WO2025114820A1

WO2025114820A1 - Piezoelectric actuator with dynamic preload force

Info

Publication number: WO2025114820A1
Application number: PCT/IB2024/061630
Authority: WO
Inventors: Kristian Gunnar ÅSNES; Patrik KENNES; Wim Bervoets
Original assignee: Cochlear Ltd
Current assignee: Cochlear Ltd
Priority date: 2023-11-28
Filing date: 2024-11-20
Publication date: 2025-06-05
Anticipated expiration: 2026-05-28

Abstract

An apparatus includes a piezoelectric stack with a plurality of piezoelectric layers and a plurality of electrodes and a housing containing the piezoelectric stack. The housing is configured to be in mechanical communication with bone tissue of a recipient's body. The apparatus further includes circuitry configured to apply non-zero voltage differences across the piezoelectric layers. The non-zero voltage differences induce changes of a length of the piezoelectric stack and include a non-zero direct current (DC) voltage component corresponding to an expansion of the length and an alternating current (AC) voltage component corresponding to time-varying expansions and contractions of the length that generate vibrational signals that propagate through the bone tissue.

Description

PIEZOELECTRIC ACTUATOR WITH DYNAMIC PRELOAD FORCE

BACKGROUND

Field

[0001] The present application relates generally to an implantable actuator for generating vibrations, and more specifically, to implantable auditory prostheses for generating auditory vibrations.

Description of the Related Art

[0002] Medical devices have provided a wide range of therapeutic benefits to recipients over recent decades. Medical devices can include internal or implantable components/de vices, external or wearable components/devices, or combinations thereof (e.g., a device having an external component communicating with an implantable component). Medical devices, such as traditional hearing aids, partially or fully-implantable hearing prostheses (e.g., bone conduction devices, mechanical stimulators, cochlear implants, etc.), pacemakers, defibrillators, functional electrical stimulation devices, and other medical devices, have been successful in performing lifesaving and/or lifestyle enhancement functions and/or recipient monitoring for a number of years.

[0003] The types of medical devices and the ranges of functions performed thereby have increased over the years. For example, many medical devices, sometimes referred to as “implantable medical devices,” now often include one or more instruments, apparatus, sensors, processors, controllers or other functional mechanical or electrical components that are permanently or temporarily implanted in a recipient. These functional devices are typically used to diagnose, prevent, monitor, treat, or manage a disease/injury or symptom thereof, or to investigate, replace or modify the anatomy or a physiological process. Many of these functional devices utilize power and/or data received from external devices that are part of, or operate in conjunction with, implantable components.

SUMMARY

[0004] In one aspect disclosed herein, an apparatus comprises a housing configured to be in mechanical communication with bone tissue of a recipient’s body. The apparatus further comprises a piezoelectric stack contained within the housing. The piezoelectric stack comprises a plurality of piezoelectric layers and a plurality of electrodes, the piezoelectric layers and the electrodes alternating with one another along a length of the piezoelectric stack such that each piezoelectric layer of the plurality of piezoelectric layers is sandwiched between a pair of electrodes of the plurality of electrodes. The apparatus further comprises circuitry in electrical communication with the plurality of electrodes. The circuitry is configured to apply non-zero voltage differences across the piezoelectric layers. The non-zero voltage differences induce changes of the length of the piezoelectric stack and comprises a non-zero direct current (DC) voltage component corresponding to an expansion of the length and an alternating current (AC) voltage component corresponding to time-varying expansions and contractions of the length that generate vibrational signals that propagate through the bone tissue.

[0005] In another aspect disclosed herein, a method comprises providing a piezoelectric actuator comprising a plurality of piezoelectric layers and a plurality of electrodes alternating with the piezoelectric layers, the piezoelectric actuator having a first actuator portion and a second actuator portion spaced from the first actuator portion by a first length along a direction with a voltage difference substantially equal to zero applied by the electrodes to the piezoelectric layers. The method further comprises providing a casing configured to contain the piezoelectric actuator. The casing has a first casing portion configured to be in mechanical communication with the first actuator portion and a second casing portion configured to be in mechanical communication with the second actuator portion. The second casing portion is spaced from the first casing portion by a distance less than the first length. The method further comprises applying a non-zero direct current (DC) voltage difference to the piezoelectric layers. The non-zero DC voltage difference causes a contraction of the piezoelectric actuator along the direction such that the second actuator portion is spaced from the first actuator portion by a second length along the direction, the second length less than the distance. The method further comprises placing the piezoelectric actuator within the casing while applying the non-zero DC voltage difference to the piezoelectric layers. The method further comprises, while the piezoelectric actuator is within the casing, applying a voltage difference substantially equal to zero to the piezoelectric layers such that the first casing portion is in mechanical communication with the first actuator portion and the second casing portion is in mechanical communication with the second actuator portion.

[0006] In another aspect disclosed herein, a method comprises providing an assembly comprising a casing containing a piezoelectric actuator comprising a plurality of piezoelectric layers and a plurality of electrodes alternating with the piezoelectric layers. The casing applies a first non-zero compressive preload force to the piezoelectric actuator and comprising a first casing portion and a second casing portion spaced from the first casing portion by a first distance along a longitudinal axis of the casing with a voltage difference substantially equal to zero applied to the piezoelectric layers. The method further comprises accessing a first wall portion configured to be affixed to the first casing portion and a second wall portion configured to be affixed to the second casing portion. The second wall portion is spaced from the first wall portion by a width, the width less than the first distance. The method further comprises applying non-zero direct current (DC) voltage differences to the piezoelectric layers. The non-zero DC voltage differences inducing a contraction of the casing along the longitudinal axis such that the second casing portion is spaced from the first casing portion by a second distance along the longitudinal axis, the second distance less than the width. The method further comprises, while applying the non-zero DC voltage differences to the piezoelectric layers, inserting the casing at least partially between the first and second wall portions. The method further comprises, after said inserting, applying voltage differences substantially equal to zero to the piezoelectric layers such that the first casing portion is in mechanical communication with the first wall portion and the second casing portion is in mechanical communication with the second wall portion.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Implementations are described herein in conjunction with the accompanying drawings, in which:

[0008] FIG. 1 schematically illustrates a cross-sectional view of an example apparatus in accordance with certain implementations described herein;

[0009] FIGs. 2A-2D schematically illustrate various example configurations of the apparatus in accordance with certain implementations described herein;

[0010] FIGs. 3 A and 3B schematically illustrate an example voltage difference A V and an implanted apparatus, respectively, for which the voltage difference AV applied to the piezoelectric element equals zero at various times during generation of the vibrational signals;

[0011] FIGs. 4A and 4B schematically illustrate an example voltage difference A V and an implanted apparatus, respectively, for which the voltage difference AV applied to the piezoelectric element is continually non-zero during generation of the vibrational signals in accordance with certain implementations described herein;

[0012] FIG. 5A is a flow diagram of an example method for fabricating an apparatus in accordance with certain implementations described herein;

[0013] FIG. 5B schematically illustrates the example method of FIG. 5 A performed using an example piezoelectric element and housing in accordance with certain implementations described herein;

[0014] FIG. 6A is a flow diagram of an example method for implanting an assembly in accordance with certain implementations described herein; and

[0015] FIG. 6B schematically illustrates the example method performed using an example assembly comprising a piezoelectric element and housing in accordance with certain implementations described herein.

DETAILED DESCRIPTION

[0016] Certain implementations described herein provide a piezoelectric actuator with a plurality of piezoelectric layers that has a non-zero compressive preload force applied to the actuator while the actuator is generating vibrational signals and not while the actuator is not generating vibrational signals. The dynamic preload force is generated by applying nonzero voltage differences to the piezoelectric layers that comprises a non-zero DC voltage component corresponding to an expansion of the piezoelectric layers and an AC voltage component corresponding to time-varying expansions and contractions to generate the vibrational signals. By applying the non-zero DC voltage component only while the actuator is generating vibrational signals, bone deformation resulting from a static preload can be reduced and/or avoided. In certain other implementations, DC voltage components corresponding to contractions of the piezoelectric layers are applied during fabrication of the actuator and/or during implantation of the actuator, resulting in a discrete mechanical preload on the actuator by the housing of the actuator and/or by the bone tissue.

[0017] The teachings detailed herein are applicable, in at least some implementations, to any type of implantable medical device (e.g., implantable vibration stimulation system or device; bone conduction auditory prosthesis) comprising a first portion implanted on or within the recipient’s body and configured to provide vibrations to a portion of the recipient’s body. Implementations can include any type of medical device that can utilize the teachings detailed herein and/or variations thereof. Furthermore, while certain implementations are described herein in the context of implantable auditory prosthesis devices, certain other implementations are compatible in the context of other implantable or nonimplantable devices or systems (e.g., bone conduction headphones; bone conduction speakers; bone conduction microphones; ultrasonic imaging).

[0018] Merely for ease of description, apparatus and methods disclosed herein are primarily described with reference to an illustrative medical device, namely an active transcutaneous or percutaneous bone conduction auditory prosthesis systems. However, the teachings detailed herein and/or variations thereof may also be used with a variety of other medical or non-medical systems that provide a wide range of therapeutic benefits to recipients, patients, or other users. In some implementations, the teachings detailed herein and/or variations thereof can be utilized in other types of devices beyond auditory prostheses that may benefit from a vibration-generating actuator able to fit within a region having restricted space and/or improved control of piezoelectric vibrations (e.g., a direction of vibration motion). Implementations can include any type of auditory prosthesis that can utilize the teachings detailed herein and/or variations thereof. Certain such implementations can be referred to as “partially implantable,” “semi-implantable,” “mostly implantable,” “fully implantable,” or “totally implantable” auditory prostheses. In some implementations, the teachings detailed herein and/or variations thereof can be utilized in other types of prostheses beyond auditory prostheses.

[0019] FIG. 1 schematically illustrates a cross-sectional view of an example apparatus 400 in accordance with certain implementations described herein. In certain implementations, the apparatus 400 comprises a portion of a transcutaneous bone conduction device implanted in a recipient in accordance with certain implementations described herein. The transcutaneous bone conduction device can be configured to compensate for conductive hearing loss, mixed hearing loss, or single-sided deafness.

[0020] The transcutaneous bone conduction device can include an external component and an implantable component which comprises the apparatus 400. For example, the implantable component can include a vibrating actuator assembly in mechanical communication with bone tissue (e.g., skull) of the recipient’s body and configured to convert electrical signals generated by at least one sound input element of the transcutaneous bone conduction device into vibrational stimuli and to deliver the vibrational stimuli to the bone tissue. For example, the vibrating actuator assembly can be rigidly attached to (e.g., in direct contact with) the outer surface of the recipient’s skull bone such that vibrations generated by the vibrating actuator assembly are transmitted to the recipient’s bone and detected by the recipient’s ossicles and/or cochlea to evoke a hearing percept. The external component and the implantable component can be in wireless communication with one another via a magnetic inductance link (e.g., comprising a communication coil of the external component in wireless communication with a communication coil of the implantable component).

[0021] In certain implementations, the at least one sound input element comprises at least one microphone configured to generate the electrical signals in response to sound received by the at least one microphone. For example, the at least one microphone can comprise at least one external microphone located outside the recipient’s body (e.g., on an outer surface of the recipient’s skin) and/or at least one internal microphone located within the recipient’s body (e.g., beneath the outer surface recipient’s skin). In certain other implementations, the at least one sound input element comprises a device configured to receive signals indicative of sound from an audiovisual device.

[0022] The transcutaneous bone conduction device can include sound processing circuitry (e.g., in the external component, in the internal component, or divided among both the external and internal components) configured to process the electrical signals from the at least one sound input element, and to provide the processed signals to the vibrating actuator to be converted into the vibrational stimuli. For example, the sound processing circuitry can receive and process the electrical signals (e.g., applying one or more of digitization, shifting, shaping, amplification, compression, filtering, and/or other signal conditioning to the electrical signals).

[0023] In certain implementations, the apparatus 400 is configured to be in mechanical communication with bone tissue 500 of a recipient’s body. For example, as shown in FIG. 1 , the apparatus 400 can be configured to be implanted at least partially within a cavity 510 (e.g., machined during a surgical procedure; naturally-occurring) in the bone tissue 500 (e.g., temporal bone; skull bone) or can be configured to be implanted within a cavity of an implantable structure in mechanical communication with a surface of bone tissue 500. In other examples, the apparatus 400 can be an implantable component of an active transcutaneous bone conduction device.

[0024] The apparatus 400 of FIG. 1 comprises a housing 420 (e.g., casing) configured to be in mechanical communication with the bone tissue 500 of a recipient’s body (e.g., implanted within the recipient’s body and at least partially within the cavity 510). The apparatus 400 further comprises a piezoelectric element 410 (e.g., piezoelectric stack) contained within the housing 420. The piezoelectric element 410 comprising a plurality of piezoelectric layers 412 and a plurality of electrodes 414, the piezoelectric layers 412 and the electrodes 414 alternating with one another along a length of the piezoelectric element 410 (e.g., axially stacked) such that each piezoelectric layer 412 of the plurality of piezoelectric layers 412 is sandwiched between a pair of electrodes 414 of the plurality of electrodes 414. The apparatus 400 further comprises circuitry 450 in electrical communication with the plurality of electrodes 414. The circuitry 450 is configured to apply non-zero voltage differences V across the piezoelectric layers 412. The non-zero voltage differences A V induce changes of the length of the piezoelectric element 410 (e.g., along a direction 460 extending from a first portion 418 of the piezoelectric element 410 to a second portion 419 of the piezoelectric element 410). The non-zero voltage differences AV comprise a non-zero direct current (DC) voltage component VDC corresponding to an expansion of the length and an alternating current (AC) voltage component VAC corresponding to time-varying expansions and contractions of the length that generate vibrational signals 470 that propagate through the bone tissue 500.

[0025] The piezoelectric element 410 (e.g., oscillator; vibrating actuator) is configured to expand and contract in the direction 460 in response to electrical voltage signals applied to the piezoelectric element 410 (e.g., the non-zero voltage difference AV). For example, the piezoelectric element 410 can comprise a column, post, or cylinder configured to expand and contract along an axial direction (e.g., substantially parallel to a longitudinal axis 416 of the piezoelectric element 410). For another example, the piezoelectric element 410 can comprise a slab, sheet, or plate configured to expand and contract along a radial direction (e.g., substantially orthogonal to the longitudinal axis 416). The longitudinal axis 416 of the piezoelectric element 410 can be an axis along a length of the piezoelectric element 410 or an axis about which the piezoelectric element 410 is at least partially symmetric. The piezoelectric element 410 can comprise a plurality of thin piezoelectric layers 412 (e.g., thickness in a range of 10 microns to 30 microns), each piezoelectric layer 412 between a pair of thin electrodes 414 (e.g., electrically conductive layers having thicknesses in a range of 2 microns to 10 microns). Thin piezoelectric layers 412 allow the applied electrical voltages to be lower than for thicker piezoelectric layers 412.

[0026] The piezoelectric element 410 is configured to generate the vibrational signals 470 (e.g., vibrational energy; vibrations; auditory vibrations) over a range of vibrational frequencies (e.g., within a range of vibrational frequencies that are perceptible by the recipient as sound; a range of 20 Hz to 20 kHz). The vibrational signals 470 can be configured to evoke a sensory percept by the recipient. For example, the vibrational signals 470 can propagate via bone conduction from the piezoelectric element 410 to an inner ear region (e.g., within the temporal bone and comprising the vestibule, the cochlea, and the semicircular canals) and/or a middle ear region (e.g., within the recipient’s head, partially bounded by the tympanic membrane and comprising the ossicles, the round window, the oval window, and the Eustachian tube) to be detected by the recipient as sound.

[0027] In certain implementations, at least one piezoelectric layer 412 of the plurality of piezoelectric layers 412 comprises a unitary (e.g., single; monolithic) component. At least one piezoelectric layer 412 of certain implementations comprises two or more sublayers in mechanical communication with one another (e.g., bonded together) into a unitary component, at least one of the sub-layers comprising a piezoelectric material (e.g., unimorph having one piezoelectric sub-layer and a non-piezoelectric sub-layer; bimorph having two or more piezoelectric sub-layers). Each piezoelectric layer 412 can be substantially perpendicular to the longitudinal axis 416 and can be configured to expand and contract along (e.g., substantially parallel to) the longitudinal axis 416. At least one piezoelectric layer 412 can comprise other non-piezoelectric materials, such as a bonding material (e.g., adhesive; epoxy; metal) between piezoelectric sub-layers, and the electrically conductive electrodes 414 (e.g., metal) that are configured to apply the non-zero voltage differences to the piezoelectric layers 412. In certain implementations, the number of piezoelectric layers 412 is selected to provide a predetermined power, size (e.g., area, thickness), stiffness, and/or resonance frequency. Examples of piezoelectric materials compatible with certain implementations described herein include but are not limited to: quartz; gallium orthophosphate; langasite; barium titanate; lead titanate; lead zirconate titanate (PZT); potassium niobate; lithium niobate; lithium tantalate; sodium tungstate; sodium potassium niobate; bismuth ferrite; sodium niobate; polyvinylidene fluoride; macro fiber composite (MFC); other piezoelectric crystals, ceramics, or polymers.

[0028] The piezoelectric element 410 can have a cross-sectional shape (e.g., circular; rectangular; square; oval; polygonal with 5, 6, 7, 8, or more sides; regular; irregular) in a plane substantially perpendicular to the longitudinal axis 416 and can be substantially symmetric about the longitudinal axis 416. The piezoelectric element 410 can have a length along the longitudinal axis 416 (e.g., in a range of 2 millimeters to 20 millimeters; in a range of 3 millimeters to 10 millimeters), a width substantially perpendicular to the length (e.g., in a range of 2 millimeters to 20 millimeters), and a thickness substantially perpendicular to the length and to the width (e.g., in a range of less than 2 millimeters; less than 1 millimeter; greater than 300 microns). Various configurations and geometries of the piezoelectric element 410 are compatible with certain implementations described herein (see, e.g., “Piezoelectric Ceramic Products: Fundamentals, Characteristics and Applications,” Physik Instruments (PI) GmbH & Co., Lederhose, Germany, www.piceramic.com, (2016)).

[0029] In certain implementations, the housing 420 is configured to hermetically seal the piezoelectric element 410 from an environment surrounding the housing 420. The housing 420 can have a length (e.g., along a longitudinal axis 422 of the housing 420) and/or a width (e.g., substantially perpendicular to the length) less than or equal to 30 millimeters (e.g., in a range of 15 millimeters to 25 millimeters; in a range of 10 millimeters to 30 millimeters), and/or a thickness less than or equal to 7 millimeters (e.g., in a range of less than or equal to 6 millimeters, in a range of less than or equal to 5 millimeters; in a range of less than or equal to 4 millimeters). The longitudinal axis 422 of the housing 420 can be an axis along the length of the housing 420 or an axis about which the housing 420 is at least partially symmetric. In certain implementations, the longitudinal axis 416 of the piezoelectric element 410 is substantially parallel to the longitudinal axis 422 of the housing 420, while in certain other implementations, the longitudinal axis 416 of the piezoelectric element 410 is substantially perpendicular to the longitudinal axis 422 of the housing 420 or at another nonzero angle relative to the longitudinal axis 422 of the housing 420. The housing 420 of certain implementations comprises at least one biocompatible material (e.g., plastic; PEEK; silicone; titanium; titanium alloy; ceramic; zirconium oxide) configured to be affixed to a bone surface of the recipient.

[0030] In certain implementations, the housing 420 is configured to be rigidly affixed to the piezoelectric element 410 and at least a portion of the housing 420 is configured to flex (e.g., expand and contract) in response to expansions and contractions of the piezoelectric element 410. For example, the housing 420 can comprise one or more walls that are sufficiently thin to be compliant (e.g., flexible; bendable; stretchable) in response to the expansion and/or contraction of the piezoelectric element 410 and one or more walls that are sufficiently thick to provide structural rigidity to support the piezoelectric element 410.

[0031] In certain implementations, the housing 420 is configured to be rigidly affixed directly to at least one wall portion (e.g., bottom wall portion 512; side wall portion 514) of the cavity 510. For example, the housing 420 can be configured to be affixed to the bone tissue by osseointegration. In certain other implementations, the housing 420 is configured to be rigidly affixed to a fixation element 480 (e.g., support assembly; bone fixture; clamp, screw, or other coupler) that is configured to be rigidly affixed to at least one wall portion (e.g., bottom wall portion 512; side wall portion 514) of the cavity 510. The fixation element 480 can comprise a substantially rigid material (e.g., metal; epoxy; bone cement) configured to transmit the vibrational signals 470 from the piezoelectric element 410 and the housing 420 to the bone tissue 500. The fixation element 480 can be configured to be affixed to the bone tissue 500 by osseointegration.

[0032] In certain implementations, the first portion 418 of the piezoelectric element 410 comprises a first end portion of the piezoelectric element 410 and the second portion 419 of the piezoelectric element 410 comprises a second end portion of the piezoelectric element 410, the first and second portions 418, 419 at opposite ends of the piezoelectric element 410 (see, e.g., FIG. 1). The second end portion can be spaced from the first end portion along the longitudinal axis 416 of the piezoelectric element 410 and the changes of the length can be along a direction 460 substantially parallel to the longitudinal axis 416. In certain other implementations, the first portion 418 comprises a first side portion of the piezoelectric element 410 and the second portion 419 comprises a second side portion of the piezoelectric element 410. The second side portion can be spaced from the first side portion and the changes of the length can be along a direction 460 substantially perpendicular to the longitudinal axis 416 of the piezoelectric element 410. In certain other implementations, the first portion 418 comprises a center portion of the piezoelectric element 410 and the second portion 419 comprises a periphery portion of the piezoelectric element 410, and the changes of the length can be along a direction 460 substantially perpendicular to the longitudinal axis 416 (e.g., in a radial direction).

[0033] FIGs. 2A-2D schematically illustrate various example configurations of the apparatus 400 in accordance with certain implementations described herein. The apparatus 400 of certain such implementations can include fixation in at least two ends of the apparatus 400 and/or other features of bone conduction actuators as described in IntT Appl. No. PCT/IB2023/050915, which is incorporated in its entirety by reference herein.

[0034] As shown in FIGs. 2A-2B, the apparatus 400 is at least partially within the cavity 510 with the housing 420 extending at a non-zero angle (e.g., substantially orthogonally) to a surface of the bone tissue 500. As shown in FIGs. 2C and 2D, the apparatus 400 is at least partially within the cavity 510 with the housing 420 extending substantially parallel to the surface of the bone tissue 500. In FIGs. 2A and 2B, the apparatus 400 further comprising a fixation element 480 configured to hold the housing 420 in the cavity 510 (e.g., to rigidly affix the housing 420 to at least one wall portion of the cavity 510 such that the housing 420 does not substantially move relative to the cavity 510 during the oscillations of the piezoelectric element 410). In FIGs. 2C and 2D, the housing 420 is configured to be held in the cavity 510 by an adhesive material (e.g., biocompatible adhesive, epoxy, or bone cement; not shown) such that the housing 420 does not substantially move relative to the cavity 510 during the oscillations of the piezoelectric element 410.

[0035] In FIGs. 2 A and 2C, the piezoelectric element 410 is configured to expand and contract in a direction 460 substantially parallel to the longitudinal axis 422 of the housing 420. In FIGs. 2B and 2D, the piezoelectric element 410 is configured to expand and contract in a direction substantially orthogonal to the longitudinal axis 422 of the housing 420 (e.g., in a radial direction for a piezoelectric element 410 having a substantially circular cross-section in a plane substantially orthogonal to the longitudinal axis 422). Other angles of the direction 460 of the expansion and contraction of the piezoelectric element 410 relative to the longitudinal axis 422 of the housing 420 are also compatible with certain implementations described herein. [0036] In FIG. 2A, the housing 420 is implanted within the recipient’s body such that the time-varying expansions and contractions of the length along the direction 460 are at a non-zero angle (e.g., substantially perpendicular) to the surface portion of the bone tissue 500 (e.g., the surface having an opening of the cavity 510). In FIGs. 2B-2D, the housing 420 is implanted within the recipient’s body such that the time-varying expansions and contractions of the length along the direction 460 are substantially parallel to the surface portion of the bone tissue 500. Other angles of the direction of the expansion and contraction of the piezoelectric element 410 relative to the surface portion of the bone tissue 500 are also compatible with certain implementations described herein.

[0037] In FIG. 2A, the housing 420 is in mechanical communication with a bottom wall portion 512 of the cavity 510 through which the vibrational signals 470 generated by the apparatus 400 propagate. In FIG. 2B (which shows a perspective view and a top view of the apparatus 400 and the cavity 510), the fixation element 480 is in mechanical communication with a side wall portion 514 of the cavity 510 through which the vibrational signals 470 generated by the apparatus 400 propagate. In FIGs. 2C and 2D (each of which shows a perspective view and a top view of the apparatus 400 and the cavity 510), the housing 420 is in mechanical communication with opposite side wall portions 514 of the cavity 510 through which the vibrational signals 470 generated by the apparatus 400 propagate. As shown in FIGs 1 and 2B-2D, for an apparatus 400 fully integrated in the bone 500, the vibrational signals 470 can propagate from the apparatus 400 in two substantially opposite directions. In the example apparatus 300 of FIGs. 2A and 2C, the piezoelectric layers 412 are axially stacked along the longitudinal axis 416 (see, e.g., FIG. 1), while in the example apparatus 300 of FIGs. 2B and 2D, the piezoelectric layers 412 are stacked in a direction substantially perpendicular to the longitudinal axis 416.

[0038] In certain implementations, the circuitry 450 comprises at least one microcontroller configured to receive data and/or control signals indicative of the non-zero voltage differences to be applied across the piezoelectric layers 412 and to generate the nonzero voltage differences in response to the received data and/or control signals. The at least one microcontroller can comprise at least one application-specific integrated circuit (ASIC) microcontroller, digital signal processing (DSP) microcontroller, generalized integrated circuits programmed by software with computer executable instructions, and/or microcontroller core. In certain implementations, the circuitry 450 comprises and/or is in operative communication with storage circuitry configured to store information (e.g., data; commands) accessed by the circuitry 450 during operation (e.g., while providing the functionality of certain implementations described herein). The storage circuitry can comprise at least one tangible (e.g., non-transitory) computer readable storage medium, examples of which include but are not limited to: read only memory (ROM); random access memory (RAM); magnetic disk storage media; optical storage media; flash memory. The storage circuitry can be encoded with software (e.g., a computer program downloaded as an application) comprising computer executable instructions for instructing the circuitry 450 (e.g., executable data access logic, evaluation logic, and/or information outputting logic). In certain implementations, the circuitry 450 executes the instructions of the software to provide functionality as described herein. The circuitry 450 of certain implementations further comprises other digital circuitry (e.g., registers; filters; output controllers; memory controllers).

[0039] In certain implementations, at least a portion of the circuitry 450 can be within the housing 420 (e.g., within the cavity 510) and/or at least a portion of the circuitry 450 can be outside the housing 420. For example, the circuitry 450 can comprise first circuitry (e.g., a first microcontroller) within the housing 420 and electrically connected to the electrodes 414 and second circuitry (e.g., a second microcontroller) in a second housing (e.g., casing) in electrical communication with the first circuitry. The second circuitry can be configured to provide power, data, and/or control signals to the first circuitry within the housing 420. In certain implementations, the second housing is implanted within the recipient’s body separate from the housing 420 and the electrical communication between the first and second circuitry is through one or more electrical conduits (e.g., wires) extending between the housing 420 and the second housing. In certain other implementations, the second housing is part of a device external to the recipient’s body (e.g., worn on the recipient’s body) and the electrical communication between the first and second circuitry is via a wireless communication channel (e.g., a transcutaneous inductive radio frequency (RF) communication link between the apparatus 400 and the external device), across which the apparatus 400 receives power, data, and/or control signals from the device. The second circuitry of certain implementations can comprise a device separate from both an externally worn device and the implanted apparatus 400 (e.g., smartphone; smart tablet; smart watch; other remote device operated by the recipient and in communication with the externally worn device and/or the implanted apparatus 400).

[0040] In certain implementations, the circuitry 450 comprises a single amplifier configured to apply both the non-zero DC voltage component and the AC voltage component across the piezoelectric layers 412. In certain other implementations, the circuitry 450 comprises a first amplifier configured to apply the non-zero DC voltage component across the piezoelectric layers 412 and a second amplifier configured to apply the AC voltage component across the piezoelectric layers 412. By having first and second amplifiers instead of a single amplifier, certain implementations can avoid having a portion of the finite voltage range (e.g., voltage headroom) of the amplifier providing the AC voltage component being used to provide the non-zero DC voltage component. The first amplifier and the second amplifier can have substantially equal resolutions or the second amplifier can have a higher resolution than does the first amplifier. In certain implementations, the first and second amplifiers are in series electrical connection with one another, while in certain other implementations, the first and second amplifiers are in parallel electrical connection with one another. For example, the first amplifier can have a sufficiently high electrical impedance such that the oscillating data signals (e.g., audio data signals) do not drive the first amplifier but do drive the second amplifier. The AC voltage component generated by the second amplifier can ride on the non-zero DC voltage component generated by the first amplifier.

[0041] In certain implementations, while the apparatus 400 is operated to generate the vibrational signals 470, the non-zero voltage difference V applied to the piezoelectric element 410 is configured such that the housing 420 continually pushes on the wall surface (e.g., surface of the bottom wall portion 512; surface of the side wall portion 514) of the cavity 510 and does not pull away from the wall surface. In this way, certain implementations are configured to continually apply a preload to the piezoelectric element 410 while the apparatus 400 is being used to generate the vibrational signals 470.

[0042] To illustrate a configuration avoided by certain implementations described herein, FIGs. 3 A and 3B schematically illustrate an example voltage difference V and an implanted apparatus 400, respectively, for which the voltage difference V applied to the piezoelectric layers 412 equals zero at various times during generation of the vibrational signals 470. As shown in FIG. 3 A, the voltage difference AV varies between + VAC and - VAC, periodically passing through zero. The dashed line in FIG. 3B shows the position of the bottom end portion of the housing 420 when the voltage difference A V equals zero. As shown in the left portion of FIG. 3B, while the voltage difference AV is greater than zero, the length of the piezoelectric element 410 would expand if not for the wall portion of the cavity 510, so the piezoelectric element 410 pushes against the wall portion of the cavity 510 and the piezoelectric element 410 experiences a non-zero compressive preload force from the wall portion. As shown in the middle portion of FIG. 3B, while the voltage difference AV equals zero, the length of the piezoelectric element 410 neither expands nor contracts, so the piezoelectric element 410 neither pushes against nor pulls from the wall portion of the cavity 510 and the piezoelectric element 410 does not experience a preload force from the wall portion. As shown in the right portion of FIG. 3B, while the voltage difference AV is less than zero, the length of the piezoelectric element 410 would contract if not adhered to the wall portion of the cavity 510, so the piezoelectric element pulls from the wall portion of the cavity 510 and the piezoelectric element 410 does not experience a preload force from the wall portion.

[0043] In contrast, FIGs. 4A and 4B schematically illustrate an example voltage difference AV and an implanted apparatus 400, respectively, for which the voltage difference AV applied to the piezoelectric layers 412 is continually non-zero during generation of the vibrational signals 470 in accordance with certain implementations described herein. As shown in FIG. 4A, the voltage difference AV varies between (+ VDC + VAC) and (+ VDC - VAC with I VDCI greater than I VAC\, SO the voltage difference AV does not pass through zero. The static non-zero DC voltage component VDC can utilize a relatively small amount (e.g., substantially equal to zero) of extra power consumption as compared to having a zero DC voltage component. The dashed line in FIG. 4B shows the position of the bottom end portion of the housing 420 if the voltage difference AV equals zero. As shown in the left portion of FIG. 4B, while the voltage difference AV is greater than + VDC, the length of the piezoelectric element 410 would expand if not for the wall portion of the cavity 510, so the piezoelectric element 410 pushes against the wall portion of the cavity 510 and the piezoelectric element 410 experiences a non-zero compressive preload force from the wall portion. As shown in the middle portion of FIG. 4B, while the voltage difference AV equals +VDC, the length of the piezoelectric element 410 would also expand if not for the wall portion of the cavity 510, so the piezoelectric element 410 pushes against the wall portion of the cavity 510 and the piezoelectric element 410 experiences a non-zero compressive preload force from the wall portion. As shown in the right portion of FIG. 4B, while the voltage difference A V is less than + VDC (but still greater than zero), the length of the piezoelectric element 410 would also expand if not for the wall portion of the cavity 510, so the piezoelectric element 410 pushes against the wall portion of the cavity 510 and the piezoelectric element 410 experiences a non-zero compressive preload force from the wall portion.

[0044] As shown in FIGs. 4A and 4B, the apparatus 400 can be operated in certain implementations by applying the non-zero voltage differences AV to the piezoelectric layers 412 to generate the vibrational signals 470, such that the housing 420 continually pushes on the wall surface (e.g., surface of the bottom wall portion 512; surface of the side wall portion 514) of the cavity 510 and does not pull away from the wall surface and a non-zero compressive preload force is continually applied to the piezoelectric element 410 while the apparatus 400 is being used to generate the vibrational signals 470.

[0045] Bone tissue does not generally support a static load and will deform over time when a load is applied. For example, a purely static preload applied by bone tissue to an implanted actuator can be accomplished during the surgical implantation of the actuator, but the preload will disappear over time as the bone tissue deforms in response to the corresponding load on the bone tissue. In certain implementations, the apparatus 400 is configured to be operated such that the non-zero (e.g., positive) DC component VDC of the voltage difference AV (and the corresponding non-zero compressive preload force) is only applied while the apparatus 400 is generating the vibrational signals 470 (e.g., when the apparatus 400 is not generating the vibrational signals 470, the preload force can be turned off to reduce the risk of bone deformation).

[0046] For example, the circuitry 450 can have a plurality of operational states. During a first operational state of the circuitry 450, the circuitry 450 applies the non-voltage differences AV across the piezoelectric layers 412 in response to data signals (e.g., audio data signals generated by at least one microphone and indicative of sound received by the at least one microphone), the piezoelectric element 410 undergoes time-varying elongations and contractions, and the resultant vibrational signals 470 are indicative of the data signals (e.g., indicative of the sound). For example, while the apparatus 400 is providing the vibrational signals 470 that evoke the sensory percept by the recipient (e.g., while data signals are received by the circuitry 450), both the non-zero DC voltage component VDC and the AC voltage component VAC can be applied across the piezoelectric layers 412, with the AC voltage component VAC indicative of the received data signals.

[0047] In certain implementations, the non-zero DC voltage component VDC can be substantially constant (e.g., static) throughout the period during which the vibrational signals 470 are generated, while in certain other implementations, the non-zero DC voltage component VDC can be dynamic (e.g., varied in response to amplitude changes of the vibrational signals 470). The non-zero DC voltage component VDC can be proportional to the strength of the vibrational signals 470 or to an average (e.g., root-mean-squared) magnitude of the AC voltage component VAC- For example, in a relatively quiet sound environment, the vibrational signals 470 indicative of the sound would have relatively small magnitudes, as would the AC voltage component VAC, SO the non-zero DC voltage component VDC could be smaller to avoid the voltage difference A V from passing through zero (e.g., the dynamic preload only as high as the amplitude of the vibrational signals 470 to avoid pull). The magnitude of the non-zero DC voltage component VDC can be set to avoid undue discomfort (e.g., from the resulting pressure applied to the bone tissue 500) being experienced by the recipient.

[0048] During a second operational state of the circuitry 450, the circuitry 450 applies a voltage difference that is substantially equal to zero across the piezoelectric layers 412. For example, while the apparatus 400 is not providing the vibrational signals 470 that evoke the sensory percept by the recipient (e.g., while data signals are not received by the circuitry 450), the voltage difference applied across the piezoelectric layers 412 can be substantially equal to zero and the compressive preload force applied to the piezoelectric element 410 by the wall portion of the cavity 510 is also substantially equal to zero. In this way, certain implementations can reduce the risk of bone deformation (e.g., by reducing the amount of time during which the non-zero compressive preload force is applied; by providing a time period during which the applied preload force is substantially zero during which the bone tissue can return to an undeformed or less deformed condition).

[0049] FIG. 5A is a flow diagram of an example method 600 for fabricating an apparatus 400 in accordance with certain implementations described herein and FIG. 5B schematically illustrates the example method 600 performed using an example piezoelectric element 410 and housing 420 in accordance with certain implementations described herein. While the method 600 is described by referring to some of the structures of the example apparatus 400 of FIGs. 1, 2A-2D, and 4A-4B, other apparatus and systems with other configurations of components can also be used to perform the method 600 in accordance with certain implementations described herein.

[0050] In an operational block 610, the method 600 comprises providing a piezoelectric actuator (e.g., piezoelectric element 410; piezoelectric stack) comprising a plurality of piezoelectric layers 412 and a plurality of electrodes 414 alternating with the piezoelectric layers 412. The actuator has a first actuator portion (e.g., first portion 418) and a second actuator portion (e.g., second portion 419) spaced from the first actuator portion. As shown in the left portion of FIG. 5B, the piezoelectric actuator has a first length Li along a direction between the first and second actuator portions when a voltage difference substantially equal to zero is applied between the first and second actuator portions.

[0051] In an operational block 620, the method 600 further comprises providing a casing (e.g., housing 420) configured to contain the piezoelectric actuator. The casing has a first casing portion 424 configured to be in mechanical communication with the first actuator portion and a second casing portion 426 configured to be in mechanical communication with the second actuator portion. The second casing portion 426 is spaced from the first casing portion 424 by a distance Di less than the first length Li.

[0052] In an operational block 630, the method 600 further comprises applying a non-zero direct current (DC) voltage difference (e.g., VDC less than zero) to the piezoelectric layers. As shown in the middle portion of FIG. 5B, the non-zero DC voltage difference causes a contraction (e.g., on the order of microns) of the piezoelectric actuator along the direction such that the second actuator portion is spaced from the first actuator portion by a second length L2 along the direction, the second length L2 less than the distance Di. In an operational block 640, the method 600 further comprises, while applying the non-zero DC voltage difference to the piezoelectric layers, placing the piezoelectric actuator within the casing. In certain implementations, the magnitude of the non-zero DC voltage difference is selected such that the second length L2 is sufficiently less than the distance Di so that the piezoelectric actuator is insertable within the casing. For example, the piezoelectric actuator can be embedded in the casing while in the shorter condition (e.g., due to the non-zero DC voltage difference) while being potted with epoxy and cured.

[0053] In an operational block 650, the method 600 further comprises, while the piezoelectric actuator is within the casing, applying a voltage difference substantially equal to zero to the piezoelectric layers such that the first casing portion 424 is in mechanical communication with the first actuator portion and the second casing portion 426 is in mechanical communication with the second actuator portion. As shown in the right portion of FIG. 5B, the casing is configured to stretch to have a distance D2 between the first and second casing portions 424, 426, the distance D2 less than the first length Li. While the voltage difference V is substantially equal to zero, the length of the piezoelectric actuator would be equal to Li if not for the first and second casing portions 424, 426, so the piezoelectric actuator pushes against the first and second casing portions 424, 426 and the first and second casing portions 424, 426 apply a non-zero compressive preload force 428 to the piezoelectric actuator upon the voltage difference between the first and second actuator portions being substantially equal to zero while the piezoelectric actuator is within the casing. This non-zero compressive preload force 428 can protect the piezoelectric actuator from experiencing tensile or pull forces (e.g., during handling and/or implantation) that could damage the piezoelectric element 410.

[0054] In certain implementations, the method 600 further comprises hermetically sealing the casing containing the piezoelectric actuator such that the piezoelectric actuator is not exposed to an environment outside the casing. In this way, the assembly of the piezoelectric actuator and the casing can apply the non-zero compressive preload force 428 to the piezoelectric actuator for voltage differences applied to the piezoelectric layers 412 that are greater than or equal to zero (e.g., in the resting mode of the assembly). Certain such implementations can avoid using an adhesive material (e.g., glue) to affix the first and second actuator portions to the first and second casing portions 424, 426.

[0055] FIG. 6A is a flow diagram of an example method 700 for implanting an assembly in accordance with certain implementations described herein and FIG. 6B schematically illustrates the example method 700 performed using an example assembly (e.g., apparatus 400) comprising a piezoelectric element 410 and housing 420 in accordance with certain implementations described herein. While the method 700 is described by referring to some of the structures of the example apparatus 400 of FIGs. 1, 2A-2D, and 4A-4B, other apparatus and systems with other configurations of components can also be used to perform the method 700 in accordance with certain implementations described herein.

[0056] In an operational block 710, the method 700 comprises providing an assembly comprising a casing (e.g., housing 420) containing a piezoelectric actuator (e.g., piezoelectric element 410; piezoelectric stack) comprising a plurality of piezoelectric layers 412 and a plurality of electrodes 414 alternating with the piezoelectric layers 412. The casing comprises a first casing portion 424 and a second casing portion 426 spaced from the first casing portion 424 by a first distance along a longitudinal axis (e.g., longitudinal axis 422) of the casing with a voltage difference substantially equal to zero applied to the piezoelectric actuator. For example, the assembly can be fabricated using the example method 600 described herein, and the first distance along the longitudinal axis can be the distance D2 described above with regard to the example method 600.

[0057] In an operational block 720, the method 700 further comprises accessing a first wall portion configured to be affixed to the first casing portion 424 and a second wall portion configured to be affixed to the second casing portion 426. The second wall portion is spaced from the first wall portion by a width W, the width W less than the first distance (e.g., less than D2). For example, the first and second wall portions can be wall portions (e.g., two opposite side wall portions 514a,b) of a cavity 510 in a surface portion of bone tissue 500 of the recipient. Accessing the first and second wall portions can comprise performing a surgical procedure to expose the bone tissue 500 and/or surgically machining the bone tissue 500 to form at least a portion of the cavity 510. For another example, the first and second wall portions can be wall portions of a cavity of a structure in mechanical communication with (e.g., mounted to) a surface portion of bone tissue 500 of the recipient. Accessing the first and second wall portions can comprise surgically implanting the structure to affix the structure to the bone tissue 500.

[0058] In an operational block 730, the method 700 further comprises applying non-zero direct current (DC) voltage differences to the piezoelectric layers. The non-zero DC voltage differences contract the casing along the longitudinal axis such that the second casing portion 426 is spaced from the first casing portion 424 by a distance D3 along the longitudinal axis, the distance D3 less than the width W (and less than the first distance). As shown in the left portion of FIG. 6B, the piezoelectric element 410 can be contracted (e.g., on the order of microns) by an applied non-zero (e.g., negative) DC voltage difference such that the first and second casing portions 424, 426 are spaced from one another by the distance Ds less than the width W between the first and second wall portions (e.g., side wall portions 514a,b). For example, the casing can adapt or follow the contraction of the piezoelectric element 410, such that the casing does not apply a compressive preload force or an expansive tension force to the piezoelectric element 410 (see, e.g., the middle portion of FIG. 5B).

[0059] In an operational block 740, the method 700 further comprises inserting the casing at least partially between the first and second wall portions while applying the non-zero DC voltage differences to the piezoelectric layers. For example, as shown in the left portion of FIG. 6B, while the non-zero (e.g., negative) DC voltage difference is applied to the piezoelectric element 410, the housing 420 can be placed within the cavity 510 with the first and second casing portions 424, 426 spaced from the two side wall portions 514a,b of the cavity 510. For example, the casing and the piezoelectric actuator can be placed between the wall portions 514a,b while in the shorter condition (e.g., due to the non-zero DC voltage difference) while being fixed with bone cement.

[0060] In an operational block 750, the method 700 further comprises, after said inserting, applying DC voltage differences substantially equal to zero to the piezoelectric layers such that the first casing portion 424 is in mechanical communication with the first wall portion and the second casing portion 426 is in mechanical communication with the second wall portion. For example, as shown in the middle portion of FIG. 6B, with the housing 420 inserted within the cavity 510 and the voltage differences across the piezoelectric layers 412 are substantially equal to zero, the first and second casing portions 424, 426 contact the two side wall portions 514a,b. Besides the non-zero compressive preload force 428 applied to the piezoelectric element 410 by the housing 420 (e.g., as described herein with regard to the method 600), upon applying zero voltage differences to the piezoelectric layers 412 within the cavity 510, the first and second wall portions (e.g., side wall portions 514a,b) of the cavity 510 can apply a non-zero compressive second preload force 520 to the casing (e.g., housing 420). This non-zero compressive second preload force 520 is also applied to the piezoelectric element 410, as shown in the middle portion of FIG. 6B (e.g., the preload force on the piezoelectric element 410 has two non-zero components: the non-zero compressive preload force 428 from the casing and the non-zero compressive second preload force 520 from the cavity 510). This second preload force 520 can improve mechanical interface and/or facilitate osseointegration of the casing with the wall portions of the cavity 510.

[0061] In certain implementations, the non-zero compressive second preload force 520 applied to the assembly (e.g., apparatus 400) by the wall portions of the cavity 510 can decrease as a function of time as the bone surface portion adapts (e.g., deforms) over time to the casing (e.g., housing 420) in response to the applied load, as described herein. For example, as shown by the right portion of FIG. 6B, over time, the cavity 510 can increase in size until the two side wall portions 514a,b are spaced apart from one another by the distance D2 between the first and second casing portions 424, 426 corresponding to a substantially zero voltage difference, such that the second preload force 520 disappears while the preload force 428 remains. In certain implementations, a non-zero DC voltage component VDC can be applied during a predetermined time period (e.g., the first three months after implantation) after implantation to create a force between the casing and the wall portions of the cavity 510 that can improve mechanical interface and/or facilitate osseointegration of the casing with the wall portions.

[0062] In certain implementations, the method 700 further comprises affixing the first casing portion 424 to the first wall portion (e.g., side wall portion 514a) and affixing the second casing portion 426 to the second wall portion (e.g., side wall portion 514b). For example, during the operational block 740, an adhesive material (e.g., bone cement) can be inserted between the first casing portion 424 and the side wall portion 514a and between the second casing portion 426 and the side wall portion 514b. For another example, during the operational block 750, an adhesive material (e.g., bone cement) can be applied to a region adjacent to where the first casing portion 424 and the side wall portion 514a contact one another and/or to a region adjacent to where the second casing portion 426 and the side wall portion 514b contact one another. In certain other implementations, the method 700 comprises avoiding applying an adhesive material to either the casing or the wall portions of the cavity 510, such that the casing and the wall portions of the cavity 510 are adhered to one another by osseointegration and/or friction between the casing and the wall portions.

[0063] Although commonly used terms are used to describe the systems and methods of certain implementations for ease of understanding, these terms are used herein to have their broadest reasonable interpretations. Although various aspects of the disclosure are described with regard to illustrative examples and implementations, the disclosed examples and implementations should not be construed as limiting. Conditional language, such as, among others, "can," "could," "might," or "may," unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain implementations include, while other implementations do not include, certain features, elements and/or steps. Thus, such conditional language is not generally intended to imply that features, elements and/or steps are in any way required for one or more implementations or that one or more implementations necessarily include logic for deciding, with or without user input or prompting, whether these features, elements and/or steps are included or are to be performed in any particular implementation. In particular, the terms “comprises” and “comprising” should be interpreted as referring to elements, components, or steps in a nonexclusive manner, indicating that the referenced elements, components, or steps may be present, or utilized, or combined with other elements, components, or steps that are not expressly referenced.

[0064] It is to be appreciated that the implementations disclosed herein are not mutually exclusive and may be combined with one another in various arrangements. In addition, although the disclosed methods and apparatuses have largely been described in the context of conventional cochlear implants, various implementations described herein can be incorporated in a variety of other suitable devices, methods, and contexts. More generally, as can be appreciated, certain implementations described herein can be used in a variety of implantable medical device contexts that can benefit from having at least a portion of the received power available for use by the implanted device during time periods in which the at least one power storage device of the implanted device unable to provide electrical power for operation of the implantable medical device.

[0065] Language of degree, as used herein, such as the terms “approximately,” “about,” “generally,” and “substantially,” represent a value, amount, or characteristic close to the stated value, amount, or characteristic that still performs a desired function or achieves a desired result. For example, the terms “approximately,” “about,” “generally,” and “substantially” may refer to an amount that is within ± 10% of, within ± 5% of, within ± 2% of, within ± 1 % of, or within ± 0.1% of the stated amount. As another example, the terms “generally parallel” and “substantially parallel” refer to a value, amount, or characteristic that departs from exactly parallel by ± 10 degrees, by ± 5 degrees, by ± 2 degrees, by ± 1 degree, or by ± 0.1 degree, and the terms “generally perpendicular” and “substantially perpendicular” refer to a value, amount, or characteristic that departs from exactly perpendicular by ± 10 degrees, by ± 5 degrees, by ± 2 degrees, by ± 1 degree, or by ± 0.1 degree. The ranges disclosed herein also encompass any and all overlap, sub-ranges, and combinations thereof. Language such as “up to,” “at least,” “greater than,” less than,” “between,” and the like includes the number recited. As used herein, the meaning of “a,” “an,” and “said” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of “in” includes “into” and “on,” unless the context clearly dictates otherwise.

[0066] While the methods and systems are discussed herein in terms of elements labeled by ordinal adjectives (e.g., first, second, etc.), the ordinal adjective are used merely as labels to distinguish one element from another (e.g., one signal from another or one circuit from one another), and the ordinal adjective is not used to denote an order of these elements or of their use.

[0067] The invention described and claimed herein is not to be limited in scope by the specific example implementations herein disclosed, since these implementations are intended as illustrations, and not limitations, of several aspects of the invention. Any equivalent implementations are intended to be within the scope of this invention. Indeed, various modifications of the invention in form and detail, in addition to those shown and described herein, will become apparent to those skilled in the art from the foregoing description. Such modifications are also intended to fall within the scope of the claims. The breadth and scope of the invention should not be limited by any of the example implementations disclosed herein, but should be defined only in accordance with the claims and their equivalents.

Claims

WHAT IS CLAIMED IS:

1. An apparatus comprising: a housing configured to be in mechanical communication with bone tissue of a recipient’s body; a piezoelectric stack contained within the housing, the piezoelectric stack comprising a plurality of piezoelectric layers and a plurality of electrodes, the piezoelectric layers and the electrodes alternating with one another along a length of the piezoelectric stack such that each piezoelectric layer of the plurality of piezoelectric layers is sandwiched between a pair of electrodes of the plurality of electrodes; and circuitry in electrical communication with the plurality of electrodes, the circuitry configured to apply non-zero voltage differences across the piezoelectric layers, the non-zero voltage differences inducing changes of the length of the piezoelectric stack, the non-zero voltage differences comprising a non-zero direct current (DC) voltage component corresponding to an expansion of the length and an alternating current (AC) voltage component corresponding to time-varying expansions and contractions of the length that generate vibrational signals that propagate through the bone tissue.

2. The apparatus of claim 1, wherein the length extends from a first end portion of the piezoelectric stack to a second end portion of the piezoelectric stack, the second end portion spaced from the first end portion along a longitudinal axis of the piezoelectric element, the changes of the length along the longitudinal axis.

3. The apparatus of claim 1 or claim 2, wherein the housing is configured to be implanted within the recipient’s body such that the time-varying expansions and contractions of the length are substantially parallel to a surface portion of the bone tissue.

4. The apparatus of claim 1 or claim 2, wherein the housing is configured to be implanted such that the time- varying expansions and contractions of the length are at a nonzero angle relative to a surface portion of the bone tissue.

5. The apparatus of any preceding claim, wherein at least one piezoelectric layer of the plurality of piezoelectric layers comprises at least one piezoelectric sub-layer and at least one non-piezoelectric sub-layer.

6. The apparatus of any preceding claim, wherein the housing and/or the bone applies a non-zero compressive preload force to the piezoelectric stack in response to the expansion of the length corresponding to the non-zero direct current (DC) voltage component.

7. The apparatus of any preceding claim, wherein the vibrational signals evoke a sensory percept by the recipient.

8. The apparatus of claim 7, wherein the circuitry has a plurality of operational states comprising: a first operational state in which the circuitry applies the non-zero voltage differences in response to audio data signals generated by at least one microphone and indicative of sound received by the at least one microphone, the vibrational signals indicative of the sound; and a second operational state in which a voltage difference applied by the circuitry across the piezoelectric layers is substantially equal to zero.

9. The apparatus of any preceding claim, wherein the apparatus comprises a bone conduction actuator.

10. A method comprising: providing a piezoelectric actuator comprising a plurality of piezoelectric layers and a plurality of electrodes alternating with the piezoelectric layers, the piezoelectric actuator having a first actuator portion and a second actuator portion spaced from the first actuator portion by a first length along a direction with a voltage difference substantially equal to zero applied by the electrodes to the piezoelectric layers; providing a casing configured to contain the piezoelectric actuator, the casing having a first casing portion configured to be in mechanical communication with the first actuator portion and a second casing portion configured to be in mechanical communication with the second actuator portion, the second casing portion spaced from the first casing portion by a distance less than the first length; applying a non-zero direct current (DC) voltage difference to the piezoelectric layers, the non-zero DC voltage difference causing a contraction of the piezoelectric actuator along the direction such that the second actuator portion is spaced from the first actuator portion by a second length along the direction, the second length less than the distance; while applying the non-zero DC voltage difference to the piezoelectric layers, placing the piezoelectric actuator within the casing; and while the piezoelectric actuator is within the casing, applying a voltage difference substantially equal to zero to the piezoelectric layers such that the first casing portion is in mechanical communication with the first actuator portion and the second casing portion is in mechanical communication with the second actuator portion.

11. The method of claim 10, wherein the casing comprises a biocompatible material configured to be affixed to a bone surface of a recipient.

12. The method of claim 11, wherein the piezoelectric actuator is configured to undergo time-varying elongations and contractions to generate vibrational signals that propagate through the bone.

13. The method of any of claims 10 to 12, wherein the first and second casing portions apply a non-zero compressive preload force to the piezoelectric actuator upon said applying the voltage difference substantially equal to zero while the piezoelectric actuator is within the casing.

14. The method of any of claims 10 to 13, further comprising hermetically sealing the casing containing the piezoelectric actuator such that the piezoelectric actuator is not exposed to an environment outside the casing.

15. A method comprising: providing an assembly comprising a casing containing a piezoelectric actuator comprising a plurality of piezoelectric layers and a plurality of electrodes alternating with the piezoelectric layers, the casing applying a first non-zero compressive preload force to the piezoelectric actuator, the casing comprising a first casing portion and a second casing portion spaced from the first casing portion by a first distance along a longitudinal axis of the casing with a voltage difference substantially equal to zero applied to the piezoelectric layers; accessing a first wall portion configured to be affixed to the first casing portion and a second wall portion configured to be affixed to the second casing portion, the second wall portion spaced from the first wall portion by a width, the width less than the first distance; applying non-zero direct current (DC) voltage differences to the piezoelectric layers, the non-zero DC voltage differences inducing a contraction of the casing along the longitudinal axis such that the second casing portion is spaced from the first casing portion by a second distance along the longitudinal axis, the second distance less than the width; while applying the non-zero DC voltage differences to the piezoelectric layers, inserting the casing at least partially between the first and second wall portions; and after said inserting, applying voltage differences substantially equal to zero to the piezoelectric layers such that the first casing portion is in mechanical communication with the first wall portion and the second casing portion is in mechanical communication with the second wall portion.

16. The method of claim 15, wherein the first and second wall portions are wall portions of a cavity in a bone surface portion of a recipient.

17. The method of claim 15, wherein the first and second wall portions are wall portions of a cavity of a structure in mechanical communication with a bone surface portion of a recipient.

18. The method claim 16 or claim 17, wherein the first and second wall portions apply a second non-zero compressive preload force to the casing upon said applying the voltage differences substantially equal to zero after said inserting.

19. The method of claim 18, wherein the second non-zero compressive preload force decreases as the bone surface portion adapts over time to the casing.

20. The method of any of claims 15 to 19, wherein the piezoelectric actuator is configured to undergo time-varying elongations and contractions to generate vibrational signals that propagate through tissue of a recipient.

21. The method of any of claims 15 to 20, further comprising affixing the first casing portion to the first wall portion and affixing the second casing portion to the second wall portion.

22. The method of any of claims 15 to 21, further comprising, after said inserting, applying second non-zero DC voltage differences to the piezoelectric layers that expands the piezoelectric actuator to improve mechanical interface and/or to facilitate osseointegration of the first and second casing portions with the first and second wall portions.

23. The method of any of claims 15 to 21 , further comprising applying second nonzero DC voltage differences to the piezoelectric layers only while the piezoelectric actuator is generating vibrational signals.