Methods and systems for mitigating unintended stimulus by neuromodulation devices

Abstract

Disclosed is an implantable device comprising a control unit and a regulator circuit. The control unit is configured to control, when the implantable device is in an active mode, a stimulus source, the stimulus source configured to deliver neural stimuli to a neural pathway of a patient via at least one stimulus electrode of a plurality of electrodes, the neural stimuli being configured to evoke neural responses from the neural pathway. The regulator circuit is connected via parasitic components to the plurality of electrodes at a connection point. The regulator circuit is configured to provide a high impedance to an AC voltage induced between a pair of the plurality of electrodes, the AC voltage resulting from application of an external oscillating electromagnetic field to the implantable device, wherein the high impedance is provided for a magnitude of the induced AC voltage that is below an activation voltage that is higher than a combined voltage drop across the parasitic components.

Description

METHODS AND SYSTEMS FOR MITIGATING UNINTENDED STIMULUS BY NEUROMODULATION DEVICES

[0001] The present application claims priority from Australian Provisional Patent Application No. 2024903203 filed on 3 October 2024, the contents of which are incorporated herein by reference in their entirety.TECHNICAL FIELD

[0002] The present disclosure relates to systems, devices, and methods for performing neuromodulation on a patient, such as for example via a neuromodulation device, and in particular to the mitigation of unintended stimuli resulting from the exposure of the neuromodulation device to an externally applied oscillating electromagnetic field.BACKGROUND OF THE INVENTION

[0003] There are a range of situations in which it is desirable to apply neural stimuli in order to alter neural function, a process known as neuromodulation. For example, neuromodulation is used to treat a variety of disorders including chronic neuropathic pain, movement disorders, and voiding disorders. A neuromodulation device applies an electrical pulse (stimulus) to neural tissue (fibres, or neurons) in order to generate a therapeutic effect. In general, the electrical stimulus generated by a neuromodulation device evokes a neural response known as an action potential in a neural fibre which then has either an inhibitory or excitatory effect. Inhibitory effects can be used to modulate an undesired process such as the transmission of pain, or excitatory effects may be used to cause a desired effect such as the contraction of a muscle.

[0004] When used to relieve neuropathic pain originating in the trunk and limbs, the electrical pulse is applied to the dorsal column (DC) of the spinal cord, a procedure referred to as spinal cord stimulation (SCS). Such a device typically comprises an implanted electrical pulse generator, and a power source such as a battery that may be transcutaneously rechargeable by wireless means, such as inductive transfer. An electrode array is connected to the pulse generator, and is implanted adjacent the target neural fibre(s) in the spinal cord, typically in the dorsal epidural space above the dorsal column. An electrical pulse of sufficient intensity applied to the target neural fibres by a stimulus electrode causes the depolarisation of neurons in the fibres, which in turn generates an action potential in the fibres. Action potentials propagate along the fibres in an orthodromic direction (in afferent fibres this means towards the head, or rostral) and in an antidromic direction (in afferent fibres this means towards the cauda, or caudal) directions. Action potentials propagating along Ap (A-beta) fibres being stimulated in this way may inhibit the transmission of pain from a region of the bodyinnervated by the target neural fibres (the dermatome) to the brain. To sustain the pain relief effects, stimuli are applied repeatedly, for example at a frequency in the range of 30 Hz - 100 Hz.

[0005] For effective and comfortable neuromodulation, it is necessary to maintain stimulus intensity above a recruitment threshold. Stimuli below the recruitment threshold will fail to recruit sufficient neurons to generate action potentials with a therapeutic effect. In some neuromodulation applications, response from a single class of fibre is desired, but the stimulus waveforms employed can evoke action potentials in other classes of fibres which cause unwanted side effects. In pain relief, it is therefore desirable to apply stimuli with intensity below a discomfort threshold, above which uncomfortable or painful percepts arise due to over-recruitment of Ap fibres or recruitment of undesired fibre classes. When recruitment is too large, A fibres produce uncomfortable sensations. Stimulation at high intensity may even recruit AS (A-delta) fibres, which are sensory nerve fibres associated with acute pain, cold and heat sensation. It is therefore desirable to maintain stimulus intensity within a therapeutic range between the recruitment threshold and the discomfort threshold.

[0006] The task of maintaining appropriate neural recruitment is made more difficult by electrode migration (change in position over time) and / or postural changes of the implant recipient (patient), either of which can significantly alter the neural recruitment arising from a given stimulus, and therefore the therapeutic range. The spinal cord itself moves within the cerebrospinal fluid (CSF) with respect to the dura and the electrode array. During postural changes, the amount of CSF and / or the distance between the spinal cord and the electrode can change significantly. This effect is so large that postural changes alone can cause a previously comfortable and effective stimulus regime to become either ineffectual or painful.

[0007] Another control problem facing neuromodulation devices of all types is achieving neural recruitment at a sufficient level for therapeutic effect, but at minimal expenditure of energy. The power consumption of the stimulation paradigm has a direct effect on battery requirements which in turn affects the device’s physical size and lifetime. For rechargeable devices, increased power consumption results in more frequent charging and, given that batteries only permit a limited number of charging cycles, this ultimately reduces the implanted lifetime of the device.

[0008] Attempts have been made to address such problems by way of feedback or closed-loop control, such as using the methods set forth in International Patent Publication No. WO2012 / 155188 by the present applicant, the content of which is incorporated herein by reference. Feedback control seeks to compensate for relative nerve / electrode movement by controlling the intensity of the delivered stimuli to maintain neural recruitment at or near a target value. The intensity of a neural response evoked by a stimulus may be used as a feedback variable representative of the amount ofneural recruitment. A signal representative of the neural response may be sensed by a measurement electrode in electrical communication with the recruited neural fibres, and processed to obtain the feedback variable. Based on the response intensity, the intensity of the applied stimulus may be adjusted to bring the response intensity closer to the target value.

[0009] It is therefore desirable to accurately measure the intensity and other characteristics of a neural response evoked by the stimulus. The action potentials generated by the depolarisation of a large number of fibres by a stimulus sum to form a measurable signal known as an evoked compound action potential (ECAP). Accordingly, an ECAP is the sum of responses from a large number of single fibre action potentials. The ECAP generated from the depolarisation of a group of similar fibres may be measured at a measurement electrode as a positive peak potential, then a negative peak, followed by a second positive peak. This morphology is caused by the region of activation passing the measurement electrode as the action potentials propagate along the individual fibres.

[0010] Approaches proposed for obtaining a neural response measurement are described by the present applicant in International Patent Publication No. WO2012 / 155183, the content of which is incorporated herein by reference.

[0011] However, neural response measurement can be a difficult task as a neural response component in the sensed signal will typically have a maximum amplitude in the range of microvolts. In contrast, a stimulus applied to evoke the response is typically several volts, and manifests in the sensed signal as crosstalk of that magnitude. Moreover, stimulus generally results in electrode artefact, which may manifest in the sensed signal as a decaying output of the order of several millivolts after the end of the stimulus. As the neural response can be contemporaneous with the stimulus crosstalk and / or the stimulus artefact, neural response measurements present a difficult challenge of measurement amplifier design. For example, to resolve a 10 pV ECAP with 1 pV resolution in the presence of stimulus crosstalk of 5 V requires an amplifier with a dynamic range of 134 dB, which is impractical in implantable devices. In practice, many non-ideal aspects of a circuit lead to artefact, and as these aspects mostly result in a time-decaying artefact waveform of positive or negative polarity, their identification and elimination can be laborious.

[0012] Evoked neural responses are less difficult to measure when they appear later in time than the artefact, or when the signal-to-noise ratio is sufficiently high. The artefact is often restricted to a time of 1 - 2 ms after the stimulus and so, provided the neural response is measured after this time window, a neural response measurement can be more easily obtained. This is the case in surgical monitoring where there are large distances (e.g. more than 12 cm for nerves conducting at 60 ms'1) between thestimulus and measurement electrodes so that the propagation time from the stimulus site to the measurement electrodes exceeds 2 ms, which is longer than the typical duration of stimulus artefact.

[0013] However, to characterize the responses from the dorsal column, high stimulation currents are required. Similarly, any implanted neuromodulation device will necessarily be of compact size, so that for such devices to monitor the effect of applied stimuli, the stimulus electrode(s) and measurement electrode(s) will necessarily be in close proximity. In such situations the measurement process must overcome artefact directly.

[0014] Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.

[0015] Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

[0016] In this specification, a statement that an element may be “at least one of’ a list of options is to be understood to mean that the element may be any one of the listed options, or may be any combination of two or more of the listed options.SUMMARY OF THE INVENTION

[0017] According to a first aspect of the present technology, there is provided an implantable device comprising: a control unit configured to control, when the implantable device is in an active mode, a stimulus source configured to deliver neural stimuli to a neural pathway of a patient via at least one stimulus electrode of a plurality of electrodes, the neural stimuli being configured to evoke neural responses from the neural pathway; and a regulator circuit connected via parasitic components to the plurality of electrodes at a connection point, wherein the regulator circuit is configured to provide a high impedance to an AC voltage induced between a pair of the plurality of electrodes, the AC voltage resulting from application of an external oscillating electromagnetic field to the implantable device, wherein the high impedance is provided for a magnitude of the induced AC voltage that is below an activation voltage that is higher than a combined voltage drop across the parasitic components.

[0018] In some embodiments, the regulator circuit is configured to provide the high impedance by providing a reduced effective capacitance at the connection point.

[0019] In some embodiments, the regulator circuit is configured to provide the reduced effective capacitance at the connection point when the implantable device is not in the active mode, and to provide a relatively higher effective capacitance at the connection point when the implantable device is in the active mode.

[0020] In some embodiments, the regulator circuit comprises at least: a voltage supply circuit configured to provide a reference voltage at an output terminal; a supply capacitor having first and second terminals respectively connected to ground and the output terminal of the voltage circuit; and one or more control elements configured to reduce the effective capacitance at the connection point by decoupling the supply capacitor from the connection point.

[0021] In some embodiments, the one or more control elements comprise a diode having an anode and a cathode respectively connected to the second terminal of the supply capacitor and the connection point.

[0022] In some embodiments, the diode is a Schottky diode.

[0023] In some embodiments, the activation voltage is determined by bias voltages of: a Zener diode having first and second terminals respectively connected to the connection point and ground; and the parasitic components of each electrode of the pair of electrodes.

[0024] In some embodiments, the one or more control elements comprise one or more switches configured to the decouple the supply capacitor from the connection point when operating in an off mode.

[0025] In some embodiments, the one or more switches comprise: a first P-channel metal-oxide semiconductor (PMOS) transistor having first and second terminals respectively connected to the output terminal of the voltage supply circuit and the connection point.

[0026] In some embodiments, the first PMOS transistor is configured to operate in the off mode when the implantable device is not in the active mode.

[0027] In some embodiments, the one or more control elements further comprise a passive pull-up element connected between the second terminal of the first PMOS transistor, and a third terminal of the first PMOS transistor, wherein the passive pull-up element is configured to operate the first PMOS transistor in the off mode when the implantable device is not in the active mode.

[0028] In some embodiments, the one or more switches further comprise a switching device connected to the third terminal of the first PMOS transistor, wherein the switching device is configured to switch the operation of the first PMOS transistor between: an on mode; and the off mode, in which the supply capacitor is respectively coupled to, and decoupled from, the connection point.

[0029] In some embodiments, the one or more switches further comprise: a second PMOS transistor having first and second terminals respectively connected to the second terminal of the first PMOS transistor and to the pair of electrodes.

[0030] In some embodiments, the switching device is controlled by the control unit.

[0031] In some embodiments, the one or more control elements are contained within a common integrated circuit of the implantable device.

[0032] In some embodiments, the externally applied oscillating electromagnetic field is generated during an Magnetic Resonance Imaging (MRI) scan of the patient.

[0033] According to a second aspect of the present technology, there is provided a method for operating an implantable device, the method comprising: when the implantable device is in an active mode, delivering neural stimuli, via at least one stimulus electrode of a plurality of electrodes of the implantable device, to a neural pathway of a patient to evoke a neural response from the neural pathway; and providing, via a regulator circuit of the implantable device, a high impedance to an AC voltage induced between a pair of the plurality of electrodes, the AC voltage resulting from an application of an external oscillating electromagnetic field to the implantable device, wherein the high impedance is provided for a magnitude of the induced AC voltage that is below an activation voltage that is higher than a combined voltage drop across one or more parasitic components connecting the regulator circuit to the plurality of electrodes.

[0034] In some embodiments, the method of the second aspect further comprises transitioning the implantable device out of the active mode in response to the application of the external oscillating electromagnetic field.

[0035] In some embodiments, providing the high impedance comprises providing a reduced effective capacitance at a connection point connecting the plurality of electrodes and the regulator circuit.

[0036] In some embodiments, the method of the second aspect further comprises providing the reduced effective capacitance at the connection point when the implantable device is not in the active mode, and providing a relatively higher effective capacitance at the connection point when the implantable device is in the active mode.

[0037] In some embodiments, providing the reduced effective capacitance at the connection point comprises using one or more control elements to decouple a supply capacitor from the connection point, wherein the supply capacitor has first and second terminals respectively connected to ground and to a voltage circuit, and wherein the voltage circuit is configured to provide a reference voltage to the second terminal of the supply capacitor.

[0038] In some embodiments, the one or more control elements comprise a diode having an anode and a cathode respectively connected to the second terminal of the supply capacitor and the connection point.

[0039] In some embodiments, the diode is a Schottky diode.

[0040] In some embodiments, the one or more control elements comprise one or more switches.

[0041] In some embodiments, the one or more switches comprise a first P-channel metal-oxide semiconductor (PMOS) transistor.

[0042] In some embodiments, the method of the second aspect further comprises operating the first PMOS transistor in the off mode when the implantable device is not in the active mode.

[0043] In some embodiments, the one or more control elements further comprise a passive pull-up element, and wherein the method comprises using the passive pull-up element to operate the first PMOS transistor in the off mode when the implantable device is not in the active mode.

[0044] In some embodiments, the one or more switches further comprise a switching device, and wherein the method further comprises using the switching device to switch the operation of the first PMOS transistor between: an on mode; and the off mode, in which the supply capacitor is respectively coupled to, and decoupled from, the connection point.

[0045] In some embodiments, the one or more switches further comprise a second PMOS transistor, and wherein the method further comprises using the second PMOS transistor to electrically isolate the voltage circuit from the connection point.

[0046] In some embodiments, the method of the second aspect further comprises controlling the switching device using a control unit of the implantable device.

[0047] In some embodiments, the activation voltage is determined by bias voltages of: a Zener diode having first and second terminals respectively connected to the connection point and ground; and one or more parasitic components connecting the regulator circuit to each electrode of the pair of electrodes.

[0048] In some embodiments, the externally applied oscillating electromagnetic field is generated during a Magnetic Resonance Imaging (MRI) scan of the patient.

[0049] According to a third aspect of the present technology, there is provided a neural stimulation system comprising: an implantable device for controllably delivering neural stimuli to a neural pathway of a patient, the device comprising: a stimulus source configured to deliver neural stimuli via at least one stimulus electrode of a plurality of electrodes, to the neural pathway, the neural stimuli being configured to evoke neural responses from the neural pathway; a control unit configured to control the stimulus source to deliver the neural stimuli; a regulator circuit connected to the pluralityof electrodes at a connection point; and a processor configured to instruct the control unit to control the stimulus source to deliver the neural stimuli to the neural pathway, wherein the system is configured to perform any of the methods described herein.

[0050] References herein to estimation, determination, comparison and the like are to be understood as referring to an automated process carried out on data by a processor operating to execute a predefined procedure suitable to effect the described estimation, determination and / or comparison step(s). The technology disclosed herein may be implemented in hardware (e.g., using digital signal processors, application specific integrated circuits (ASICs) or field programmable gate arrays (FPGAs)), or in software (e.g., using instructions tangibly stored on non-transitory computer-readable media for causing a data processing system to perform the steps described herein), or in a combination of hardware and software. The disclosed technology can also be embodied as computer-readable code on a computer-readable medium. The computer-readable medium can include any data storage device that can store data which can thereafter be read by a computer system. Examples of the computer-readable medium include read-only memory ("ROM"), random-access memory ("RAM"), magnetic tape, optical data storage devices, flash storage devices, or any other suitable storage devices. The computer-readable medium can also be distributed over network-coupled computer systems so that the computer-readable code is stored and / or executed in a distributed fashion.

[0051] The present technology has been developed primarily for use in or with neuromodulation of the spinal cord and will be described hereinafter mostly with reference to this application. However, it will be appreciated that the present technology is not limited to this particular field of use, and may be applied in other neuromodulation contexts, including but not limited to sacral nerve stimulation, pudendal nerve stimulation, deep brain stimulation, stimulation of other parts of the peripheral and central nervous system. It will further be appreciated that the present technology may be applied for treatment of conditions other than chronic pain, including but not limited to movement disorders, Crohn’s disease, rheumatoid arthritis, diabetes, Reynaud’s phenomenon, pelvic floor disorders, chronic inflammatory conditions, migraine, stroke, or depression.BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Notwithstanding any other implementations which may fall within the scope of the present invention, one or more implementations of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0053] Fig. 1 schematically illustrates an implanted spinal cord stimulator, according to one implementation of the present technology;

[0054] Fig. 2 is a block diagram of the stimulator of Fig. 1 ;

[0055] Fig. 3 is a schematic illustrating interaction of the implanted stimulator of Fig. 1 with a bundle of target nerve fibres;

[0056] Fig. 4a illustrates an idealised activation plot for one posture of a patient undergoing neural stimulation;

[0057] Fig. 4b illustrates the variation in the activation plots with changing posture of the patient;

[0058] Fig. 5 is a schematic illustrating elements and inputs of a closed-loop neural stimulation (CLNS) system, according to one implementation of the present technology;

[0059] Fig. 6 illustrates the typical form of an electrically evoked compound action potential (ECAP) of a healthy subject;

[0060] Fig. 7 is a block diagram of a neural stimulation therapy system including the implanted stimulator of Fig. 1 according to one implementation of the present technology;

[0061] Fig. 8 is an illustration of the stimulus pulses delivered by a stimulation program with four interleaved stimulation sets (stimsets);

[0062] Fig. 9 is a schematic illustrating elements and inputs of a closed-loop neural stimulation (CLNS) system with multiple stimsets;

[0063] Fig. 10a is a schematic of a circuit model for unintended stimulus by a conventional neuromodulation device in response to an AC voltage induced by an externally applied oscillating electromagnetic field;

[0064] Fig. 10b is a graph showing an unintended stimulus current flowing through a pair of electrodes and neural tissue as a function of the magnitude of the induced AC voltage according to the circuit model of Fig. 10a;

[0065] Fig. 11 is a schematic of a circuit model of an implantable neuromodulation device, according to some embodiments of the proposed technology;

[0066] Fig. 12 illustrates a device that is a first example of the device of Fig. 11 configured for mitigating unintended stimuli, according to some embodiments of the proposed technology;

[0067] Fig. 13 is a graph of an unintended stimulus current as a function of the magnitude of the induced AC voltage for the circuit model of Fig. 10a and for the device of Fig. 12;

[0068] Fig. 14 illustrates a device that is a second example of the device of Fig. 11 configured for mitigating unintended stimuli, according to some embodiments of the proposed technology;

[0069] Fig. 15 is a graph of an unintended stimulus current as a function of the magnitude of the induced AC voltage for the circuit model of Fig. 10a and for the device of Fig. 14;

[0070] Fig. 16 illustrates a device that is a third example of the device of Fig. 11 configured for mitigating unintended stimuli, according to some embodiments of the proposed technology;

[0071] Fig. 17 is a graph of an unintended stimulus current as a function of the magnitude of the induced AC voltage for the circuit model of Fig. 10a and for the device of Fig. 16;

[0072] Fig. 18 illustrates a device that is a fourth example of the device of Fig. 11 configured for mitigating unintended stimuli, according to some embodiments of the proposed technology;

[0073] Fig. 19 is a graph of an unintended stimulus current as a function of the magnitude of the induced AC voltage for the circuit model of Fig. 10a and for the device of Fig. 18;

[0074] Fig. 20 illustrates a device that is a fifth example of the device of Fig. 11 configured for mitigating unintended stimuli, according to some embodiments of the proposed technology; and

[0075] Fig. 21 illustrates a device that is a sixth example of the device of Fig. 11 configured for mitigating a unintended stimuli, according to some embodiments of the proposed technology.DETAILED DESCRIPTION OF THE PRESENT TECHNOLOGYDevices and systems for neuromodulation

[0076] Fig. 1 schematically illustrates an implanted spinal cord stimulator 100 in a patient 108, according to one implementation of the present technology. Stimulator 100 comprises an electronics module 110 housed within a conductive case, implanted at a suitable location. In one implementation, stimulator 100 is implanted in the patient’s lower abdominal area or posterior superior gluteal region. In other implementations, the electronics module 110 is implanted in other locations, such as in a flank or sub-clavicularly. The electronics module 110 is configured to electrically connect to an electrode assembly, typically comprising an electrode array 150 implanted within the epidural space and connected to the module 110 by a suitable lead. The electrode array 150 may comprise one or more electrodes such as electrode pads on a paddle lead, circular (e.g., ring) electrodes surrounding the body of a percutaneous lead, conformable electrodes, cuff electrodes, segmented electrodes, or any other type of electrodes capable of forming unipolar, bipolar or multipolar electrode configurations for stimulation and measurement. The electrodes may pierce or affix directly to the tissue itself.

[0077] Numerous aspects of the operation of implanted stimulator 100 may be programmable by an external computing device 192, which may be operable by a user such as a clinician or the patient 108. Moreover, implanted stimulator 100 serves a data gathering role, with gathered data being communicated to external device 192 via a transcutaneous communications channel 190. Communications channel 190 may be active on a substantially continuous basis, at periodic intervals, at non-periodic intervals, or upon request from the external device 192. External device 192 may thus provide a clinical interface configured to program the implanted stimulator 100 and recover data stored on the implanted stimulator 100. This configuration is achieved by program instructionscollectively referred to as the Clinical Programming Application (CPA) and stored in an instruction memory of the clinical interface.

[0078] Fig. 2 is a block diagram of the stimulator 100. Electronics module 110 contains a battery 112 and a telemetry module 114. In implementations of the present technology, any suitable type of transcutaneous communications channel 190, such as infrared (IR), radiofrequency (RF), capacitive and / or inductive transfer, may be used by telemetry module 114 to transfer power and / or data to and from the electronics module 110 via communications channel 190. Module controller 116 has an associated memory 118 storing one or more of clinical data 120, clinical settings 121, control programs 122, and the like. Controller 116 is configured by control programs 122, sometimes referred to as firmware, to control a pulse generator 124 to generate stimuli, such as in the form of electrical pulses, in accordance with the clinical settings 121. Electrode selection module 126 switches the generated pulses to the selected electrode(s) of electrode array 150, for delivery of the pulses to the tissue surrounding the selected electrode(s). Measurement circuitry 128, which may comprise an amplifier and / or an analog-to-digital converter (ADC), is configured to process signals comprising neural responses sensed at measurement electrode(s) of the electrode array 150 as selected by electrode selection module 126.

[0079] Fig. 3 is a schematic illustrating interaction of the implanted stimulator 100 with a bundle of target nerve fibres 180 in the patient 108. In the implementation illustrated in Fig. 3 the target fibres 180 may be located in the spinal cord, however in alternative implementations the stimulator 100 may be positioned adjacent any target neural tissue including a peripheral nerve, visceral nerve, sacral nerve, parasympathetic nerve or a brain structure. Electrode selection module 126 selects a stimulus electrode 2 of electrode array 150 through which to deliver a pulse from the pulse generator 124 to surrounding neural tissue including target fibres 180. A pulse may comprise one or more phases, e.g. a monophasic pulse comprises one phase, and a biphasic stimulus pulse 160 comprises two phases. Electrode selection module 126 also selects a return electrode 4 ofthe electrode array 150 for stimulus current return in each phase, to maintain a zero net charge transfer. An electrode may act as both a stimulus electrode and a return electrode over a complete multiphasic stimulus pulse. The use of two electrodes in this manner for delivering and returning current in each stimulus phase is referred to as bipolar stimulation. Alternative embodiments may apply other forms of bipolar stimulation, or may use a greater number of stimulus and / or return electrodes. By contrast, in monopolar stimulation, current is returned through the conductive case of the stimulator 100, which may therefore be configured and function as an electrode though it is not physically part of the electrode array 150. The set of stimulus electrodes and return electrodes is referred to as the stimulus electrodeconfiguration. Electrode selection module 126 is illustrated as connecting to a ground 130 of the pulse generator 124 to enable stimulus current return via the return electrode 4. However, other connections for current return may be used in other implementations.

[0080] Delivery of an appropriate stimulus via electrodes 2 and 4 to the target fibres 180 evokes a neural response 170 comprising an evoked compound action potential (ECAP) which will propagate along the target fibres 180 as illustrated at a rate known as the conduction velocity. The ECAP may be evoked for therapeutic purposes, which in the case of a spinal cord stimulator for chronic pain may be associated with paresthesia at a desired location. To this end, the electrodes 2 and 4 are used to deliver stimuli periodically at any therapeutically suitable frequency, for example 30 Hz, although other frequencies may be used including frequencies as high as the kHz range. In alternative implementations, stimuli may be delivered in a non-periodic manner such as in bursts, or sporadically, as appropriate for the patient 108. To program the stimulator 100 to the patient 108, a clinician may cause the stimulator 100 to deliver stimuli of various configurations which seek to produce a sensation that may be experienced by the patient as paresthesia. When a stimulus electrode configuration is found which evokes paresthesia in a location and of a size which is congruent with the area of the patient’s body affected by pain and of a quality that is comfortable for the patient, the clinician or the patient nominates that configuration for ongoing use. The therapy parameters may be loaded into the memory 118 of the stimulator 100 as the clinical settings 121.

[0081] Fig. 6 illustrates the typical form of an ECAP 600 of a healthy subject, as recorded at a single measurement electrode referenced to the system ground 130 or referenced to an indifferent electrode. Such configurations are referred to as single-ended ECAP measurement. The shape and duration of the single-ended ECAP 600 shown in Fig. 6 is predictable because it is a result of the ion currents produced by the ensemble of fibres depolarising and generating action potentials (APs) in response to stimulation. The evoked action potentials (EAPs) generated synchronously among a large number of fibres sum to form the ECAP 600. The ECAP 600 generated from the synchronous depolarisation of a group of similar fibres comprises a positive peak Pl, then a negative peak Nl, followed by a second positive peak P2. This shape is caused by the region of activation passing the measurement electrode as the action potentials propagate along the individual fibres.

[0082] The ECAP may be recorded differentially using two measurement electrodes, as illustrated in Fig. 3. Differential ECAP measurements are less subject to common-mode noise on the surrounding tissue than single-ended ECAP measurements. Depending on the polarity of recording, a differential ECAP may take an inverse form to that shown in Fig. 6, i.e. a form having two negative peaks Nl and N2, and one positive peak Pl. Alternatively, depending on the distance between the twomeasurement electrodes, a differential ECAP may resemble the time derivative of the ECAP 600, or more generally the difference between the ECAP 600 and a time-delayed copy thereof.

[0083] The ECAP 600 may be characterised by any suitable characteristic(s) of which some are indicated in Fig. 6. The amplitude ofthe positive peak Pl is Ap and occurs at time Tp . The amplitude of the positive peak P2 is Api and occurs at time Tpi. The amplitude of the negative peak Pl is Am and occurs at time Tm. The peak-to-peak amplitude is Api + Am. A recorded ECAP will typically have a maximum peak-to-peak amplitude in the range of microvolts and a duration of 2 to 3 ms.

[0084] The stimulator 100 is further configured to measure the intensity of ECAPs 170 propagating along target fibres 180, whether such ECAPs are evoked by the stimulus from electrodes 2 and 4, or otherwise evoked. To this end, any electrodes of the array 150 may be selected by the electrode selection module 126 to serve as recording electrode 6 and reference electrode 8, whereby the electrode selection module 126 selectively connects the chosen electrodes to the inputs of the measurement circuitry 128. Thus, signals sensed by the measurement electrodes 6 and 8 subsequent to the respective stimuli are passed to the measurement circuitry 128, which may comprise a differential amplifier and an analog -to-digital converter (ADC), as illustrated in Fig. 3. The recording electrode and the reference electrode are referred to as the measurement electrode configuration. The measurement circuitry 128 for example may operate in accordance with the teachings of the above- mentioned International Patent Publication No. WO2012 / 155183.

[0085] Signals sensed by the measurement electrodes 6, 8 and processed by measurement circuitry 128 are further processed by an ECAP detector implemented within controller 116, configured by control programs 122, to obtain information regarding the effect of the applied stimulus upon the target fibres 180. In some implementations, the sensed signals are processed by the ECAP detector in a manner which measures and stores one or more characteristics from each evoked neural response or group of evoked neural responses contained in the sensed signal. In one such implementation, the characteristics comprise a peak-to-peak ECAP amplitude in microvolts (pV). For example, the sensed signals may be processed by the ECAP detector to determine the peak-to-peak ECAP amplitude in accordance with the teachings of International Patent Publication No. WO2015 / 074121, the contents of which are incorporated herein by reference. Alternative implementations of the ECAP detector may measure and store an alternative characteristic from the neural response, or may measure and store two or more characteristics from the neural response.

[0086] Stimulator 100 applies stimuli over a potentially long period such as days, weeks, or months and during this time may store characteristics of neural responses, clinical settings, target response intensity, and other operational parameters in memory 118. To effect suitable SCS therapy, stimulator100 may deliver tens, hundreds or even thousands of stimuli per second, for many hours each day. Each neural response or group of responses generates one or more characteristics such as a measure of the intensity of the neural response. Stimulator 100 thus may produce such data at a rate of tens or hundreds of Hz, or even kHz, and over the course of hours or days this process results in large amounts of clinical data 120 which may be stored in the memory 118. Memory 118 is however necessarily of limited capacity and care is thus required to select compact data forms for storage into the memory 118, to ensure that the memory 118 is not exhausted before such time that the data is expected to be retrieved wirelessly by external device 192, which may occur only once or twice a day, or less.

[0087] An activation plot, or growth curve, is an approximation to the relationship between stimulus intensity (e.g. an amplitude of the current pulse 160) and intensity of neural response 170 evoked by the stimulus (e.g. an ECAP amplitude). Fig. 4a illustrates an idealised activation plot 402 for one posture of the patient 108. The activation plot 402 shows a linearly increasing ECAP amplitude for stimulus intensity values above a threshold 404 referred to as the ECAP threshold. The ECAP threshold exists because of the binary nature of fibre recruitment; if the field strength is too low, no fibres will be recruited. However, once the field strength exceeds a threshold, fibres begin to be recruited, and their individual evoked action potentials are independent of the strength of the field. The ECAP threshold 404 therefore reflects the field strength at which significant numbers of fibres begin to be recruited, and the increase in response intensity with stimulus intensity above the ECAP threshold reflects increasing numbers of fibres being recruited. Below the ECAP threshold 404, the ECAP amplitude may be taken to be zero. Above the ECAP threshold 404, the activation plot 402 has a positive, approximately constant slope indicating a linear relationship between stimulus intensity and the ECAP amplitude. Such a relationship may be modelled in piecewise linear form as:

[0088] where 5 is the stimulus intensity, d is the ECAP amplitude, T is the ECAP threshold and S is the slope of the activation plot (referred to herein as the patient sensitivity) above the ECAP threshold T. The sensitivity S and the ECAP threshold T are the key parameters of the activation plot 402.

[0089] Fig. 4a also illustrates a discomfort threshold 408, which is a stimulus intensity above which the patient 108 experiences uncomfortable or painful stimulation. Fig. 4a also illustrates a perception threshold 410. The perception threshold 410 corresponds to an ECAP amplitude that is barely perceptible by the patient. There are a number of factors which can influence the position of the perception threshold 410, including the posture of the patient. Perception threshold 410 may correspond to a stimulus intensity that is greater than the ECAP threshold 404, as illustrated in Fig.4a, if patient 108 does not perceive low levels of neural activation. Conversely, the perception threshold 410 may correspond to a stimulus intensity that is less than the ECAP threshold 404, if the patient has a high perception sensitivity to lower levels of neural activation than can be detected in an ECAP, or if the signal-to-noise ratio of the ECAP is low.

[0090] For effective and comfortable operation of an implantable neuromodulation device such as the stimulator 100, it is desirable to maintain stimulus intensity within a therapeutic range. A stimulus intensity within a therapeutic range 412 is above the ECAP threshold 404 and below the discomfort threshold 408. In principle, it would be straightforward to measure these limits and ensure that stimulus intensity, which may be closely controlled, always falls within the therapeutic range 412. However, the activation plot, and therefore the therapeutic range 412, varies with the posture of the patient 108.

[0091] Fig. 4b illustrates the variation in the activation plots with changing posture of the patient. A change in posture of the patient may cause a change in impedance of the electrode-tissue interface or a change in the distance between electrodes and the spinal cord. While the activation plots for only three postures, 502, 504 and 506, are shown in Fig. 4b, the activation plot for any given posture can he between or outside the activation plots shown, on a continuously varying basis depending on posture. Consequently, as the patient’s posture changes, the ECAP threshold changes, as indicated by the ECAP thresholds 508, 510, and 512 for the respective activation plots 502, 504, and 506. Additionally, as the patient’s posture changes, the patient sensitivity also changes, as indicated by the varying slopes of activation plots 502, 504, and 506. In general, as the distance between the stimulus electrodes and the spinal cord increases, the ECAP threshold increases and the sensitivity decreases. The activation plots 502, 504, and 506 therefore correspond to increasing distance between stimulus electrodes and spinal cord, and decreasing patient sensitivity.

[0092] To keep the applied stimulus intensity within the therapeutic range as patient posture varies, in some implementations an implantable neuromodulation device such as the stimulator 100 may adjust the applied stimulus intensity based on a feedback variable that is determined from one or more measured ECAP characteristics. In one implementation, the device may adjust the stimulus intensity to maintain the measured ECAP amplitude at or near a target response intensity. For example, the device may calculate an error between a target ECAP amplitude and a measured ECAP amplitude, and adjust the applied stimulus intensity to bring the measured ECAP amplitude closer to the target ECAP amplitude, such as by adding the scaled error to the current stimulus intensity. A neuromodulation device that operates by adjusting the applied stimulus to maintain a feedback variable at or near a target value is said to be operating in closed-loop mode and will also be referredto as a closed-loop neural stimulation (CLNS) device. By adjusting the applied stimulus intensity to maintain the measured ECAP amplitude at an appropriate target response intensity, such as a target ECAP amplitude 520 illustrated in Fig. 4b, a CLNS device will generally keep the stimulus intensity within the therapeutic range as patient posture varies.

[0093] A CLNS device comprises a pulse generator that takes a stimulus intensity value and converts it into a neural stimulus comprising a sequence of electrical pulses according to a predefined stimulation pattern. The stimulation pattern is parametrised by multiple stimulus parameters including stimulus amplitude, pulse width, number of phases, order of phases, number of stimulus electrode poles (two for bipolar, three for tripolar etc.), and stimulus rate or frequency. At least one of the stimulus parameters, for example the stimulus amplitude, is controlled by the feedback loop.

[0094] In an example CLNS system, a user sets a target response intensity, and the CLNS device performs proportional -integral -differential (PID) control. In some implementations, the differential contribution is disregarded and the CLNS device uses a first order integrating feedback loop. The pulse generator produces stimulus in accordance with a stimulus intensity parameter, which evokes a neural response in the patient. The intensity of an evoked neural response (e.g. an ECAP) is measured by the CLNS device and compared to the target response intensity.

[0095] The measured neural response intensity, and its deviation from the target response intensity, is used by the feedback loop to determine possible adjustments to the stimulus intensity parameter to maintain the neural response at the target intensity. If the target intensity is properly chosen, the patient receives consistently comfortable and therapeutic stimulation through posture changes and other perturbations to the stimulus / response behaviour.

[0096] Fig. 5 is a schematic illustrating elements and inputs of a closed-loop neural stimulation (CLNS) system 300, according to one implementation of the present technology. The system 300 comprises a pulse generator 312 which converts a stimulus intensity parameter (for example a stimulus current amplitude) s. in concert with a set of predefined stimulus parameters, to a neural stimulus comprising a sequence of electrical pulses on the stimulus electrodes (not shown in Fig. 5). According to one implementation, the predefined stimulus parameters comprise the number and order of phases, the number of stimulus electrode poles, the pulse width, and the stimulus rate or frequency.

[0097] The generated stimulus crosses from the electrodes to the spinal cord, which is represented in Fig. 5 by the dashed box 308. The box 309 represents the evocation of a neural response y by the stimulus as described above. The box 311 represents the evocation of an artefact signal a, which is dependent on stimulus intensity and other stimulus parameters, as well as the electrical environment of the measurement electrodes. Various sources of measurement noise n, as well as the artefact a,may add to the evoked response y at the summing element 313 to form the sensed signal r, including: electrical noise from external sources such as 50 Hz mains power; electrical disturbances produced by the body such as neural responses evoked not by the device but by other causes such as peripheral sensory input; EEG; EMG; and electrical noise from measurement circuitry 318.

[0098] The neural recruitment arising from the stimulus is affected by mechanical changes, including posture changes, walking, breathing, heartbeat and so on. Mechanical changes may cause impedance changes, or changes in the location and orientation of the nerve fibres relative to the electrode array(s) . As described above, the intensity of the evoked response provides a measure of the recruitment of the fibres being stimulated. In general, the more intense the stimulus, the more recruitment and the more intense the evoked response. An evoked response typically has a maximum amplitude in the range of microvolts, whereas the voltage resulting from the stimulus applied to evoke the response is typically several volts.

[0099] Measurement circuitry 318, which may be identified with measurement circuitry 128, amplifies the sensed signal r (including evoked neural response, artefact, and measurement noise), and samples the amplified sensed signal r to capture a “signal window” 319 comprising a predetermined number of samples of the amplified sensed signal r. The ECAP detector 320 processes the signal window 319 and outputs a measured neural response intensity d. In one implementation, the neural response intensity comprises a peak-to-peak ECAP amplitude. The measured response intensity d (an example of a feedback variable) is input into the feedback controller 310. The feedback controller 310 comprises a comparator 324 that compares the measured response intensity d to a target ECAP amplitude as set by the target ECAP controller 304 and provides an indication of the difference between the measured response intensity d and the target ECAP amplitude. This difference is the error value, e.

[0100] The feedback controller 310 calculates an adjusted stimulus intensity parameter, s. with the aim of maintaining a measured response intensity d equal to the target ECAP amplitude . Accordingly, the feedback controller 310 adjusts the stimulus intensity parameter .s' to minimise the error value, e. In one implementation, the controller 310 utilises a first order integrating function, using a gain element 336 and an integrator 338, in order to provide suitable adjustment to the stimulus intensity parameter .v. According to such an implementation, the current stimulus intensity parameter .s' may be determined by the feedback controller 310 as s = f K edt (2)

[0101] where K is the gain of the gain element 336 (the controller gain). This relation may also be represented as6s = Ke (3)

[0102] where 8s is an adjustment to the current stimulus intensity parameter s.

[0103] A target ECAP amplitude is input to the feedback controller 310 via the target ECAP controller 304. In one embodiment, the target ECAP controller 304 provides an indication of a specific target ECAP amplitude. In another embodiment, the target ECAP controller 304 provides an indication to increase or to decrease the present target ECAP amplitude. The target ECAP controller 304 may comprise an input into the CLNS system 300, via which the patient or clinician can input a target ECAP amplitude, or indication thereof. The target ECAP controller 304 may comprise memory in which the target ECAP amplitude is stored, and from which the target ECAP amplitude is provided to the feedback controller 310.

[0104] A clinical settings controller 302 provides clinical settings to the system 300, including the feedback controller 310 and the stimulus parameters for the pulse generator 312 that are not under the control of the feedback controller 310. In one example, the clinical settings controller 302 may be configured to adjust the controller gain K of the feedback controller 310 to adapt the feedback loop to patient sensitivity. The clinical settings controller 302 may comprise an input into the CLNS system 300, via which the patient or clinician can adjust the clinical settings. The clinical settings controller 302 may comprise memory in which the clinical settings are stored, and are provided to components of the system 300.

[0105] In some implementations, two clocks (not shown) are used, being a stimulus clock operating at the stimulus frequency (e.g. 60 Hz) and a sample clock for sampling the sensed signal r (for example, operating at a sampling frequency of 16 kHz). As the ECAP detector 320 is linear, only the stimulus clock affects the dynamics of the CLNS system 300. On the next stimulus clock cycle, the pulse generator 312 generates a stimulus in accordance with the adjusted stimulus intensity s. Accordingly, there is a delay of one stimulus clock cycle before the stimulus intensity is updated in light of the error value e.

[0106] Fig. 7 is a block diagram of a neural stimulation system 700. The neural stimulation system 700 is centred on a neuromodulation device 710. In one example, the neuromodulation device 710 may be implemented as the stimulator 100 of Fig. 1, implanted within a patient (not shown). The neuromodulation device 710 is connected wirelessly to a remote controller (RC) 720. The remote controller 720 is a portable computing device that provides the patient with control of their stimulation in the home environment by allowing control of the functionality of the neuromodulation device 710, including one or more of the following functions: enabling or disabling stimulation; adjustment ofstimulus intensity or target response intensity; and selection of a stimulation control program from the control programs stored on the neuromodulation device 710.

[0107] The charger 750 is configured to recharge a rechargeable power source of the neuromodulation device 710. The recharging is illustrated as wireless in Fig. 7 but may be wired in alternative implementations.

[0108] The neuromodulation device 710 is wirelessly connected to a Clinical System Transceiver (CST) 730. The wireless connection may be implemented as the transcutaneous communications channel 190 of Fig. 1. The CST 730 acts as an intermediary between the neuromodulation device 710 and the Clinical Interface (CI) 740, to which the CST 730 is connected. A wired connection is shown in Fig. 7, but in other implementations, the connection between the CST 730 and the CI 740 is wireless.

[0109] The CI 740 may be implemented as the external computing device 192 of Fig. 1. The CI 740 is configured to program the neuromodulation device 710 and recover data stored on the neuromodulation device 710. This configuration is achieved by program instructions collectively referred to as the Clinical Programming Application (CPA) and stored in an instruction memory of the CI 740.

[0110] For some patients, it is beneficial for a neural stimulation therapy program to comprise multiple stimulation sets. A stimulation set (“stimsef ’) is a set of stimulus and return electrodes, or more precisely a stimulus electrode configuration (SEC), along with the stimulus parameters that govern the stimulation pulses delivered via that SEC.

[0111] Fig. 8 is an illustration 800 of the stimulus pulses delivered by a stimulation program with four interleaved stimsets. The stimulus pulse train delivered according to each stimset is illustrated on a separate, but vertically aligned, horizontal axis representing time. All the stimulus pulse trains are delivered at the same stimulus frequency. (It is not a requirement that all the stimulus pulse trains for the respective stimsets are delivered at the same stimulus frequency; however it is so represented in Fig. 8 for ease of illustration.) The first stimulus pulse 810, delivered according to the first stimset, is illustrated as a biphasic, anodic-first stimulus pulse, though many other stimulus pulse types are contemplated. The second, third, and fourth stimulus pulses 820, 830, and 840, delivered according to the second, third, and fourth stimsets in the program respectively, are also biphasic, anodic-first stimulus pulses with different pulse widths and different amplitudes. Each stimulus pulse is illustrated as delayed in time by a constant amount (the inter-stimulus interval, or ISI, 815) from the stimulus pulse delivered according to the preceding stimset. However, this is not to be interpreted as limiting, since the intervals between the pulses in the various stimsets may be different. Because all thestimulus pulse trains in Fig. 8 are delivered at the same stimulus frequency, the four stimulus pulses 810, 820, 830, 840 form a cycle that repeats indefinitely without any change to the relative timing of the pulses from the different stimsets. The fifth stimulus pulse 850 is a subsequent pulse in the pulse train delivered according to the first stimset and is therefore illustrated on the same time axis as the first stimulus pulse 810, and the cycle repeats thereafter. The stimulus period 890 is the period of repetition of the full cycle and is equal to the reciprocal of the stimulus frequency.

[0112] Also illustrated is an evoked neural response in the form of an evoked compound action potential (ECAP) 860 as sensed via a predetermined measurement electrode configuration (MEC) on a common time axis with the stimulus pulses. The illustrated ECAP 860 is evoked by the fourth stimulus pulse 840. A closed-loop neural stimulation (CLNS) system programmed with multiple interleaved stimsets, as illustrated in Fig. 8, may be based on measurements of the ECAP 860. That is to say, closed-loop adjustments to the stimulus parameters of all stimsets may all be based on measurements of the ECAP 860 from a single stimset, referred to as the applied stimset. In Fig. 8, the final stimset in the cycle is the applied stimset.

[0113] If the ISI 815 is short, ECAPs evoked by the first three stimulus pulses 810, 820, and 830 are potentially obscured by stimulus crosstalk and / or artefact from the stimulus pulses 820, 830, and 840. Therefore, if the ISI 815 is short, only the final stimset in the cycle may evoke a measurable ECAP. If the ISI 815 is greater than the refractory period and sufficiently long that ECAPs evoked by the earlier stimsets are not obscured by stimulus crosstalk and artefact from the other stimulus pulses in the cycle, any of the stimsets in the cycle may evoke a measurable ECAP.

[0114] Fig. 9 is a schematic illustrating elements and inputs of a multi-stimset CLNS system 900 with multiple stimsets. The multi-stimset CLNS system 900 is the same as the CLNS system 300 of Fig. 5, with like numbers indicating like elements, with the addition of three further stimsets. The four stimsets are labelled A, B, C, and D and are delivered by pulse generators 312A, 312B, 312C, and 312D (the latter of which corresponds to the pulse generator 312 in the CLNS system 300) according to respective stimulus intensity parameters SA, SB, SC, and SD, and via respective SECs. The pulses delivered by the pulse generators 312A, 312B, 312C, and 312D correspond to the stimulus pulses 810, 820, 830, and 840 of Fig. 8. Stimset D, delivered by the pulse generator 312D, is delivered last in the cycle and is the applied stimset, from which the ECAP is measured. In the implementation of Fig. 8, the stimulus intensity parameter SD for stimset D is scaled by ratios RA, RB, and Rc to obtain the stimulus intensity parameters SA, SB, and sc for stimsets A, B, and C respectively. The ratios RA, RB, and Rc are fixed at the ratios of the respective stimulus intensities at which the respective stimsets were originally programmed, to the originally programmed stimulus intensity of the applied stimsetD. In such an implementation, the stimulus intensity parameters SA, SB, and sc always remain in fixed ratio with the applied stimulus intensity parameter so and with each other. This is referred to as ratiometric adjustment. So for example, if the originally programmed stimulus intensities were 1 mA, 2 mA, 4 mA, and 6 mA for the four stimsets A, B, C, and D respectively, the ratios RA, RB, and Rc are fixed at programming time at 1 / 6, 1 / 3, and 2 / 3 respectively and form part of the clinical settings 121 of the multi-stimset program. If during therapy the feedback controller 310 adjusts the applied stimset intensity parameter SD to 6.6 mA, the stimulus intensity parameters SA, SB, and sc of the nonapplied stimsets are automatically adjusted to 1.1 mA, 2.2 mA, and 4.4 mA respectively. The clinical settings controller 302 provides to the pulse generators 312A, 312B, 312C, and 312D the stimulus parameters that are not under the control of the feedback controller 310.

[0115] It may be seen from Fig. 9 that the adjustments to the stimulus intensity parameters after each stimulus cycle are all in fixed proportion. A ratiometric multi-stimset CLNS system therefore emulates a CLNS system with four separate feedback loops driven by the four stimsets, wherein each loop has the same controller gain. A ratiometric multi-stimset CLNS system is effective to maintain the responses evoked by each stimset at a constant neural response intensity on the condition that when the patient moves to a new posture, the threshold and slope of all activation plots, both for applied and non-applied stimsets, move in a proportional manner. (See Fig. 4b for examples of activation plots for a given stimset in different postures.)Unintended stimulus from externally applied oscillating electromagnetic fields

[0116] Neuromodulation devices, and particularly implantable neuromodulation devices such as the stimulator 100, may be exposed to externally applied oscillating electromagnetic fields. One example situation where the case, the electrodes, and / or the respective lead wires of the stimulator 100 are exposed to an externally applied oscillating electromagnetic field is during a magnetic resonance imaging (MRI) scan of the patient. An MRI scanner produces one or more time-varying fields, such as a gradient field or the field associated with radio frequency (RF) magnetic excitation pulses to alter the alignment of specified nuclei within a particular volume or plane to be imaged within the patient. This scenario is challenging because the stimulator 100 is susceptible to developing unintended electromagnetic field-induced effects in relation to its conducting elements, such as between any pair of the electrodes 2, 4, 6, 8 of electrode array 150 (i.e., via the respective lead wires), and / or between any electrode and the conductive case of the stimulator 100 (i.e., where the case acts as an “electrode”).

[0117] The stimulator 100 is configured to operate in an active mode during which neural stimuli are delivered via the stimulus electrodes to a neural pathway of a patient, and an evoked neural responseis subsequently measured from a signal captured by the one or more measurement electrodes. Outside of the stimuli provided to the neural pathway by the stimulator 100 in the active mode, it is desired to prevent any stimulus to the neural pathway. In some implementations, the stimulator 100 may be transitioned between the active mode, and a “stock” or “standby” mode during which the electrodes are electrically inactive.

[0118] Fig. 10a schematically illustrates a circuit model 1000 for unintended stimulus by a neuromodulation device (stimulator) 100 in response to an externally applied oscillating electromagnetic field (e.g. as resulting from an MRI scan). Stimulator 100 comprises stimulation electrodes 2 and 4 of an electrode array 150, each connected to respective lead wires 1012 and 1014, which in turn are connected to the stimulator 100 at connection ports 1020 and 1040 of the stimulator 100. Lead wires 1012 and 1014 each have a distal end placed into contact with neural tissue 1001 of the patient to permit the delivery of a neural stimulus, as described above. The connection of each electrode 2 and 4 to the stimulator 100 has an associated parasitic effect, as represented by a respective set of one or more parasitic components 1024 and 1044. As shown in Fig. 10a, the sets of components 1024 and 1044 include respective pairs of diodes 1024a, 1024b and 1044a, 1044b. In the configuration of Fig. 10a, the respective pairs of diodes 1024a, 1024b and 1044a, 1044b have forward bias voltages of Vbias= -0.6V.

[0119] An externally applied oscillating electromagnetic field, such as for example the gradient field generated by an MRI scan, acts on a surface S of the neural tissue 1001 inducing an AC voltage over the lead wires 1012 and 1014. The induced AC voltage VACis proportional to the rate of change of the magnetic flux density <bBof the applied field according towhere B is the instantaneous magnetic flux density vector. If B is uniform over surface S, then the magnetic flux <bBcan be approximated as <bB= | B | A where A is the area of the surface S of the neural tissue 1001. That is, for a gradient field varying at an excitation frequency (e.g., 2 kHz) the induced AC voltage Vacwill manifest as a train of biphasic square pulses at the excitation frequency.

[0120] Fig. 10a illustrates a power supply circuit 1003 comprising: a battery 1009 configured to provide a DC supply of VBat; and a charge pump 1002 or similar circuit, collectively configured to provide a reference voltage Vrej- at connection node 1008 during normal operation of the stimulator 100. However, when an externally applied oscillating electromagnetic field is known or expected to be present, such as during an MRI scan, the stimulator 100 enters stock mode, disabling the chargepump 1002, which effectively becomes a high impedance element between the DC supply 1009 and the connection node 1008.

[0121] In this configuration, the application of an external oscillating electromagnetic field causing an induced AC voltage Vacbetween electrodes 2 and 4 with a magnitude V (e.g. a peak-zero amplitude) of greater than or equal to the combined voltage drop Vparaacross the sets of parasitic components 1024, 1044 (e.g., Vpara= 0.6 X 2 = 1.2 V) causes an unintended stimulus current tim t° Sow through the tissue 1001, which results in an unintended stimulus to the patient. The minimum magnitude Vmin at which unintended stimulus currents start to flow, which in Fig. 10a is equal to the parasitic voltage Vpara, is referred to herein as the activation voltage. The current flow tim through the tissue induced by an oscillating electromagnetic field is alternating and pulsed in synchrony with the induced AC voltage Vac. However, the diodes 1024a, 1024b and 1044a, 1044b rectify the alternating pulsed current to a unidirectional pulsed current into the connection node 1008.

[0122] The supply capacitor 1006 is connected to node 1008 and charges on each cycle as a result of the unidirectional pulsed current flow into the node 1008. Eventually, the charge on the supply capacitor 1006 increases to a value that is sufficient to increase the reference potential Vref to V — Vparaf°rthe induced AC voltage VAC. Once this occurs, there is a small but finite current that flows into the supply capacitor 1006 in every cycle to restore any discharge between cycles of induced AC voltage Vac. This small but finite current is the steady-state unintended stimulus current Istim-

[0123] Fig. 10b is a graph 1050 showing the peak-zero unintended stimulus current Istimflowing through the pair of electrodes 2, 4 and neural tissue 1001 as a function of the magnitude V. An unintended stimulus corresponding to a current Istim> 0 occurs for V > Vparawhich increases with the value of V (shown in plot 1052 of graph 1050). This issue is exacerbated in implementations of the power supply circuit 1003 that include a Zener diode (not shown) connected to the connection node 1008 (e.g., with a reverse breakdown voltage of VZener= 18.6 V) to protect the device 100. The supply capacitor 1006 will charge to an upper limit of Vzenerand no further, so any excess of the magnitude V above Vzener+ Vparawill be dropped across the neural tissue 1001, leading to significantly greater unintended stimulus current Istimthan would be the case without the Zener diode. That is, in this scenario the value of Istimwill increase more sharply with the excess of the magnitude V over Vzener+ Vpara(as shown in plot 1054 of Fig. 10b).

[0124] It is therefore desired to develop neuromodulation devices and techniques that mitigate unintended stimulus currents resulting from an externally applied oscillating electromagnetic field,such as, for example, the gradient field that is experienced by a patient during an MRI scan, or that at least provide a useful alternative.Overview of the disclosed technology

[0125] Disclosed herein are methods, devices, and systems for mitigating unintended stimulus currents that result from externally induced AC voltages occurring between electrodes of a neuromodulation device. In some scenarios, the AC voltages are induced during an MRI scan of the patient, or from or other activity that results in the application of an external oscillating electromagnetic field to at least a pair of the electrodes, the corresponding leads, and / or the case of the device.

[0126] Fig. 11 shows a circuit model of an implantable neuromodulation device 1100 according to embodiments of the disclosed technology. Neuromodulation device 1100 comprises an electronics module 110 that includes a regulator circuit 1101 configured to control a reference voltage or an impedance presented to a pair of electrodes 2, 4 of the electrode array 150 at a connection point 1108, in the presence of an AC voltage induced between the corresponding electrode leads (i.e., lead wires 1012, 1014), thereby mitigating an unintended stimulus current flowing through neural tissue 1001 in electrical contact with the electrodes 2, 4 (i.e., via the lead wires 1012, 1014) as a result of the induced AC voltage.

[0127] According to a first implementation of the proposed technology, the regulator circuit 1101 is configured to sustain a reference voltage at the connection point 1108. For example, the one or more control elements 1104 augment the voltage circuit 1102 to sustain the reference voltage at point 1108, thereby increasing the activation voltage Vmin (a minimum value of the magnitude V of the induced AC voltage Vacthat is required to cause a substantial unintended stimulus current Istimto flow through the neural tissue 1001).

[0128] In some embodiments, the regulator circuit 1101 includes one or more voltage-dependent current control elements 1104 (e.g., one or more diodes) configured relative to the voltage circuit 1102 to passively increase the activation voltage Vmin. In other embodiments, the regulator circuit 1101 comprises one or more control elements 1104 configured to adjust the output of the voltage circuit 1102 actively in response to the detection of the induced AC voltage.

[0129] In some embodiments, the regulator circuit 1101 controls the reference voltage of the connection point 1108 based on whether the neuromodulation device 1100 is in an active mode, during which neural stimuli are applied via one or more stimulus electrodes of the electrode array 150, or alternatively in a standby (or stock) mode. This advantageously eliminates the need for detection of the induced AC voltage resulting from an externally applied oscillating electromagneticfield, thereby reducing the complexity and the power consumption of the regulator circuit 1101 in such embodiments relative to other embodiments.

[0130] According to a second implementation of the proposed technology, the regulator circuit 1101 is configured to mitigate an unintended stimulus current, induced as a result of application of an externally applied oscillating electromagnetic field to electrical conductors (e.g., lead wires and / or the case) connected to at least the pair of electrodes 2, 4 of the neural stimulation device 100, by providing a high impedance to a corresponding AC voltage induced between the pair of electrodes 2, 4. The regulator circuit 1101 provides a high impedance to the AC voltage induced between the pair of electrodes 2, 4 (or any other pair of the electrical conductors) that is less than the activation voltage Vmin where the activation voltage Vmin is higher than the parasitic voltage Vpara. This contrasts with the activation voltage Vmin for the circuit model 1000, in which the activation voltage Vmin is equal to the parasitic voltage Vpara. The result will be to mitigate the unintended stimulus current induced as a result of application of the externally applied oscillating electromagnetic field.

[0131] The regulator circuit 1101 is configured to provide the high impedance by providing a reduced effective capacitance at the connection point 1108. In some examples, the regulator circuit 1101 is configured to decouple the supply capacitor 1106 from the connection point 1108.

[0132] For example, the regulator circuit 1101 may comprise components including: a voltage circuit 1102 configured to provide a supply voltage to its output terminal; a supply capacitor 1106 having a first terminal connected to ground and a second terminal connected to an input terminal of the control elements 1104; and one or more control elements 1104 connected between the voltage circuit 1102 and the connection point 1108. The one or more control elements 1104 control the electrical connection between the second terminal of the supply capacitor 1106 and the connection point 1108. According to the second implementation, the one or more control elements 1104 reduce the effective capacitance provided by the supply capacitor 1106 to the connection point 1108 by decoupling the supply capacitor 1106 from the connection point 1108 (i.e., to substantially reduce or prevent the charging of the supply capacitor 1106 by the induced AC voltage).

[0133] The decoupling of the supply capacitor 1106 reduces the effective capacitance observed by the electrodes 2, 4 at the connection point 1108. The decoupling may be achieved by control elements 1104 that comprise one or more diodes (e.g., Schottky diodes) or one or more other elements operating as switches (e.g., metal -oxide semiconductor transistors) arranged with respective electrical connections to the supply capacitor 1106 and the connection point 1108, as described herein.

[0134] In some embodiments, the regulator circuit 1101 is configured to provide the reduced effective capacitance to the connection point 1108 when the implantable device 1100 is not in the active mode,and to provide a relatively higher effective capacitance to the connection point 1108 when the implantable device 100 is in the active mode. For example, the one or more control elements 1104 of the regulator circuit 1101 may receive an input indicating that the neuromodulation device 100 is not in the active mode, and subsequently reduce the capacitance at the connection point 1108 without regard to the corresponding induced AC voltage Cacbetween the electrodes. However, it will be understood that the operation of the regulator circuit 1101 to provide a high impedance to mitigate an unintended stimulus current is not necessarily dependent on the mode of the device 1100. That is, advantageously, the regulator circuit 1101 may be configured to mitigate unintended stimulus currents when the neuromodulation device 1100 is in the active mode or any other mode.

[0135] The proposed technology for mitigating unintended stimulus resulting from an externally applied oscillating electromagnetic field are described herein with respect to a pair of stimulus electrodes 2 and 4 of the device 1100 (as shown in Fig. 11). However, it will be appreciated that in some embodiments the electrode array 150 may comprise only one stimulus electrode and at least one other electrode, for example to sense a signal evoked by a neural stimulus delivered via the stimulus electrode(s). Further, it will be appreciated that other electrodes of the electrode array 150 of device 100 are similarly configured to electrodes 2 and 4 with respect to the techniques proposed herein below. Therefore, the mitigation of an unintended stimulus, as resulting from an AC voltage being induced by an externally applied oscillating electromagnetic field, may be achieved between any pair of the plurality of electrodes 2, 4, 6, 8 of electrode array 150, and / or an electrode together with the case (that electrically acts as an electrode), according to the same principles as described for electrodes 2, 4.Mitigation of unintended stimulus by sustaining a reference voltage

[0136] Fig. 12 illustrates a device 1200 that is a first example of the device 1100 configured for mitigating unintended stimuli according to the first implementation of the proposed technology. Voltage circuit 1102 as depicted in Fig. 11 comprises a DC voltage source 1209, such as a battery, providing a DC voltage Vbatto a power supply circuit 1202, such as for example a charge pump. The charge pump 1202 has a first terminal configured to receive the DC voltage from the battery 1209 and an output terminal. The one or more control elements 1104 as depicted in Fig. 11 comprise a diode 1205 having a first terminal (anode) connected to the first terminal of the charge pump 1202, and a second terminal (cathode) connected to the connection point 1108. The one or more control elements 1104 also connect the output terminal of the charge pump 1202 and the supply capacitor 1106 to the connection point 1108.

[0137] The forward bias drop Vbiasof the diode 1205 sustains the reference voltage Vref at the connection point 1108 at Vbat— Vbias. The activation voltage Vmin required for unintended stimulus is therefore given by Vref+ Vpara= Vbat- Vbias+ Vpara.

[0138] Fig. 13 is a graph 1300 of the peak-zero unintended stimulus current Istimas a function of the magnitude V of the induced AC voltage Vacfor the conventional circuit model 1000 of Fig. 10a (as shown by plots 1302 and 1304 for a circuit model without and with a Zener diode respectively) and for the device 1200 of Fig. 12 (i.e., as shown in plot 1306). In the example device 1200 of Fig. 12, the diode 1205 has a forward bias voltage of Vbias= 0.6 V and the battery voltage is Vbat= 3.7 V. The activation voltage Vmin required for unintended stimulus is therefore given by 3.7 — 0.6 + 1.2 = 4.3 V. As observed by comparing plot 1302 with plot 1306 in graph 1300, the proposed device 1200 advantageously reduces the stimulus current Istimfor magnitudes V above the electrode parasitic voltage Vpara(1.2 V).

[0139] Fig. 14 illustrates a device 1400 that is a second example of the device 1100 configured for mitigating unintended stimuli according to the first implementation of the proposed technology. Voltage circuit 1102 comprises a DC voltage source 1409, such as a battery, providing a DC voltage VBatto a power supply circuit 1402. The power supply circuit 1402 has a first terminal configured to receive the DC voltage from the battery 1409 and an output terminal. The power supply circuit 1402 may be a switched mode power supply (SMPS), a charge pump, or any other circuit configured to supply an auxiliary voltage VAux.

[0140] The one or more control elements 1104 comprise an AC voltage detector 1405 having first and second terminals connected to the electrodes 2, 4, and a third terminal connected to an enable input of the power supply circuit 1402. The one or more control elements 1104 also connect the output terminal of the power supply circuit 1402 and the supply capacitor 1106 to the connection point 1108. The AC voltage detector 1405 is configured to detect the AC voltage of magnitude V induced between the electrodes 2, 4 (i.e., based on a signal received over the first and second terminals of the detector 1405). In response to detecting a value of the magnitude V above a predetermined level (e.g., set at or close to 0 volts), the AC voltage detector 1405 enables the power supply circuit 1402 which subsequently sustains the voltage at the connection point 1108 at the auxiliary voltage VAux(e.g,. 15 V). The activation voltage Vmin is thereby increased to VAux+ Vpara.

[0141] Fig. 15 is a graph 1500 of the peak-zero unintended stimulus current Istimas a function of the magnitude V for the conventional circuit model 1000 of Fig. 10a (as shown by plots 1502 and 1504 for a circuit model without and with a Zener diode respectively) and for the device 1400 of Fig. 14 (i.e., as shown in plot 1506). In the example device 1400 of Fig. 14, the auxiliary voltage is VAux=15 V. The activation voltage Vmin required for unintended stimulus is therefore given by 15 + 1.2 = 16.2 V. As observed by comparing plot 1502 with plot 1506 in graph 1500, the example device 1400 provides a further reduction in the stimulus current Istimfor magnitudes V above the electrode parasitic voltage Vpara(1.2 V).

[0142] For a sufficiently high auxiliary voltage VAux, the circuit 1400 advantageously mitigates the unintended stimulus current for a larger magnitude V compared to the first example device 1200. Further, in some examples the auxiliary voltage may be adjusted (e.g., with a predetermined variance relative to a base value, such as VAux= 15 + 5 V) to permit a corresponding adjustment of the degree to which the regulator circuit 1101 is effective to mitigate the unintended stimulus currents.

[0143] Fig. 16 illustrates a device 1600 that is a third example of the device 1100 configured for mitigating unintended stimuli according to the first implementation of the proposed technology. Voltage circuit 1102 comprises a DC voltage source 1609, such as a battery, providing a voltage VBatto a power supply circuit 1602. The power supply circuit 1602 has a first terminal connected to the battery 1609 and an output terminal. The power supply circuit 1602 may be a switched mode power supply (SMPS), a charge pump, or any other circuit configured to supply an auxiliary voltage VAux.

[0144] The one or more control elements 1104 comprise a mode switch 1605 connected to an enable input of the power supply circuit 1602. The one or more control elements 1104 also connect the output terminal of the power supply circuit 1602 and the supply capacitor 1106 to the connection point 1108. Example device 1600 is similar to example device 1400 of Fig. 14, where the AC voltage detector 1405 is replaced by the mode switch 1605, which enables the power supply circuit 1602 conditionally based on the mode of the neuromodulation device 1600. For example, in response to the neuromodulation device 100 being in a mode other than the active mode, the mode switch 1605 enables the power supply circuit 1602 which subsequently sustains the voltage at the connection point 1108 at the auxiliary voltage VAux. thereby increasing the activation voltage Vmin at the connection point 1108 to y4UX+ Vpara. In response to the neuromodulation device 1600 being in the active mode, the mode switch 1605 disables the power supply circuit 1602 such that the activation voltage Vmin at the connection point 1108 returns to Vpara.

[0145] Fig. 17 is a graph 1700 of the peak-zero unintended stimulus current Istimas a function of the magnitude V for the conventional circuit model 1000 of Fig. 10a (as shown by plots 1702 and 1704 for a circuit model without and with a Zener diode respectively) and for the device 1600 of Fig. 16 (i.e., as shown in plot 1706). In the example device 1600 of Fig. 16, the auxiliary voltage is VAux= 15 V. The performance of the device 1600 is identical to that of device 1400, while removing theneed for an AC voltage detector 1405 to conditionally enable the power supply circuit 1402 based on detection of the induced AC voltage above the predetermined level.Mitigation of unintended stimulus by providing a high impedance

[0146] Fig. 18 illustrates a device 1800 that is a fourth example of the device 1100 configured for mitigating unintended stimuli according to the second implementation of the proposed technology. Voltage circuit 1102 comprises a voltage supply circuit 1802. The voltage supply circuit 1802 has a first terminal connected to ground and an output terminal. The voltage supply circuit 1802 is configured to be enabled when the device 100 is in active mode, and disabled when the device 100 is in stock mode.

[0147] The one or more control elements 1104 of the regulator circuit 1101 include a diode 1803 with first and second terminals (anode and cathode) respectively connected to the output terminal of the voltage supply circuit 1802 and the connection point 1108. The one or more control elements 1104 also connect the output terminal of the voltage supply circuit 1802 to the supply capacitor 1106, so as to conditionally decouple the supply capacitor 1106 from the connection point 1108. In some examples, the diode 1803 is a Schottky diode . The diode 1803 is configured to be reverse biased when the implantable device is in stock mode, i.e. the voltage supply circuit 1802 is disabled, thereby decoupling the supply capacitor 1106 from the connection point 1108 and substantially preventing current flow between the connection point 1108 and the supply capacitor 1106.

[0148] In the example device 1800, regulator circuit 1101 further includes a Zener diode 1805 having first and second terminals respectively connected to the connection point 1108 and ground. The parallel arrangement of the diode 1803 and Zener diode 1805 provides a high impedance to the induced AC voltage between the electrodes 2, 4, when its magnitude V is below the activation voltage Vmin determined by the reverse breakdown voltage of the Zener diode 1805, via the sets of parasitic components 1024, 1044 of each electrode 2, 4. The provision of the high impedance, by reducing the effective capacitance at point 1108, substantially reduces or eliminates the steady-state charging of the supply capacitor 1106 by an induced AC voltage of magnitude V above the activation voltage Vmin of the circuit model 1000 which is equal to the electrode parasitic voltage Vpara, and hence mitigates the unintended stimulus current Istimoccurring with the circuit model 1000.

[0149] Fig. 19 is a graph 1900 of the peak-zero unintended stimulus current Istimas a function of the magnitude V for the conventional circuit model 1000 of Fig. 10a with a Zener diode (as shown by plot 1904) and for the device 1800 of Fig. 18 with Zener diode 1805 (i.e., as shown by plot 1906). As observed by comparing plot 1904 with plot 1906 in graph 1900, the device 1800 provides a reduction or elimination of the unintended stimulus current IsLfmfor magnitudes V above the activation voltageVmin of the circuit model 1000 which is equal to the electrode parasitic voltage Vpara(1.2 V) and less than the activation voltage Vmin of the device 1800. For example, using a Zener diode 1805 with a reverse breakdown voltage of VZener= 18.6 V, the unintended stimulus currents are substantially reduced for magnitudes of the induced AC voltage that are less than the activation voltage Vmin of the device 1800 of 19.8 V (i.e., Vpara+ Vzener). That is, the device 1800 has a substantially increased activation voltage (i.e., 19.8 V) compared to that of the circuit model 1000 (i.e., 1.2 V). As shown in plot 1906 of Fig. 19, for a magnitude V above the activation voltage Vmin, the unintended stimulus current Istimof the device 1800 will increase sharply with the excess of magnitude V over the activation voltage Vmin, similarly to (though slightly less than) the conventional circuit model 1000 (as shown in plot 1904 of Fig. 19).

[0150] In the absence of a Zener diode 1805 in the device 1800, the unintended stimulus current Istimwould be essentially zero for magnitudes V up to the reverse breakdown voltage of the diode 1803.

[0151] The device 1800 therefore provides improved performance over the conventional circuit model 1000, both without and with a Zener diode, and also over devices 1200, 1400, and 1600 which mitigate the unintended stimulus currents by sustaining the reference voltage at the connection point 1108 when the device 100 is not in the active mode. Further, the use of a Schottky diode as the diode 1803 advantageously provides a high reverse breakdown voltage and a fast response to robustly decouple the supply capacitor 1106 from the connection point 1108 while having a low forward bias voltage (e.g., Fbjas~0.1 V), thereby minimising power dissipation in the diode 1803 during conduction (i.e., when the device is in the active mode).

[0152] It is noted that decoupling the supply capacitor 1106 with the diode 1803 still leaves some parasitic capacitance (e.g., a few pF) to ground in parallel with the Zener diode 1805. This will charge much more quickly (i.e., of the order of 1 million times) than the supply capacitor 1106 (with a capacitance typically of a few pF). However, the rectified currents needed to sustain the charging of the parasitic capacitance at each cycle are sufficiently small that they do not provide a low impedance to an induced AC voltage that has a magnitude Flower than the activation voltage Vmin. That is, when V is below VZener+ Vpara, the induced unintended stimulus current Istimis much lower than the value shown in plot 1904 of Fig. 19, and is substantially close to the value of zero depicted in plot 1906.

[0153] Fig. 20 illustrates a device 2000 that is a fifth example of the device 1100 configured for mitigating unintended stimuli according to the second implementation of the proposed technology. The device 2000 includes a voltage supply circuit 1802, supply capacitor 1106, and Zener diode 1805 connected as in the example device 1800.

[0154] In the device 2000, the one or more control elements 1104 include a P-channel metal-oxide semiconductor (PMOS) transistor 2003 having drain and source terminals respectively connected to the output terminal of the voltage supply circuit 1802, and to the connection point 1108. The PMOS transistor 2003 operates as a voltage-controlled switch to selectively couple or decouple the supply capacitor 1106 to the connection point 1108 (i.e., when the PMOS transistor 2003 is transitioned to an on or off mode respectively).

[0155] As depicted in Fig. 20, the one or more control elements 1104 further include a passive pull- up element 2004 connected between the second terminal (i.e., source) of the PMOS transistor 2003, and a third terminal (i.e., gate) of the PMOS transistor 2003. In some examples, the passive pull-up element 2004 is a resistor configured to set the PMOS transistor 2003 to the off mode, for example when the device 100 is not in the active mode, thus decoupling the supply capacitor 1106.

[0156] The gate of the PMOS transistor 2003 is controlled by a switching device, which in the device 2000 is aN-channel metal-oxide semiconductor (NMOS) transistor 2009 operating as an active pulldown switch. The NMOS transistor 2009 is configured to switch the operation of the PMOS transistor 2003 between an on mode and the off mode, in which the supply capacitor 1106 is respectively coupled to, and decoupled from, the connection point 1108.

[0157] Fig. 21 illustrates a device 2100 that is a sixth example of the device 1100 configured for mitigating unintended stimuli according to the second implementation of the proposed technology. The device 2100 includes a modification to the device 2000 in which the one or more control elements 1104 include a second PMOS transistor 2107 having source and drain terminals respectively connected to the source of the first PMOS transistor 2103 and connection point 1108. The gate of the second PMOS transistor 2107 is connected to the passive pull-up element 2104 and the gate of the first PMOS transistor 2103. The second PMOS transistor 2107 operates in tandem with the first PMOS transistor 2103, the operation of which is controlled by an NMOS transistor 2109 in the device 2100, to increase the degree of electrical isolation between the supply capacitor 1106 and the connection point 1108 in the off mode.

[0158] Compared to the device 1800, the device 2000 is advantageous in that the PMOS transistor 2003 provides a reduced forward voltage drop compared to the (Schottky) diode 1803 when turned on to couple the supply capacitor 1106, thereby reducing the power consumption of the circuit (as a trade-off for an increase in the complexity of the circuit). Further, in either of the MOS transistorbased devices 2000, 2100, the NMOS transistor 2009, 2109 may be optionally controlled by an onboard control unit (e .g . , controller 116) of the neuromodulation device 2000, 2100 based on the mode of the device 2000, 2100.

[0159] In some implementations of devices 1800, 2000 and 2100, the one or more control elements 1104 (e.g., diode 1803; PMOS transistors 2003, 2103, 2107; and NMOS transistors 2009, 2109) are contained within a common integrated circuit of the electronics module 110. The common integrated circuit also contains the sets 1024, 1044 of the one or more parasitic components of the respective electrodes 2, 4. This may be advantageous in reducing the total Printed Circuit Board Assembly (PCBA) area required by the components of the neuromodulation devices 1800, 2000 and 2100.

[0160] It will be appreciated by persons skilled in the art that numerous variations and / or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not limiting or restrictive.INDUSTRIAL APPLICABILITY

[0161] It is apparent from the above that the arrangements described are applicable to the health care industries.LABEL LIST

Claims

CLAIMS:

1. An implantable device comprising: a control unit configured to control, when the implantable device is in an active mode, a stimulus source configured to deliver neural stimuli to a neural pathway of a patient via at least one stimulus electrode of a plurality of electrodes, the neural stimuli being configured to evoke neural responses from the neural pathway; and a regulator circuit connected via parasitic components to the plurality of electrodes at a connection point, wherein the regulator circuit is configured to provide a high impedance to an AC voltage induced between a pair of the plurality of electrodes, the AC voltage resulting from application of an external oscillating electromagnetic field to the implantable device, wherein the high impedance is provided for a magnitude of the induced AC voltage that is below an activation voltage that is higher than a combined voltage drop across the parasitic components.

2. The implantable device of claim 1, wherein the regulator circuit is configured to provide the high impedance by providing a reduced effective capacitance at the connection point.

3. The implantable device of claim 2, wherein the regulator circuit is configured to provide the reduced effective capacitance at the connection point when the implantable device is not in the active mode, and to provide a relatively higher effective capacitance at the connection point when the implantable device is in the active mode.

4. The implantable device of claim 3, wherein the regulator circuit comprises at least: a voltage supply circuit configured to provide a reference voltage at an output terminal; a supply capacitor having first and second terminals respectively connected to ground and the output terminal of the voltage circuit; and one or more control elements configured to reduce the effective capacitance at the connection point by decoupling the supply capacitor from the connection point.

5. The implantable device of claim 4, wherein the one or more control elements comprise a diode having an anode and a cathode respectively connected to the second terminal of the supply capacitor and the connection point.

6. The implantable device of claim 5, wherein the diode is a Schottky diode.

7. The implantable device of claim 4, wherein the one or more control elements comprise one or more switches configured to the decouple the supply capacitor from the connection point when operating in an off mode.

8. The implantable device of claim 7, wherein the one or more switches comprise: a first P-channel metal -oxide semiconductor (PMOS) transistor having first and second terminals respectively connected to the output terminal of the voltage supply circuit and the connection point.

9. The implantable device of claim 8, wherein the first PMOS transistor is configured to operate in the off mode when the implantable device is not in the active mode.

10. The implantable device of claim 9, wherein the one or more control elements further comprise a passive pull-up element connected between the second terminal of the first PMOS transistor, and a third terminal of the first PMOS transistor, wherein the passive pull-up element is configured to operate the first PMOS transistor in the off mode when the implantable device is not in the active mode.

11. The implantable device of any one of claims 8 to 10, wherein the one or more switches further comprise a switching device connected to the third terminal of the first PMOS transistor, wherein the switching device is configured to switch the operation of the first PMOS transistor between: an on mode; and the off mode, in which the supply capacitor is respectively coupled to, and decoupled from, the connection point.

12. The implantable device of claim 11, wherein the one or more switches further comprise: a second PMOS transistor having first and second terminals respectively connected to the second terminal of the first PMOS transistor and to the pair of electrodes.

13. The implantable device of any one of claims 11 to 12, wherein the switching device is controlled by the control unit.

14. The implantable device of any one of claims 8 to 13, wherein the one or more control elements are contained within a common integrated circuit of the implantable device.

15. The implantable device of any one of claims 1 to 14, wherein the activation voltage is determined by bias voltages of: a Zener diode having first and second terminals respectively connected to the connection point and ground; and the parasitic components of each electrode of the pair of electrodes.

16. The implantable device of any one of claims 1 to 15, wherein the externally applied oscillating electromagnetic field is generated during an Magnetic Resonance Imaging (MRI) scan of the patient.

17. A method for operating an implantable device, the method comprising: when the implantable device is in an active mode, delivering neural stimuli, via at least one stimulus electrode of a plurality of electrodes of the implantable device, to a neural pathway of a patient to evoke a neural response from the neural pathway; and providing, via a regulator circuit of the implantable device, a high impedance to an AC voltage induced between a pair of the plurality of electrodes, the AC voltage resulting from an application of an external oscillating electromagnetic field to the implantable device, wherein the high impedance is provided for a magnitude of the induced AC voltage that is below an activation voltage that is higher than a combined voltage drop across one or more parasitic components connecting the regulator circuit to the plurality of electrodes.

18. The method of claim 17, further comprising transitioning the implantable device out of the active mode in response to the application of the external oscillating electromagnetic field.

19. The method of any one of claims 17 to 18, wherein providing the high impedance comprises providing a reduced effective capacitance at a connection point connecting the plurality of electrodes and the regulator circuit.

20. The method of claim 19, further comprising providing the reduced effective capacitance at the connection point when the implantable device is not in the active mode, and providing a relatively higher effective capacitance at the connection point when the implantable device is in the active mode.

21. The method of claim 20, wherein providing the reduced effective capacitance at the connection point comprises using one or more control elements to decouple a supply capacitor from the connection point, wherein the supply capacitor has first and second terminals respectivelyconnected to ground and to a voltage circuit, and wherein the voltage circuit is configured to provide a reference voltage to the second terminal of the supply capacitor.

22. The method of claim 21, wherein the one or more control elements comprise a diode having an anode and a cathode respectively connected to the second terminal of the supply capacitor and the connection point.

23. The method of claim 22, wherein the diode is a Schottky diode.

24. The method of claim 21, wherein the one or more control elements comprise one or more switches.

25. The method of claim 24, wherein the one or more switches comprise a first P-channel metal- oxide semiconductor (PMOS) transistor.

26. The method of claim 25, further comprising operating the first PMOS transistor in the off mode when the implantable device is not in the active mode.

27. The method of claim 26, wherein the one or more control elements further comprise a passive pull-up element, and wherein the method comprises using the passive pull-up element to operate the first PMOS transistor in the off mode when the implantable device is not in the active mode.

28. The method of any one of claims 25 to 27, wherein the one or more switches further comprise a switching device, and wherein the method further comprises using the switching device to switch the operation of the first PMOS transistor between: an on mode; and the off mode, in which the supply capacitor is respectively coupled to, and decoupled from, the connection point.

29. The method of claim 28, wherein the one or more switches further comprise a second PMOS transistor, and wherein the method further comprises using the second PMOS transistor to electrically isolate the voltage circuit from the connection point.

30. The method of any one of claims 28 to 29, further comprising controlling the switching device using a control unit of the implantable device.

31. The method of any one of claims 17 to 30, wherein the activation voltage is determined by bias voltages of: a Zener diode having first and second terminals respectively connected to theconnection point and ground; and one or more parasitic components connecting the regulator circuit to each electrode of the pair of electrodes.

32. The method of any one of claims 17 to 31, wherein the externally applied oscillating electromagnetic field is generated during a Magnetic Resonance Imaging (MRI) scan of the patient.

33. A neural stimulation system comprising: an implantable device for controllably delivering neural stimuli to a neural pathway of a patient, the device comprising: a stimulus source configured to deliver neural stimuli via at least one stimulus electrode of a plurality of electrodes, to the neural pathway, the neural stimuli being configured to evoke neural responses from the neural pathway; a control unit configured to control the stimulus source to deliver the neural stimuli; a regulator circuit connected to the plurality of electrodes at a connection point; and a processor configured to instruct the control unit to control the stimulus source to deliver the neural stimuli to the neural pathway, wherein the system is configured to perform the method of any one of claims 17 to 32.

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