, using folded cascode topology 114, we create an adjustable, high-voltage compliant, constant current source. Note that it will be obvious to a person having ordinary skills in the art that the circuits shown in 112, 114, and 116 can be realized using either bipolar junction transistor or field effect transistor technology. Further, the above configuration can be implemented using either discrete components or fabricated as a monolithic integrated circuit (IC). Still further, while this embodiment presents specific examples of current source and current mirror circuits, the above embodiment can also be realized using alternative configurations, by a person skilled in the art.

(16) In 116, when the folded-cascode and output transistors QP.sub.0, QP.sub.2 and QN.sub.0, QN.sub.2 are subjected to large reverse voltages, when they are in cutoff mode and the complementary current mirror circuit is active. Hence, these current transistors must be selected with collector-emitter breakdown voltage (V.sub.CEO) greater than HV power supply, i.e. V.sub.CEO2*|HV|. For example, transistors FZT758 and FZT658 from Diodes Inc., have V.sub.CEO>|400V|, which is typically in the range for neuromuscular stimulation. The remaining current mirror transistors, namely QP.sub.1, QP.sub.3, Q.sub.P4, Q.sub.N1, Q.sub.N3, and Q.sub.N4 can be implemented with smaller V.sub.CEO requirement, however it is recommended to use same transistors as Q.sub.P2 and Q.sub.N2, for matching the DC current gains.

(17) In the quiescent mode when no stimulation is being generated, the output driver is in cutoff and the maximum power dissipation occurs in the VCCS. In this conventional Howland pump using a high-voltage rated opamp (e.g. PA341DF from Cirrus Logic) with quiescent current 2.5 mA and supply voltage 150V, the quiescent power dissipation is 750 mW. While in the present embodiment using a low-voltage rate opamp with similar output current capacity (e.g. LM7321 from Texas Instruments), having quiescent current 1.1 mA and 15V supply, the quiescent power dissipation is 33 mW. Thus the present VCCS implementation is 95% more efficient than conventional Howland pump based designs.

(18) To ensure safe operation of this embodiment during neuromuscular stimulation, the output current I.sub.OUT can be limited through a fuse before being delivered through CH.sub.1, as shown in FIG. 1. Additionally, in an emergency situation, switch SW can immediately divert the current away from CH.sub.1 by shunting the channel with low impedance load R.sub.P. During normal operation, current I.sub.OUT can be measured in its return path using a series resistor R.sub.S, a current sense amplifier U.sub.5, and an analog-to-digital converter U.sub.6. Further, in some embodiments, the current measured can be used as a feedback signal to adjust the load current I.sub.L and maintain desired output current I.sub.OUT.

(19) FIG. 2 illustrates one instance of the embodiment of FIG. 1 to electrically stimulate the forearm muscles of a patient. In FIG. 2, many aspects of CH.sub.1 (FIG. 1) are hidden and some aspect of its output driving stage is reproduced in 210 for illustration purposes. A second stimulation channel CH.sub.2 and its corresponding output driving stage 212, is illustrated for stimulating a second muscle segment. Lead wires of CH.sub.1 and CH.sub.2 are respectively connected to an antagonistic muscle pair, namely flexor digitorum and extensor digitorum muscles, using a pair surface electrodes (e.g. Model 650 from Covidien Inc.). In one embodiment, the current enters the muscles through one of the surface electrodes 216 or 220 placed over the muscle's belly and returns through either or both of the distally placed surface electrodes 214 and 218. In another embodiment, the entry and return paths of the currents are interchanged depending on the polarity of the current waveform.

(20) In FIG. 3A, the timing profiles for stimulation channels CH.sub.1 & CH.sub.2 are shown. In the example illustrated in FIG. 3A, the periods and amplitudes of positive and negative phases are configured such that the waveforms of CH.sub.1 and CH.sub.2 are asymmetrical and symmetrical, respectively. In the prior art, it is known that the strength of muscle contractions are proportional to the amplitude and duration of the stimulation currents. Therefore, by adjusting the amplitude and duration of the waveforms of CH.sub.1 and CH.sub.2, we can control the response of individual muscles. Further, the stimulation repetition rate or frequency 310 (FIG. 3A) can be configured by the microcontroller to ensure sustained muscle contractions. In some embodiments, the negative and positive phases can be separated by an inter-pulse interval 312 to delay the onset of muscle fatigue. In the version shown in FIG. 3A, CH.sub.1 and CH.sub.2 occur sequentially separated by an inter-channel delay 314, such that the ensuing muscle contractions do not overlap. In other versions, the trains of pulses can be programmed to generate sustained contraction in one antagonistic muscle after another, allowing the patient for example, to completely open and then close their hand. In yet other versions, the antagonistic muscles can contract simultaneously and with different strengths, to produce a sustained but damped contraction in one direction.

(21) In FIG. 3B, sample waveforms recorded from the disclosed NMES system in FIG. 1 are shown. The left and right plots of FIG. 3B correspond to asymmetrical and symmetrical current waveforms, generated for different amplitudes ranging from 10 mA-70 mA, for a load impedance of 2 kilo-ohms (k). The negative phase duration was set at 500 micro-seconds (s), and for the asymmetrical waveforms the positive pulse duration and amplitude were adjusted to maintain equal charge per phase. In this example, circuits 110 and 112 (FIG. 1) were powered with LV=15V, whereas the output-driving stage 116 was powered with HV=150V. To generate the low and high voltage supplies, step-down and step-up DC-DC converters were used respectively, which were operated by two 24V batteries. In the literature several topologies of DC-DC converters are known and a person skilled in the art can implement it using alternative topologies, requiring even lower voltage batteries.

(22) The microcontroller's normal sequence of execution is briefly described in the flowcharts of FIGS. 4A and 4B. At the start, the user uses a graphical user interface (GUI) to specify the stimulation parameters such as pulse-duration, amplitude, inter-phase delay, etc. Depending on the desired waveform shape, either symmetrical 410 or asymmetrical 412 phase durations and amplitude are configured. In 412, the phase and amplitude of positive and negative phases can be scaled by parameter (k), in order to maintain equal charge balance for each phase. For a given pulse duration, the microcontroller computes the number of cycles for which the voltage on each waveform generator signal must remain HIGH. A digital Timer keeps track of the number of microcontroller cycles executed and after the desired interval, the voltage is returned to LOW. The sequence of operations are repeated in a loop, until the desired the number of stimulation pulses have been generated. Prior to activating the stimulator, the step-up DC-DC converter is turned on and a few milliseconds (ms) delay is introduced, so as to reach the desired compliance voltage. Any time during the program execution, if the emergency OFF switch is pressed by the patient or a fault occurs, the program interrupts the microcontroller and shunts all stimulation channels using low impedance loads.

DETAILED DESCRIPTIONFIGS. 5-7OTHER EMBODIMENTS

(23) FIG. 5 illustrates a second embodiment of the disclosed NMES device. In this setup, the return path of channel CH.sub.1 via conductor 510 is connected to the folded cascode buffer stage 512 of channel CH.sub.2 and the return path of CH.sub.2 completes the circuit. When the PNP transistor-based current mirror circuit 514 of CH.sub.1 sources current, it causes the NPN transistor-based current mirror to simultaneously sink current through CH.sub.2. Likewise, when the complementary circuit 516 sinks current through CH.sub.1, it causes circuit 518 to source current to CH.sub.2. Alternately, when current is applied at electrode sites 522 and 524 by CH.sub.1, an equal and inverted current is simultaneously applied at electrode sites 526 and 528 by CH.sub.2. This will cause the muscles connected to CH.sub.1 and CH.sub.2 to simultaneously contract. Thus, in this embodiment using a single voltage-controlled current source (VCCS) we can simultaneously deliver equal amounts of current to both muscles. This is depicted in the illustrative example shown FIG. 5, where the biceps and flexor digitorum muscles can be simultaneously contracted using a single VCCS. This movement is useful during simultaneous reaching and grasping tasks, e. g. eating with a spoon.

(24) While it is possible to directly connect multiple muscles using series or parallel combinations and stimulate them with a single stimulator, this approach has significant limitations. Firstly, in the case when the muscles are connected in series, the tissue impedance of each muscle will add up and the effective tissue impedance will increase. In this case, to maintain voltage compliance in the presence of large effective impedance, a higher power supply will be required which will increase the device cost. Secondly, for the case when multiple muscles are connected in parallel, the current through each individual muscles will be reduced according to Kirchhoff's current law, which will reduce the overall stimulation efficacy. Thus, our multistage current driver circuit is beneficial over conventional series or parallel topologies for simultaneously stimulating multiple muscle groups.

(25) In FIG. 6, the embodiment from FIG. 5 is extended to N muscles such that the return path of previous output stage drives current into the next output stage. Thus, using a plurality of output drivers connected in series, a single VCCS can drive equal currents through channels CH.sub.1, CH.sub.2, up to CH.sub.N and simultaneously activate multiple muscles, as represented in FIG. 6 by 610, 612, and 614, respectively. This embodiment can be employed for movements that require simultaneous activation of muscles synergies, for e.g. in sit to stand training, in bilateral upper-limb training for holding a glass in one hand and pouring water with another, and in gait training for simultaneous activation lower-limb muscles during the stance and swing phases.

(26) In another embodiment illustrated in FIG. 7, a network of switches 710, 712, and 714 are used to selectively connect CH.sub.1 with both, either or none of the subsequent output drivers controlling stimulation channels CH.sub.2 and CH.sub.3. This provides the NMES device the capability to selectively activate or deactivate certain muscles pairs. When both channels CH.sub.2 and CH.sub.3 are connected in the return path of CH1, variable resistors 716 and 718 can be used to proportionally divide the current I.sub.1, such that I.sub.1=I.sub.2+I.sub.3. This embodiment will allow for simultaneous stimulation of a large muscle that requires higher current intensity and multiple smaller muscles that require lower current intensities, e.g. stimulating the forearm and hand muscles during grasping.