Functional electrical stimulation devices and methods of operating the same

Abstract

Functional electrical stimulation devices and methods of operating the same are described. For example, a device includes a high-efficiency power boost circuit configured to step a low battery voltage to a boosted output voltage, an open-loop charge pump circuit coupled to the power boost circuit to further increase the boosted voltage, and a switched capacitor output stage to generate high-slew-rate, charge-balanced stimulation pulses. The switched capacitor stage may produce symmetric or asymmetric bi-phasic pulses with adjustable amplitudes, widths, and frequencies over an operating range. The boost and charge pump stages may operate in discontinuous conduction and discontinuous voltage modes to achieve soft switching and improved efficiency at high conversion ratios. The devices may be incorporated into wearable electrical stimulators using textile electrodes. Methods are also provided for operating the power boost circuit, the charge pump circuit, and the switched capacitor stage to deliver controlled stimulation to electrically excitable tissue.

Description

FUNCTIONAL ELECTRICAL STIMULATION DEVICES AND METHODS OFOPERATING THE SAMEFIELD

[0001] Embodiments of the present disclosure generally relate to the field of functional electrical stimulation, and in particular to functional electrical stimulation devices and methods of operating the same.BACKGROUND

[0002] Functional electrical stimulation may be a treatment configured to generate electrical impulses to activate user muscle or nerves. The electrical impulses may cause muscle contractions. In some examples, functional electrical stimulation operations include generating charge-balanced low-energy pulses to depolarize one or more nerve membranes. Such low-energy pulses may be transmitted to a user until an activation threshold value is reached, and action potentials may be triggered for causing user muscle contractions.SUMMARY

[0003] Embodiments of functional electrical stimulation devices are described in the present disclosure. Embodiments of the devices are configured to include a quasi- resonant boost stage for stepping up battery voltage to an intermediate bus voltage. In some examples, a 1 :2 charge pump configured in an open loop configuration further boosts the intermediate bus voltage.

[0004] For reducing charge required for user stimulation, a downstream switched capacitor stage is configured for generating high slew rate pulses. The switched capacitor stage may be configured to provide net zero-charge pulses, thereby providing configurable current pulse frequency, amplitude, or duration.

[0005] Embodiments of functional stimulation devices described herein may be integrated with textile devices, such as textile electrode devices, among other example textile devices.

[0006] In one aspect, the present disclosure describes a functional electrical stimulation device. The device may include: a power boost circuit configured to generate a boost stage output voltage; a charge pump circuit coupled to the power boost circuit configured in an open loop for further boosting the boost stage output voltage; and a switched capacitor circuit coupled to the charge pump circuit and configured to provide an output stage generating high slew rate pulses.

[0007] In this respect, before explaining at least one embodiment in detail, it is to be understood that the embodiments are not limited in application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting.

[0008] Many further features and combinations thereof concerning embodiments described herein will appear to those skilled in the art following a reading of the present disclosure.DESCRIPTION OF THE FIGURES

[0009] In the figures, embodiments are illustrated by way of example. It is to be expressly understood that the description and figures are only for the purpose of illustration and as an aid to understanding.

[0010] Embodiments will now be described, by way of example only, with reference to the attached figures, wherein in the figures:

[0011] FIG. 1 illustrates a block diagram of an electrical stimulation system, in accordance with an embodiment of the present disclosure;

[0012] FIG. 2 illustrates a schematic of a GaN-based hybrid electrical stimulator, in accordance with embodiments of the present disclosure;

[0013] FIG. 3 illustrates a quasi-resonant boost switch configuration during a first state, in accordance with an embodiment of the present disclosure;

[0014] FIG. 4 illustrates a quasi-resonant boost switch configuration during a second state, in accordance with an embodiment of the present disclosure;

[0015] FIG. 5 illustrates a quasi-resonant boost switch configuration during a third state, in accordance with an embodiment of the present disclosure;

[0016] FIG. 6 illustrates switch sequence intervals of a quasi-resonant boost switch configuration, in accordance with embodiments of the present disclosure;

[0017] FIG. 7 illustrates a switched capacitor circuit, in accordance with embodiments of the present disclosure;

[0018] FIG. 8 illustrates a charge injection state of the switched capacitor circuit, in accordance with embodiments of the present disclosure;

[0019] FIG. 9 illustrates a charge withdrawal state of the switched capacitor circuit, in accordance with embodiments of the present disclosure;

[0020] FIG. 10 illustrates an idle state of the switched capacitor circuit, in accordance with embodiments of the present disclosure;

[0021] FIG. 11 illustrates a charge-balanced symmetric bi-phasic pulse generated by the switched capacitor circuit, in accordance with embodiments of the present disclosure;

[0022] FIG. 12 illustrates a charge-balanced asymmetric bi-phasic pulse generated by the switched capacitor circuit, in accordance with embodiments of the present disclosure;

[0023] FIG. 13 illustrates a wearable electric stimulator device, in accordance with an embodiment of the present disclosure;

[0024] FIG. 14 illustrates a waveform of a converter stepping up a voltage by a power boost circuit and a charge pump circuit, in accordance with an embodiment of the present disclosure;

[0025] FIG. 15 illustrates charge-balanced high-current symmetric bi-phasic pulses, in accordance with an embodiment of the present disclosure;

[0026] FIG. 16 illustrates charge-balanced high-current asymmetric bi-phasic pulses, in accordance with an embodiment of the present disclosure;

[0027] FIG. 17 illustrates positive and negative slew rate measurements, in accordance with embodiments of the present disclosure;

[0028] FIG. 18 illustrates medium and high frequency asymmetric bi-phasic pulse generation current, in accordance with embodiments of the present disclosure; and

[0029] FIG. 19 illustrates power stage efficiency measurements, in accordance with embodiments of the present disclosure.DETAILED DESCRIPTION

[0030] Functional electrical stimulation (FES) may include methods configuring electric stimulators (ES) to inject charge-balanced low-energy pulses to depolarize a nerve membrane, until the activation threshold is reached. Action potentials may be triggered for generation of muscle contractions. Some applications of FES may include therapies for spinal cord injury, muscle relaxation, or restoration of lost motor functions by neuroprostheses among other example applications. Depending on the application and tissue type, electric pulses having varying attributes may be required.

[0031] In some scenarios, small muscle groups may require smaller amplitudes than deeper and larger muscles. In some scenarios of generating bi-phasic current pulses for artificial stimulation of electrically excitable tissue, ES may generate a very high voltage from a much lower cell voltage to create an action potential across the highly resistive tissues. Further, the frequency of these pulses applied to muscle tissues may be relativelylow, typically in the range of 20-70 Hz. Accordingly, the dc-dc converter may need to step up the cell voltage (3-3.6 V) to output voltages ranging between 60 V-200 V at low load conditions.

[0032] FIG. 1 illustrates a block diagram of a portable ES, in accordance with embodiments of the present disclosure. A high-voltage dc-dc step-up converter boosts the unregulated input battery voltage to a relatively high voltage, in some cases as high as 200 V. This may be required to inject sufficient current (in range of 1 mA-150 mA) to stimulate the relatively high resistive tissues. Through a user interface, a user may select to choose and create one or more stimulation sequences. It communicates with the pulse logic block. Data may be transmitted to the central controller, which communicates with the pulse generator block for setting appropriate pulse type, amplitude, width, and frequency.

[0033] In some examples, depending on the architecture of pulse generator stage, electric stimulators may be divided into three categories: (1) voltage source-based (VS)

[0011] ,

[0024] , (2) current source-based (OS)

[0011] ,

[0025] , and (3) charge-based

[0022] ,

[0023] . VS may be easier to implement but may need to be controlled to inject the same levels of current with changing tissue impedance. On the other hand, CS provide stable current stimulation levels, but the slew rates (SR) of the pulses are slow and may not be desirable. Chargebased stimulators, proposed in [1], [2], may provide better control of pulses and may be more efficient.

[0034] In some examples, boost converters may include efficiency drop at high conversion ratios and low power levels due to the high voltage-current overlap losses in the main switch during turn-off. Example switched-capacitor-based charge pumps operate with higher efficiency, but the input-to-output conversion ratios may be in discrete steps, thereby providing lower flexibility with adjusting the output voltage, which in turn is required to inject controlled current pulse amplitude in the tissue whose impedance can vary.

[0035] In some scenarios, through tissue impedance modeling, slew rate (SR) of the pulse may be inversely proportional to the amount of charge that needs to be injected to create and maintain action potential for muscle contraction. In some embodiments, rectangular charge-balanced pulses may provide the highest repetition rate, but their generation is technically more complex because it may require an H-bridge current output stage or a negative supply voltage.

[0036] In some examples, dc-dc boost converters may suffer from high voltage-current overlap switching losses, which degrade efficiency when operating at high output voltage and low current levels. Also, the limited SR of VS and CS based solutions require more charge to create action potential for muscle contraction.

[0037] In some examples, ES devices may be configured with a limited range of pulse shape and amplitudes because they are optimized for specific clinical applications

[0011] ,

[0018] . Therefore, these solutions may not be well suited for several emerging applications requiring multichannel transcutaneous battery-powered portable hardware capable of providing a wide variety of pulses over long operating periods. Compared to example devices

[0019] -

[0023] , embodiments of the present disclosure include devices with an output stage having a much wider variety of stimulation pulses and lower power consumption.

[0038] In some embodiments, functional electrical stimulation devices may include an efficient quasi-resonant boost stage configured to step up the battery voltage to an intermediate bus voltage. A 1 :2 charge pump operating in open loop may be configured to further boost the voltage. To reduce the charge required for stimulation, the device includes a downstream switched capacitor stage capable of generating very high SR pulses. The SC stage may inherently provide net zero-charge pulses and offers a high degree of freedom in terms of pulse frequency, amplitude, and duration adjustments. FIG. 2 illustrates a schematic of portions of a functional electrical stimulation device, in accordance with embodiments of the present disclosure.

[0039] In some embodiments, a power stage may include four switches (Q1-Q4), an inductor (L), and a flying capacitor (Ct). Operation of such a converter block is describedherein. In some embodiments, Ct is quasi-resonantly fully charged and discharged with L in series, operating in discontinuous voltage mode (DVM). This operation may create zero voltage across the switches during the switch transitions, zero-voltage switching (ZVS). L may operate in discontinuous conduction mode (DCM), providing zero-current switching (ZCS) at multiple switch transitions. Combining DVM of Ct and DCM of L, fully soft-switching is achieved for all the switches, therefore greatly reducing switching losses at low power levels.

[0040] Reference is made to FIGS. 3 to 6, which illustrate features of a converter block, in accordance with embodiments of the present disclosure. The switching sequence may be illustrated based on six time intervals. In FIG. 6, the example time intervals are indicated as intervals 1 to 6, which correspond to durations of time where the converter block may be transitioning through three switch state configurations. The example switch state configurations are illustrated in FIGS. 3 to 5. At the beginning of a switching period, inductor current, i_L, is zero and voltage across Ct is also zero.

[0041] Referring to FIG. 3, interval 1 (v_x = 0) starts when switches Q1 and Q2 are turned ON with ZCS, and the converter is in state A. L is linearly charged by IJn for a time T_ON.

[0042] Referring to FIG. 4, interval 2 (v_x = V_cr) starts when Q1 is turned OFF and Q3 is turned ON, both with ZVS. The converter is in state B, due to the state of voltage across Ct. Ct is resonantly charged for a time T_OFF such that at the end of this interval V_cr = V_boost and i_L = 0.

[0043] Interval 3 is a resting period; no energy is transferred, and the converter sits idle.

[0044] Again, at interval 4 (v_x = 0), L is linearly charged by IJn, and the converter is in state A, illustrated in FIG. 3.

[0045] At interval 5 (v_x = V_boost - V_cr), the converter is in state C. Ct is resonantly discharged, and energy is transferred to the output, FIG. 5. Q2 is turned OFF and Q4 is turned ON with ZVS due to the state of voltage across Ct.

[0046] Interval 6 may be configured for V_boost regulation: no switching takes place until V_boost falls below the reference voltage, V REF.

[0047] Switch selection: In a switching period, switches Q1 and Q2 conduct for the majority of the time, and Q3 and Q4 conduct briefly, just to transfer energy to and from the flying capacitor. Therefore, to optimize conduction losses, switches with proper sizes are configured. In the current topology, the switches must be rated for full output voltage, V_boost. In high V_boost scenarios, this presents challenges since on-resistance (R_on) increases with the square of the blocking voltage (V_BV) for a given switch area [4], GaN devices may offer lower R_on when compared to MOSFETs for the same switch size and V_BV. Therefore, GaN may be a promising switch category for this high-voltage and high power-density application.

[0048] In some embodiments, a configuration to power up gate drivers for vertically stacked series switches may be cascaded bootstrapping [5]. There, the bootstrap capacitor of the upper gate driver is charged using the bootstrap capacitor and diode of the lower gate driver when the lower switch is ON. In some embodiments, the voltage across the N-th bootstrap capacitor may be expressed as:V_boot,N = V_drive - (N - 1) V_D, where V_drive is the drive voltage applied to the lowest switch and V_D is one diode drop.

[0049] In some embodiments, GaN devices may be sensitive to variations in gate drive voltage and have a very small tolerance around their nominal V_GS value of 5 V (e.g., 4.5 V-6 V [6]). Therefore, for cascaded GaN switches, embodiments may be configured to keep the gate drive voltage close to 5 V. A 200 V half-bridge gate driver from Texas Instruments, LMG1210 [7], which is specifically designed to drive GaN switches, is used to drive Q1 and Q2. To provide drive voltage to stacked gate drivers, a double charge pump technique was introduced in [8]. This technique utilizes the internal switches of the gate driver to form a 1 :2 charge pump and double the bootstrap voltage of the lower switches to compensate for diode drops. In some embodiments, resistive dividers may be used to step down the boosted voltage to make it suitable for GaN drivers. But here,the boosted voltage is utilized for driving MOSFETs Q3 and Q4, which have a much higher allowable V_GS range.

[0050] Switched capacitor stage: FIGS. 11 and 12 illustrate pulse shapes predominantly found in ESs. The pulse parameters include stimulation or active pulse amplitude (l_A), charge balance or discharging pulse amplitude (l_0), duration of active and discharge pulses (T_h and T_0), pulse frequency or repetition rate (1 / T_PULSE), and slew rate (SR = AI_A / T_rise), not explicitly illustrated.

[0051] For a wide range of applications, the pulse generator block illustrated in FIG. 1 may provide independent and wide range of control of all these pulse parameters. In FIG. 7, the switched capacitor stage [9] comprises two capacitors (C_P and C_N) connected in series and two switches (O_P and O_N) connected in half-bridge configuration. Differential electrode connection points for the tissue, V_TISSUE+ and V_TISSUE-, may be tapped at the connection points of the capacitors and switches. This output stage is capable of generating a wide variety of current pulses: (a) symmetric and asymmetric biphasic pulses; (b) adjustable current amplitudes in accordance with the tissue impedance; (c) adjustable pulse widths; (d) wide range of pulse frequency, ranging from a few hertz to hundreds of kilohertz; (e) theoretically infinite current slew rate, limited only by switch turn-ON speed and on-resistance; and (f) intrinsic charge-balanced pulses.

[0052] In some scenarios, intrinsic charge balance of pulses arises from the fact that tissue current may flow through the midpoint of the capacitor divider, thereby enforcing net charge to be zero over a switching period. High slew rate of the pulses may occur because as soon as switches O_P or O_N change their configuration, instantaneous voltages V_P or V_N are applied across the tissue terminals.

[0053] FIGS. 8 to 10 illustrate the operating states of the SC stage. In the idle state shown in FIG. 10, when switches O_P and O_N are OFF, capacitors C_P and C_N maintain voltages V_P and V_N respectively in accordance with capacitor voltage division rules, and the lower voltage will appear across the larger capacitor.

[0054] In the charge injection state shown in FIG. 8, O_P is turned ON and a positive voltage, V_P, appears instantaneously across the tissue terminals.

[0055] In the charge withdrawal state shown in FIG. 9, O_N is turned ON and a negative voltage with magnitude V_N appears instantaneously across the tissue terminals, thereby generating charge-balanced symmetric / asymmetric, high-slew-rate, bi-phasic pulses.

[0056] In some embodiments, in the low-frequency regime of 20 Hz-70 Hz, usually used for tissue stimulation, the voltages V_P and V_N will depend on the size of the capacitors C_P and C_N:V_P + V_N = V_out, and C_P V_P = C_N V_N, if the time constant T = R_TISSUE (C_P || C_N) » T_d, where T_d is the time duration of the pulse (T_h or T_0 in FIG. 11 or 12). Therefore, by varying the capacitor divider value, symmetric (FIG. 11 ) or asymmetric (FIG. 12) bi-phasic pulses can be generated. In the high-frequency regime, i.e., kHz range, where T « T_d, if pulse frequency is constant, due to the charge balance of C_P and C_N, voltages V_P and V_N will selfadjust according to T_h and T_0. Therefore, just by changing T_h and T_0, symmetric and asymmetric bi-phasic pulses can be easily generated: l_A T_h = l_0-T_0. To adjust current amplitudes l_A and l_0 with changing tissue impedance, V_out can be varied accordingly, which in turn will vary V_P and V_N: l_A = V_P / R_TISSUE and l_0 = V_N / R-TISSUE.

[0057] To illustrate embodiments of the present disclosure, practical implementation and experimental results are described. FIG. 13 illustrates discrete prototypes of the hybrid GaN-MOS boost stage and bi-phasic SC stage, in accordance with embodiments of the present disclosure. The discrete prototypes may be used to validate the operation of the introduced portable electric stimulator solution (23 mm x 20 mm x 8 mm). QR boost and charge pump stages may be configured on a single PCB and the bi-phasic SC stage may be configured on another PCB. The PCBs may be configured such that the PCBs may be stacked on top of either board.

[0058] In the present example, voltage generated by the boost stage may be transferred to the bi-phasic stage using interconnection pins. For simple and replicable implementation, the controller is implemented around an FPGA board using simple counters to generate appropriate ON / OFF-time gating signals based on feedback from the output voltage comparator.

[0059] FIG. 14 illustrates basic operating waveforms of the boost stage operating at 3.6 V-60 V, in accordance with embodiments of the present disclosure. Further, the operating waveforms of the boost stage may include a charge pump configured to double it to around 120 V (not shown in FIG. 14) and supplying 15 mA of load current.

[0060] Referring again to FIG. 6, the experimental waveforms closely follow the ideal operating waveforms. L is first linearly charged by IJn, then Ct is resonantly charged to V_boost. Again, L is linearly charged and then energy stored in Ct is resonantly transferred to the output.

[0061] FIGS. 15 to 18 illustrate experimental operation of the switched-capacitor-based bi-phasic stage, in accordance with embodiments of the present disclosure. High- frequency, low pulse-width and high-current supply pulses may be challenging to generate in wearable ESs. The tissue is modelled as a resistor of 1 kQ.

[0062] FIG. 15 illustrates the generation of ±75 mA symmetric bi-phasic pulses at a pulse frequency of 25 kHz and tissue activation time of 1 .6 ps.

[0063] FIG. 16 illustrates the generation of +125 mA and -25 mA asymmetric bi-phasic pulses at a pulse frequency of 56 kHz and tissue activation time of 400 ns.

[0064] FIG. 17 illustrates the rise and fall slew rates of +4.25 kV / ps and -3.65 kV / ps. Discrepancy in rise and fall SR may be due to unequal R_on of the switches O_P and O_N of the GaN-based integrated half-bridge (IHB), LMG2610. For over-current protection, the current sense pin of the IHB can be utilized to detect faults and accordingly shut down the system.

[0065] FIG. 18 illustrates the approximate maximum frequency at which the bi-phasic stage can generate pulses. This frequency may be higher for lower activation currents, as then the charge injected would be lower and discharge pulses may be shorter. Therefore, the SC stage may generate high-current pulses for big muscles and low- current pulses for smaller muscles (by adjusting V_out) and higher frequency for smoother movement and HF-10 therapies.

[0066] To illustrate the power processing efficiency of the utilized topology, for high conversion ratios and low load currents, converter efficiency results with the lowest input battery voltage of 3 V, boosting it up to 90 V, 120 V, and 150 V, and load currents ranging from 1 mA to 25 mA are illustrated in FIG. 19. The converter provides operation at conversion ratios exceeding 30*, while maintaining much better efficiencies than conventional high step-up, low power solutions

[0010] . The inductor operates in DCM and the flying capacitor in DVM across the entire load range. By soft-switching, high efficiencies for high step-up ratios at low power levels are achieved.

[0067] In a general aspect, the present disclosure provides a functional electrical stimulation device including a power boost circuit configured to generate a boost stage output voltage from an input battery voltage, a charge pump circuit coupled to the power boost circuit and configured in an open loop to further boost the boost stage output voltage, and a switched capacitor circuit coupled to the charge pump circuit and configured to provide an output stage that generates high-slew-rate, charge-balanced stimulation pulses.

[0068] In some embodiments, the charge pump circuit comprises a double charge pump configured to supply a drive voltage to stacked gate drivers to provide a 1 :2 boost ratio. The power boost circuit may comprise four switches, an inductor, and a flying capacitor, wherein the flying capacitor operates in a discontinuous voltage mode and the inductor operates in a discontinuous conduction mode. In some embodiments, at least one of the switches of the power boost circuit comprises a gallium nitride device. In some embodiments, the switched capacitor circuit comprises two capacitors connected in series and two switches arranged in a half-bridge configuration. In some embodiments,the output stage of the switched capacitor circuit is configured to generate symmetric biphasic pulses, asymmetric bi-phasic pulses, adjustable stimulation current amplitudes, adjustable stimulation pulse widths, stimulation pulse frequencies ranging from a few hertz to hundreds of kilohertz, stimulation pulses having slew rates determined at least in part by switch turn-on speed and on-resistance, or stimulation pulses having intrinsic charge balance.

[0069] In another general aspect, the present disclosure provides a functional electrical stimulation system including the above device, at least one electrode configured to be placed in contact with electrically excitable tissue, and a controller configured to control operation of the power boost circuit, the charge pump circuit, and the switched capacitor circuit to generate stimulation sequences having selected pulse amplitudes, widths, and frequencies.

[0070] In a further general aspect, the present disclosure provides a method of operating a functional electrical stimulation device including generating, by a power boost circuit, a boost stage output voltage from an input battery voltage, further boosting, by an openloop charge pump circuit coupled to the power boost circuit, the boost stage output voltage to form an intermediate high voltage, generating, by a switched capacitor circuit coupled to the charge pump circuit, high-slew-rate bi-phasic stimulation pulses, and applying the stimulation pulses to electrically excitable tissue through at least one electrode.

[0071] In some embodiments, the boost stage output voltage is between 60 V and 200 V. In some embodiments, the device is configured to deliver stimulation currents between 1 mA and 150 mA. In some embodiments, the electrode comprises a textile electrode integrated into a wearable garment. In some embodiments, the controller is configured to regulate the boost stage output voltage based on feedback from an output voltage comparator. In some embodiments, the switched capacitor circuit is configured to generate stimulation pulses having rise or fall slew rates of at least 3 kV per microsecond. In some embodiments, the method includes generating symmetric or asymmetric charge- balanced bi-phasic current pulses. In some ^ ibodiments, the power boost circuit and thecharge pump circuit are arranged to provide zero-voltage switching or zero-current switching during at least one switching interval of a stimulation cycle.

[0072] The term “connected” or “coupled to” may include both direct coupling (in which two elements that are coupled to each other contact each other) and indirect coupling (in which at least one additional element is located between the two elements).

[0073] Although the embodiments have been described in detail, it should be understood that various changes, substitutions, and alterations can be made herein without departing from the scope. Moreover, the scope of the present disclosure is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods, and steps described in the specification.

[0074] As one of ordinary skill in the art will readily appreciate from the disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.

[0075] The description provides many example embodiments of the inventive subject matter. Although each embodiment represents a single combination of inventive elements, the inventive subject matter is considered to include all possible combinations of the disclosed elements. Thus, if one embodiment comprises elements A, B, and C, and a second embodiment comprises elements B and D, then the inventive subject matter is also considered to include other remaining combinations of A, B, C, or D, even if not explicitly disclosed.

[0076] As can be understood, the examples described above and illustrated are intended to be exemplary only.

[0077] Applicant notes that the described embodiments and examples are illustrative and non-limiting. Practical implementation of the features may incorporate a combination of some or all of the aspects, and features described herein should not be taken as indications of future or existing product plans. Applicant partakes in both foundational and applied research, and in some cases, the features described are developed on an exploratory basis.REFERENCES

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[0102]

[0024] J. Simpson and M. Ghovanloo, “An experimental study of voltage, current, and charge controlled stimulation front-end circuitry,” in Proc. IEEE Int. Symp. Circuits Syst., May 2007, pp. 325-328.

[0103]

[0025] P. H. Peckham and J. S. Knutson, “Functional electrical stimulation for neuromuscular applications,” Annu. Rev. Biomed. Eng., vol. 7, pp. 327-360, Aug. 2005.

Claims

WHAT IS CLAIMED IS:1 . A functional electrical stimulation device comprising: a power boost circuit configured to generate a boost stage output voltage from an input battery voltage; a charge pump circuit coupled to the power boost circuit and configured in an open loop to further boost the boost stage output voltage; and a switched capacitor circuit coupled to the charge pump circuit and configured to provide an output stage that generates high-slew-rate, charge-balanced stimulation pulses.

2. The device of claim 1 , wherein the charge pump circuit comprises a double charge pump configured to supply a drive voltage to stacked gate drivers to provide a 1 :2 boost ratio.

3. The device of claim 1 , wherein the power boost circuit comprises four switches, an inductor, and a flying capacitor, wherein the flying capacitor operates in a discontinuous voltage mode and the inductor operates in a discontinuous conduction mode.

4. The device of claim 3, wherein at least one of the switches comprises a gallium nitride device.

5. The device of claim 1 , wherein the switched capacitor circuit comprises two capacitors connected in series and two switches arranged in a half-bridge configuration.

6. The device of claim 5, wherein the output stage is configured to generate at least one of: symmetric bi-phasic pulses, asymmetric bi-phasic pulses, adjustable stimulation current amplitudes, adjustable stimulation pulse widths, stimulation pulse frequencies ranging from a few hertz to hundreds of kilohertz, stimulation pulses having slew rates determined at least in part by switch turn-on speed and on-resistance, or stimulation pulses having intrinsic charge balance.

7. A functional electrical stimulation system comprising: the device of claim 1 ; at least one electrode configured to be placed in contact with electrically excitabletissue; and a controller configured to control operation of the power boost circuit, the charge pump circuit, and the switched capacitor circuit to generate stimulation sequences having selected pulse amplitudes, widths, and frequencies.

8. A method of operating a functional electrical stimulation device comprising: generating, by a power boost circuit, a boost stage output voltage from an input battery voltage; further boosting, by an open-loop charge pump circuit coupled to the power boost circuit, the boost stage output voltage to form an intermediate high voltage; generating, by a switched capacitor circuit coupled to the charge pump circuit, high- slew-rate bi-phasic stimulation pulses; and applying the stimulation pulses to electrically excitable tissue through at least one electrode.

9. The device of claim 1 , wherein the boost stage output voltage is between 60 V and 200 V.

10. The device of claim 1 , configured to deliver stimulation currents between 1 mA and 150 mA.11 .The system of claim 7, wherein the at least one electrode comprises a textile electrode integrated into a wearable garment.

12. The device of claim 1 , wherein the controller is configured to regulate the boost stage output voltage based on feedback from an output voltage comparator.

13. The device of claim 1 , wherein the switched capacitor circuit is configured to generate stimulation pulses having rise or fall slew rates of at least 3 kV per microsecond.

14. The method of claim 8, wherein generating the stimulation pulses comprises producing symmetric or asymmetric charge-balanced bi-phasic current pulses.

15. The device of claim 1 , wherein the power boost circuit and the charge pump circuit are arranged to provide zero-voltage switching or zero-current switching during at least one switching interval of a stimulation cycle.

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