Systems and methods for compliance control in medical devices

Abstract

The implantable device may include an adjustable voltage source and a primary current source electrically coupled to tissue. For the implantable device to properly treat tissue, the voltage across the primary current source may need to exceed a compliance voltage. The implantable device may include an auxiliary current source and a control circuit configured to electrically connect one or more of the one or more auxiliary current sources in parallel with the primary current source if the voltage across the primary current source is less than the compliance voltage for the primary current source. If the voltage across the primary current source is less than the compliance voltage after the auxiliary current source is connected, the control circuit may adjust the supply voltage from the adjustable voltage source.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of priority to U.S. Provisional Patent Application No. 63 / 443,669, filed February 6, 2023, the contents of which are incorporated herein by reference in their entirety.

[0002] Technical Field FIELD OF THE DISCLOSURE The present disclosure relates to systems and methods for controlling voltage across components of medical devices, particularly components of implantable medical devices configured to electrically stimulate tissue of a subject. [Background technology]

[0003] Implantable medical devices can be used to electrically stimulate tissue in a patient to treat various medical conditions. For example, implantable devices may be used to stimulate cardiac tissue, the brain, peripheral nerves, or organs for therapeutic treatment.

[0004] Many implantable devices use a current source to electrically stimulate tissue. For the current source to function in this manner, the voltage across the current source must exceed a critical voltage value known as the compliance voltage. If the voltage across the current source in an implantable device falls below the compliance voltage, the current source will not deliver the desired amount of current to the tissue, and as a result, the patient may not receive the correct treatment.

[0005] Conventional methods for ensuring that current sources remain compliant involve driving the current sources at voltages much higher than their compliance voltage. However, driving current sources at high voltages requires high power draw from the power source powering the current sources. The lifetime of implantable devices, especially small devices with limited energy storage capabilities, may be significantly reduced if the power draw from the device's power source is substantially increased. Implantable devices with shorter lifetimes must be replaced or recharged frequently, which may impose significant mental, physical, and financial burdens on patients. Summary of the Invention

[0006] An implantable device is provided having a compliance control system for maintaining the voltage across a current source at a level sufficient to provide effective treatment to a patient while ensuring that power draw by the current source is minimized. If the implantable device detects that a current source is out of compliance, the device may connect one or more auxiliary current sources in parallel with the non-compliant current source. Connecting an auxiliary current source in parallel with the non-compliant current source may reduce the voltage required by the current source to deliver the desired current, thereby returning the current source to compliance. Alternatively, the device may directly adjust the gate voltage of the non-compliant current source. If the voltage across the current source remains below the compliance voltage after the auxiliary current source is connected or the gate voltage is adjusted, the implantable device may increase the power supply voltage until the current source returns to a compliant state.

[0007] The ensemble of voltage regulation techniques described above may allow for the voltage across a current source within an implanted device to be gradually varied to drive the current source into compliance without overtaxing the device's power reserves. By minimizing the energy required by the device to treat a patient, the systems and methods provided herein may reduce the rate at which the device needs to be recharged or replaced. In addition to the practical benefits provided by increased energy efficiency (e.g., reduced material consumption from the manufacture of implanted devices), the described implanted devices may significantly improve a patient's quality of life by reducing the amount of time they spend interacting with or otherwise thinking about their implant.

[0008] A first embodiment of an implantable device may include a primary current source electrically coupled to an adjustable voltage source and tissue, one or more auxiliary current sources configured to be electrically connected in parallel with the primary current source, and control circuitry configured to electrically connect one or more of the one or more auxiliary current sources in parallel with the primary current source if a voltage across the primary current source is less than a compliance voltage for the primary current source.

[0009] In one or more examples, a first embodiment of an implantable device includes a primary current sink configured to be electrically coupled to the primary current source through the tissue and one or more auxiliary current sinks configured to be electrically connected in parallel with the primary current sink, and the control circuit is configured to electrically connect one or more of the one or more auxiliary current sinks in parallel with the primary current sink if a voltage across the primary current sink is less than a compliance voltage for the primary current sink.

[0010] A second embodiment of the implantable device may include a primary current source electrically coupled to an adjustable voltage source and tissue, and a control circuit configured to adjust a gate voltage of the primary current source if a voltage across the primary current source is less than a compliance voltage for the primary current source.

[0011] In one or more examples, a second embodiment of the implantable device includes a primary current sink configured to be electrically coupled to the primary current source through the tissue, and the control circuit is configured to adjust a gate voltage of the primary current sink if a voltage across the primary current sink is less than a compliance voltage for the primary current sink.

[0012] In some examples of the first embodiment of the implantable device or the second embodiment of the implantable device, the primary current source is configured to function as a current sink.

[0013] In some examples of the first embodiment of the implantable device, the one or more auxiliary current sources are configured to function as a current sink.

[0014] In some examples of the first embodiment of the implantable device, the control circuit is configured to adjust the adjustable voltage source if the voltage across the primary current source is less than the compliance voltage for the primary current source after the one or more auxiliary current sources are connected in parallel with the primary current source.

[0015] In some examples of the first embodiment of the implantable device, the control circuit is configured to adjust the adjustable voltage source if the voltage across the primary current sink is less than the compliance voltage for the primary current sink after the one or more auxiliary current sinks are connected in parallel with the primary current sink.

[0016] In some examples of the second embodiment of the implantable device, the control circuit is configured to adjust the adjustable voltage source if the voltage across the primary current source after the gate voltage of the primary current source is adjusted is less than the compliance voltage for the primary current source.

[0017] In some examples of the second embodiment of the implantable device, the control circuit is configured to adjust the adjustable voltage source if the voltage across the primary current sink after the gate voltage of the primary current sink is adjusted is less than the compliance voltage for the primary current sink.

[0018] In some examples of the first or second embodiment of the implantable device, the adjustable voltage source is configured to receive energy from an external device.

[0019] In some examples of the first or second embodiment of the implantable device, the implantable device includes one or more ultrasound transducers, and the adjustable voltage source is configured to receive energy from ultrasound emitted by the external device.

[0020] In some examples of the first or second embodiments of the implantable device, the implantable device includes a radio frequency (RF) antenna, and the adjustable voltage source is configured to receive energy from RF emitted by the external device.

[0021] In some examples of the first or second embodiment of the implantable device, the control circuit is configured to perform periodic measurements of the adjustable voltage across the primary current source.

[0022] In some examples of the first or second embodiment of the implantable device, the control circuit is configured to perform a single measurement of the voltage across the primary current source after a predetermined period of time.

[0023] In some examples of the first or second embodiment of the implantable device, the control circuit is configured to perform periodic measurements of the voltage across the primary current sink.

[0024] In some examples of the first or second embodiment of the implantable device, the control circuit is configured to perform a single measurement of the voltage across the primary current sink after a predetermined period of time.

[0025] In some examples of the first or second embodiment of the implantable device, the control circuit is configured to receive a command from an external device configured to cause the control circuit to measure the voltage across the primary current source.

[0026] In some examples of the first or second embodiment of the implantable device, the implantable device includes an ultrasonic transducer and the control circuit receives the commands from ultrasonic waves emitted by the external device.

[0027] In some examples of the first or second embodiment of the implantable device, the implantable device includes a radio frequency (RF) antenna, and the control circuit is configured to receive the command from radio waves emitted by the external device.

[0028] In some examples of the first or second embodiment of the implantable device, the control circuit is configured to receive a command from an external device configured to cause the control circuit to measure the voltage across the primary current sink.

[0029] In some examples of the first or second embodiment of the implantable device, the implantable device includes an ultrasonic transducer and the control circuit receives the commands from ultrasonic waves emitted by the external device.

[0030] In some examples of the first or second embodiment of the implantable device, the implantable device includes a radio frequency (RF) antenna, and the control circuit is configured to receive the command from radio waves emitted by the external device.

[0031] In some examples of the first or second embodiment of the implantable device, the control circuitry is configured to communicate with an external device using ultrasound backscatter.

[0032] In some examples of the first or second embodiment of the implantable device, the control circuitry is configured to communicate with an external device using radio wave backscatter.

[0033] In some examples of the first or second embodiment of the implantable device, the primary current source is a transistor.

[0034] In some examples of the first or second embodiment of the implantable device, the primary current source comprises a PMOS transistor.

[0035] In some examples of the first or second embodiment of the implantable device, the primary current source comprises an NMOS transistor.

[0036] In some examples of the first or second embodiment of the implantable device, the primary current sink is a transistor.

[0037] In some examples of the first or second embodiment of the implantable device, the primary current sink is an NMOS transistor.

[0038] In some examples of the first or second embodiment of the implantable device, the primary current source is an active feedback current source.

[0039] In some examples of the first or second embodiment of the implantable device, the primary current sink is an active feedback current sink.

[0040] In some examples of the first or second embodiment of the implantable device, the primary current source is configured to provide a higher current than the one or more auxiliary current sources.

[0041] In some examples of the first embodiment of the implantable device, the one or more auxiliary current sources comprise one or more transistors.

[0042] In some examples of the first embodiment of the implantable device, the one or more auxiliary current sources comprise one or more active feedback current sources.

[0043] In some examples of the first embodiment of the implantable device, the one or more auxiliary current sources are each electrically coupled to one or more switches that are electrically coupled in parallel to the primary current source and configured to be controlled by the control circuit.

[0044] In some examples of the first embodiment of the implantable device, the primary current sink is configured to receive a higher current than the one or more auxiliary current sinks.

[0045] In some examples of the first embodiment of the implantable device, the one or more auxiliary current sinks comprise one or more transistors.

[0046] In some examples of the first embodiment of the implantable device, the one or more auxiliary current sinks comprise one or more active feedback current sinks.

[0047] In some examples of the first embodiment of the implantable device, the one or more auxiliary current sinks are each electrically coupled to one or more switches that are electrically coupled in parallel to the primary current sink and configured to be controlled by the control circuit.

[0048] In some examples of the first or second embodiment of the implantable device, the primary current source is coupled to a first electrode, and the first electrode is configured to be attached to the tissue.

[0049] In some examples of the first or second embodiment of the implantable device, the primary current sink is coupled to a second electrode, the second electrode being configured to be attached to the tissue.

[0050] In some examples of the first or second embodiment of the implantable device, the tissue is neural tissue.

[0051] In some examples of the first or second embodiment of the implantable device, the neural tissue is brain tissue.

[0052] In some examples of the first or second embodiment of the implantable device, the nerve tissue is a peripheral nerve.

[0053] In some examples of the first or second embodiment of the implantable device, the peripheral nerve is the splenic nerve.

[0054] In some examples of the first or second embodiment of the implantable device, the tissue is organ tissue.

[0055] In some examples of the first or second embodiment of the implantable device, the organ tissue is cardiac tissue.

[0056] In some examples of the first or second embodiment of the implantable device, the organ tissue is bladder tissue.

[0057] In some examples of the first or second embodiment of the implantable device, the organ tissue is stomach tissue.

[0058] In some examples of the first or second embodiment of the implantable device, the organ tissue is muscle tissue.

[0059] In some examples of the first or second embodiment of the implantable device, the compliance voltage for the primary current source depends on the electrical resistance of the tissue.

[0060] In some examples of the first or second embodiment of the implantable device, the compliance voltage for the primary current sink depends on the electrical resistance of the tissue.

[0061] In some examples of the first or second embodiment of the implantable device, the adjustable voltage source comprises one or more capacitors.

[0062] A first embodiment of a method for operating an implantable device may include providing a supply voltage to a primary current source electrically coupled to tissue; measuring a voltage across the primary current source; determining that the voltage across the primary current source is less than a compliance voltage for the primary current source; and electrically connecting one or more auxiliary current sources in parallel with the primary current source if the voltage across the primary current source is less than the compliance voltage for the primary current source.

[0063] In some examples, a first embodiment of a method includes providing a supply voltage to a primary current sink electrically coupled to the primary current source through the tissue, measuring a voltage across the primary current sink, determining that the voltage across the primary current sink is less than a compliance voltage for the primary current sink, and if the voltage across the primary current sink is less than the compliance voltage for the primary current sink, electrically connecting one or more auxiliary current sinks in parallel with the primary current sink.

[0064] A second embodiment of a method for controlling an implantable device may include providing a supply voltage to a primary current source electrically coupled to tissue, measuring a voltage across the primary current source, determining that the voltage across the primary current source is less than a compliance voltage for the primary current source, and adjusting a gate voltage of the primary current source if the voltage across the primary current source is less than a compliance voltage for the primary current source.

[0065] In some examples, a second embodiment of a method includes supplying the supply voltage to a primary current sink electrically coupled to the primary current source through the tissue, measuring a voltage across the primary current sink, determining that the voltage across the primary current sink is less than a compliance voltage for the primary current sink, and adjusting a gate voltage of the primary current sink if the voltage across the primary current sink is less than the compliance voltage for the primary current sink.

[0066] In some examples, a first embodiment of a method includes adjusting the supply voltage if the voltage across the primary current source is less than the compliance voltage for the primary current source after the one or more auxiliary current sources are connected in parallel with the primary current source.

[0067] In some examples, a first embodiment of a method includes adjusting the supply if the voltage across the primary current sink is less than the compliance voltage for the primary current sink after the one or more auxiliary current sinks are connected in parallel with the primary current sink.

[0068] In some examples, a second embodiment of the method includes adjusting the supply voltage if the voltage across the primary current source is less than the compliance voltage for the primary current source after the gate voltage for the primary current source is adjusted.

[0069] In some examples, a second embodiment of a method includes adjusting the supply voltage if the voltage across the primary current sink is less than the compliance voltage for the primary current sink after the gate voltage for the primary current sink is adjusted.

[0070] In some examples, the first or second embodiment of the method includes receiving energy from an external device.

[0071] In some examples of the first or second embodiment of the method, the energy is received from ultrasound emitted by the external device.

[0072] In some examples of the first or second embodiment of the method, the energy is received from radio waves emitted by the external device.

[0073] In some examples, the first or second embodiment of the method includes performing periodic measurements of the voltage across the primary current source.

[0074] In some examples, the first or second embodiment of the method includes performing a single measurement of the voltage across the primary current source after a predetermined period of time.

[0075] In some examples, the first or second embodiment of the method includes performing periodic measurements of the voltage across the primary current sink.

[0076] In some examples, the first or second embodiment of the method includes performing a single measurement of the voltage across the primary current sink after a predetermined period of time.

[0077] In some examples, the first or second embodiment of the method includes receiving a command from an external device indicating that the voltage across the primary current source should be measured, and measuring the voltage across the primary current source in response to receiving the command from the external device.

[0078] In some examples of the first or second embodiment of the method, the command is encoded in ultrasound waves emitted by the external device.

[0079] In some examples of the first or second embodiment of the method, the command is encoded in radio waves emitted by the external device.

[0080] In some examples, the first or second embodiment of the method includes receiving a command from an external device indicating that the voltage across the primary current sink should be measured, and measuring the voltage across the primary current sink in response to receiving the command from the external device.

[0081] In some examples of the first or second embodiment of the method, the command is encoded in ultrasound waves emitted by the external device.

[0082] In some examples of the first or second embodiment of the method, the command is encoded in radio waves emitted by the external device.

[0083] In some examples, the first or second embodiment of the method includes communicating with an external device using ultrasound backscatter.

[0084] In some examples, the first or second embodiment of the method includes communicating with an external device using radio wave backscatter.

[0085] In some examples of the first embodiment of the method, connecting the one or more auxiliary current sources in parallel with the primary current source includes controlling one or more switches electrically coupled to the one or more auxiliary current sources and electrically coupled in parallel with the primary current source.

[0086] In some examples of the first embodiment of the method, connecting the one or more auxiliary current sinks in parallel with the primary current sink includes controlling one or more switches electrically coupled to the one or more auxiliary current sinks and electrically coupled in parallel with the primary current sink.

[0087] In some examples of the first or second embodiment of the method, the tissue is neural tissue.

[0088] In some examples of the first or second embodiment of the method, the neural tissue is brain tissue.

[0089] In some examples of the first or second embodiment of the method, the nerve tissue is a peripheral nerve.

[0090] In some examples of the first or second embodiment of the method, the peripheral nerve is the splenic nerve.

[0091] In some examples of the first or second embodiment of the method, the tissue is organ tissue.

[0092] In some examples of the first or second embodiment of the method, the organ tissue is cardiac tissue.

[0093] In some examples of the first or second embodiment of the method, the organ tissue is bladder tissue.

[0094] In some examples of the first or second embodiment of the method, the organ tissue is stomach tissue.

[0095] In some examples of the first or second embodiment of the method, the organ tissue is muscle tissue.

[0096] In some examples of the first or second embodiment of the method, the compliance voltage for the primary current source depends on the electrical resistance of the tissue.

[0097] In some examples of the first or second embodiment of the method, the compliance voltage for the primary current sink depends on the electrical resistance of the tissue.

[0098] A first embodiment of a non-transitory computer-readable storage medium may store instructions for operating an implantable device that, when executed by control circuitry of the implantable device, cause the implantable device to: provide a supply voltage to a primary current source electrically coupled to tissue; measure a voltage across the primary current source; determine that the voltage across the primary current source is less than a compliance voltage for the primary current source; and, if the voltage across the primary current source is less than the compliance voltage for the primary current source, electrically connect one or more auxiliary current sources in parallel with the primary current source.

[0099] In some examples of the first embodiment of the non-transitory computer-readable storage medium, the instructions, when executed, cause the device to: supply a supply voltage to a primary current sink electrically coupled to the primary current source through the tissue; measure a voltage across the primary current sink; determine that the voltage across the primary current sink is less than a compliance voltage for the primary current sink; and, if the voltage across the primary current sink is less than the compliance voltage for the primary current sink, electrically connect one or more auxiliary current sinks in parallel with the primary current sink.

[0100] A second embodiment of a non-transitory computer-readable storage medium may store instructions for operating an implantable device that, when executed by control circuitry of the implantable device, cause the implantable device to provide a supply voltage to a primary current source electrically coupled to tissue, measure a voltage across the primary current source, determine that the voltage across the primary current source is less than a compliance voltage for the primary current source, and adjust a gate voltage of the primary current source if the voltage across the primary current source is less than the compliance voltage for the primary current source.

[0101] In some examples of the second embodiment of the non-transitory computer-readable storage medium, the instructions, when executed, cause the implantable device to supply the supply voltage to a primary current sink electrically coupled to the primary current source through the tissue, measure a voltage across the primary current sink, determine that the voltage across the primary current sink is less than a compliance voltage for the primary current sink, and adjust a gate voltage of the primary current sink if the voltage across the primary current sink is less than the compliance voltage for the primary current sink.

[0102] In some examples of the first embodiment of the non-transitory computer-readable storage medium, the instructions, when executed, cause the implantable device to: In some examples of the first embodiment of the implantable device, the control circuit adjusts the adjustable voltage source if the voltage across the primary current source is less than the compliance voltage for the primary current source after the one or more auxiliary current sources are connected in parallel with the primary current source.

[0103] In some examples of the first embodiment of the non-transitory computer-readable storage medium, the instructions, when executed, cause the embedded device to adjust the adjustable voltage source if the voltage across the primary current sink is less than the compliance voltage for the primary current sink after the one or more auxiliary current sinks are connected in parallel with the primary current sink.

[0104] In some examples of the second embodiment of the non-transitory computer-readable storage medium, the instructions, when executed, cause the embedded device to adjust the adjustable voltage source if the voltage across the primary current source is less than the compliance voltage for the primary current source after the gate voltage of the primary current source is adjusted.

[0105] In some examples of the second embodiment of the non-transitory computer-readable storage medium, the instructions, when executed, cause the embedded device to adjust the supply voltage if the voltage across the primary current sink is less than the compliance voltage for the primary current sink after the gate voltage for the primary current sink is adjusted.

[0106] In some examples of the first or second embodiment of the non-transitory computer-readable storage medium, the instructions, when executed, cause the implanted device to receive energy from an external device.

[0107] In some examples of the first or second embodiment of the non-transitory computer-readable storage medium, the energy is received from ultrasound waves emitted by the external device.

[0108] In some examples of the first or second embodiment of the non-transitory computer-readable storage medium, the energy is received from radio waves emitted by the external device.

[0109] In some examples of the first or second embodiment of the non-transitory computer-readable storage medium, the instructions, when executed, cause the implantable device to perform periodic measurements of the voltage across the primary current source.

[0110] In some examples of the first or second embodiment of the non-transitory computer-readable storage medium, the instructions, when executed, cause the implantable device to perform a single measurement of the voltage across the primary current source after a predetermined period of time.

[0111] In some examples of the first or second embodiment of the non-transitory computer-readable storage medium, the instructions, when executed, cause the implantable device to perform periodic measurements of the voltage across the primary current sink.

[0112] In some examples of the first or second embodiment of the non-transitory computer-readable storage medium, the instructions, when executed, cause the implantable device to perform a single measurement of the voltage across the primary current sink after a predetermined period of time.

[0113] In some examples of the first or second embodiment of the non-transitory computer-readable storage medium, the instructions, when executed, cause the implantable device to receive a command from an external device indicating that the voltage across the primary current source should be measured, and measure the voltage across the primary current source in response to receiving the command from the external device.

[0114] In some examples of the first or second embodiment of the non-transitory computer-readable storage medium, the command is encoded in ultrasound waves emitted by the external device.

[0115] In some examples of the first or second embodiment of the non-transitory computer-readable storage medium, the command is encoded in radio waves emitted by the external device.

[0116] In some examples of the first or second embodiment of the non-transitory computer-readable storage medium, the instructions, when executed, cause the embedded device to receive a command from an external device indicating that the voltage across the primary current sink should be measured, and measure the voltage across the primary current sink in response to receiving the command from the external device.

[0117] In some examples of the first or second embodiment of the non-transitory computer-readable storage medium, the command is encoded in ultrasound waves emitted by the external device.

[0118] In some examples of the first or second embodiment of the non-transitory computer-readable storage medium, the command is encoded in radio waves emitted by the external device.

[0119] In some examples of the first or second embodiment of the non-transitory computer-readable storage medium, the instructions, when executed, cause the implantable device to communicate with an external device using ultrasound backscatter.

[0120] In some examples of the first or second embodiment of the non-transitory computer-readable storage medium, the instructions, when executed, cause the embedded device to communicate with an external device using radio wave backscatter.

[0121] In some examples of the first or second embodiment of the non-transitory computer-readable storage medium, connecting the one or more auxiliary current sources in parallel with the primary current source includes controlling one or more switches electrically coupled to the one or more auxiliary current sources and electrically coupled in parallel with the primary current source.

[0122] In some examples of the first embodiment of the non-transitory computer-readable storage medium, connecting the one or more auxiliary current sinks in parallel with the primary current sink includes controlling one or more switches electrically coupled to the one or more auxiliary current sinks and electrically coupled in parallel with the primary current sink.

[0123] In some examples of the first or second embodiment of the non-transitory computer-readable storage medium, the tissue is neural tissue.

[0124] In some examples of the first or second embodiment of the non-transitory computer-readable storage medium, the neural tissue is brain tissue.

[0125] In some examples of the first or second embodiment of the non-transitory computer-readable storage medium, the nervous tissue is a peripheral nerve.

[0126] In some examples of the first or second embodiment of the non-transitory computer-readable storage medium, the peripheral nerve is a splenic nerve.

[0127] In some examples of the first or second embodiment of the non-transitory computer-readable storage medium, the tissue is organ tissue.

[0128] In some examples of the first or second embodiment of the non-transitory computer-readable storage medium, the organ tissue is cardiac tissue.

[0129] In some examples of the first or second embodiment of the non-transitory computer-readable storage medium, the organ tissue is bladder tissue.

[0130] In some examples of the first or second embodiment of the non-transitory computer-readable storage medium, the organ tissue is stomach tissue.

[0131] In some examples of the first or second embodiment of the non-transitory computer-readable storage medium, the organ tissue is muscle tissue.

[0132] In some examples of the first or second embodiment of the non-transitory computer-readable storage medium, the compliance voltage for the primary current source depends on the electrical resistance of the tissue.

[0133] In some examples of the first or second embodiment of the non-transitory computer-readable storage medium, the compliance voltage for the primary current sink depends on the electrical resistance of the tissue. [Brief explanation of the drawings]

[0134] Various aspects of the disclosed systems and methods are set forth with particularity in the appended claims. A better understanding of the features and advantages of the disclosed systems and methods will be obtained by reference to the detailed description of exemplary embodiments and the accompanying drawings.

[0135] [Figure 1] 1 illustrates an exemplary system including an implantable device configured to electrically stimulate tissue of a subject, according to some embodiments.

[0136] [Figure 2A] 1 illustrates an exemplary implantable device having components configured for compliance control, according to some embodiments.

[0137] [Figure 2B] 1 illustrates another exemplary implantable device having components configured for compliance control, according to some embodiments.

[0138] [Figure 3] 1 illustrates a cross-sectional view of an exemplary metal oxide semiconductor field effect transistor (MOSFET).

[0139] [Figure 4] 1 illustrates a cross-sectional view of an exemplary complementary metal-oxide-semiconductor (CMOS) transistor.

[0140] [Figure 5] 1 shows a circuit diagram of an exemplary common-emitter amplifier.

[0141] [Figure 6] 1 shows a circuit diagram of an exemplary current mirror.

[0142] [Figure 7A] 1 shows a schematic representation of a compliance control system for an implantable device, according to some embodiments.

[0143] [Figure 7B] 1 shows a schematic representation of a second compliance control system for an implantable device, according to some embodiments.

[0144] [Figure 8] 1 illustrates an exemplary method for compliance control in an implantable device, according to some embodiments.

[0145] [Figure 9] 1 illustrates another exemplary method for compliance control in an implantable device, according to some embodiments.

[0146] [Figure 10] 1 illustrates an exemplary method for electrically stimulating tissue using an implantable device having a compliance control system, according to some embodiments.

[0147] [Figure 11A]1 illustrates an exemplary external device for communicating with and providing energy to an implanted device, according to some embodiments.

[0148] [Figure 11B] 1 illustrates another exemplary external device for communicating with and providing energy to an implantable device, according to some embodiments. DETAILED DESCRIPTION OF THE INVENTION

[0149] Implantable devices for electrically stimulating a subject's tissue can be used to treat a variety of medical problems, ranging from chronic pain to cancer. These devices may use a current source to deliver electrical current to the tissue being treated. In order for the current source to deliver current, the voltage across the current source must be maintained at a level above a critical threshold, referred to as the compliance voltage. If the voltage across the current source within the implantable device falls below the compliance voltage for the current source, i.e., if the current source is out of compliance, the implantable device may not be able to properly provide therapy to the subject. Therefore, a compliance control system is essential for implantable devices to provide effective electrical stimulation therapy. Increasing the voltage output from the voltage source far above the compliance voltage can ensure that the current source maintains compliance, but it can cause a rapid depletion of available energy, which is not an appropriate solution for low-energy implantable devices.

[0150] Thus, systems and methods for compliance control in implantable medical devices are described. An ensemble of voltage regulation mechanisms may be employed to control the voltage across current sources within the implantable device. The provided systems and methods may ensure that the current sources remain compliant while electrical stimulation therapy is being delivered by the implant, while simultaneously ensuring that energy consumption by the current sources is minimized to maximize the lifespan of the implant. Furthermore, the described compliance control systems and methods may be employed to address imbalances in compliance voltage between multiple current sources present within a single implantable device, which may arise due to differences in characteristics between the multiple current sources or interactions between the current sources and the tissue being treated.

[0151] definition The term "current source" may be used hereinafter to refer to an electronic circuit or component thereof that delivers current, or an electrical circuit or component thereof that absorbs current. An electrical circuit or component thereof that absorbs current is also referred to as a "current sink" to further specify the function of a particular current source circuit. Any component referred to as a "current source" may, in some embodiments, function as a current sink.

[0152] As used in this document, the singular forms "a," "an," and "the" include plural references unless the context clearly dictates otherwise.

[0153] References herein to "about" a value or parameter include (and describe) a variation of that value or parameter per se. For example, a reference to "about X" includes a description of "X."

[0154] It is understood that aspects and variations of the invention described herein include "consisting of" and / or "consisting essentially of" aspects and variations.

[0155] When a range of values ​​or values ​​is provided, it is understood that each intervening value between the upper and lower limit of that range, and any other stated or intervening value in that stated range, is encompassed within the scope of the disclosure. When a stated range includes an upper or lower limit, ranges excluding any of the included limits are also included in the disclosure.

[0156] The section headings used herein are for organizational purposes only and should not be construed as limiting the subject matter described. The description is presented to enable one skilled in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the described embodiments will be readily apparent to those skilled in the art, and the generic principles herein may be applied to other embodiments. Thus, the present invention is not intended to be limited to the embodiments shown but is to be accorded the widest scope consistent with the principles and features described herein.

[0157] The drawings illustrate processes according to various embodiments. In the exemplary processes, some blocks are optionally combined, the order of some blocks is optionally changed, and some blocks are optionally omitted. In some instances, additional steps may be performed in combination with the exemplary processes. Accordingly, the operations described (and described in more detail below) are exemplary in nature and, therefore, should not be considered limiting.

[0158] Finally, it will be understood that the features and preferences described in connection with "embodiments" are separate preferences and not limited to that particular embodiment, but may be freely combined with features from other embodiments where technically feasible to form any preferred combination of features. The description is presented to enable one skilled in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the described embodiments will be readily apparent to those skilled in the art, and the general principles herein may be applied to other embodiments. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features described herein. Moreover, section headings are provided for organizational purposes and should not be construed as limiting.

[0159] The entire disclosures of the patents and publications referenced in this application are incorporated herein by reference for all purposes. To the extent that any reference incorporated by reference conflicts with the present disclosure, the present disclosure shall control.

[0160] Implantable devices for electrically stimulating tissue Implantable devices for electrically stimulating tissue of a subject may be used to treat a wide variety of medical problems (e.g., cancer, autoimmune diseases such as Crohn's disease, etc.). FIG. 1 provides a schematic representation of an implantable device for electrically stimulating tissue of a subject. As shown, implantable device 100 may be implanted within a patient's body 108. Implantable device 100 may include one or more electrodes 106 configured to attach to tissue 104 within body 108. Each electrode 106 may be electrically coupled to a current source within implantable device 100. The current source may deliver an electric current to tissue 104 to treat the tissue 104 for the medical condition.

[0161] The tissue 104 stimulated by the implantable device 100 may be nerve tissue, organ tissue, muscle tissue, connective tissue, or epithelial tissue. For example, the tissue 104 may be brain tissue, peripheral nerve tissue (e.g., splenic nerve tissue), heart tissue, bladder tissue, colon tissue, rectal tissue, or stomach tissue. The implantable device 100 may deliver an electric current to the tissue 104 to produce a physiological response in the tissue 104 to provide therapy to the patient. In some embodiments, the implantable device 100 may be completely implanted within the body 108, as shown in FIG. 1 , while in other embodiments, the implantable device 100 may be partially implanted within the body 108.

[0162] The overall size of implantable device 100 may be small (e.g., relative to the subject in which it is implanted or the tissue to which electrodes 106 are attached) to facilitate implantation of implantable device 100 in body 108. In its longest dimension, implantable device 100 may have a length of about 0.5-1.5 mm, about 1.5-2.5 mm, about 2.5-3.5 mm, about 3.5-4.5 mm, or about 4.5-5.5 mm. In some embodiments, the length of the longest dimension of implantable device 100 may be less than or equal to 0.5 mm, 1.0 mm, 1.5 mm, 2.0 mm, 2.5 mm, 3.0 mm, 3.5 mm, 4.0 mm, 4.5 mm, 5.0 mm, or 5.5 mm. The total volume of implantable device 100 may be about 0.5-1.5 mm. 3 , about 1.5~2.5mm 3 , about 2.5~3.5mm 3 , about 3.5~4.5mm 3 Or about 4.5 to 5.5 mm 3 In some embodiments, the total volume of the implantable device 100 may be less than 0.5 mm 3 , 1.0mm 3 , 1.5mm 3 , 2.0mm 3 , 2.5mm 3 , 3.0mm 3 , 3.5mm 3 , 4.0mm 3 , 4.5mm3 , 5.0mm 3 or 5.5 mm 3 It may be less than or equal to.

[0163] To minimize size, implantable device 100 may include only a small energy storage device (e.g., a small capacitor). In some implementations, the energy storage device is a capacitor. In some implementations, the energy storage device is a battery. This energy storage device may need to be periodically charged in order for implantable device 100 to provide therapy to the patient. Energy for charging the energy storage device may be provided to implantable device 100 by an external device 102. In some embodiments, external device 102 may be external to body 108 in which implantable device 100 is implanted, as shown in FIG. 1 . Implantable device 100 may be configured to receive energy from external device 102 whenever external device 102 is located in a location proximate to the location in body 108 where implantable device 100 is located. Alternatively, external device 102 may be a separate device that is (at least partially) implanted in body 108. In such a case, external device 102 may be programmed to periodically provide energy to implantable device 100. The external device 102 may also be configured to be remotely controlled by a device such as a smartphone or computer (e.g., the patient's smartphone) and may send energy to the implantable device 100 whenever an appropriate command is received from the remote device.

[0164] Compliance Control System To prevent interruptions to a patient's treatment, an implantable device, such as the implantable device 100 shown in FIG. 1, may include a system for compliance control of a current source that delivers current to tissue being treated. The compliance control system may include one or more components or mechanisms for monitoring the voltage across each current source within the implantable device. Whenever the voltage across a current source is determined to be lower than the compliance voltage across the current source, the compliance control system may be configured to incrementally adjust the voltage across the current source until the current source is returned to a compliant state. Initially, the adjustments to the voltage across the current source may be relatively small. If the small adjustments are insufficient to return the current source to compliance, one or more larger adjustments to the voltage across the current source may be made. Incrementally adjusting the voltage in the above manner may minimize energy consumption by the current source.

[0165] An exemplary schematic diagram of an implantable device 200 is provided in FIGS. 2A-2B. Similar to the implantable device 100 shown in FIG. 1, the implantable device 200 may be configured to be implanted in a subject to electrically stimulate tissue. When the implantable device 200 is implanted in a subject, an electrical current for stimulating the tissue may be delivered to the tissue by a primary current source 208, which may be configured to electrically couple to the tissue. In some embodiments, the electrical coupling between the primary current source 208 and the tissue may be facilitated by a first electrode 206a. The primary current source 208 may receive energy from an adjustable voltage source 218 that includes an energy storage unit 224. The adjustable voltage source may be any circuit or circuit component capable of providing an adjustable output voltage. In some implementations, the output voltage is digitally adjusted, for example, by a control circuit. Exemplary adjustable voltage sources include, but are not limited to, a DC-DC converter, an AC-DC converter, or an adjustable voltage regulator. In some implementations, the energy storage unit is a capacitor. In some implementations, the energy storage unit is a battery.

[0166] Optionally, implantable device 200 may include a primary current sink 210. Primary current sink 210 may be configured to electrically couple to primary current source 208 through tissue to absorb current being delivered through the tissue by primary current source 208. In some embodiments, electrical coupling between primary current sink 210 and tissue may be facilitated by second electrode 206b. Similar to primary current source 208, primary current sink 210 may receive energy from an adjustable voltage source 218.

[0167] A control circuit 204 in the implantable device 200 may be configured to monitor the voltage across the primary current source 208. In one example of an implantable device 200 (illustrated in FIG. 2A ), the control circuit 204 may be configured to electrically connect one or more auxiliary current sources 212 in parallel with the primary current source 208 if the control circuit 204 detects that the voltage across the primary current source 208 is less than the compliance voltage for the primary current source 208. The auxiliary current sources 212 may be current sources configured to deliver a current that is less than the current delivered by the primary current source 208. When the control circuit 204 electrically connects the auxiliary current sources 212 in parallel with the primary current source 208, the voltage required by the primary current source 208 to deliver the amount of current necessary to effectively stimulate tissue may be reduced, and thus, connecting a sufficient number of auxiliary current sources 212 to the primary current source 208 can force the primary current source 208 into a compliant state.

[0168] The total number of auxiliary current sources 212 available to connect to the primary current source 208 may depend on various factors, including the size of the implantable device 200, the amount of energy the energy storage unit 212 can store, and the type of treatment being provided by the implantable device 200. In some embodiments, the implantable device 200 may include at least one, at least two, at least three, at least four, at least five, at least ten, at least fifteen, or at least twenty auxiliary current sources 212. Alternatively, in some embodiments, the implantable device 200 may include fewer than two, fewer than three, fewer than four, fewer than five, fewer than ten, fewer than fifteen, or fewer than twenty auxiliary current sources 212. In some examples of the implantable device 200, each auxiliary current source 212 may be configured to deliver the same current, while in other examples of the implantable device 200, at least two auxiliary current sources 212 may be configured to deliver different currents. The total number of auxiliary current sources 212 and the current delivered by the auxiliary current sources 212 may affect the sensitivity of the adjustments that may be made by the control circuit 204 to the voltage across the primary current source 208 .

[0169] 2B , the control circuit 204 may not include the auxiliary current source 212. In such cases, the control circuit 204 may be configured to directly control the voltage of the primary current source 208 (e.g., by adjusting the gate voltage of the primary current source 208 if the primary current source 208 is implemented as a transistor) if the control circuit 204 detects that the voltage across the primary current source 208 is less than the compliance voltage for the primary current source 208. In some embodiments, the control circuit 204 may be configured to adjust the voltage at the primary current source 208 by about 0.001 V, about 0.005 V, about 0.010 V, about 0.020 V, about 0.030 V, about 0.040 V, about 0.050 V, about 0.060 V, about 0.070 V, about 0.080 V, about 0.090 V, about 0.100 V, about 0.500 V, about 1.00 V, about 5.00 V, or about 10.00 V. Optionally, the control circuit 204 may be configured to adjust the voltage at the primary current source 208 by more than 0.001 V, 0.010 V, 0.100 V, 1.00 V, or 10.00 V.

[0170] It is possible that connecting auxiliary current sources 212 in parallel with primary current source 208 (as shown in FIG. 2A) or adjusting the voltage at primary current source 208 (as shown in FIG. 2B) may not be sufficient to bring primary current source 208 into compliance. Control circuit 204 may be configured to adjust the voltage of adjustable voltage source 218 to increase the voltage across primary current source 208 if control circuit 204 continues to detect that the voltage across primary current source 208 is less than the compliance voltage for primary current source 208 after control circuit 204 has connected all auxiliary current sources 212 to primary current source 208 or after adjusting the voltage at primary current source 208 by a maximum amount. In some embodiments, control circuit 204 may be configured to adjust the voltage of adjustable voltage source 218 by about 0.001 V, about 0.005 V, about 0.010 V, about 0.020 V, about 0.030 V, about 0.040 V, about 0.050 V, about 0.060 V, about 0.070 V, about 0.080 V, about 0.090 V, about 0.100 V, about 0.500 V, about 1.00 V, about 5.00 V, or about 10.00 V. Optionally, control circuit 204 may be configured to adjust the voltage of adjustable voltage source 218 by more than 0.001 V, 0.010 V, 0.100 V, 1.00 V, or 10.00 V.

[0171] If implantable device 200 includes a primary current sink 210, control circuit 204 may similarly be configured to monitor and, if necessary, adjust the voltage across primary current sink 210. The compliance control mechanisms described above with respect to primary current source 208 may also be applied to primary current sink 210. Specifically, control circuit 204 may be configured to connect one or more auxiliary current sinks 214 in parallel with primary current sink 210 (FIG. 2A) if control circuit 204 detects that the voltage across primary current sink 210 is less than the compliance voltage for primary current sink 210, or to adjust the voltage at primary current sink 210 directly (FIG. 2B) if implantable device 200 does not include an auxiliary current sink. The control circuit 204 may be configured to adjust the voltage of the adjustable voltage source 218 to increase the voltage across the primary current sink 210 if the control circuit 204 continues to detect that the voltage across the primary current sink 210 is less than a compliance voltage for the primary current sink 210 after the control circuit 204 has connected all of the auxiliary current sinks 214 to the primary current sink 210 or after it has adjusted the voltage at the primary current sink 210 by a maximum amount.

[0172] The control circuitry 204 may be configured to control the flow and storage of energy and information within the implantable device 200, in addition to controlling the compliance control mechanism for the primary current source 208 (and, if present, the primary current sink 210). Similar to the implantable device 100 shown in FIG. 1 , the implantable device 200 may be configured to receive energy from and communicate with an external device 202. The external device 202 may be located in any area outside (i.e., separate from) the implantable device 200. For example, the external device 202 may be external to the subject in which the implantable device 200 is located. Alternatively, the external device may be implanted within the subject along with the implantable device 200, at a location within the subject that is spatially separated from the implantable device 200.

[0173] External device 202 may be configured to wirelessly transfer energy to implantable device 200. The energy transferred from external device 202 to implantable device 200 may be received by transducer 220. Transducer 220 may be configured to convert the energy received from external device 202 into an electric current. This electric current may be sent to AC / DC rectifier 216, which may convert the electric current from an alternating current (AC) signal to a direct current (DC) signal. The DC signal may then be sent to adjustable voltage source 218 to charge energy storage 224.

[0174] One or more examples of implantable device 200 may receive energy from ultrasound waves emitted by external device 202. Accordingly, transducer 220 may be an ultrasound transducer (e.g., a bulk piezoelectric transducer, a piezoelectric micromachined transducer, or a capacitive micromachined transducer) configured to convert energy carried by the ultrasound waves emitted by external device 202 into a current signal. Other examples of implantable device 200 may receive energy from radio frequency (RF) waves emitted by external device 202. In these examples, transducer 220 may include one or more RF antennas or one or more RF coils configured to convert energy carried by the RF waves into a current signal.

[0175] As described, control circuitry 204 may be configured to monitor the voltage across primary current source 208 (and, if applicable, the voltage across primary current sink 210). These voltage measurements may be at least temporarily stored by control circuitry 204. Additionally, control circuitry 204 may collect information related to implantable device 200, such as information regarding the timing or frequency at which tissue is stimulated by implantable device 200. A patient (or others involved in the patient's treatment, such as the patient's physician) may occasionally desire to read and evaluate the information gathered by control circuitry 204, for example, to ensure that implantable device 200 is functioning properly and providing appropriate therapy to the patient. Accordingly, control circuitry 204 may be configured to communicate with external device 202 using one or more components of implantable device 200.

[0176] In some embodiments, the transmission of information from implantable device 200 to external device 202 may rely on a backscatter communication protocol. As described, external device 202 may be configured to transmit a wireless signal (e.g., an ultrasound signal or an RF signal) to implantable device 200. When a patient desires to read information from implantable device 200, control circuitry 204 may cause modulation circuitry 222 to encode the information in a current flowing through implantable device 200. The encoded current may be sent from modulation circuitry 222 to transducer 220, which may convert the encoded current into the same type of coded wireless signal (e.g., an ultrasound signal or an RF signal) emitted by external device 202. This coded wireless signal (a “backscatter” signal) may be emitted by transducer 220 and then received by external device 202. The external device 202 may then decode the backscatter signal to read the encoded information.

[0177] In addition to or as an alternative to backscatter communication, the transfer of information from implantable device 299 to external device 202 may rely on an active communication protocol. In such an embodiment, control circuitry 204 may cause transducer 220 to generate and transmit signals that encode information. External device 202 may receive and decode the signals to read the encoded information.

[0178] Current Source Implementation There are various possible implementations of current sources, such as primary current source 208 and primary current sink 210, of implantable device 200 shown in Figures 2A-2B. In some embodiments, one or more of primary current source 208 and primary current sink 210 may be transistors. Exemplary transistors that may be utilized to implement primary current source 208 and / or primary current sink 210 include metal-oxide-semiconductor field-effect transistors (MOSFETs), such as NMOS transistors, PMOS transistors, and CMOS transistors.

[0179] An exemplary MOSFET 300 is shown in FIG. 3. The MOSFET 300 may include multiple semiconductor regions: a source region 306, a drain region 308, and a body region 310. The source region 306 and the drain region 308 may be of the same semiconductor type (e.g., doped with impurities that provide charge carriers of a first type), and the body region 310 may be of a second semiconductor type (e.g., doped with impurities that provide charge carriers of a second type). The gate electrode 302 may be spatially separated from the source region 306 and the drain region 308 by an insulating layer 304. When a voltage (a “gate” voltage) is applied between the gate electrode 302 and the source region 306, charge carriers in the body region 310 may be forced away from the gate electrode 302, and a conductive channel may be formed between the source region 306 and the drain region 308. If the voltage between the gate electrode 302 and the source region 306 is below a threshold voltage value (the "threshold voltage" for MOSFET 300), only a small current may flow between the source region 306 and the drain region 308 through the conduction channel. However, if the voltage between the gate electrode 302 and the source region 306 exceeds the threshold voltage, a current may be generated in the conduction channel. Increasing the voltage between the gate electrode 302 and the source region 306 above the threshold voltage may cause an increase in the current generated in the conduction channel.

[0180] The semiconductor regions that form a MOSFET may be n-type or p-type semiconductors. An n-type semiconductor may be an intrinsic semiconductor doped with an electron donor (e.g., phosphorus or arsenic) to form a semiconductor with electrons as the majority charge carriers. On the other hand, a p-type semiconductor may be an intrinsic semiconductor doped with an electron acceptor (e.g., boron or gallium) to form a semiconductor with electron acceptors (or "holes") as the majority charge carriers. A MOSFET with a p-type source region, an n-type body region, and a p-type drain region is a "PMOS" transistor, while a MOSFET with an n-type source region, a p-type body region, and an n-type drain region is an "NMOS" transistor.

[0181] 2A-2B, the primary current source 208 of the implantable device 200 may be a PMOS or NMOS transistor, while the primary current sink 210 may be an NMOS transistor. In such cases, the threshold voltage of the MOSFETs used to implement the primary current source 208 and / or the primary current sink 210 may affect the compliance voltage of the primary current source 208 and / or the primary current sink 210.

[0182] If the compliance control system of implantable device 200 includes auxiliary current source 212 (as shown in FIG. 2A ), auxiliary current source 212 may be implemented using the same MOSFET type used to implement primary current source 208 (e.g., auxiliary current source 212 may comprise PMOS or NMOS transistors). Similarly, if the compliance control system of implantable device 200 includes auxiliary current sink 214, auxiliary current sink 214 may be implemented using the same MOSFET type used to implement primary current sink 210, i.e., NMOS transistors.

[0183] The current source may also be implemented using complementary metal-oxide-semiconductor (CMOS) transistors, a type of MOSFET formed from a combination of a PMOS transistor and an NMOS transistor. An exemplary CMOS transistor 400 is shown in FIG. 4. The CMOS transistor 400 may include a p-type body region 410 modified to include an n-type well 412. The NMOS transistor 402 may be formed on the p-type body region 410, and the PMOS transistor 404 may be formed on the n-type well 412. If a high voltage is applied to the NMOS gate 406 and the PMOS gate 408, the NMOS transistor 402 conducts current and the PMOS transistor 404 does not conduct current. Alternatively, if a low voltage is applied to the NMOS gate 406 and the PMOS gate 408, the NMOS transistor 402 does not conduct current and the PMOS transistor 404 does conduct current.

[0184] If implantable device 200 does not include auxiliary current source 212 or auxiliary current sink 214 (as shown in FIG. 2B ), control circuit 204 may be configured to directly adjust the voltage at primary current source 208 and / or primary current sink 210 when control circuit 204 determines that primary current source 208 and / or primary current sink 210 are out of compliance. If one or more of primary current source 208 and primary current sink 210 are implemented using MOSFETs, adjusting the voltage at primary current source 208 and / or primary current sink 210 may include adjusting the gate voltage (i.e., the voltage between the gate and source regions) of the respective MOSFETs.

[0185] In other examples of implantable device 200, one or more of primary current source 208 and primary current sink 210 may be implemented using current sources (or sinks) that can dynamically adjust their resistance to account for changes in current. Such current sources may be circuits that include transistors, such as MOSFETs.

[0186] Circuit diagrams of exemplary current sources are shown in Figures 5 and 6. Specifically, Figure 5 shows a circuit diagram of a common-source amplifier, and Figure 6 shows a circuit diagram of a current mirror (which may have various implementations, including a cascade implementation, a low-power implementation, a Wilder implementation, a Wilson implementation, etc.). Other examples of circuits that can function as current sources include operational amplifier current sources, linear dropout regulators, common-base amplifiers, common-gate amplifiers, and constant current diodes.

[0187] If primary current source 208 is implemented using an active current source and implantable device 200 includes auxiliary current source 212 (as shown in FIG. 2A), auxiliary current source 212 may also be implemented using an active current source. Similarly, if primary current sink 210 is implemented using an active current sink and implantable device 200 includes auxiliary current sink 214, auxiliary current sink 214 may also be implemented using an active current sink.

[0188] Compliance Control Circuit As described, an implantable device (e.g., implantable device 200 shown in FIGS. 2A-2B) may be configured to incrementally adjust the voltage across a primary current source if the primary current source is out of compliance. In some embodiments, incrementally adjusting the voltage across the primary current source includes connecting one or more auxiliary current sources in parallel with the primary current source to drive the primary current source into a compliant state by reducing the current required by the primary current source to deliver an effective stimulation current. In other examples, incrementally adjusting the voltage across the primary current source involves directly adjusting the voltage at the primary current source (e.g., by adjusting the gate voltage of the primary current source).

[0189] FIG. 7A shows a circuit diagram of an exemplary system for controlling the compliance of a primary current source 704 and a primary current sink 706 in an implantable device. The primary current source 704 may be electrically coupled to an adjustable voltage source 702. When the primary current source 704 is compliant, the primary current source 704 may deliver a stimulation current to tissue 712 (represented as a resistance in FIG. 7 ). The primary current sink 706 may be electrically coupled to the primary current source 704 through the tissue 712. When the primary current sink 706 is compliant, the primary current sink 706 may absorb the current delivered by the primary current source 704 after passing through the tissue 712. In some embodiments, capacitors 708 and 710 may be located in the current flow path between the primary current source 704 and the tissue 712 and between the tissue 712 and the primary current sink 706, respectively, for charge balance.

[0190] If the voltage across the primary current source 704 falls below the compliance voltage for the primary current source 704, one or more auxiliary current sources 714 may be electrically connected in parallel with the primary current source 704. The auxiliary current sources 714 may be connected by a control circuit of the implantable device (e.g., the control circuit 204 shown in FIGS. 2A-2B ). In some embodiments, each auxiliary current source 714 may be electrically coupled to a switch 716, which may then be electrically connected in parallel with the primary current source 704. The control circuit of the implantable device may control the connection between the auxiliary current source 714 and the primary current source 704 by closing the switch 716. If connecting the auxiliary current source 714 does not return the primary current source 704 to compliance, the control circuit may adjust the voltage of the adjustable voltage source 702.

[0191] Similarly, one or more auxiliary current sinks 718 may be electrically connected in parallel with the primary current sink 706 if the voltage across the primary current sink 706 falls below the compliance voltage for the primary current sink 706. The auxiliary current sinks 718 may be connected by the implantable device's control circuitry (e.g., the control circuit 204 shown in FIGS. 2A-2B ). In some embodiments, each auxiliary current sink 718 may be electrically coupled to a switch 720, which may then be electrically connected in parallel with the primary current sink 706. The implantable device's control circuitry may control the connection between the auxiliary current sinks 714 and the primary current source 706 by closing the switch 720. If connecting the auxiliary current sinks 718 does not return the primary current sink 706 to compliance, the control circuitry may adjust the voltage of the adjustable voltage source 702.

[0192] 7B shows a circuit diagram of a second exemplary system for controlling compliance of a primary current source 704 and a primary current sink 706 in an implantable device. In this case, the control circuitry of the implantable device may be configured to directly adjust the voltage at the primary current source 704 and / or the primary current sink 706 whenever either the primary current source 704 or the primary current sink 706 is out of compliance, rather than incrementally adjusting the voltage across the primary current source 704 and / or the primary current sink 706 by connecting an auxiliary current source 714 and / or an auxiliary current sink 718.

[0193] Compliance Control Method An exemplary compliance control method 800 is shown in Figure 8. One or more steps of 800 may be performed by one or more components (e.g., a control circuit such as control circuit 204 shown in Figures 2A-2B) of an implantable device for providing electrical stimulation therapy (e.g., implantable device 200 shown in Figures 2A-2B) to control the compliance of a primary current source and a primary current sink used by the implantable device to deliver electrical current to tissue being treated.

[0194] To initiate treatment of the tissue, a supply voltage may be applied to a primary current source electrically coupled to the tissue (step 802). The supply voltage may be provided by an adjustable voltage source within the implantable device electrically coupled to the primary current source. In some embodiments, the adjustable voltage source may be controlled by control circuitry of the implantable device.

[0195] Initially, the supply voltage may be the maximum voltage that the adjustable voltage source is configured to provide. Once the maximum supply voltage is supplied to the primary current source, the control circuit may reduce the supply voltage (step 804) and monitor the voltage across the primary current source (step 806). In some embodiments, the supply voltage may be reduced by approximately 0.01 V, approximately 0.1 V, approximately 0.5 V, or approximately 1 V. In some embodiments, the supply voltage may be reduced by 0.01 V or less, 0.1 V or less, 0.5 V or less, or 1 V or less. In some embodiments, the supply voltage may be reduced by 0.01 V or more, 0.1 V or more, 0.5 V or more, or 1 V or more.

[0196] Measurement of the voltage across the primary current source may be performed periodically at predetermined times after the supply voltage is initially applied to the primary current source in step 802. For example, the first measurement of the voltage across the primary current source may be performed about 1 second, about 5 seconds, about 10 seconds, about 30 seconds, about 1 minute, about 2 minutes, about 3 minutes, about 5 minutes, about 10 minutes, about 30 minutes, about 1 hour, about 5 hours, about 10 hours, or about 1 day after the voltage is initially applied to the primary current source in step 802. In some embodiments, the first measurement of the voltage across the primary current source may be performed less than 1 second or more than 1 day after the voltage is initially applied to the primary current source in step 802. In other embodiments, the implantable device may be configured to receive a command from an external device indicating that the voltage across the primary current source should be measured. Measurement of the voltage across the primary current source may be performed whenever such a command is received.

[0197] In some embodiments, the timing of measuring the voltage across the primary current source may depend on the treatment the implantable device is providing or the amount of time that has elapsed since the initial implantation of the implantable device. For example, immediately after implantation of an implantable device, the voltage requirements of the primary current source may be unstable as the tissue surrounding the implantable device heals (e.g., due to scar tissue accumulation or tissue encapsulation). To ensure that the primary current source remains compliant, the voltage across the primary current source may be measured frequently. As time passes and the subject's body recovers from implantation, the voltage requirements of the primary current source may stabilize, and therefore, the voltage across the primary current source may need to be measured less frequently.

[0198] Monitoring the voltage across the primary current source may include, for each measurement of the voltage across the primary current source, determining whether the voltage across the primary current source is less than a compliance voltage for the primary current source. In some embodiments, the compliance voltage for the primary current source may be a predetermined voltage value stored in control circuitry of the implantable device. The compliance voltage may depend on characteristics of the primary current source (e.g., if the primary current source includes a MOSFET, the compliance voltage may depend on a threshold voltage for the primary current source), treatment requirements (e.g., amperage or power required to stimulate tissue to provide appropriate therapy to the patient), and tissue characteristics (e.g., electrical resistance of the tissue). While monitoring the voltage across the primary current source, the control circuitry may compare the measured voltage across the primary current source to the stored compliance voltage value to determine whether the primary current source is compliant.

[0199] If the voltage across the primary current source is greater than or equal to the compliance voltage for the primary current source (i.e., if the primary current source is compliant / compliant, as indicated by the label "C" in FIG. 8 ), method 800 may return to step 804, and the supply voltage may be further reduced. On the other hand, if the voltage across the primary current source is less than the compliance voltage for the primary current source (i.e., if the primary current source is out of compliance / non-compliant, as indicated by the label "NC" in FIG. 8 ), method 800 may proceed to step 808, and an auxiliary current source may be electrically connected in parallel with the primary current source to reduce the voltage required by the primary current source to supply the desired stimulation current. The implanted device's control circuitry may connect the auxiliary current source to the primary current source by controlling one or more switches.

[0200] After the auxiliary current sources are connected to the primary current sources in step 808, the voltage across the primary current sources may again be measured (step 810), and a determination may be made as to whether the voltage across the primary current sources is less than the compliance voltage for the primary current sources. If the voltage across the primary current sources is greater than or equal to the compliance voltage for the primary current sources (i.e., if the primary current sources are in a compliant (“C”) state), method 800 may return to step 806. The voltage across the primary current sources may be monitored throughout the treatment period to ensure that the primary current sources remain compliant. However, if the voltage across the primary current sources is still below the compliance voltage (i.e., if the primary current sources are in a non-compliant (“NC”) state) and one or more auxiliary current sources remain unconnected to the primary current sources, method 800 may return to step 808. Method 800 may repeat steps 808-810 until the primary current sources are made compliant or until all of the available auxiliary current sources are connected to the primary current sources.

[0201] If the primary current source remains out of compliance after all of the available auxiliary current sources have been connected to the primary current source, the supply voltage may be incrementally increased (step 822). The control circuit may increase the supply voltage by approximately 0.001 V, approximately 0.01 V, approximately 0.1 V, approximately 1 V, approximately 2 V, approximately 3 V, approximately 4 V, or approximately 5 V. The voltage across the primary current source may then be measured (step 826). If the primary current source remains in a non-compliant (“NC”) state, method 800 may return to step 822 and the supply voltage may again be incrementally increased. If the primary current source is in a compliant (“C”) state, method 800 may proceed to step 828 or step 830.

[0202] If method 800 proceeds to step 828, all of the auxiliary current sources that were electrically connected to the primary current source before step 822 may be disconnected from the primary current source. This may cause the primary current source to become non-compliant. One by one, the auxiliary current sources may be reconnected in parallel with the primary current source while the voltage across the primary current source is monitored until the primary current source returns to compliance. This may ensure that only the auxiliary current sources necessary for the primary current source to remain compliant are connected, which may in turn reduce the total energy consumption of the implanted device.

[0203] Alternatively, if method 800 proceeds to step 830, the auxiliary current sources that were electrically connected to the primary current source before step 822 may be disconnected from the primary current source one by one. The voltage across the primary current source may be monitored while each auxiliary current source is disconnected. If disconnecting an auxiliary current source causes the primary current source to become non-compliant, the most recently disconnected auxiliary current source may be reconnected in parallel with the primary current source to return the primary current source to a compliant state. Similar to step 828, this may ensure that only the auxiliary current sources necessary for the primary current source to remain compliant are connected, which may in turn reduce the total energy consumption of the implanted device.

[0204] As shown in FIG. 8 , method 800 may include steps for controlling the compliance of the primary current sink as well as the compliance of the primary current source. These steps may be substantially identical to steps 802-810 and, in some embodiments, may be performed in parallel with steps 802-810. First, in step 812, a supply voltage may be supplied to the primary current sink, which may be electrically coupled to the primary current source through the tissue undergoing treatment. This initially supplied voltage may be the maximum voltage that the adjustable voltage source is configured to provide. The supply voltage may be incrementally reduced (step 814), and the voltage across the primary current sink may be monitored (step 816). Step 816, similar to step 804, may be performed periodically after the supply voltage is initially supplied to the primary current sink. In some embodiments, the timing of step 816, similar to step 804, may depend on the treatment the implanted device is providing or the amount of time that has elapsed since the implanted device was initially implanted.

[0205] Based on the measurement of the voltage across the primary current sink, a determination may be made as to whether the voltage across the primary current sink is less than a compliance voltage for the primary current sink. In some embodiments, the compliance voltage for the primary current sink may be a predetermined voltage value stored in the control circuitry of the implantable device. The compliance voltage may depend on the characteristics of the primary current sink (e.g., if the primary current sink includes a MOSFET, the compliance voltage may depend on a threshold voltage for the primary current sink), the treatment requirements (e.g., the amperage or power required to stimulate the tissue to provide appropriate therapy to the patient), and the characteristics of the tissue (e.g., the electrical resistance of the tissue).

[0206] If the voltage across the primary current sink is greater than or equal to the compliance voltage for the primary current sink (i.e., if the primary current sink is compliant / compliant, as indicated by the label "C" in FIG. 8), method 800 may return to step 814, and the supply voltage may be further reduced. If the voltage across the primary current sink is less than the compliance voltage for the primary current sink (i.e., if the primary current sink is out of compliance / non-compliant, as indicated by the label "NC" in FIG. 8), method 800 may proceed to step 818, and an auxiliary current sink may be electrically connected in parallel with the primary current sink to reduce the voltage required by the primary current sink. The implantable device's control circuitry may connect the auxiliary current sink to the primary current sink by controlling one or more switches.

[0207] After the auxiliary current sink is connected to the primary current sink in step 818, the voltage across the primary current sink may again be measured (step 820), and a determination may be made as to whether the voltage across the primary current sink is less than the compliance voltage for the primary current sink. If the voltage across the primary current sink is equal to or greater than the compliance voltage for the primary current sink, method 800 may return to step 816. The voltage across the primary current sink may be monitored throughout the treatment period to ensure that the primary current sink remains compliant. However, if the voltage across the primary current sink remains below the compliance voltage and one or more auxiliary current sinks remain unconnected to the primary current sink, method 800 may return to step 818. Method 800 may repeat steps 818-820 until the primary current sink is made compliant or until all of the available auxiliary current sinks are connected to the primary current sink.

[0208] If the primary current sink remains out of compliance after all of the available auxiliary current sinks have been connected to the primary current sink, the supply voltage may be incrementally increased (step 822). The control circuit may increase the supply voltage by approximately 0.001 V, approximately 0.01 V, approximately 0.1 V, approximately 1 V, approximately 2 V, approximately 3 V, approximately 4 V, or approximately 5 V. The voltage across the primary current sink may then be measured (step 832). If the primary current sink remains in a non-compliant (“NC”) state, method 800 may return to step 822 and the supply voltage may again be incrementally increased. If the primary current sink is in a compliant (“C”) state, method 800 may proceed to step 834 or step 836.

[0209] If method 800 proceeds to step 834, all of the auxiliary current sinks that were electrically connected to the primary current sink before step 822 may be disconnected from the primary current sink. This may cause the primary current sink to become non-compliant. One by one, the auxiliary current sinks may be reconnected in parallel with the primary current sink while the voltage across the primary current sink is monitored until the primary current sink returns to compliance. This may ensure that only the auxiliary current sinks necessary for the primary current sink to remain compliant are connected, which may in turn reduce the total energy consumption of the implantable device.

[0210] Alternatively, if method 800 proceeds to step 836, the auxiliary current sinks that were electrically connected to the primary current sink before step 822 may be disconnected from the primary current sink one-by-one. The voltage across the primary current sink may be monitored while each auxiliary current sink is disconnected. If disconnecting an auxiliary current sink causes the primary current sink to become non-compliant, the most recently disconnected auxiliary current sink may be reconnected in parallel with the primary current sink to return the primary current sink to a compliant state. Similar to step 834, this may ensure that only auxiliary current sinks necessary for the primary current sink to remain compliant are connected, which may in turn reduce the total energy consumption of the implanted device.

[0211] A second exemplary method 900 for compliance control is shown in FIG. 9. Method 900 may be used to control the compliance of a current source that includes a MOSFET and consequently has an associated gate voltage. Similar to method 800 shown in FIG. 8, method 900 may be applied to both the primary current source (steps 902-910 and 922) and the primary current sink (steps 912-922). Steps 902-906 and parallel steps 912-916 are substantially identical to steps 802-806 and steps 912-816, respectively, of method 800.

[0212] Method 900 branches from method 800 at steps 908 / 918. If it is determined in step 906 that the voltage across the primary current source is less than the compliance voltage for the primary current source, then the gate voltage of the primary current source may be adjusted in step 908. Similarly, if it is determined in step 916 that the voltage across the primary current sink is less than the compliance voltage for the primary current sink, then the gate voltage of the primary current sink may be adjusted in step 918. In some embodiments, the gate voltage of the primary current source and / or primary current sink may be adjusted in small voltage increments, for example, in increments of about 0.001 V, about 0.01 V, about 0.1 V, about 1 V, or about 5 V.

[0213] After the gate voltage of the primary current source is adjusted in step 908, the voltage across the primary current source may be measured again (step 910), and a determination may be made as to whether the voltage across the primary current source is less than the compliance voltage for the primary current source. If the voltage across the primary current source is equal to or greater than the compliance voltage for the primary current source, method 900 may return to step 910. The voltage across the primary current source may be measured periodically to ensure that the primary current source remains compliant while the implantable device is delivering treatment. Similarly, after the gate voltage of the primary current sink is adjusted in step 918, the voltage across the primary current sink may be measured again (step 920), and a determination may be made as to whether the voltage across the primary current sink is less than the compliance voltage for the primary current sink. If the voltage across the primary current sink is equal to or greater than the compliance voltage for the primary current sink, method 900 may return to step 916. The voltage across the primary current source may be measured periodically to ensure that the primary current sink remains compliant while the implantable device is delivering treatment.

[0214] On the other hand, if the primary current source remains out of compliance after the gate voltage of the primary current source is adjusted and the gate voltage of the primary current source is still adjustable, method 900 may return to step 908. If the gate voltage of the primary current source is no longer adjustable, the supply voltage may be increased until the primary current source is compliant (step 922). Similarly, if the primary current sink remains out of compliance after the gate voltage of the primary current sink is adjusted and the gate voltage of the primary current sink is still adjustable, method 900 may return to step 918.

[0215] If the gate voltage of the primary current source and / or the gate voltage of the primary current sink are no longer adjustable, the supply voltage may be incrementally increased (step 922). The control circuit may increase the supply voltage by approximately 0.001 V, approximately 0.01 V, approximately 0.1 V, approximately 1 V, approximately 2 V, approximately 3 V, approximately 4 V, or approximately 5 V. The voltage across the primary current source and / or the voltage across the primary current sink may then be measured (step 926 / step 930). If the primary current source and / or the primary current sink remain in a non-compliant (“NC”) state, method 900 may return to step 922, and the supply voltage may again be incrementally increased.

[0216] On the other hand, if the primary current source is in a compliant (“C”) state, method 900 may proceed from step 926 to step 928, and the gate voltage of the primary current source may be adjusted until a minimum gate voltage required for the primary current source to remain compliant is identified. The gate voltage of the primary current source may be maintained at that minimum gate voltage value to ensure that the primary current source remains compliant and minimize the total energy consumption to the implantable device.

[0217] Similarly, if the primary current sink is in a compliant (“C”) state, method 900 may proceed from step 930 to step 932, and the gate voltage of the primary current sink may be adjusted until a minimum gate voltage necessary for the primary current sink to remain compliant is identified. The gate voltage of the primary current sink may be maintained at that minimum gate voltage value to ensure the primary current sink remains compliant and minimize total energy consumption to the implantable device.

[0218] FIG. 10 shows a method 1000 for providing stimulation therapy to a patient using an implantable device. At the beginning of a treatment cycle, an external device may be used to charge the implantable device. Thus, in step 1002, energy may be received by the implantable device from the external device. Once energy is received by the implantable device, a supply voltage may be provided to a primary current source (and / or primary current sink) within the implantable device. At this point, the voltage across the primary current source and primary current sink may be controlled to ensure that the primary current source (sink) is in compliance (step 1004). In some embodiments, the voltage across the primary current source (sink) may be controlled using one or more of the methods described above, such as method 800 (shown in FIG. 8) or method 900 (shown in FIG. 9).

[0219] Once the primary current source and primary current sink become compliant, the tissue undergoing treatment may be electrically stimulated (1006). The primary current source, which may be electrically coupled to the tissue (e.g., via an electrode), may deliver a stimulation current to the tissue. In some embodiments, the current delivered to the tissue by the primary current source may be transmitted through the tissue and absorbed by the primary current sink (e.g., through a second electrode). Electrically stimulating the tissue may produce a beneficial response within the tissue that addresses a medical problem. For example, if the tissue being treated is bladder tissue, electrically stimulating the tissue may help alleviate incontinence.

[0220] In some embodiments, the implantable device may be configured to record and store treatment information associated with the therapy being delivered (step 1008). For example, the implantable device may record stimulation duration, stimulation current, stimulation voltage, tissue resistance, or the time treatment began. In some embodiments, the implantable device may detect one or more physiological signals associated with the tissue being treated, such as temperature, pressure, pH, or analyte concentration. The information recorded and stored by the implantable device may be transmitted to an external device in step 1010 for further analysis and evaluation, for example, using a backscatter communication protocol.

[0221] Communication with external devices and energy transfer The implantable devices described herein may be configured to receive energy from and communicate with an external device, such as an interrogator. As mentioned above, the external device with which the implantable device communicates may be any device (e.g., an interrogator) that is separate from the implantable device. Communicating energy and information with such an external device may allow the size of various components of the implantable device (e.g., energy storage) to be reduced, thereby minimizing the overall size of the implantable device.

[0222] 11A shows an exemplary external device 1100 for providing energy to and communicating with an implanted device. An exemplary schematic diagram of an exemplary external device 1100 for providing energy to and communicating with an implanted device is shown in FIG. 11B.

[0223] The external device 1100 may include one or more transducers 1102 for wireless communication with the implanted device. In some embodiments, the one or more transducers 1102 may include an ultrasound transducer. The ultrasound transducer may be configured to be ultrasonically coupled to the subject's skin to facilitate ultrasonic communication between the external device 1100 and the implanted device. An ultrasound coupling gel or alternative coupling medium may be used to ultrasonically couple the external device 1100 to the skin.

[0224] The external device 1100 may include a transceiver circuit 1104, a data interface 1106, an embedded controller 1108, and a power source 1110. The implantable device may be configured to rely on power transmission from the external device 1100. The power transmission from the external device 1100 may be used to power the implantable device to initiate electrical stimulation of tissue by the implantable device.

[0225] Optionally, external device 1100 may be controlled using a separate computer system, such as a mobile device (e.g., a smartphone or a table). The computer system may communicate wirelessly with external device 1100, for example, through a network connection, a radio frequency (RF) connection, or Bluetooth. The computer system, in some embodiments, may turn external device 1100 on or off or analyze information encoded in backscattered waves (e.g., ultrasound backscattered waves or radio frequency backscattered waves) received by external device 1100 from the implanted device.

[0226] The implanted device and external device 1100 may communicate with each other wirelessly, for example using ultrasound or radio frequency waves. The communication may be one-way (e.g., the external device 1100 sends information to the implanted device or the implanted device sends information to the external device 1100) or two-way (e.g., the external device 1100 sends information to the implanted device and the implanted device sends information to the external device 1100). Physical contact between the patient and the external device 1100 may allow the external device 1100 to receive measurements from the implanted device.

[0227] In some embodiments, the implantable device may include one or more ultrasound transducers with piezoelectric crystals configured to receive commands from ultrasound energy sent from the external device 1100. The implantable device may decode pulse interval coded commands sent from the external device 1100 and may passively send data to the external device via amplitude modulated backscatter communication.

[0228] To encode physiological signal information in the backscatter, the current flowing through the ultrasound transducer of the implanted device may be modulated as a function of the encoded information. The current modulation may encode an analog or digital signal. The information may be encoded by changes in the amplitude, frequency, or phase of the backscattered ultrasound.

[0229] The backscattered waves may be digitized by the implanted device. For example, the implanted device may include an oscilloscope or an analog-to-digital converter (ADC) and / or memory capable of digitally encoding information of current (or impedance) fluctuations. The digitized current fluctuations capable of encoding information are received by a wireless communication system, which then transmits the digitized ultrasound. The digitized data may be compressed from the analog data, for example, by using singular value decomposition (SVD) and least-squares-based compression. Reducing the size of the digitized signal may enable more efficient reporting of the information encoded in the backscatter. Compression may be performed by a correlator or pattern detection algorithm. The backscattered signal may undergo a series of nonlinear transformations, such as a fourth-order Butterworth bandpass filter rectified integration of the backscattered domain, to generate a reconstructed data point at a single time instance. Such transformations may be performed in either hardware (i.e., hard-coded) or software.

[0230] In one or more examples, ultrasonic communication may be established when the piezoelectric crystal of the implanted device is approximately 5 mm + / - 20% away from the external device 1100. In some embodiments, ultrasonic communication may be established when the surface of the piezoelectric crystal is at most approximately 3 mm, 5 mm, 7 mm, or 9 mm away from the surface of the external device 1100. In some embodiments, ultrasonic communication may be established when the surface of the piezoelectric crystal is at least approximately 1 mm, 2 mm, or 3 mm away from the surface of the external device 1100. In some embodiments, ultrasonic communication may be established when the surface of the piezoelectric crystal is approximately 1 mm to 9 mm, 2 mm to 7 mm, or 3 mm to 5 mm away from the surface of the external device 1100. Once established, ultrasonic communication may withstand typical involuntary movements by the patient for a short period of time.

[0231] In some embodiments, one or more transducers 1102 of external device 1100 may be configured to send commands to the implanted device. Commands from external device 1100 may instruct the implanted device to reset itself, enter a particular mode, set device parameters, or initiate a transmission sequence.

[0232] Instructions from the external device 1100 to the implanted device may be carried by an ultrasound carrier. The ultrasound carrier generated by the ultrasound transducer of the external device 1100 may include a series of ultrasound pulses having a varying number of carrier periods. The number of carrier periods may encode information specific to the implanted device. For example, based on the number of carrier periods, the information may include instructions for the implanted device to initiate a data transmission sequence. The transmission sequence may include steps for measuring data related to the implanted device (e.g., a voltage across a current source within the implanted device) and for encoding the data as ultrasound backscatter.

[0233] In some embodiments, communication between the external device 1100 and the implanted device may rely on a pulse-echo technique for transmitting and receiving ultrasound waves. In the pulse-echo technique, the external device 1100 may transmit a series of interrogation pulses at a predetermined frequency and then receive backscattered echoes from the implanted device. The pulses may be square, rectangular, triangular, sawtooth, or sinusoidal. The pulse output may be two-level (GND and POS), three-level (GND, NEG, POS), five-level, or any other multi-level (e.g., using a 24-bit DAC). In some embodiments, pulses are transmitted continuously by the external device 1100 during operation. In some embodiments, when pulses are transmitted continuously by the external device 1100, some of the transducers 1102 may be configured to receive ultrasound waves and some of the transducers 1102 on the interrogator may be configured to transmit ultrasound waves. The transducers configured to receive ultrasound waves and the transducers configured to transmit ultrasound waves may be on the same transducer array or on different transducer arrays of the external device 1100. In some embodiments, the transducers 1102 on the external device 1100 may alternatively be configured to transmit or receive ultrasound waves. For example, the transducers may cycle between transmitting one or more pulses and rest periods. The transducers may be configured to transmit ultrasound waves when transmitting one or more pulses and to receive ultrasound waves during the rest periods.

[0234] Additional details regarding backscatter communications are provided herein, and additional examples are provided in WO 2018 / 009905, WO 2018 / 009908, WO 2018 / 0091010, WO 2018 / 009911, WO 2018 / 009912, International Patent Application No. PCT / US2019 / 028381, International Patent Application No. PCT / US2019 / 028385, and International Patent Application No. PCT / 2019 / 048647, each of which is incorporated by reference herein for all purposes.

[0235] Illustrative Embodiments The following embodiments are illustrative and are not intended to limit the scope of any inventions described herein.

[0236] Embodiment 1. An implantable device comprising: a primary current source electrically coupled to the adjustable voltage source and to the tissue; one or more auxiliary current sources configured to be electrically connected in parallel with the primary current source; and a control circuit configured to electrically connect one or more of the one or more auxiliary current sources in parallel with the primary current source if a voltage across the primary current source is less than a compliance voltage for the primary current source.

[0237] Embodiment 2. The implantable device of embodiment 1, a primary current sink configured to be electrically coupled to the primary current source through the tissue; one or more auxiliary current sinks configured to be electrically connected in parallel with the primary current sink; the control circuit is configured to electrically connect one or more of the one or more auxiliary current sinks in parallel with the primary current sink if the voltage across the primary current sink is less than a compliance voltage for the primary current sink,

[0238] Embodiment 3. An implantable device, comprising: a primary current source electrically coupled to the adjustable voltage source and to the tissue; and a control circuit configured to adjust a gate voltage of the primary current source if a voltage across the primary current source is less than a compliance voltage for the primary current source.

[0239] Embodiment 4. An implantable device as described in embodiment 3, comprising a primary current sink configured to be electrically coupled to the primary current source through the tissue, and the control circuit configured to adjust a gate voltage of the primary current sink if the voltage across the primary current sink is less than a compliance voltage for the primary current sink.

[0240] Embodiment 5. An implantable device as described in embodiment 1 or 3, wherein the primary current source is configured to function as a current sink.

[0241] Embodiment 6. An implantable device as described in embodiment 1, wherein the one or more auxiliary current sources are configured to function as current sinks.

[0242] Embodiment 7. An implantable device as described in any one of embodiments 1, 2, 5 and 6, wherein the control circuit is configured to adjust the adjustable voltage source if the voltage across the primary current source is less than the compliance voltage for the primary current source after the one or more auxiliary current sources are connected in parallel with the primary current source.

[0243] Embodiment 8. An implantable device as described in embodiment 2, wherein the control circuit is configured to adjust the adjustable voltage source if the voltage across the primary current sink is less than the compliance voltage for the primary current sink after the one or more auxiliary current sinks are connected in parallel with the primary current sink.

[0244] Embodiment 9. An implantable device as described in any one of embodiments 3 to 5, wherein the control circuit is configured to adjust the adjustable voltage source if the voltage across the primary current source after the gate voltage of the primary current source is adjusted is less than the compliance voltage for the primary current source.

[0245] Embodiment 10. An implantable device as described in embodiment 4, wherein the control circuit is configured to adjust the adjustable voltage source if the voltage across the primary current sink after the gate voltage of the primary current sink is adjusted is less than the compliance voltage for the primary current sink.

[0246] Embodiment 11. An implantable device as described in any one of embodiments 1 to 10, wherein the adjustable voltage source is configured to receive energy from an external device.

[0247] Embodiment 12. An implantable device as described in embodiment 11, wherein the implantable device comprises one or more ultrasonic transducers and the adjustable voltage source is configured to receive energy from ultrasonic waves emitted by the external device.

[0248] Embodiment 13. An implantable device as described in embodiment 11, wherein the implantable device comprises a radio frequency (RF) antenna and the adjustable voltage source is configured to receive energy from RF emitted by the external device.

[0249] Embodiment 14. An implantable device as described in any one of embodiments 1 to 13, wherein the control circuit is configured to perform periodic measurements of the adjustable voltage across the primary current source.

[0250] Embodiment 15. An implantable device as described in embodiment 14, wherein the control circuit is configured to perform a single measurement of the voltage across the primary current source after a predetermined period of time.

[0251] Embodiment 16. An implantable device described in any one of embodiments 2, 4 and 7 to 15, wherein the control circuit is configured to perform periodic measurements of the voltage across the primary current sink.

[0252] Embodiment 17. An implantable device as described in embodiment 16, wherein the control circuit is configured to perform a single measurement of the voltage across the primary current sink after a predetermined period of time.

[0253] Embodiment 18. An implantable device as described in any one of embodiments 1 to 17, wherein the control circuit is configured to receive a command from an external device configured to cause the control circuit to measure the voltage across the primary current source.

[0254] Embodiment 19. An implantable device as described in embodiment 18, wherein the implantable device comprises an ultrasonic transducer and the control circuit is configured to receive the command from ultrasonic waves emitted by the external device.

[0255] Embodiment 20. An implantable device as described in embodiment 18, wherein the implantable device is provided with a radio frequency (RF) antenna and the control circuit is configured to receive the command from radio waves emitted by the external device.

[0256] Embodiment 21. An implantable device as described in any one of embodiments 2, 4 and 7 to 20, wherein the control circuit is configured to receive a command from an external device configured to cause the control circuit to measure the voltage across the primary current sink.

[0257] Embodiment 22. An implantable device as described in embodiment 21, wherein the implantable device comprises an ultrasonic transducer and the control circuit is configured to receive the command from ultrasonic waves emitted by the external device.

[0258] Embodiment 23. An implantable device as described in embodiment 21, wherein the implantable device is provided with a radio frequency (RF) antenna and the control circuit is configured to receive the command from radio waves emitted by the external device.

[0259] Embodiment 24. An implantable device as described in any one of embodiments 1 to 23, wherein the control circuit is configured to communicate with an external device using ultrasonic backscatter.

[0260] Embodiment 25. An implantable device as described in any one of embodiments 1 to 23, wherein the control circuit is configured to communicate with an external device using radio wave backscatter.

[0261] Embodiment 26. An implantable device described in any one of embodiments 1 to 25, wherein the primary current source is a transistor.

[0262] Embodiment 27. An implantable device as described in embodiment 26, wherein the primary current source comprises a PMOS transistor.

[0263] Embodiment 28. An implantable device according to embodiment 26 or 27, wherein the primary current source comprises an NMOS transistor.

[0264] Embodiment 29. An implantable device described in any one of embodiments 2, 4 and 7 to 28, wherein the primary current sink is a transistor.

[0265] Embodiment 30. An implantable device as described in embodiment 29, wherein the primary current sink comprises an NMOS transistor.

[0266] Embodiment 31. An implantable device described in any one of embodiments 1 to 30, wherein the primary current source is an active feedback current source.

[0267] Embodiment 32. An implantable device described in any one of embodiments 2, 4 and 7 to 31, wherein the primary current sink is an active feedback current sink.

[0268] Embodiment 33. An implantable device described in any one of embodiments 1, 2, 5 to 8 and 11 to 32, wherein the primary current source is configured to supply a higher current than the one or more auxiliary current sources.

[0269] Embodiment 34. An implantable device described in any one of embodiments 1, 2, 5 to 8 and 11 to 33, wherein the one or more auxiliary current sources include one or more transistors.

[0270] Embodiment 35. An implantable device described in any one of embodiments 1, 2, 5 to 8 and 11 to 34, wherein the one or more auxiliary current sources include one or more active feedback current sources.

[0271] Embodiment 36. An implantable device described in any one of embodiments 1, 2, 5 to 8 and 11 to 35, wherein the one or more auxiliary current sources are each electrically coupled to one or more switches electrically coupled in parallel to the primary current source and configured to be controlled by the control circuit.

[0272] Embodiment 37. An implantable device described in any one of embodiments 2, 7, 8 and 11 to 36, wherein the primary current sink is configured to receive a higher current than the one or more auxiliary current sinks.

[0273] Embodiment 38. An implantable device described in any one of embodiments 2, 7, 8 and 11 to 37, wherein the one or more auxiliary current sinks include one or more transistors.

[0274] Embodiment 39. An implantable device described in any one of embodiments 2, 7, 8 and 11 to 38, wherein the one or more auxiliary current sinks include one or more active feedback current sinks.

[0275] Embodiment 40. An implantable device described in any one of embodiments 2, 7, 8 and 11 to 39, wherein the one or more auxiliary current sinks are each electrically coupled to one or more switches electrically coupled in parallel to the primary current sink and configured to be controlled by the control circuit.

[0276] Embodiment 41. An implantable device described in any one of embodiments 1 to 40, wherein the primary current source is coupled to a first electrode, and the first electrode is configured to be attached to the tissue.

[0277] Embodiment 42. An implantable device described in any one of embodiments 2, 4 and 7 to 41, wherein the primary current sink is coupled to a second electrode, and the second electrode is configured to be attached to the tissue.

[0278] Embodiment 43. An implantable device described in any one of embodiments 1 to 42, wherein the tissue is neural tissue.

[0279] Embodiment 44. An implantable device as described in embodiment 43, wherein the neural tissue is brain tissue.

[0280] Embodiment 45. An implantable device as described in embodiment 43, wherein the nerve tissue is a peripheral nerve.

[0281] Embodiment 46. An implantable device according to embodiment 45, wherein the peripheral nerve is the splenic nerve.

[0282] Embodiment 47. An implantable device described in any one of embodiments 1 to 42, wherein the tissue is organ tissue.

[0283] Embodiment 48. An implantable device as described in embodiment 47, wherein the organ tissue is cardiac tissue.

[0284] Embodiment 49. An implantable device as described in embodiment 47, wherein the organ tissue is bladder tissue.

[0285] Embodiment 50. An implantable device as described in embodiment 47, wherein the organ tissue is stomach tissue.

[0286] Embodiment 51. An implantable device as described in embodiment 47, wherein the organ tissue is muscle tissue.

[0287] Embodiment 52. An implantable device described in any one of embodiments 1 to 51, wherein the compliance voltage for the primary current source depends on the electrical resistance of the tissue.

[0288] Embodiment 53. An implantable device described in any one of embodiments 2, 4 and 7 to 52, wherein the compliance voltage for the primary current sink depends on the electrical resistance of the tissue.

[0289] Embodiment 54. An implantable device described in any one of embodiments 1 to 53, wherein the adjustable voltage source includes one or more capacitors.

[0290] Embodiment 55. A method for operating an implantable device, comprising: providing a supply voltage to a primary current source electrically coupled to the tissue; measuring a voltage across the primary current source; determining that the voltage across the primary current source is less than a compliance voltage for the primary current source; and electrically connecting the one or more auxiliary current sources in parallel with the primary current source if the voltage across the primary current source is less than the compliance voltage for the primary current source.

[0291] Embodiment 56. The method of embodiment 55, providing the supply voltage to a primary current sink electrically coupled to the primary current source through the tissue; measuring a voltage across the primary current sink; determining that the voltage across the primary current sink is less than a compliance voltage for the primary current sink; and electrically connecting the one or more auxiliary current sinks in parallel with the primary current sink if the voltage across the primary current sink is less than the compliance voltage for the primary current sink.

[0292] Embodiment 57. A method for controlling an implantable device, comprising: providing a supply voltage to a primary current source electrically coupled to the tissue; measuring a voltage across the primary current source; determining that the voltage across the primary current source is less than a compliance voltage for the primary current source; adjusting a gate voltage of the primary current source if the voltage across the primary current source is less than a compliance voltage for the primary current source.

[0293] Embodiment 58. The method of embodiment 57, providing the supply voltage to a primary current sink electrically coupled to the primary current source through the tissue; measuring a voltage across the primary current sink; determining that the voltage across the primary current sink is less than a compliance voltage for the primary current sink; adjusting a gate voltage of the primary current sink if the voltage across the primary current sink is less than the compliance voltage for the primary current sink.

[0294] Embodiment 59. A method as described in embodiment 55 or 56, comprising adjusting the supply voltage if the voltage across the primary current source is less than the compliance voltage for the primary current source after the one or more auxiliary current sources are connected in parallel with the primary current source.

[0295] Embodiment 60. The method of embodiment 56, comprising adjusting the supply if the voltage across the primary current sink is less than the compliance voltage for the primary current sink after the one or more auxiliary current sinks are connected in parallel with the primary current sink.

[0296] Embodiment 61. A method as described in embodiment 57 or 58, comprising adjusting the supply voltage if the voltage across the primary current source after the gate voltage for the primary current source is adjusted is less than the compliance voltage for the primary current source.

[0297] Embodiment 62. The method of embodiment 58, comprising adjusting the supply voltage if the voltage across the primary current sink is less than the compliance voltage for the primary current sink after the gate voltage for the primary current sink is adjusted.

[0298] Embodiment 63. A method according to any one of embodiments 55 to 62, comprising receiving energy from an external device.

[0299] Embodiment 64. The method described in embodiment 63, wherein the energy is received from ultrasound emitted by the external device.

[0300] Embodiment 65. The method of embodiment 53, wherein the energy is received from radio waves emitted by the external device.

[0301] Embodiment 66. A method according to any one of embodiments 55 to 65, comprising performing periodic measurements of the voltage across the primary current source.

[0302] Embodiment 67. A method as described in embodiment 66, comprising performing a single measurement of the voltage across the primary current source after a predetermined period of time.

[0303] Embodiment 68. A method according to any one of embodiments 56 and 58 to 67, comprising performing periodic measurements of the voltage across the primary current sink.

[0304] Embodiment 69. The method of embodiment 68, comprising performing a single measurement of the voltage across the primary current sink after a predetermined period of time.

[0305] Embodiment 70. The method of any one of embodiments 55 to 69, receiving a command from an external device indicating that the voltage across the primary current source should be measured; measuring the voltage across the primary current source in response to receiving the command from the external device.

[0306] Embodiment 71. The method described in embodiment 70, wherein the command is encoded in ultrasound emitted by the external device.

[0307] Embodiment 72. The method of embodiment 70, wherein the command is encoded in radio waves emitted by the external device.

[0308] Embodiment 73. The method according to any one of embodiments 56 and 58 to 72, receiving a command from an external device indicating that the voltage across the primary current sink should be measured; measuring the voltage across the primary current sink in response to receiving the command from the external device.

[0309] Embodiment 74. The method described in embodiment 73, wherein the command is encoded in ultrasound emitted by the external device.

[0310] Embodiment 75. The method of embodiment 73, wherein the command is encoded in radio waves emitted by the external device.

[0311] Embodiment 76. A method according to any one of embodiments 55 to 75, comprising communicating with an external device using ultrasonic backscatter.

[0312] Embodiment 77. A method according to any one of embodiments 55 to 76, comprising communicating with an external device using radio wave backscatter.

[0313] Embodiment 78. A method according to any one of embodiments 55, 56, 59, 60 and 63 to 77, wherein connecting the one or more auxiliary current sources in parallel with the primary current source includes controlling one or more switches electrically coupled to the one or more auxiliary current sources and electrically coupled in parallel with the primary current source.

[0314] Embodiment 79. A method according to any one of embodiments 56, 59, 60 and 63 to 78, wherein connecting the one or more auxiliary current sinks in parallel with the primary current sink includes controlling one or more switches electrically coupled to the one or more auxiliary current sinks and electrically coupled in parallel with the primary current sink.

[0315] Embodiment 80. A method according to any one of embodiments 55 to 78, wherein the tissue is nervous tissue.

[0316] Embodiment 81. The method of embodiment 80, wherein the neural tissue is brain tissue.

[0317] Embodiment 82. The method of embodiment 80, wherein the nerve tissue is a peripheral nerve.

[0318] Embodiment 83. The method of embodiment 82, wherein the peripheral nerve is the splenic nerve.

[0319] Embodiment 84. A method according to any one of embodiments 55 to 79, wherein the tissue is organ tissue.

[0320] Embodiment 85. The method described in embodiment 84, wherein the organ tissue is cardiac tissue.

[0321] Embodiment 86. The method of embodiment 84, wherein the organ tissue is bladder tissue.

[0322] Embodiment 87. The method of embodiment 84, wherein the organ tissue is stomach tissue.

[0323] Embodiment 88. The method of embodiment 84, wherein the organ tissue is muscle tissue.

[0324] Embodiment 89. A method according to any one of embodiments 55 to 88, wherein the compliance voltage for the primary current source depends on the electrical resistance of the tissue.

[0325] Embodiment 90. A method according to any one of embodiments 56 and 58 to 88, wherein the compliance voltage for the primary current sink depends on the electrical resistance of the tissue.

[0326] Embodiment 91. A non-transitory computer-readable storage medium storing instructions for operating an implantable device, the instructions, when executed by control circuitry of the implantable device, causing the implantable device to: providing a supply voltage to a primary current source electrically coupled to the tissue; measuring a voltage across the primary current source; determining that the voltage across the primary current source is less than a compliance voltage for the primary current source; and electrically connecting the one or more auxiliary current sources in parallel with the primary current source if the voltage across the primary current source is less than the compliance voltage for the primary current source.

[0327] Embodiment 92. The non-transitory computer-readable storage medium of embodiment 91, wherein the instructions, when executed, cause the device to: providing the supply voltage to a primary current sink electrically coupled to the primary current source through the tissue; measuring a voltage across the primary current sink; determining that the voltage across the primary current sink is less than a compliance voltage for the primary current sink; and electrically connecting the one or more auxiliary current sinks in parallel with the primary current sink if the voltage across the primary current sink is less than the compliance voltage for the primary current sink.

[0328] Embodiment 93. A non-transitory computer-readable storage medium storing instructions for operating an implantable device, the instructions, when executed by control circuitry of the implantable device, causing the implantable device to: providing a supply voltage to a primary current source electrically coupled to the tissue; measuring a voltage across the primary current source; determining that the voltage across the primary current source is less than a compliance voltage for the primary current source; and adjusting a gate voltage of the primary current source if the voltage across the primary current source is less than a compliance voltage for the primary current source.

[0329] Embodiment 94. The non-transitory computer-readable storage medium of embodiment 93, wherein the instructions, when executed, cause the embedded device to: providing the supply voltage to a primary current sink electrically coupled to the primary current source through the tissue; measuring a voltage across the primary current sink; determining that the voltage across the primary current sink is less than a compliance voltage for the primary current sink; and adjusting a gate voltage of the primary current sink if the voltage across the primary current sink is less than a compliance voltage for the primary current sink.

[0330] Embodiment 95. A non-transitory computer-readable storage medium as described in embodiment 91 or 92, comprising adjusting the supply voltage if the voltage across the primary current source is less than the compliance voltage for the primary current source after the one or more auxiliary current sources are connected in parallel with the primary current source.

[0331] Embodiment 96. A non-transitory computer-readable storage medium as described in embodiment 92, comprising adjusting the supply if the voltage across the primary current sink is less than the compliance voltage for the primary current sink after the one or more auxiliary current sinks are connected in parallel with the primary current sink.

[0332] Embodiment 97. A non-transitory computer-readable storage medium as described in embodiment 93 or 94, comprising adjusting the supply voltage if the voltage across the primary current source is less than the compliance voltage for the primary current source after the gate voltage for the primary current source is adjusted.

[0333] Embodiment 98. A non-transitory computer-readable storage medium as described in embodiment 94, comprising adjusting the supply voltage if the voltage across the primary current sink is less than the compliance voltage for the primary current sink after the gate voltage for the primary current sink is adjusted.

[0334] Embodiment 99. A non-transitory computer-readable storage medium according to any one of embodiments 92 to 98, comprising receiving energy from an external device.

[0335] Embodiment 100. A non-transitory computer-readable storage medium as described in embodiment 99, wherein the energy is received from ultrasound emitted by the external device.

[0336] Embodiment 101. A non-transitory computer-readable storage medium as described in embodiment 99, wherein the energy is received from radio waves emitted by the external device.

[0337] Embodiment 102. A non-transitory computer-readable storage medium according to any one of embodiments 91 to 101, comprising performing periodic measurements of the voltage across the primary current source.

[0338] Embodiment 103. A non-transitory computer-readable storage medium as described in embodiment 102, comprising performing a single measurement of the voltage across the primary current source after a predetermined period of time.

[0339] Embodiment 104. A non-transitory computer-readable storage medium according to any one of embodiments 92, 94 to 103, comprising performing periodic measurements of the voltage across the primary current sink.

[0340] Embodiment 105. A non-transitory computer-readable storage medium as described in embodiment 104, comprising performing at least one measurement of the voltage across the primary current sink after a predetermined period of time.

[0341] Embodiment 106. A non-transitory computer-readable storage medium according to any one of embodiments 91 to 105, comprising: receiving a command from an external device indicating that the voltage across the primary current source should be measured; and measuring the voltage across the primary current source in response to receiving the command from the external device.

[0342] Embodiment 107. A non-transitory computer-readable storage medium as described in embodiment 106, wherein the command is encoded in ultrasound emitted by the external device.

[0343] Embodiment 108. A non-transitory computer-readable storage medium as described in embodiment 106, wherein the command is encoded in radio waves emitted by the external device.

[0344] Embodiment 109. A non-transitory computer-readable storage medium according to any one of embodiments 92 and 94 to 108, comprising: receiving a command from an external device indicating that the voltage across the primary current sink should be measured; and measuring the voltage across the primary current sink in response to receiving the command from the external device.

[0345] Embodiment 110. A non-transitory computer-readable storage medium as described in embodiment 109, wherein the command is encoded in ultrasound emitted by the external device.

[0346] Embodiment 111. A non-transitory computer-readable storage medium as described in embodiment 109, wherein the command is encoded in radio waves emitted by the external device.

[0347] Embodiment 112. A non-transitory computer-readable storage medium according to any one of embodiments 91 to 111, comprising communicating with an external device using ultrasonic backscatter.

[0348] Embodiment 113. A non-transitory computer-readable storage medium according to any one of embodiments 91 to 111, comprising communicating with an external device using radio wave backscatter.

[0349] Embodiment 114. A non-transitory computer-readable storage medium described in any one of embodiments 91, 92, 95, 96 and 99 to 113, wherein connecting the one or more auxiliary current sources in parallel with the primary current source includes controlling one or more switches electrically coupled to the one or more auxiliary current sources and electrically coupled in parallel with the primary current source.

[0350] Embodiment 115. A non-transitory computer-readable storage medium described in any one of embodiments 92, 95, 96 and 99 to 114, wherein connecting the one or more auxiliary current sinks in parallel with the primary current sink includes controlling one or more switches electrically coupled to the one or more auxiliary current sinks and electrically coupled in parallel with the primary current sink.

[0351] Embodiment 116. A non-transitory computer-readable storage medium described in any one of embodiments 91 to 115, wherein the tissue is nervous tissue.

[0352] Embodiment 117. A non-transitory computer-readable storage medium as described in embodiment 116, wherein the neural tissue is brain tissue.

[0353] Embodiment 118. A non-transitory computer-readable storage medium as described in embodiment 116, wherein the nervous tissue is a peripheral nerve.

[0354] Embodiment 119. A non-transitory computer-readable storage medium according to embodiment 118, wherein the peripheral nerve is the splenic nerve.

[0355] Embodiment 120. A non-transitory computer-readable storage medium according to any one of embodiments 91 to 115, wherein the tissue is organ tissue.

[0356] Embodiment 121. A non-transitory computer-readable storage medium as described in embodiment 120, wherein the organ tissue is cardiac tissue.

[0357] Embodiment 122. A non-transitory computer-readable storage medium according to embodiment 120, wherein the organ tissue is bladder tissue.

[0358] Embodiment 123. A non-transitory computer-readable storage medium according to embodiment 120, wherein the organ tissue is stomach tissue.

[0359] Embodiment 124. A non-transitory computer-readable storage medium according to embodiment 120, wherein the organ tissue is muscle tissue.

[0360] Embodiment 125. A non-transitory computer-readable storage medium described in any one of embodiments 91 to 124, wherein the compliance voltage for the primary current source depends on the electrical resistance of the tissue.

[0361] Embodiment 126. A non-transitory computer-readable storage medium described in any one of embodiments 92 and 94 to 125, wherein the compliance voltage for the primary current sink depends on the electrical resistance of the tissue.

[0362] The foregoing description has been set forth with reference to specific embodiments and / or examples for purposes of explanation. However, the illustrative discussion above is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible in light of the above teachings. The embodiments were chosen and described in order to best explain the principles of the technology and their practical applications, thereby enabling those skilled in the art to best utilize the technology and various embodiments with various modifications as suited to the particular use contemplated.

[0363] Although the present disclosure and examples have been fully described with reference to the accompanying drawings, it should be noted that various changes and modifications will become apparent to those skilled in the art. Such changes and modifications will be understood to be included within the scope of the present disclosure and examples, as defined by the claims. Finally, the entire disclosures of the patents and publications referenced in this application are hereby incorporated by reference.

[0364] Any of the systems, methods, techniques and / or features disclosed herein may be combined, in whole or in part, with any other system, method, technique and / or feature disclosed herein.

Claims

1. 1. An implantable device comprising: a primary current source electrically coupled to the adjustable voltage source and to the tissue; one or more auxiliary current sources configured to be electrically connected in parallel with the primary current source; and a control circuit configured to electrically connect one or more of the one or more auxiliary current sources in parallel with the primary current source if a voltage across the primary current source is less than a compliance voltage for the primary current source.

2. 10. The implantable device of claim 1, a primary current sink configured to be electrically coupled to the primary current source through the tissue; one or more auxiliary current sinks configured to be electrically connected in parallel with the primary current sink; The implantable device, wherein the control circuit is configured to electrically connect one or more of the one or more auxiliary current sinks in parallel with the primary current sink if the voltage across the primary current sink is less than a compliance voltage for the primary current sink.

3. 1. An implantable device comprising: a primary current source electrically coupled to the adjustable voltage source and to the tissue; and a control circuit configured to adjust a gate voltage of the primary current source if a voltage across the primary current source is less than a compliance voltage for the primary current source.

4. 4. An implantable device as described in claim 3, comprising a primary current sink configured to be electrically coupled to the primary current source through the tissue, and wherein the control circuit is configured to adjust a gate voltage of the primary current sink if a voltage across the primary current sink is less than a compliance voltage for the primary current sink.

5. 4. An implantable device according to claim 1 or 3, wherein the primary current source is configured to act as a current sink.

6. 10. The implantable device of claim 1, wherein the one or more auxiliary current sources are configured to function as a current sink.

7. 7. An implantable device as described in any one of claims 1, 2, 5 and 6, wherein the control circuit is configured to adjust the adjustable voltage source if the voltage across the primary current source is less than the compliance voltage for the primary current source after the one or more auxiliary current sources are connected in parallel with the primary current source.

8. 3. The implantable device of claim 2, wherein the control circuit is configured to adjust the adjustable voltage source if the voltage across the primary current sink is less than the compliance voltage for the primary current sink after the one or more auxiliary current sinks are connected in parallel with the primary current sink.

9. 6. An implantable device as claimed in any one of claims 3 to 5, wherein the control circuit is configured to adjust the adjustable voltage source if the voltage across the primary current source after the gate voltage of the primary current source is adjusted is less than the compliance voltage for the primary current source.

10. 5. The implantable device of claim 4, wherein the control circuit is configured to adjust the adjustable voltage source if the voltage across the primary current sink after the gate voltage of the primary current sink is adjusted is less than the compliance voltage for the primary current sink.

11. 11. An implantable device according to any preceding claim, wherein the adjustable voltage source is configured to receive energy from an external device.

12. 12. The implantable device of claim 11, wherein the implantable device comprises one or more ultrasound transducers, and the adjustable voltage source is configured to receive energy from ultrasound emitted by the external device.

13. 12. The implantable device of claim 11, wherein the implantable device comprises a radio frequency (RF) antenna, and the adjustable voltage source is configured to receive energy from RF emitted by the external device.

14. 14. An implantable device according to any preceding claim, wherein the control circuit is configured to perform periodic measurements of the adjustable voltage across the primary current source.

15. 15. The implantable device of claim 14, wherein the control circuitry is configured to perform a single measurement of the voltage across the primary current source after a predetermined period of time.

16. 16. An implantable device according to any one of claims 2, 4 and 7 to 15, wherein the control circuitry is configured to perform periodic measurements of the voltage across the primary current sink.

17. 17. The implantable device of claim 16, wherein the control circuitry is configured to perform a single measurement of the voltage across the primary current sink after a predetermined period of time.

18. 18. An implantable device as described in any one of claims 1 to 17, wherein the control circuit is configured to receive a command from an external device configured to cause the control circuit to measure the voltage across the primary current source.

19. 20. The implantable device of claim 18, wherein the implantable device comprises an ultrasonic transducer and the control circuitry is configured to receive the command from ultrasonic waves emitted by the external device.

20. 20. The implantable device of claim 18, wherein the implantable device comprises a radio frequency (RF) antenna, and the control circuitry is configured to receive the commands from radio waves emitted by the external device.

21. 21. An implantable device as described in any one of claims 2, 4 and 7 to 20, wherein the control circuit is configured to receive a command from an external device configured to cause the control circuit to measure the voltage across the primary current sink.

22. 22. The implantable device of claim 21, wherein the implantable device comprises an ultrasonic transducer and the control circuitry is configured to receive the command from ultrasonic waves emitted by the external device.

23. 22. The implantable device of claim 21, wherein the implantable device comprises a radio frequency (RF) antenna, and the control circuitry is configured to receive the commands from radio waves emitted by the external device.

24. 24. An implantable device according to any preceding claim, wherein the primary current source is a transistor.

25. 25. An implantable device according to any one of claims 2, 4 and 7 to 24, wherein the primary current sink is a transistor.

26. 26. An implantable device according to any preceding claim, wherein the primary current source is an active feedback current source.

27. 27. An implantable device according to any one of claims 2, 4 and 7 to 26, wherein the primary current sink is an active feedback current sink.

28. 28. An implantable device as claimed in any one of claims 1, 2, 5-8 and 11-27, wherein the primary current source is configured to supply a higher current than the one or more auxiliary current sources.

29. 29. An implantable device according to any one of claims 1, 2, 5-8 and 11-28, wherein the one or more auxiliary current sources comprise one or more transistors.

30. 30. An implantable device as claimed in any one of claims 1, 2, 5-8 and 11-29, wherein the one or more auxiliary current sources comprise one or more active feedback current sources.

31. 31. An implantable device as described in any one of claims 1, 2, 5-8 and 11-30, wherein the one or more auxiliary current sources are each electrically coupled to one or more switches electrically coupled in parallel to the primary current source and configured to be controlled by the control circuit.

32. 32. An implantable device according to any one of claims 2, 7, 8 and 11 to 31, wherein the primary current sink is configured to receive a higher current than the one or more auxiliary current sinks.

33. 33. An implantable device according to any one of claims 2, 7, 8 and 11 to 32, wherein the one or more auxiliary current sinks comprise one or more transistors.

34. 34. An implantable device according to any one of claims 2, 7, 8 and 11 to 33, wherein the one or more auxiliary current sinks comprise one or more active feedback current sinks.

35. 35. An implantable device as described in any one of claims 2, 7, 8 and 11 to 34, wherein the one or more auxiliary current sinks are each electrically coupled to one or more switches electrically coupled in parallel to the primary current sink and configured to be controlled by the control circuit.

36. 36. An implantable device according to any one of claims 1 to 35, wherein the primary current source is coupled to a first electrode, the first electrode being configured to be attached to the tissue.

37. 37. An implantable device as described in any one of claims 2, 4 and 7 to 36, wherein the primary current sink is coupled to a second electrode, the second electrode being configured to be attached to the tissue.

38. 38. An implantable device according to any one of claims 1 to 37, wherein the compliance voltage for the primary current source depends on the electrical resistance of the tissue.

39. 39. An implantable device according to any one of claims 2, 4 and 7 to 38, wherein the compliance voltage for the primary current sink depends on the electrical resistance of the tissue.

40. 40. An implantable device according to any one of claims 1 to 39, wherein the adjustable voltage source comprises one or more capacitors.

41. 1. A method for operating an implantable device, comprising: providing a supply voltage to a primary current source electrically coupled to the tissue; measuring a voltage across the primary current source; determining that the voltage across the primary current source is less than a compliance voltage for the primary current source; and electrically connecting the one or more auxiliary current sources in parallel with the primary current source if the voltage across the primary current source is less than the compliance voltage for the primary current source.

42. 42. The method of claim 41 , providing the supply voltage to a primary current sink electrically coupled to the primary current source through the tissue; measuring a voltage across the primary current sink; determining that the voltage across the primary current sink is less than a compliance voltage for the primary current sink; and electrically connecting the one or more auxiliary current sinks in parallel with the primary current sink if the voltage across the primary current sink is less than the compliance voltage for the primary current sink.

43. 1. A method for controlling an implantable device, comprising: providing a supply voltage to a primary current source electrically coupled to the tissue; measuring a voltage across the primary current source; determining that the voltage across the primary current source is less than a compliance voltage for the primary current source; adjusting a gate voltage of the primary current source if the voltage across the primary current source is less than a compliance voltage for the primary current source.

44. 44. The method of claim 43, providing the supply voltage to a primary current sink electrically coupled to the primary current source through the tissue; measuring a voltage across the primary current sink; determining that the voltage across the primary current sink is less than a compliance voltage for the primary current sink; adjusting a gate voltage of the primary current sink if the voltage across the primary current sink is less than the compliance voltage for the primary current sink.

45. 43. A method as claimed in claim 41 or 42, comprising adjusting the supply voltage if the voltage across the primary current source after the one or more auxiliary current sources are connected in parallel with the primary current source is less than the compliance voltage for the primary current source.

46. 43. The method of claim 42, comprising adjusting the supply if the voltage across the primary current sink is less than the compliance voltage for the primary current sink after the one or more auxiliary current sinks are connected in parallel with the primary current sink.

47. 45. A method according to claim 43 or 44, comprising adjusting the supply voltage if the voltage across the primary current source is less than the compliance voltage for the primary current source after the gate voltage for the primary current source has been adjusted.

48. 45. The method of claim 44, comprising adjusting the supply voltage if the voltage across the primary current sink is less than the compliance voltage for the primary current sink after the gate voltage for the primary current sink is adjusted.

49. 49. A method according to any one of claims 41 to 48, comprising performing periodic measurements of the voltage across the primary current source.

50. 50. The method of claim 49, comprising performing a single measurement of the voltage across the primary current source after a predetermined period of time.

51. 51. A method according to any one of claims 42 and 44 to 50, comprising performing periodic measurements of the voltage across the primary current sink.

52. 52. The method of claim 51, comprising performing a single measurement of the voltage across the primary current sink after a predetermined period of time.

53. 53. The method of any one of claims 41 to 52, comprising: receiving a command from an external device indicating that the voltage across the primary current source should be measured; measuring the voltage across the primary current source in response to receiving the command from the external device.

54. 54. The method of any one of claims 42 and 44 to 53, comprising: receiving a command from an external device indicating that the voltage across the primary current sink should be measured; measuring the voltage across the primary current sink in response to receiving the command from the external device.

55. 55. The method of any one of claims 41, 42 and 45-54, wherein connecting the one or more auxiliary current sources in parallel with the primary current source comprises controlling one or more switches electrically coupled to the one or more auxiliary current sources and electrically coupled in parallel with the primary current source.

56. 56. The method of any one of claims 42 and 45-55, wherein connecting the one or more auxiliary current sinks in parallel with the primary current sink comprises controlling one or more switches electrically coupled to the one or more auxiliary current sinks and electrically coupled in parallel with the primary current sink.

57. 57. The method of any one of claims 41 to 56, wherein the tissue is neural tissue or organ tissue.

58. 58. The method of any one of claims 41 to 57, wherein the tissue is bladder tissue.

59. 59. The method of any one of claims 41 to 58, wherein the compliance voltage for the primary current source depends on the electrical resistance of the tissue.

60. 60. The method of any one of claims 42 and 44-59, wherein the compliance voltage for the primary current sink depends on the electrical resistance of the tissue.

61. 1. A non-transitory computer-readable storage medium storing instructions for operating an implantable device, the instructions, when executed by control circuitry of the implantable device, causing the implantable device to: providing a supply voltage to a primary current source electrically coupled to the tissue; measuring a voltage across the primary current source; determining that the voltage across the primary current source is less than a compliance voltage for the primary current source; and electrically connecting the one or more auxiliary current sources in parallel with the primary current source if the voltage across the primary current source is less than the compliance voltage for the primary current source.

62. 1. A non-transitory computer-readable storage medium storing instructions for operating an implantable device, the instructions, when executed by control circuitry of the implantable device, causing the implantable device to: providing a supply voltage to a primary current source electrically coupled to the tissue; measuring a voltage across the primary current source; determining that the voltage across the primary current source is less than a compliance voltage for the primary current source; and adjusting a gate voltage of the primary current source if the voltage across the primary current source is less than a compliance voltage for the primary current source.

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