Remote control of power or polarity selection for a neural stimulator

An implantable neural stimulator includes one or more electrodes, at least one antenna, and one or more circuits connected to the at least one antenna. The one or more electrodes are configured to apply one or more electrical pulses to excitable tissue. The antenna is configured to receive one or more input signals containing polarity assignment information and electrical energy, the polarity assignment information designating polarities for the electrodes. The one or more circuits are configured to control an electrode interface such that the electrodes have the polarities designated by the polarity assignment information; create one or more electrical pulses using the electrical energy contained in the input signal; and supply the one or more electrical pulses to the one or more electrodes through the electrode interface so that the one or more electrical pulses are applied according to the polarities designated by the polarity assignment information.

TECHNICAL FIELD

This description is related to implanted neural stimulators.

BACKGROUND

Neural modulation of neural tissue in the body by electrical stimulation has become an important type of therapy for chronic disabling conditions, such as chronic pain, problems of movement initiation and control, involuntary movements, dystonia, urinary and fecal incontinence, sexual difficulties, vascular insufficiency, heart arrhythmia and more. Electrical stimulation of the spinal column and nerve bundles leaving the spinal cord was the first approved neural modulation therapy and been used commercially since the 1970s. Implanted electrodes are used to pass pulsatile electrical currents of controllable frequency, pulse width and amplitudes. Two or more electrodes are in contact with neural elements, chiefly axons, and can selectively activate varying diameters of axons, with positive therapeutic benefits. A variety of therapeutic intra-body electrical stimulation techniques are utilized to treat neuropathic conditions that utilize an implanted neural stimulator in the spinal column or surrounding areas, including the dorsal horn, dorsal root ganglia, dorsal roots, dorsal column fibers and peripheral nerve bundles leaving the dorsal column or brain, such as vagus-, occipital-, trigeminal, hypoglossal-, sacral-, and coccygeal nerves.

SUMMARY

In one aspect, an implantable neural stimulator includes one or more electrodes, at least one antenna, and one or more circuits connected to at least one antenna. The one or more electrodes are configured to apply one or more electrical pulses to excitable tissue. The antenna is configured to receive one or more input signals containing polarity assignment information and electrical energy, with the polarity assignment information designating polarities for each of the electrodes. The one or more circuits are configured to control an electrode interface such that the electrodes have the polarities designated by the polarity assignment information; create one or more electrical pulses using the electrical energy contained in the input signal; and supply the one or more electrical pulses to the one or more electrodes through the electrode interface such that the one or more electrodes apply the one or more electrical pulses to excitable tissue according to the polarities designated by the polarity assignment information.

Implementations of this and other aspects may include the following features. The polarities designated by the polarity assignment information may include a negative polarity, a positive polarity, or a neutral polarity. The electrical pulses include a cathodic portion and an anodic portion. The electrode interface may include a polarity routing switch network. The polarity routing switch network may include a first input that receives the cathodic portion of the electrical pulses and a second input that receives the anodic portion of the electrical pulses. The polarity routing switch network may be configured to route the cathodic portion to electrodes with a negative polarity, route the anodic portion to electrodes with a positive polarity, and disconnect electrodes with a neutral polarity from the electrical pulses.

The one or more circuits may include a register with an output coupled to a selection input of the polarity routing switch network. The register may be configured to store the polarity assignment information and send the stored polarity assignment information from the register output to the selection input of the polarity routing switch network to control the polarity routing switch network to route the cathodic portion to electrodes with a negative polarity, route the anodic portion to electrodes with a positive polarity, and disconnect electrodes with a neutral polarity from the electrical pulses.

The one or more circuits include a power-on reset circuit and a capacitor, wherein the capacitor may store a charge using a portion of the electrical energy contained in the one or more input signals, and wherein the capacitor may be configured to energize the power-on reset circuit to reset the register contents when the implanted neural stimulator loses power.

The at least one antenna may be configured to transmit, to the separate antenna through electrical radiative coupling, one or more stimulus feedback signals. The one or more circuits may be configured to generate a stimulus feedback signal. The stimulus feedback signal may indicate one or more parameters associated with the one or more electrical pulses applied to the excitable tissue by the one or more electrodes. The parameters may include the power being delivered to the tissue and an impedance at the tissue.

The one or more circuits may include a current sensor configured to sense an amount of current being delivered to the tissue and a voltage sensor configured to sense a voltage being delivered to the tissue. The current sensor may include a resistor placed in serial connection with an anodic branch of the polarity routing switch network, and the anodic portion of the electrical pulses may be transported over the anodic branch. The current sensor and the voltage sensor are coupled to an analog controlled carrier modulator, the modulator being configured to communicate the sensed current and voltage to the separate antenna.

The at least one antenna may include a first antenna and a second antenna. The first antenna may be configured to receive an input signal containing the electrical energy. The second antenna may be configured to transmit the stimulus feedback signal to the separate antenna through electrical radiative coupling. The second antenna may be further configured to receive an input signal containing the polarity assignment information. The transmission frequency of the second antenna may be higher than a resonant frequency of the first antenna. The transmission frequency of the second antenna may be a second harmonic of the resonant frequency of the first antenna. The transmission frequency and the resonant frequency are in a range from about 300 MHz to about 6 GHz. The at least one antenna may be between about 0.1 mm and about 7 cm in length and between about 0.1 mm to about 3 mm in width. The at least one antenna may be a dipole antenna.

The one or more circuits may additionally include a rectifying circuit configured to rectify the input signal received by the first antenna to generate the one or more electrical pulses. The rectifying circuit may be coupled to a RC-timer to shape the one or more electrical pulses. The rectifying circuit may include at least one full wave bridge rectifier. The full wave bridge rectifier may include several diodes, each of which may be less than 100 micrometers in length.

In another aspect, system includes a RF pulse generator module. The RF pulse generator module includes an antenna module and one or more circuits coupled to the antenna module.

The antenna module is configured to send one or more input signals to at least one antenna in an implantable neural stimulator through electrical radiative coupling. The one or more input signal contain electrical energy and polarity assignment information that designates polarity assignments of one or more electrodes in the implantable neural stimulator. The implantable neural stimulator is configured to control an electrode interface such that the electrodes have the polarities designated by the polarity assignment information, create one or more electrical pulses suitable for stimulation of neural tissue using the electrical energy contained in the input signal, and supply the one or more electrical pulses to the one or more electrodes through the electrode interface such that the one or more electrodes apply the one or more electrical pulses to neural tissue with the polarities designated by the polarity assignment information. The antenna module is further configured to receive one or more signals from the at least one antenna in an implantable neural stimulator through the electrical radiative coupling.

The one or more circuits are configured to generate the one or more input signals and send the one or more input signals to the antenna module; extract a stimulus feedback signal from one or more signals received by the antenna module, the stimulus feedback signal being sent by the implantable neural stimulator and indicating one or more parameters of the one or more electrical pulses; and adjust parameters of the input signal based on the stimulus feedback signal.

Implementations of this and other aspects may include the following features. The antenna module may be configured to transmit portions of the input signal containing electrical energy using a different carrier frequency than portions of the input signal containing information encoding the polarity assignments of one or more electrodes.

The antenna module may include a first antenna configured to operate at a first frequency to transmit an input signal containing the electrical energy and a second antenna configured to operate at a second frequency to receive the one or more signals from the at least one antenna of the implantable neural stimulator. The second frequency may be, for example, a second harmonic frequency of the first frequency.

Various implementations may be inherently low in cost compared to existing implantable neural modulation systems, and this may lead to wider adoption of neural modulation therapy for patients in need as well as reduction in overall cost to the healthcare system.

DETAILED DESCRIPTION

In various implementations, a neural stimulation system may be used to send electrical stimulation to targeted nerve tissue by using remote radio frequency (RF) energy with neither cables nor inductive coupling to power the passive implanted stimulator. The targeted nerve tissues may be, for example, in the spinal column including the spinothalamic tracts, dorsal horn, dorsal root ganglia, dorsal roots, dorsal column fibers, and peripheral nerves bundles leaving the dorsal column or brainstem, as well as any cranial nerves, abdominal, thoracic, or trigeminal ganglia nerves, nerve bundles of the cerebral cortex, deep brain and any sensory or motor nerves.

For instance, in some implementations, the neural stimulation system may include a controller module, such as an RF pulse generator module, and a passive implanted neural stimulator that contains one or more dipole antennas, one or more circuits, and one or more electrodes in contact with or in proximity to targeted neural tissue to facilitate stimulation. The RF pulse generator module may include an antenna and may be configured to transfer energy from the module antenna to the implanted antennas. The one or more circuits of the implanted neural stimulator may be configured to generate electrical pulses suitable for neural stimulation using the transferred energy and to supply the electrical pulses to the electrodes so that the pulses are applied to the neural tissue. For instance, the one or more circuits may include wave conditioning circuitry that rectifies the received RF signal (for example, using a diode rectifier), transforms the RF energy to a low frequency signal suitable for the stimulation of neural tissue, and presents the resulting waveform to an electrode array. The one or more circuits of the implanted neural stimulator may also include circuitry for communicating information back to the RF pulse generator module to facilitate a feedback control mechanism for stimulation parameter control. For example, the implanted neural stimulator may send to the RF pulse generator module a stimulus feedback signal that is indicative of parameters of the electrical pulses, and the RF pulse generator module may employ the stimulus feedback signal to adjust parameters of the signal sent to the neural stimulator.

FIG. 1depicts a high-level diagram of an example of a neural stimulation system. The neural stimulation system may include four major components, namely, a programmer module102, a RF pulse generator module106, a transmit (TX) antenna110(for example, a patch antenna, slot antenna, or a dipole antenna), and an implanted wireless neural stimulator114. The programmer module102may be a computer device, such as a smart phone, running a software application that supports a wireless connection114, such as Bluetooth®. The application can enable the user to view the system status and diagnostics, change various parameters, increase/decrease the desired stimulus amplitude of the electrode pulses, and adjust feedback sensitivity of the RF pulse generator module106, among other functions.

The RF pulse generator module106may include communication electronics that support the wireless connection104, the stimulation circuitry, and the battery to power the generator electronics. In some implementations, the RF pulse generator module106includes the TX antenna embedded into its packaging form factor while, in other implementations, the TX antenna is connected to the RF pulse generator module106through a wired connection108or a wireless connection (not shown). The TX antenna110may be coupled directly to tissue to create an electric field that powers the implanted neural stimulator module114. The TX antenna110communicates with the implanted neural stimulator module114through an RF interface. For instance, the TX antenna110radiates an RF transmission signal that is modulated and encoded by the RF pulse generator module110. The implanted wireless neural stimulator module114contains one or more antennas, such as dipole antenna(s), to receive and transmit through RF interface112. In particular, the coupling mechanism between antenna110and the one or more antennas on the implanted neural stimulation module114is electrical radiative coupling and not inductive coupling. In other words, the coupling is through an electric field rather than a magnetic field.

Through this electrical radiative coupling, the TX antenna110can provide an input signal to the implanted neural stimulation module114. This input signal contains energy and may contain information encoding stimulus waveforms to be applied at the electrodes of the implanted neural stimulator module114. In some implementations, the power level of this input signal directly determines an applied amplitude (for example, power, current, or voltage) of the one or more electrical pulses created using the electrical energy contained in the input signal. Within the implanted wireless neural stimulator114are components for demodulating the RF transmission signal, and electrodes to deliver the stimulation to surrounding neuronal tissue.

The RF pulse generator module106can be implanted subcutaneously, or it can be worn external to the body. When external to the body, the RF generator module106can be incorporated into a belt or harness design to allow for electric radiative coupling through the skin and underlying tissue to transfer power and/or control parameters to the implanted neural stimulator module114, which can be a passive stimulator. In either event, receiver circuit(s) internal to the neural stimulator module114can capture the energy radiated by the TX antenna110and convert this energy to an electrical waveform. The receiver circuit(s) may further modify the waveform to create an electrical pulse suitable for the stimulation of neural tissue, and this pulse may be delivered to the tissue via electrode pads.

In some implementations, the RF pulse generator module106can remotely control the stimulus parameters (that is, the parameters of the electrical pulses applied to the neural tissue) and monitor feedback from the wireless neural stimulator module114based on RF signals received from the implanted wireless neural stimulator module114. A feedback detection algorithm implemented by the RF pulse generator module106can monitor data sent wirelessly from the implanted wireless neural stimulator module114, including information about the energy that the implanted wireless neural stimulator module114is receiving from the RF pulse generator and information about the stimulus waveform being delivered to the electrode pads. In order to provide an effective therapy for a given medical condition, the system can be tuned to provide the optimal amount of excitation or inhibition to the nerve fibers by electrical stimulation. A closed loop feedback control method can be used in which the output signals from the implanted wireless neural stimulator module114are monitored and used to determine the appropriate level of neural stimulation current for maintaining effective neuronal activation, or, in some cases, the patient can manually adjust the output signals in an open loop control method.

FIG. 2depicts a detailed diagram of an example of the neural stimulation system. As depicted, the programming module102may comprise user input system202and communication subsystem208. The user input system221may allow various parameter settings to be adjusted (in some cases, in an open loop fashion) by the user in the form of instruction sets. The communication subsystem208may transmit these instruction sets (and other information) via the wireless connection104, such as Bluetooth or Wi-Fi, to the RF pulse generator module106, as well as receive data from module106.

For instance, the programmer module102, which can be utilized for multiple users, such as a patient's control unit or clinician's programmer unit, can be used to send stimulation parameters to the RF pulse generator module106. The stimulation parameters that can be controlled may include pulse amplitude, pulse frequency, and pulse width in the ranges shown in Table 1. In this context the term pulse refers to the phase of the waveform that directly produces stimulation of the tissue; the parameters of the charge-balancing phase (described below) can similarly be controlled. The patient and/or the clinician can also optionally control overall duration and pattern of treatment.

The implantable neural stimulator module114or RF pulse generator module114may be initially programmed to meet the specific parameter settings for each individual patient during the initial implantation procedure. Because medical conditions or the body itself can change over time, the ability to re-adjust the parameter settings may be beneficial to ensure ongoing efficacy of the neural modulation therapy.

The programmer module102may be functionally a smart device and associated application. The smart device hardware may include a CPU206and be used as a vehicle to handle touchscreen input on a graphical user interface (GUI)204, for processing and storing data.

The RF pulse generator module106may be connected via wired connection108to an external TX antenna110. Alternatively, both the antenna and the RF pulse generator are located subcutaneously (not shown).

The signals sent by RF pulse generator module106to the implanted stimulator114may include both power and parameter-setting attributes in regards to stimulus waveform, amplitude, pulse width, and frequency. The RF pulse generator module106can also function as a wireless receiving unit that receives feedback signals from the implanted stimulator module114. To that end, the RF pulse generator module106may contain microelectronics or other circuitry to handle the generation of the signals transmitted to the stimulator module114as well as handle feedback signals, such as those from the stimulator module114. For example, the RF pulse generator module106may comprise controller subsystem214, high-frequency oscillator218, RF amplifier216, a RF switch, and a feedback subsystem212.

The controller subsystem214may include a CPU230to handle data processing, a memory subsystem228such as a local memory, communication subsystem234to communicate with programmer module102(including receiving stimulation parameters from programmer module), pulse generator circuitry236, and digital/analog (D/A) converters232.

The controller subsystem214may be used by the patient and/or the clinician to control the stimulation parameter settings (for example, by controlling the parameters of the signal sent from RF pulse generator module106to neural stimulator module114). These parameter settings can affect, for example, the power, current level, or shape of the one or more electrical pulses. The programming of the stimulation parameters can be performed using the programming module102, as described above, to set the repetition rate, pulse width, amplitude, and waveform that will be transmitted by RF energy to the receive (RX) antenna238, typically a dipole antenna (although other types may be used), in the wireless implanted neural stimulator module214. The clinician may have the option of locking and/or hiding certain settings within the programmer interface, thus limiting the patient's ability to view or adjust certain parameters because adjustment of certain parameters may require detailed medical knowledge of neurophysiology, neuroanatomy, protocols for neural modulation, and safety limits of electrical stimulation.

The controller subsystem214may store received parameter settings in the local memory subsystem228, until the parameter settings are modified by new input data received from the programming module102. The CPU206may use the parameters stored in the local memory to control the pulse generator circuitry236to generate a stimulus waveform that is modulated by a high frequency oscillator218in the range from 300 MHz to 8 GHz. The resulting RF signal may then be amplified by RF amplifier226and then sent through an RF switch223to the TX antenna110to reach through depths of tissue to the RX antenna238.

In some implementations, the RF signal sent by TX antenna110may simply be a power transmission signal used by stimulator module114to generate electric pulses. In other implementations, a telemetry signal may also be transmitted to the stimulator module114to send instructions about the various operations of the stimulator module114. The telemetry signal may be sent by the modulation of the carrier signal (through the skin if external, or through other body tissues if the pulse generator module106is implanted subcutaneously). The telemetry signal is used to modulate the carrier signal (a high frequency signal) that is coupled onto the implanted antenna(s)238and does not interfere with the input received on the same lead to power the implant. In one embodiment the telemetry signal and powering signal are combined into one signal, where the RF telemetry signal is used to modulate the RF powering signal, and thus the implanted stimulator is powered directly by the received telemetry signal; separate subsystems in the stimulator harness the power contained in the signal and interpret the data content of the signal.

The RF switch223may be a multipurpose device such as a dual directional coupler, which passes the relatively high amplitude, extremely short duration RF pulse to the TX antenna110with minimal insertion loss while simultaneously providing two low-level outputs to feedback subsystem212; one output delivers a forward power signal to the feedback subsystem212, where the forward power signal is an attenuated version of the RF pulse sent to the TX antenna110, and the other output delivers a reverse power signal to a different port of the feedback subsystem212, where reverse power is an attenuated version of the reflected RF energy from the TX Antenna110.

During the on-cycle time (when an RF signal is being transmitted to stimulator114), the RF switch223is set to send the forward power signal to feedback subsystem. During the off-cycle time (when an RF signal is not being transmitted to the stimulator module114), the RF switch223can change to a receiving mode in which the reflected RF energy and/or RF signals from the stimulator module114are received to be analyzed in the feedback subsystem212.

The feedback subsystem212of the RF pulse generator module106may include reception circuitry to receive and extract telemetry or other feedback signals from the stimulator114and/or reflected RF energy from the signal sent by TX antenna110. The feedback subsystem may include an amplifier226, a filter224, a demodulator222, and an A/D converter220.

The feedback subsystem212receives the forward power signal and converts this high-frequency AC signal to a DC level that can be sampled and sent to the controller subsystem214. In this way the characteristics of the generated RF pulse can be compared to a reference signal within the controller subsystem214. If a disparity (error) exists in any parameter, the controller subsystem214can adjust the output to the RF pulse generator106. The nature of the adjustment can be, for example, proportional to the computed error. The controller subsystem214can incorporate additional inputs and limits on its adjustment scheme such as the signal amplitude of the reverse power and any predetermined maximum or minimum values for various pulse parameters.

The reverse power signal can be used to detect fault conditions in the RF-power delivery system. In an ideal condition, when TX antenna110has perfectly matched impedance to the tissue that it contacts, the electromagnetic waves generated from the RF pulse generator106pass unimpeded from the TX antenna110into the body tissue. However, in real-world applications a large degree of variability may exist in the body types of users, types of clothing worn, and positioning of the antenna110relative to the body surface. Since the impedance of the antenna110depends on the relative permittivity of the underlying tissue and any intervening materials, and also depends on the overall separation distance of the antenna from the skin, in any given application there can be an impedance mismatch at the interface of the TX antenna110with the body surface. When such a mismatch occurs, the electromagnetic waves sent from the RF pulse generator106are partially reflected at this interface, and this reflected energy propagates backward through the antenna feed.

The dual directional coupler RF switch223may prevent the reflected RF energy propagating back into the amplifier226, and may attenuate this reflected RF signal and send the attenuated signal as the reverse power signal to the feedback subsystem212. The feedback subsystem212can convert this high-frequency AC signal to a DC level that can be sampled and sent to the controller subsystem214. The controller subsystem214can then calculate the ratio of the amplitude of the reverse power signal to the amplitude of the forward power signal. The ratio of the amplitude of reverse power signal to the amplitude level of forward power may indicate severity of the impedance mismatch.

In order to sense impedance mismatch conditions, the controller subsystem214can measure the reflected-power ratio in real time, and according to preset thresholds for this measurement, the controller subsystem214can modify the level of RF power generated by the RF pulse generator106. For example, for a moderate degree of reflected power the course of action can be for the controller subsystem214to increase the amplitude of RF power sent to the TX antenna110, as would be needed to compensate for slightly non-optimum but acceptable TX antenna coupling to the body. For higher ratios of reflected power, the course of action can be to prevent operation of the RF pulse generator106and set a fault code to indicate that the TX antenna110has little or no coupling with the body. This type of reflected-power fault condition can also be generated by a poor or broken connection to the TX antenna. In either case, it may be desirable to stop RF transmission when the reflected-power ratio is above a defined threshold, because internally reflected power can lead to unwanted heating of internal components, and this fault condition means the system cannot deliver sufficient power to the implanted wireless neural stimulator and thus cannot deliver therapy to the user.

The controller242of the stimulator114may transmit informational signals, such as a telemetry signal, through the antenna238to communicate with the RF pulse generator module106during its receive cycle. For example, the telemetry signal from the stimulator114may be coupled to the modulated signal on the dipole antenna(s)238, during the on and off state of the transistor circuit to enable or disable a waveform that produces the corresponding RF bursts necessary to transmit to the external (or remotely implanted) pulse generator module106. The antenna(s)238may be connected to electrodes254in contact with tissue to provide a return path for the transmitted signal. An A/D (not shown) converter can be used to transfer stored data to a serialized pattern that can be transmitted on the pulse modulated signal from the internal antenna(s)238of the neural stimulator.

A telemetry signal from the implanted wireless neural stimulator module114may include stimulus parameters such as the power or the amplitude of the current that is delivered to the tissue from the electrodes. The feedback signal can be transmitted to the RF pulse generator module116to indicate the strength of the stimulus at the nerve bundle by means of coupling the signal to the implanted RX antenna238, which radiates the telemetry signal to the external (or remotely implanted) RF pulse generator module106. The feedback signal can include either or both an analog and digital telemetry pulse modulated carrier signal. Data such as stimulation pulse parameters and measured characteristics of stimulator performance can be stored in an internal memory device within the implanted neural stimulator114, and sent on the telemetry signal. The frequency of the carrier signal may be in the range of at 300 MHz to 8 GHz.

In the feedback subsystem212, the telemetry signal can be down modulated using demodulator222and digitized by being processed through an analog to digital (A/D) converter220. The digital telemetry signal may then be routed to a CPU230with embedded code, with the option to reprogram, to translate the signal into a corresponding current measurement in the tissue based on the amplitude of the received signal. The CPU230of the controller subsystem214can compare the reported stimulus parameters to those held in local memory228to verify the stimulator(s)114delivered the specified stimuli to tissue. For example, if the stimulator reports a lower current than was specified, the power level from the RF pulse generator module106can be increased so that the implanted neural stimulator114will have more available power for stimulation. The implanted neural stimulator114can generate telemetry data in real time, for example, at a rate of 8 kbits per second. All feedback data received from the implanted lead module114can be logged against time and sampled to be stored for retrieval to a remote monitoring system accessible by the health care professional for trending and statistical correlations.

The sequence of remotely programmable RF signals received by the internal antenna(s)238may be conditioned into waveforms that are controlled within the implantable stimulator114by the control subsystem242and routed to the appropriate electrodes254that are placed in proximity to the tissue to be stimulated. For instance, the RF signal transmitted from the RF pulse generator module106may be received by RX antenna238and processed by circuitry, such as waveform conditioning circuitry240, within the implanted wireless neural stimulator module114to be converted into electrical pulses applied to the electrodes254through electrode interface252. In some implementations, the implanted stimulator114contains between two to sixteen electrodes254.

The waveform conditioning circuitry240may include a rectifier244, which rectifies the signal received by the RX antenna238. The rectified signal may be fed to the controller242for receiving encoded instructions from the RF pulse generator module106. The rectifier signal may also be fed to a charge balance component246that is configured to create one or more electrical pulses based such that the one or more electrical pulses result in a substantially zero net charge at the one or more electrodes (that is, the pulses are charge balanced). The charge-balanced pulses are passed through the current limiter248to the electrode interface252, which applies the pulses to the electrodes254as appropriate.

The current limiter248insures the current level of the pulses applied to the electrodes254is not above a threshold current level. In some implementations, an amplitude (for example, current level, voltage level, or power level) of the received RF pulse directly determines the amplitude of the stimulus. In this case, it may be particularly beneficial to include current limiter248to prevent excessive current or charge being delivered through the electrodes, although current limiter248may be used in other implementations where this is not the case. Generally, for a given electrode having several square millimeters surface area, it is the charge per phase that should be limited for safety (where the charge delivered by a stimulus phase is the integral of the current). But, in some cases, the limit can instead be placed on the current, where the maximum current multiplied by the maximum possible pulse duration is less than or equal to the maximum safe charge. More generally, the limiter248acts as a charge limiter that limits a characteristic (for example, current or duration) of the electrical pulses so that the charge per phase remains below a threshold level (typically, a safe-charge limit).

In the event the implanted wireless neural stimulator114receives a “strong” pulse of RF power sufficient to generate a stimulus that would exceed the predetermined safe-charge limit, the current limiter248can automatically limit or “clip” the stimulus phase to maintain the total charge of the phase within the safety limit. The current limiter248may be a passive current limiting component that cuts the signal to the electrodes254once the safe current limit (the threshold current level) is reached. Alternatively, or additionally, the current limiter248may communicate with the electrode interface252to turn off all electrodes254to prevent tissue damaging current levels.

A clipping event may trigger a current limiter feedback control mode. The action of clipping may cause the controller to send a threshold power data signal to the pulse generator106. The feedback subsystem212detects the threshold power signal and demodulates the signal into data that is communicated to the controller subsystem214. The controller subsystem214algorithms may act on this current-limiting condition by specifically reducing the RF power generated by the RF pulse generator, or cutting the power completely. In this way, the pulse generator106can reduce the RF power delivered to the body if the implanted wireless neural stimulator114reports it is receiving excess RF power.

The controller250of the stimulator205may communicate with the electrode interface252to control various aspects of the electrode setup and pulses applied to the electrodes254. The electrode interface252may act as a multiplex and control the polarity and switching of each of the electrodes254. For instance, in some implementations, the wireless stimulator106has multiple electrodes254in contact with tissue, and for a given stimulus the RF pulse generator module106can arbitrarily assign one or more electrodes to 1) act as a stimulating electrode, 2) act as a return electrode, or 3) be inactive by communication of assignment sent wirelessly with the parameter instructions, which the controller250uses to set electrode interface252as appropriate. It may be physiologically advantageous to assign, for example, one or two electrodes as stimulating electrodes and to assign all remaining electrodes as return electrodes.

Also, in some implementations, for a given stimulus pulse, the controller250may control the electrode interface252to divide the current arbitrarily (or according to instructions from pulse generator module106) among the designated stimulating electrodes. This control over electrode assignment and current control can be advantageous because in practice the electrodes254may be spatially distributed along various neural structures, and through strategic selection of the stimulating electrode location and the proportion of current specified for each location, the aggregate current distribution in tissue can be modified to selectively activate specific neural targets. This strategy of current steering can improve the therapeutic effect for the patient.

In another implementation, the time course of stimuli may be arbitrarily manipulated. A given stimulus waveform may be initiated at a time T_start and terminated at a time T_final, and this time course may be synchronized across all stimulating and return electrodes; further, the frequency of repetition of this stimulus cycle may be synchronous for all the electrodes. However, controller250, on its own or in response to instructions from pulse generator106, can control electrode interface252to designate one or more subsets of electrodes to deliver stimulus waveforms with non-synchronous start and stop times, and the frequency of repetition of each stimulus cycle can be arbitrarily and independently specified.

For example, a stimulator having eight electrodes may be configured to have a subset of five electrodes, called set A, and a subset of three electrodes, called set B. Set A might be configured to use two of its electrodes as stimulating electrodes, with the remainder being return electrodes. Set B might be configured to have just one stimulating electrode. The controller250could then specify that set A deliver a stimulus phase with 3 mA current for a duration of 200 us followed by a 400 us charge-balancing phase. This stimulus cycle could be specified to repeat at a rate of 60 cycles per second. Then, for set B, the controller250could specify a stimulus phase with 1 mA current for duration of 500 us followed by a 800 us charge-balancing phase. The repetition rate for the set-B stimulus cycle can be set independently of set A, say for example it could be specified at 25 cycles per second. Or, if the controller250was configured to match the repetition rate for set B to that of set A, for such a case the controller250can specify the relative start times of the stimulus cycles to be coincident in time or to be arbitrarily offset from one another by some delay interval.

In some implementations, the controller250can arbitrarily shape the stimulus waveform amplitude, and may do so in response to instructions from pulse generator106. The stimulus phase may be delivered by a constant-current source or a constant-voltage source, and this type of control may generate characteristic waveforms that are static, e.g. a constant-current source generates a characteristic rectangular pulse in which the current waveform has a very steep rise, a constant amplitude for the duration of the stimulus, and then a very steep return to baseline. Alternatively, or additionally, the controller250can increase or decrease the level of current at any time during the stimulus phase and/or during the charge-balancing phase. Thus, in some implementations, the controller250can deliver arbitrarily shaped stimulus waveforms such as a triangular pulse, sinusoidal pulse, or Gaussian pulse for example. Similarly, the charge-balancing phase can be arbitrarily amplitude-shaped, and similarly a leading anodic pulse (prior to the stimulus phase) may also be amplitude-shaped.

As described above, the stimulator114may include a charge-balancing component246. Generally, for constant current stimulation pulses, pulses should be charge balanced by having the amount of cathodic current should equal the amount of anodic current, which is typically called biphasic stimulation. Charge density is the amount of current times the duration it is applied, and is typically expressed in the units uC/cm2. In order to avoid the irreversible electrochemical reactions such as pH change, electrode dissolution as well as tissue destruction, no net charge should appear at the electrode-electrolyte interface, and it is generally acceptable to have a charge density less than 30 uC/cm2. Biphasic stimulating current pulses ensure that no net charge appears at the electrode after each stimulation cycle and the electrochemical processes are balanced to prevent net dc currents. Neural stimulator114may be designed to ensure that the resulting stimulus waveform has a net zero charge. Charge balanced stimuli are thought to have minimal damaging effects on tissue by reducing or eliminating electrochemical reaction products created at the electrode-tissue interface.

A stimulus pulse may have a negative-voltage or current, called the cathodic phase of the waveform. Stimulating electrodes may have both cathodic and anodic phases at different times during the stimulus cycle. An electrode that delivers a negative current with sufficient amplitude to stimulate adjacent neural tissue is called a “stimulating electrode.” During the stimulus phase the stimulating electrode acts as a current sink. One or more additional electrodes act as a current source and these electrodes are called “return electrodes.” Return electrodes are placed elsewhere in the tissue at some distance from the stimulating electrodes. When a typical negative stimulus phase is delivered to tissue at the stimulating electrode, the return electrode has a positive stimulus phase. During the subsequent charge-balancing phase, the polarities of each electrode are reversed.

In some implementations, the charge balance component246uses a blocking capacitor(s) placed electrically in series with the stimulating electrodes and body tissue, between the point of stimulus generation within the stimulator circuitry and the point of stimulus delivery to tissue. In this manner, a resistor-capacitor (RC) network may be formed. In a multi-electrode stimulator, one charge-balance capacitor(s) may be used for each electrode or a centralized capacitor(s) may be used within the stimulator circuitry prior to the point of electrode selection. The RC network can block direct current (DC), however it can also prevent low-frequency alternating current (AC) from passing to the tissue. The frequency below which the series RC network essentially blocks signals is commonly referred to as the cutoff frequency, and in one embodiment the design of the stimulator system may ensure the cutoff frequency is not above the fundamental frequency of the stimulus waveform. In this embodiment of the present invention, the wireless stimulator may have a charge-balance capacitor with a value chosen according to the measured series resistance of the electrodes and the tissue environment in which the stimulator is implanted. By selecting a specific capacitance value the cutoff frequency of the RC network in this embodiment is at or below the fundamental frequency of the stimulus pulse.

In other implementations, the cutoff frequency may be chosen to be at or above the fundamental frequency of the stimulus, and in this scenario the stimulus waveform created prior to the charge-balance capacitor, called the drive waveform, may be designed to be non-stationary, where the envelope of the drive waveform is varied during the duration of the drive pulse. For example, in one embodiment, the initial amplitude of the drive waveform is set at an initial amplitude Vi, and the amplitude is increased during the duration of the pulse until it reaches a final value k*Vi. By changing the amplitude of the drive waveform over time, the shape of the stimulus waveform passed through the charge-balance capacitor is also modified. The shape of the stimulus waveform may be modified in this fashion to create a physiologically advantageous stimulus.

In some implementations, the wireless neural stimulator module114may create a drive-waveform envelope that follows the envelope of the RF pulse received by the receiving dipole antenna(s)238. In this case, the RF pulse generator module106can directly control the envelope of the drive waveform within the wireless neural stimulator114, and thus no energy storage may be required inside the stimulator itself. In this implementation, the stimulator circuitry may modify the envelope of the drive waveform or may pass it directly to the charge-balance capacitor and/or electrode-selection stage.

In some implementations, the implanted neural stimulator114may deliver a single-phase drive waveform to the charge balance capacitor or it may deliver multiphase drive waveforms. In the case of a single-phase drive waveform, for example, a negative-going rectangular pulse, this pulse comprises the physiological stimulus phase, and the charge-balance capacitor is polarized (charged) during this phase. After the drive pulse is completed, the charge balancing function is performed solely by the passive discharge of the charge-balance capacitor, where is dissipates its charge through the tissue in an opposite polarity relative to the preceding stimulus. In one implementation, a resistor within the stimulator facilitates the discharge of the charge-balance capacitor. In some implementations, using a passive discharge phase, the capacitor may allow virtually complete discharge prior to the onset of the subsequent stimulus pulse.

In the case of multiphase drive waveforms the wireless stimulator may perform internal switching to pass negative-going or positive-going pulses (phases) to the charge-balance capacitor. These pulses may be delivered in any sequence and with varying amplitudes and waveform shapes to achieve a desired physiological effect. For example, the stimulus phase may be followed by an actively driven charge-balancing phase, and/or the stimulus phase may be preceded by an opposite phase. Preceding the stimulus with an opposite-polarity phase, for example, can have the advantage of reducing the amplitude of the stimulus phase required to excite tissue.

In some implementations, the amplitude and timing of stimulus and charge-balancing phases is controlled by the amplitude and timing of RF pulses from the RF pulse generator module106, and in others this control may be administered internally by circuitry onboard the wireless stimulator114, such as controller250. In the case of onboard control, the amplitude and timing may be specified or modified by data commands delivered from the pulse generator module106.

FIG. 3is a flowchart showing an example of an operation of the neural stimulator system. In block302, the wireless neural stimulator114is implanted in proximity to nerve bundles and is coupled to the electric field produced by the TX antenna110. That is, the pulse generator module106and the TX antenna110are positioned in such a way (for example, in proximity to the patient) that the TX antenna110is electrically radiatively coupled with the implanted RX antenna238of the neural stimulator114. In certain implementations, both the antenna110and the RF pulse generator106are located subcutaneously. In other implementations, the antenna110and the RF pulse generator106are located external to the patient's body. In this case, the TX antenna110may be coupled directly to the patient's skin.

Energy from the RF pulse generator is radiated to the implanted wireless neural stimulator114from the antenna110through tissue, as shown in block304. The energy radiated may be controlled by the Patient/Clinician Parameter inputs in block301. In some instances, the parameter settings can be adjusted in an open loop fashion by the patient or clinician, who would adjust the parameter inputs in block301to the system.

The wireless implanted stimulator114uses the received energy to generate electrical pulses to be applied to the neural tissue through the electrodes238. For instance, the stimulator114may contain circuitry that rectifies the received RF energy and conditions the waveform to charge balance the energy delivered to the electrodes to stimulate the targeted nerves or tissues, as shown in block306. The implanted stimulator114communicates with the pulse generator106by using antenna238to send a telemetry signal, as shown in block308. The telemetry signal may contain information about parameters of the electrical pulses applied to the electrodes, such as the impedance of the electrodes, whether the safe current limit has been reached, or the amplitude of the current that is presented to the tissue from the electrodes.

In block310, the RF pulse generator106detects amplifies, filters and modulates the received telemetry signal using amplifier226, filter224, and demodulator222, respectively. The A/D converter230then digitizes the resulting analog signal, as shown in312. The digital telemetry signal is routed to CPU230, which determines whether the parameters of the signal sent to the stimulator114need to be adjusted based on the digital telemetry signal. For instance, in block314, the CPU230compares the information of the digital signal to a look-up table, which may indicate an appropriate change in stimulation parameters. The indicated change may be, for example, a change in the current level of the pulses applied to the electrodes. As a result, the CPU may change the output power of the signal sent to stimulator114so as to adjust the current applied by the electrodes254, as shown in block316.

Thus, for instance, the CPU230may adjust parameters of the signal sent to the stimulator114every cycle to match the desired current amplitude setting programmed by the patient, as shown in block318. The status of the stimulator system may be sampled in real time at a rate of 8 kbits per second of telemetry data. All feedback data received from the stimulator114can be maintained against time and sampled per minute to be stored for download or upload to a remote monitoring system accessible by the health care professional for trending and statistical correlations in block318. If operated in an open loop fashion, the stimulator system operation may be reduced to just the functional elements shown in blocks302,304,306, and308, and the patient uses their judgment to adjust parameter settings rather than the closed looped feedback from the implanted device.

FIG. 4depicts a flow chart showing an example of an operation of the system when the current level at the electrodes254is above a threshold limit. In certain instances, the implanted wireless neural stimulator114may receive an input power signal with a current level above an established safe current limit, as shown in block402. For instance, the current limiter248may determine the current is above an established tissue-safe limit of amperes, as shown in block404. If the current limiter senses that the current is above the threshold, it may stop the high-power signal from damaging surrounding tissue in contact with the electrodes as shown in block406, the operations of which are as described above in association withFIG. 2.

A capacitor may store excess power, as shown in block408. When the current limiter senses the current is above the threshold, the controller250may use the excess power available to transmit a small 2-bit data burst back to the RF pulse generator106, as shown in block410. The 2-bit data burst may be transmitted through the implanted wireless neural stimulator's antenna(s)238during the RF pulse generator's receive cycle, as shown in block412. The RF pulse generator antenna110may receive the 2-bit data burst during its receive cycle, as shown in block414, at a rate of 8 kbps, and may relay the data burst back to the RF pulse generator's feedback subsystem212which is monitoring all reverse power, as shown in block416. The CPU230may analyze signals from feedback subsystem202, as shown in block418and if there is no data burst present, no changes may be made to the stimulation parameters, as shown in block420. If the data burst is present in the analysis, the CPU230can cut all transmission power for one cycle, as shown in block422.

If the data burst continues, the RF pulse generator106may push a “proximity power danger” notification to the application on the programmer module102, as shown in block424. This proximity danger notification occurs because the RF pulse generator has ceased its transmission of power. This notification means an unauthorized form of energy is powering the implant above safe levels. The application may alert the user of the danger and that the user should leave the immediate area to resume neural modulation therapy, as shown in block426. If after one cycle the data burst has stopped, the RF pulse generator106may slowly ramp up the transmission power in increments, for example from 5% to 75% of previous current amplitude levels, as shown in block428. The user can then manually adjust current amplitude level to go higher at the user's own risk. During the ramp up, the RF pulse generator106may notify the application of its progress and the application may notify the user that there was an unsafe power level and the system is ramping back up, as shown in block430.

FIG. 5is a diagram showing examples of signals that may be used to detect an impedance mismatch. As described above, a forward power signal and a reverse power signal may be used to detect an impedance mismatch. For instance, a RF pulse502generated by the RF pulse generator may pass through a device such as a dual directional coupler to the TX antenna110. The TX antenna110then radiates the RF signal into the body, where the energy is received by the implanted wireless neural stimulator114and converted into a tissue-stimulating pulse. The coupler passes an attenuated version of this RF signal, forward power510, to feedback subsystem212. The feedback subsystem212demodulates the AC signal and computes the amplitude of the forward RF power, and this data is passed to controller subsystem214. Similarly the dual directional coupler (or similar component) also receives RF energy reflected back from the TX antenna110and passes an attenuated version of this RF signal, reverse power512, to feedback subsystem212. The feedback subsystem212demodulates the AC signal and computes the amplitude of the reflected RF power, and this data is passed to controller subsystem214.

In the optimal case, when the TX antenna110may be perfectly impedance-matched to the body so that the RF energy passes unimpeded across the interface of the TX antenna110to the body, and no RF energy is reflected at the interface. Thus, in this optimal case, the reverse power512may have close to zero amplitude as shown by signal504, and the ratio of reverse power512to forward power510is zero. In this circumstance, no error condition exists, and the controller214sets a system message that operation is optimal.

In practice, the impedance match of the TX antenna204to the body may not be optimal, and some energy of the RF pulse502is reflected from the interface of the TX antenna110and the body. This can occur for example if the TX antenna110is held somewhat away from the skin by a piece of clothing. This non-optimal antenna coupling causes a small portion of the forward RF energy to be reflected at the interface, and this is depicted as signal506. In this case, the ratio of reverse power512to forward power510is small, but a small ratio implies that most of the RF energy is still radiated from the TX antenna110, so this condition is acceptable within the control algorithm. This determination of acceptable reflection ratio may be made within controller subsystem214based upon a programmed threshold, and the controller subsystem214may generate a low-priority alert to be sent to the user interface. In addition, the controller subsystem214sensing the condition of a small reflection ratio, may moderately increase the amplitude of the RF pulse502to compensate for the moderate loss of forward energy transfer to the implanted wireless neural stimulator114.

During daily operational use, the TX antenna110might be accidentally removed from the body entirely, in which case the TX antenna will have very poor coupling to the body (if any). In this or other circumstances, a relatively high proportion of the RF pulse energy is reflected as signal508from the TX antenna110and fed backward into the RF-powering system. Similarly, this phenomenon can occur if the connection to the TX antenna is physically broken, in which case virtually 100% of the RF energy is reflected backward from the point of the break. In such cases, the ratio of reverse power512to forward power510is very high, and the controller subsystem214will determine the ratio has exceeded the threshold of acceptance. In this case, the controller subsystem214may prevent any further RF pulses from being generated. The shutdown of the RF pulse generator module106may be reported to the user interface to inform the user that stimulation therapy cannot be delivered.

FIG. 6is a diagram showing examples of signals that may be employed during operation of the neural stimulator system. According to some implementations, the amplitude of the RF pulse602received by the implanted wireless neural stimulator114can directly control the amplitude of the stimulus630delivered to tissue. The duration of the RF pulse608corresponds to the specified pulse width of the stimulus630. During normal operation the RF pulse generator module106sends an RF pulse waveform602via TX antenna110into the body, and RF pulse waveform608may represent the corresponding RF pulse received by implanted wireless neural stimulator114. In this instance the received power has an amplitude suitable for generating a safe stimulus pulse630. The stimulus pulse630is below the safety threshold626, and no error condition exists. In another example, the attenuation between the TX antenna110and the implanted wireless neural stimulator114has been unexpectedly reduced, for example due to the user repositioning the TX antenna110. This reduced attenuation can lead to increased amplitude in the RF pulse waveform612being received at the neural stimulator114. Although the RF pulse602is generated with the same amplitude as before, the improved RF coupling between the TX antenna110and the implanted wireless neural stimulator114can cause the received RF pulse612to be larger in amplitude. Implanted wireless neural stimulator114in this situation may generate a larger stimulus632in response to the increase in received RF pulse612. However, in this example, the received power612is capable of generating a stimulus632that exceeds the prudent safety limit for tissue. In this situation, the current limiter feedback control mode can operate to clip the waveform of the stimulus pulse632such that the stimulus delivered is held within the predetermined safety limit626. The clipping event628may be communicated through the feedback subsystem212as described above, and subsequently controller subsystem214can reduce the amplitude specified for the RF pulse. As a result, the subsequent RF pulse604is reduced in amplitude, and correspondingly the amplitude of the received RF pulse616is reduced to a suitable level (non-clipping level). In this fashion, the current limiter feedback control mode may operate to reduce the RF power delivered to the body if the implanted wireless neural stimulator114receives excess RF power.

In another example, the RF pulse waveform606depicts a higher amplitude RF pulse generated as a result of user input to the user interface. In this circumstance, the RF pulse620received by the implanted wireless neural stimulator14is increased in amplitude, and similarly current limiter feedback mode operates to prevent stimulus636from exceeding safety limit626. Once again, this clipping event628may be communicated through the feedback subsystem212, and subsequently controller subsystem214may reduce the amplitude of the RF pulse, thus overriding the user input. The reduced RF pulse604can produce correspondingly smaller amplitudes of the received waveforms616, and clipping of the stimulus current may no longer be required to keep the current within the safety limit. In this fashion, the current limiter feedback may reduce the RF power delivered to the body if the implanted wireless neural stimulator114reports it is receiving excess RF power.

FIG. 7is a flow chart showing a process for the user to control the implantable wireless neural stimulator through the programmer in an open loop feedback system. In one implementation of the system, the user has a wireless neural stimulator implanted in their body, the RF pulse generator106sends the stimulating pulse power wirelessly to the stimulator114, and an application on the programmer module102(for example, a smart device) is communicating with the RF pulse generator106. In this implementation, if a user wants to observe the current status of the functioning pulse generator, as shown in block702, the user may open the application, as shown in block704. The application can use Bluetooth protocols built into the smart device to interrogate the pulse generator, as shown in block706. The RF pulse generator106may authenticate the identity of the smart device and serialized patient assigned secure iteration of the application, as shown in block708. The authentication process may utilize a unique key to the patient specific RF pulse generator serial number. The application can be customized with the patient specific unique key through the Manufacturer Representative who has programmed the initial patient settings for the stimulation system, as shown in block720. If the RF pulse generator rejects the authentication it may inform the application that the code is invalid, as shown in block718and needs the authentication provided by the authorized individual with security clearance from the device manufacturer, known as the “Manufacturer's Representative,” as shown in block722. In an implementation, only the Manufacturer's Representative can have access to the security code needed to change the application's stored RF pulse generator unique ID. If the RF pulse generator authentication system passes, the pulse generator module106sends back all of the data that has been logged since the last sync, as shown in block710. The application may then register the most current information and transmit the information to a 3rd party in a secure fashion, as shown in712. The application may maintain a database that logs all system diagnostic results and values, the changes in settings by the user and the feedback system, and the global runtime history, as shown in block714. The application may then display relevant data to the user, as shown in block716; including the battery capacity, current program parameter, running time, pulse width, frequency, amplitude, and the status of the feedback system.

FIG. 8is another example flow chart of a process for the user to control the wireless stimulator with limitations on the lower and upper limits of current amplitude. The user wants to change the amplitude of the stimulation signal, as shown in block802. The user may open the application, as show in block704and the application may go through the process described inFIG. 7to communicate with the RF pulse generator, authenticate successfully, and display the current status to the user, as shown in block804. The application displays the stimulation amplitude as the most prevalent changeable interface option and displays two arrows with which the user can adjust the current amplitude. The user may make a decision based on their need for more or less stimulation in accordance with their pain levels, as shown in block806. If the user chooses to increase the current amplitude, the user may press the up arrow on the application screen, as shown in block808. The application can include safety maximum limiting algorithms, so if a request to increase current amplitude is recognized by the application as exceeding the preset safety maximum, as shown in block810, then the application will display an error message, as shown in block812and will not communicate with the RF pulse generator module106. If the user presses the up arrow, as shown in block808and the current amplitude request does not exceed the current amplitude maximum allowable value, then the application will send instructions to the RF pulse generator module106to increase amplitude, as shown in block814. The RF pulse generator module106may then attempt to increase the current amplitude of stimulation, as shown in block816. If the RF pulse generator is successful at increasing the current amplitude, the RF pulse generator module106may perform a short vibration to physically confirm with the user that the amplitude is increased, as shown in block818. The RF pulse generator module106can also send back confirmation of increased amplitude to the application, as shown in block820, and then the application may display the updated current amplitude level, as shown in block822.

If the user decides to decrease the current amplitude level in block806, the user can press the down arrow on the application, as shown in block828. If the current amplitude level is already at zero, the application recognizes that the current amplitude cannot be decreased any further, as shown in block830and displays an error message to the user without communicating any data to the RF pulse generator, as shown in block832. If the current amplitude level is not at zero, the application can send instructions to the RF pulse generator module106to decrease current amplitude level accordingly, as shown in block834. The RF pulse generator may then attempt to decrease current amplitude level of stimulation RF pulse generator module106and, if successful, the RF pulse generator module106may perform a short vibration to physically confirm to the user that the current amplitude level has been decreased, as shown in block842. The RF pulse generator module106can send back confirmation of the decreased current amplitude level to the application, as shown in block838. The application then may display the updated current amplitude level, as indicated by block840. If the current amplitude level decrease or increase fails, the RF pulse generator module106can perform a series of short vibrations to alert user, and send an error message to the application, as shown in block824. The application receives the error and may display the data for the user's benefit, as shown in block826.

FIG. 9is yet another example flow chart of a process for the user to control the wireless neural stimulator114through preprogrammed parameter settings. The user wants to change the parameter program, as indicated by block902. When the user is implanted with a wireless neural stimulator or when the user visits the doctor, the Manufacturer's Representative may determine and provide the patient/user RF pulse generator with preset programs that have different stimulation parameters that will be used to treat the user. The user will then able to switch between the various parameter programs as needed. The user can open the application on their smart device, as indicated by block704, which first follows the process described inFIG. 7, communicating with the RF pulse generator module106, authenticating successfully, and displaying the current status of the RF pulse generator module106, including the current program parameter settings, as indicated by block812. In this implementation, through the user interface of the application, the user can select the program that they wish to use, as shown by block904. The application may then access a library of pre-programmed parameters that have been approved by the Manufacturer's Representative for the user to interchange between as desired and in accordance with the management of their indication, as indicated by block906. A table can be displayed to the user, as shown in block908and each row displays a program's codename and lists its basic parameter settings, as shown in block910, which includes but is not limited to: pulse width, frequency, cycle timing, pulse shape, duration, feedback sensitivity, as shown in block912. The user may then select the row containing the desired parameter preset program to be used, as shown in block912. The application can send instructions to the RF pulse generator module106to change the parameter settings, as shown in block916. The RF pulse generator module106may attempt to change the parameter settings154. If the parameter settings are successfully changed, the RF pulse generator module106can perform a unique vibration pattern to physically confirm with the user that the parameter settings were changed, as shown in block920. Also, the RF pulse generator module106can send back confirmation to the application that the parameter change has been successful, as shown in block922, and the application may display the updated current program, as shown in block924. If the parameter program change has failed, the RF pulse generator module106may perform a series of short vibrations to alert the user, and send an error message to the application, as shown in block926, which receives the error and may display to the user, as shown in block928.

FIG. 10is still another example flow chart of a process for a low battery state for the RF pulse generator module106. In this implementation, the RF pulse generator module's remaining battery power level is recognized as low, as shown in block1002. The RF pulse generator module106regularly interrogates the power supply battery subsystem210about the current power and the RF pulse generator microprocessor asks the battery if its remaining power is below threshold, as shown in block1004. If the battery's remaining power is above the threshold, the RF pulse generator module106may store the current battery status to be sent to the application during the next sync, as shown in block1006. If the battery's remaining power is below threshold the RF pulse generator module106may push a low-battery notification to the application, as shown in block1008. The RF pulse generator module106may always perform one sequence of short vibrations to alert the user of an issue and send the application a notification, as shown in block1010. If there continues to be no confirmation of the application receiving the notification then the RF pulse generator can continue to perform short vibration pulses to notify user, as shown in block1010. If the application successfully receives the notification, it may display the notification and may need user acknowledgement, as shown in block1012. If, for example, one minute passes without the notification message on the application being dismissed the application informs the RF pulse generator module106about lack of human acknowledgement, as shown in block1014, and the RF pulse generator module106may begin to perform the vibration pulses to notify the user, as shown in block1010. If the user dismisses the notification, the application may display a passive notification to switch the battery, as shown in block1016. If a predetermined amount of time passes, such as five minutes for example, without the battery being switched, the application can inform the RF pulse generator module106of the lack of human acknowledgement, as shown in block1014and the RF pulse generator module106may perform vibrations, as shown in block1010. If the RF pulse generator module battery is switched, the RF pulse generator module106reboots and interrogates the battery to assess power remaining, as shown in block1018. If the battery's power remaining is below threshold, the cycle may begin again with the RF pulse generator module106pushing a notification to the application, as shown in block1008. If the battery's power remaining is above threshold the RF pulse generator module106may push a successful battery-change notification to the application, as shown in block1020. The application may then communicate with the RF pulse generator module106and displays current system status, as shown in block1022.

FIG. 11is yet another example flow chart of a process for a Manufacturer's Representative to program the implanted wireless neural stimulator. In this implementation, a user wants the Manufacturer's Representative to set individual parameter programs from a remote location different than where the user is, for the user to use as needed, as shown in block1102. The Manufacturer's Representative can gain access to the user's set parameter programs through a secure web based service. The Manufacturer's Representative can securely log into the manufacturer's web service on a device connected to the Internet, as shown in block1104. If the Manufacturer's Representative is registering the user for the first time in their care they enter in the patient's basic information, the RF pulse generator's unique ID and the programming application's unique ID, as shown in block1106. Once the Manufacturer's Representative's new or old user is already registered, the Manufacturer's Representative accesses the specific user's profile, as shown in block1108. The Manufacturer's Representative is able to view the current allotted list of parameter programs for the specific user, as shown in block1110. This list may contain previous active and retired parameter preset programs, as shown in block1112. The Manufacturer's Representative is able to activate/deactivate preset parameter programs by checking the box next to the appropriate row in the table displayed, as shown in block1114. The Manufacturer's Representative may then submit and save the allotted new preset parameter programs, as shown in block1116. The user's programmer application may receive the new preset parameter programs at the next sync with the manufacturer's database.

FIG. 12is a circuit diagram showing an example of a wireless neural stimulator, such as stimulator114. This example contains paired electrodes, comprising cathode electrode(s)1208and anode electrode(s)1210, as shown. When energized, the charged electrodes create a volume conduction field of current density within the tissue. In this implementation, the wireless energy is received through a dipole antenna(s)238. At least four diodes are connected together to form a full wave bridge rectifier1202attached to the dipole antenna(s)238. Each diode, up to 100 micrometers in length, uses a junction potential to prevent the flow of negative electrical current, from cathode to anode, from passing through the device when said current does not exceed the reverse threshold. For neural stimulation via wireless power, transmitted through tissue, the natural inefficiency of the lossy material may lead to a low threshold voltage. In this implementation, a zero biased diode rectifier results in a low output impedance for the device. A resistor1204and a smoothing capacitor1206are placed across the output nodes of the bridge rectifier to discharge the electrodes to the ground of the bridge anode. The rectification bridge1202includes two branches of diode pairs connecting an anode-to-anode and then cathode to cathode. The electrodes1208and1210are connected to the output of the charge balancing circuit246.

FIG. 13is a circuit diagram of another example of a wireless neural stimulator, such as stimulator114. The example shown inFIG. 13includes multiple electrode control and may employ full closed loop control. The stimulator includes an electrode array254in which the polarity of the electrodes can be assigned as cathodic or anodic, and for which the electrodes can be alternatively not powered with any energy. When energized, the charged electrodes create a volume conduction field of current density within the tissue. In this implementation, the wireless energy is received by the device through the dipole antenna(s)238. The electrode array254is controlled through an on-board controller circuit242that sends the appropriate bit information to the electrode interface252in order to set the polarity of each electrode in the array, as well as power to each individual electrode. The lack of power to a specific electrode would set that electrode in a functional OFF position. In another implementation (not shown), the amount of current sent to each electrode is also controlled through the controller242. The controller current, polarity and power state parameter data, shown as the controller output, is be sent back to the antenna(s)238for telemetry transmission back to the pulse generator module106. The controller242also includes the functionality of current monitoring and sets a bit register counter so that the status of total current drawn can be sent back to the pulse generator module106.

At least four diodes can be connected together to form a full wave bridge rectifier302attached to the dipole antenna(s)238. Each diode, up to 100 micrometers in length, uses a junction potential to prevent the flow of negative electrical current, from cathode to anode, from passing through the device when said current does not exceed the reverse threshold. For neural stimulation via wireless power, transmitted through tissue, the natural inefficiency of the lossy material may lead to a low threshold voltage. In this implementation, a zero biased diode rectifier results in a low output impedance for the device. A resistor1204and a smoothing capacitor1206are placed across the output nodes of the bridge rectifier to discharge the electrodes to the ground of the bridge anode. The rectification bridge1202may include two branches of diode pairs connecting an anode-to-anode and then cathode to cathode. The electrode polarity outputs, both cathode1208and anode1210are connected to the outputs formed by the bridge connection. Charge balancing circuitry246and current limiting circuitry248are placed in series with the outputs.

FIG. 14is a block diagram showing an example of control functions1405and feedback functions1430of a wireless implantable neural stimulator1400, such as the ones described above or further below. An example implementation of the implantable neural stimulator1400may be implanted lead module114, as discussed above in association withFIG. 2. Control functions1405include functions1410for polarity switching of the electrodes and functions1420for power-on reset.

Polarity switching functions1410may employ, for example, a polarity routing switch network to assign polarities to electrodes254. The assignment of polarity to an electrode may, for instance, be one of: a cathode (negative polarity), an anode (positive polarity), or a neutral (off) polarity. The polarity assignment information for each of the electrodes254may be contained in the input signal received by wireless implantable neural stimulator1400through Rx antenna238from RF pulse generator module106. Because a programmer module102may control RF pulse generator module106, the polarity of electrodes254may be controlled remotely by a programmer through programmer module102, as shown inFIG. 2.

Power-on reset functions1420may reset the polarity assignment of each electrode immediately on each power-on event. As will be described in further detail below, this reset operation may cause RF pulse generator module106to transmit the polarity assignment information to the wireless implantable neural stimulator1400. Once the polarity assignment information is received by the wireless implantable neural stimulator1400, the polarity assignment information may be stored in a register file, or other short term memory component. Thereafter the polarity assignment information may be used to configure the polarity assignment of each electrode. If the polarity assignment information transmitted in response to the reset encodes the same polarity state as before the power-on event, then the polarity state of each electrode can be maintained before and after each power-on event.

Feedback functions1430include functions1440for monitoring delivered power to electrodes254and functions1450for making impedance diagnosis of electrodes254. For example, delivered power functions1440may provide data encoding the amount of power being delivered from electrodes254to the excitable tissue and tissue impedance diagnostic functions1450may provide data encoding the diagnostic information of tissue impedance. The tissue impedance is the electrical impedance of the tissue as seen between negative and positive electrodes when a stimulation current is being released between negative and positive electrodes.

Feedback functions1430may additionally include tissue depth estimate functions1460to provide data indicating the overall tissue depth that the input radio frequency (RF) signal from the pulse generator module, such as, for example, RF pulse generator module106, has penetrated before reaching the implanted antenna, such as, for example, RX antenna238, within the wireless implantable neural stimulator1400, such as, for example, implanted lead module114. For instance, the tissue depth estimate may be provided by comparing the power of the received input signal to the power of the RF pulse transmitted by the RF pulse generator106. The ratio of the power of the received input signal to the power of the RF pulse transmitted by the RF pulse generator106may indicate an attenuation caused by wave propagation through the tissue. For example, the second harmonic described below may be received by the RF pulse generator106and used with the power of the input signal sent by the RF pulse generator to determine the tissue depth. The attenuation may be used to infer the overall depth of wireless implantable neural stimulator1400underneath the skin.

The data from blocks1440,1450, and1460may be transmitted, for example, through Tx antenna110to RF pulse generator106, as illustrated inFIGS. 1 and 2.

As discussed above in association withFIGS. 1,2,12, and13, a wireless implantable neural stimulator1400may utilize rectification circuitry to convert the input signal (e.g., having a carrier frequency within a range from about 800 MHz to about 6 GHz) to a direct current (DC) power to drive the electrodes254. Some implementations may provide the capability to regulate the DC power remotely. Some implementations may further provide different amounts of power to different electrodes, as discussed in further detail below.

FIG. 15is a schematic showing an example of a wireless implantable neural stimulator1500with components to implement control and feedback functions as discussed above in association withFIG. 14. An RX antenna1505receives the input signal. The RX antenna1505may be embedded as a dipole, microstrip, folded dipole or other antenna configuration other than a coiled configuration, as described above. The input signal has a carrier frequency in the GHz range and contains electrical energy for powering the wireless implantable neural stimulator1500and for providing stimulation pulses to electrodes254. Once received by the antenna1505, the input signal is routed to power management circuitry1510. Power management circuitry1510is configured to rectify the input signal and convert it to a DC power source. For example, the power management circuitry1510may include a diode rectification bridge such as the diode rectification bridge1202illustrated inFIG. 12. The DC power source provides power to stimulation circuitry1511and logic power circuitry1513. The rectification may utilize one or more full wave diode bridge rectifiers within the power management circuitry1510. In one implementation, a resistor can be placed across the output nodes of the bridge rectifier to discharge the electrodes to the ground of the bridge anode, as illustrated by the shunt register1204inFIG. 12.

FIG. 16shows an example pulse waveform generated by the MFS sent to the power management circuitry1510of the wireless implantable neural stimulator1500. This can be a typical pulse waveform generated by the RF pulse generator module106and then passed on the carrier frequency. The pulse amplitude is ramped over the pulse width (duration) from a value ranging from −9 dB to +6 dB. In certain implementations, the ramp start and end power level can be set to any range from 0 to 60 dB. The gain control is adjustable and can be an input parameter from RF pulse generator module106to the stimulation power management circuitry1510. The pulse width, Pw, can range from 100 to 300 microseconds (μs) in some implementations, as shown inFIG. 16. In other implementations not shown, the pulse width can be between about 5 microseconds (5 us) and about 10 milliseconds (10 ms). The pulse frequency (rate) can range from about 5 Hz to 120 Hz as shown. In some implementations not shown, the pulse frequency can be below 5 Hz, and as high as about 10,000 Hz.

Returning toFIG. 15, based on the received waveform, stimulation circuitry1511creates the stimulation waveform to be sent to the electrodes254to stimulate excitable tissues, as discussed above. In some implementations, stimulation circuitry1511may route the waveform to pulse-shaping resistor-capacitor (RC) timer1512to shape each travelling pulse waveform. An example RC-timer can be the shunt resistor1204and smoothing resistor1206, as illustrated inFIG. 12and as discussed above. The pulse-shaping RC timer1512can also be used to, but is not limited to, inverting the pulse to create a pre-anodic dip or provide a slow ramping in waveform.

Once the waveform has been shaped, the cathodic energy—energy being transmitted over the cathodic branch1515of the polarity routing switch network1523—is routed through the passive charge balancing circuitry1518to prevent the build-up of noxious chemicals at the electrodes254, as discussed above. Cathodic energy is then routed to input1, block1522, of polarity routing switch network1521. Anodic energy—energy being transmitted over the anodic branch1514of the polarity routing switch network1523—is routed to input2, block1523, of polarity routing switch network1521. Thereafter, the polarity routing switch network1521delivers the stimulation energy in the form of cathodic energy, anodic energy, or no energy, to the each of the electrodes254, depending on the respective polarity assignment, which is controlled based on a set of bits stored in the register file1532. The bits stored in the register file1532are output to a selection input1534of the polarity routing switch network1523, which causes input1or input2to be routed to the electrodes as appropriate.

Turning momentarily toFIG. 17, a schematic of an example of a polarity routing switch network1700is shown. As discussed above, the cathodic (−) energy and the anodic energy are received at input1(block1522) and input2(block1523), respectively. Polarity routing switch network1700has one of its outputs coupled to an electrode of electrodes254which can include as few as two electrodes, or as many as sixteen electrodes. Eight electrodes are shown in this implementation as an example.

Polarity routing switch network1700is configured to either individually connect each output to one of input1or input2, or disconnect the output from either of the inputs. This selects the polarity for each individual electrode of electrodes254as one of: neutral (off), cathode (negative), or anode (positive). Each output is coupled to a corresponding three-state switch1730for setting the connection state of the output. Each three-state switch is controlled by one or more of the bits from the selection input1750. In some implementations, selection input1750may allocate more than one bits to each three-state switch. For example, two bits may encode the three-state information. Thus, the state of each output of polarity routing switch device1700can be controlled by information encoding the bits stored in the register1532, which may be set by polarity assignment information received from the remote RF pulse generator module106, as described further below.

Returning toFIG. 15, power and impedance sensing circuitry may be used to determine the power delivered to the tissue and the impedance of the tissue. For example, a sensing resistor1518may be placed in serial connection with the anodic branch1514. Current sensing circuit1519senses the current across the resistor1518and voltage sensing circuit1520senses the voltage across the resistor. The measured current and voltage may correspond to the actual current and voltage applied by the electrodes to the tissue.

As described below, the measured current and voltage may be provided as feedback information to RF pulse generator module106. The power delivered to the tissue may be determined by integrating the product of the measured current and voltage over the duration of the waveform being delivered to electrodes254. Similarly, the impedance of the tissue may be determined based on the measured voltage being applied to the electrodes and the current being applied to the tissue. Alternative circuitry (not shown) may also be used in lieu of the sensing resistor1518, depending on implementation of the feature and whether both impedance and power feedback are measured individually, or combined.

The measurements from the current sensing circuitry1519and the voltage sensing circuitry1520may be routed to a voltage controlled oscillator (VCO)1533or equivalent circuitry capable of converting from an analog signal source to a carrier signal for modulation. VCO1533can generate a digital signal with a carrier frequency. The carrier frequency may vary based on analog measurements such as, for example, a voltage, a differential of a voltage and a power, etc. VCO1533may also use amplitude modulation or phase shift keying to modulate the feedback information at the carrier frequency. The VCO or the equivalent circuit may be generally referred to as an analog controlled carrier modulator. The modulator may transmit information encoding the sensed current or voltage back to RF pulse generator106.

Antenna1525may transmit the modulated signal, for example, in the GHz frequency range, back to the RF pulse generator module106. In some embodiments, antennas1505and1525may be the same physical antenna. In other embodiments, antennas1505and1525may be separate physical antennas. In the embodiments of separate antennas, antenna1525may operate at a resonance frequency that is higher than the resonance frequency of antenna1505to send stimulation feedback to RF pulse generator module106. In some embodiments. antenna1525may also operate at the higher resonance frequency to receive data encoding the polarity assignment information from RF pulse generator module106.

Antenna1525may be a telemetry antenna1525which may route received data, such as polarity assignment information, to the stimulation feedback circuit1530. The encoded polarity assignment information may be on a band in the GHz range. The received data may be demodulated by demodulation circuitry1531and then stored in the register file1532. The register file1532may be a volatile memory. Register file1532may be an 8-channel memory bank that can store, for example, several bits of data for each channel to be assigned a polarity. Some embodiments may have no register file, while some embodiments may have a register file up to 64 bits in size. The information encoded by these bits may be sent as the polarity selection signal to polarity routing switch network1521, as indicated by arrow1534. The bits may encode the polarity assignment for each output of the polarity routing switch network as one of: + (positive), − (negative), or 0 (neutral). Each output connects to one electrode and the channel setting determines whether the electrode will be set as an anode (positive), cathode (negative), or off (neutral).

Returning to power management circuitry1510, in some embodiments, approximately 90% of the energy received is routed to the stimulation circuitry1511and less than 10% of the energy received is routed to the logic power circuitry1513. Logic power circuitry1513may power the control components for polarity and telemetry. In some implementations, the power circuitry1513, however, does not provide the actual power to the electrodes for stimulating the tissues. In certain embodiments, the energy leaving the logic power circuitry1513is sent to a capacitor circuit1516to store a certain amount of readily available energy. The voltage of the stored charge in the capacitor circuit1516may be denoted as Vdc. Subsequently, this stored energy is used to power a power-on reset circuit1516configured to send a reset signal on a power-on event. If the wireless implantable neural stimulator1500loses power for a certain period of time, for example, in the range from about 1 millisecond to over 10 milliseconds, the contents in the register file1532and polarity setting on polarity routing switch network1521may be zeroed. The wireless implantable neural stimulator1500may lose power, for example, when it becomes less aligned with RF pulse generator module106. Using this stored energy, power-on reset circuit1540may provide a reset signal as indicated by arrow1517. This reset signal may cause stimulation feedback circuit1530to notify RF pulse generator module106of the loss of power. For example, stimulation feedback circuit1530may transmit a telemetry feedback signal to RF pulse generator module106as a status notification of the power outage. This telemetry feedback signal may be transmitted in response to the reset signal and immediately after power is back on neural stimulator1500. RF pulse generator module106may then transmit one or more telemetry packets to implantable wireless neutral stimulator. The telemetry packets contain polarity assignment information, which may be saved to register file1532and may be sent to polarity routing switch network1521. Thus, polarity assignment information in register file1532may be recovered from telemetry packets transmitted by RF pulse generator module106and the polarity assignment for each output of polarity routing switch network1521may be updated accordingly based on the polarity assignment information.

The telemetry antenna1525may transmit the telemetry feedback signal back to RF pulse generator module106at a frequency higher than the characteristic frequency of an RX antenna1505. In one implementation, the telemetry antenna1525can have a heightened resonance frequency that is the second harmonic of the characteristic frequency of RX antenna1505. For example, the second harmonic may be utilized to transmit power feedback information regarding an estimate of the amount of power being received by the electrodes. The feedback information may then be used by the RF pulse generator in determining any adjustment of the power level to be transmitted by the RF pulse generator106. In a similar manner, the second harmonic energy can be used to detect the tissue depth. The second harmonic transmission can be detected by an external antenna, for example, on RF pulse generator module106that is tuned to the second harmonic. As a general matter, power management circuitry1510may contain rectifying circuits that are non-linear device capable of generating harmonic energies from input signal. Harvesting such harmonic energy for transmitting telemetry feedback signal could improve the efficiency of wireless implantable neural stimulator1500.FIGS. 18A and 18Band the following discussion demonstrate the feasibility of utilizing the second harmonic to transmit telemetry signal to RF pulse generator module106.

FIGS.18A and18BB respectively show an example full-wave rectified sine wave and the corresponding spectrum. In particular, a full-wave rectified 915 MHz sine wave is being analyzed. In this example, the second harmonic of the 915 MHz sine wave is an 1830 MHz output harmonic. This harmonic wave may be attenuated by the amount of tissue that the harmonic wave needs to pass through before reaching the external harmonic receiver antenna. In general, an estimation of the power levels during the propagation of the harmonic wave can reveal the feasibility of the approach. The estimation may consider the power of received input signal at the receiving antenna (e.g., at antenna1505and at 915 MHz), the power of the second harmonic radiated from the rectified 915 MHz waveform, the amount of attenuation for the second harmonic wave to propagate through the tissue medium, and an estimation of the coupling efficiency for the harmonic antenna. The average power transmitted in Watts can be estimated by Equation 1:
Pt=PkDuC
Pr=(Pt/Aant)(1−{Γ}2)Lλ2Grη/4π)  (1)

Table 1 below tabulates the denotations of each symbol and the corresponding value used in the estimation.

In estimating L, the loss due to the attenuation in the tissue, attenuations from the fundamental (for the forward path to the implanted lead module114) and second harmonics (for the reverse path from the implanted lead module113) may be considered. The plane wave attenuation is given by the following equation (2) and Table 2:

TABLE 2Output power loss for 915 MHz and1830 MHz harmonic at 1 cm depth.Freq(MHz)rS/m)neper/m)Power loss0.915e941.3290.8716925.0300.6061.83e938.8231.196535.7730.489

The worst case assumption for coupling of the harmonics wave to the external receive antenna is that the power radiated at the harmonic frequency by the implanted telemetry antenna (e.g., telemetry antenna1625) is completely absorbed by external receive antenna. This worst case scenario can be modeled by the following equation (3) and Table 3:
Pnr=PtLnLna(3)

where

Pnr=nth Harmonic Antenna Received Power (W)

Pt=Total Received power of Implant (W)

Ln=Power of nth Harmonic of Implant Power (W)

Lna=Attenuation Loss Factor

TABLE 3Output total power and received harmonicpower for the 2ndharmonic.Pt(W)LnLnaPnr(W)dBm0.356.24210.4890.042216.3

In sum, the reduction of power levels has been estimated to be about 10 dB utilizing these developed equations. This includes the attenuation of a 915 MHz plane wave that propagates through tissue depths from 1 cm to 6 cm. The average received power, Pr, at 915 MHz is 0.356 W. The power in the second harmonic (1830 MHz) is about −6.16 dB, as obtained from a SPICE simulation using a full wave rectified 915 MHz sine wave. The estimate of 10 dB means a reduction of a factor of 10, which is acceptable for field operations. Thus, the feasibility of utilizing the second harmonic frequency to transmit the telemetry feedback signal back to the RF pulse generator module106has been demonstrated.

FIG. 19is a flow chart illustrating an example of operations of control and feedback functions of the neural stimulator. The operations are described with respect to the wireless implantable neural stimulator1500, although the operations may be performed by other variations of a wireless implantable neural stimulator, such as the ones described above.

RF pulse generator module106transmits one or more signals containing electrical energy (1900). RF pulse generator module106may also be known as a microwave field stimulator (MFS) in some implementations. The signal may be modulated at a microwave frequency band, for example, from about 800 MHz to about 6 GHz.

The input signal containing electrical energy is received by RX antenna1505of the neural stimulator1500(1910). As discussed above, RX antenna1505may be embedded as a dipole, microstrip, folded dipole or other antenna configuration other than a coiled configuration.

The input signal is rectified and demodulated by the power management circuitry1510, as shown by block1911. Some implementations may provide waveform shaping and, in this case, the rectified and demodulated signal is passed to pulse shaping RC timer (1912). Charge balancing may be performed by charge balancing circuit1518to provide a charged balanced waveform (1913). Thereafter, the shaped and charge balanced pulses are routed to electrodes254(1920), which deliver the stimulation to the excitable tissue (1921).

In the meantime, the current and voltage being delivered to the tissue is measured using the current sensor1519and voltage sensor1520(1914). These measurements are modulated and amplified (1915) and transmitted to the RF pulse generator module106from telemetry antenna1525(1916). In some embodiments, the telemetry antenna1525and RX antenna1505may utilize the same physical antenna embedded within the neural stimulator1500. The RF pulse generator module106may use the measured current and voltage to determine the power delivered to the tissue, as well as the impedance of the tissue.

For example, the RF pulse generator module106may store the received feedback information such as the information encoding the current and voltage. The feedback information may be stored, for instance, as a present value in a hardware memory on RF pulse generator module106. Based on the feedback information, RF pulse generator module106may calculate the impedance value of the tissue based on the current and voltage delivered to the tissue.

In addition, RF pulse generator module106may calculate the power delivered to the tissue based on the stored current and voltage (1950). The RF pulse generator module106can then determine whether power level should be adjusted by comparing the calculated power to the desired power stored, for example, in a lookup table stored on the RF pulse generator module106(1917). For example, the look-up table may tabulate the optimal amount of power that should be delivered to the tissue for the position of the receive antenna1505on neural stimulator1500relative to the position of the transmit antenna on RF pulse generator module106. This relative position may be determined based on the feedback information. The power measurements in the feedback information may then be correlated to the optimal value to determine if a power level adjustment should be made to increase or decrease the amplitude of stimulation of the delivered power to the electrodes. The power level adjustment information may then enable the RF pulse generator module106to adjust parameters of transmission so that the adjusted power is provided to the RX antenna1505.

In addition to the input signal containing electrical energy for stimulation, the RF pulse generator module106may send an input signal that contains telemetry data such as polarity assignment information (1930). For instance, upon power on, the RF pulse generator module106may transmit data encoding the last electrode polarity settings for each electrode before RF pulse generator module106was powered off. This data may be sent to telemetry antenna1525as a digital data stream embedded on the carrier waveform. In some implementations, the data stream may include telemetry packets. The telemetry packets are received from the RF pulse generator module106and subsequently demodulated (1931) by demodulation circuit1531. The polarity setting information in the telemetry packets is stored in the register file1532(1932). The polarity of each electrode of electrodes254is programmed according to the polarity setting information stored in the register file1532(1933). For example, the polarity of each electrode may be set as one of: anode (positive), cathode (negative), or neutral (off).

As discussed above, upon a power-on reset, the polarity setting information is resent from the RF pulse generator module106to be stored in the register file1532(1932). This is indicated by the arrow1932to1916. The information of polarity setting stored in the register file1532may then be used to program the polarity of each electrode of electrodes254(1933). The feature allows for re-programming of a passive device remotely from the RF pulse generator module106at the start of each powered session, thus obviating the need of maintaining CMOS memory within the neural stimulator1500.