IMPLANTABLE WIRELESS ACCOUSTIC STIMULATORS WITH HIGH ENERGY CONVERSION EFFICIENCIES

Abstract

Receiver-stimulator with folded or rolled up assembly of piezoelectric components, causing the receiver-stimulator to operate with a high degree of isotropy are disclosed. The receiver-stimulator comprises piezoelectric components, rectifier circuitry, and at least two stimulation electrodes. Isotropy allows the receiver-stimulator to be implanted with less concern regarding the orientation relative the transmitted acoustic field from an acoustic energy source.

Claims

1-20. (canceled)

21. An implantable receiver-stimulator for delivering electrical stimulation to tissue of a patient, comprising: a transducer configured to receive first acoustic energy from a controller-transmitter and/or second acoustic energy from a diagnostic ultrasound source and to convert the first acoustic energy and/or the second acoustic energy to a first electrical output; a circuit coupled to the transducer to receive the first electrical output and to produce a second electrical output; and electrodes electrically coupled to the circuit and configured to deliver the second electrical output to the tissue of the patient, wherein the circuit is configured to produce the second electrical output by— delivering to the electrodes a first portion of the first electrical output produced by the transducer corresponding to the first acoustic energy from the controller-transmitter, and filtering out a second portion of the first electrical output produced by the transducer corresponding to the second acoustic energy from the diagnostic ultrasound source.

22. The implantable receiver-stimulator of claim 21 wherein filtering out the second portion of the first electrical output substantially prevents delivery of the second portion of the first electrical output to the electrodes.

23. The implantable receiver-stimulator of claim 21 wherein the tissue of the patient is cardiac tissue.

24. The implantable receiver-stimulator of claim 21 wherein the circuit includes a low pass filter configured to pass the first portion of the first electrical output to the electrodes and to eliminate the second portion of the first electrical output.

25. The implantable receiver-stimulator of claim 24 wherein the circuit further includes a rectifier configured to receive the first portion of the first electrical output from the low pass filter and to rectify the first portion of the first electrical output.

26. The implantable receiver-stimulator of claim 21 wherein the first acoustic energy has a lower frequency than the second acoustic energy.

27. The implantable receiver-stimulator of claim 21 wherein the first acoustic energy is between about 800 kHz to 1.3 MHz, and wherein the second acoustic energy is between about 2 MHz to 10 MHz.

28. The implantable receiver-stimulator of claim 21 wherein the first acoustic energy has a different pulse duration than the second acoustic energy.

29. The implantable receiver-stimulator of claim 21 wherein the first acoustic energy has a longer pulse duration than the second acoustic energy.

30. The implantable receiver-stimulator of claim 21 wherein the transducer is a piezoelectric transducer.

31. An implantable receiver-stimulator for delivering electrical stimulation to tissue of a patient, comprising: a transducer configured to receive first acoustic energy from a first acoustic transmitter and/or second acoustic energy from a second acoustic transmitter and to convert the first and/or second acoustic energy to an electrical output; a filter electrically coupled to the transducer, wherein the filter is configured to (a) receive the electrical output from the transducer, (b) pass a first portion of the electrical output corresponding to the first acoustic energy, and (c) substantially eliminate a second portion of the electrical output corresponding to the second acoustic energy; and an electrode electrically coupled to the filter, wherein the electrode is configured to receive the first portion of the electrical output from the filter and to deliver the first portion of the electrical output to the tissue of the patient.

32. The implantable receiver-stimulator of claim 31 wherein the first acoustic transmitter is configured to be implanted in the patient, and wherein the second acoustic transmitter is a diagnostic ultrasound source.

33. The implantable receiver-stimulator of claim 31 wherein the first acoustic energy has a lower frequency than the second acoustic energy.

33. The implantable receiver-stimulator of claim 31 wherein the filter is a low pass filter configured to pass electrical signals having a frequency of less than about 1.3 MHz.

34. The implantable receiver-stimulator of claim 31 wherein the filter is a low pass filter configured to substantially eliminate electrical signals having a frequency of greater than about 2 MHz.

35. The implantable receiver-stimulator of claim 31, further comprising a rectifier electrically coupled between the filter and the electrode, wherein the rectifier is configured to rectify the first portion of the electrical output and to pass the rectified first portion of the electrical output to the electrode for delivery to the tissue.

36. The implantable receiver-stimulator of claim 31, further comprising: a rectifier electrically coupled to the filter, wherein the rectifier is configured to rectify the first portion of the electrical output; and a voltage limiter electrically coupled to the rectifier, wherein the voltage limiter is configured to (a) receive the rectified first portion of the electrical output, (b) limit a voltage of the rectified first portion of the electrical output, and (c) pass the voltage-limited and rectified first portion of the electrical output to the electrode for delivery to the tissue.

37. A method of delivering electrical stimulation to tissue of a patient, the method comprising: receiving first acoustic energy from a first acoustic transmitter and second acoustic energy from a second acoustic transmitter; converting the first and second acoustic energy to an electrical output; passing a first portion of the electrical output corresponding to the first acoustic energy to an electrode; substantially eliminating a second portion of the electrical output corresponding to the second acoustic energy; and delivering, via the electrode, the first portion of the electrical output to the tissue of the patient.

38. The method of claim 37 wherein the first acoustic energy has a lower frequency than the second acoustic energy.

39. The method of claim 37 wherein receiving the first acoustic energy includes receiving the first acoustic energy from a controller-transmitter implanted in the patient, and wherein receiving the second acoustic energy includes receiving the second acoustic energy from a diagnostic ultrasound source.

40. The method of claim 37 wherein receiving the first and second acoustic energy includes receiving the first and second acoustic energy at a receiver-stimulator implanted in the patient.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The invention has other advantages and features which will be more readily apparent from the following detailed description of the invention and the appended claims, when taken in conjunction with the accompanying drawings, in which:

[0016] FIG. 1A shows a receiver-stimulator with the anchoring mechanism retracted.

[0017] FIG. 1B shows the receiver-stimulator with the anchoring mechanism deployed.

[0018] FIG. 1C shows the receiver-stimulator cross-section in the retracted state.

[0019] FIG. 1D shows the receiver-stimulator cross-section in the deployed state.

[0020] FIG. 2 is a flow diagram illustrating a method for manufacturing a receiver-stimulator.

[0021] FIG. 3A shows a can assembly with piezoelectric components and rectifiers organized in an exemplary arrangement.

[0022] FIG. 3B is a side-view of the can assembly and exemplary piezoelectric components.

[0023] FIG. 3C shows a folded can assembly.

[0024] FIG. 3D is a cross-sectional view of the folded can assembly.

[0025] FIGS. 3E-3I show an alternative can assembly with piezoelectric components and rectifiers organized in an exemplary arrangement.

[0026] FIG. 4A shows a partial receiver-stimulator assembly with two end caps and an axle assembly.

[0027] FIG. 4B shows a polyimide, polyester, or liquid crystal polymer (LCP) flex circuit for the end cap.

[0028] FIG. 4C shows an end cap assembly.

[0029] FIG. 4D is a cross-sectional view of an end cap assembly.

[0030] FIG. 5A is a flow diagram illustrating a method for assembling an end cap.

[0031] FIG. 5B is a flow diagram illustrating a method for assembling an axle assembly.

[0032] FIG. 6A shows an electrical circuit with harvesting elements connected in parallel.

[0033] FIG. 6B shows an electrical circuit with harvesting elements connected in series.

DETAILED DESCRIPTION OF THE INVENTION

[0034] In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the invention. It will be apparent, however, to one skilled in the art that the invention can be practiced without these specific details. 100351 In a first aspect, the present invention provides an implantable receiver-stimulator (hereinafter also abbreviated as “R-S” and also referred to as “stimulator”) device which is capable of wirelessly harvesting acoustic power from an acoustic field delivered from an acoustic source physically separate from the R-S, and converting that acoustic power to electrical power, rectifying the electrical power, and delivering an electrical output between two stimulation electrodes.

[0035] FIGS. 1A-1D are diagrams illustrating an implantable receiver-stimulator 101, constructed in accordance with the principles of the present embodiments. FIGS. 1C and 1D are cross-sectional views of the R-S 101 device as shown in FIGS. 1A and 1B, respectively.

[0036] The R-S 101 comprises an axle assembly 104 containing a moveable/retractable needle assembly 107 with an anchoring mechanism 102 at its distal end and a detachment mechanism 103 at its proximal end. The anchoring mechanism 102 enables the R-S 101 to be attached to a desired location at the treatment site. The axle assembly 104 comprises an internal cathode feed-through 106 to the needle assembly 107. The needle assembly 107 is configured to move axially in R-S 101 through the axle assembly 104. The proximal end of the R-S 101 may comprise mechanical or electro-mechanical arrangements to engage and disengage the R-S 101 from a delivery catheter.

[0037] The needle assembly 107, including the anchoring and detachment mechanisms 102 and 103, is configured to start out in a retracted state as shown in FIGS. 1A and 1C. When the R-S 101 is to be permanently attached to patient tissue, the needle assembly 107 assumes a deployed state as the detachment mechanism 103 is pushed axially forward, for example by a delivery catheter. As shown in FIGS. 1B and 1D, pushing the needle assembly 107 axially forward will insert the anchoring mechanism 102 into patient tissue (delivery catheter not shown).

[0038] In the retracted state shown in FIGS. 1A and 1C, the anchoring mechanism 102 is retracted inside the R-S 101 with one or more barbs 105 held in place along the tip of the needle assembly 107, and the detachment mechanism 103 is extended outwards at the proximal end of the R-S 101.

[0039] In the deployed state shown in FIGS. 1B and ID, the anchoring mechanism 102 is pushed distally outwards, allowing its barbs 105 to fan out radially and thereby attach the R-S 101 to patient tissue.

[0040] As shown in FIGS. 1C and 1D, the R-S 101 comprises a hermetically sealed enclosure 110. The enclosure 110 has an outer surface 111 and an inner surface 112, constructed using an electrically conductive base, such as titanium, upon which a circuit layer is assembled on the inner surface 112 of the device. In one embodiment, the circuit comprises a thin film ceramic insulating layer and a thin film metal layer. In another embodiment, the circuit is composed of materials having substantially similar acoustic impedances as surrounding materials residing in the acoustic path between the acoustic energy source and the enclosure. Such surrounding materials include patient tissue through which the acoustic waves travel, and elements of the R-S 101 itself.

[0041] A plurality of piezoelectric components 120 are affixed to and organized on the inner surface 112 in which an affixed side/face of the piezoelectric component 120 forms an aperture 150 with the wall of the enclosure 110. In response to an acoustic field coupling through the aperture 150 to the piezoelectric component 120, the piezoelectric component 120 converts acoustic power to electrical power. The internal spaces 160 between the piezoelectric components 120 contain materials of low acoustic impedance, such as air, or vacuum making these surfaces non-isotropic. As shown in FIG. 3A, the inner surface 112 further contains a plurality of rectifier circuits 121 each electrically connected to a corresponding piezoelectric component 120. Together, a piezoelectric component, an associated aperture and a rectifier function as a harvesting element.

[0042] In one embodiment, the output of each of the harvesting elements is connected in parallel as illustrated by the abbreviated electrical circuit in FIG. 6A or connected in series as illustrated by the abbreviated electrical circuit in FIG. 6B, and optionally also comprising protection circuitry (not shown) to protect the rectifiers from voltages exceeding a threshold. In one embodiment, such a voltage threshold is approximately 3 to 4 V. In one embodiment, the protection circuitry comprises a Zener diode or a series of GaAs Schottky diodes.

[0043] In an embodiment where the stimulation electrodes are of a polarizing type, a direct current (DC) charge can build up on the electrodes unless a discharge path is provided. In traditional pacemakers, the pacing electrodes are shorted using a switch to remove any residual charge on the electrodes when not delivering a stimulus pulse. In embodiments where rectifiers are used to convert an alternating current (AC) signal into a suitable electrical output pulse, there is no discharge path back through the rectifier. Therefore, a discharge path may be provided by using a bleed resistor connected between the stimulation electrodes. Preferably, the bleed resistor is 5-10 times the load impedance in order to provide a sufficient discharge path while not wasting energy that is intended for delivery to the tissue.

[0044] In one embodiment, the R-S 101 comprises circuitry to limit the electrical output to the stimulation electrodes in order to prevent the electrical output from exceeding certain currents that would be harmful to the tissue or would have harmful or undesirable side effects for the patient.

[0045] While the aperture 150 of a harvesting element is determined in part by the surface area of the piezoelectric component 120 exposed to the acoustic field through the enclosure wall 110, a number of other factors contribute to the effective acoustic aperture, including piezoelectric component dimensions and materials, mechanical properties of the coupling surface of the enclosure 110 wall to which the piezoelectric component is affixed, and proximity of neighboring harvesting elements.

[0046] In another consideration for the R-S 101 to attain high efficiency in converting the acoustic field to electrical power, the source impedance of the combined harvesting elements needs to be matched with the load impedance, dictated by the tissue characteristics and the electrical characteristics of the stimulation electrodes. This invention allows matching the impedance of the R-S 101 (source impedance) to the load impedance by a judicious electrical arrangement of the harvesting elements, which could be in series, parallel, or a combination thereof. Another way the source impedance can be manipulated is by changing the dimensions of the individual piezoelectric components 120, for example, changing the cross-section of a piezoelectric component 120 while keeping its height constant changes the impedance of that piezoelectric component. 100481 The inner surface 112 may also contain additional circuitry that connects the output from the harvesting elements to a pair of stimulation electrodes, a cathode 113 and an anode 114 (see FIGS. 1C and 1D). The electrodes are located on the exterior surface of, or otherwise attached to or extended from, the R-S 101 device in electrical contact with the tissue. As described below, in one embodiment, the cathode 113 is part of the tissue engagement mechanism located at the tip of the R-S 101, and the anode 114 is a portion of or the entire exterior surface of the R-S 101. In one embodiment, the R-S 101 device is covered with woven polyester (or other similarly suitable material) to promote tissue in growth.

[0047] The R-S 101 is manufactured by attaching and wire bonding a plurality of piezoelectric components 120 and rectifiers 121 to a flat sheet that becomes the enclosure 110 by folding or rolling the sheet to produce an R-S “can” assembly. This assembly process is described below and represented by FIG. 2 and depicted in FIGS. 3A-3D and in an alternative arrangement of components in FIGS. 3E-3I. As can be appreciated any number of arrangements of components can be made. The can assembly 300 (shown in FIG. 3 A or alternatively in FIGS. 3E and 3F) is folded to produce an octagonal (or rolled to produce an otherwise substantially cylindrical or tubular) can structure 310, as shown in FIGS. 3C and 3D or alternatively in FIGS. 3G-3I. This process will now be described in more detail.

[0048] FIG. 2 is a flow diagram illustrating a method for manufacturing an R-S 101, in accordance with embodiments of the present invention. At step 201, a sheet 301 (see FIG. 3A) is masked to avoid depositing insulation or circuit material onto edges that will later be welded. In one embodiment, the sheet 301 is made of thin titanium. Alternatively, it is contemplated that the sheet 301 may be made of biocompatible material such as low carbon stainless steel, polyimide, polyester, liquid crystal polymer (LCP), or the like, or non-biocompatible material coated with a biocompatible material such as those mentioned above.

[0049] Optionally, many sheets 301 can be fabricated out of a larger sheet. In such an embodiment, all the steps described below up to the folding up of the can assembly can be performed on the larger sheet, after which individual can assemblies 300 can be cut from the larger sheet and folded individually.

[0050] At step 202, the sheet 301 is coated with a thin electrical insulation layer, such as a very thin ceramic layer. Alternatively, a non-conductive polymer layer such as polyimide can be used instead, or a flexible circuit can be laminated on the sheet 301 to achieve the same results.

[0051] At step 203, the mask is removed, and at step 204 the sheet 301 is masked again, this time to form a circuit pattern. At step 205, the masked sheet 301 is coated with a conductive material. It is contemplated that gold or gold over nickel or other such conductive materials will work particularly well, due to their high conductivity and suitability for wire bonding.

[0052] At step 206, the mask is removed to reveal the circuit pattern. As shown in FIG. 3A, the circuit pattern comprises positive and negative conductive traces 302 and 303 which will be connected to the positive and negative outputs of the rectifiers 121. Additional layers of insulator and conductor could be added to form more complex circuitry similar to conventional circuit boards.

[0053] At step 207, the sheet 301 is masked to create half depth fold lines by etching. The masking and the fold lines are preferably created on the outer surface 111 of the sheet 301 to avoid interfering with the circuit on the inner surface 112. However, the masking and fold lines can also be created on the inner surface 112 (circuit side), for example, such that they do not interfere with the circuit. Alternatively, circuit traces may be created across the fold lines. Alternatively, the rectifiers could be attached with flip chip connections eliminating a substantial fraction of the wire bonds and simplifying routing.

[0054] Additionally, half depth outlines may be etched on either outer or inner surfaces that define locations for the piezoelectric components 120. These outlines also contribute to defining the aperture 150 for a piezoelectric component 120, thereby providing mechanical interruption or isolation, as well as defining an effective aperture which may be larger than the widths of the piezoelectric components 120. Such acoustic apertures 150 may be defined in a number of alternative ways, for example, by etching away (a) surface material that lies outside of the aperture boundaries and leaving material inside of the aperture boundaries unetched, (b) surface material that lies inside of the aperture boundaries and leaving material outside of the aperture boundaries unetched, (c) material at the aperture boundaries (of a certain boundary width) and leaving material inside and outside of the boundaries unetched, or (d) by leaving the aperture boundaries (or a certain boundary width) unetched and etching surface material that lies inside and outside of the aperture boundaries. While such apertures may be etched on the inner surface 112 or the outer surface 111 of the sheet 301, they are preferably etched on the outer surface 111, thereby leaving a smooth surface on the inner surface 112 for attaching the piezoelectric components 120. Alternatively, the surface may be modified by a process other than etching, as known to those of ordinary skill in the art. The masking and etching of fold lines and aperture geometry could also be done before plating with insulator and circuitry layers. Acoustic apertures could also be created by attaching other materials such as frames or cylinders of titanium, ceramic or other material to the substrate in the space not occupied by the piezoelectric components.

[0055] At step 208 the inner surface of the sheet 301 is masked to also create outlines of individual can assemblies 300. At step 209, the sheet 301 is chemically etched to create fold lines 304 (as shown in FIG. 3A) and outlines of individual cans. At step 210, the masks are removed, and at step 211 the piezoelectric components 120 and rectifiers 121 are mounted onto the sheet 301. In one embodiment, they are mounted using an adhesive. The bottom electrode coating on the piezoelectric components makes electrical contact with the circuit either by intimate mechanical contact at high points when the adhesive is thin enough, by using an electrically conductive adhesive, by reflowing of indium or tin lead alloys to make the electrical and mechanical connection (similar to conventional surface mounting practice), or by using similar techniques known to those of ordinary skill in the art.

[0056] At step 212, the piezoelectric components 120 are wire bonded to their respective rectifiers 121, and the positive and negative outputs of the rectifiers 121 are wire bonded to the positive and negative conductive traces 302 and 303 on the sheet 301. While wire bonding works well due to the minimal influence it has on the resonant structure, other attachment methods, such as tab bonding, flip chip connections, or surface mount technology (SMT) methods would work for other connections made to the piezoelectric components 120 and/or the rectifiers 121.

[0057] FIG. 3B is a side-view of the flat sheet can assembly 300 of FIG. 3A, showing wire bonds 311 from the inputs of the rectifiers 121 to circuit pads on the sheet 301 which are connected to piezoelectric components 120, wire bonds 312 from the positive outputs of the rectifiers 121 to the positive conductive traces 302 (see FIG. 3A), and wire bonds 313 from the negative outputs of the rectifiers 121 to the negative conductive traces 303 (see FIG. 3A). In the particular exemplary configuration shown, the harvesting elements are connected in parallel. However, any combination of circuit configuration may be obtained by the layout of the circuit mask on the sheet 301, including series or parallel configurations or combinations thereof, as will be obvious to one skilled in the art.

[0058] Once the can assembly 300 is complete, it is folded into an octagonal structure or otherwise folded or rolled to a substantially cylindrical can structure 310 as dictated by the fold lines, as shown in FIG. 3C. FIG. 3D is a cross-sectional view of the folded can structure 310 of FIG. 3C showing a compact and an optimal arrangement of the piezoelectric components and the rectifiers. This spatial arrangement allows packing a high number of piezoelectric components into a given volume. The fold lines act to isolate the folding strains away from the attachment points of the piezoelectric components 120 and rectifiers 121. In one embodiment, the can assembly 300 comprises an interlocking joint structure 320 (also known as a “dado joint”), as shown in FIGS. 3A, 3E, 3F, and 3G. This interlocking joint structure 320 helps align the edges of the folded can structure 301 as they are brought together through folding before the edges are welded together. This allows for maximizing the amount of material immediately adjoining the weld, thereby providing for a complete and substantially hole-free seam upon completion of welding. FIG. 3G shows the folded can structure 310 and the interlocking joint structure 320 prior to welding. FIG. 3H shows the welded can structure 310 with a welding seam 321 where the interlocking joint structure 320 provided alignment of the can structure edges.

[0059] It is an advantageous aspect that the composite structure of the R-S 101 can operate with a high degree of isotropy even if the individual harvesting elements have a low isotropy. This is due to the diversity of aperture orientations of the harvesting elements. For any given spatial orientation of the R-S 101 relative to the acoustic field, there are harvesting elements whose apertures are oriented such that they are able to harvest a large portion of the acoustic power that is impingent on them. For example, in an R-S 101 with an octagonal cross-section as shown in FIGS. 3C and 3D, there are four differently oriented harvesting elements, each contributing a potentially different energy output, but in aggregate contributing to a substantially constant overall power output of the R-S 101 as the direction of the incoming acoustic field is varied.

[0060] In one embodiment, the electrical output of the rectifiers 121 is used to directly stimulate tissue. In an alternative embodiment, the R-S 101 further comprises processing circuitry that manipulates the electrical output converted by the rectifiers 121 to produce an electrical signal that stimulates tissue. The processing circuitry manipulates the electrical output such that it is suitable for the particular stimulation application at hand, such as cardiac pacing, nerve stimulation, brain stimulation, voluntary muscle stimulation, pain amelioration, or the like. Such manipulation may involve summing or conditioning the electrical signals from the individual rectifiers 121 to produce the biologically stimulating electrical output.

[0061] As described above, the R-S 101 further comprises at least two electrodes to stimulate patient tissue: a cathode 113 and an anode 114. The cathode 113 and anode 114 are electrically connected as illustrated in the abbreviated electrical circuits of FIGS. 6A or 6B to negative and positive outputs of the rectifiers 121 (or of the processing circuitry, in embodiments where such circuitry is used in the R-S 101). Either or both of the electrodes 113 and 114 may be mounted on the exterior surface of, or otherwise attached to the R-S 101, in some instances forming a portion of the device enclosure.

[0062] In the exemplary embodiments shown in FIGS. 1A-1D, the cathode 113 is disposed on the needle assembly 107 and is routed through a feed-through 106 in the axle assembly 104 at the distal tip of the R-S 101. FIG. 1C shows the tip of the cathode 113 exposed at the distal tip of the axle assembly 104, and the body of the cathode 113 extending through the axle assembly 104 and into the R-S 101 to electrically connect with a flexible conductive coil 115 that is connected to the negative output of the rectifiers 121 (the connection between the conductive coil 115 and rectifiers 121 not shown). FIG. 1D shows the flexible conductive coil 115 stretched as the axle assembly 104 is pushed axially forward, maintaining an electrical connection between the cathode 113 and the negative output of the rectifiers 121 or electrical signal processing circuitry. In embodiments where a processing circuitry is used, the conductive coil 115 is connected to the negative output of the processing circuitry. The anode 114 is represented by a portion of the device enclosure and extends into the R-S 101 to electrically connect with the positive output of the rectifiers 121 electrical signal processing circuitry (this internal connection is not shown).

[0063] According to the present embodiments, the length of the R-S 101 is preferably about 4-12 mm, more preferably about 6-10 mm, and most preferably about 8 mm; the diameter of the R-S 101 is preferably about 3-16 French (1.0 to 5.3mm), more preferably about 5-12 French (1.7 to 4.0 mm), and most preferably about 6-8 French (2.0 to 2.7 mm); the operating frequency of the R-S 101 is preferably about 200 kH-3 MHz, more preferably about 600 kHz-1.8 MHz, and most preferably about 950 kHz-1.2 MHz; and the number of harvesting elements in the R-S 101 is preferably about 6-200, more preferably about 30-100, and most preferably about 40-60.

[0064] As described above, the implantable R-S 101 devices of the present embodiments are also capable of functioning at a high degree of isotropy. This means that the composite structure of the R-S 101 device produces output electric power that is constant or nearly constant as the relative orientation of the R-S 101 to the acoustic source is varied. It is contemplated that the electric power produced by the R-S 101 device and delivered to the tissue in proportion to the incident acoustic intensity impinging on the R-S 101 will be such that the minimum effective area preferably is no more than −6 dB, more preferably is no more than −3 dB, and most preferably no more than −1 dB from the maximum effective area as the orientation of the receiver-stimulator varies relative to that of the acoustic source.

[0065] The R-S 101 assembly may comprise one or more piezoelectric components 120 in the shape of a cuboid, a post, a cylinder, or a structure with a hexagonal construction or the like, having a pair of transducer electrodes formed over opposed surfaces thereof. The cuboid is a preferred embodiment, since a structure with a square or rectangular cross-section is easy to manufacture. Additionally, a cuboid shape satisfies the requirement of being able to pack the most number of piezoelectric components 120 into a given volume. In a first exemplary embodiment, the piezoelectric component 120 may be composed of a single-crystal or polycrystalline ceramic piezoelectric material. In a preferred mode, the piezoelectric components operate in resonance, and more preferably in a thickness mode resonance. Also in a preferred embodiment, the natural structural resonance of the R-S 101 body will overlap the resonance of the transducer. One advantage of using single-crystal piezoelectric material is that the piezoelectric components can be smaller compared to using polycrystalline ceramic piezoelectric material, due to the lower velocity of sound in single-crystal piezoelectric materials. When the piezoelectric material is formed in the shape of a cuboid, the opposed transducer electrodes may typically be formed over the two opposing square surfaces of the piezoelectric component, although transducer electrodes over the other surfaces may also be used.

[0066] In a still further embodiment of the implantable R-S 101 of the present invention, the R-S 101 comprises a plurality of individual harvesting elements containing piezoelectric components 120, which themselves will typically have a maximum dimension that is approximately one-half wavelength of the expected acoustic wave frequency, but the cumulative lateral dimensions of the R-S 101 will preferably be much greater than a single wavelength.

[0067] The harvesting elements 120 in the can structure 310 have apertures which are arranged substantially orthogonal to the longitudinal axis of the can structure 310. The least favorable direction of the propagation of acoustic energy is when the energy propagates substantially parallel to this longitudinal axis. Therefore, in an optional embodiment, the R-S 101 further comprises a plurality of end cap harvesting elements 120 which are organized such that their apertures are substantially orthogonal to the long axis of the can structure 310. This orthogonal arrangement allows the combined output of the plurality of harvesting elements of the R-S 101 to maintain a more constant output power when the acoustic energy propagates substantially parallel to the longitudinal axis of the can structure 310.

[0068] FIG. 4A is a cross-sectional illustration of an exemplary embodiment of an axle assembly 104 with end caps. The axle assembly 104 has an end cap 501 at the proximal end and another end cap 501 at the distal end. The end caps 501 comprise harvesting elements whose apertures 150 are arranged orthogonal to the apertures of harvesting elements 120 on the sides of the can structure 310 described above. The can structure 310 is not shown in FIG. 4A in order to more clearly illustrate the end caps 501. The can structure 310 would be placed between the end caps 501.

[0069] FIG. 5A is a flow diagram illustrating a method for assembling an end cap 501, in accordance with an embodiment of the present invention. At step 601, an end cap flex circuit 510 is folded and bonded into an end cap structure. FIG. 4B shows an exemplary end cap flex circuit 510. At step 603, a plurality of piezoelectric components 120 and rectifiers 121 are bonded to the flex circuit 510, as shown in FIG. 4C and in the cross-section view of FIG. 4D. At step 605, the piezoelectric components 120 are wire bonded to their respective rectifiers 121, and the positive and negative outputs of the rectifiers 121 are wire bonded to positive and negative conductive traces on the end cap flex circuit 510 (similar to the positive and negative conductive traces of the can assembly 300, as described above).

[0070] FIG. 5B is a flow diagram illustrating a method for assembling an axle assembly 104, in accordance with an embodiment of the present invention. At step 611, an electrical lead (serving as the cathode 113) is brazed to the internal cathode feed-through 106 of the axle assembly 104. The axle assembly 104 is coated with a conductive material at its distal tip, and at step 613 an electrical connection is made from the electrical lead 113 to the coated axle assembly 104 tip. At step 615, one or more internal tubes are assembled and welded to the internal cathode feed-through 106. The internal tubes complete the hermetic enclosure while allowing a through hole for the axle assembly 104 to pass and be deployed. At step 617, the end cap assemblies 501 are welded to the internal tubes.

[0071] In a another aspect of the present invention, methods for transmitting an acoustic field to an implanted R-S 101 comprise implanting an R-S 101, typically formed as an assembly of multiple harvesting elements, the R-S 101 having a high degree of isotropy as described above in connection with the devices of the present invention; directing an acoustic field to the implanted R-S 101 from an acoustic source, which may be implanted or located externally, to focus or maximize the acoustic field on the R-S 101; using the harvesting elements that are exposed to the acoustic field through their aperture 150 to transfer acoustic power to their associated piezoelectric components 120 which in turn convert the acoustic power to create electrical power; using the rectifiers 121 to produce an electrical output from the electrical power that is delivered to stimulation electrodes in electrical contact with tissue; and transmitting the acoustic field for sufficient time to produce sufficient electrical energy to stimulate the tissue. The electrical energy flowing between the stimulation electrodes of the R-S 101 may possess specific characteristics of voltage, current, waveform, and the like. These electrical characteristics will be selected to stimulate the target cardiac tissue, nerve tissue, brain tissue, voluntary muscle tissue, bone tissue, or the like.

[0072] While the above is a complete description of the preferred embodiments of the invention, various alternatives, modifications, and equivalents may be used. Therefore, the above description should not be taken as limiting the scope of the invention which is defined by the appended claims.