HK1197897B - Neural stimulator system - Google Patents

Description

Neurostimulator system

Cross Reference to Related Applications

This application claims the benefit of U.S. provisional application 61/437561 filed on 28/1/2011, which is hereby incorporated by reference in its entirety.

Technical Field

The present description relates to implanted neural stimulators.

Background

Neuromodulation of neural tissue in the body by electrical stimulation has become an important type of treatment for chronic disabling conditions such as chronic pain, problems with motor initiation and control, involuntary movement, dystonia, urinary and fecal incontinence, sexual difficulties, vascular insufficiency, cardiac arrhythmias, and the like. Electrical stimulation of the spinal column and the nerve bundles exiting the spinal cord was the first approved neuromodulation therapy and has begun commercial use since the seventies of the twentieth century. The implanted electrodes are used to deliver pulsating current of controllable frequency, pulse width and amplitude. Two or more electrodes contact the neuron, primarily the axon, and are capable of selectively activating axons of different diameters, with positive therapeutic benefit. A variety of therapeutic in vivo electrical stimulation techniques are used to treat neurological disorders, these techniques utilizing neurostimulators implanted in the spinal column or peripheral regions, including the dorsal horn, dorsal root ganglia, dorsal roots, dorsal column fibers, and peripheral nerve bundles leaving the dorsal column or brain, such as the vagus, occipital, trigeminal, hypoglossal, sacral, and caudal nerves.

Disclosure of Invention

In one aspect, an implantable neurostimulator includes one or more electrodes, a first antenna, and one or more circuits. The one or more electrodes are configured to apply one or more electrical pulses to neural tissue. The first antenna is a dipole antenna and is configured to: receiving an input signal containing electrical energy from a second antenna by electrical radiative coupling, the second antenna being physically separate from the implantable neurostimulator; and transmitting one or more feedback signals to the second antenna through electrical radiative coupling. The one or more circuits are connected to the dipole antenna and configured to generate one or more electrical pulses suitable for stimulating neural tissue using the electrical energy contained in the input signal; supplying the one or more electrical pulses to the one or more electrodes, thereby causing the one or more electrodes to apply the one or more electrical pulses to neural tissue; generating a stimulation feedback signal indicative of one or more parameters of the one or more electrical pulses applied to the neural tissue by the one or more electrodes; and sending the stimulus feedback signal to the dipole antenna, thereby causing the dipole antenna to transmit the stimulus feedback signal to the second antenna through electrical radiative coupling.

Implementations of this and other aspects may include the following features. The input signal may also contain information-encoded stimulation parameters for the one or more electrical pulses, and the one or more circuits are configured to generate the electrical pulses based on the information-encoded stimulation parameters. The one or more parameters may include an amplitude of the one or more electrical pulses or an impedance of the one or more electrodes. The one or more circuits may be configured such that a level of the input signal directly determines an amplitude of the one or more electrical pulses applied by the one or more electrodes to the neural tissue.

The one or more circuits may be configured to limit a characteristic of the one or more electrical pulses applied to the neural tissue by the one or more electrodes such that a unit-phase charge induced by the one or more electrical pulses remains below a threshold level; generating a limiting feedback signal when the unit-phase charge induced by the one or more electrical pulses would exceed the threshold level if the one or more circuits did not limit the characteristics of the one or more electrical pulses applied to the neural tissue by the one or more electrodes, such that the unit-phase charge induced by the one or more electrical pulses remains below the threshold level; and sending the limit feedback signal to the dipole antenna, thereby causing the dipole antenna to transmit the limit feedback signal to the second antenna through electrical radiative coupling. The characteristic of the one or more pulses applied to the neural tissue by the one or more electrodes may be a current level, and the threshold level may be a current threshold level.

The one or more circuits may be configured to generate the one or more electrical pulses such that the one or more electrical pulses result in a net charge of substantially zero. In order to generate the one or more electrical pulses such that the one or more electrical pulses result in a substantially zero net charge, the one or more circuits may comprise at least one capacitor in series with the one or more electrodes.

The one or more circuits may include: a waveform adjustment component for generating the one or more electrical pulses suitable for stimulating neural tissue using the electrical energy contained in the input signal; an electrode interface connected to a waveform adjustment circuit, the electrode interface configured to receive the one or more electrical pulses from the waveform adjustment circuit and supply the one or more electrical pulses to the one or more electrodes; and a controller connected to the electrode interface, the controller configured to generate and transmit the stimulation feedback signals to the dipole antenna. The waveform adjusting means may include: a rectifier connected to the dipole antenna, the rectifier configured to receive the input signal from the dipole antenna and generate a rectified electrical waveform based on the input signal; a charge balancing component configured to generate the one or more electrical pulses based on the rectified electrical waveform such that the one or more electrical pulses result in substantially zero net charge at the one or more electrodes; and a charge limiter configured to limit a characteristic of the one or more electrical pulses such that a unit-phase charge induced by the one or more electrical pulses remains below a threshold level, wherein the limited electrical pulses are transmitted from the charge limiter to the electrode interface.

The one or more electrodes may include a plurality of electrodes, and the one or more circuits may be configured to selectively designate each of the electrodes to function as a stimulation electrode, to function as a return electrode, or to be inoperative.

The electrodes, dipole antenna, and one or more circuits may be configured and geometrically arranged to be located in one of: the epidural space of the spine, near, below or above the dura mater of the spine, in tissue immediately adjacent to the spine, in tissue near the dorsal horn, dorsal root ganglia, dorsal root, dorsal column fibers and/or peripheral nerve bundles exiting the dorsal column of the spine, the celiac, thoracic and trigeminal ganglia, peripheral nerves, deep brain structures, cerebral cortical surfaces and sensory or motor nerves.

The implantable neurostimulator may not include an internal power source. The one or more circuits may include only passive components. The input signal may have a carrier frequency in a range from about 300MHz to about 8 GHz.

In another aspect, a system includes a controller module. The controller module includes a first antenna and one or more circuits. The first antenna is configured to transmit an input signal containing electrical energy to a second antenna through electrical radiative coupling. The second antenna is a dipole antenna and is located in an implantable neural stimulator configured to generate one or more electrical pulses suitable for stimulating neural tissue using the input signal, wherein the implantable neural stimulator is separate from the controller module. The first antenna is also configured to receive one or more signals from the dipole antenna. The one or more circuits are configured to generate the input signal and transmit the input signal to the dipole antenna; extracting a stimulation feedback signal from one or more signals received by the first antenna, the stimulation feedback signal being transmitted by the implantable neurostimulator and indicative of one or more parameters of the one or more electrical pulses; and adjusting a parameter of the input signal based on the stimulus feedback signal.

Implementations of this and other aspects may include one or more of the following features. For example, the one or more parameters of the electrical pulses may include an amplitude of the one or more electrical pulses as applied to the neural tissue, and the one or more circuits are configured to adjust the power of the input signal based on the amplitude of the one or more electrical pulses. The one or more circuits may be configured to obtain a forward power signal reflecting an amplitude of a signal transmitted to the first antenna; obtaining a reverse power signal reflecting the amplitude of the reflected portion of the signal sent to the first antenna; determining a mismatch value indicative of an impedance mismatch magnitude based on the forward power signal and the reverse power signal; and adjusting a parameter of the input signal based on the mismatch value.

The system may include the implantable neural stimulator. The implantable neural stimulator may include: one or more electrodes configured to apply the one or more electrical pulses to neural tissue; and one or more circuits. The one or more circuits may be configured to generate the one or more electrical pulses; supplying the one or more electrical pulses to the one or more electrodes, thereby causing the one or more electrodes to apply the one or more electrical pulses to neural tissue; generating the stimulus feedback signal; and sending the stimulus feedback signal to the dipole antenna, thereby causing the dipole antenna to transmit the stimulus feedback signal to the first antenna through electrical radiative coupling.

The input signal may also contain information-encoded stimulation parameters for the one or more electrical pulses, and the implantable neurostimulator is configured to generate the one or more electrical pulses based on the information-encoded stimulation parameters. The one or more parameters of the one or more electrical pulses may include an amplitude of the one or more electrical pulses or an impedance of the one or more electrodes. The one or more circuits of the implantable neurostimulator may be configured such that the level of the input signal directly determines the amplitude of the one or more electrical pulses applied to the neural tissue by the one or more electrodes.

The one or more circuits of the implantable neurostimulator may be configured to limit a characteristic of the one or more electrical pulses applied to the neural tissue by the one or more electrodes such that a unit-phase charge induced by the one or more electrical pulses remains below a threshold level; generating a limiting feedback signal when the unit-phase charge induced by the one or more electrical pulses would exceed the threshold level if the one or more circuits did not limit the characteristics of the one or more electrical pulses applied to the neural tissue by the one or more electrodes, such that the unit-phase charge induced by the one or more electrical pulses remains below the threshold level; and sending the limit feedback signal to the dipole antenna, thereby causing the dipole antenna to transmit the limit feedback signal to the second antenna through electrical radiative coupling. The characteristic of the one or more pulses applied to the neural tissue by the one or more electrodes may be a current level, and the threshold level may be a current threshold level. The one or more circuits of the controller module may be configured to receive the limit feedback signal from the dipole antenna; and attenuating the input signal in response to receiving the limit feedback signal.

The one or more circuits may be configured to generate the one or more electrical pulses such that the one or more electrical pulses result in a net charge of substantially zero. To generate the one or more electrical pulses such that the one or more electrical pulses result in a net charge of substantially zero, the one or more circuits of the implantable neurostimulator may include at least one capacitor in series with the one or more electrodes.

The one or more circuits of the implantable neurostimulator may include: a waveform adjustment component for generating the one or more electrical pulses suitable for stimulating neural tissue using electrical energy contained in the input signal; an electrode interface connected to a waveform adjustment circuit, the electrode interface configured to receive the one or more electrical pulses from the waveform adjustment circuit and supply the one or more electrical pulses to the one or more electrodes; and a controller connected to the electrode interface, the controller configured to generate and transmit the stimulation feedback signals to the dipole antenna. The waveform adjusting means may include: a rectifier connected to the dipole antenna, the rectifier configured to receive the input signal from the dipole antenna and generate a rectified electrical waveform based on the input signal; a charge balancing component configured to generate the one or more electrical pulses based on the rectified electrical waveform such that the one or more electrical pulses result in substantially zero net charge at the one or more electrodes; and a charge limiter configured to limit a characteristic of the one or more electrical pulses such that a unit-phase charge induced by the one or more electrical pulses remains below a threshold level, wherein the limited electrical pulses are transmitted to the electrode interface through the charge limiter.

The implantable neural stimulator may include a plurality of electrodes. The one or more circuits of the controller module may be configured to generate control signals that specify which electrodes act as stimulation electrodes, which electrodes act as return electrodes, and which electrodes are inactive; and sending the control signal to the first antenna, thereby causing the first antenna to transmit the control signal to the dipole antenna through electrical radiative coupling. The one or more circuits of the implantable neurostimulator may be configured to selectively designate each of the electrodes to function as a stimulation electrode, a return electrode, or be inoperative based on the control signal.

The implantable neurostimulator may not include an internal power source. The one or more circuits of the implantable neurostimulator may include only passive components. The input signal has a carrier frequency in a range from about 300MHz to about 8 GHz.

In another aspect, a method includes implanting a neurostimulator within a patient's body, thereby positioning one or more electrodes of the neurostimulator to apply electrical pulses to neural tissue. The neurostimulator includes a first antenna configured for receiving an input signal comprising electrical energy. The first antenna is a dipole antenna. The neurostimulator is configured for: generating one or more electrical pulses suitable for stimulating the neural tissue using the electrical energy contained in the input signal; supplying the one or more electrical pulses to the one or more electrodes, thereby causing the one or more electrodes to apply the one or more electrical pulses to the neural tissue; generating a stimulation feedback signal indicative of one or more parameters of the one or more electrical pulses applied to the neural tissue by the one or more electrodes; and transmitting the stimulus feedback signal from the dipole antenna to a second antenna through electrical radiative coupling. The method further comprises positioning a controller module proximate the patient body, wherein the controller module is connected to the second antenna; and operating the controller module such that the controller module: generating and transmitting the input signal to the second antenna such that the second antenna transmits the input signal to the dipole antenna within the implanted neurostimulator via electrical radiative coupling; extracting the stimulation feedback signal from one or more signals received by the second antenna; and adjusting a parameter of the input signal based on the stimulus feedback signal.

Implementations of this and other aspects may include one or more of the following features. For example, the parameter may include an amplitude of the one or more electrical pulses or an impedance of the one or more electrodes. The neurostimulator may be configured for generating the one or more electrical pulses such that the one or more electrical pulses result in a substantially zero net charge within the patient's body. The neurostimulator may be configured to selectively designate one or more electrodes as stimulating electrodes, as return electrodes, or inactive.

Implanting the neurostimulator may include implanting the neurostimulator at one of the following locations within the patient's body: the epidural space of the spine, near, below or above the dura mater of the spine, in tissue immediately adjacent to the spine, in tissue near the dorsal horn, dorsal root ganglia, dorsal root, dorsal column fibers and/or peripheral nerve bundles exiting the dorsal column of the spine, the celiac, thoracic and trigeminal ganglia, peripheral nerves, deep brain structures, cerebral cortical surfaces and sensory or motor nerves.

The implanted neural stimulator may not include an internal power source. The implanted neurostimulator may include at least one capacitor in series with one or more electrodes.

Implementations of the techniques described herein may include one or more of the following advantages. For example, embodiments may avoid many failure modes associated with implanted pulse generator modules connected to electrodes by physical leads, such as: loss of electrical continuity due to mechanical bending, mechanical dislodgement due to natural body movement, impact of the lead electrode assembly on tissue, infection, and uncomfortable stimulation.

Various embodiments may be useful for neuromodulation therapies involving the brain. Can stimulate the brain area to help treat chronic pain symptoms, aid in movement disorders, clinical depression, control epilepsy, and the like. The cerebral cortex is the target of neurostimulation where the stimulation electrodes are positioned outside the dura mater. Various embodiments may employ lead/electrode volumes that are more than ten times smaller than electrodes currently used for such stimulation. Currently such electrodes may require the creation of a large hole in the skull bone of 1.0sq mm or more in diameter. Some embodiments are capable of ejecting from a very small infusion lumen, such as a typical 22 gauge (gauge) needle used in laparoscopic or endoscopic placement. Thus, some embodiments may employ a hole in the skull that is much smaller than current devices. If several stimulators are to be inserted, a catheter can be placed through the holes and guided through the removable probe, and the stimulators can be pushed out of the catheter placed at their respective locations.

Deep Brain Stimulation (DBS) is used to treat symptoms caused by chronic pain, movement disorders, obsessive-compulsive disorders, and epilepsy. Target sites for electrode placement for the treatment of chronic pain with DBS include sensory thalamus and periventricular gray matter. Target sites in the brain for treatment of motor disorder symptoms such as parkinson's disease include the ventral intermediate thalamus, subthalamic nucleus and the medial globus pallidus. The hypothalamus is one target site for electrode placement for treatment of epileptic symptoms with DBS. Placement of various embodiments deep in the brain may result in minimal acute trauma or chronic reactions due to the small size of the stimulator.

Applying this technique near the spinal cord may include the following advantages: ease of insertion, elimination of compensating leads, and no need to be able to implant a pulse generator to administer chronic treatments. Spinal cord stimulation is used to treat chronic neuropathic pain, especially low back pain and radiculopathy, vascular insufficiency in the feet or hands, angina pectoris, etc. Various embodiments of this technique may allow placement of the electrodes in the epidural space, between the dura and arachnoid (which is standard practice in the prior art), or in the subdural intrathecal space, since significant reactions and scarring may be minimal. Insertion in any of these spaces may be accomplished by: from a 22 gauge needle ejection device or from a catheter ejection device guided into place by a removable probe. In some embodiments, once in place, there is no need for any other skin incision extension arrangement, receiver, or implanted pulse generator. Due to the small size and absence of compensating leads for energy delivery, various embodiments of wireless neuromodulation systems may have significant advantages that allow placement with minimal trauma and allow long-term effective treatment where larger implantable devices can cause more scar tissue and tissue reaction, affecting efficacy and safety.

Various embodiments may have inherently low costs compared to existing implantable neuromodulation systems, and this may allow patients in need thereof to more broadly employ neuromodulation therapy, as well as reduce the overall cost of healthcare systems.

The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims.

Drawings

Fig. 1 depicts a schematic illustration of an example of a wireless neurostimulation system.

Fig. 2 depicts a detailed illustration of an example of a wireless neurostimulation system.

Fig. 3 is a flow chart illustrating an example of the operation of a wireless neurostimulator system.

Fig. 4 depicts a flow chart showing an example of the operation of the system when the current level at the electrode is above a threshold limit.

Fig. 5 is a diagram illustrating an example of a signal that may be used to detect an impedance mismatch.

Fig. 6 is a diagram illustrating an example of signals that may be employed during operation of a wireless neurostimulator system.

Figure 7 is a flow chart illustrating a process by which a user controls an implantable wireless neurostimulator through an external programmer in an open loop feedback system.

Fig. 8 is another example flow chart of a process for a user to control a wireless stimulator with limits on upper and lower limits of current amplitude.

Fig. 9 is yet another example flow chart of a process for a user to control a wireless neurostimulator through preprogrammed parameter settings.

Fig. 10 is yet another example flow diagram for a process for a low battery state for an RF pulse generator module.

Fig. 11 is yet another example flow diagram of a process for programming an implanted wireless neurostimulator for a manufacturer's representative.

Fig. 12 is a circuit diagram showing an example of a wireless neural stimulator.

Fig. 13 is a circuit diagram showing another example of a wireless neural stimulator.

Detailed Description

In various embodiments, the neurostimulation system can be used to deliver electrical stimulation to target neural tissue by using remote Radio Frequency (RF) energy, neither cable nor inductive coupling, to power a passive implanted stimulator. The target neural tissue may, for example, be in the spinal column, including the spinothalamic ducts, dorsal horn, dorsal root ganglia, dorsal roots, dorsal column fibers and peripheral nerve bundles exiting the dorsal column or brainstem, as well as any cranial nerves, the celiac, thoracic or trigeminal nerves, nerve bundles of the cerebral cortex, deep brain and any sensory or motor nerves.

For example, in some embodiments, a neurostimulation system may include a controller module, such as an RF pulse generator module, and a passive implanted neurostimulator including one or more dipole antennas, one or more circuits, and one or more electrodes in contact with or adjacent to target 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 antenna. The one or more circuits of the implanted neural stimulator may be configured to use the transmitted energy to generate electrical pulses suitable for neural stimulation and supply the electrical pulses to the electrodes to apply the pulses to the neural tissue. For example, the one or more circuits may include a wave modification circuit that rectifies the received RF signal (e.g., using a diode rectifier), transforms the RF energy into a low frequency signal suitable for stimulating neural tissue, and provides the resulting waveform to the electrode array. The one or more circuits of the implanted neurostimulator may also include circuitry for sending information back to the RF pulse generator module to facilitate a feedback control mechanism for stimulation parameter control. For example, the implanted neurostimulator may send a stimulation feedback signal to the RF pulse generator module indicative of a parameter of the electrical pulse, and the RF pulse generator module may employ the stimulation feedback signal to adjust a parameter of a signal sent to the neurostimulator.

Fig. 1 depicts a schematic illustration of an example of a neurostimulation system. The neurostimulation system may include four main components, namely, a programmer module 102, an RF pulse generator module 106, a Transmit (TX) antenna 110 (e.g., a patch antenna, a slot antenna, or a dipole antenna), and an implanted wireless neurostimulator 114. Programmer module 102 may support operations such asSuch as a smart phone, to a software application of the wireless connection 114. The application can enable a user to view system conditions and diagnostics, change various parameters, among other functionsThe desired stimulation amplitude of the electrode pulses is increased/decreased and the feedback sensitivity of the RF pulse generator module 106 is adjusted.

The RF pulse generator module 106 may include communications electronics supporting the wireless connection 104, stimulation circuitry, and a battery to power the generator electronics. In some embodiments, the RF pulse generator module 106 includes a TX antenna embedded in its packaging form factor (packaging form factor), while in other embodiments, the TX antenna is connected to the RF pulse generator module 106 by a wired connection 108 or a wireless connection (not shown). The TX antenna 110 may be coupled directly to tissue to generate an electric field that powers the implanted neurostimulator module 114. The TX antenna 110 communicates with an implanted neurostimulator module 114 through an RF interface. For example, the TX antenna 110 radiates an RF transmit signal that is modulated and encoded by the RF pulse generator module 110. The implanted wireless neurostimulator module 114 contains one or more antennas, such as dipole antenna(s), for receiving and transmitting through the RF interface 112. In particular, the coupling mechanism between the antenna 110 and the one or more antennas on the implanted neural stimulation module 114 is electrical radiative coupling, rather than inductive coupling. In other words, the coupling is through an electric field rather than a magnetic field.

Through this electrical radiative coupling, the TX antenna 110 can provide an input signal to the implanted neural stimulation module 114. Such input signals contain energy and may contain information-encoded stimulation waveforms to be applied at the electrodes of the implanted neurostimulator module 114. In some embodiments, the power level of such an input signal directly determines the applied amplitude (e.g., power, current, or voltage) of one or more electrical pulses generated using the electrical energy contained in the input signal. Within the implanted wireless neurostimulator 114 are components for demodulating the RF transmission signal and electrodes for delivering stimulation to the surrounding neuronal tissue.

The RF pulse generator module 106 can be implanted subcutaneously or it can be worn outside the body. When external to the body, the RF generator module 106 can be incorporated into a strap or harness design to allow electrical radiative coupling through the skin and underlying tissue to deliver power and/or control parameters to the implanted neurostimulator module 114, which implanted neurostimulator module 114 can be a passive stimulator. In either case, the receiver circuit(s) inside the neurostimulator module 114 can capture the energy radiated by the TX antenna 110 and convert such energy into an electrical waveform. The receiver circuit(s) may further modify the waveform to generate electrical pulses suitable for stimulating neural tissue, and such pulses may be delivered to the tissue via the electrode pad.

In some embodiments, the RF pulse generator module 106 is capable of remotely controlling stimulation parameters (i.e., parameters of the electrical pulses applied to the neural tissue) and monitoring feedback from the wireless neurostimulator module 114 based on RF signals received from the implanted wireless neurostimulator module 114. The feedback detection algorithm implemented by the RF pulse generator module 106 can monitor data wirelessly transmitted from the implanted wireless neurostimulator module 114, including information about the energy being received by the implanted wireless neurostimulator module 114 from the RF pulse generator and information about the stimulation waveform delivered to the electrode pad. In order to provide effective treatment for a given medical condition, the system can be adapted to provide an optimal amount of stimulation or inhibition to the nerve fibers by electrical stimulation. A closed loop feedback control method can be used in which the output signal from the implanted wireless neurostimulator module 114 is monitored and can be used to determine the appropriate level of neurostimulation current for maintaining effective neuron activation, or in some cases, the patient can manually adjust the output signal in an open loop control method.

Fig. 2 depicts a detailed diagram of an example of a neurostimulation system. As depicted, programming module 102 may include a user input system 202 and a communication subsystem 208. The user input system 221 may allow a user to adjust various parameter settings in the form of an instruction set (in some cases, in an open loop manner). The communication subsystem 208 may transmit these instruction sets (as well as other information) to the RF pulse generator module 106 via a wireless connection 104, such as Bluetooth or WiFi, and receive data from the module 106.

For example, a programmer module 102 that can be utilized by multiple users, such as a patient's control unit or a clinician's programmer unit, can be used to send stimulation parameters to the RF pulse generator module 106. 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 the tissue stimulation; the parameters of the charge balance phase can be similarly controlled (as described below). The patient and/or clinician can also optionally control the overall duration and mode of treatment.

The implantable neurostimulator module 114 or the RF pulse generator module 114 may be initially programmed to meet specific parameter settings for each individual patient during the initial implantation procedure. Because the medical condition or the body itself can change over time, the ability to readjust the parameter settings may be beneficial to ensure uninterrupted efficacy of the neuromodulation therapy.

Programmer module 102 may be functionally a smart device and associated application. The smart device hardware may include a CPU206 and is used as a vehicle for manipulating touch screen inputs on a Graphical User Interface (GUI) 204 for processing and storing data.

The RF pulse generator module 106 may be connected to an external TX antenna 110 via a wired connection 108. Alternatively, both the antenna and the RF pulse generator are located subcutaneously (not shown).

The signal sent by the RF pulse generator module 106 to the implanted stimulator 114 may include both power and parameter setting attributes regarding stimulation waveform, amplitude, pulse width, and frequency. The RF pulse generator module 106 can also function as a wireless receiving unit that receives feedback signals from the implanted stimulator module 114. To this end, the RF pulse generator module 106 may contain microelectronics or other circuitry to handle the generation of signals transmitted to the stimulator module 114, as well as to handle feedback signals, such as those from the stimulator module 114. For example, the RF pulse generator module 106 may include a controller subsystem 214, a high frequency oscillator 218, an RF amplifier 216, an RF switch, and a feedback subsystem 212.

Controller subsystem 214 may include a CPU230 for handling data processing, a memory subsystem 228 such as local memory, a communication subsystem 234 in communication with programmer module 102 (including receiving stimulation parameters from the programmer module), pulse generator circuitry 236, and a digital-to-analog (D/a) converter 232.

The controller subsystem 214 may be used by the patient and/or clinician to control stimulation parameter settings (e.g., by controlling parameters of signals sent from the RF pulse generator module 106 to the neurostimulator module 114). These parameter settings can affect, for example, the power, current level, or shape of one or more electrical pulses. As described above, the programming of stimulation parameters can be performed using the programming module 102 to set the repetition rate, pulse width, amplitude, and waveform to be transmitted by the RF energy to the Receive (RX) antenna 238 in the wirelessly implanted neurostimulator module 214, the Receive (RX) antenna 238 typically being a dipole antenna (although other types can be used). The clinician may have the ability to lock and/or hide the selection of particular settings within the programmer interface, thereby limiting the patient's ability to view or adjust particular parameters, as adjusting particular parameters may require detailed medical knowledge of neurophysiology, neuroanatomy, neuromodulation protocols, and electrical stimulation safety limitations.

The controller subsystem 214 may store the received parameter settings in the local memory subsystem 228 until the parameter settings are modified by new input data received from the programming module 102. The CPU206 may control the pulse generator circuit 236 using parameters stored in the local memory to generate stimulation waveforms modulated by the high frequency oscillator 218 in the 300MHz to 8GHz range. The resulting RF signal may then be amplified by RF amplifier 226 and then sent through RF switch 223 to TX antenna 110 to reach RX antenna 238 through tissue depth.

In some embodiments, the RF signal transmitted by TX antenna 110 may simply be a power transmit signal used by stimulator module 114 to generate electrical pulses. In other embodiments, telemetry signals may also be transmitted to stimulator module 114 to send instructions regarding various operations of stimulator module 114. The telemetry signal may be transmitted by modulating a carrier signal (through the skin if external, or through other body tissue if the pulse generator module 106 is implanted subcutaneously). The telemetry signal is used to modulate a carrier signal (high frequency signal) coupled to the implanted antenna(s) 238 and does not interfere with inputs received on the same leads that 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 so that the implanted stimulator is powered directly by the received telemetry signal; the independent subsystems in the stimulator utilize the power contained in the signal and interpret the data content of the signal.

The RF switch 223 may be a multi-purpose device, such as a bidirectional coupler, that delivers relatively high amplitude, extremely short duration RF pulses to the TX antenna 110 with minimal insertion loss, while simultaneously providing two low level outputs to the feedback subsystem 212; one output delivers a forward power signal to the feedback subsystem 212 where it is an attenuated version of the RF pulses sent to the TX antenna 110, and the other output delivers a reverse power signal to a different port of the feedback subsystem 212 where it is an attenuated version of the reflected RF energy from the TX antenna 110.

During the on-cycle time (when an RF signal is being transmitted to the stimulator 114), the RF switch 223 is set to send a forward power signal to the feedback subsystem. During the off period time (when no RF signal is being transmitted to the stimulator module 114), the RF switch 223 can change to a receive mode in which reflected RF energy and/or RF signals from the stimulator module 114 are received for analysis in the feedback subsystem 212.

The feedback subsystem 212 of the RF pulse generator module 106 may include receive circuitry to receive and extract telemetry or other feedback signals from the stimulator 114 and/or reflected RF energy from the signals transmitted by the TX antenna 110. The feedback subsystem may include an amplifier 226, a filter 224, a demodulator 222, and an a/D converter 220.

Feedback subsystem 212 receives the forward power signal and converts this high frequency AC signal to a DC level that can be sampled and sent to controller subsystem 214. In this manner, the characteristics of the generated RF pulses can be compared to a reference signal within the controller subsystem 214. If there is an inconsistency (error) in any of the parameters, the controller subsystem 214 can adjust the output to the RF pulse generator 106. The nature of the adjustment can be proportional to the error of the calculation, for example. The controller subsystem 214 can incorporate additional inputs and limit its adjustment scheme, such as signal amplitude of reverse power and any predetermined maximum or minimum values for various pulse parameters.

The reverse power signal can be used to detect a fault condition in the RF power delivery system. In an ideal case, the electromagnetic waves generated from the RF pulse generator 106 are transmitted unimpeded from the TX antenna 110 into the body tissue when the TX antenna 110 has a perfectly matched impedance to the tissue in contact therewith. However, in real world applications, there may be a large degree of variability in the type of user's body, the type of clothing worn, and the positioning of the antenna 110 relative to the body surface. Since the impedance of the antenna 110 depends on the relative permittivity of the underlying tissue and any intervening materials, and also on the total separation distance of the antenna from the skin, in any given application, there can be an impedance mismatch at the interface of the TX antenna 110 with the body surface. When such a mismatch occurs, the electromagnetic wave transmitted from the RF pulse generator 106 is partially reflected at this interface, and this reflected energy propagates back through the antenna feed.

The bi-directional coupler RF switch 223 may prevent reflected RF energy from propagating back into the amplifier 226 and may attenuate such reflected RF signal and send the attenuated signal as a reverse power signal to the feedback subsystem 212. The feedback subsystem 212 can convert this high frequency AC signal to a DC level that can be sampled and sent to the controller subsystem 214. The controller subsystem 214 can then calculate a ratio of the amplitude of the reverse power signal to the amplitude of the forward power signal. The ratio of the amplitude of the reverse power signal to the amplitude level of the forward power may indicate the severity of the impedance mismatch.

To sense an impedance mismatch condition, the controller subsystem 214 can measure the reflected power ratio in real time, and the controller subsystem 214 can modify the level of RF power generated by the RF pulse generator 106 according to a preset threshold for such measurement. For example, for a moderate level of reflected power, the action process can be used for controller subsystem 214 to increase the magnitude of the RF power sent to TX antenna 110, which is needed to compensate for slightly non-optimal but acceptable TX antenna-to-body coupling. For higher reflected power ratios, the action process can be to block operation of the RF pulse generator 106 and set a fault code to indicate that the TX antenna 110 is barely coupled to the body. This type of reflected power failure condition can also result from a poor or broken connection to the TX antenna. In either case, it may be desirable to stop the RF emission when the reflected power ratio is above a defined threshold, since internally reflected power can cause undesirable heating of internal components, and such a fault condition means that the system is unable to deliver sufficient power to the implanted wireless neurostimulator, and thus, unable to deliver therapy to the user.

The controller 242 of the stimulator 114 may transmit an informational signal, such as a telemetry signal, through the antenna 238 to communicate with the RF pulse generator module 106 during a receive cycle of the RF pulse generator module 106. For example, during the on and off states of the transistor circuit, telemetry signals from the stimulator 114 may be coupled to signals modulated on the dipole antenna(s) 238 to enable or disable such waveforms: the waveforms produce the corresponding RF burst signals necessary to transmit to an external (or remotely implanted) pulse generator module 106. The antenna(s) 238 may be connected to an electrode 254 in contact with tissue to provide a return path for the transmitted signal. An a/D (not shown) converter can be used to transfer the stored data to a serial pattern that can be transmitted on a pulsed signal from the internal antenna(s) 238 of the neurostimulator. The telemetry signals from the implanted wireless neurostimulator module 114 may include stimulation parameters such as power or amplitude of current delivered from the electrodes to the tissue. A feedback signal can be transmitted to the RF pulse generator module 116 to indicate the stimulation intensity at the nerve bundle by coupling a signal to the implanted RX antenna 238, the implanted RX antenna 238 radiating a telemetry signal to the external (or remotely implanted) RF pulse generator module 106. The feedback signal can include any or both of analog and digital telemetry pulse modulated carrier signals. Data such as stimulation pulse parameters and measured stimulator performance characteristics can be stored in an internal memory device within the implanted neurostimulator 114 and can be transmitted on telemetry signals. The frequency of the carrier signal may be in the range from 300MHz to 8 GHz.

In the feedback subsystem 212, the telemetry signal can be down-modulated using a demodulator 222 and digitized by processing through an analog-to-digital (a/D) converter 220. The digital telemetry signal may then be routed to the CPU230 with the embedded code by reprogramming the received signal based on its amplitude, translating into a selection of corresponding current measurements in the tissue. The CPU230 of the controller subsystem 214 can compare the reported stimulation parameters to those saved in the local memory 228 to verify the specified stimulation delivered to the tissue by the stimulator(s) 114. For example, if the stimulator reports a lower current than specified, the power level from the RF pulse generator module 106 can be increased so that the implanted neurostimulator 114 will have more power available for stimulation. The implanted neurostimulator 114 can generate telemetry data in real-time, for example, at a rate of 8kbit per second. All feedback data received from the implanted lead module 114 can be recorded over time and sampled for storage to a remote monitoring system for retrieval, which can be accessed by a healthcare professional for trend analysis and statistical correlation.

The sequence of remote programmable RF signals received by the internal antenna(s) 238 may be adjusted into a waveform that: the waveforms can be controlled by the control subsystem 242 within the implantable stimulator 114 and routed to the appropriate electrodes 254 placed in proximity to the tissue to be stimulated. For example, RF signals transmitted from RF pulse generator module 106 may be received by RX antenna 238 and processed by circuitry within implanted wireless neurostimulator module 114, such as waveform adjustment circuitry 240, to be converted into electrical pulses applied to electrodes 254 through electrode interface 252. In some embodiments, the implanted stimulator 114 includes between two and sixteen electrodes 254.

The waveform adjustment circuit 240 may include a rectifier 244, the rectifier 244 rectifying the signal received by the RX antenna 238. The rectified signal may be fed to the controller 242 for receiving encoded instructions from the RF pulse generator module 106. The rectifier signal may also be fed to a charge balancing component 246, the charge balancing component 246 being configured to generate the one or more electrical pulses based on the one or more electrical pulses resulting in a substantially zero net charge at the one or more electrodes (i.e., the pulses are charge balanced). The charge balanced pulses are transmitted through current limiter 248 to electrode interface 252, which electrode interface 252 applies the pulses to electrode 254 as appropriate.

Current limiter 248 ensures that the current level of the pulse applied to electrode 254 is not above the threshold current level. In some embodiments, the amplitude (e.g., current level, voltage level, or power level) of the received RF pulses directly determines the amplitude of the stimulation. In this case, it may be particularly beneficial to include a current limiter 248 to prevent delivery of excessive current or charge through the electrodes, but the current limiter 248 may be used in other embodiments where this is not the case. Generally, for a given electrode having a surface area of several square millimeters, what should be limited for safety is the unit phase charge (where the charge delivered by the stimulation phase is the integral of the current). In some cases, however, the limit may 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, limiter 248 acts as a charge limiter that limits the characteristics (e.g., current or duration) of the electrical pulses such that the unit phase charge remains below a threshold level (typically, a safe charge limit).

In the event that the implanted wireless neurostimulator 114 receives a "strong" pulse of RF power sufficient to generate stimulation that would exceed a predetermined safe charge limit, the current limiter 248 can automatically limit or "clip" the stimulation phase to maintain the total charge of that phase within safe limits. Current limiter 248 may be a passive current limiting component that, once a safe current limit (threshold current level) is reached, shuts off the signal to electrode 254. Alternatively, or additionally, the current limiter 248 may communicate with the electrode interface 252 to turn off all of the electrodes 254 to prevent current levels from damaging tissue.

The shortening event may trigger a current limiter feedback control mode. The act of shortening may cause the controller to send a threshold power data signal to the pulse generator 106. Feedback subsystem 212 detects the threshold power signal and demodulates the signal into data that is communicated to controller subsystem 214. The controller subsystem 214 algorithm may act on such current limiting conditions by specifically reducing the RF power generated by the RF pulse generator or by completely shutting off power. In this manner, if the implanted wireless neurostimulator 114 reports that it is receiving too much RF power, the pulse generator 106 can reduce the RF power delivered to the body.

The controller 250 of the stimulator 205 may communicate with the electrode interface 252 to control various aspects of the electrode settings and the pulses applied to the electrodes 254. Electrode interface 252 may act as a multiplexer and control the polarity and switching of each of electrodes 254. For example, in some embodiments, the wireless stimulator 106 has a plurality of electrodes 254 in contact with tissue, and for a given stimulation, the RF pulse generator module 106 can arbitrarily assign one or more electrodes to 1) act as stimulation electrodes, 2) act as return electrodes, or 3) not operate by communicating wirelessly transmitted assignment and parameter instructions that the controller 250 uses as appropriate to set the electrode interface 252. It may be physiologically advantageous to assign, for example, one or two electrodes as stimulation electrodes and all the remaining electrodes as return electrodes.

Also, in some embodiments, for a given stimulation pulse, controller 250 may control electrode interface 252 to divide the current arbitrarily between the designated stimulation electrodes (or according to instructions from pulse generator module 106). Such control of electrode assignment and current control can be advantageous because in practice, the electrodes 254 can be spatially distributed along a variety of neural structures, and by strategic selection of stimulation electrode locations and the proportion of current specified for each location, the aggregate current distribution in the 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 embodiment, the timing of the stimulation can be manipulated arbitrarily. A given stimulation waveform may be initiated at a "T _ start" time and terminated at a "T _ end" time, and such a time-course may be synchronized across all stimulation and return electrodes; moreover, the repetition frequency of such stimulation cycles may be synchronized for all electrodes. However, controller 250, on its own or in response to instructions from pulse generator 106, can control electrode interface 252 to specify that one or more subsets of electrodes deliver stimulation waveforms at asynchronous start and stop times, and the repetition frequency of each stimulation cycle can be arbitrary and can be specified independently.

For example, a stimulator having eight electrodes may be configured to have a subset of five electrodes referred to as set a and a subset of three electrodes referred to as set B. Set a may be configured to use two of its electrodes as stimulation electrodes, the remaining as return electrodes. Set B may be configured to have only one stimulation electrode. Controller 250 can then specify that set a delivers a stimulation phase of 200 μ s duration with 3mA current, followed by a charge balance phase of 400 μ s. Such stimulation cycles can be specified to repeat at a rate of 60 cycles per second. Thereafter, for set B, the controller 250 can specify a stimulation phase of 500 μ s duration with a current of 1mA, followed by a charge balance phase of 800 μ s. The repetition rate for set B stimulation cycles can be set independently of set a, i.e. for example, the repetition rate for set B stimulation cycles can be specified as 25 cycles per second. Alternatively, if the controller 250 is configured to match the repetition rate for set B with the repetition rate for set a, for such cases, the controller 250 can specify that the relative start times of the stimulation periods coincide in time or are arbitrarily offset from each other by some delay interval.

In some embodiments, the controller 250 can arbitrarily set the shape of the stimulation waveform amplitude, and can do so in response to instructions from the pulse generator 106. The stimulation phase may be delivered by a constant current source or a constant voltage source, and this type of control may generate a static characteristic waveform, e.g., a constant current source generates a characteristic rectangular pulse in which the current waveform has a very steep increase, a constant amplitude over the duration of the stimulation, and a very steep return to baseline thereafter. Alternatively, or additionally, controller 250 can increase or decrease the current level at any time during the stimulation phase and/or during the charge balancing phase. Thus, in some embodiments, controller 250 is capable of delivering arbitrarily shaped stimulation waveforms, such as triangular pulses, sinusoidal pulses, or, for example, gaussian pulses. Similarly, the charge balance phase can be arbitrary amplitude-shaped, and similarly, the leading edge anodal pulse (prior to the stimulation phase) can also be amplitude-shaped.

As described above, the stimulator 114 may include a charge balancing component 246. In general, for a constant current stimulation pulse, the pulse should be charge balanced by having a cathodic current amount that should be equal to the anodic current amount, which is commonly referred to as biphasic stimulation. Charge density is the amount of current multiplied by the duration of time it is applied, typically in μ C/cm²Is expressed in units. To avoid irreversible electrochemical reactions, such as pH changes, electrode dissolution, and tissue destruction, no net charge should appear at the electrode-electrolyte interface, and generally have less than 30 μ C/cm²Is acceptable. The biphasic stimulation current pulses ensure that no net charge is present at the electrodes after each stimulation cycle and that the electrochemical processes are balanced preventing a net dc current. The neurostimulator 114 may be designed to ensure that the resulting stimulation waveform has a net charge of zero. The charge-balanced stimulus is believed to have minimal injurious effects on the tissue by reducing or eliminating the electrochemical reaction products generated at the electrode-tissue interface.

The stimulation pulse may have a negative voltage or current, referred to as the cathodic phase of the waveform. The stimulation electrodes may have both cathodic and anodic phases at different times during the stimulation cycle. An electrode that delivers a negative current of sufficient magnitude to stimulate adjacent neural tissue is referred to as a "stimulating electrode". During the stimulation phase, the stimulation electrodes act as current sinks. One or more additional electrodes, referred to as "return electrodes," act as a current source. The return electrode is placed at some other location in the tissue at a distance from the stimulating electrode. When a typical negative stimulation phase is delivered to the tissue at the stimulation electrode, the return electrode has a positive stimulation phase. During the subsequent charge balance phase, the polarity of each electrode is reversed.

In some embodiments, the charge balancing component 246 uses dc blocking capacitor(s) placed in electrical series with the stimulation electrodes and the body tissue between the stimulation generation point within the stimulator circuit and the point of delivering stimulation to the tissue. In this way, a resistor-capacitor (RC) network may be formed. In a multi-electrode stimulator, one charge balancing capacitor may be used for each electrode, or the centralized capacitor(s) may be used within the stimulator circuitry prior to the electrode-selected point. The RC network can block Direct Current (DC), however it can also prevent low frequency Alternating Current (AC) from passing to the tissue. This frequency is commonly referred to as the cut-off frequency: the series RC network below the lesion frequency essentially blocks the signal, and in one embodiment, the design of the stimulator system may ensure that the lesion cut-off frequency is not higher than the fundamental frequency of the stimulation waveform. In such embodiments of the invention, the wireless stimulator may have a charge balancing capacitor with a value selected 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 cut-off frequency of the RC network in such an embodiment is at or below the fundamental frequency of the stimulation pulse.

In other embodiments, the cut-off frequency may be selected to be at or above the fundamental frequency of stimulation, in which case the stimulation waveform, referred to as the drive waveform, generated prior to the charge balance capacitor may be designed to be unstable, where the envelope of the drive waveform varies over 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 varying the amplitude of the drive waveform over time, the shape of the stimulus waveform across the charge balance capacitor is also modified. The shape of the stimulation waveform can be modified in this manner to produce a physiologically beneficial stimulus.

In some embodiments, the wireless neurostimulator module 114 may generate a drive waveform envelope that follows the envelope of the RF pulses received by the receive dipole antenna(s) 238. In this case, the RF pulse generator module 106 is able to directly control the envelope of the drive waveform within the wireless neurostimulator 114, so no energy storage may be required within the stimulator itself. In such embodiments, the stimulator circuitry may modify the envelope of the drive waveform or may transfer it directly to the charge balance capacitor and/or the electrode selection platform.

In some embodiments, the implanted neurostimulator 114 may deliver a single-phase drive waveform to the charge balance capacitor or it may deliver a multi-phase drive waveform. In the case of a single phase drive waveform, for example, a negative going rectangular pulse, such pulse includes a physiological stimulus phase and the charge balance capacitor is polarized (charged) during such phase. After completion of the drive pulse, the charge balancing function is performed solely by the passive discharge of the charge balancing capacitor, where its charge is dissipated in the tissue in an opposite polarity with respect to the previous stimulus. In one embodiment, a resistor within the stimulator facilitates discharge of the charge balance capacitor. In some embodiments, using a passive discharge phase, the capacitor may be allowed to discharge almost completely before starting the subsequent stimulation pulse.

In the case of a multiphase drive waveform, the wireless stimulator may implement internal switches to deliver negative 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 the desired physiological effect. For example, the stimulation phase may be followed by an actively driven charge balance phase, and/or the stimulation phase may be followed by an opposite phase. Having a phase of opposite polarity before stimulation can have the advantage of reducing the amplitude of the stimulation phase required to excite the tissue, for example.

In some embodiments, the amplitude and timing of the stimulation and charge balance phases are controlled by the amplitude and timing of the RF pulses from the RF pulse generator module 106, and in other embodiments, such control may be administered internally by circuitry on board the wireless stimulator 114, such as the controller 250. The case of on-board control, the amplitude and timing may be specified or modified by data commands delivered from the pulse generator module 106.

Fig. 3 is a flow chart illustrating an example of operation of the neurostimulator system. In block 302, the wireless neural stimulator 114 is implanted adjacent to the nerve fascicles and coupled to the electric field generated by the TX antenna 110. That is, the pulse generator module 106 and the TX antenna 110 are positioned (e.g., adjacent to the patient) in such a way that the TX antenna 110 is electrically radiatively coupled with the implanted RX antenna 238 of the neurostimulator 114. In a particular embodiment, the antenna 110 and the RF pulse generator 106 are both located subcutaneously. In other embodiments, the antenna 110 and the RF pulse generator 106 are located outside the patient's body. In this case, the TX antenna 110 may be directly coupled to the patient's skin.

As shown in block 304, energy from the RF pulse generator is radiated from the antenna 110 through the tissue to the implanted wireless neural stimulator 114. The energy of the radiation may be controlled by the patient/clinician parameter input in block 301. In some cases, the parameter settings can be adjusted in an open-loop manner by the patient or clinician who would adjust the parameters entered into the system in block 301.

The wirelessly implanted stimulator 114 uses the received energy to generate electrical pulses to be applied to the neural tissue via the electrodes 238. For example, the stimulator 114 may contain circuitry that rectifies the received RF energy and adjusts the waveform to charge balance the energy delivered to the electrodes to stimulate the target nerve or tissue, as shown in block 306. The implanted stimulator 114 communicates with the pulse generator 106 using the antenna 238 to send telemetry signals, as shown in block 308. The telemetry signal may contain information about parameters of the electrical pulse applied to the electrode, such as the impedance of the electrode, whether a safe current limit has been reached, or the magnitude of the current provided to the tissue from the electrode.

In block 310, the RF pulse generator 106 amplifies, filters, and modulates the received telemetry signal using the amplifier 226, the filter 224, and the demodulator 222, respectively. The a/D converter 230 then digitizes the resulting analog signal, as shown in 312. The digital telemetry signal is routed to the CPU230, and the CPU230 determines whether the parameters of the signal sent to the stimulator 114 need to be adjusted based on the digital telemetry signal. For example, in block 314, the CPU230 compares the information of the digital signal to a look-up table, which may indicate an appropriate change in the stimulation parameter. The change in indication may be, for example, a change in the current level of the pulse applied to the electrode. As a result, the CPU may vary the output power of the signal sent to the stimulator 114 in order to adjust the current applied by the electrodes 254, as shown in block 316.

Thus, for example, the CPU230 may adjust the parameters of the signal sent to the stimulator 114 each cycle to match the desired current amplitude setting programmed by the patient, as shown in block 318. The state of the stimulator system may be sampled in real time at a rate of 8kbit per second of telemetry data. In block 318, all feedback data received from the stimulator 114 can be maintained in time and sampled per minute for storage for download or loading to a remote monitoring system that can be accessed by a healthcare professional for trend analysis and statistical correlation. If operated in an open loop manner, the operation of the stimulator system may be reduced to only the functional elements shown in blocks 302, 304, 306, and 308, and the patient uses their determinations to adjust the parameter settings, rather than closed loop feedback from the implanted device.

Fig. 4 depicts a flowchart showing an example of the operation of the system when the current level at the electrode 254 is above a threshold limit. In some cases, the implanted wireless neurostimulator 114 may receive an input power signal with a current level above an established safe current limit, as shown in block 402. For example, the current limiter 248 may determine that the current is above the established tissue safety limit amperage, as shown in block 404. If the current limiter senses that the current is above the threshold, it may stop the high power signal from damaging the surrounding tissue in contact with the electrode, as shown in block 406, which operates as described above in connection with FIG. 2.

The capacitor may store the remaining power as shown in block 408. When the current limiter senses that the current is above the threshold, the controller 250 may send back a small 2-bit data burst to the RF pulse generator 106 using the remaining power available, as shown in block 410. The 2-bit data burst may be transmitted via the antenna(s) 238 of the implanted wireless neural stimulator during the receive cycle of the RF pulse generator, as shown in block 412. The RF pulse generator antenna 110 may receive the 2-bit data burst at a rate of 8kbps during its receive period, as shown in block 414, and the RF pulse generator antenna 110 may relay the data burst back to the feedback subsystem 212 of the RF pulse generator, which feedback subsystem 212 is monitoring all reverse power, as shown in block 416. The CPU230 may analyze the signal from the feedback subsystem 202 as shown in block 418 and may not make any changes to the stimulation parameters if no data bursts are present as shown in block 420. If there is a data burst in the analysis, the CPU230 can turn off all transmit power for one cycle, as shown in block 422.

If the data burst continues, RF pulse generator 106 may push a notification of a "proximity power hazard" to the application on programmer module 102, as shown in block 424. This proximity hazard notification occurs because the RF pulse generator has stopped its power transmission. This notification means that an unauthorized form of energy is powering the implant above a safe level. The application may prompt the user for danger and that the user should leave the vicinity to resume neuromodulation therapy, as shown in block 426. If the data burst ceases after one period, the RF pulse generator 106 may slowly boost the transmit power in increments, for example, from 5% to 75% of the previous current amplitude level, as shown in block 428. The user can then manually adjust the current amplitude level to be responsible for making the current amplitude level higher by the user himself. During the reinforcement process, the RF pulse generator 106 may inform the application of its progress, and the application may inform the user that an unsafe power level exists and the system is reinforcing back, as shown in block 430.

Fig. 5 is a diagram illustrating an example of a signal that may be used to detect an impedance mismatch. As described above, the forward power signal and the reverse power signal may be used to detect impedance mismatches. For example, the RF pulse 502 generated by the RF pulse generator may pass through a device such as a bi-directional coupler to the TX antenna 110. The TX antenna 110 then radiates RF signals into the body where energy is received by the implanted wireless neurostimulator 114 and converted into pulses that stimulate tissue. The coupler delivers an attenuated version of this RF signal, forward power 510, to the feedback subsystem 212. The feedback subsystem 212 demodulates the AC signal and calculates the magnitude of the forward RF power, and this data is passed to the controller subsystem 214. Similarly, the bi-directional coupler (or similar component) also receives RF energy reflected back from TX antenna 110 and transmits an attenuated version of such RF signal, i.e., reverse power 512, to feedback subsystem 212. The feedback subsystem 212 demodulates the AC signal and calculates the magnitude of the reflected RF power, and this data is passed to the controller subsystem 214.

In an optimal case, when the TX antenna 110 can be perfectly impedance matched to the body, the RF energy is made to pass unimpeded through the interface of the TX antenna 110 to the body, and no RF energy is reflected at the interface. Thus, in this optimal case, the reverse power 512 may have a magnitude close to zero, as shown by signal 504, and the ratio of reverse power 512 to forward power 510 is zero. In such an environment, there are no error conditions and the controller 214 sets the system messages to operate optimally.

In implementation, the impedance matching of TX antenna 204 to the body may not be optimal and some of the energy of RF pulse 502 is reflected from the interface of TX antenna 110 and the body. This can occur, for example, if TX antenna 110 is held away from the skin by a distance of about one garment. This non-optimal antenna coupling causes a small portion of the forward RF energy to be reflected at the interface, and this is shown as signal 506. In this case, the ratio of reverse power 512 to forward power 510 is small, and a small ratio implies that most of the RF energy is still radiated from TX antenna 110, and is therefore acceptable within the control algorithm. This determination of an acceptable reflectance ratio may be made within the controller subsystem 214 based on programmed thresholds, and the controller subsystem 214 may generate low priority prompts that are sent to the user interface. Further, the controller subsystem 214 sensing the small reflectance condition may moderately increase the amplitude of the RF pulse 502 to compensate for a moderate loss of forward energy transmitted to the implanted wireless neurostimulator 114.

During everyday operational use, the TX antenna 110 may be accidentally removed completely from the body, in which case the TX antenna will have very poor coupling (if any) with the body. In this or other environments, a relatively high proportion of the RF pulse energy is reflected from the TX antenna 110 as a signal 508 and fed back into the RF power supply system. Similarly, this phenomenon can occur if the connection to the TX antenna is physically broken, in which case almost 100% of the RF energy is reflected back from the point of breakage. In this case, the ratio of reverse power 512 to forward power 510 is very high and controller subsystem 214 will determine that the ratio has exceeded the acceptance threshold. In this case, the controller subsystem 214 may prevent any further RF pulses from being generated. The shutdown of the RF pulse generator module 106 may be reported to the user interface to inform the user that stimulation therapy cannot be delivered.

Fig. 6 is a diagram illustrating an example of signals that may be employed during operation of a neurostimulator system. According to some embodiments, the amplitude of the RF pulses 602 received by the implanted wireless neurostimulator 114 can directly control the amplitude of the stimulation 630 delivered to the tissue. The duration of the RF pulses 608 corresponds to the specified pulse width of the stimulus 630. During normal operation, the RF pulse generator module 106 sends an RF pulse waveform 602 into the body via the TX antenna 110, and the RF pulse waveform 608 may represent a corresponding RF pulse received by the implanted wireless neurostimulator 114. In this case, the received power has an amplitude suitable for generating the safety stimulation pulse 630. Stimulation pulse 630 is below safety threshold 626 and no error condition exists. In another example, the attenuation between the TX antenna 110 and the implanted wireless neurostimulator 114 is unexpectedly reduced, for example, as a result of the user repositioning the TX antenna 110. This reduced attenuation can result in an increase in the amplitude of the RF pulse waveform 612 received at the neurostimulator 114. Although the generated RF pulse 602 has the same amplitude as before, the improved RF coupling between the TX antenna 110 and the implanted wireless neurostimulator 114 can make the received RF pulse 612 amplitude larger. The implanted wireless neurostimulator 114 in this condition may generate greater stimulation 632 in response to an increase in the received RF pulses 612. However, in this example, the received power 612 is capable of generating a stimulus 632 that exceeds prudent safety limits for the tissue. In this case, the current limiter feedback control mode can be used to shorten the waveform of the stimulation pulse 632, thereby keeping the delivered stimulation within the predetermined safety limit 626. The shortening event 628 may be communicated through the feedback subsystem 212 as described above, and subsequently, the controller subsystem 214 can reduce the amplitude specified for the RF pulse. As a result, the subsequent RF pulse 604 decreases in amplitude and correspondingly, the amplitude of the received RF pulse 616 is reduced to an appropriate level (non-shortened level). In this manner, if the implanted wireless neurostimulator 114 receives excessive RF power, the current limiter feedback control mode may be used to reduce the RF power delivered to the body.

In another example, the RF pulse waveform 606 depicts a higher amplitude RF pulse generated as a result of output to a user of the user interface. In this environment, the amplitude of the RF pulses 620 received by the implanted wireless neurostimulator 14 increases, and similarly, the current limiter feedback mode is used to prevent the stimulation 636 from exceeding the safety limit 626. Again, such a shortening event 628 may be communicated through the feedback subsystem 212, and subsequently, the controller subsystem 214 may reduce the amplitude of the RF pulse, thereby ignoring the user input. The reduced RF pulse 604 can produce a correspondingly smaller amplitude of the received waveform 616, and the shortening of the stimulation current may no longer be required to keep the current within safe limits. In this way, if the implanted wireless neurostimulator 114 reports that it is receiving excessive RF power, the current limiter feedback may reduce the RF power delivered to the body.

Figure 7 is a flow chart illustrating a process by which a user controls an implantable wireless neurostimulator via a programmer in an open loop feedback system. In one embodiment of the system, the user has an implanted wireless neurostimulator in his body, the RF pulse generator 106 wirelessly transmits the stimulating pulse power to the stimulator 114, and the application on the programmer module 102 (e.g., a smart device) communicates with the RF pulse generator 106. In such an embodiment, if the user wants to observe the current state of the pulse generator being operated, as shown in block 702, the user may open the application, as shown in block 704. The application can use the bluetooth protocol built into the smart device to interrogate the pulse generator, as shown in block 706. The RF pulse generator 106 may authenticate the identity of the smart device and the identity of the assigned concatenated patient safety iterations of the application, as shown in block 708. The authentication process may utilize a unique key for the RF pulse generator serial number that is specific to the patient. The application may be customized with a patient-specific unique key by a manufacturer representative who has programmed the stimulation system with initial patient settings, as shown in block 720. If the RF pulse generator denies authentication, it may notify the application that the code is invalid, as shown in block 718, and requires authentication provided by an authorized individual with a security clearance from the device manufacturer known as "manufacturer representative," as shown in block 722. In one embodiment, only the manufacturer represents the security code required to be able to access the stored RF pulse generator unique ID that changes the application. If the RF pulse generator authentication system passes, the pulse generator module 106 sends back all the data that has been recorded since the last synchronization, as shown in block 710. The application may then register the most current information and transmit the information to the third party in a secure manner, as shown in 712. The application may maintain a database that records all system diagnostic results and values, setting changes made by the user and feedback system, and the global run-time history, as shown in block 714. The application may then display the relevant data to the user, as shown in block 716; including battery capacity, current program parameters, run time, pulse width, frequency, amplitude, and state of the feedback system.

Fig. 8 is another example flow chart of a process for a user to control a wireless stimulator through limits on upper and lower limits of current amplitude. The user wants to change the amplitude of the stimulation signal as shown in block 802. The user may open an application, as shown in block 704, and the application may proceed with the process described in fig. 7 to communicate with the RF pulse generator, authenticate successfully, and display the current status to the user, as shown in block 804. The application displays the stimulation amplitude as the most common interface-changeable 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 based on their pain level, as shown in block 806. If the user chooses to increase the current magnitude, the user may press the up arrow on the application screen, as shown in block 808. The application can include a safe maximum limit algorithm so if the application identifies the request to increase the current amplitude as exceeding a preset safe maximum, as shown in block 810, the application will display an error message, as shown in block 812, and will not communicate with the RF pulse generator module 106. If the user presses the up arrow, as shown in block 808, and the current magnitude request does not exceed the current magnitude maximum allowed value, then the application will send an instruction to the RF pulse generator module 106 to increase the magnitude, as shown in block 814. The RF pulse generator module 106 may then attempt to increase the current amplitude of the stimulation, as shown in block 816. If the RF pulse generator successfully increases the current amplitude, the RF pulse generator module 106 may perform a short vibration to physically confirm with the user that the amplitude increased, as shown in block 818. The RF pulse generator module 106 can also send back an acknowledgement of the increased amplitude to the application as shown in block 820, and the application can then display the updated current amplitude level as shown in block 822.

If the user decides to decrease the current amplitude level in block 806, the user can press the down arrow on the application, as shown in block 828. If the current amplitude level is already at zero, the application recognizes that the current amplitude cannot be further reduced, as shown in block 830, and displays an error message to the user without transmitting any data to the RF pulse generator, as shown in block 832. If the current amplitude level is not at zero, the application can send instructions to the RF pulse generator module 106 to decrease the current amplitude level accordingly, as shown in block 834. The RF pulse generator may then attempt to reduce the current amplitude level of the stimulation RF pulse generator module 106, and if successful, the RF pulse generator module 106 may perform a short vibration to physically confirm to the user that the current amplitude level has been reduced, as shown in block 842. The RF pulse generator module 106 can send back confirmation of the reduced current amplitude level to the application as shown in block 838. The application may then display the updated current magnitude level, as indicated at block 840. If the current amplitude level fails to decrease or increase, the RF pulse generator module 106 can perform a series of short vibrations to alert the user and send an error message to the application, as shown in block 824. The application receives the error and may display data for the benefit of the user, as shown in block 826.

Fig. 9 is yet another example flowchart of a process for a user to control the wireless neurostimulator 114 through preprogrammed parameter settings. The user wants to change the parameter program as indicated by block 902. When the user implants the wireless neurostimulator or when the user visits a physician, the manufacturer's representative may determine and provide the patient/user RF pulse generator with a preset program with different stimulation parameters to be used for treating the user. The user will then be able to switch between the various parameter programs as desired. The user can open an application on their smart device as indicated at block 704, which first follows the process described in fig. 7, communicate with the RF pulse generator module 106, authenticate successfully, and display the current status of the RF pulse generator module 106, including the current program parameter settings, as indicated at block 812. In such an embodiment, the user can select the program they wish to use through the user interface of the application, as shown in block 904. The application may then access a library of preprogrammed parameters that have been approved by the manufacturer's representative for the user to alternate between desired and managed according to the user's instructions, as indicated in block 906. The user can be presented with a table as shown in block 908, each row displaying the code name of the program and listing its basic parameter settings, as shown in block 910, including, but not limited to: pulse width, frequency, cycle timing, pulse shape, duration, feedback sensitivity, as shown in block 912. The user may then select a row containing the desired parameter preset program to be used, as shown in block 912. The application can send instructions to the RF pulse generator module 106 to change the parameter settings as shown in block 916. The RF pulse generator module 106 may attempt to change the parameter settings 154. If the parameter setting is successfully changed, the RF pulse generator module 106 can execute a unique vibration pattern to physically confirm with the user that the parameter setting was changed, as shown in block 920. Also, the RF pulse generator module 106 can send back an acknowledgement to the application that: the parameter change has been successful, as shown in block 922, and the application may display the updated current program, as shown in block 924. If the parameter program change fails, the RF pulse generator module 106 may perform a series of short vibrations to alert the user and send an error message to the application, as shown in block 926, which receives the error and may display it to the user, as shown in block 928.

Fig. 10 is yet another example flow chart of a process for a low battery state of the RF pulse generator module 106. In such an embodiment, the remaining battery power level of the RF pulse generator module is identified as low, as shown in block 1002. The RF pulse generator module 106 periodically queries the power supply battery subsystem 210 for current power and the RF pulse generator microprocessor queries the battery whether its remaining power is below a threshold, as shown in block 1004. If the remaining power of the battery is above the threshold, the RF pulse generator module 106 may store the current battery state to be sent to the application during the next synchronization, as shown in block 1006. If the remaining power of the battery is below the threshold, the RF pulse generator module 106 may push a low battery notification to the application, as shown in block 1008. The RF pulse generator module 106 may always perform a sequence of short vibrations to alert the user of a problem and send a notification to the application, as shown in block 1010. If the application continues to receive no confirmation of notification, the RF pulse generator can continue to perform short vibration pulses to notify the user, as shown in block 1010. If the application successfully receives the notification, it may display the notification and may require user confirmation, as shown in block 1012. If, for example, a minute has elapsed without a notification message on the revoked application, the application notifies the RF pulse generator module 106 of the lack of human confirmation, as shown in block 1014, and the RF pulse generator module 106 may begin executing a vibration pulse to notify the user, as shown in block 1010. If the user dismisses the notification, the application may display a passive notification to switch the battery, as shown in block 1016. If a predetermined amount of time has elapsed, for example five minutes has elapsed, the battery has not been switched, the application can notify the RF pulse generator module 106 of the lack of human confirmation, as shown in block 1014, and the RF pulse generator module 106 can perform a vibration, as shown in block 1010. If the RF pulse generator module battery is switched, the RF pulse generator module 106 restarts and interrogates the battery to assess the remaining power, as shown in block 1618. If the remaining power of the battery is below the threshold, the cycle may begin again, with the RF pulse generator module 106 pushing a notification to the application, as shown in block 1008. If the remaining power of the battery is above the threshold, the RF pulse generator module 106 may push a notification of a successful battery change to the application, as shown in block 1620. The application may then communicate with the RF pulse generator module 106 and display the current system state, as shown in block 1022.

Fig. 11 is a flowchart of yet another example of a process for programming an implanted wireless neurostimulator on behalf of a manufacturer. In such an embodiment, the user wants the manufacturer's representative to set the individual parameter programs from a remote location different from the user's location for use by the user as desired, as shown in block 1102. The manufacturer's representative can gain access to the user's parameter-setting program through a secure network-based service. The manufacturer represents a manufacturer network service that can securely log onto a device connected to the internet, as shown in block 1104. If the manufacturer's representative registers the user for the first time in his care, he enters basic information for the patient, the unique ID of the RF pulse generator and the unique ID of the programming application, as shown in block 1106. Once the new or old user represented by the manufacturer has registered, the manufacturer's representative accesses the profile of the particular user, as shown in block 1108. The manufacturer's representative can view the current distribution list of parameter programs for a particular user, as shown in block 1110. Such a list may contain parameter presets that were previously active and inactive, as shown in block 1112. The manufacturer's representative can activate/deactivate the preset parameter program by checking the box next to the appropriate row in the displayed table, as shown in block 1114. The manufacturer's representative may then submit and save the assigned new preset parameter programs, as shown in block 1116. The user's programmer application may receive the new preset parameter program at the next synchronization with the manufacturer's database.

Fig. 12 is a circuit diagram illustrating an example of a wireless neurostimulator, such as stimulator 114. Such an example includes pairs of electrodes including cathode electrode(s) 1208 and anode electrode(s) 1210, as shown. When energized, the charged electrodes generate a volume conduction field of current density within the tissue. In this embodiment, wireless energy is received by dipole antenna(s) 238. At least four diodes are connected together to form a full wave bridge rectifier 1202 attached to dipole antenna(s) 238. Each diode, up to 100 microns in length, uses a junction potential to prevent the flow of negative current through the device from the cathode to the anode when the current does not exceed a reverse threshold. The natural inefficiency of lossy materials for nerve stimulation via wireless power, transmitted through tissue, can result in a low threshold voltage. In such an embodiment, the zero-biased diode rectifier forms a low output impedance for the device. A resistor 1204 and a smoothing capacitor 1206 are placed across the output nodes of the bridge rectifier to discharge the electrodes to the ground of the anode of the bridge. The rectifier bridge 1202 comprises two branches of a diode pair connecting the anode to the anode and then the cathode to the cathode. The electrodes 1208 and 1210 are connected to the output of the charge balancing circuit 246.

Fig. 13 is a circuit diagram of another example of a wireless neurostimulator, such as stimulator 114. The example shown in fig. 13 includes multiple electrode control and may employ full closed loop control. The stimulator includes an electrode array 254 in which the polarity of the electrodes can be assigned as either a cathode or an anode, and for this assignment the electrodes can be powered or not powered with any energy. When energized, the charged electrodes generate a volume conduction field of current density within the tissue. In such an embodiment, the device receives wireless energy through dipole antenna(s) 238. The array of electrodes 254 is controlled by the on-board controller circuit 242, and the on-board controller circuit 242 sends appropriate bit information to the electrode interface 252 to set the polarity of each electrode in the array and to power each individual electrode. The lack of power to a particular electrode will place that electrode in a functional off position. In another embodiment (not shown), the amount of current sent to each electrode is also controlled by controller 242. The controller current, polarity, and power state parameter data, shown as controller output, is sent back to the antenna(s) 238 for telemetry transmission back to the pulse generator module 106. The controller 242 also includes current monitoring functionality and sets the bit register counter so that the status of the total current drawn can be sent back to the pulse generator module 106.

At least four diodes can be connected together to form a full wave bridge rectifier 302 attached to the dipole antenna(s) 238. Each diode, up to 100 microns in length, uses a junction potential to prevent the flow of negative current through the device from cathode to anode when the current does not exceed a reverse threshold. The natural inefficiency of lossy materials for nerve stimulation via wireless power, transmitted through tissue, can result in a low threshold voltage. In such an embodiment, the zero-biased diode rectifier forms a low output impedance for the device. A resistor 1204 and a smoothing capacitor 1206 are placed across the output nodes of the bridge rectifier to discharge the electrodes to the ground of the anode of the bridge. The rectifier bridge 1202 may include two branches of a diode pair connecting the anode to the anode and then the cathode to the cathode. The polarity output of the electrodes, the cathode 1208 and the anode 1210 are all connected to outputs formed by bridge connections. A charge balancing circuit 246 and a current limiting circuit 248 are placed in series with the output.

A number of embodiments have been described. Nevertheless, it will be understood that various modifications may be made. Accordingly, other implementations are within the scope of the following claims.

Claims (16)

1. A neurostimulation system, comprising:

an external controller module, comprising:

a first antenna configured to:

sending an input signal containing electrical energy by electrical radiative coupling to a second antenna, the second antenna being a dipole antenna and located in an implantable neurostimulator, the implantable neurostimulator having no battery and being configured to use the electrical energy contained in the input signal alone to generate one or more electrical pulses suitable for stimulating neural tissue to apply the one or more electrical pulses to the neural tissue, measure one or more parameters of the one or more electrical pulses applied to the neural tissue, store energy from the electrical energy in the input signal on the implantable neurostimulator, and transmit a feedback signal by using the stored energy alone, the feedback signal including (i) a stimulation feedback signal indicative of the measured parameters, and (ii) an indication that a characteristic of the one or more electrical pulses has been limited such that it is induced by the one or more electrical pulses applied at an electrode A limiting feedback signal that the unit phase charge will not exceed a threshold level, wherein the implantable neurostimulator is separate from the external controller module, and

receiving one or more signals from the second antenna; and

one or more circuits configured to:

generating the input signal and transmitting the input signal to the first antenna;

extracting a stimulus feedback signal from one or more signals from the second antenna received by the first antenna;

adjusting a parameter of the input signal based on the stimulus feedback signal; and

attenuating the input signal in response to the limiting feedback signal being received such that unit phase charge induced by the one or more electrical pulses remains below the threshold level.

2. The neurostimulation system of claim 1, wherein the one or more parameters of the electrical pulses comprise an amplitude of the one or more electrical pulses applied to the neural tissue, and the one or more circuits are configured for adjusting the power of the input signal based on the amplitude of the one or more electrical pulses.

3. The neurostimulation system of claim 1, wherein the one or more circuits are configured for:

obtaining a forward power signal reflecting an amplitude of a Radio Frequency (RF) signal transmitted to the first antenna;

obtaining a reverse power signal reflecting an amplitude of a reflected portion of a radio frequency signal transmitted to the first antenna;

determining a mismatch value indicative of a degree of impedance mismatch based on the forward power signal and the reverse power signal; and is

Adjusting the parameter of the input signal based on the mismatch value.

4. The neurostimulation system of claim 1, further comprising the implantable neurostimulator, the implantable neurostimulator comprising:

one or more electrodes configured to apply the one or more electrical pulses to neural tissue;

one or more circuits configured to:

generating the one or more electrical pulses;

supplying the one or more electrical pulses to the one or more electrodes, thereby causing the one or more electrodes to apply the one or more electrical pulses to neural tissue;

generating the stimulus feedback signal; and

sending the stimulus feedback signal to the second antenna, thereby causing the second antenna to transmit the stimulus feedback signal to the first antenna through electrical radiative coupling.

5. The neurostimulation system of claim 4, wherein the input signal further comprises information encoding stimulation parameters for the one or more electrical pulses, and the implantable neurostimulator is configured for generating the one or more electrical pulses based on the information encoding stimulation parameters.

6. The neurostimulation system of claim 4, wherein the one or more parameters of the one or more electrical pulses comprise an amplitude of the one or more electrical pulses or an impedance of the one or more electrodes.

7. The neurostimulation system of claim 4, wherein the one or more circuits of the implantable neurostimulator are configured such that the level of the input signal directly determines the amplitude of the one or more electrical pulses applied to the neural tissue by the one or more electrodes.

8. The neurostimulation system of claim 6, wherein the one or more circuits of the implantable neurostimulator are configured for:

limiting the characteristic of the one or more electrical pulses applied to the neural tissue by the one or more electrodes such that the unit-phase charge induced by the one or more electrical pulses remains below the threshold level;

generating the limit feedback signal when the unit-phase charge induced by the one or more electrical pulses would exceed the threshold level if the one or more circuits did not limit the characteristics of the one or more electrical pulses applied to the neural tissue by the one or more electrodes, such that the unit-phase charge induced by the one or more electrical pulses remains below the threshold level; and is

Sending the limit feedback signal to the second antenna, thereby causing the second antenna to transmit the limit feedback signal to the first antenna through electrical radiative coupling.

9. The neurostimulation system of claim 8, wherein the characteristic of the one or more pulses applied to the neural tissue by the one or more electrodes is a current level, and the threshold level is a current threshold level.

10. The neurostimulation system of claim 4, wherein the one or more electrical circuits are configured for generating the one or more electrical pulses such that the one or more electrical pulses result in a net charge of substantially zero.

11. The neurostimulation system of claim 10, wherein to generate the one or more electrical pulses such that the one or more electrical pulses result in a substantially zero net charge, the one or more circuits of the implantable neurostimulator comprise at least one capacitor in series with the one or more electrodes.

12. The neurostimulation system of claim 4, wherein the one or more circuits of the implantable neurostimulator comprise:

a waveform adjustment component for generating the one or more electrical pulses suitable for stimulating neural tissue using electrical energy contained in the input signal;

an electrode interface connected to a waveform adjustment circuit, the electrode interface configured to receive the one or more electrical pulses from the waveform adjustment circuit and supply the one or more electrical pulses to the one or more electrodes; and

a controller connected to the electrode interface, the controller configured to generate the stimulation feedback signal and transmit the stimulation feedback signal to the second antenna.

13. The neurostimulation system of claim 12, wherein the waveform adjustment component comprises:

a rectifier connected to the second antenna, the rectifier configured to receive the input signal from the second antenna and generate a rectified electrical waveform based on the input signal;

a charge balancing component configured to generate the one or more electrical pulses based on the rectified electrical waveform such that the one or more electrical pulses result in substantially zero net charge at the one or more electrodes; and

a charge limiter configured to limit the characteristic of the one or more electrical pulses such that unit phase charges induced by the one or more electrical pulses remain below a threshold level, wherein the limited electrical pulses are transmitted to the electrode interface through the charge limiter.

14. The neurostimulation system of claim 4, wherein:

the implantable neural stimulator includes a plurality of electrodes;

the one or more circuits of the external controller module are configured to:

generating control signals specifying which electrodes act as stimulation electrodes, which electrodes act as return electrodes positioned further from the neural tissue being stimulated than the stimulation electrodes, and which electrodes are not operational; and

sending the control signal to the first antenna, thereby causing the first antenna to transmit the control signal to the second antenna through electrical radiative coupling; and is

The one or more circuits of the implantable neurostimulator are configured to selectively designate each of the electrodes to function as a stimulation electrode, a return electrode, or be inoperative based on the control signals.

15. The neurostimulation system of claim 4, wherein the implantable neurostimulator does not include an internal power source.

16. The neurostimulation system of claim 4, wherein the one or more circuits of the implantable neurostimulator comprise only passive components.