System and method for sensing stimulation electrode tissue impedance

Abstract

The present invention relates to a method comprising: transmitting a first set of radio frequency (RF) pulses to an implantable wireless stimulator device such that current is generated from the first set of RF pulses and flows through a calibrated internal load on the implantable wireless stimulator device; recording a first set of RF reflection measurements in response to the current flowing through the calibrated internal load; transmitting a second set of RF pulses to the implantable wireless stimulator device such that stimulation current is generated from the second set of RF pulses and flows through an electrode of the implantable wireless stimulator device to tissue surrounding the electrode; recording a second set of RF reflection measurements in response to the stimulation current flowing through the electrode to the surrounding tissue; and characterizing electrode-tissue impedance by comparing the second set of RF reflection measurements with the first set of RF reflection measurements.

Description

[0001] Related Applications

[0002] This application claims the benefit of U.S. Provisional Application No. 62 / 624,982, filed on February 1, 2018, which is incorporated by reference in its entirety. Technical Field

[0003] This application generally relates to systems and methods for the operation of an implantable stimulator device that has been implanted within a subject. Background Art

[0004] Regulation of excitable tissues in the body by electrical stimulation has become an important therapy for patients with chronic disabling conditions, including pain, initiation and control of movement, involuntary movement, vascular insufficiency, arrhythmia, and various other forms. A variety of therapeutic in vivo electrical stimulation techniques can be used to provide therapeutic relief for these conditions. For example, a device can be used to deliver stimulation signals to excitable tissues, record vital signs, perform pacing or defibrillation operations, record action potential activity from target tissues, control drug release from timed release capsules or drug pump units, or interface with the auditory system to assist hearing. Summary of the invention

[0005] In one aspect, some embodiments provide a method for adjusting stimulation of surrounding tissue by an implantable wireless stimulator device, the method comprising: transmitting a first set of radio frequency (RF) pulses to the implantable wireless stimulator device from an external pulse generator and via electrical radiation coupling, such that current is generated from the first set of RF pulses and flows through a calibrated internal load on the implantable wireless stimulator device; recording a first set of RF reflection measurements on the external pulse generator in response to the current flowing through the calibrated internal load; transmitting a second set of radio frequency (RF) pulses to the implantable wireless stimulator device from the external pulse generator and via electrical radiation coupling, such that stimulation current is generated from the second set of RF pulses and flows through electrodes of the implantable wireless stimulator device to tissue surrounding the electrodes; recording a second set of RF reflection measurements on the external pulse generator in response to the stimulation current flowing through the electrodes to the surrounding tissue; and characterizing electrode-tissue impedance by comparing the second set of RF reflection measurements with the first set of RF reflection measurements.

[0006] Embodiments may include one or more of the following features. In response to characterizing the electrode-tissue impedance as resistive, the method may include adjusting one or more input pulses to be transmitted by the external pulse generator to the implantable wireless stimulator device so that the stimulation current generated from the input pulses on the implantable wireless stimulator device is adjusted to compensate for the resistive electrode-tissue impedance. Adjusting the input pulses may include: maintaining steady-state delivery of power to the implantable wireless stimulator device so that electrical energy is extracted from the input pulses as fast as electrical energy is consumed by the implantable wireless stimulator device to (i) generate the stimulation current having one or more pulse parameters that have been changed to accommodate the resistive electrode-tissue impedance, and (ii) delivering the stimulation current from the electrode on the implantable wireless stimulator device to the surrounding tissue. The pulse parameters may include: pulse width, pulse amplitude, and pulse frequency.

[0007] The method may include: in response to characterizing the electrode-tissue impedance as capacitive, adjusting one or more input pulses to be transmitted by the external pulse generator to the implantable wireless stimulator device such that a stimulation current generated from the input pulses and delivered to the surrounding tissue by the electrodes on the implantable wireless stimulator device is adjusted to compensate for the capacitive electrode-tissue impedance. Adjusting the input pulses may include: maintaining a steady-state delivery of power to the implantable wireless stimulator device such that electrical energy is extracted from the input pulses as quickly as electrical energy is consumed by the implantable wireless stimulator device to (i) generate the stimulation current having one or more pulse parameters that have been altered to accommodate the capacitive electrode-tissue impedance, and (ii) deliver the stimulation current from the electrodes on the implantable wireless stimulator device to the surrounding tissue. The pulse parameters may include: pulse width, pulse amplitude, and pulse frequency.

[0008] The method may further include: based on the results of characterizing the electrode-tissue impedance, automatically selecting a stimulation session by determining an input pulse to be transmitted by the external pulse generator to the implantable wireless stimulator device so that a stimulation current is generated on the implantable wireless stimulator device and delivered to the surrounding tissue through the electrode on the implantable wireless stimulator device. Determining the input pulse may include: updating the second set of radio frequency (RF) pulses to obtain an updated second set of RF reflection measurements; and comparing the updated second set of RF reflection measurements with the first set of RF reflection measurements. The updating and comparing may be performed iteratively until the desired RF reflection measurements are obtained.

[0009] The method may further include automatically performing a fault check based on the results of characterizing the electrode-tissue impedance. Automatically performing a fault check may include detecting damaged wires in a circuit leading to the electrode on the implantable wireless stimulator device.

[0010] In another aspect, some embodiments provide a system comprising: an implantable wireless stimulator device comprising: a first non-inductive antenna; one or more electrodes; and a circuit between the first non-inductive antenna and the one or more electrodes, the circuit comprising: a calibrated internal load representing a predetermined load condition on the one or more electrodes; an external pulse generator comprising: a second non-inductive antenna configured to: transmit a first set of radio frequency (RF) pulses to the first non-inductive antenna on the implantable wireless stimulator device via electrical radiation coupling, so that current is generated from the first set of RF pulses and flows through the calibrated internal load on the implantable wireless stimulator device; and transmit a second non-inductive antenna to the first non-inductive antenna on the implantable wireless stimulator device via electrical radiation coupling. a set of radio frequency (RF) pulses transmitted to the first non-inductive antenna on the implantable wireless stimulator device, so that stimulation current is generated from the second set of RF pulses and flows through the electrode of the implantable wireless stimulator device to the tissue surrounding the electrode; and a reflection sensor subsystem, which is coupled to the second non-inductive antenna and is configured to: obtain a first set of RF reflection measurements in response to the current flowing through the calibrated internal load; and obtain a second set of RF reflection measurements in response to the stimulation current flowing through the electrode to the surrounding tissue; and a signal processor, which communicates with the reflection sensor subsystem and is configured to: characterize electrode-tissue impedance by comparing the second set of RF reflection measurements with the first set of RF reflection measurements.

[0011] Implementations may include one or more of the following features. The reflection sensor subsystem includes: a directional coupler coupled to the second non-inductive antenna and configured to detect a radio frequency (RF) signal reflected from the first non-inductive antenna; and a radio frequency (RF) phase detector coupled to the directional coupler and configured to detect a phase difference between the RF signal reflected from the first non-inductive antenna and the RF signal transmitted from the second non-inductive antenna to the first non-inductive antenna. The reflection sensor subsystem may further include: an analog-to-digital converter (ADC) coupled to the directional coupler and configured to convert the RF signal reflected from the first non-inductive antenna into a digital record.

[0012] The signal processor may be a digital signal processor. The signal processor may be further configured to: in response to characterizing the electrode-tissue impedance as resistive, adjust one or more input pulses to be transmitted by the external pulse generator to the implantable wireless stimulator device, so that one or more stimulation pulses generated from the input pulses and delivered to the surrounding tissue through the electrodes on the implantable wireless stimulator device are adjusted to compensate for the resistive electrode-tissue impedance. The signal processor may be further configured to: in response to characterizing the electrode-tissue impedance as capacitive, adjust one or more input pulses to be transmitted by the external pulse generator to the implantable wireless stimulator device, so that the stimulation current generated from the input pulses and delivered to the surrounding tissue through the electrodes on the implantable wireless stimulator device is adjusted to compensate for the capacitive electrode-tissue impedance. The signal processor may be further configured to: automatically select a stimulation session based on the result of characterizing the electrode-tissue impedance by determining an input pulse to be transmitted by the external pulse generator to the implantable wireless stimulator device so that a stimulation current is generated on the implantable wireless stimulator device and delivered to the surrounding tissue through the electrodes on the implantable wireless stimulator device. The signal processor may be further configured to: automatically perform a fault check based on the result of characterizing the electrode-tissue impedance. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 A high-level diagram depicting an example of a wireless stimulation system.

[0014] Figure 2 A more detailed diagram depicting an example of a wireless stimulation system.

[0015] Figure 3A is an illustration of an example of an implementation of a microwave field stimulator (MFS) transmitter for wireless power transfer to an implanted dipole antenna.

[0016] Figure 3B is another illustration of another example of an implementation with a directional coupler and a power detector.

[0017] Figures 4A to 4D Examples illustrating normalized reflectance versus normalized power for various loading conditions.

[0018] Figure 5 An example of a flowchart is shown below.

[0019] Like reference symbols in the various drawings indicate like elements. DETAILED DESCRIPTION

[0020] In various implementations, systems and methods are disclosed for applying one or more electrical pulses to target excitable tissues (e.g., nerves) for treating chronic pain, inflammation, arthritis, sleep apnea, seizures, incontinence, cancer-related pain, incontinence, movement initiation and control problems, involuntary movements, vascular insufficiency, arrhythmias, obesity, diabetes, craniofacial pain (e.g., migraine or cluster headaches), and other diseases. In a particular embodiment, a device can be used to send electrical energy to target neural tissue using remote radio frequency (RF) energy without cables or inductive coupling to power a wireless stimulator device. Target nerves can include, but are not limited to, the spinal cord and surrounding areas, including the dorsal horn, dorsal root ganglia, exiting nerve roots, ganglia, dorsal column fibers and peripheral nerve bundles exiting the dorsal column and brain, such as the vagus nerve, occipital, trigeminal nerve, hypoglossal nerve, sacral, coccygeal nerve, and the like.

[0021] The wireless stimulation system may include an implantable stimulator device having one or more electrodes and one or more inductive antennas (e.g., dipole or patch antennas), and internal circuitry for detecting pulse commands and rectification of RF power. The system may further include an external controller and antenna for transmitting radio frequency or microwave energy from an external source to the implantable stimulator device without cables or inductive coupling to provide power.

[0022] In various embodiments, the wireless implantable stimulator device is powered wirelessly (and therefore does not require a wired connection) and contains the circuitry necessary to receive pulse instructions from a source outside the body. For example, various embodiments employ an internal dipole (or other) antenna configuration to receive RF power through electrical radiation coupling, and the received RF power is used to power the implantable stimulator device. This allows such a device to generate current capable of stimulating a neural bundle without a physical connection to an implantable pulse generator (IPG) or the use of an inductive coil.

[0023] In some embodiments, the passive repeater module can be configured as an implantable device to couple the electromagnetic energy radiated from an external transmitting antenna to a wireless implantable stimulator device. In one example, the implantable device includes two monopole coupler arms connected to each other by a cable. A monopole coupler arm is implanted in a parallel configuration with the external transmitting antenna so that the linearly polarized electromagnetic waves radiated from the external transmitting antenna are received by this monopole coupler arm. The received electromagnetic waves can be propagated to another monopole coupler arm by the cable. This monopole coupler arm can radiate the received electromagnetic energy to the receiving antenna of the stimulator device in a reciprocal manner. In order to effectively radiate the received electromagnetic energy to the receiving antenna of the stimulator device, this other monopole coupler arm can be used again to align parallel to the receiving antenna. In some cases, the length of the monopole arm and the length of the cable can be cut to improve the transmission efficiency, for example, at a specific operating frequency.

[0024] Some embodiments utilize non-battery wireless power transmission implants, which are a new class of devices that can be constructed in a very small form factor, thereby achieving minimal surgical incisions and potentially unlimited product life without the limitations and complexity associated with battery-powered devices. However, wireless power transmission faces various challenges. The size of the implanted antenna is ideally very small to pass through a needle or cannula in order to achieve minimally invasive surgery. Typically, a small antenna receives less RF power than a larger antenna, which means that the power transmission efficiency to a very small antenna may be poor. Complicating the problem is that the RF power that can be delivered by an external transmission source is limited, because the specific absorption rate (SAR) of RF in the human body must be kept within safe limits. Thus, the best power transmission efficiency (or minimum path loss) must be maintained during wireless power transmission of an implantable medical device. In order to achieve optimal power transmission, the external transmitting antenna must be aligned relative to the implant in a favorable position on the body. Estimating the position of the implant has historically been feasible only with the use of medical imaging systems (such as x-rays or ultrasound). Some embodiments disclosed herein achieve positioning of the in-situ receiver antenna without the use of complex and expensive medical imaging techniques.

[0025] Additional descriptions of exemplary wireless systems for providing neural stimulation to a patient may be found in commonly assigned published PCT applications PCT / US2012 / 23029, filed on January 28, 2011, and published on August 2, 2012, PCT / US2012 / 32200, filed on April 11, 2011, and published on October 11, 2012, PCT / US2012 / 48903, filed on January 28, 2011, and published on February 7, 2013, PCT / US2012 / 50633, filed on August 12, 2011, and published on February 21, 2013, and PCT / US2012 / 55746, filed on September 15, 2011, and published on March 21, 2013, the entire disclosures of which are incorporated by reference.

[0026] Figure 1 A high-level diagram depicting an example of a wireless stimulation system. The wireless stimulation 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 neural stimulator 114. The programmer module 102 may be a processor that supports wireless connectivity 104 (e.g., ) software application. The application can enable the user to view system status and diagnostics, change various parameters, increase / decrease the desired stimulation amplitude of the electrical pulses, and adjust the feedback sensitivity of the RF pulse generator module 106, among other functions.

[0027] The RF pulse generator module 106 may include communication electronics supporting the wireless connection 104 and a battery for powering the generator electronics. In some embodiments, the RF pulse generator module 106 includes a TX antenna embedded in its package form factor, while in other embodiments, the TX antenna is connected to the RF pulse generator module 106 via a wired connection 108 or a wireless connection (not shown). The TX antenna 110 can be directly coupled to the tissue to generate an electric field that powers the implanted neurostimulator 114. The TX antenna 110 communicates with the implanted neurostimulator 114 through an RF interface. For example, the TX antenna 110 radiates an RF transmission signal modulated and encoded by the RF pulse generator module 106. The implanted wireless stimulator device of the implanted neurostimulator 114 contains one or more antennas, such as a dipole antenna, for receiving and transmitting via the RF signal 112. In particular, the coupling mechanism between the antenna 110 and the one or more antennas of the implanted neurostimulator 114 utilizes electrical radiation coupling rather than inductive coupling. In other words, the coupling is through an electric field rather than a magnetic field.

[0028] Through this electrical radiation coupling, the TX antenna 110 can provide an input signal to the implanted neurostimulator 114. This input signal contains energy and may contain an information-encoded stimulation waveform to be applied at the electrodes of the implanted neurostimulator 114. In some embodiments, the power level of this 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 neurostimulator 114 are components for demodulating the RF transmission signal and electrodes for delivering stimulation to the peripheral neural tissue.

[0029] The RF pulse generator module 106 may be implanted subcutaneously, or it may be worn externally. When externally, the RF pulse generator module 106 may be incorporated into a belt or harness design to allow electrical radiation coupling through the skin and underlying tissue to transmit power and / or control parameters to the implanted neurostimulator 114. In either case, the receiver circuit inside the implanted neurostimulator 114 may capture the energy radiated by the TX antenna 110 and convert this energy into an electrical waveform. The receiver circuit may further modify the waveform to produce electrical pulses suitable for stimulating neural tissue.

[0030] In some embodiments, the RF pulse generator module 106 can remotely control stimulation parameters (i.e., parameters of the electrical pulses applied to the neural tissue) and monitor feedback from the implanted neural stimulator 114 based on the RF signal received from the implanted neural stimulator 114. The feedback detection algorithm implemented by the RF pulse generator module 106 can monitor data sent wirelessly from the implanted neural stimulator 114, including information about the energy received from the RF pulse generator by the implanted neural stimulator 114 and information about the stimulation waveform delivered to the electrodes. In order to provide effective therapy for a given medical condition, the system can be tuned to provide the optimal amount of stimulation or inhibition to the neural fibers through electrical stimulation. A closed loop feedback control method can be used, in which the output signal from the implanted neural stimulator 114 is monitored and used to determine the appropriate neural stimulation level for maintaining effective therapy, or in some cases, open loop control can be used.

[0031] Figure 2 Detailed diagram depicting an example of a wireless stimulation system. As depicted, programmer module 102 may include user input system 221 and communication subsystem 208. User input system 221 may allow various parameter settings to be adjusted (in some cases, in an open loop manner) by a user in the form of an instruction set. Communication subsystem 208 may transmit these instruction sets (and other information) to RF pulse generator module 106 via wireless connection 104 (e.g., Bluetooth or Wi-Fi), and may receive data from RF pulse generator module 106.

[0032] For example, a programmer module 102 that can be utilized for 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 controllable stimulation parameters can include pulse amplitude, pulse frequency, and pulse width within the ranges shown in Table 1. In this context, the term pulse refers to the phase of the waveform that directly produces stimulation to the tissue; the parameters of the charge balance phase (described below) can be similarly controlled. The patient and / or clinician can also optionally control the overall duration and pattern of treatment.

[0033] Stimulation parameters Table 1

[0034] Pulse amplitude: 0 to 25mA

[0035] Pulse frequency: 0 to 20000Hz

[0036] Pulse width: 0 to 2ms

[0037] The RF pulse generator module 106 may be initially programmed to meet each individual patient's specific parameter settings during the initial implantation procedure. Because medical conditions or tissue properties may change over time, the ability to readjust parameter settings may be beneficial to ensure continued efficacy of neuromodulation therapy.

[0038] Programmer module 102 is functionally a smart device and associated applications. The smart device hardware may include a CPU 206 and may be used as a medium to handle touch screen input on a graphical user interface (GUI) 204 for processing and storing data.

[0039] RF pulse generator module 106 may be connected to external TX antenna 110 via wired connection 108. Alternatively, both the antenna and RF pulse generator are positioned subcutaneously (not shown).

[0040] The signals sent by the RF pulse generator module 106 to the implanted neural stimulator 114 may include both power and parameter setting properties regarding the stimulation waveform, amplitude, pulse width, and frequency. The RF pulse generator module 106 may also serve as a wireless receiving unit that receives feedback signals from the implanted neural stimulator 114. To this end, the RF pulse generator module 106 may contain microelectronics or other circuitry to handle the generation of signals for the implanted neural stimulator 114 and to handle feedback signals (e.g., feedback signals from the implanted neural stimulator 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.

[0041] The controller subsystem 214 may include a CPU 230 for handling data processing, a local memory subsystem 228 (e.g., local memory), a communication subsystem 234 for communicating with the programmer module 102 (including receiving stimulation parameters from the programmer module), a pulse generator circuit system 236, and a digital-to-analog (D / A) converter 232.

[0042] The controller subsystem 214 may be used by the patient and / or clinician to control stimulation parameter settings (e.g., by controlling parameters of a signal sent from the RF pulse generator module 106 to the implanted neurostimulator 114). These parameter settings may affect, for example, the power, current level, or shape of one or more electrical pulses. Programming of stimulation parameters may be performed using the programmer module 102 (as described above) to set the repetition rate, pulse width, amplitude, and waveform to be transmitted by the RF energy to the receiving (RX) antenna 238 (typically a dipole antenna (but other types may be used)) in the implanted neurostimulator 114. The clinician may choose to lock and / or hide specific settings within the programmer interface, thereby limiting the patient's ability to view or adjust specific parameters, as adjusting specific parameters may require detailed medical knowledge of neurophysiology, neuroanatomy, neuromodulation protocols, and safety limits of electrical stimulation.

[0043] The controller subsystem 214 may store the received parameters in the local memory subsystem 228 until the parameters are modified by new data received from the programmer module 102. The CPU 206 may use the parameters stored in the local memory to control the RF pulse generator circuit system 236 to generate a pulse timing waveform modulated by the high frequency oscillator 218 in the range from 300 MHz to 8 GHz (preferably between about 700 MHz and 5.8 GHz and more preferably between about 800 MHz and 1.3 GHz). The resulting RF signal may then be amplified by the RF amplifier 226 and then sent to the TX antenna 110 through the RF switch 223 to pass through the tissue depth to the RX antenna 238.

[0044] In some embodiments, the RF signal transmitted by the TX antenna 110 may simply be a power transmission signal used by the wireless stimulation device to generate electrical pulses via the implanted neurostimulator 114. In other embodiments, a digital signal may also be transmitted to the implanted neurostimulator 114 to send instructions regarding the configuration of the implanted neurostimulator 114. The digital signal is used to modulate a carrier signal coupled to the implanted antenna 238 and does not interfere with input received on the same stimulator device to power the device. In one embodiment, the digital signal and the power signal are combined into one signal, wherein the digital signal is used to modulate the RF power signal, and thus the wireless stimulation device is powered directly by the received digital signal; a separate subsystem in the wireless stimulation device handles the power contained in the signal and interprets the data content of the signal.

[0045] The RF switch 223 may be a multi-purpose device, such as a dual redirecting coupler, which passes RF pulses to the TX antenna 110 with minimal insertion loss while providing two low-level outputs to the feedback subsystem 212; one output delivers a forward power signal to the feedback subsystem 212, where the forward power signal is an attenuated version of the RF pulse 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 the reverse power is an attenuated version of the RF energy reflected from the TX antenna 110. The RF energy and / or RF signal reflected from the implanted neurostimulator 114 is processed in the feedback subsystem 212.

[0046] 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 implanted neurostimulator 114 and / or RF energy reflected from a signal 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.

[0047] 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 way, the characteristics of the generated RF pulses can be compared to a reference signal within controller subsystem 214. If there is an inconsistency (error) in any parameter, controller subsystem 214 can make adjustments to the output of RF pulse generator module 106. The nature of the adjustment can be, for example, proportional to the calculated error. Controller subsystem 214 can incorporate additional inputs and constraints on its adjustment scheme, such as the signal amplitude of the reverse power and any predetermined maximum or minimum values ​​for various pulse parameters.

[0048] The reverse power signal can be used, for example, to detect fault conditions in the RF power delivery system. In ideal conditions, when the TX antenna 110 has a perfectly matched impedance to the tissue in contact with it, the electromagnetic waves generated from the RF pulse generator module 106 pass unimpeded from the TX antenna 110 into the body tissue. However, in real-world applications, there may be a large degree of variation in the user's body shape, 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 subcutaneous tissue and any intermediate materials, and also on the overall separation distance of the antenna from the skin, in any given application, there may be an impedance mismatch at the interface of the TX antenna 110 and the body surface. When this mismatch occurs, the electromagnetic waves sent from the RF pulse generator module 106 are partially reflected at this interface, and the energy of this reflection is propagated in the reverse direction through the antenna feed.

[0049] The dual redirection coupler RF switch 223 can prevent the reflected RF energy from propagating back into the amplifier 226, and can attenuate this 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 the 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 can indicate the severity of the impedance mismatch.

[0050] To sense the impedance mismatch condition, the controller subsystem 214 may measure the reflected power ratio in real time, and based on a preset threshold of this measurement, the controller subsystem 214 may modify the RF power level generated by the RF pulse generator module 106. For example, for moderate reflected power, the course of action may be for the controller subsystem 214 to increase the amplitude of the RF power sent to the TX antenna 110, as needed to compensate for the slightly less than ideal but acceptable TX antenna coupling to the body. For higher reflected power ratios, the course of action may be to prevent operation of the RF pulse generator module 106 and set a fault code to indicate that the TX antenna 110 has little or no coupling to the body. This type of reflected power fault condition may also be caused by a poor or broken connection to the TX antenna. In either case, it may be desirable to stop RF transmission when the reflected power ratio is above a defined threshold, because the internally reflected power may cause undesirable heating of internal components, and this fault condition means that the system is unable to deliver sufficient power to the implanted wireless stimulation device and therefore unable to provide therapy to the user.

[0051] The controller 242 of the implanted neurostimulator 114 can communicate with the RF pulse generator module 106 by transmitting information signals (e.g., telemetry signals) through the antenna 238. For example, the telemetry signal from the implanted neurostimulator 114 can be coupled to its dipole antenna 238. The antenna 238 can be connected to an electrode 254 that contacts the tissue to provide a return path for the reflected signal. An A / D (not shown) converter can be used to transfer the stored data to a sequenced pattern that can be transmitted on the pulse modulated signal from the internal antenna 238 of the implanted neurostimulator 114.

[0052] The telemetry signal from the implanted neurostimulator 114 may include stimulation parameters, such as the power or amplitude of the current delivered from the electrode to the tissue. The feedback signal may be transmitted to the RF pulse generator module 106 to indicate the stimulation intensity at the nerve bundle by coupling the signal to the implanted RX antenna 238, which radiates the telemetry signal to the external (or remotely implanted) RF pulse generator module 106. The feedback signal may include one or both of analog and digital telemetry pulse modulated carrier signals. Data such as stimulation pulse parameters and measured characteristics of stimulator performance may be stored in an internal memory device within the implanted neurostimulator 114 and may be sent via the telemetry signal. The frequency of the carrier signal may be in the range of 300 MHz to 8 GHz (preferably between about 700 MHz and 5.8 GHz, and more preferably between about 800 MHz and 1.3 GHz).

[0053] In the feedback subsystem 212, the telemetry signal may be down-modulated using a demodulator 222 and digitized by an analog / digital (A / D) converter 220. The digital telemetry signal may then be routed to a CPU 230 for interpretation. The CPU 230 of the controller subsystem 214 may compare the reported stimulation parameters with the stimulation parameters maintained in the local memory subsystem 228 to verify that the implanted neurostimulator 114 delivers the specified stimulation to the tissue. For example, if the wireless stimulation device reports that the current is lower than specified, the power level from the RF pulse generator module 106 may be increased so that the implanted neurostimulator 114 will have more power available for stimulation. The implanted neurostimulator 114 may alternatively generate telemetry data in real time, for example, at a rate of 8 Kbits per second. All feedback data received from the implanted neurostimulator 114 may be recorded for time and sampled to be stored for retrieval by a remote monitoring system that may be accessed by a health care professional.

[0054] The RF signal received by the internal antenna 238 may be conditioned into a waveform that is controlled by the controller 242 within the implanted neurostimulator 114 and routed to the appropriate electrodes 254 placed proximate to the tissue to be stimulated. For example, an RF signal transmitted from the RF pulse generator module 106 may be received by the RX antenna 238 and processed by circuitry (e.g., waveform conditioning circuitry 240) within the implanted neurostimulator 114 to be converted into electrical pulses that are applied to the electrodes 254 through the electrode interface 252. In some embodiments, the implanted neurostimulator 114 contains between two and sixteen electrodes 254.

[0055] The waveform conditioning circuitry 240 may include a rectifier 244. The rectified signal may be fed to a controller 250 for receiving encoded instructions from the RF pulse generator module 106. The rectifier signal may also be fed to a charge balancing component 246, which is configured to generate one or more electrical pulses such that the one or more electrical pulses produce charge balanced electrical stimulation at one or more electrodes. The charge balanced pulses pass through a current limiter 248 to an electrode interface 252, which optionally applies the pulses to an electrode 254.

[0056] The current limiter 248 ensures that the current level of the pulse applied to the electrode 254 is not higher than the threshold current level. In some embodiments, the amplitude (e.g., current level, voltage level, or power level) of the received RF pulse directly determines the amplitude of the stimulation. In this case, it may be particularly beneficial to include the current limiter 248 to prevent excess current or charge from being delivered through the electrode, but the current limiter 248 may be used in other embodiments. In general, for a given electrode having a surface area of ​​several square millimeters, it is the charge per phase (where the charge delivered by the stimulation phase is the integral of the current) that should be limited for safety reasons. However, in some cases, the limit may instead be applied only to the current amplitude. The current limiter 248 may automatically limit or "shorten" the stimulation phase to maintain the phase within safe limits.

[0057] The controller 250 of the implanted neurostimulator 114 may communicate with the electrode interface 252 to control various aspects of the electrode settings and the pulses applied to the electrodes 254. The electrode interface 252 may act as a multiplexer and control the polarity and switching of each of the electrodes 254. For example, in some embodiments, the implanted neurostimulator 114 has multiple electrodes 254 in contact with the tissue, and for a given stimulation, the RF pulse generator module 106 may designate one or more electrodes to 1) serve as a stimulation electrode, 2) serve as a return electrode, or 3) be inactive. The designation may be performed by sending instructions to the RF pulse generator module 106 of the implanted neurostimulator 114.

[0058] Moreover, in some embodiments, for a given stimulation pulse, the controller 250 can control the electrode interface 252 to divide the current among the designated stimulation electrodes. This control of electrode assignment and current control can be advantageous because, in practice, the electrodes 254 can be spatially distributed along various neural structures, and through strategic selection of stimulation electrode locations and current ratios designated for each location, the aggregate current distribution in the tissue can be modified to selectively activate designated neural targets. This current steering strategy can improve the therapeutic effect on the patient.

[0059] In another embodiment, the time course of stimulation can be manipulated. A given stimulation waveform can be started and terminated at a selected time, and this time course can be synchronized across all stimulation and return electrodes; in addition, the repetition frequency of this stimulation cycle for all electrodes can be synchronized. However, the controller 250, by itself or in response to instructions from the RF pulse generator module 106, can control the electrode interface 252 to specify one or more subsets of electrodes to deliver stimulation waveforms with asynchronous start and stop times, and can arbitrarily and independently specify the repetition frequency of each stimulation cycle.

[0060] In some embodiments, the controller 250 can shape the stimulation waveform amplitude arbitrarily, and it can do so in response to instructions from the RF pulse generator module 106. The stimulation phase can be delivered by a constant current source or a constant voltage source, and this type of control can produce a static characteristic waveform, for example, a constant current source produces a characteristic rectangular pulse, where the current waveform has a very steep rise, a constant amplitude for the duration of the stimulation, and then a very steep return to the baseline. Alternatively, or in addition, the controller 250 can increase or decrease the current level at any time during the stimulation phase and / or during the charge balance phase. Thus, in some embodiments, the controller 250 can deliver an arbitrarily shaped stimulation waveform, such as (for example) a triangular pulse, a sine pulse, or a Gaussian pulse. Similarly, the charge balance phase can be shaped by an arbitrary amplitude, and similarly, the leading anode pulse (before the stimulation phase) can also be amplitude shaped.

[0061] As described above, the implanted neurostimulator 114 may include a charge balancing component 246. The biphasic stimulation current pulses ensure that no net charge is present at the electrodes after each stimulation cycle, and the electrochemical processes are balanced to prevent net DC current. The implanted neurostimulator 114 may be configured to ensure that the resulting stimulation waveform has a net zero charge. Charge balanced stimulation is believed to have minimal damaging effects on tissue by reducing or eliminating electrochemical reaction products generated at the electrode-tissue interface.

[0062] In some embodiments, the charge balancing component 246 uses a DC blocking capacitor placed in electrical series with the stimulation electrodes and body tissue between the stimulation generation point within the stimulator circuitry and the stimulation delivery point to the tissue. In a multi-electrode stimulator, one charge balancing capacitor may be used for each electrode, or a centralized capacitor may be used within the stimulator circuitry before the electrode selection point. The stimulation waveform generated before the charge balancing capacitor, referred to as the drive waveform, may be controlled so that its amplitude changes during the duration of the drive pulse. The shape of the stimulation waveform may be modified in this manner to produce physiologically favorable stimulation.

[0063] In some implementations, the implanted neural stimulator 114 may generate a drive waveform envelope that follows the envelope of the RF pulses received by the receive dipole antenna 238. In this case, the RF pulse generator module 106 may directly control the envelope of the drive waveform within the implanted neural stimulator 114, and therefore, no energy storage may be required within the stimulator itself. In this implementation, the stimulator circuitry may modify the envelope of the drive waveform or may pass it directly to the charge balancing capacitor and / or electrode selection stage.

[0064] In some embodiments, the implanted neurostimulator 114 may deliver a single-phase drive waveform to the charge balancing capacitor or it may deliver a multi-phase drive waveform. In the case of a single-phase drive waveform (e.g., a negative-going rectangular pulse), this pulse includes a physiological stimulation phase, and the charge balancing capacitor is polarized (charged) during this phase. After the drive pulse is completed, the charge balancing function is performed alone by the passive discharge of the charge balancing capacitor, wherein its charge is dissipated through the tissue with a polarity opposite to the previous stimulation. In one embodiment, a resistor within the stimulator facilitates the discharge of the charge balancing capacitor.

[0065] In the case of a multi-phase drive waveform, the wireless stimulator can perform internal switching to pass either negative-going or positive-going pulses (phases) to the charge-balancing capacitor. These pulses can be delivered in any order and with different amplitudes and waveform shapes to achieve the desired physiological effect.

[0066] In some embodiments, the amplitude and timing of the stimulation and charge balancing phases are controlled by the amplitude and timing of the RF pulses from the RF pulse generator module 106, and in other embodiments, this control may be managed internally by on-board circuitry (e.g., controller 250) of the implanted neurostimulator 114. In the on-board control case, the amplitude and timing may be specified or modified by data commands delivered from the RF pulse generator module 106.

[0067] refer to Figures 3A to 3B, some embodiments use a microwave field stimulator (MFS) transmitter for wireless power transfer, as illustrated in system-level diagram 300. The MFS may include a digital signal processor 301, a gain control 302, a phase-locked loop 303, a gated amplifier 304, a phase-amplitude input matching network 305, a boost regulator 306, a radio frequency (RF) amplifier 307, a pulse amplitude harmonic filter 308, an antenna 309, a tissue boundary 310, a passive neurostimulator 311, a directional coupler 312, an analog-to-digital converter (ADC) 313, and a receiving dipole antenna 314. As illustrated, implanted electrodes may be used to deliver pulsating currents with controllable frequency, pulse width, and amplitude. A variety of therapeutic in vivo electrical stimulation techniques may be used to treat diseases known to respond to neuromodulation.

[0068] The digital signal processor 301 may generate pulse parameters such as pulse width, amplitude, and repetition rate. The digital signal processor 301 may feed the pulse parameters to a gain control 302 which may include a digital to analog converter (DAC). The gain control 302 may generate an RF envelope 302A to a gated amplifier 304. The digital signal processor 301 may feed a phase locked loop 303 under a stimulus timing control 301A, which is a voltage signal that drives a crystal XTAL 303 to generate an RF carrier burst 303B. The RF carrier burst 303B arrives at the gated amplifier to modulate the RF envelope 302A so that an RF pulse 304A is generated to feed a pulse amplitude input matching network 305.

[0069] The output from the pulse amplitude input matching network 305 is provided to an RF amplifier 307 under a bias voltage from a boost regulator 306. Subsequently, a harmonic filter 308 mitigates harmonic distortion and feeds the filtered output as a high power RF pulse to an antenna 309. The high power RF pulse is transmitted from the antenna 309 through the skin layer 310 to the receiver dipole antenna 314 of the passive neural stimulator 311 so that therapy is applied at the tissue electrodes.

[0070] In some implementations, the Rx antenna 238 and the Tx antenna 110 can exhibit mutual coupling. In some implementations, the mutual coupling of the Rx antenna 238 and the Tx antenna 110 can be observed for the purpose of evaluating the status of the implanted neurostimulator 114.

[0071] In some implementations, the estimated geometry factor may be included in a measurement normalization that may account for variations in the mutual coupling of various thicknesses of tissue separating the Rx antenna 238 and the Tx antenna 110 .

[0072] Some embodiments incorporate RF complex impedance measurement via a Γ sensor subsystem, which may include an RF phase detector 312A. Figure 3BThe reflected RF signal is received at antenna 110 and routed to analog-to-digital converter (ADC) 313 via directional coupler 312, and the RF impedance or reflection coefficient can be calculated from this signal.

[0073] In some embodiments, the wireless stimulation system 100 can utilize RF reflection measurements to obtain Figure 3A The impedance at the electrode-tissue interface. At the implantation site, the extracellular environment around the implanted electrode can be altered due to insertion-related injuries and the presence of the electrode (foreign material) in the tissue, both of which promote the formation of scar tissue, a tight cell sheath, and an extracellular matrix around the implant. Some studies have found that this encapsulated tissue can change the electrical impedance relative to normal (or uninjured) tissue. Since changes in electrode-tissue impedance can change the effectiveness of the implanted neural stimulator 114, it is powerful to have the wireless stimulation system 100 have the ability to assess the impedance of the electrode-tissue interface.

[0074] In some embodiments, based on the inferred impedance of the electrode-tissue interface, the stimulation strategy can be modified to compensate for the impedance. For example, if the electrode-tissue impedance is found to be highly resistive, the wireless stimulation system 100 can compensate by providing a higher voltage to the current driver within the implanted neurostimulator 114.

[0075] If through Figures 1 to 2 Discussed in detail, the RF signal 112 is received by the Rx antenna 238 of the stimulator and then rectified via an RF to DC bridge (rectifier 244) connected to the feed point of the Rx antenna. The received energy is stored in a capacitor implanted in the neural stimulator 114. The energy stored in the capacitor is a function of the charge held by the capacitor and the voltage across the capacitor.

[0076] In some embodiments, Figure 3B The signal received at the Γ sensor is processed to infer the charge state of the capacitor in the implanted neural stimulator 114. In more detail, the time-varying current and voltage at the rectifier and capacitor are used to produce a variable RF impedance at the feed point of the Rx antenna 238. When the capacitor has a low charge, the RF current flows freely across the rectifier 244, which is a near RF short circuit at the feed point. When the capacitor is close to fully charged, the RF current is hindered, so that there is a near RF open circuit at the feed point. It follows that the complex impedance at the feed point of the Rx antenna 238 is an indicator of the charge state of the capacitor in the implanted neural stimulator 114. Because the dynamic impedance at the Rx antenna is coupled to the Tx antenna 110, the impedance at the Rx antenna can be given by Figure 3BThe Γ sensor observes, and based on this measurement, the RF pulse generator module 106 can infer the charge of the capacitor implanted in the neural stimulator 114.

[0077] In some implementations, the transmitted RF signal 112 may be strategically selected to maintain the voltage on a capacitor in the implanted neurostimulator 114 at a desired constant level. For example, the RF pulse rate and width may be strategically selected to maintain a steady-state delivery of power to the stimulator so that energy is delivered at the same rate as it is consumed by the stimulator circuitry.

[0078] In some embodiments, the charge state of the capacitor implanted in the neurostimulator 114 is an indicator of the current operating state and environment of the stimulator. The voltage at the capacitor will decay in proportion to the rate at which energy is consumed by the load connected to the capacitor. The load can encompass the load at the electrodes of the stimulator (tissue) and the load of the circuitry associated with transferring charge from the capacitor to the electrodes.

[0079] In some embodiments, the rate at which charge is consumed from a capacitor in an implanted neurostimulator 114 depends on the stimulation parameters, the electrode-tissue load, and the internal circuitry of the stimulator. With this dependency, the rate at which charge is consumed from the capacitor can be used to determine the impedance of the electrode-tissue interface. The rate of charge consumption can reveal the RF impedance characteristics of the Rx antenna 238 from which the electrode-tissue impedance can be extracted. For example, if the electrode-tissue impedance is primarily resistive and sufficiently low (e.g., z=300 to 500Ω), then the expected stimulation current will be driven into the target tissue and the charge on the capacitor will be consumed at the expected rate. In contrast, if the electrode-tissue impedance has a high value resistance or is dominated by series capacitance, then the expected stimulation current may not be delivered. Examples of high value resistances are described below. Figure 4D If the programmed stimulation current is not delivered to the tissue, the charge on the capacitor will be depleted at a slower rate than expected. In both cases, the rate at which the charge is depleted can reveal the RF impedance characteristics of the Rx antenna, and from this rate, inferences can be made about the impedance of the electrode-tissue interface.

[0080] In some embodiments, circuitry implanted within the neurostimulator 114 may allow the current driver circuit to be connected to a calibrated internal load.

[0081] In some embodiments, the calibrated internal load in the implanted neurostimulator 114 can be programmed to a specific impedance value. In these embodiments, the calibrated internal load can be placed anywhere on the implanted neurostimulator 114.

[0082] In some embodiments, the implanted neurostimulator 114 may drive current into a calibrated internal load while either the current or the load is swept through a series of values, and a corresponding series of unique complex RF reflection coefficients may be captured for reference. Subsequently, when the stimulator is configured to drive current through an unknown electrode-tissue load, an RF reflection coefficient curve may be captured and compared to the series of reference curves. By matching the curve of the unknown electrode-tissue load to the curve of the known load, the electrode-tissue impedance may be inferred.

[0083] In some embodiments, circuitry implanted within the neurostimulator 114 can facilitate a system self-test to determine the suitability of the wireless stimulation system 100 to provide stimulation therapy. For example, for a system self-test, the stimulator can drive various currents to an internal load and, for each current level, sweep the average RF power while observing RF reflections. These measurements can be used as a reference during the self-test to compare with electrode-tissue load measurements.

[0084] For analytical purposes, it is useful to observe the change in the measured variable (RF reflection, or reverse RF voltage) relative to its initial value and the change in the independent variable (average RF power) relative to this initial value. For example, for the measured variable V, the change can be (VV min ). In addition, it may be advantageous to normalize the data. Figures 4A to 4B The data presented in the figure are normalized to reference measurements as follows: The maximum and minimum RF reflections (reverse RF voltage, V max and V min ) is extracted and the difference is used as a normalization factor. For example, the subsequent measured variable changes are normalized as follows:

[0085] v=(VV min ) / (V max -V min ),

[0086] Where V is the measured reverse RF voltage, and v (lowercase) implies normalization. Similarly, the independent variable change is normalized as follows:

[0087] p=(PP min ) / (P max -P min ),

[0088] Where P is the average transmitted RF power, and p (lower case) implies normalization.

[0089] Figure 4AReference measurements are shown, normalized change in RF reflection versus RF average power for three different currents driven into the internal load. The curves show: 1) minimum current through the internal load (circles), 2) median current through the internal load (triangles), 3) maximum current through the internal load (squares). In some cases, the minimum value may be about 0.1 mA; the median value may be about 6 mA; and the maximum value may be about 12 mA. In the case of low current, the charge on the capacitor steadily accumulates higher as the average RF power increases. This is indicated by the rising trajectory of the RF reflection on the graph (circles). In the case of the median current, the charge on the capacitor remains relatively flat as the average RF power increases. This is indicated by the relatively flat trajectory of the RF reflection on the graph (triangles). In the case of high current, the charge on the capacitor remains relatively flat (but at different levels) as the average RF power increases. This is indicated by the relatively flat trajectory of the RF reflection on the graph (squares) (at higher levels).

[0090] In some embodiments, the wireless stimulation system 100 self-test may include various fault checks. For example, when the implanted neural stimulator 114 is powered on but not programmed to drive stimulation current, the RF reflection should be similar to that for Figure 4A The minimum current condition shows RF reflection. Figure 4B , where the RF reflection of the unconfigured stimulator (dashed line) is superimposed on Figure 4A on the chart.

[0091] In addition, based on Figure 4A , it is feasible to systematically test for faults in the stimulation current driver (e.g., damaged electrodes). For example, the implanted neural stimulator 114 may be programmed to drive stimulation current through selected electrode pairs and compare the RF reflections of each pair to Figure 4A When the stimulator is programmed to drive a given stimulation current through the tissue, the RF reflection should be similar to the RF reflection when the same current is driven through the internal load. However, if, for example, the wire at the electrode is broken, then the current through the circuit may be blocked, meaning that the RF reflection should be similar to Figure 4A The minimum current curve. Figure 4C This is shown in Figure 4A Plot the reference measurements.

[0092] Furthermore, during normal operation, tissue impedance may be unknown, however, it will likely be within the expected range and a fault check can verify this. Figure 4DThe RF reflection is shown when a 500Ω resistor is connected in series with the electrode and a mid-value current is delivered (dashed line). The results show that the RF reflection falls within the expected range. However, if the results show, for example, RF reflection superimposed with a low current curve, then a fault would be obvious.

[0093] By capturing the reflected RF signal and applying the analysis methods described herein, it is possible to measure the impedance at the electrode-tissue interface. Furthermore, by measuring the impedance, the system can adjust the stimulation parameters to compensate, thereby maintaining the therapeutic effect of the stimulation.

[0094] Figure 5 An example of a flowchart 500 for implementing stimulation adjustments on an implantable wireless stimulator based on sensing of tissue electrode impedance is shown. As illustrated by step 502, a first set of radio frequency (RF) pulses are transmitted from an external pulse generator (e.g., RF pulse generator module 106) to an implantable wireless stimulator device (e.g., implanted neural stimulator 114) via non-inductive electrical radiation coupling. Figure 1 3, current is generated from the first set of RF pulses and flows through a calibrated internal load on the implantable wireless stimulator device. The calibrated internal load represents a load condition that is predetermined and imposed on (eg, electrodes on the implanted neurostimulator 114).

[0095] In response to the current flowing through the calibrated internal load, the flowchart 500 continues with recording a first set of RF reflection measurements on the external pulse generator 504. This recording measures the RF signal reflected from the implanted neural stimulator 114 and received by the RF pulse generator module 106, for example.

[0096] Next, a second set of radio frequency (RF) pulses is transmitted from the external pulse generator and via electrical radiation coupling to the implantable wireless stimulator device, so that stimulation current is generated from the second set of RF pulses and flows through the electrodes of the implantable wireless stimulator device to the tissue surrounding the electrodes 506. Here, the stimulation current flows through the stimulator circuitry, the electrodes, and the electrode-tissue interface.

[0097] In response to the stimulation current flowing through the electrodes to the surrounding tissue, a second set of RF reflection measurements are recorded on the external pulse generator 508. This second set of reflection measurements is based on the RF signal reflected from the implanted neural stimulator 114 and received by the RF pulse generator module 106.

[0098] By comparing the second set of RF reflection measurements with the first set of RF reflection measurements, the electrode-tissue impedance is characterized 510. When the electrode-tissue impedance is characterized as resistive, one or more input pulses to be transmitted by the external pulse generator to the implantable wireless stimulator device can be adjusted so that the stimulation current generated from these input pulses on the implantable wireless stimulator device is similarly adjusted to compensate for the resistive electrode-tissue impedance. When the electrode-tissue impedance is characterized as capacitive, one or more input pulses to be transmitted by the external pulse generator to the implantable wireless stimulator device can be adjusted so that the stimulation current generated from these input pulses on the implantable wireless stimulator device is similarly adjusted to compensate for the capacitive electrode tissue impedance. Here, the adjustment of the input pulses involves maintaining a steady-state delivery of power to the implantable wireless stimulator device so that electrical energy is extracted from the input pulses as fast as electrical energy is consumed to generate a stimulation current having one or more pulse parameters that have been changed to accommodate the resistive electrode-tissue impedance. Such stimulation current is delivered from the electrode to the surrounding tissue. Examples of pulse parameters include: pulse width, pulse amplitude, and pulse frequency.

[0099] Based on the results of the characterized electrode-tissue impedance, a stimulation session may be automatically selected. Selecting a session may include: determining an input pulse to be transmitted by an external pulse generator to the implantable wireless stimulator device such that a stimulation current is generated on the implantable wireless stimulator device and delivered to surrounding tissue by electrodes on the implantable wireless stimulator device in a manner that maintains therapy consistency, for example, even if electrode-tissue impedance changes. In one example, the second set of radio frequency (RF) pulses may be updated to obtain an updated second set of RF reflection measurements; and then, the updated second set of RF reflection measurements may be compared to the first set of RF reflection measurements. In this example, the updating and comparing steps may be iteratively performed until the desired RF reflection measurements are obtained.

[0100] The characterization step may also result in automatic fault checking based on the results from this characterization. In one example, the automatic fault checking includes: automatically detecting damaged wires in the circuit leading to electrodes on the implantable wireless stimulator device, such as (for example) Figure 4C In the description.

[0101] Several embodiments have been described. Nevertheless, it should be understood that various modifications can be made. Therefore, other embodiments are within the scope of the following claims.

Claims

1. A method for adjusting stimulation of surrounding tissue by an implantable wireless stimulator device, the method comprising: transmitting, from an external pulse generator and via electrical radiation coupling, one or more radio frequency input signals to the implantable wireless stimulator device, the one or more radio frequency input signals comprising energy to be applied to tissue surrounding one or more electrodes of the implantable wireless stimulator device; storing the energy from the one or more radio frequency input signals as charge in one or more capacitors of the implantable wireless stimulator device; receiving, at the external pulse generator, one or more reflected radio frequency signals representing a reflected radio frequency signal of each of the one or more transmitted radio frequency input signals; processing the one or more reflected RF signals to determine a rate at which charge of the one or more capacitors is depleted; and Electrode-tissue impedance is characterized based on the rate at which the charge is consumed.

2. The method according to claim 1, further comprising: In response to characterizing the electrode-tissue impedance as resistive, the one or more RF input signals to be transmitted by the external pulse generator to the implantable wireless stimulator device are adjusted so that stimulation pulses generated from the one or more RF input signals and delivered to the surrounding tissue through the one or more electrodes on the implantable wireless stimulator device are adjusted to compensate for the resistive electrode-tissue impedance.

3. The method of claim 2, wherein adjusting the one or more RF input signals comprises: Maintaining steady-state delivery of power to the implantable wireless stimulator device allows electrical energy to be extracted from the one or more RF input signals as quickly as electrical energy is consumed by the implantable wireless stimulator device to (i) generate the stimulation pulses having one or more pulse parameters that have been altered to accommodate the resistive electrode-tissue impedance, and (ii) deliver the stimulation pulses from the one or more electrodes on the implantable wireless stimulator device to the surrounding tissue.

4. The method of claim 3, wherein the pulse parameters include: Pulse width, pulse amplitude, and pulse frequency.

5. The method according to claim 1, further comprising: In response to characterizing the electrode-tissue impedance as capacitive, the one or more RF input signals to be transmitted by the external pulse generator to the implantable wireless stimulator device are adjusted so that stimulation pulses generated from the one or more RF input signals and delivered to the surrounding tissue through the one or more electrodes on the implantable wireless stimulator device are adjusted to compensate for the capacitive electrode-tissue impedance.

6. The method of claim 5, wherein adjusting the one or more RF input signals comprises: Maintaining steady-state delivery of power to the implantable wireless stimulator device allows electrical energy to be extracted from the one or more RF input signals as quickly as it is consumed by the implantable wireless stimulator device to (i) generate the stimulation pulses having one or more pulse parameters that have been altered to accommodate the capacitive electrode-tissue impedance, and (ii) deliver the stimulation pulses from the one or more electrodes on the implantable wireless stimulator device to the surrounding tissue.

7. The method of claim 6, wherein the pulse parameters include: Pulse width, pulse amplitude, and pulse frequency.

8. The method according to claim 1, further comprising: Based on the results of characterizing the electrode-tissue impedance, a stimulation session is automatically selected by determining the one or more RF input signals to be transmitted by the external pulse generator to the implantable wireless stimulator device, so that stimulation pulses are generated from the one or more RF input signals and delivered to the surrounding tissue through the one or more electrodes on the implantable wireless stimulator device.

9. The method of claim 1, wherein the one or more reflected radio frequency signals are indicative of a rate at which the charge of the one or more capacitors is consumed when the charge is applied to the surrounding tissue.

10. The method of claim 1, wherein a power level of each of the one or more radio frequency input signals when transmitted is different.

11. The method of claim 10, wherein the power level of each of the one or more RF input signals is increased compared to a power level of a previously transmitted RF input signal.

12. The method of claim 1, further comprising transmitting the one or more radio frequency input signals to maintain a desired voltage of the one or more capacitors.

13. A wireless stimulation system, comprising: An implantable wireless stimulator device comprising: a first non-inductive antenna; one or more electrodes; and one or more capacitors electrically connected to the first non-inductive antenna and to the one or more electrodes; An external pulse generator comprising: A second non-inductive antenna configured to: transmitting, via electrical radiation coupling, one or more radio frequency input signals to the first non-inductive antenna on the implantable wireless stimulator device, the one or more radio frequency input signals comprising energy to be applied to tissue surrounding the one or more electrodes of the implantable wireless stimulator device; and receiving a reflected radio frequency signal for each of the one or more radio frequency input signals transmitted; and a circuit coupled to the second non-inductive antenna and configured to: processing the one or more reflected radio frequency signals to determine a rate at which charge of the one or more capacitors is depleted; and Electrode-tissue impedance is characterized based on the rate at which the charge is consumed, wherein the one or more capacitors are configured to store energy transmitted with the one or more radio frequency input signals.

14. The wireless stimulation system of claim 13, wherein the circuit comprises: a directional coupler coupled to the second non-inductive antenna and configured to detect the one or more reflected radio frequency signals reflected from the first non-inductive antenna; and A radio frequency phase detector coupled to the directional coupler and configured to detect a phase difference between the one or more reflected radio frequency signals reflected from the first non-inductive antenna and the one or more radio frequency input signals transmitted from the second non-inductive antenna to the first non-inductive antenna.

15. The wireless stimulation system of claim 14, wherein the circuit further comprises: An analog-to-digital converter (ADC) is coupled to the directional coupler and is configured to convert the one or more reflected radio frequency signals reflected from the first non-inductive antenna into a digital record.

16. The wireless stimulation system of claim 13, wherein the circuit is further configured to: processing the one or more reflected radio frequency signals to determine a variable complex impedance of the first non-inductive antenna; inferring a variable state of charge of the one or more capacitors based on the variable complex impedance; and A rate at which the charge of the one or more capacitors is consumed is determined based on the variable state of charge of the one or more capacitors.

17. The wireless stimulation system of claim 13, wherein the circuit is further configured to: In response to characterizing the electrode-tissue impedance as resistive, one or more of the one or more RF input signals to be transmitted by the external pulse generator to the implantable wireless stimulator device are adjusted so that one or more stimulation pulses generated from the one or more RF input signals and delivered to surrounding tissue through the one or more electrodes on the implantable wireless stimulator device are adjusted to compensate for the resistive electrode-tissue impedance.

18. The wireless stimulation system of claim 13, wherein the circuit is further configured to: In response to characterizing the electrode-tissue impedance as capacitive, one or more of the one or more RF input signals to be transmitted by the external pulse generator to the implantable wireless stimulator device are adjusted so that the stimulation current generated from the one or more RF input signals and delivered to the surrounding tissue through the one or more electrodes on the implantable wireless stimulator device is adjusted to compensate for the capacitive electrode-tissue impedance.

19. The wireless stimulation system of claim 13, wherein the circuit is further configured to: Based on the results of characterizing the electrode-tissue impedance, a stimulation session is automatically selected by determining that the one or more RF input signals to be transmitted by the external pulse generator to the implantable wireless stimulator device allow stimulation pulses to be generated from the one or more RF input signals and delivered to surrounding tissue through the one or more electrodes on the implantable wireless stimulator device.

20. The wireless stimulation system of claim 13, wherein the one or more reflected radio frequency signals are indicative of a rate at which the charge of the one or more capacitors is consumed when the charge is applied to the tissue.

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