Frequency Modulated Communications

Abstract

A method and system for low power communication using ultrasound is described herein. The method may include receiving ultrasound transmitted by an interrogator at one or more ultrasound transducers of an implantable device, and emitting ultrasound backscatter including encoded data from the one or more ultrasound transducers of the implantable device, the data being encoded in the ultrasound backscatter by modulating a frequency of the ultrasound backscatter. The device may include an ultrasound transducer configured to receive the ultrasound and emit the ultrasound backscatter, a switch configured to modulate a frequency of the emitted ultrasound backscatter, and circuitry configured to operate the switch to encode data in the emitted ultrasound backscatter based on frequency.

Description

[Technical field]

[0001] [CROSS REFERENCE TO RELATED APPLICATIONS] This application claims priority to U.S. Patent Application No. 63 / 322,987, filed March 23, 2022, the entire contents of which are incorporated herein by reference.

[0002] [Technical field] Methods and systems for low power communication using ultrasonic backscatter communication are described herein, as well as implantable devices that employ such methods and systems. [Background technology]

[0003] Backscatter ultrasound communication is a low-power communication method for transmitting data. Such low-power methods are preferred for communication between a patient-implantable device and an external device (referred to herein as an interrogator). A typical communication between an interrogator and an implanted device can include emission of backscatter ultrasound pulses based on amplitude modulation at the implantable device. However, amplitude modulation suffers from noise issues (such as passive reflections from environmental structures) and therefore suffers from low signal-to-noise (SNR), resulting in digital communication errors between the interrogator and the implantable device. Alternatives to amplitude-modulated backscatter communication include active transmission and cancellation methods. However, as further described below, these alternatives can introduce significant drawbacks.

[0004] Active transmission generally relies on an internal battery to power the signal radiation outside the body. Active implants extend the read range, allow more variation in the transmitted information, and eliminate problems from passive reflections. Although active transmission avoids the problem of amplitude modulation, the introduction of a battery increases the required size of the implant and may introduce new challenges regarding battery recharging, safety, and cost.

[0005] To limit environmental interference passive reflections from environmental interference, cancellation methods can be implemented. Cancellation methods transmit a passive pulse to "learn" the reflecting environment, and then transmit a calibrated active pulse to determine the modulation by the implant. Using the difference between these two reflections helps to limit the effect of environmental structures that may also reflect ultrasound that causes passive reflections. While this limits the problems from passive reflections, these cancellation methods do not take into account environmental interference and changes that occur after the training pulse during actual data transfer.

[0006] Thus, overcoming passive reflections during ultrasonic backscatter communication between an interrogator and an implantable device remains a challenge. Summary of the Invention

[0007] Amplitude modulation can be used to encode data into the ultrasonic backscatter to transmit information from the implantable device to an external device. However, as discussed in the previous section, communications based on amplitude modulation are subject to noise from passive reflections and other sources of reflected interference that can adversely affect the communications. Thus, a need remains to encode data into the ultrasonic backscatter to enable low power and low noise communications between the implantable device and the interrogator.

[0008] A method and system for low power communication using ultrasound is described herein. The method may include receiving ultrasound transmitted by an interrogator at one or more ultrasound transducers of an implantable device, and emitting ultrasound backscatter from the one or more ultrasound transducers of the implantable device, the ultrasound backscatter including encoded data, the data being encoded in the ultrasound backscatter by modulating a frequency of the ultrasound backscatter. The device may include an ultrasound transducer configured to receive the ultrasound and emit the ultrasound backscatter, a switch configured to modulate a frequency of the emitted ultrasound backscatter, and a circuit (optionally an integrated circuit) configured to operate the switch to encode data in the emitted ultrasound backscatter based on frequency.

[0009] A method for low power communication using ultrasound may include receiving ultrasound transmitted by an interrogator at one or more ultrasound transducers of an implantable device, and emitting ultrasound backscatter from the one or more ultrasound transducers of the implantable device, the data being encoded into the ultrasound backscatter by modulating a frequency of the ultrasound backscatter. The method may include selecting a switching frequency from a plurality of different switching frequencies to modulate the frequency of the ultrasound backscatter, the switching frequency being configured to encode the data into the ultrasound backscatter. In some embodiments, each switching frequency is associated with a different predetermined bit pattern. Modulating the frequency of the ultrasound backscatter may include switching the implantable device at the switching frequencies.

[0010] In some embodiments, each predetermined bit pattern comprises two or more digital bits. In some embodiments, the predetermined bit patterns associated with the different switching frequencies are Gray coded.

[0011] The switching frequency may be based at least on instructions received from the interrogator via ultrasound.

[0012] The different switching frequencies may be selected based at least on the intensity of the ultrasonic backscatter at a given frequency.

[0013] The method may further include detecting sensor information from one or more sensors of the implantable device, the data including one or more of the sensor information, and the method including selecting a switching frequency from a plurality of different switching frequencies based at least on the sensor information. The sensor information may include, for example, one or more of a power level, a pH, a temperature, a pressure, an electrophysiological pulse, and an analyte concentration. In some embodiments, the analyte concentration includes an oxygen level.

[0014] The method may include determining operational status information of the implantable device, the data including the determined operational status information, and the method may include selecting a switching frequency from a plurality of different switching frequencies based at least on the determined operational status information.

[0015] The method may include selecting a sequence of two or more switching frequencies from a plurality of different switching frequencies, each switching frequency associated with a different predetermined bit pattern, for encoding the data.

[0016] The method further includes decoding the received ultrasonic backscatter at the interrogator, where decoding the ultrasonic backscatter includes analyzing a frequency of arrival of the received ultrasonic backscatter at the interrogator to determine a calculated switching frequency of a switch that modulates the frequency of the ultrasonic backscatter, matching the calculated switching frequency to a predetermined bit pattern, and recording the predetermined bit pattern as data transferred from the implantable device. In some embodiments, decoding the backscatter comprises a bit error rate of up to 6%.

[0017] The method may include adjusting a parameter of ultrasound transmitted by the interrogator based on ultrasound backscatter received from the implantable device.

[0018] In some embodiments, modulating the frequency of the ultrasound backscatter may include switching the implantable device between an open state and a closed state at a switching frequency from a plurality of different switching frequencies. For example, the closed state may short out the ultrasound transducer. The open state may be a load configuration in which electrical energy is collected from the ultrasound and maximum power is transferred from the ultrasound to the implantable device to power the implantable device.

[0019] The method may include switching a switch of the implantable device between an open state and a closed state, the closed state configured to periodically (e.g., sinusoidally) vary the acoustic energy reflected by the implantable device.

[0020] The method may include instructing the implantable device by an interrogator via ultrasound to cause the implantable device to perform one or more functions. For example, the one or more functions may include determining an average incident power from the ultrasound.

[0021] The method may further include transmitting ultrasound from the interrogator.

[0022] The method may further include receiving ultrasonic backscatter at the interrogator.

[0023] In some embodiments of the method, the interrogator comprises one or more transducers for transmitting ultrasound and receiving ultrasound backscatter.

[0024] A device for communicating via ultrasound may include an ultrasonic transducer configured to receive ultrasound and emit ultrasonic backscatter, a switch configured to modulate a frequency of the emitted ultrasonic backscatter, and a circuit (optionally an integrated circuit) configured to operate the switch to encode data of the emitted ultrasonic backscatter based on frequency.

[0025] In some embodiments of the device, the switch is switchable at a switching frequency from a plurality of different switching frequencies, and the data encoded in the emitted ultrasound backscatter is based on the switching frequency, with each switching frequency associated with a different predetermined bit pattern. In some embodiments, each predetermined bit pattern comprises two or more digital bits. In some embodiments, the predetermined bit patterns associated with the plurality of different switching frequencies may be Gray coded.

[0026] The interrogator may be configured to transmit ultrasound including instructions, and the switching frequency may be based at least on instructions received from the interrogator via ultrasound.

[0027] In some embodiments, the switching frequency is based at least on the relative maximum intensity of ultrasonic backscatter receivable at the interrogator.

[0028] The device may further include one or more sensors configured to detect information (e.g., sensor information), the data including the detected information, and the switching frequency from the plurality of different switching frequencies may be based at least on the detected information. The information may include, for example, a power level of the device. In some embodiments, the one or more sensors comprise one or more of a pressure sensor, a pH sensor, a temperature sensor, and an analyte sensor.

[0029] In some embodiments, the device may include a status indicator, and the switching frequency from the plurality of different switching frequencies is based on at least one or more statuses of the status indicator.

[0030] In some embodiments, the switch is capable of switching through a sequence of two or more switching frequencies from a plurality of different switching frequencies to encode data, each switching frequency may be associated with a different predefined bit pattern, for example.

[0031] In some embodiments, an interrogator is configured to receive the ultrasonic backscatter, decode the ultrasonic backscatter by analyzing the frequency of arrival of the ultrasonic backscatter to determine a calculated switching frequency of the switch, align the calculated switching frequency to a predefined bit pattern, and record the predefined bit pattern as data transferred from the device. In some embodiments, the interrogator is capable of analyzing the ultrasonic backscatter with a bit rate error of up to 6% in order to decode the ultrasonic backscatter.

[0032] In some embodiments, ultrasound is transmitted by an interrogator, the interrogator being configured to adjust parameters of the ultrasound transmitted by the interrogator based on the ultrasound backscatter received from the device.

[0033] In some embodiments, the switch is switchable between an open state and a closed state at a switching frequency from a plurality of different switching frequencies. The closed state can, for example, short out one or more ultrasonic transducers of the device. In some embodiments, the open state is a load configuration in which electrical energy is collected from the ultrasonic waves and maximum power is transferred from the ultrasonic waves to the device to power the device. In some embodiments, switching the switch between the open state and the closed state is configured to periodically (e.g., sinusoidally) vary the acoustic energy reflected by the device.

[0034] In some embodiments of the device, ultrasound transmitted from the interrogator is configured to instruct the device to perform one or more functions, which may include, for example, determining the average incident power from the ultrasound.

[0035] In some embodiments, the interrogator comprises one or more transducers configured to transmit ultrasound and receive ultrasound backscatter.

[0036] In some embodiments, the device is an implantable device.

[0037] In some embodiments, the device includes two or more electrodes capable of emitting an electrical pulse to tissue, hi some embodiments, the device includes two or more electrodes capable of detecting an electrophysiological pulse.

[0038] Further described herein is a system for communicating via ultrasound, the system including a device and an interrogator, the interrogator including one or more ultrasound transducers configured to receive emitted ultrasound backscatter and transmit ultrasound to the device, and a processor configured to decode the emitted ultrasound backscatter. [Brief description of the drawings]

[0039] The present disclosure will now be described, by way of example only, with reference to the accompanying drawings.

[0040] [Figure 1] 13 shows an example of how a switch of an implantable device can be operated to modulate the frequency of ultrasound backscatter according to some embodiments.

[0041] [Diagram 2]An example is shown of modulating the frequency of ultrasonic backscatter based on the frequency of a carrier signal and the switching frequency of a switch. The carrier signal is shown in the top panel, and the switch states are shown in the second panel. The backscatter signal generated from mixing the carrier signal with the switch states is shown in the third panel.

[0042] [Diagram 3] 1 illustrates examples of envelope-extracted backscattering at different switching frequencies, represented by Frequency 0 and Frequency 1 in Table 1, according to some embodiments.

[0043] [Figure 4] 4 shows a flow chart illustrating a method 400 for detecting a signal in an interrogator, according to some embodiments.

[0044] [Diagram 5] 1 shows a plot of bit error rate (BER) for each transducer channel of a 16 channel interrogator according to some embodiments.

[0045] [Figure 6] 13 shows an example plot of power levels read from a piezoelectric voltage-based frequency modulation in an implantable device using the thresholds in Table 2, according to some embodiments.

[0046] [Figure 7] 1 shows an example of switching frequency calculated for received ultrasound backscatter strength vs. channel at the interrogator, according to some embodiments. As shown in this figure, different switching frequencies have higher or lower signal-to-noise depending on the physical properties of the ultrasound transducer (e.g., piezoelectric crystal) used.

[0047] [Figure 8]FIG. 1 shows an example flow diagram illustrating a steering method for a closed loop power transfer algorithm utilizing the communication methods described herein, in accordance with some embodiments.

[0048] [Figure 9] 1 shows a flow diagram illustrating a method 1000 for low-power communication using ultrasound, according to some embodiments.

[0049] [Figure 10] 1 illustrates an interrogator in communication with an implantable device, according to some embodiments. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0050] Noise caused by passive reflections and other reflective interference sources can alter the accuracy and quality of data communication encoded by amplitude modulation of ultrasonic backscatter. Described herein are devices and systems that enable ultrasonic communication that is more protected against passive reflections and other reflective interference sources. Additionally, methods for using the devices and systems for low power and low noise ultrasonic communication are described. The devices and systems include an ultrasonic transducer configured to receive ultrasonic waves and emit ultrasonic backscatter, and include a switch for modulating the frequency of the emitted ultrasonic backscatter. The switch can operate to modulate the frequency of the ultrasonic backscatter such that data is encoded into the emitted ultrasonic backscatter. Encoding data at different frequencies or frequency sequences rather than at different amplitudes provides a method of ultrasonic data communication that is more protected against passive reflections and other reflective interference.

[0051] The use of ultrasound to operate and power implantable devices may be advantageous over other approaches because biological tissue absorbs ultrasound at a significantly lower rate than other types of waves, such as RF waves. This property of ultrasound may allow the device to be implanted at greater depths within the subject, as well as reduce tissue heating due to energy absorbed by the tissue.

[0052] As used herein, the singular forms "a," "an," and "the" include plural references unless the context clearly dictates otherwise.

[0053] Reference herein to "about" or "approximately" a value or parameter includes (and describes) variations that are directed to the value or parameter itself. For example, a description that refers to "about X" includes a description of "X."

[0054] It will be understood that the embodiments and variations of the invention described herein include those that "consist of" and / or "consist essentially of".

[0055] When a range of 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 those included limits are also included in the disclosure.

[0056] It is to be understood that one, some, or all of the features of the various embodiments described herein may be combined to form other embodiments of the present invention. The headings used herein are for organizational purposes only and should not be construed as limiting the subject matter described.

[0057] The features and preferences described above in connection with the "embodiments" are individual preferences and are not limited to that particular embodiment only, and they can be freely combined with features from other embodiments that are technically feasible to form preferred combinations of features. The description is presented to enable one of ordinary skill 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.

[0058] The disclosures of all publications, patents, and patent applications mentioned herein are each incorporated herein by reference in their entirety. To the extent that any reference incorporated by reference conflicts with the present disclosure, the present disclosure shall control.

[0059] As further described herein, a device for communicating via ultrasound can include an ultrasonic transducer, a switch, and a circuit (e.g., an integrated circuit) configured to operate the switch. The ultrasonic transducer can receive ultrasonic waves (e.g., ultrasonic waves transmitted by a separate device such as an interrogator) and emit ultrasonic backscatter (e.g., which can be received by the interrogator). Without intervention, the frequency of the ultrasonic backscatter depends on the frequency of the incident ultrasonic waves. However, the switch is configured to modulate the frequency of the emitted ultrasonic backscatter. For example, the switch can short the ultrasonic transducer at a desired frequency causing modulation of the ultrasonic backscatter frequency. The circuit can operate the switch to encode data in the emitted ultrasonic backscatter based on the frequency. For example, different frequencies can be used to communicate different bit patterns.

[0060] The data transmitted by the device can be information about the device, e.g., device status, or data collected by the device (e.g., sensor information, such as information indicative of electrophysiological signals, pH, temperature, pressure, or other physiological information sensed by one or more sensors of the device). According to some embodiments, the implantable device can include two or more electrodes capable of detecting electrical pulses in the subject, e.g., electrophysiological pulses transmitted by nerves. According to some embodiments, the implantable device can include two or more electrodes capable of emitting electrical pulses to tissues of the body.

[0061] According to some embodiments, the implantable device may include an ultrasound transducer capable of receiving ultrasound emitted by the interrogator, converting mechanical energy of the received ultrasound into electrical energy to power the implantable device, and emitting ultrasound backscatter to the interrogator. The interrogator may comprise one or more transducers for transmitting ultrasound and receiving ultrasound backscatter. According to some embodiments, data may be encoded into the ultrasound backscatter by modulating the frequency of the ultrasound backscatter. Encoding data into the ultrasound backscatter in this manner allows for low noise transmission through relatively noisy environments (such as within the body) compared to communication protocols that rely on amplitude modulation to encode data into the ultrasound backscatter. For example, encoding data using amplitude modulation suffers from noise issues such as passive reflections that can change the amplitude of the ultrasound backscatter. However, encoding data using frequency modulation cannot be altered by passive reflections, thus eliminating the noise factor and the need to calibrate the frequency modulation to account for such environmental noise. According to some embodiments, the data encoded into the ultrasound backscatter may be related to the instantaneous power impinging on the ultrasound transducer of the implantable device. According to some embodiments, the data encoded in the ultrasound backscatter may relate to, for example, sensor data from one or more sensors of the implantable device, any acknowledgment responses from the implantable device, and / or the operational status of the implantable device. The one or more sensors may be configured to detect power levels, pH, temperature, pressure, electrophysiological pulses, and analyte concentrations (such as oxygen levels). Thus, the sensor data of the one or more sensors may include power levels, pH, temperature, pressure, electrophysiological pulses, and analyte concentrations (such as oxygen levels).

[0062] The implantable device can include a switch for modulating the frequency of the ultrasound backscatter to encode data in the ultrasound backscatter emitted from the implantable device. The ultrasound backscatter can include a pattern that is unique to the implantable device. According to some embodiments, the pattern can include a signature of the implantable device that can be used to distinguish the implantable device from other devices.

[0063] According to some embodiments, data may be encoded into the ultrasound backscatter by selecting a switching frequency of a switch of the implantable device. The switching frequency may be selected from a plurality of different switching frequencies, and each switching frequency may be associated with a predefined bit pattern. The number of digital bits in any one of the predefined bit patterns may be based on a binary logarithm of some of the plurality of different switching frequencies. The binary logarithm may be used to calculate the number of digital bits required to encode a message. For example, if there are 16 different switching frequencies that can be generated, the binary logarithm of 16 (which is 4) specifies that, given the 16 different switching frequencies, four digital bits can be communicated per transmission. By operating the switch to modulate the frequency of the ultrasound backscatter based on the selected switching frequency, data associated with the selected switching frequency may be encoded into the ultrasound backscatter. According to some embodiments, the switching frequency may be a sequence of different frequencies.

[0064] According to some embodiments, at least four different switching frequencies are available, for example, 4 to 256 different switching frequencies are available (e.g., 4 to 8, 8 to 16, 16 to 32, 32 to 64, 64 to 128, or 128 to 256 different switching frequencies are available).

[0065] According to some embodiments, the selection of the switching frequency may be based on one or more of the sensor data, the strength of the ultrasonic backscatter at a given frequency, and an operating state of the implantable device. According to some embodiments, the operating state of the implantable device may indicate, for example, that the implantable device is powered on by ultrasonic waves from the interrogator, the interrogator is interrogating the implantable device for data, the implantable device is responding to the interrogation, measurements made by the implantable device are complete, a fault in the implantable device has been detected, and the implantable device is receiving a particular power level from the interrogator.

[0066] According to some embodiments, the switching frequency is the frequency at which a switch of the implantable device shorts out an ultrasound transducer of the device. For example, the implantable device may include a switch and a digital logic component that determines the state of the switch (e.g., whether the switch should be open or closed). According to some embodiments, the closed state may be a short state. The switching frequency may be selected by toggling the switch between an open state and a closed state at a selected switching frequency. According to some embodiments, when the switch is in a closed state, the implantable device cannot harvest energy from the incoming ultrasound waves (e.g., may be stored in an energy storage circuit) because no energy flows to the implantable device. In the closed state, the implantable device may be configured to redirect at least some energy from the incoming ultrasound waves. According to some embodiments, when the switch is in an open state, the implantable device may be in a load configuration in which electrical energy is harvested from the ultrasound waves and maximum power is transferred from the ultrasound waves to the implantable device to power the implantable device. Switching the switch between open and closed states can be configured to periodically (eg, sinusoidally) vary the acoustic energy reflected by the implantable device.

[0067] According to some embodiments, the ultrasound backscatter is emitted at a frequency, which can provide multiple bits of information by encoding multiple bits into the frequency. As described above, data such as multiple bits can be encoded into a frequency by selecting a switching frequency of the implantable device associated with the multiple bits and modulating the frequency emitted from the implantable device using the selected switching frequency. For example, Table 1 below lists examples of different backscatter frequency symbols, each associated with a different bit pattern. Each backscatter frequency and corresponding bit pattern is associated with a switching frequency. In the example of Table 1, the number of possible backscatter frequency symbols to select is 16, and therefore the number of digital bits per question can be 4. According to some embodiments, the possible number of backscatter frequency symbols can be up to 32 or 64. [Table 1]

[0068] In the example of Table 1, the association between bit pattern and frequency is Gray coded, so that adjacent frequencies differ by one bit. Alternative coding schemes are possible, with the most appropriate one depending on the typical systematic errors of the frequency detector.

[0069] The selected switching frequency may be selected from a plurality of predetermined switching frequencies, which may be based on the frequency of the ultrasound received by the implantable device. For example, in some embodiments, the plurality of predetermined switching frequencies is within about 10%, within about 15%, within about 20%, within about 25%, or within about 30% of the frequency of the ultrasound received by the implantable device.

[0070] According to some embodiments, data encoded ultrasound backscatter may be emitted from the implantable device when the implantable device is activated from ultrasound received from the interrogator. The ultrasound backscatter emitted from the implantable device may be received and decoded at the interrogator. According to some embodiments, the interrogator may be configured to analyze the ultrasound backscatter to determine a frequency or change in frequency of the ultrasound backscatter. According to some embodiments, the interrogator may be configured to determine a switching frequency of a switch of the implantable device. Determining may include calculating the switching frequency. Based on the determined switching frequency, the interrogator may be configured to associate the switching frequency with a predefined digital bit pattern to decode encoded data in the ultrasound backscatter. According to some embodiments, the interrogator may be preprogrammed to associate the determined switching frequency with a corresponding predefined digital bit pattern to decode the ultrasound backscatter. According to some embodiments, the implantable device may be preprogrammed with one or more different switching frequencies, each of which may be associated with a different predefined bit pattern. Similarly, the interrogator may be pre-programmed with one or more different switching frequencies for decoding ultrasonic backscatter.

[0071] According to some embodiments, decoding the ultrasonic backscatter may include a bit error rate of up to 6%. According to some embodiments, decoding the ultrasonic backscatter may include a bit error rate of up to 5%. According to some embodiments, decoding the ultrasonic backscatter may include a bit error rate of up to 4%. According to some embodiments, decoding the ultrasonic backscatter may include a bit error rate of up to 3%. According to some embodiments, decoding the ultrasonic backscatter may include a bit error rate of up to 2%.

[0072] Ultrasonic communication between the implantable device and the interrogator can enable closed-loop communication. That is, the interrogator can change or adjust parameters of the outgoing ultrasound in response to encoded data received from the implantable device and decoded at the interrogator. For example, depending on the power level received at the implantable device, the frequency of the emitted ultrasound backscatter can be related to the power level received at the implantable device. In this way, the implantable device can inform the interrogator how much instantaneous power is hitting the implantable device via the frequency of the emitted ultrasound backscatter. In response, the interrogator can steer to adjust and track how well the ultrasound from the interrogator is hitting the implantable device over time. Similarly, the implantable device can respond according to the ultrasound received at the implantable device from the interrogator. A given response can include selecting a switching frequency based at least on instructions received at the implantable device from the interrogator. According to some embodiments, the instructions may cause the implantable device to perform one or more functions, which may include one or more of determining the average incident power received from the ultrasound, sensing sensor information (electrophysiological pulses, pH, oxygen levels, etc.), and performing high sampling rates of sensor data streams such as electrophysiological voltage time series (rather than pulse sensing).

[0073] According to some embodiments, the implantable device and the interrogator may be programmed with the same multiple different switching frequencies. For example, the implantable device may be programmed with multiple different switching frequencies from which one or more switching frequencies may be selected. Each switching frequency may be associated with a predefined bit pattern configured to communicate a message to an external device, such as an interrogator. The interrogator may be configured to align the determined switching frequency with one of the multiple different switching frequencies to determine a message from the implantable device. For example, if the determined switching frequency aligns with a first switching frequency of the multiple different switching frequencies, the interrogator may be configured to interpret the predefined bit pattern of the first switching frequency as a message from the implantable device. According to some embodiments, for each of the multiple different switching frequencies, the different predefined bit patterns associated with each different switching frequency are Gray coded.

[0074] The implantable device may include one or more piezoelectric crystals having different resonant operating modes. According to some embodiments, optimal signal-to-noise transfer from the implantable device to the interrogator may be achieved by modulating the frequency of the ultrasound backscatter such that the frequency corresponds to the resonant operating mode of the one or more piezoelectric crystals. For example, a particular frequency of ultrasound backscatter may achieve a particular backscatter strength receivable at the interrogator. The strength of the particular backscatter strength may be a relative maximum strength. According to some embodiments, the particular frequency may correspond to a characteristic (e.g., a relative minimum) of the impedance data of the one or more piezoelectric crystals.

[0075] According to some embodiments, one or more characteristics of the backscatter intensity and impedance data can be used to determine a process for selecting a set of switching frequencies that can modulate the frequency of the ultrasound backscatter, and thus, according to some embodiments, the implantable device can include an ultrasound transducer configured to receive ultrasound from the interrogator and emit ultrasound backscatter. The implantable device can also include a switch configured to modulate the frequency of the ultrasound backscatter and a controller, such as an application specific integrated circuit (ASIC), configured to communicate data to the interrogator using the ultrasound backscatter. In general, there is a finite amount of incident acoustic energy originating from the interrogator, which is then reflected by the implantable device towards the interrogator. When the implant's ultrasound transducer (such as a piezoelectric circuit) is shorted, the maximum amount of incoming acoustic energy is reflected. By adding a matched load to the piezoelectric circuit of the implantable device, electrical energy can be collected and maximum power can be transmitted to the ASIC. According to some embodiments, the matched load can be a resistor.

[0076] To conserve energy, switching the load causes the reflected energy to change. Traditionally, the change in reflected energy can be detected, allowing digital communication by having high amplitude reflected energy represent a 1 and low amplitude reflected energy (when the energy is being collected by the ASIC) represent a 0. Traditional amplitude modulated communication schemes work to transmit 1s and 0s, but have limitations due to environmental interference and low data rates between the implant and the interrogator. Alternatively, the devices and systems described herein are configured to adjust the reflected energy by modulating the frequency of the reflected energy based on the frequency at which the piezoelectric circuit is shorted.

[0077] Rather than via individual high and low amplitude 0s and 1s, the devices and systems described herein communicate by changing the frequency of the load change in the piezoelectric circuit, so that multiple data bits can be communicated via frequency modulated ultrasonic backscatter. For example, an ultrasonic signal can be transmitted from an interrogator to the piezoelectric circuit at 1.5 MHz. The piezoelectric circuit can then be switched at some switching frequency between a short and a load configuration such that the reflected acoustic energy varies periodically (e.g., sinusoidally). Rather than detecting the modulation amplitude at some threshold, a frequency analysis can be performed at the interrogator, and the switching frequency of the piezoelectric circuit can be estimated by analyzing the ultrasonic backscatter signal, as shown in FIG. 4. The switching frequency can represent a set number of bits that can be recorded as data to be transferred for interrogation.

[0078] FIG. 1 illustrates an example of how a switch of an implantable device can operate to modulate the frequency of ultrasonic backscatter, according to some embodiments. FIG. 1 illustrates a plot of the switching behavior of a switch with amplitude (voltage) on the y-axis and time (microseconds) on the x-axis. The diagram represents the voltage on the piezoelectric crystal of the implantable device as a result of the switching frequency applied by the ASIC. The pattern on the piezoelectric crystal is reflected in the ultrasonic backscatter emitted from the implantable device, and in the example of FIG. 1 the carrier frequency is 1.5 MHZ and the switch is configured to short the implantable device at a frequency of 125 kHz. When the switch is closed, for example in region A, the emitted ultrasonic backscatter intensity has a minimum amplitude, indicating that the piezoelectric crystal of the implantable device is shorted. When the switch is open, for example in region B, the emitted backscatter intensity is at a maximum amplitude. The ultrasonic backscatter received by the interrogator can identify the 125 kHz frequency segment and the bit pattern associated with the 125 kHz frequency can be decoded (e.g., using a lookup table).

[0079] Another example of modulating the frequency of the ultrasound backscatter based on the frequency of the carrier signal and the switching frequency of the switch is provided in FIG. 2. FIG. 2 shows what happens to the carrier signal (top panel) in the implantable device in the frequency domain based on the switch state of the implantable device (second panel). Specifically, the top panel represents the input signal that is input to the implantable device. The second panel ("Implant Switch State") represents the state of the switch of the implantable device as determined by a digital logic controller configured to operate the switch. An example of a digital logic controller can be an application specific integrated circuit (ASIC), a microcontroller, a field programmable gate array (FPGA), or other similar component. Specifically, the second panel shows the switch being operated to step between two states at a particular switching frequency. The particular switching frequency of the switch affects how the ultrasound backscatter (reflected acoustic energy) is modulated. The third panel ("Mixed Signal") shows an example of an ultrasound backscatter signal generated from mixing the carrier signal (top panel) with the switch state (second panel). By switching the switch at a switching frequency, the frequency of the ultrasonic backscattered signal is modulated. Such frequency modulation may be more robust than amplitude modulation, since amplitude modulation can be corrupted by passive environmental interference.

[0080] The bottom panel of Figure 2 shows a different representation of the ultrasonic backscatter signal shown in the third panel. The plot shown in the bottom panel is called the frequency domain and shows that by multiplying the carrier signal and the switch state signal together, different frequency content can be generated. In the example shown in the bottom panel, the frequency of the switch is the low frequency peak ("sw") and the frequency of the carrier signal is the higher frequency ("x"). By multiplying the frequency of the switch with the frequency of the carrier signal, new frequency components can be generated, shown in orange ("x mixed"). After mixing (orange signal in the bottom panel), the frequency of the ultrasonic backscatter signal is "mixed" or shifted by the switching frequency + and -. The frequency content of the ultrasonic backscatter signal received at the interrogator remains centered at 1.5 MHz, but the amplitude of the backscatter changes at certain frequencies. In this case, the frequency is detectable even though some amplitude changes are not detected due to typical amplitude modulation problems.

[0081] According to some embodiments, a digital bit can be encoded by associating a particular bit (or bit pattern) with a backscatter frequency, or (more preferably) with a switching frequency of a switch. For example, assume that an implantable device is capable of backscattering one of 16 different backscatter frequencies, as shown in Table 1 above. FIG. 3 shows an example of envelope-extracted ultrasound backscatter at different switching frequencies, represented by Frequency 0 (300.0 kHz) and Frequency 1 (57.5 kHz) in Table 1, according to some embodiments. The y-axis is intensity proportional to the ultrasound pressure received by the interrogator, and the x-axis is time (in microseconds). The curves in FIG. 3 represent the backscatter frequencies receivable from the implantable device at the interrogator. Comparing the two frequencies shown, Frequency 0 is a relatively high frequency and Frequency 1 is a relatively low frequency oscillation.

[0082] FIG. 4 shows a flow chart illustrating a method 400 for detecting a signal in an interrogator according to some embodiments. The method 400 may be applied to receive and analyze different frequencies. In step 410, the interrogator generates a signal. The signal may be focused to a specific point for a beamformer if it is a multi-element transducer. In step 420, the original signal or the focused signal may be processed in a beamformer. The beamformer may be used to emphasize a specific spatial region of the received signal. In step 430, an envelope extraction of the received signal may be performed. In step 440, a spectral analysis of the envelope extraction may be performed. The spectral analysis may include a frequency decomposition of the received signal into different frequency components. An example of such a spectral analysis includes a fast Fourier transform. In step 450, the frequency may be determined based on the output of the spectral analysis.

[0083] According to some embodiments, the interrogator includes an array of ultrasonic transducers. The array of transducers may include channels configured to steer the focus of the ultrasound and steer from where the ultrasonic backscattered signals are received. Each channel is a signal path that includes a transducer and physical elements (electronic components) of the interrogator. Each channel may receive different information, for example, based on where each channel is physically located on the transducer array. Each channel may also transmit different signals that focus the beam to different points. In this way, the use of multiple channels allows for signal diversity in a multi-element system, and the signals can be combined in a meaningful way. For example, different chatter may be received in each of the different channels depending on the angle or distance of the implantable device from the transducer array. The different chatter may be used to put together a single picture of the communication received from the implantable device.

[0084] FIG. 5 illustrates a plot of bit error rate (BER) for each transducer channel of an interrogator including 16 channels, according to some embodiments. The y-axis is a scale from 0 to 1, and the x-axis is an array of transducer channels in the interrogator. Different channels of the interrogator are configured to receive ultrasonic backscatter at different locations in space. In the example of FIG. 5, a 16-element transducer was used to receive ultrasonic backscatter. According to some embodiments, ultrasonic backscatter can be acquired simultaneously across all channels, and thus, the variation in performance across the transducer array can be attributed to spatial variations in the backscatter characteristics. The BER can be calculated by counting the number of received bits that do not match the expected bits. The lower BER illustrated in FIG. 5 indicates that the majority of the decoded bits were correctly decoded. An example of a correct decode is when a zero bit is transmitted to the interrogator and a zero bit is received by the interrogator and decoded as a zero bit.

[0085] Overall, Figure 5 illustrates an example of an ultrasound communication protocol with relatively few errors in which some channels (such as channel 1 and channel 2) have relatively high bit error rates. According to some embodiments, the relative bit errors may be due to the physical location of these elements relative to the implant. According to some embodiments, the signal quality may be based on the spatial geometry of the transducer and the channels.

[0086] Despite channel 2 having a relatively high bit error rate compared to the other channels, one or more channels with a relatively low bit error rate can be selected to obtain a view of the communication signal from the other channels. According to some embodiments, in order to compensate for one or more channels with a relatively high bit error rate, one or more other channels can be selected in a multi-element system based on a signal quality criterion. According to some embodiments, in order to compensate for one or more channels with a relatively high bit error, a beamformer can be used to combine information from all channels into a weighted average signal. The term "relatively high bit error rate" is intended to be for certain channels / frequencies that have no or very low bit error rate and is not intended to indicate that the bit error rate of those channels is unusable or would otherwise impair communications.

[0087] According to some embodiments, advantages of ultrasonic communication as described herein include low error rate (see FIG. 5) and low power, which allows for effective closed-loop communication. Closed-loop communication can be used to power an implantable device that may be implanted deep in the body. According to some embodiments, depending on the input voltage to the piezoelectric crystal of the implantable device, the implantable device can backscatter. In this way, if the interrogator configured to determine how much power is hitting the implantable device can perform electronic steering to compensate or adjust the power depending on the requirements of the implantable device. Thus, closed-loop communication can include transmitting power via ultrasound from the interrogator to the implantable device, emitting ultrasonic backscatter from the implantable device, and adjusting the frequency of the ultrasonic backscatter based on the voltage level input at the implantable device. An example of closed-loop communication includes charging the implantable device at a target rate based on the charge level of the implantable device.

[0088] Frequency modulation of the ultrasonic backscatter can be applied to convey information about the variation of the piezoelectric voltage at the implantable device. The variation of the piezoelectric voltage indicates the variation of the input power (current x voltage). Such variation can occur in a handheld ultrasonic link. Table 2 below shows exemplary voltage range readings that can be associated with a particular switching (or shorting) frequency of the piezoelectric element at the implantable device. At the interrogator, the ultrasonic backscatter can be received and decoded by associating the frequency of the ultrasonic backscatter and / or the switching frequency at the implantable device with a predefined bit pattern as described above. Each bit pattern can represent a power level that can be interpreted by the interrogator. For example, if the implantable device has an input voltage of less than half a volt, the implantable device can set the switching frequency to 50,000 Hz. The interrogator can be configured to calculate the switching frequency, and if it calculates that the switching frequency is less than 55,000, the interrogator can record that the voltage on the piezoelectric on the implantable device is less than half a volt and can reference the power level zero for discretization. Figure 6 shows an example plot of power levels read from a piezoelectric voltage-based frequency modulation in an implantable device using the thresholds in Table 2. The data and correlations such as those shown in Table 2 and Figure 6 can be used to effectively charge an implantable device based on its charging needs. [Table 2]

[0089] According to some embodiments, plotting the magnitude of the ultrasound backscatter frequency versus the calculated switching frequency can show trends that may be indicative of the physical properties of the piezoelectric in the implantable device. Figure 7 shows an example of the ultrasound backscatter intensity received at the interrogator versus the switching frequency calculated for a channel, according to some embodiments. As shown in this figure, different switching frequencies have higher or lower signal-to-noise depending on the physical properties of the ultrasound transducer (e.g., piezoelectric crystal) used.

[0090] According to some embodiments, the switching frequency can be determined by a controller of the implantable instrument. The determination can be random or based on the piezoelectric characteristics of the implantable device. For example, assume that it is desired to have a large magnitude of ultrasound backscatter at the interrogator. In the example of FIG. 7, there is a relative maximum (local maximum) in the magnitude of the backscatter near 60,000 Hz. Thus, to achieve a high magnitude of ultrasound backscatter at the interrogator, the switching frequency can be selected to have a region that includes a local maximum in the backscatter intensity. That is, depending on the application, the switching frequency can be selected based at least in part on the strength of the ultrasound backscatter at the interrogator.

[0091] According to some embodiments, a switching frequency close to the resonant characteristic of the piezoelectric crystal of the implantable device can potentially result in a local maximum in backscatter at the detector. According to some embodiments, a larger ultrasonic backscatter magnitude at the detector can lead to a lower bit error rate. According to some embodiments, the implantable device can be calibrated to select a switching frequency based on the magnitude received at the interrogator, and the selected switching frequency can correspond to a particular region of the piezoelectric impedance spectrum.

[0092] According to some embodiments, a closed loop power delivery algorithm may be implemented using the communication protocols described herein. Figure 8 shows an example flow diagram illustrating a steering method 800 for closed loop communication, according to some embodiments. Method 800 outlines the processing of power level data in a closed loop system. That is, method 800 illustrates how power level data may be communicated.

[0093] In step 810, electronic steering may be set in the interrogator. In step 820, the electronic steering may be communicated to a transducer array in the interrogator. The transducer array may emit ultrasound waves to the implantable device. In step 830, the implantable device may receive ultrasound waves, measure incident power, and emit information-containing ultrasound backscatter. In step 840, one or more ultrasound transducers in the interrogator may receive ultrasound backscatter from the implantable device, and a processor in the interrogator may decode the ultrasound backscatter to determine the incident power received at the implantable device. In step 850, new steering parameters may be calculated in the interrogator based on the determined incident power received at the implantable device.

[0094] 9 shows a flow diagram illustrating a method 900 for low-power communication using ultrasound, according to some embodiments. The method 900 may be implemented by an implantable device as described herein.

[0095] At step 910, one or more ultrasound transducers of the implantable device may receive ultrasound transmitted by the interrogator. The ultrasound may be transmitted by one or more ultrasound transducers of the interrogator.

[0096] At step 920, one or more ultrasound transducers of the implantable device can emit ultrasound backscatter that includes encoded data. Data can be encoded into the ultrasound backscatter by modulating the frequency of the ultrasound backscatter.

[0097] According to some embodiments, the implantable device may include a switch configured to open and close a piezoelectric circuit of the implantable device. According to some embodiments, when the switch is closed, the circuit is shorted. The controller may be configured to operate the switch between an open state and a closed state and to select a switching frequency at which the switch changes between the open state and the closed state. According to some embodiments, the method 900 may include selecting a switching frequency from a plurality of different switching frequencies. As further described above, each switching frequency may be associated with a predefined bit pattern that may include a plurality of bits. The association between each switching frequency and the predefined bit pattern may be Gray coded.

[0098] According to some embodiments, the implantable device may include one or more sensors configured to detect sensor information, such as one or more of a power level, pH, temperature, pressure, electrophysiological pulse, and analyte concentration (such as oxygen level). According to some embodiments, the method 900 may include detecting the sensor information by one or more sensors of the implantable device. The implantable device may include a status indicator configured to determine an operational state of the implantable device. The operational state may include, for example, whether the implantable device is on, off, in a measurement mode, functioning well, has encountered an error, is fully charged, or requires more power to reach a fully charged level. According to some embodiments, the selection of the switching frequency may be based on one or more of instructions from the interrogator, sensor information detected by one or more sensors of the implantable device, and an operational state of the implantable device.

[0099] According to some embodiments, the method 900 can include decoding the ultrasonic backscatter at the interrogator. The decoding can include analyzing the incoming ultrasonic backscatter to calculate a frequency of the ultrasonic backscatter and a switching frequency of a switch in the implantable device. The calculated switching frequency can be matched to a predefined bit pattern that can be recorded as data transferred from the device.

[0100] According to some embodiments, the implantable device can include one or more first electrodes configured to emit an electrical pulse to tissue. According to some embodiments, the implantable device can include one or more second electrodes configured to detect an electrophysiological pulse. According to some embodiments, the method 1000 can include emitting an electrical pulse and detecting an electrophysiological pulse.

[0101] FIG. 10 shows an interrogator 1010 in communication with an implantable device 1020. An external ultrasound transceiver emits ultrasound waves ("carrier waves") that can pass through tissue. The carrier waves induce mechanical vibrations in an ultrasound transducer (e.g., bulk piezoelectric transducer, PMUT, or CMUT). A voltage is generated across the ultrasound transducer, giving a current that flows through a circuit (e.g., an integrated circuit) on the implantable device. The current flowing through the ultrasound transducer backscatters ultrasound waves to a transducer on the implantable device. In some embodiments, a circuit modulates the frequency of the ultrasound transducer to encode information by shorting the circuit at a switching frequency, and the resulting ultrasound backscattered waves encode the information. The backscattered waves can be detected by the interrogator and analyzed to interpret the information encoded in the ultrasound backscatter. According to some embodiments, the implantable device 1020 can include one or more sensors. According to some embodiments, the implantable device 1020 can include one or more electrodes.

[0102] The foregoing description has been described with reference to specific embodiments for purposes of illustration. However, the above exemplary discussion 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 have been 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.

[0103] 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 should 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 mentioned in this application are incorporated herein by reference.

Claims

1. 1. A method for communicating using ultrasound, comprising: receiving ultrasound waves transmitted by the interrogator at one or more ultrasound transducers of the implantable device; emitting ultrasonic backscatter comprising encoded data from the one or more ultrasonic transducers of the implantable device, wherein the data is encoded onto the ultrasonic backscatter by modulating a frequency of the ultrasonic backscatter.

2. 2. The method of claim 1, comprising selecting a switching frequency from a plurality of different switching frequencies to modulate the frequency of the ultrasonic backscatter, the switching frequency being configured to encode the data into the ultrasonic backscatter.

3. The method of claim 2 , wherein each switching frequency is associated with a different predetermined bit pattern.

4. The method of claim 2 or 3, wherein modulating the frequency of the ultrasonic backscatter comprises switching the implantable device at the switching frequency.

5. 5. The method of claim 2, wherein each predetermined bit pattern comprises two or more digital bits.

6. The method of claim 2 , wherein the predetermined bit patterns associated with the plurality of different switching frequencies are Gray coded.

7. 7. The method of claim 2, wherein the switching frequency is selected based at least on instructions received from the interrogator via the ultrasound.

8. The method of claim 2 , wherein the plurality of different switching frequencies are selected based at least on a strength of the ultrasonic backscatter at a given frequency.

9. 9. The method of claim 1, further comprising: detecting sensor information from one or more sensors of the implantable device, the data including the sensor information; and selecting the switching frequency from the plurality of different switching frequencies based on at least the sensor information.

10. The method of claim 9 , wherein the sensor information includes one or more of a power level, a pH, a temperature, a pressure, an electrophysiological pulse, and an analyte concentration.

11. The method of claim 10 , wherein the analyte concentration comprises an oxygen level.

12. 12. The method of claim 1, comprising: determining operational state information of the implantable device, the data comprising determined operational state information; and selecting the switching frequency from the plurality of different switching frequencies based on at least the determined operational state information.

13. 13. The method of claim 1, comprising selecting a sequence of two or more switching frequencies from a plurality of different switching frequencies for encoding the data, each switching frequency being associated with a different predetermined bit pattern.

14. receiving, at the interrogator, the ultrasonic backscatter containing the encoded data; decoding the received ultrasonic backscatter at the interrogator, where decoding the ultrasonic backscatter includes analyzing an incident frequency of the received ultrasonic backscatter at the interrogator to determine a calculated switching frequency of a switch that modulates the frequency of the ultrasonic backscatter, matching the calculated switching frequency to a predetermined bit pattern, and recording the predetermined bit pattern as decoded data transmitted from the implantable device; 14. The method of any one of claims 1 to 13, comprising:

15. The method of claim 14 , wherein the decoded data has a bit error rate of up to 6%.

16. 16. The method of claim 1, comprising adjusting parameters of the ultrasound transmitted by the interrogator based on the ultrasound backscatter received from the implantable device.

17. 17. The method of any one of claims 1 to 16, wherein modulating the frequency of the ultrasonic backscatter comprises switching a switch of the implantable device between an open state and a closed state at a switching frequency from a plurality of different switching frequencies.

18. 20. The method of claim 17, wherein the closed state shorts the one or more ultrasound transducers of the implantable device.

19. 19. The method of claim 17 or 18, wherein the open state is a load configuration in which electrical energy is harvested from the ultrasound and maximum power is transmitted from the ultrasound to the implantable device to power the implantable device.

20. 20. The method of any one of claims 17 to 19, wherein switching the switch of the implantable device between the open state and the closed state is configured to periodically vary the acoustic energy reflected by the implantable device.

21. 21. The method of any one of claims 1 to 20, comprising transmitting ultrasound from the interrogator to the implantable device, the ultrasound instructing the implantable device to perform one or more functions.

22. 22. The method of claim 21, wherein the one or more functions include determining an average incident power from the ultrasound.

23. 21. The method of any one of claims 1 to 20, comprising transmitting the ultrasound from the interrogator to the implantable device.

24. 24. A method according to any one of claims 1 to 23, comprising receiving the ultrasonic backscatter at the interrogator.

25. 25. The method of any one of claims 1 to 24, wherein the interrogator comprises one or more transducers for transmitting the ultrasound and receiving the ultrasound backscatter.

26. 1. A device for communicating via ultrasound, comprising: an ultrasonic transducer configured to receive ultrasonic waves and emit ultrasonic backscatter; a switch configured to modulate the frequency of the emitted ultrasonic backscatter; a circuit configured to operate a switch to encode data into the emitted ultrasonic backscatter based on the frequency; Including, the device.

27. 27. The device of claim 26, wherein the switch is switchable at a switching frequency from a plurality of different switching frequencies, and the data encoded into the emitted ultrasound backscatter is based on the switching frequencies, each switching frequency being associated with a different predetermined bit pattern.

28. 30. The device of claim 27, wherein each predetermined bit pattern comprises two or more digital bits.

29. 30. The device of claim 27, wherein the predetermined bit patterns associated with the multiple different switching frequencies are Gray coded.

30. 30. A device according to any one of claims 27 to 29, wherein the switching frequency is based at least on a maximum intensity of ultrasonic backscatter receiveable by an interrogator.

31. 31. The device of claim 27, comprising one or more sensors configured to detect sensor information, the data including detected sensor information, and the switching frequency from the plurality of different switching frequencies is based at least on the detected sensor information.

32. The device of claim 31 , wherein the sensor information includes a power level of the device.

33. 33. The device of claim 31 or 32, wherein the one or more sensors comprise one or more of a pressure sensor, a pH sensor, a temperature sensor, and an analyte sensor.

34. 34. The device of claim 27, comprising a status indicator, the switching frequency from the plurality of different switching frequencies being based on at least one or more statuses of the status indicator.

35. 35. The device of claim 27, wherein the switch is switchable at a sequence of two or more switching frequencies of a plurality of different switching frequencies to encode the data.

36. 36. The device of claim 35, wherein each switching frequency is associated with a different predetermined bit pattern.

37. 37. The device of any one of claims 26 to 36, wherein the switch is switchable between an open state and a closed state at a switching frequency of a plurality of different switching frequencies.

38. 38. The device of claim 37, wherein the closed state shorts the ultrasound transducer.

39. 39. The device of claim 37 or 38, wherein the open state is a load configuration in which electrical energy is harvested from the ultrasound and maximum power is transferred from the ultrasound to the device to power the device.

40. 40. A device according to any one of claims 37 to 39, wherein toggling the switch between the open and closed states is configured to periodically vary acoustic energy reflected by the device.

41. 41. The device of any one of claims 26 to 40, wherein the device is an implantable device.

42. 42. The device of any one of claims 26 to 41, comprising two or more electrodes configured to emit an electrical pulse to tissue.

43. 43. A device according to any one of claims 26 to 42, comprising two or more electrodes configured to detect electrophysiological pulses.

44. 1. A device for communicating via ultrasound, comprising: A device according to any one of claims 26 to 43; one or more ultrasound transducers configured to receive the emitted ultrasound backscatter and transmit ultrasound to a device; a processor configured to decode the emitted ultrasound backscatter; an interrogator including: Including, the system.

45. 45. The system of claim 44, wherein an interrogator is configured to transmit the ultrasound including instructions, and the switching frequency is selected based at least on the instructions received by the device.

46. 46. ​​The system of claim 44 or 45, wherein the interrogator is configured to decode the ultrasonic backscatter by analysing the incident frequency of the ultrasonic backscatter to determine a calculated switching frequency of the switch, matching the calculated switching frequency to a predetermined bit pattern, and recording the predetermined bit pattern as data transmitted from the device.

47. 47. The system of claim 46, wherein decoding the ultrasonic backscatter results in a bit rate error of up to 6%.

48. 48. The system of any one of claims 44 to 47, wherein the interrogator is configured to adjust parameters of the ultrasound transmitted by the interrogator based on the ultrasound backscatter received from the device.

49. 49. The system of any one of claims 44 to 48, wherein the ultrasound transmitted from an interrogator is configured to instruct the device to perform one or more functions.

50. 50. The system of claim 49, wherein the one or more functions include determining an average incident power from the ultrasound.

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