Ultrasonic transducers for implantable devices

Abstract

An implantable device may include a housing with an acoustic window, a piezoelectric transducer, and a fluid filler. The piezoelectric transducer and the fluid filler may be contained within the housing. The transducer may be attached to an interior surface of the acoustic window such that ultrasonic waves passing through the acoustic window do not pass through a fluid before reaching the piezoelectric transducer. The acoustic window and the piezoelectric transducer may be configured to acoustically resonate at approximately the same resonance frequency. The fluid filler may have an acoustic impedance that is not approximately equal to an acoustic impedance of the piezoelectric transducer.

Description

ULTRASONIC TRANSDUCERS FOR IMPLANTABLE DEVICESCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to and the benefit of U.S. Provisional Patent Application No. 63 / 580,698, filed September 5, 2023, the contents of which are incorporated herein in its entirety.FIELD

[0002] The present disclosure relates to ultrasonic transducers for use in medical devices, in particular ultrasonic transducers for use in implantable medical devices.BACKGROUND

[0003] Implantable medical devices can be used treat various medical conditions and to collect physiological data that provides insight into patient health and disease prognosis. For example, implantable devices for stimulating nerve tissue can be used to treat cancers, and implantable devices for monitoring intraocular pressure may be used to monitor the health of patients with glaucoma.

[0004] Many implantable devices have limited energy storage capabilities. In order to receive power, such devices may use ultrasonic transducers to harvest energy from ultrasonic waves emitted by a second device external to the implant. The same ultrasonic transducers may be used to transmit information from the implant, for example to an external interrogator device. These ultrasonic transducers are typically formed from piezoelectric materials, such as piezoceramics, that have high electromechanical coupling coefficients. The bioincompatibility of many piezoelectric materials necessitates encapsulating the ultrasonic transducers in a biocompatible housing to prevent the piezoelectric material from coming into contact with tissue.

[0005] While encapsulating an ultrasonic transducer in a housing may reduce the risk of the piezoelectric material damaging surrounding tissue, significant energy losses may occur as the ultrasonic waves pass between the housing and the transducer, reducing the transducer’s efficiency. The reduced efficiency may result in insufficient power transfer, which may increase the amount of time that the implantable device requires to harvest enough energy to function or may prevent the implant from turning on. This may burden the patient by extending or preventing treatment or measurement sessions. Additionally, lowtransducer efficiency may limit the amount and quality of information that the transducer can communicate, reducing the utility of the implant.SUMMARY

[0006] Provided herein is an implantable device with a piezoelectric ultrasonic transducer that is attached to an internal surface of the device’s biocompatible housing. The connection between the transducer and the housing may be such that ultrasonic waves transmitted to the implant through the housing do not pass through a fluid before reaching the piezoelectric transducer. This may significantly reduce energy losses from the ultrasonic waves as well as simplify the fabrication process, particularly for larger implants. In addition, the transducer and the acoustic window region of the implant housing to which the transducer is attached may be configured to acoustically resonate at about the same resonance frequency. This may maximize the amount of energy that is transferred from the ultrasonic waves to the implant during energy harvesting sessions.

[0007] The described implantable device may also be to communicate information using ultrasonic waves. In particular, the implant may communicate using backscatter ultrasonic waves that are produced by the piezoelectric transducer when current that was generated by the transducer in response to received ultrasonic waves flows back through the transducer. Modulating this current as a function of the information to be communicated may encode the information in the ultrasonic backscatter as the backscatter is emitted by the piezoelectric transducer. The backscatter waves emitted by the transducer and transmitted through the acoustic window may be retrieved by an external device and subsequently decoded to extract the information. Alternatively, the implant may actively generate ultrasonic waves to communicate information. To ensure that the ultrasonic waves emitted by the piezoelectric transducer pass through the acoustic window, the implant housing may contain a fluid fdler (e.g., atmospheric air) configured to act as a backing layer to the transducer by reflecting ultrasonic waves emitted by the transducer that are directed inward toward the center of the implantable device back outward toward the acoustic window.

[0008] Material characteristics of the acoustic window that increase the operational bandwidth of the transducer and, as a result, improve the transducer’s communication capabilities are also provided. In some embodiments, adding a second biocompatible material to the biocompatible material that forms the acoustic window, either by disposing a layer of the second material on the external-facing surface of the first material, or by forming a composite between the first housing layer material and the second material on the externalsurface of the housing layer. Adding the second material to the housing layer may cause the effective acoustic impedance of the acoustic window to more closely match the acoustic impedance of the tissue in which the implantable device is implanted. This may reduce the proportion of ultrasonic waves transmitted through the tissue to the device that are reflected by the device as well as the proportion of ultrasonic waves transmitted from the device through the tissue that are reflected by the tissue, thereby increasing the efficiency of ultrasonic wave transmission at the resonance frequency of the piezoelectric transducer and increasing the spread of ultrasonic frequencies that the transducer can receive and transmit.

[0009] An implantable device may comprise a housing comprising an acoustic window and a piezoelectric transducer contained within the housing and attached to an interior surface of the acoustic window such that ultrasonic waves passing through the acoustic window do not pass through a fluid before reaching the piezoelectric transducer. The acoustic window and the piezoelectric transducer may be configured to acoustically resonate at approximately the same resonance frequency.

[0010] The thickness of the acoustic window may be approximately equal to an integer multiple of a quarter of a wavelength in the acoustic window of sound at the resonance frequency. In particular, the thickness of the acoustic window may be approximately equal to half of the wavelength in the acoustic window of sound at the resonance frequency. The ratio of the thickness of the acoustic window to the wavelength in the acoustic window of sound at the resonance frequency may be less than 0.02.

[0011] Similarly, the thickness of the piezoelectric transducer may be approximately equal an integer multiple of a quarter of a wavelength in the piezoelectric transducer of sound at the resonance frequency. More specifically, the thickness of the piezoelectric transducer may be approximately equal to half of the wavelength in the piezoelectric transducer of sound at the resonance frequency.

[0012] The implantable device may also include a circuit that is electrically coupled to the piezoelectric transducer and is configured to control one or more implant functions. The circuit can be electrically coupled to the piezoelectric transducer by one or more wires. Alternatively, the circuit can be connected to a printed circuit board (PCB) and electrically coupled to the piezoelectric transducer by a plurality of conductive studs bonded to the PCB and to one or more electrodes of the piezoelectric transducer. The conductive studs may include metal studs or conductive epoxy studs.

[0013] In some embodiments, the implantable device includes a printed circuit board (PCB) comprising a first portion disposed between and bonded to the interior surface of theacoustic window and the piezoelectric transducer and a second portion connected to the first portion and to the circuit. The PCB may electrically couple the piezoelectric transducer to the circuit. The thickness of the first portion of the PCB may be approximately equal to an integer multiple of a quarter of a wavelength in PCB of sound at the resonance frequency. Specifically, the thickness of the first portion of the PCB may be approximately equal to a quarter of a wavelength in the bonding layer of sound at the resonance frequency. The PCB may have a cutout portion between the first portion and the second portion that contains a fluid filler.

[0014] The housing of the implantable device may also contain a fluid filler. A surface of the piezoelectric transducer may be in contact with the fluid filler. The fluid filler may have an acoustic impedance that is not approximately equal to an acoustic impedance of the piezoelectric transducer. The fluid filler may be a gaseous fluid and may comprise air or helium gas. Alternatively, the fluid filler may be a liquid fluid.

[0015] The implantable device may comprise a bonding layer disposed between the and bonded to the piezoelectric transducer and the interior surface of the acoustic window. The bonding layer may comprise an epoxy resin. The thickness of the bonding layer may be less than or equal to one hundredth of a wavelength in the bonding layer of sound at the resonance frequency.

[0016] In some embodiments, an electrode of the piezoelectric transducer may be metallically fused to the interior surface of the acoustic window. The acoustic window can also be an electrode of the piezoelectric transducer.

[0017] The piezoelectric transducer may generate electric current when ultrasonic waves with frequencies approximately equal to the resonance frequency are transmitted to the implantable device by an external device. The electric current generated by the piezoelectric transducer may provide power to the circuit. The circuit may include an energy storage device, and the electric current generated by the piezoelectric transducer may charge the energy storage device. The circuit may be configured to modulate current flowing through the piezoelectric transducer to communicate data from the implantable device, and the piezoelectric transducer may emit ultrasonic waves encoding the data in response to the modulated electric current. The data may include information about a physiological signal detected by the implantable device, or a treatment provided by the implantable device.

[0018] The circuit may comprise one or more sensors configured to detect a physiological signal. The one or more sensors may comprise at least one of: a pressure sensor, a pH sensor, a temperature sensor, a sensor configured to detect an electrophysiological pulse, a sensorconfigured to detect a presence of an analyte, a sensor configured to detect a concentration of an analyte, a sensor configured to measure an evoked action potential in a brain, or a sensor configured to measure a local field potential in a brain.

[0019] The housing of the implantable device may comprise titanium, stainless steel, or nitinol. The piezoelectric transducer may include lead zirconate titanate (PZT).

[0020] The acoustic window may comprise a first material and a second material. The first material and the second material can be layered; the second material may form the outermost layer. The first material and the second material can also form a composite wherein the second material may be disposed within a plurality of cavities in the first material. The acoustic impedance of the second material may be less than the acoustic impedance of the first material. The first material may comprise titanium and the second material may comprise a polymer. The polymer may include poly(methyl methacrylate) (PMMA) or poly etheretherketone (PEEK).

[0021] The implantable device may be configured to be fully implantable. In particular, the implantable device may be configured to be implanted in or attached to a tissue or an organ.

[0022] A system provided herein may comprise any of the embodiments of the implantable device described above as well as an external device configured to generate ultrasonic waves at approximately the resonance frequency. The external device may be configured to receive ultrasonic waves emitted by the piezoelectric transducer in the implantable device. The external device may be configured to be operated outside of a subject in which the implantable device is implanted.

[0023] A method provided herein may comprise generating, using an external device, ultrasonic waves with a frequency approximately equal to a resonance frequency of: an acoustic window in a housing of an implantable device and a piezoelectric transducer contained within the housing of the implantable device and attached to an interior surface of the acoustic window. Additionally, the method may comprise transmitting the ultrasonic waves from the external device to the piezoelectric transducer through the acoustic window such that the ultrasonic waves do not pass through a fluid after passing through the acoustic window before reaching the piezoelectric transducer and generating, using the piezoelectric transducer, in response to receiving the ultrasonic waves, an electric current. The method can further comprise powering a circuit for controlling one or more implant functions using the electric current generated by the piezoelectric transducer and charging an energy storagedevice in the implantable device using the electric current generated by the piezoelectric transducer.

[0024] In some embodiments, the method includes emitting, using the piezoelectric transducer, ultrasonic waves encoding information about a physiological signal detected by the implantable device, or a treatment provided by the implantable device, transmitting the ultrasonic waves encoding the information through the acoustic window, and receiving the ultrasonic waves using the external device. The ultrasonic waves emitted by the piezoelectric transducer may include ultrasonic backscatter waves. Emitting the ultrasonic waves encoding the information may comprise modulating, using the implantable device, electric current flowing through the piezoelectric transducer to encode the information in the electric current. Modulating the electric current flowing through the piezoelectric transducer may comprise modulating a frequency, an amplitude, or a phase of the electric current flowing through the piezoelectric transducer. The method can additionally include extracting, using the external device, the information encoded in the ultrasonic waves and transmitting the extracted information to a user device.

[0025] The method may include detecting, using the implantable device, a physiological signal in a subject. The physiological signal may be a pressure, a pH, a temperature, an electrophysiological pulse, a concentration of an analyte, an evoked action potential in a brain, or a local field potential in a brain. Additionally or alternatively, the method can include stimulating, using the implantable device, a tissue or an organ in a subject.

[0026] Finally, provided are methods of manufacturing an implantable device. A method of manufacturing an implantable device may comprise attaching a piezoelectric transducer to an interior surface of an acoustic window in a housing of the implantable device such that ultrasonic waves passing through the acoustic window do not pass through a fluid before reaching the piezoelectric transducer. The acoustic window and the piezoelectric transducer may be configured to acoustically resonate at approximately the same resonance frequency. The method of manufacturing the implantable device may further comprise positioning a circuit configured to control one or more implant functions adjacent to a second interior surface of the housing that is opposite to the interior surface of the acoustic window and electrically coupling the circuit to the piezoelectric transducer.

[0027] In some embodiments, the method of manufacturing may include filling a region between the circuit and the piezoelectric transducer with a fluid filler. The fluid filler may be a gaseous fluid and may comprise air or helium gas. Alternatively, the fluid filler may be a liquid fluid.

[0028] The method of manufacturing the implantable device can comprise forming the acoustic window from a first material and a second material. Forming the acoustic window may comprise layering the second material on top of the first material or disposing the first material as a base layer, creating (i.e., forming) a plurality of cavities in the base layer, and disposing the second material within each of the plurality of cavities.BRIEF DESCRIPTION OF THE FIGURES

[0029] 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 can be obtained by reference to the detailed description of illustrative embodiments and the accompanying drawings.

[0030] FIG. 1 shows a cross-sectional view of an implantable device with a piezoelectric transducer that is attached to an interior surface of an acoustic window in the device housing, according to some embodiments.

[0031] FIG. 2A shows an exemplary piezoelectric transducer, according to some embodiments.

[0032] FIG. 2B shows an example electrode pattern for a piezoelectric transducer, according to some embodiments.

[0033] FIG. 2C shows an example electrode pattern for a piezoelectric transducer, according to some embodiments.

[0034] FIG. 3A shows a first example of an electrical connection between a piezoelectric transducer and implant control circuitry in an implantable device, according to some embodiments.

[0035] FIG. 3B shows an additional example of an electrical connection between a piezoelectric transducer and implant control circuitry in an implantable device, according to some embodiments.

[0036] FIG. 3C shows an additional example of an electrical connection between a piezoelectric transducer and implant control circuitry in an implantable device, according to some embodiments.

[0037] FIG. 4A shows an exploded cross-sectional view of a piezoelectric transducer and an acoustic window in an implantable device, according to some embodiments.

[0038] FIG. 4B shows an exploded cross-sectional view of a piezoelectric transducer, an acoustic window, and a bonding layer that attaches the transducer to the acoustic window, according to some embodiments.

[0039] FIG. 4C shows an exploded cross-sectional view of a piezoelectric transducer, an acoustic window, and a printed circuit board that attaches the transducer to the acoustic window, according to some embodiments.

[0040] FIG. 5A shows a cross-sectional view of a layered acoustic window, according to some embodiments.

[0041] FIG. 5B shows a cross-sectional view of a composite acoustic window, according to some embodiments.

[0042] FIG. 5C shows a top-down view of a composite acoustic window, according to some embodiments.

[0043] FIG. 6 shows a method for communicating with an implantable device, according to some embodiments.

[0044] FIG. 7 shows a method for transmitting information from an implantable device, according to some embodiments.

[0045] FIG. 8 shows a method of manufacturing an implantable device, according to some embodiments.

[0046] FIG. 9 shows example data indicating the effect of bonding layer thickness on the performance of a piezoelectric transducer.

[0047] FIG. 10 shows example data indicating the harvesting performance of a titanium - PZT composite transducer.DETAILED DESCRIPTION

[0048] Implantable medical devices can be used treat various medical conditions and to collect physiological data that provides insight into patient health and disease prognosis. Many such implants contain ultrasonic transducers that can harvest energy from ultrasonic waves emitted by a device external to the implant (e.g., a medical interrogator). These implants may also utilize the ultrasonic transducer to exchange information and data with the external device using ultrasonic waves.

[0049] Ultrasonic transducers typically contain a piezoelectric material. However, piezoelectric materials used for ultrasonic transduction may not be medical grade and thus may be damaged by, or cause damage to, biological tissue with which they are brought into direct physical contact. Many piezoelectric materials contain toxic substances (e.g., lead) that could cause health problems in a subject if brought into contact with the subject’s tissue. An ultrasonic transducer in an implantable device must therefore be encased in a medical grade housing to avoid undue harm to the subject and to ensure proper transducer function.

[0050] When a piezoelectric transducer for an implantable device is placed within a housing, any ultrasonic waves transmitted to the transducer must first pass through several layers of material (including the housing and, if the device has been implanted, portions of the subject’s body) before being received by the transducer, which may cause the ultrasonic waves to lose significant amounts of energy. Likewise, any ultrasonic waves emitted by the transducer must pass through several layers of material (including the housing and, if the device has been implanted, portions of the subject’s body) before being received by an external device. This can cause information encoded in the ultrasonic waves by the implantable device to be inaccurately transmitted or lost entirely. Surrounding the transducer with a liquid (e.g., an ultrasonic gel) may facilitate acoustic coupling between the implant housing and the transducer and mitigate energy losses, but filling the implant with the liquid during manufacturing without damaging other implant components may be a complex and costly process.

[0051] Provided herein is an implantable device with a piezoelectric ultrasonic transducer that is attached to an internal surface of the device’s biocompatible housing. Ultrasonic waves transmitted to the implant may induce vibrations in the implant housing that transfer directly to the piezoelectric transducer through the point(s) of contact between the housing and the transducer. That is, the connection between the transducer and the housing may be such that ultrasonic waves transmitted to the implant through the housing do not pass through a fluid before reaching the piezoelectric transducer, which may significantly reduce energy losses from the ultrasonic waves as well as simplify the simplify the fabrication process, particularly for larger implants. Energy losses may be further mitigated by configuring the transducer and the acoustic window region of the implant housing to which the transducer is attached to acoustically resonate at about the same resonance frequency. In this configuration, when ultrasonic waves at approximately the resonance frequency of the acoustic window and the transducer are transmitted to the implant, the vibrations induced in the acoustic window and in the transducer intensify, thereby maximizing the amount of energy that is transferred from the ultrasonic waves to the implant.

[0052] In addition to receiving ultrasonic waves produced by a source external to the implant, the implantable device may be configured to communicate information using ultrasonic waves. The implantable device, in some embodiments, encodes information in backscatter ultrasonic waves that are produced by the piezoelectric transducer when current that was generated by the transducer in response to received ultrasonic waves flows back through the transducer. Modulating this current (e.g., using a physiological signal sensor inthe implant) as it flows through the transducer may modulate and encode information in the ultrasonic backscatter. In other embodiments, the implantable device may generate ultrasonic waves directly by producing and applying an information-encoded electric current to the piezoelectric transducer.

[0053] Ultrasonic waves emitted by the transducer for information-transfer purposes (including modulated ultrasonic backscatter and directly generated ultrasonic waves) may be retrieved by an external device after passing through the acoustic window. To ensure that these ultrasonic waves pass through the acoustic window, the housing may contain a fluid filler (e.g., atmospheric air) that serves as a backing material for the transducer by reflecting ultrasonic waves emitted by the transducer that are directed inward toward the center of the implantable device back outward toward the acoustic window. This may increase the strength and quality of the signals that are received by the external device.

[0054] Material characteristics of the acoustic window that increase the operational bandwidth of the transducer and, as a result, improve the transducer’s communication capabilities are also provided. In some embodiments, adding a second biocompatible material to the biocompatible material that forms the acoustic window, either by disposing a layer of the second material on the external-facing surface of the first material, or by forming a composite between the first housing layer material and the second material on the external surface of the housing layer. Adding the second material to the housing layer may cause the acoustic impedance of the housing layer to more closely match the acoustic impedance of the tissue in which the implantable device is implanted. This may reduce the proportion of ultrasonic waves transmitted through the tissue to the device that are reflected by the device as well as the proportion of ultrasonic waves transmitted from the device through the tissue that are reflected by the tissue, thereby increasing the spread of ultrasonic frequencies that the transducer can receive and transmit.Definitions

[0055] As used herein, the singular forms “a”, “an”, and “the” include the plural reference unless the context clearly dictates otherwise.

[0056] Reference to “about” or “approximately” a value or parameter herein includes (and describes) variations of up to 10% of that value or parameter per se. For example, description referring to “approximately X” or “about X” includes description of “X” as well as description of “X + / - 10%”.

[0057] It is understood that aspects and variations of the invention described herein include “consisting of’ and / or “consisting essentially of’ aspects and variations.

[0058] When a range of values or values is provided, it is to be 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 present disclosure. Where the stated range includes upper or lower limits, ranges excluding either of those included limits are also included in the present disclosure.

[0059] The disclosures of all publications, patents, and patent applications referred to herein are each hereby incorporated by reference in their entireties. To the extent that any reference incorporated by reference conflicts with the instant disclosure, the instant disclosure shall control.

[0060] The section headings used herein are for organization purposes only and are not to be construed as limiting the subject matter described. 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 persons 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 embodiment shown but is to be accorded the widest scope consistent with the principles and features described herein.

[0061] The figures 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 examples, additional steps may be performed in combination with the exemplary processes. Accordingly, the operations as illustrated (and described in greater detail below) are exemplary by nature and, as such, should not be viewed as limiting.

[0062] Finally, it is to be understood that features and preferences described in relation to “embodiments” are distinct preferences and are not limited only to that particular embodiment; they may be freely combined with features from other embodiments, where technically feasible, and may 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 persons skilled in the art and the generic principles herein may be applied to other embodiments. Thus, the present invention isnot intended to be limited to the embodiment shown but is to be accorded the widest scope consistent with the principles and features described herein.Device

[0063] The provided implantable device comprises a piezoelectric ultrasonic transducer that is attached to an acoustic window portion of the implant housing in which it is contained. Attaching the transducer to the acoustic window, particularly if the acoustic window is configured to acoustically resonate at about the same frequency as the transducer resonates, may help preserve the quality and strength of acoustic signals that are received by the implant (e.g., for energy harvesting purposes) or transmitted from the implant (e.g., for communication purposes) while simultaneously preventing direct contact between the piezoelectric transducer and the body part in which the implantable device is implanted.

[0064] FIG. 1 depicts a cross-sectional side view of an exemplary implantable device 100. As discussed, implantable device 100 may include a housing 102 comprising an acoustic window 104 as well as a piezoelectric transducer 108 that is attached to an internal surface of acoustic window 104. Housing 102 may also contain a fluid filler 108. Implant control electronics 113, which may include an application-specific integrated circuit (ASIC) 110 configured to control one or more functions of implantable device 100, discrete printed circuit board assemblies (PCBAs) 115, or a combination thereof, may be electrically coupled to transducer 108.

[0065] Implantable device 100 may be configured to be partially or entirely implanted in a body part. For example, implantable device 100 may be configured to be partially or entirely implanted in an eye, a stomach, a bladder, a brain, a muscle, an intestine, or a heart. Implantable device 100 can also be configured to attach to a body part. For instance, implantable device 100 may be configured to attach to a nerve, a bone, or a surface of an organ. Implantable device 100 may comprise one or more components configured to monitor physiological conditions associated with a body part (e.g., pressure sensors, temperature sensors, pH sensors, etc.) and / or one or more components configured to deliver treatment to a body part (e.g., electrodes configured to electrically stimulate a body part, fluid delivery systems configured to deliver a medication to a body part, etc.). These components may be connected to or controlled by circuit 110. Circuit 110 can also include additional components such as energy storage devices (e.g., rechargeable batteries or capacitors), power management units, current modulation circuits, digital electronic devices, and processors.

[0066] Housing 102 may entirely encapsulate implantable device 100 and thus may come into direct physical contact with biological tissue when implantable device 100 is implanted within a subject. In order to prevent damage to the tissue, housing 102 - including acoustic window 104 - may be formed from one or more medical grade or biocompatible materials. These materials may be materials that do not produce a toxic or immune reaction with the subject’s tissue; example materials include titanium, stainless steel, polymethylmethacrylate (PMMA), polypropylene (PP), medical grade silicone, and hydroxyapatite. In some embodiments, housing 102 comprises a composite of multiple biocompatible or medical grade materials.

[0067] Acoustic window 104 may be a region of housing 102 that is configured to allow acoustic signals (e.g., ultrasonic waves) to pass (relatively) uninhibited into and out of housing 102. In various embodiments, acoustic window 104 may have a particular physical or material characteristics that facilitate the passage of acoustic signals. These characteristics may distinguish acoustic window 104 from other regions of housing 102.

[0068] Piezoelectric transducer 106 may comprise a piezoelectric material - i.e., a solid material that, when mechanically deformed, generates an electric voltage and, in the presence of an electric voltage, mechanically deforms. Specifically, piezoelectric transducer 106 may be formed from a piezoelectric material that can be deformed by and can produce (as a result of induced mechanical deformations) ultrasonic waves. Piezoelectric transducer 106 may be attached to an internal surface of acoustic window 104 such that acoustic signals (e.g., ultrasonic waves) passing through acoustic window 104 do not pass through a fluid before reaching transducer 106; as such, any acoustic signals traveling through acoustic window 104 may be transmitted directly to piezoelectric transducer 106 through the points of contact between acoustic window 104 and transducer 106.

[0069] Additionally, the resonance frequency of acoustic window 104 and the resonance frequency of piezoelectric transducer 106 may be approximately the same. For example, the resonance frequency of the acoustic window may be within 10%, within 9%, within 8%, within 7%, within 6%, within 5%, within 4%, within 3%, within 2%, within 1%, within 0.5%, within 0.25%, within 0.1%, within 0.01%, or within 0.001% of the resonance frequency of the piezoelectric transducer. When acoustic window 104 is driven at the resonance frequency (e.g., by ultrasonic waves), acoustic window 104 may resonate - i.e., may oscillate with a higher amplitude than when the same driving force is applied at a different, non-resonant frequency. The resonance of acoustic window 104 may, in turn, drive piezoelectric transducer 106 at the resonance frequency, causing piezoelectric transducer 106to resonate, as well. The amount of voltage generated by piezoelectric transducer 106 (due to the mechanical deformations in the piezoelectric material caused by the ultrasonic waves) may increase significantly when piezoelectric layer 106 resonates and may be sufficient to induce a current in circuit 110 that can provide power to the various components of circuit 110.

[0070] In addition to receiving externally applied ultrasonic waves, transducer 106 may be configured to emit ultrasonic waves, for example to communicate with external device 112. To cause transducer 106 to emit ultrasonic waves, circuit 110 may apply a voltage across transducer 110. The emitted ultrasonic waves may be transmitted through acoustic window 104 and off of implantable device 100, where they may be received by a receiver (e.g., another ultrasonic transducer) external to implantable device 100 (e.g., a receiver in external device 112). Fluid filler 108, which may have an acoustic impedance that is not approximately equal to (e.g., that is higher or lower than) the acoustic impedance of piezoelectric transducer 106, may cause ultrasonic waves emitted through surface 106b of piezoelectric transducer 106 to be reflected back through surface 106a of piezoelectric layer 106, where they may pass out of device 100 via acoustic window 104. That is, fluid filler 108 may serve as a backing material for transducer 106. This may increase the amount of energy that transducer 102 can use to transmit acoustic signals in order to, for example, communicate information from implantable device 100 to external device 116.

[0071] As shown in FIG. 1, fluid filler 108 may be in contact with a surface 106(b) of piezoelectric transducer 106 that is opposite to the transducer surface 106(a) that is attached to acoustic window 104. In some embodiments, fluid filler 108 surrounds piezoelectric transducer on all sides except for the surface 106(a) that is attached to acoustic window 104.

[0072] The region within housing 102 that contains fluid filler 108 may have a thickness approximately equal to a positive multiple of the wavelength in fluid filler 108 of sound at the resonance frequency of acoustic window 104 and piezoelectric transducer 106. That is, fluid filler 108 may constitute a backing layer of transducer 106 that has a thickness approximately equal to mAF. where m is a number greater than or approximately equal to one and AFis the wavelength in fluid filler 108 of sound at the resonance frequency of acoustic window 104 and transducer 106. For example, fluid filler 108 may constitute a backing layer of transducer 106 that has a thickness approximately equal to AF, 2AF, 3AF, 4AF, 5AF. 6AF, 7AF, 8AF. 9AF, or 10dF. More specifically, fluid filler 108 may constitute a backing layer of transducer 106 that has a thickness within 10%, within 9%, within 8%, within 7%, within 6%, within 5%,within 4%, within 3%, within 2%, within 1%, within 0.5%, within 0.25%, within 0.1%, within 0.01%, or within 0.001% of ZF, 22F. 3AF. 4AF, 5AF, 6AF, 7AF, 8AF, 9AF, or 10AF. In other embodiments, fluid filler constitutes a backing layer of transducer 106 that has a thickness greater than 2AF, 3AF. 4AF, 5AF, 6AF, 7AF, 8AF, 9AF, or 10AF.

[0073] Fluid filler 108 may be or may comprise a gaseous fluid or a liquid fluid. For example, fluid filler 108 may comprise atmospheric air or helium gas. The acoustic impedance of fluid filler 108 may be characterized by the acoustic reflection coefficient at the interface between fluid filler 108 and transducer 106, which, as shown in the equation below, depends on both the acoustic impedance of fluid filler 108 (ZF) and the acoustic impedance of transducer 106 (ZP)ZF— ZPZF+ ZPIn some embodiments, the acoustic impedance of fluid filler 108 may be such that the magnitude of the acoustic reflection coefficient at the interface between fluid filler 108 and piezoelectric transducer 106 is approximately equal to 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, or 0.99. More specifically, the acoustic impedance of fluid filler 108 may be such that the magnitude of the acoustic reflection coefficient at the interface between fluid filler 108 and piezoelectric transducer 106 is within 10%, within 9%, within 8%, within 7%, within 6%, within 5%, within 4%, within 3%, within 2%, within 1%, within 0.5%, within 0.25%, within 0.1%, within 0.01%, or within 0.001% of 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.75, 0.8, 0.85, 0.9, 0.95, or 0.99.Piezoelectric Transducer

[0074] The piezoelectric transducer, as noted, may be a device configured to convert acoustic energy (e.g., in the form of ultrasonic waves) to electrical energy (and / or vice versa) by leveraging the piezoelectric effect. An exploded cross-sectional view of piezoelectric transducer 106 of implantable device 100 is illustrated in FIG. 2A. As shown, transducer 106 may comprise a piezoelectric material 214 and one or more electrodes 216. Electrodes 216 may allow transducer 106 to be electrically coupled to other implant components such as circuit 110 (shown in FIG. 1).

[0075] Piezoelectric material 214 may be or may comprise any material that exhibits the piezoelectric effect, including piezoelectric crystals, piezoelectric ceramics, piezoelectric semiconductors, and piezoelectric polymers. Example materials include lead zirconate titanate (Pb[ZrxTii-x]O3, with 0 < x < 1; also known as PZT), potassium niobate (KNbO.fl.sodium tungstate (Na2WOs), lead titanate (PbTiCh), lithium niobate (LiNbO.fi. polyvinylidene fluoride (PVDF), and barium titanate (BaTiCh, also known as BTO).

[0076] Electrodes 216 may be formed from an electrical conductor (e.g., copper, gold, silver, nickel alloys, titanium alloys, etc.) and can be arranged in various patterns on the outer surface(s) of piezoelectric material 214. In some embodiments, as shown in FIG. 2B, electrodes 216 are in a solid or surface pattern, wherein one electrode 216 is disposed on a first surface of piezoelectric material 214 and another electrode 216 is disposed on a second surface of piezoelectric material 214 that opposes the first surface. Alternatively, as shown in FIG. 2C, electrodes 216 can be in a wrap-around pattern wherein both electrodes 216 are disposed on the same surface of piezoelectric material 214 and are separated by an insulating band. In some embodiments, an electrode 216 of piezoelectric transducer 106 may also serve as the acoustic window (e.g., acoustic window 104) of the implantable device.Electrical coupling between transducer and implant circuitry

[0077] The piezoelectric transducer may be electrically coupled to the implant control circuitry, as illustrated in FIG. 1. The electrical coupling may enable currents to be induced in the circuit by voltages generated in the transducer as a result of received ultrasonic waves. These currents can be used to power various circuit components (sensors, electrodes, etc.) or to charge energy storage devices (e.g., capacitors). Similarly, the electrical coupling may allow the circuit to encode information collected by the circuit (e.g., physiological signal data measured by a sensor) in ultrasonic waves emitted by the piezoelectric transducer by modulating currents flowing through the piezoelectric transducer.

[0078] FIGS. 3A-3C depict various ways in which piezoelectric transducer 106 and circuit 110 of implantable device 100 may be electrically coupled. A first option, as shown in FIG. 3 A, may be connecting transducer to circuit 110 using one or more wires 318. In other embodiments, a printed circuit board (PCB) 320 can be used to electrically couple circuit 110 to transducer 106 (FIGS. 3B-3C). Transducer 106 may be connected to electrical connectors on PCB 320 by one or more conductive (e.g., gold, conductive epoxy, etc.) studs 322 (FIG. 3B), for example using a flip chip bonding process. Alternatively, as shown in FIG. 3C, transducer 106 may be electrically coupled to a first portion 320(a) of PCB 320 that is disposed between acoustic window 104 and transducer 106, while circuit 110 may be electrically coupled to a second portion 320(b) of PCB 320 that is connected to first portion 320(a) by a connecting portion 320(d). In this embodiment, PCB 320 comprises a cutoutportion 320(c) between first portion 320(a) and second portion 320(b) that contains fluid filler 108.

[0079] In either of the embodiments illustrated in FIGS. 3B-3C, conductive traces in PCB 320 (not shown) may electrically couple transducer 106 to circuit 110. PCB 320 can be a ceramic printed circuit board, a flexible printed circuit board, or a combination thereof. In the embodiment illustrated in FIG. 3C, specifically, connecting region 320(d) that connects first region 320(a) to second region 320(b) may be flexible, while first and second regions 320(a)- (b) may be ceramic.Connecting the acoustic window and the piezoelectric transducer

[0080] As discussed, the piezoelectric transducer may be contained within the implant housing and attached to an interior surface of the acoustic window. The attachment between the piezoelectric transducer and the acoustic window may be such that ultrasonic waves passing through the acoustic window do not pass through a fluid before reaching the piezoelectric transducer. This may substantially simplify the manufacturing process for the implantable device by removing the need for an acoustic coupling fluid between the transducer and the interior surface of the acoustic window. Furthermore, attaching the piezoelectric transduce the acoustic window may enable acoustic signals to transmit directly between the transducer and the acoustic window through the attachment points, which may reduce energy losses and improve acoustic transmission quality.

[0081] Piezoelectric transducer 106 may be attached to an interior surface of acoustic window 104 in implantable device 100 in various ways. In some embodiments, as shown in FIG. 4A, transducer 106 is attached directly to acoustic window 104, for example by metallically fusing an electrode of transducer 106 to the interior surface of acoustic window 104, or by configuring an electrode of transducer 106 to simultaneously serve as acoustic window 104. In other embodiments, as shown in FIG. 4B, transducer 106 may be bonded to acoustic window 104 by a layer of bonding material 424 (e.g., by a layer of an epoxy resin). In other embodiments, as shown in FIG. 4C, transducer 106 and acoustic window 104 may be connected to different sides of the same printed circuit board (PCB) 320.

[0082] Additionally, as previously noted, the acoustic window and the piezoelectric transducer may be configured to acoustically resonate at approximately the same resonance frequency. More specifically, in various embodiments, the resonance frequency of the acoustic window is within 10%, within 9%, within 8%, within 7%, within 6%, within 5%, within 4%, within 3%, within 2%, within 1%, within 0.5%, within 0.25%, within 0.1%,within 0.01%, or within 0.001% of the resonance frequency of the piezoelectric transducer. The resonance frequency of the acoustic window and the transducer may depend on the ultrasonic frequencies the external device used to communicate with and power the implant can generate as well as the part of and / or location within the body in which the implantable device is configured to be implanted.

[0083] In some embodiments, the resonance frequency of the acoustic window and the piezoelectric transducer is approximately 0.5 MHz, 1 MHz, 2 MHz, 3 MHz, 4 MHz, 5 MHz, 6 MHz, 7 MHz, 8 MHz, 9 MHz, 10 MHz, 11 MHz, 12 MHz, 13 MHz, 14 MHz, or 15 MHz, or any value therebetween. For example, the resonance frequency of the acoustic window and the piezoelectric transducer may be within 10%, within 9%, within 8%, within 7%, within 6%, within 5%, within 4%, within 3%, within 2%, within 1%, within 0.5%, within 0.25%, within 0.1%, within 0.01%, or within 0.001% of 0.5 MHz, 1 MHz, 2 MHz, 3 MHz, 4 MHz, 5 MHz, 6 MHz, 7 MHz, 8 MHz, 9 MHz, 10 MHz, 11 MHz, 12 MHz, 13 MHz, 14 MHz, or 15 MHz. In some embodiments, the resonance frequency of the acoustic window and the piezoelectric transducer is less than or equal to about 10 MHz, 15 MHz, 20 MHz, or 25 MHz. In some embodiments, the resonance frequency of the acoustic window and the piezoelectric transducer is greater than or equal to about 0.25 MHz, 0.5 MHz, 0.75 MHz, or 1 MHz.

[0084] Various acoustic properties of the acoustic window, including the resonance frequency of the acoustic window, may depend on the thickness of the acoustic window. Likewise, various acoustic properties of the piezoelectric transducer, including the resonance frequency of the piezoelectric transducer, may depend on the thickness of the piezoelectric transducer. As shown in FIGS. 4A-4C, acoustic window 104 and piezoelectric transducer 106 may be configured to acoustically resonate at approximately the same frequency f when acoustic window 104 has a thickness approximately equal to an integer (n) multiple of a quarter of a wavelength in acoustic window 104 (AW) of sound at frequency f and when piezoelectric transducer 106 has a thickness approximately equal to an integer (n) multiple of a quarter of a wavelength in piezoelectric transducer 106 (AP) of sound at frequency f. In other words, resonance of acoustic window 104 and piezoelectric transducer 106 at approximately the same frequency f may be achieved if:Acoustic Window Thickness « n — n e Z +4 pPiezoelectric Transducer Thickness ~ n— , n £ Z +4 where:Z+ = nonnegative integers, i.e., integers greater than or equal to 0 ^AW^AW=~y~ >VAW = speed of sound in acoustic window vPP= —, vP= speed of sound in piezoelectric transducer

[0085] In some embodiments, acoustic window 104 has a thickness within 10%, within 9%, within 8%, within 7%, within 6%, within 5%, within 4%, within 3%, within 2%, within 1%, within 0.5%, within 0.25%, within 0.1%, within 0.01%, or within 0.001% of an integer multiple of a quarter of a wavelength in acoustic window 104 of sound at the resonance frequency. Similarly, in some embodiments, piezoelectric transducer 106 has a thickness within 10%, within 9%, within 8%, within 7%, within 6%, within 5%, within 4%, within 3%, within 2%, within 1%, within 0.5%, within 0.25%, within 0.1%, within 0.01%, or within 0.001% of an integer multiple of a quarter of a wavelength in piezoelectric transducer 106 of sound at the resonance frequency.

[0086] Different acoustic window and piezoelectric transducer thicknesses may be useful for different implant types. In some embodiments, acoustic window 104 has a thickness approximately equal to half of the wavelength in acoustic window 104 of sound at the resonance frequency (i.e., in some embodiments, acoustic window 104 has a thickness approximately equal to - . For example, acoustic window 104 may have a thickness within10%, within 9%, within 8%, within 7%, within 6%, within 5%, within 4%, within 3%, within 2%, within 1%, within 0.5%, within 0.25%, within 0.1%, within 0.01%, or within 0.001% of half of the wavelength in acoustic window 104 of sound at the resonance frequency.Similarly, in some embodiments, piezoelectric transducer 106 has a thickness approximately equal to half of the wavelength in piezoelectric transducer 106 of sound at the resonance frequency (i.e., in some embodiments, piezoelectric transducer 106 has a thickness approximately equal to )- For example, piezoelectric transducer 106 may have a thickness within 10%, within 9%, within 8%, within 7%, within 6%, within 5%, within 4%, within 3%, within 2%, within 1%, within 0.5%, within 0.25%, within 0.1%, within 0.01%, or within 0.001% of half of the wavelength in piezoelectric transducer 106 of sound at the resonance frequency.

[0087] In other embodiments, acoustic window 104 has a smaller thickness relative to the wavelength in acoustic window 104 of sound at the resonance frequency. For example, acoustic window 104 may have a thickness that approximates the “zero” multiple of one quarter of the wavelength in acoustic window 104 of sound at the resonance frequency (i.e., athickness approximately equal to In these cases, a ratio of thethickness of acoustic window 104 to the wavelength in acoustic window 104 of sound at the resonance frequency may be on the order of 0.001, 0.005, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, or 0.1.

[0088] In embodiments where the acoustic window and the piezoelectric transducer are bonded together by a bonding layer, the thickness of the bonding layer may be significantly less than the wavelength in the bonding layer of sound at the resonance frequency in order to reduce the amount of energy that is lost by sound waves as they travel through the bonding layer. As shown in FIG. 4B, bonding layer 424 may have a thickness less than or equal to one hundredth of a wavelength in bonding layer 424 (AB) of sound at the resonance frequency f of acoustic window 104 and piezoelectric transducer 106. For example, bonding layer 424 may have athickness approximately equal tovaluetherebetween. More specifically, bonding layer 424 may have a thickness within 10%, within 9%, within 8%, within 7%, within 6%, within 5%, within 4%, within 3%, within 2%, within 1%, within 0.5%, within 0.25%, within 0.1%, within 0.01%, or within 0.001% ofto^B / 200' ^B^250’

[0089] During manufacturing of the implantable device, the amount of bonding material deposited may vary. In some embodiments, a predetermined thickness (e.g., in microns) or a predetermined volume of the bonding material (e.g., epoxy resin) is deposited. The thickness of the bonding layer may be adjusted subsequently to ensure that the thickness of the bonding layer is, e.g., approximately equal to an appropriate multiple of the wavelength in the bonding material of sound at the resonance frequency of the acoustic window and the piezoelectric transducer. Adjusting the thickness of the bonding layer following deposition of the bonding material may comprise removing a slice of the bonding material and measuring the thickness of the slice, for example using a microscope.

[0090] If the acoustic window and the piezoelectric transducer are connected by a portion of a printed circuit board (PCB), the PCB may be configured to acoustically resonate at about the same resonance frequency as the acoustic window and the piezoelectric transducer. As depicted in FIG. 4C, portion 320(a) of PCB 320 that connects acoustic window 104 and piezoelectric transducer 106 may have a thickness approximately equal to an integer (n)multiple of a quarter of a wavelength in PCB 320 (PCB) of sound at the resonance frequency f of acoustic window 104 and piezoelectric transducer 106. For instance, portion 320(a) of PCB 320 may have a thickness within 10%, within 9%, within 8%, within 7%, within 6%, within 5%, within 4%, within 3%, within 2%, within 1%, within 0.5%, within 0.25%, within 0.1%, within 0.01 %, or within 0.001 % of an integer multiple of a quarter of a wavelength in PCB 320 of sound at the resonance frequency of acoustic window 104 and piezoelectric transducer 106. For example, PCB portion 320(a) may have a thickness approximately equal to (e.g., within 10%, within 9%, within 8%, within 7%, within 6%, within 5%, within 4%, within 3%, within 2%, within 1%, within 0.5%, within 0.25%, within 0.1%, within 0.01%, or within 0.001% of) ^PCBI or ^PCBf

[0091] Minimizing the thicknesses of all implant components, including the acoustic window, the piezoelectric transducer, and (if present) the bonding layer or the PCB may reduce the overall volume of the implantable device. Smaller implants may be more easily implanted in body parts, particularly small or sensitive body parts such as eyes.Material properties of the acoustic window

[0092] The housing of the implantable device, including the acoustic window, may separate the internal circuitry and components of the implantable device - including the bioincompatible piezoelectric material in the ultrasonic transducer - from the biological tissue of the subject in which the implantable device is implanted. Acoustic signals (e.g., ultrasonic waves) transmitted to the ultrasonic transducer or emitted by the ultrasonic transducer may, therefore, encounter an interface between the acoustic window and the subject’s tissue. To prevent reflection of acoustic signals at this interface, the acoustic window may be configured such that the difference between the acoustic impedance of the acoustic window and the acoustic impedance of the surrounding biological tissue is minimized.

[0093] FIG. 5A illustrates a cross-sectional side view of an example implementation of acoustic window 104. As shown, acoustic window 104 may comprise a first layer 526 formed from a first material and a second layer 528 formed from a second material. The material from which layer 526 is formed may be the same material from which the rest of the implantable device’s housing is formed. This first material may be highly durable and may have an acoustic impedance (Z1) that is greater than the acoustic impedance (Zr) of the tissue that will surround the implantable device once the device is implanted. The second material from which layer 528 is formed may have an acoustic impedance (Z2) that is less than theacoustic impedance of layer 526 but greater than the acoustic impedance (ZT) of the tissue that will the implantable device. That is, the relationships between the acoustic impedances of each layer of acoustic window 104 and the acoustic impedance of the surrounding tissue may be:

[0094] Second layer 528 may therefore reduce the acoustic impedance gradient between layer 526 and the surrounding tissue by providing a region of intermediate acoustic impedance between layer 526 and the surrounding tissue. This may allow a wider band of acoustic frequencies to be transmitted between the ultrasonic transducer and the surrounding tissue, which may, for example, increase the efficiency with which the ultrasonic transducer can communicate information from the implantable device.

[0095] As discussed, the acoustic window may be configured resonate at about a frequency f . If the acoustic window has the layered structure shown in FIG. 5A, then both layer 526 and layer 528 may be configured to resonate at frequency f. Resonance of layer 526 at a frequency f may be achieved if layer 526 has a thickness approximately equal to (e.g., within 10%, within 9%, within 8%, within 7%, within 6%, within 5%, within 4%, within 3%, within 2%, within 1%, within 0.5%, within 0.25%, within 0.1%, within 0.01%, or within 0.001% of) an integer (n) multiple of a quarter of a wavelength in layer 526 (AAW1) of sound at frequency f. Likewise, resonance of layer 528 at a frequency f may be achieved if layer 528 has a thickness approximately equal to (e.g., within 10%, within 9%, within 8%, within 7%, within 6%, within 5%, within 4%, within 3%, within 2%, within 1%, within 0.5%, within 0.25%, within 0.1%, within 0.01%, or within 0.001% of) an integer (n) multiple of a quarter of a wavelength in layer 528 (AAW2) of sound at frequency f. In some embodiments, layer 528 may have a thickness approximately equal to (e.g., within 10%, within 9%, within 8%, within 7%, within 6%, within 5%, within 4%, within 3%, within 2%, within 1%, within 0.5%, within 0.25%, within 0.1%, within 0.01%, or within 0.001% of) one or two times a quarter of a wavelength in layer 528 (AAW2) of sound at frequency f.

[0096] FIG. 5B and FIG. 5C respectively depict a cross-sectional side view and a surface view of a second example implementation of acoustic window 104. In this implementation, acoustic window 104 is formed from a composite of a first material 526 and a second material 528. First material 526 may be the same material from which the rest of the implantable device’s housing is formed; as such, it may be highly durable and may have an acoustic impedance (Z1) that is greater than the acoustic impedance (ZT) of the tissue thatwill surround the implantable device once the device is implanted. Second material 528 may have an acoustic impedance (Z2) that is less than the acoustic impedance of first material 526 but greater than the acoustic impedance (ZT) of the tissue that will the implantable device. That is, the relationships between the acoustic impedances of each material that makes up acoustic window 104 and the acoustic impedance of the surrounding tissue may be:

[0097] Second material 528 may therefore reduce the acoustic impedance gradient between acoustic window 104 and the surrounding tissue, which may allow a wider band of acoustic frequencies to be transmitted between the ultrasonic transducer and the surrounding tissue, increasing the efficiency with which the ultrasonic transducer can communicate information from the implantable device. The composite between first material 526 and second material 528 may be formed by disposing second material 528 in a plurality of cavities in a surface formed from first material 526.

[0098] In the example implementation of acoustic window 104 shown in FIG. 5 A as well as the example implementation of acoustic window 104 shown in FIG. 5B, the first material may be a durable biocompatible material such as titanium, stainless steel, or nitinol. The second material may be a biocompatible polymer. Example polymers include poly(methyl methacrylate) (PMMA) and polyetheretherketone (PEEK).Methods of use

[0099] In the described implantable device, the piezoelectric transducer is attached to an interior surface of the acoustic window in the device housing. To reduce energy loss by ultrasonic waves that pass between the acoustic window and the piezoelectric transducer and improve ultrasound transmission quality, the piezoelectric transducer and the acoustic window may be configured to acoustically resonate at approximately the same resonance frequency. The frequency at which the transducer and the acoustic window are configured to resonate may be a frequency that an external device (e.g., external device 112 shown in FIG. 1) is configured to produce and / or receive (e.g., using a second ultrasonic transducer in the external device).

[0100] FIG. 6 provides a method 600 for powering an implantable device using ultrasonic waves transmitted by a second device that is external to the implant. First, the external device may be used to generate ultrasonic waves at (approximately) the resonance frequency of the acoustic window and the piezoelectric transducer in the implantable device (step 602). Forexample, the external device may be used to generate ultrasonic waves within 10%, within 9%, within 8%, within 7%, within 6%, within 5%, within 4%, within 3%, within 2%, within 1%, within 0.5%, within 0.25%, within 0.1%, within 0.01%, or within 0.001% ofthe resonance frequency of the acoustic window and the piezoelectric transducer. The external device may be controlled by a user (e.g., the subject in which the implantable device is implanted, or the subject’s doctor). Ultrasonic waves may be produced by an ultrasonic transducer (e.g., a piezoelectric transducer) in the external device. These ultrasonic waves may then be transmitted from the external device to the piezoelectric transducer through the implant’s acoustic window such that the ultrasonic waves do not pass through a fluid while traveling between the acoustic window and the transducer (step 604).

[0101] Transmitting the ultrasonic waves from the external device to the implant transducer may involve positioning the external device on or near the subject at a location proximal to the location of the implant. If the implantable device is implanted on or in the subject’s bladder, for instance, the external device may be placed against the subject’s abdomen during ultrasound transmission. A coupling fluid (e.g., ultrasound gel or water) may be disposed between the external device and the subject to improve transmission quality.

[0102] As the ultrasonic waves pass through the acoustic window, the acoustic window may resonate. Since the piezoelectric transducer is attached to the acoustic window and configured to resonate at approximately the same resonance frequency, the resonant response of the acoustic window to the ultrasonic waves may induce a resonant response in the piezoelectric transducer. The transducer may then generate an electric current (step 606). This electric current may be used to power a circuit (e.g., circuit 110 shown in FIG. 1) configured to control one or more implant functions (step 608). The current may be transmitted from the piezoelectric transducer to the circuit via an electrical connection between the circuit and the transducer electrodes.

[0103] In some embodiments, the circuit comprises an energy storage device (e.g., a rechargeable battery or a capacitor) that the circuit charges using the current produced by the piezoelectric transducers. The circuit may also comprise sensors for detecting physiological signals (e.g., temperature sensors, pH sensors, pressure sensors, etc.), devices for delivering treatment (e.g., stimulation electrodes), or a combination thereof. Upon receipt of current produced by the piezoelectric transducer, the circuit may be configured to provide power to a sensor or to a treatment device in order to initiate a physiological signal measurement or a treatment session. For example, the circuit may provide power to a sensor to cause the sensor to detect a pressure, a pH, a temperature, an electrophysiological pulse, a concentration of ananalyte, an evoked action potential in a brain, or a local field potential in a brain, or may provide power to a treatment device to cause the treatment device to stimulate a nerve (e.g., a splenic nerve). The circuit may comprise a memory system in which information associated with a detected physiological signal or a treatment may be recorded and stored.

[0104] In addition to providing power to the implantable device, the external device may be configured to communicate with the implantable device. This may allow users to extract information associated with the implant, such as information associated with a detected physiological signal or a delivered treatment. In some embodiments, the circuit in the implantable device is configured to generate a current encoding information to be communicated. The generated current may be transmitted to the piezoelectric transducer to cause the transducer to generate ultrasonic waves. The ultrasonic waves generated by the transducer may be transmitted through the acoustic window and into the subject’s tissue. After passing through the subject’s tissue, the ultrasonic waves may be received by the external device. Any information encoded in the ultrasonic waves may then be extracted and provided to a user, for example by transmitting the information from the external device to a user device such as a computer or a phone.

[0105] In other embodiments, the implantable device is configured to communicate using an ultrasonic backscatter protocol, as shown in exemplary method 700 depicted in FIG. 7. As described, when the piezoelectric transducer receives ultrasonic waves (e.g., from the external device), a voltage across the transducer may be generated. This voltage may impart a current that flows through the electronic components of the implantable device, including through the transducer itself. As this current flows back through the piezoelectric transducer, vibrations in the piezoelectric material may be generated, causing the transducer to emit backscatter ultrasonic waves. By modulating the current that is already flowing through the piezoelectric transducer, information to be communicated may be encoded in the backscatter waves (step 702).

[0106] The implant circuitry may modulate the current flowing through the piezoelectric transducer by modulating the frequency of the current, the amplitude of the current, the phase of the current, or a combination thereof. In some embodiments, the current is directly modulated by an analog component of the circuit, for example a sensor that detects a physiological signal. The current may also be modulated to encode a digital signal, which may be controlled by a digital core (e.g., a digital circuit) in the implant circuitry. This modulated current may, in turn, modulate and encode the information in backscattered ultrasonic waves. These ultrasonic backscatter waves may then be emitted by thepiezoelectric transducer (step 704) and subsequently transmitted through the acoustic window (step 706), after which they may be received by the external device (step 708). The information encoded in the ultrasonic backscatter waves may then be extracted and provided to a user, for example by transmitting the information from the external device to a user device such as a computer or a phone (step 710).

[0107] Additional details about backscatter communication are provided herein, and additional examples are provided in WO 2018 / 009905; WO 2018 / 009908; WO 2018 / 0091010; WO 2018 / 009911; WO 2018 / 009912; WO / 2019 / 204769; WO / 2019 / 204773; and WO / 2020 / 047152; each of which is incorporated herein by reference for all purposes.

[0108] Whenever the piezoelectric transducer emits ultrasonic waves (regardless of whether said ultrasonic waves are produced directly or as a result of a backscatter protocol), a portion of the ultrasonic waves may be transmitted out of a surface of the transducer that is not directly attached to the internal surface of the acoustic window (e.g., out of surface 106(b) of transducer 106 shown in FIG. 1). These ultrasonic waves may encounter the fluid filler contained within the implant housing. The fluid filler, which may have an acoustic impedance that is different than the acoustic impedance of the piezoelectric transducer, may reflect the ultrasonic waves back through the piezoelectric transducer so that they pass through the acoustic window and out of the implant. This may improve the quality of the acoustic signals that the implantable device is able to produce.Methods of manufacturing the implantable device

[0109] An exemplary method 800 for manufacturing the described implantable device is provided in FIG. 8. As shown, the piezoelectric transducer may be attached to an interior surface of the acoustic window in the housing of the implantable device (step 802). The transducer can be attached to the acoustic window directly, for example by metallically fusing one of the transducer electrodes to the interior surface of the acoustic window, or by configuring the acoustic window to serve as one of the transducer electrodes. Alternatively, the transducer may be bonded to the acoustic window by a layer of a bonding material such as an epoxy resin. In other embodiments, the transducer and the acoustic window can be connected by a printed circuit board (PCB). On or adjacent to a second interior surface of the housing (i.e., a surface that is not the interior surface of the acoustic window to which the transducer is attached), a circuit configured to control one or more of the implant’s functions may be positioned (step 804). In some embodiments, this second interior surface opposes the interior surface of the acoustic window. Once the circuit is positioned, it may be electricallycoupled to the piezoelectric transducer (step 806), for example using one or more wires, or by connecting the piezoelectric transducer to a printed circuit board (PCB) to which the circuit is also connected.

[0110] The implant housing may be filled with a fluid filler (step 808) that a different acoustic impedance than the piezoelectric transducer. The fluid filler may be a gaseous fluid or a liquid fluid. If the fluid is a gaseous fluid (e.g., atmospheric air), the housing may be filled with the fluid filler by immersing the implantable device in the gas during execution of method 800. The fluid may also be pumped or injected into the implant housing.[oni] In some embodiments, the acoustic window is formed before the piezoelectric transducer is attached. The acoustic window may be formed from a first material (e.g., titanium) and a second material (e.g., a polymer). Forming the acoustic window from the first and second materials can involve layering the second material on top of the first material, as shown in FIG. 5A. Alternatively, the first material may be used to form base layer comprising a plurality of cavities. The second material may then be disposed in each of the plurality of cavities to form a composite of the first and second materials.EXEMPLARY EMBODIMENTS

[0112] Embodiment 1. An implantable device comprising: a housing comprising an acoustic window; and a piezoelectric transducer contained within the housing and attached to an interior surface of the acoustic window such that ultrasonic waves passing through the acoustic window do not pass through a fluid before reaching the piezoelectric transducer; wherein the acoustic window and the piezoelectric transducer are configured to acoustically resonate at approximately the same resonance frequency.

[0113] Embodiment 2. The implantable device of embodiment 1, wherein a thickness of the acoustic window is approximately equal to an integer multiple of a quarter of a wavelength in the acoustic window of sound at the resonance frequency.

[0114] Embodiment 3. The implantable device of embodiment 2, wherein the thickness of the acoustic window is approximately equal to half of the wavelength in the acoustic window of sound at the resonance frequency.

[0115] Embodiment 4. The implantable device of embodiment 2, wherein a ratio of the thickness of the acoustic window to the wavelength in the acoustic window of sound at the resonance frequency is less than 0.02.

[0116] Embodiment 5. The implantable device of any one of embodiments 1-4, wherein a thickness of the piezoelectric transducer is approximately equal an integer multiple of a quarter of a wavelength in the piezoelectric transducer of sound at the resonance frequency.

[0117] Embodiment 6. The implantable device of embodiment 5, wherein the thickness of the piezoelectric transducer is approximately equal to half of the wavelength in the piezoelectric transducer of sound at the resonance frequency.

[0118] Embodiment 7. The implantable device of any one of embodiments 1-6, comprising a circuit electrically coupled to the piezoelectric transducer and configured to control one or more implant functions.

[0119] Embodiment 8. The implantable device of embodiment 7, wherein the circuit is electrically coupled to the piezoelectric transducer by one or more wires.

[0120] Embodiment 9. The implantable device of embodiment 7, wherein the circuit is connected to a printed circuit board (PCB), and wherein the circuit is electrically coupled to the piezoelectric transducer by a plurality of conductive studs bonded to the PCB and to one or more electrodes of the piezoelectric transducer.

[0121] Embodiment 10. The implantable device of embodiment 9, wherein the conductive studs comprise metal studs or conductive epoxy studs.

[0122] Embodiment 11. The implantable device of embodiment 7, comprising a printed circuit board (PCB) comprising: a first portion disposed between and bonded to the interior surface of the acoustic window and the piezoelectric transducer; and a second portion connected to the first portion and to the circuit; wherein the PCB electrically couples the piezoelectric transducer to the circuit.

[0123] Embodiment 12. The implantable device of embodiment 11, wherein a thickness of the first portion of the PCB is approximately equal to an integer multiple of a quarter of a wavelength in PCB of sound at the resonance frequency.

[0124] Embodiment 13. The implantable device of embodiment 12, wherein the thickness of the first portion of the PCB is approximately equal to a quarter of a wavelength in the bonding layer of sound at the resonance frequency.

[0125] Embodiment 14. The implantable device of any one of embodiments 11-13, wherein the PCB comprises a cutout portion between the first portion and the second portion, and wherein the cutout portion contains a fluid filler.

[0126] Embodiment 15. The implantable device of any one of embodiments 1-10, wherein the housing contains a fluid filler.

[0127] Embodiment 16. The implantable device of embodiment 14 or 15, wherein a surface of the piezoelectric transducer is in contact with the fluid fdler.

[0128] Embodiment 17. The implantable device of any one of embodiments 14-16, wherein the fluid filler has an acoustic impedance that is not approximately equal to an acoustic impedance of the piezoelectric transducer.

[0129] Embodiment 18. The implantable device of any one of embodiments 14-17, wherein the fluid filler is a gaseous fluid.

[0130] Embodiment 19. The implantable device of embodiment 18, wherein the fluid filler comprises air.

[0131] Embodiment 20. The implantable device of embodiment 18, wherein the fluid filler comprises helium gas.

[0132] Embodiment 21. The implantable device of any one of embodiments 14-17, wherein the fluid filler is a liquid fluid.

[0133] Embodiment 22. The implantable device of any one of embodiments 1-10 or 15, comprising a bonding layer disposed between the and bonded to the piezoelectric transducer and the interior surface of the acoustic window.

[0134] Embodiment 23. The implantable device of embodiment 22, wherein the bonding layer comprises an epoxy resin.

[0135] Embodiment 24. The implantable device of embodiment 22 or 23, wherein a thickness of the bonding layer is less than or equal to one hundredth of a wavelength in the bonding layer of sound at the resonance frequency.

[0136] Embodiment 25. The implantable device of any one of embodiments 1-10 or 15, wherein an electrode of the piezoelectric transducer is metallically fused to the interior surface of the acoustic window.

[0137] Embodiment 26. The implantable device of any one of embodiments 1-10 or 15, wherein the acoustic window is an electrode of the piezoelectric transducer.

[0138] Embodiment 27. The implantable device of any one of embodiments 7-14, wherein the piezoelectric transducer generates electric current when ultrasonic waves with frequencies approximately equal to the resonance frequency are transmitted to the implantable device by an external device, and wherein the electric current generated by the piezoelectric transducer provides power to the circuit.

[0139] Embodiment 28. The implantable device of embodiment 27, wherein the circuit comprises an energy storage device, and the electric current generated by the piezoelectric transducer charges the energy storage device.

[0140] Embodiment 29. The implantable device of any one of embodiments 7-14 or 27- 28, wherein the circuit is configured to modulate current flowing through the piezoelectric transducer to communicate data from the implantable device.

[0141] Embodiment 30. The implantable device of embodiment 29, wherein the piezoelectric transducer emits ultrasonic waves encoding the data in response to the modulated electric current.

[0142] Embodiment 31. The implantable device of embodiment 29 or 30, wherein the data comprises information about a physiological signal detected by the implantable device or a treatment provided by the implantable device.

[0143] Embodiment 32. The implantable device of any one of embodiments 7-14 or 27- 31, wherein the circuit comprises one or more sensors configured to detect a physiological signal.

[0144] Embodiment 33. The implantable device of embodiment 32, wherein the one or more sensors comprise at least one of: a pressure sensor, a pH sensor, a temperature sensor, a sensor configured to detect an electrophysiological pulse, a sensor configured to detect a presence of an analyte, a sensor configured to detect a concentration of an analyte, a sensor configured to measure an evoked action potential in a brain, or a sensor configured to measure a local field potential in a brain.

[0145] Embodiment 34. The implantable device of any one of embodiments 1-33, wherein the housing comprises titanium.

[0146] Embodiment 35. The implantable device of any one of embodiments 1-33, wherein the housing comprises stainless steel.

[0147] Embodiment 36. The implantable device of any one of embodiments 1-33, wherein the housing comprises nitinol.

[0148] Embodiment 37. The implantable device of any one of embodiments 1-36, wherein the acoustic window comprises a first material and a second material.

[0149] Embodiment 38. The implantable device of embodiment 37, wherein the first material and the second material are layered, wherein the second material forms the outermost layer.

[0150] Embodiment 39. The implantable device of embodiment 37, wherein the first material and the second material form a composite.

[0151] Embodiment 40. The implantable device of embodiment 39, wherein the second material is disposed within a plurality of cavities in the first material.

[0152] Embodiment 41. The implantable device of any one of embodiments 37-40, wherein the acoustic impedance of the second material is less than the acoustic impedance of the first material.

[0153] Embodiment 42. The implantable device of any one of embodiments 37-41, wherein the first material comprises titanium and the second material comprise a polymer.

[0154] Embodiment 43. The implantable device of embodiment 42, wherein the polymer comprises poly(methyl methacrylate) (PMMA).

[0155] Embodiment 44. The implantable device of embodiment 42, wherein the polymer comprises poly etheretherketone (PEEK).

[0156] Embodiment 45. The implantable device of any one of embodiments 1-44, wherein the piezoelectric transducer comprises lead zirconate titanate (PZT).

[0157] Embodiment 46. The implantable device of any one of embodiments 1-45, wherein the device is configured to be fully implantable.

[0158] Embodiment 47. The implantable device of any one of embodiments 1-46, wherein the device is configured to be implanted in or attached to a tissue or an organ.

[0159] Embodiment 48. A system comprising the implantable device of any one of embodiments 1-47, and an external device configured to generate ultrasonic waves at approximately the resonance frequency.

[0160] Embodiment 49. The system of embodiment 48, wherein the external device is configured to receive ultrasonic waves emitted by the piezoelectric transducer in the implantable device.

[0161] Embodiment 50. The system of embodiments 48 or 49, wherein the external device is configured to be operated outside of a subject in which the implantable device is implanted.

[0162] Embodiment 51. A method comprising: generating, using an external device, ultrasonic waves with a frequency approximately equal to a resonance frequency of an acoustic window in a housing of an implantable device, and a piezoelectric transducer contained within the housing of the implantable device and attached to an interior surface of the acoustic window; transmitting the ultrasonic waves from the external device to the piezoelectric transducer through the acoustic window such that the ultrasonic waves do not pass through a fluid after passing through the acoustic window before reaching the piezoelectric transducer; andgenerating, using the piezoelectric transducer, in response to receiving the ultrasonic waves, an electric current.

[0163] Embodiment 52. The method of embodiment 51, comprising powering a circuit for controlling one or more implant functions using the electric current generated by the piezoelectric transducer.

[0164] Embodiment 53. The method of embodiment 51 or 52, comprising charging an energy storage device in the implantable device using the electric current generated by the piezoelectric transducer.

[0165] Embodiment 54. The method of any one of embodiments 51-53, comprising: emitting, using the piezoelectric transducer, ultrasonic waves encoding information about a physiological signal detected by the implantable device or a treatment provided by the implantable device; transmitting the ultrasonic waves encoding the information through the acoustic window; and receiving the ultrasonic waves using the external device.

[0166] Embodiment 55. The method of embodiment 54, wherein the ultrasonic waves emitted by the piezoelectric transducer comprise ultrasonic backscatter waves.

[0167] Embodiment 56. The method of embodiment 54 or 55, wherein emitting the ultrasonic waves encoding the information comprises modulating, using the implantable device, electric current flowing through the piezoelectric transducer to encode the information in the electric current.

[0168] Embodiment 57. The method of any one of embodiments 54-56, wherein modulating the electric current flowing through the piezoelectric transducer comprises modulating a frequency, an amplitude, or a phase of the electric current flowing through the piezoelectric transducer.

[0169] Embodiment 58. The method of any one of embodiments 51-57 comprising: extracting, using the external device, the information encoded in the ultrasonic waves; and transmitting the extracted information to a user device.

[0170] Embodiment 59. The method of any one of embodiments 51-58, comprising detecting, using the implantable device, a physiological signal in a subject.

[0171] Embodiment 60. The method of embodiment 59, wherein the physiological signal is a pressure, a pH, a temperature, an electrophysiological pulse, a concentration of an analyte, an evoked action potential in a brain, or a local field potential in a brain.

[0172] Embodiment 61. The method of any one of embodiments 51-60, comprising stimulating, using the implantable device, a tissue or an organ in a subject.

[0173] Embodiment 62. A method of manufacturing an implantable device, the method comprising: attaching a piezoelectric transducer to an interior surface of an acoustic window in a housing of the implantable device such that ultrasonic waves passing through the acoustic window do not pass through a fluid before reaching the piezoelectric transducer, wherein the acoustic window and the piezoelectric transducer are configured to acoustically resonate at approximately the same resonance frequency; positioning a circuit configured to control one or more implant functions adjacent to a second interior surface of the housing that is opposite to the interior surface of the acoustic window; and electrically coupling the circuit to the piezoelectric transducer.

[0174] Embodiment 63. The method of embodiment 62, comprising filling a region between the circuit and the piezoelectric transducer with a fluid filler.

[0175] Embodiment 64. The method of embodiment 63, wherein the fluid filler is a gaseous fluid.

[0176] Embodiment 65. The method of embodiment 64, wherein the fluid filler comprises air.

[0177] Embodiment 66. The method of embodiment 64, wherein the fluid filler comprises helium gas.

[0178] Embodiment 67. The method of embodiment 64, wherein the fluid filler is a liquid fluid.

[0179] Embodiment 68. The method of any one of embodiments 62-67, comprising forming the acoustic window from a first material and a second material.

[0180] Embodiment 69. The method of embodiment 68, wherein forming the acoustic window comprises layering the second material on top of the first material.

[0181] Embodiment 70. The method of embodiment 69, wherein forming the acoustic window comprises: disposing the first material as a base layer; forming a plurality of cavities in the base layer; and disposing the second material within each of the plurality of cavities.EXAMPLES

[0182] The following examples are intended to further demonstrate to one skilled in the art how to make and use the embodiments described herein. The provided examples are not intended to limit the scope of the invention as described herein.Example 1

[0183] 1 mm thick piezoelectric transducers were attached to 1.4 mm thick acoustic windows using layers of bonding material of varying thicknesses (0.1 pm, 10 pm, and 30 pm). Ultrasonic waves were then transmitted to each transducer. As the ultrasonic waves were transmitted to the transducers, the frequency of the ultrasound was varied between approximately 1.4 MHz and 2.5 MHz. The amount of power produced by each transducer in response to the ultrasonic waves was measured while sweeping the frequency of the ultrasonic waves to determine the effect of bonding layer thickness on transducer performance.

[0184] FIG. 9 shows a plot of the power produced by each transducer as the frequency of ultrasonic waves transmitted to the piezoelectric transducers was varied. All transducers resonated at approximately 2.2 MHz. At the resonance frequency, the transducer that was bonded to the acoustic window with the thinnest bonding layer (the bonding layer with a thickness of 0.1 pm) exhibited the strongest response, producing over 2 mW of power. The transducer that was bonded to the acoustic window with the second thinnest bonding layer (the bonding layer with a thickness of 10 pm) exhibited the second-strongest response, producing about 1.5 mW of power. The transducer that was bonded to the acoustic window with the thickest bonding layer (the bonding layer with a thickness of 30 pm) exhibited the weakest response, producing a little over 0.5 mW of power. This data therefore indicates that, if a bonding layer is used to attach a piezoelectric transducer to an acoustic window in an implantable device, the thickness of the bonding layer should be minimized.Example 2

[0185] In this example, the energy harvesting performance of a PZT piezoelectric transducer attached to a titanium acoustic window was analyzed. Ultrasonic waves with a frequency of 2.05 MHz and an intensity of 720 mW / cm2were transmitted to the piezoelectric transducer through the acoustic window. The voltage across and power harvested by the piezoelectric transducer were measured while varying the load resistance of a circuit electrically coupled to and configured to be powered by the piezoelectric transducer toidentify the load resistance at which maximum power transfer to the piezoelectric transducer occurred.

[0186] FIG. 10 shows a plot of the power harvested by the piezoelectric transducer and the voltage across the piezoelectric transducer as the load resistance of the circuit electrically coupled to the transducer was varied. The amount of power harvested by the piezoelectric transducer peaked at approximately 75 ohms. This data indicates that the amount of power harvested by a PZT piezoelectric transducer may be maximized by configuring the load resistance of the circuit to be approximately 75 ohms.

Claims

CLAIMSWhat is claimed is:

1. An implantable device comprising: a housing comprising an acoustic window; and a piezoelectric transducer contained within the housing and attached to an interior surface of the acoustic window such that ultrasonic waves passing through the acoustic window do not pass through a fluid before reaching the piezoelectric transducer; wherein the acoustic window and the piezoelectric transducer are configured to acoustically resonate at approximately the same resonance frequency.

2. The implantable device of claim 1, wherein a thickness of the acoustic window is approximately equal to an integer multiple of a quarter of a wavelength in the acoustic window of sound at the resonance frequency.

3. The implantable device of claim 2, wherein the thickness of the acoustic window is approximately equal to half of the wavelength in the acoustic window of sound at the resonance frequency.

4. The implantable device of claim 2, wherein a ratio of the thickness of the acoustic window to the wavelength in the acoustic window of sound at the resonance frequency is less than 0.02.

5. The implantable device of any one of claims 1-4, wherein a thickness of the piezoelectric transducer is approximately equal an integer multiple of a quarter of a wavelength in the piezoelectric transducer of sound at the resonance frequency.

6. The implantable device of claim 5, wherein the thickness of the piezoelectric transducer is approximately equal to half of the wavelength in the piezoelectric transducer of sound at the resonance frequency.

7. The implantable device of any one of claims 1-6, comprising a circuit electrically coupled to the piezoelectric transducer and configured to control one or more implant functions.

8. The implantable device of claim 7, wherein the circuit is electrically coupled to the piezoelectric transducer by one or more wires.

9. The implantable device of claim 7, wherein the circuit is connected to a printed circuit board (PCB), and wherein the circuit is electrically coupled to the piezoelectric transducer by a plurality of conductive studs bonded to the PCB and to one or more electrodes of the piezoelectric transducer.

10. The implantable device of claim 9, wherein the conductive studs comprise metal studs or conductive epoxy studs.

11. The implantable device of claim 7, comprising a printed circuit board (PCB) comprising: a first portion disposed between and bonded to the interior surface of the acoustic window and the piezoelectric transducer; and a second portion connected to the first portion and to the circuit; wherein the PCB electrically couples the piezoelectric transducer to the circuit.

12. The implantable device of claim 11, wherein a thickness of the first portion of the PCB is approximately equal to an integer multiple of a quarter of a wavelength in PCB of sound at the resonance frequency.

13. The implantable device of claim 12, wherein the thickness of the first portion of the PCB is approximately equal to a quarter of a wavelength in the bonding layer of sound at the resonance frequency.

14. The implantable device of any one of claims 11-13, wherein the PCB comprises a cutout portion between the first portion and the second portion, and wherein the cutout portion contains a fluid filler.

15. The implantable device of any one of claims 1-10, wherein the housing contains a fluid filler.

16. The implantable device of claim 14 or 15, wherein a surface of the piezoelectric transducer is in contact with the fluid fdler.

17. The implantable device of any one of claims 14-16, wherein the fluid fdler has an acoustic impedance that is not approximately equal to an acoustic impedance of the piezoelectric transducer.

18. The implantable device of any one of claims 14-17, wherein the fluid fdler is a gaseous fluid.

19. The implantable device of claim 18, wherein the fluid fdler comprises air.

20. The implantable device of claim 18, wherein the fluid fdler comprises helium gas.

21. The implantable device of any one of claims 14-17, wherein the fluid fdler is a liquid fluid.

22. The implantable device of any one of claims 1-10 or 15, comprising a bonding layer disposed between the and bonded to the piezoelectric transducer and the interior surface of the acoustic window.

23. The implantable device of claim 22, wherein the bonding layer comprises an epoxy resin.

24. The implantable device of claim 22 or 23, wherein a thickness of the bonding layer is less than or equal to one hundredth of a wavelength in the bonding layer of sound at the resonance frequency.

25. The implantable device of any one of claims 1-10 or 15, wherein an electrode of the piezoelectric transducer is metallically fused to the interior surface of the acoustic window.

26. The implantable device of any one of claims 1-10 or 15, wherein the acoustic window is an electrode of the piezoelectric transducer.

27. The implantable device of any one of claims 7-14, wherein the piezoelectric transducer generates electric current when ultrasonic waves with frequencies approximately equal to the resonance frequency are transmitted to the implantable device by an external device, and wherein the electric current generated by the piezoelectric transducer provides power to the circuit.

28. The implantable device of claim 27, wherein the circuit comprises an energy storage device, and the electric current generated by the piezoelectric transducer charges the energy storage device.

29. The implantable device of any one of claims 7-14 or 27-28, wherein the circuit is configured to modulate current flowing through the piezoelectric transducer to communicate data from the implantable device.

30. The implantable device of claim 29, wherein the piezoelectric transducer emits ultrasonic waves encoding the data in response to the modulated electric current.

31. The implantable device of claim 29 or 30, wherein the data comprises information about a physiological signal detected by the implantable device or a treatment provided by the implantable device.

32. The implantable device of any one of claims 7-14 or 27-31, wherein the circuit comprises one or more sensors configured to detect a physiological signal.

33. The implantable device of claim 32, wherein the one or more sensors comprise at least one of: a pressure sensor, a pH sensor, a temperature sensor, a sensor configured to detect an electrophysiological pulse, a sensor configured to detect a presence of an analyte, a sensor configured to detect a concentration of an analyte, a sensor configured to measure an evoked action potential in a brain, or a sensor configured to measure a local field potential in a brain.

34. The implantable device of any one of claims 1-33, wherein the housing comprises titanium.

35. The implantable device of any one of claims 1-33, wherein the housing comprises stainless steel.

36. The implantable device of any one of claims 1-33, wherein the housing comprises nitinol.

37. The implantable device of any one of claims 1-36, wherein the acoustic window comprises a first material and a second material.

38. The implantable device of claim 37, wherein the first material and the second material are layered, wherein the second material forms the outermost layer.

39. The implantable device of claim 37, wherein the first material and the second material form a composite.

40. The implantable device of claim 39, wherein the second material is disposed within a plurality of cavities in the first material.

41. The implantable device of any one of claims 37-40, wherein the acoustic impedance of the second material is less than the acoustic impedance of the first material.

42. The implantable device of any one of claims 37-41, wherein the first material comprises titanium and the second material comprise a polymer.

43. The implantable device of claim 42, wherein the polymer comprises poly(methyl methacrylate) (PMMA).

44. The implantable device of claim 42, wherein the polymer comprises polyetheretherketone (PEEK).

45. The implantable device of any one of claims 1-44, wherein the piezoelectric transducer comprises lead zirconate titanate (PZT).

46. The implantable device of any one of claims 1-45, wherein the device is configured to be fully implantable.

47. The implantable device of any one of claims 1-46, wherein the device is configured to be implanted in or attached to a tissue or an organ.

48. A system comprising the implantable device of any one of claims 1-47, and an external device configured to generate ultrasonic waves at approximately the resonance frequency.

49. The system of claim 48, wherein the external device is configured to receive ultrasonic waves emitted by the piezoelectric transducer in the implantable device.

50. The system of claims 48 or 49, wherein the external device is configured to be operated outside of a subject in which the implantable device is implanted.

51. A method comprising : generating, using an external device, ultrasonic waves with a frequency approximately equal to a resonance frequency of: an acoustic window in a housing of an implantable device, and a piezoelectric transducer contained within the housing of the implantable device and attached to an interior surface of the acoustic window; transmitting the ultrasonic waves from the external device to the piezoelectric transducer through the acoustic window such that the ultrasonic waves do not pass through a fluid after passing through the acoustic window before reaching the piezoelectric transducer; and generating, using the piezoelectric transducer, in response to receiving the ultrasonic waves, an electric current.

52. The method of claim 51, comprising powering a circuit for controlling one or more implant functions using the electric current generated by the piezoelectric transducer.

53. The method of claim 51 or 52, comprising charging an energy storage device in the implantable device using the electric current generated by the piezoelectric transducer.

54. The method of any one of claims 51-53, comprising: emitting, using the piezoelectric transducer, ultrasonic waves encoding information about a physiological signal detected by the implantable device or a treatment provided by the implantable device; transmitting the ultrasonic waves encoding the information through the acoustic window; and receiving the ultrasonic waves using the external device.

55. The method of claim 54, wherein the ultrasonic waves emitted by the piezoelectric transducer comprise ultrasonic backscatter waves.

56. The method of claim 54 or 55, wherein emitting the ultrasonic waves encoding the information comprises modulating, using the implantable device, electric current flowing through the piezoelectric transducer to encode the information in the electric current.

57. The method of any one of claims 54-56, wherein modulating the electric current flowing through the piezoelectric transducer comprises modulating a frequency, an amplitude, or a phase of the electric current flowing through the piezoelectric transducer.

58. The method of any one of claims 51-57 comprising: extracting, using the external device, the information encoded in the ultrasonic waves; and transmitting the extracted information to a user device.

59. The method of any one of claims 51-58, comprising detecting, using the implantable device, a physiological signal in a subject.

60. The method of claim 59, wherein the physiological signal is a pressure, a pH, a temperature, an electrophysiological pulse, a concentration of an analyte, an evoked action potential in a brain, or a local field potential in a brain.

61. The method of any one of claims 51-60, comprising stimulating, using the implantable device, a tissue or an organ in a subject.

62. A method of manufacturing an implantable device, the method comprising: attaching a piezoelectric transducer to an interior surface of an acoustic window in a housing of the implantable device such that ultrasonic waves passing through the acoustic window do not pass through a fluid before reaching the piezoelectric transducer, wherein the acoustic window and the piezoelectric transducer are configured to acoustically resonate at approximately the same resonance frequency; positioning a circuit configured to control one or more implant functions adjacent to a second interior surface of the housing that is opposite to the interior surface of the acoustic window; and electrically coupling the circuit to the piezoelectric transducer.

63. The method of claim 62, comprising filling a region between the circuit and the piezoelectric transducer with a fluid filler.

64. The method of claim 63, wherein the fluid filler is a gaseous fluid.

65. The method of claim 64, wherein the fluid filler comprises air.

66. The method of claim 64, wherein the fluid filler comprises helium gas.

67. The method of claim 64, wherein the fluid filler is a liquid fluid.

68. The method of any one of claims 62-67, comprising forming the acoustic window from a first material and a second material.

69. The method of claim 68, wherein forming the acoustic window comprises layering the second material on top of the first material.

70. The method of claim 69, wherein forming the acoustic window comprises: disposing the first material as a base layer; forming a plurality of cavities in the base layer; and disposing the second material within each of the plurality of cavities.

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