Energy harvesters with fixed and movable couplings

Abstract

Devices for energy harvesting are provided. In some embodiments, a device includes a housing configured to be implanted within a patient, a power source positioned within the housing, and an energy harvesting mechanism positioned within the housing and configured to charge the power source. The energy harvesting mechanism can include a first piezoelectric element including a first portion fixedly coupled to a support within the housing and a second portion fixedly coupled to a harvester mass. The energy harvesting mechanism can also include a second piezoelectric element including a first portion coupled to the support and a second portion coupled to the harvester mass, such that: (a) the first portion of the second piezoelectric element is slidably coupled to the support, (b) the second portion of the second piezoelectric element is slidably coupled to the harvester mass, or (c) both (a) and (b).

Description

[0001]Atty Ref. No. A0010209WO01 ENERGY HARVESTERS WITH FIXED AND MOVABLE COUPLINGS This application claims the benefit of U.S. Provisional Patent Application Serial No.63 / 512,434, filed July 7, 2023, the entire content of which is incorporated herein by reference. TECHNICAL FIELD The present technology generally relates to medical devices, and in particular, to energy harvesters with fixed and movable couplings. BACKGROUND Various types of implantable medical devices have been developed for monitoring or treating one or more conditions of a patient. For example, a cardiac pacemaker can monitor a patient’s heart activity and provide therapeutic electrical stimulation to the heart via electrodes. The electrical stimulation provided by the cardiac pacemaker can include signals such as pacing pulses to address abnormal cardiac rhythms (e.g., bradycardia). Some types of cardiac pacemakers are implanted at a distance from the heart and are coupled to one or more leads that extend intravascularly into the heart to position the electrodes in contact with cardiac tissue. However, the leads may be prone to fracture, which may result in unreliable or incorrect pacing, and may require replacement of the lead or even the entire pacemaker. Some types of cardiac pacemakers are sized to be completely implanted within one of the chambers of the heart, and may include electrodes integrated with or attached to the device housing rather than leads. Such pacemakers can be less invasive than traditional pacemakers and can avoid complications associated with lead fracture. However, the relatively small size of such pacemakers may limit the types of power sources that can be incorporated into the device. BRIEF DESCRIPTION OF THE DRAWINGS Many aspects of the present disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to Atty Ref. No. A0010209WO01 scale. Instead, emphasis is placed on illustrating clearly the principles of the present disclosure. FIG. 1 illustrates a pacing device implanted in the heart of a patient, in accordance with embodiments of the present technology. FIG. 2 is a perspective view of a pacing device configured in accordance with embodiments of the present technology. FIG.3 is a side view of another pacing device configured in accordance with embodiments of the present technology. FIG.4 is a schematic block diagram illustrating electronic components of a pacing device configured in accordance with embodiments of the present technology. FIG. 5A is a perspective view of a device including an energy harvesting mechanism, in accordance with embodiments of the present technology. FIG.5B is a side cross-sectional view of the device of FIG.5A. FIG.5C is a closeup perspective view of a portion of the device of FIG.5A. FIG.6 is a partially schematic side cross-sectional view of a device including an energy harvesting mechanism and two supports, in accordance with embodiments of the present technology. FIG. 7A is a side cross-sectional view of a device including an energy harvesting mechanism, in accordance with embodiments of the present technology. FIG.7B is a partially schematic side cross-sectional view of a portion of the device of FIG.7A. FIG. 8A is a partially schematic side cross-sectional view of a device including an energy harvesting mechanism and a roller assembly, in accordance with embodiments of the present technology. FIG.8B is a partially schematic end cross-sectional view of a portion of the device of FIG.8A. Atty Ref. No. A0010209WO01 FIG.9A is a partially schematic illustration of a device including an energy harvesting mechanism and a roller assembly, in accordance with embodiments of the present technology. FIG.9B is a partially schematic illustration of a device including an energy harvesting mechanism and a roller assembly, in accordance with embodiments of the present technology. FIG.10 is a partially schematic end view of three piezoelectric elements with different principal bending directions, in accordance with embodiments of the present technology. DETAILED DESCRIPTION The present technology relates to energy harvesters for implantable devices. In some embodiments, for example, a device includes a housing configured to be implanted within a patient, a power source positioned within the housing, and an energy harvesting mechanism positioned within the housing and configured to charge the power source. The energy harvesting mechanism can include a first piezoelectric element and a second piezoelectric element that are each coupled to a support within the housing and to a harvester mass. The first piezoelectric element can be fixedly (e.g., rigidly) coupled to the support and to the harvester mass. The second piezoelectric element can be movably (e.g., slidably) coupled to the support, to the harvester mass, or both. The embodiments disclosed herein can provide various advantages compared to conventional devices and systems for energy harvesting. For instance, a single piezoelectric beam that is fixed at both ends may not be sufficiently stiff to best utilize the imposed mechanical motion for a given harvester mass. However, a system including multiple piezoelectric beams that are fixed at both ends may be too stiff (e.g., two piezoelectric beams may exhibit more than double the stiffness of a single beam) and / or may exhibit non-optimal loading of the piezoelectric beams. In contrast, the combination of fixed and movable couplings described herein allows for more gradual increases in stiffness (e.g., a piezoelectric beam that is fixed at both ends combined with another piezoelectric beam that is slidable at one or both ends may exhibit approximately double the stiffness of a single beam). Moreover, the combination of fixed and movable couplings can be used to Atty Ref. No. A0010209WO01 tune the overall stiffness of the energy harvesting mechanism and, thus, the resonant frequency of the energy harvesting mechanism. For example, an energy harvesting mechanism including at least one movable coupling can exhibit a lower stiffness and a higher resonant frequency than an energy harvesting mechanism that includes fixed couplings only. This approach can improve energy harvesting efficiency by allowing more piezoelectric elements to be incorporated into a single device, while maintaining the resonant frequency within a range suitable for harvesting from physiological motion. Embodiments of the present disclosure will be described more fully hereinafter with reference to the accompanying drawings in which like numerals represent like elements throughout the several figures, and in which example embodiments are shown. Embodiments of the claims may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. The examples set forth herein are non-limiting examples and are merely examples among other possible examples. As used herein, the terms “vertical,” “lateral,” “upper,” and “lower” can refer to relative directions or positions of features of the embodiments disclosed herein in view of the orientation shown in the Figures. For example, “upper” or “uppermost” can refer to a feature positioned closer to the top of a page than another feature. These terms, however, should be construed broadly to include embodiments having other orientations, such as inverted or inclined orientations where top / bottom, over / under, above / below, up / down, and left / right can be interchanged depending on the orientation. The headings provided herein are for convenience only and do not interpret the scope or meaning of the claimed present technology. Embodiments under any one heading may be used in conjunction with embodiments under any other heading. I. Overview of Implantable Pacing Devices FIGS.1–4 provide a general overview of implantable devices configured in accordance with embodiments of the present technology. Specifically, FIG. 1 illustrates a pacing device implanted in a patient’s heart, FIG.2 illustrates an example configuration for a pacing device, FIG. 3 illustrates another example configuration for a pacing device, and FIG.4 illustrates electronic components that can be included in a pacing device. Any of the Atty Ref. No. A0010209WO01 features of the embodiments of FIGS.1–4 can be combined with each other and / or with any of the other embodiments described herein. Referring first to FIG.1, which illustrates a pacing device 100 implanted in the heart H of a patient, the device 100 is configured to monitor activity of the heart H and provide electrical stimulation (e.g., pacing signals) to the heart H. In some embodiments, the device 100 is a leadless intracardiac pacemaker configured to be implanted entirely within a heart chamber, such as entirely within the right atrium (RA), entirely within the right ventricle (RV), entirely within the left atrium (LA), or entirely within the left ventricle (LV). The device 100 can be implanted at any of a variety of locations to sense and / or deliver therapy to any chamber or chambers of the heart H. For example, as shown in FIG.1, the device 100 can be a right atrial intracardiac pacemaker that is implanted in the RA of the patient’s heart H in a target implant region T (e.g., the triangle of Koch). The target implant region T can lie between the bundle of His and the coronary sinus, and / or can be adjacent to the tricuspid valve. In other embodiments, the device 100 can instead be configured as a right ventricular intracardiac pacemaker that is implanted in the RV of the heart H, with the target implant region T lying along the endocardial wall at or near the apex of the RV. The device 100 can include a housing 102 having a size and form factor suitable for transvenous delivery into the heart H via a catheter. In the illustrated embodiment, the housing 102 has an elongate shape extending from a distal portion 104 to a proximal portion 106. The housing 102 can have a generally cylindrical shape (e.g., pill- shaped or capsule-shaped), a generally prismatic shape (e.g., a rectangular prism), or any other suitable shape. The housing 102 can define an interior cavity that contains the electronic components of the device 100 (e.g., circuitry, power source, sensors). The device 100 can include a fixation mechanism 108 to secure the device 100 to the tissue of the heart H. For example, the fixation mechanism 108 can include one or more fixation elements configured to penetrate into tissue, such as one or more tines, coils, barbs, etc. In the illustrated embodiment, the fixation mechanism 108 is coupled to and extends outwardly from the distal portion 104 of the housing 102. Accordingly, when the device 100 is implanted, the distal portion 104 can be positioned in contact with or in close proximity to the cardiac tissue, while the proximal portion 106 can be spaced apart Atty Ref. No. A0010209WO01 from the cardiac tissue. In other embodiments, however, the fixation mechanism 108 can be located at a different portion of the device 100. The device 100 also includes a plurality of electrodes configured to sense electrical activity of the heart H and / or deliver electrical therapy to the heart H. For example, the device 100 can include two, three, four, five, six, seven, eight, nine, ten, or more electrodes. Each electrode can be positioned at any suitable portion of the device 100, such as on or coupled to the housing 102 (e.g., the distal portion 104, the proximal portion 106, an intermediate location between the distal portion 104 and proximal portion 106), or on or coupled to the fixation mechanism 108. In some embodiments, the device 100 includes one or more electrodes (e.g., cathodes) that directly contact the cardiac tissue (e.g., of a single heart chamber or multiple heart chambers) to sense the activity thereof and / or deliver electrical therapy thereto. Such electrode(s) can be located at the distal portion 104 of the housing 102 and / or incorporated into the fixation mechanism 108, for example. The device 100 can also include at least one electrode (e.g., an anode and / or return electrode) that does not directly contact cardiac tissue. Such electrode(s) can be located at portions of the housing 102 that are spaced apart from cardiac tissue, such as the proximal portion 106. Optionally, a single electrode may serve as a cathode for certain operations, and may serve as an anode and / or return electrode for other operations. In some embodiments, the device 100 is operably coupled to an external device 110 shown schematically) via bidirectional wireless communication, such as BLUETOOTH®, Wi-Fi, Medical Implant Communication Service (MICS), or other radiofrequency communication technique. The external device 110 can be a computing device or system that is located outside of the patient’s body, and can be used in a healthcare setting (e.g., in a clinic, hospital or other medical facility), at the patient’s home, or suitable combinations thereof. The external device 110 can be configured to control various operational parameters of the device 100, such as therapy parameters (e.g., pacing control parameters such as pacing interval), sensing parameters, power management parameters, etc. For instance, the external device 110 can transmit control signals to the device 100 to program one or more operational parameters of the device 100. Optionally, the external device 110 can display information relating to and / or received from the device 100, such as intracardiac electrogram (EGM) signals obtained by the device 100, motion sensor signals acquired by the device 100, operational parameters of the device 100, etc. In some Atty Ref. No. A0010209WO01 embodiments, the external device 110 transmits information received from the device 100 to another computing device or system (e.g., a computer, laptop, workstation, mobile device, server, remote patient management system) for display, processing, and / or storage, using any suitable wired or wireless communication technique. The external device 110 can serve as a “programmer” that allows a physician, patient, or other individual to monitor and / or control the operations of the device 100. Although FIG.1 illustrates a single device 100, the present technology is also applicable to implantable systems including multiple devices 100 implanted at different locations in the heart H. For example, an implantable system can include a first device 100 in the RA and a second device 100 in the RV. In such embodiments, each device 100 can independently have any of the features described herein. FIG.2 is a perspective view of a pacing device 200 configured in accordance with embodiments of the present technology. The device 200 is configured to be implanted within a chamber of a heart of the patient to monitor activity of the heart and / or provide electrical therapy (e.g., pacing therapy) to the heart. The device 200 includes a housing 202 having a size and form factor that allows the device 200 to be entirely implanted within a single chamber of the patient’s heart. In the illustrated embodiment, the housing 202 has an elongate shape (e.g., a generally cylindrical shape, a generally prismatic shape) extending between a distal end 204 and proximal end 206. The housing 202 can define a hermetically sealed internal cavity for housing the electronic components of the device 200. The housing 202 can also include an attachment mechanism 208 (e.g., at the proximal end 206) configured to temporarily engage with a delivery tool during implantation and / or extraction of the device 200. The housing 202 can be formed partially or entirely from a conductive material, such as titanium or titanium alloy, stainless steel, MP35N (a non-magnetic nickel- cobalt-chromium-molybdenum alloy), a platinum alloy, or other biocompatible metal or metal alloy, or other suitable conductive material. Alternatively or in combination, the housing 202 can be formed partially or entirely from a nonconductive (e.g., insulative) material, such as ceramic, glass, sapphire, silicone, polyurethane, epoxy, acetyl co-polymer plastics, polyether ether ketone (PEEK), a liquid crystal polymer, other biocompatible polymer, or other suitable nonconductive material. Atty Ref. No. A0010209WO01 The device 200 can include a plurality of electrodes 210a–210c configured to sense electrical activity of the heart and / or deliver electrical stimulation to the heart. In the illustrated embodiment, for example, the device 200 includes a first electrode 210a and a second electrode 210b at or proximate to the distal end 204 of the housing 202, and a third electrode 210c on the housing 202. The first and second electrodes 210a, 210b can be configured as cathode electrodes that directly contact cardiac tissue, e.g., a distal end of the first electrode 210a can be configured to rest within a ventricular myocardium of the patient, and the second electrode 210b can be configured to contact an atrial endocardium of the patient. The third electrode 210c can be configured as an anode and / or return electrode that does not directly contact cardiac tissue. As shown in FIG.2, the first electrode 210a can be an elongate structure that extends from the distal end 204 of the housing 202 to penetrate through the wall tissue of a first heart chamber (e.g., the chamber in which the device 200 is implanted) into wall tissue of a second, different heart chamber. For example, in some embodiments, the device 200 is implanted in the RA with the distal end 204 oriented toward the LV (e.g., similar to the arrangement of the device 100 in FIG.1), and the first electrode 210a extends through the wall tissue of the RA and into the wall tissue of the LV. In the illustrated embodiment, the first electrode 210a is configured as a coil (e.g., a helical and / or spiral coil), while in other embodiments, the first electrode 210a can have a different form factor (e.g., an elongate dart, barb, tine, or other tissue penetrating element). The first electrode 210a can include a proximal end that is coupled to the distal end 204 of the housing 202, and a free distal end that is not attached to the housing 202. The distal end of the first electrode 210a can have a conical, hemi-spherical, or slanted edge distal tip with a narrow tip diameter (e.g., less than 1 mm) for penetrating into and through tissue layers. In some embodiments, the distal end of the first electrode 210a can have a sharpened or angular tip, and / or sharpened or beveled edges, but the degree of sharpness can be constrained to avoid a cutting action that could lead to lateral displacement of the distal end of the first electrode 210a and undesired tissue trauma. The second electrode 210b can be a structure that extends from the distal end 204 of the housing 202 to contact the wall tissue of the first heart chamber without penetrating the wall tissue. The second electrode 210b can be located proximal to the first electrode 210a. The second electrode 210b can be configured as a coil (e.g., a partial helical Atty Ref. No. A0010209WO01 and / or spiral coil that does not form a full turn), loop, button, pad, or any other suitable form factor. The second electrode 210b can include a proximal end that is coupled to the distal end 204 of the housing 202, and a distal end that may or may not be coupled to the housing 202. In some embodiments, the second electrode 210b is configured to flexibly maintain contact with wall tissue of the heart chamber in which the device 200 is implanted, (e.g., the RA endocardium), despite variations in the tissue surface and / or in the distance between the distal end 204 of the housing 202 and the tissue surface, which may occur as the wall tissue moves during the cardiac cycle. Accordingly, the second electrode 210b can be flexible and / or have spring-like properties, e.g., the second electrode 210b can have a spring bias that urges at least a portion of the second electrode 210b away from the distal end 204 of the housing 202 and toward the wall tissue of the heart chamber to maintain consistent contact. The first and second electrodes 210a, 210b can each be formed of an electrically conductive material, such as titanium, platinum, iridium, tantalum, or alloys thereof. The first electrode 210a can include one or more insulative coatings (e.g., parylene, polyurethane, silicone, epoxy) that reduce the electrically conductive surface area of the first electrode 210a to define a first electrically active region 212 (e.g., at or near the distal end of the first electrode 210a). The second electrode 210b can include one or more insulative coatings (e.g., parylene, polyurethane, silicone, epoxy) that reduce the electrically conductive surface area of the second electrode 210b to define a second electrically active region 214 (e.g., at an intermediate region between the proximal and distal ends of the second electrode 210b). This approach can increase the electrical impedance of the first and second electrodes 210a, 210b, and thereby reduce the current delivered during a pacing pulse, which can conserve the power used by the device 200. In some embodiments, the first and second electrodes 210a, 210b include an electrically conductive material coating (e.g., TiN) on the first and second electrically active regions 212, 214, respectively, to define the active regions. The first and second electrodes 210a, 210b can be made of the same materials, or can be made of different materials. All, substantially all, or a portion of the housing 202 can serve as a third electrode 210c (e.g., an anode and / or return electrode) during pacing and / or sensing. In some embodiments, the third electrode 210c partially or fully circumscribes a portion of the housing 202 at or near the proximal end 206. Although FIG.2 illustrates the third electrode Atty Ref. No. A0010209WO01 210c as a singular band, in other embodiments, the third electrode 210c can include multiple segments spaced a distance apart along a longitudinal axis 216 of the housing 202 and / or around a perimeter of the housing 202. Additionally, the third electrode 210c can also be located at other positions along the housing 202, e.g., located at or near the distal end 204 or at other positions along the longitudinal axis 216. In embodiments where the housing 202 is formed from a conductive material, one or more portions of the housing 202 can be electrically insulated by a nonconductive material, such as a coating of parylene, polyurethane, silicone, epoxy or other biocompatible polymer, or other suitable material. For the portions of the housing 202 without the nonconductive material, one or more discrete areas of the housing 202 with conductive material can be exposed to define the third electrode 210c. In embodiments where the housing 202 is formed from a nonconductive material, a conductive material can be applied to one or more discrete areas of the housing 202 to form the third electrode 210c. Optionally, the third electrode 210c can be a discrete component (e.g., a ring electrode) that is coupled to the housing 202. The electrodes 210a–210c can be used to sense electrical activity of one or more heart chambers and / or to deliver electrical stimulation to one or more heart chambers. For example, the first electrode 210a can be paired with the second electrode 210b or the third electrode 210c to for sensing ventricular signals and delivering ventricular pacing pulses. As another example, the second electrode 210b can be paired with the first electrode 210a or the third electrode 210c for sensing atrial signals and delivering pacing pulses to the atrial myocardium. In a further example, the third electrode 210c can be paired at different times with both the first electrode 210a and the second electrode 210b for either ventricular or atrial functionality, respectively. As yet another example, the first electrode 210a and the second electrode 210b can be paired with each other with different polarities for atrial and ventricular functionality. In some embodiments, the second electrode 210b is configured as an atrial cathode electrode for delivering pacing pulses to the atrial tissue at a target implant region in combination with the third electrode 210c. The second electrode 210b and the third electrode 210c can also be used to sense atrial P-waves for use in controlling atrial pacing pulses (e.g., delivered in the absence of a sensed P-wave) and for controlling atrial- Atty Ref. No. A0010209WO01 synchronized ventricular pacing pulses delivered using the first electrode 210a as a cathode and the third electrode 210c as the return anode. The configuration of the electrodes 210a– 210c illustrated in FIG.2 allows the device 200 to sense cardiac signals from and / or deliver cardiac pacing to one or more chambers of the heart. For example, the present technology can facilitate the delivery of A-V synchronous pacing using a single device 200 implanted within a single heart chamber (e.g., the RA). The device 200 can include a fixation mechanism 218 configured to fix the device 200 to cardiac tissue at a target implant region (e.g., the triangle of Koch). In the illustrated embodiment, the first electrode 210a and / or second electrode 210b at the distal end 204 of the housing 202 can serve as the fixation mechanism 218. In other embodiments, the fixation mechanism 218 can be a different component than the first electrode 210a and / or the second electrode 210b, such one or more separate barbs, tines, coils, darts, etc. FIG.3 is a side view of another pacing device 300 configured in accordance with embodiments of the present technology. The device 300 is configured to be implanted within a chamber of a heart of a patient to monitor activity of the heart and / or to provide electrical therapy to the heart. In the embodiment shown in FIG.3, the device 300 includes a housing 302, a plurality of fixation tines 304, a first electrode 306a, and a second electrode 306b. The housing 302 can have a size and form factor that allows the device 300 to be entirely implanted within a chamber of a heart of a patient. For example, as shown in FIG. 3, the housing 302 has a generally cylindrical (e.g., pill-shaped or capsule-shaped), elongate form factor extending between a distal end 308 and a proximal end 310. The housing 302 contains electronic components of the device 300, and can be hermetically or near-hermetically sealed to prevent fluid ingress into the housing 302. The materials used to form the housing 302 can include any of the conductive and nonconductive materials described above with respect to FIG.2. The device 300 can include a fixation mechanism configured to fix the device 300 to cardiac tissue at a target implant region (e.g., the endocardial wall near the apex of the RV). In the illustrated embodiment, the device 300 includes a plurality of fixation tines 304 extending from the distal end 308 of the housing 302 and configured to engage with cardiac tissue to secure the housing 302 at a fixed position within the chamber of the heart. Atty Ref. No. A0010209WO01 The fixation tines 304 can be configured to anchor the housing 302 to the cardiac tissue such that the device 300 moves along with the cardiac tissue during cardiac contractions. The device 300 can include any suitable number of fixation tines 304, such as one, two, three, four, five, or more fixation tines 304. The fixation tines 304 can be fabricated from any suitable material, such as a shape memory material (e.g., Nitinol). Alternatively or in combination, the device 300 can be fixed to cardiac tissue using other types of fixation mechanisms, such as, but not limited to, barbs, coils, darts, and the like. Optionally, the device 300 can include an attachment mechanism configured to temporarily couple the device 300 to a delivery tool, e.g., for delivery and / or extraction of the device 300. In the illustrated embodiment, for example, the proximal end 310 includes a flange 318 that defines an opening. The flange 318 can be attached to a tether (e.g., by threading the tether through the opening) that extends through an elongate shaft (e.g., a catheter) to implant or extract the device 300. In some embodiments, the device 300 is configured to sense electrical activity of the heart and / or deliver electrical stimulation to the heart via the first electrode 306a and second electrode 306b (collectively, “electrodes 306”). The first electrode 306a can serve as a cathode configured to electrically contact cardiac tissue and deliver pacing pulses thereto, and the second electrode 306b can serve as an anode and / or a return electrode. Optionally, the device 300 can be equipped with multiple cathode electrodes. Such multiple cathode electrodes can be configured to electrically contact and deliver pacing pulses to cardiac tissue of a single heart chamber, or cardiac tissue of multiple heart chambers. In some such embodiments, the multiple cathode electrodes are configured to electrically contact and deliver pacing pulses to cardiac tissue of different heart chambers. For example, one cathode electrode can be configured to electrically contact and deliver pacing pulses to atrial tissue, and another cathode electrode may be configured to electrically contact and deliver pacing pulses to ventricular tissue. The electrodes 306 can be configured in many different ways. For example, one or both of the electrodes 306 can be discrete components that are mechanically coupled to the housing 302. As another example, one or both of the electrodes 306 can be defined by an outer portion of the housing 302 that is electrically conductive. The electrodes 306 can be electrically isolated from each other. In some embodiments, a portion of the housing Atty Ref. No. A0010209WO01 302 is covered by or formed from an insulative material to isolate the electrodes 306 from each other and / or to provide a desired size and shape for one or both of the electrodes 306. The electrodes 306 can be electrically coupled to at least some of the internal electronic components of the device 300 within the housing 302 (e.g., sensing circuitry, electrical stimulation circuitry, or both). In the illustrated embodiment, the first electrode 306a is located at the distal end 308 of the housing 302. The first electrode 306a may be referred to as a tip electrode, and the fixation tines 304 can be configured to anchor the device 300 to cardiac tissue such that the first electrode 306a maintains contact with the cardiac tissue. In some examples, the housing 302 includes an end cap 312 at the distal end 308, and the end cap 312 includes a feedthrough assembly to electrically couple the first electrode 306a to the electronics within the housing 302, while electrically isolating the first electrode 306a from the remaining portions of the housing 302, e.g., including the second electrode 306b and / or other conductive portions of the housing 302 The second electrode 306b can be located on the housing 302 away from (e.g., proximal to) the first electrode 306a. As shown in FIG.3, the housing 302 includes a first portion 314 and a second portion 316, with the first portion 314 being located proximal to the end cap 312, and the second portion 316 being located proximal to the first portion 314. The second portion 316 can optionally define at least part of a power source case that houses a power source (e.g., a battery) of the pacing device 300. In some embodiments, the second electrode 306b is located on the second portion 316, while in other embodiments, the second electrode 306b is located on the first portion 314. In some embodiments, the second electrode 306b is a conductive portion of the housing 302 (e.g., an annular portion of the housing 302 that is made partially or entirely from a conductive material). Additionally or alternatively, the second electrode 306b can be a conductive material that is coated onto the material of the housing 302, or a discrete component (e.g., a ring electrode) that is coupled to the housing 302. The remaining portions of the housing 302 can include or be coated with an insulative material so that the second electrode 306b is electrically isolated from the rest of the housing 302 and / or from the first electrodes 306a. Atty Ref. No. A0010209WO01 FIG.4 is a schematic block diagram illustrating electronic components of a pacing device 400 configured in accordance with embodiments of the present technology. Any of the electronic components shown in FIG. 4 can be incorporated into any of the embodiments of implantable devices described herein, such as the device 100 of FIG.1, the device 200 of FIG.2, or the device 300 of FIG.3. As shown in FIG.4, the device 400 includes a plurality of electrodes 402a– 402c that are electrically coupled to components within a housing 404 of the device 400. Although the device 400 is illustrated and described herein as having three electrodes 402a– 402c (e.g., similar to the device 200 of FIG.2), in other embodiments, the device 400 can be modified to include a different number of electrodes, such as two electrodes (e.g., similar to the device 300 of FIG.3) or any other suitable number of electrodes. At least some of the electrodes 402a–402c can be configured to contact tissue of one or more heart chambers, as described elsewhere herein. For example, as discussed above with respect to FIG.2, the first electrode 402a can be configured to electrically contact and deliver electrical signals to tissue of a first heart chamber (e.g., ventricular tissue), and the second electrode 402b can be configured to electrically contact and deliver electrical signals to tissue of a second, different heart chamber (e.g., atrial tissue). The third electrode 402c can be an anode and / or return electrode that does not electrically contact heart tissue. Optionally, either the first electrode 402a or the second electrode 402b can be omitted, or the device 400 can include additional electrodes that electrically contact and deliver electrical signals to tissue of a heart chamber (e.g., the first heart chamber, the second heart chamber, or another heart chamber). The device 400 includes a plurality of electronic components within the housing 404, such as switch circuitry 406, sensing circuitry 408, therapy generation circuitry 410, one or more sensors 412, processing circuitry 414, communication circuitry 416, memory 418, and / or a power source 420. The various circuitry can be or include programmable or fixed function circuitry configured to perform the operations described herein. One or more of the components of the device 400 shown in FIG.4 can be part of an electronics assembly. For example, one or more of the switch circuitry 406, sensing circuitry 408, therapy generation circuitry 410, sensor(s) 412, processing circuitry 414, Atty Ref. No. A0010209WO01 communication circuitry 416, and / or memory 418 can be mounted on a circuit board of an electronics assembly of the device 400. The switch circuitry 406 can include one or more switches (e.g., a switch matrix, switch arrays, or other collection of switches), multiplexers, transistors, and / or other electrical circuitry. The switch circuitry 406 can selectively couple one or more of the electrodes 402a–402c to other components of the device 400 (e.g., the sensing circuitry 408 and / or the therapy generation circuitry 410). The subset of the electrodes 402a–402c to be used can depend on the particular operation of the device 400 that is being performed, such as whether the device 400 is sensing or delivering therapy, the locations of the heart being monitored or treated, etc. In some embodiments, the processing circuitry 414 determines which subset of the electrodes 402a–402c should be used for a particular operation, and controls the switch circuitry 406 to selectively couple those electrodes to the appropriate components of the device 400. The sensing circuitry 408 can monitor signals from at least one of electrodes 402a–402c to monitor electrical activity of the heart, impedance, and / or other electrical phenomena. Sensing can be performed to determine heart rates and / or heart rate variability; and / or to detect ventricular dyssynchrony, arrhythmias (e.g., tachyarrhythmias), and / or other electrical signals. The sensing circuitry 408 can include filters, amplifiers, analog-to- digital converters, and / or other circuitry configured to sense cardiac electrical signals via one or more of the electrodes 402a–402c. In some embodiments, the switch circuitry 406 as controlled by the processing circuitry 414 selectively couples the sensing circuitry 408 to selected combinations of the electrodes 402a–402c, e.g., to selectively sense the electrical activity of one or more chambers of the heart. For example, the switch circuitry 406 can couple each of the first electrode 402a and the second electrode 402b (in combination with the third electrode 402c) to respective sensing channels provided by the sensing circuitry 408 to sense electrical signals from the cardiac tissues in electrical contact with the first electrode 402a (e.g., ventricular tissue) and the second electrodes 402b (e.g., atrial tissue), respectively. In some embodiments, the sensing circuitry 408 is configured to detect events, (e.g., depolarizations) within the cardiac electrical signals, and to provide indications thereof to the processing circuitry 414. In this manner, the processing circuitry 414 can determine the Atty Ref. No. A0010209WO01 timing of atrial and / or ventricular depolarizations, and can control the delivery of cardiac pacing (e.g., AV synchronized cardiac pacing) based thereon. The therapy generation circuitry 410 can generate electrical stimulation signals, such as cardiac pacing pulses. The therapy generation circuitry 410 can be electrically coupled to one or more of the electrodes 402a–402c to deliver pulses to a portion of cardiac muscle within the heart via one or more of the electrodes 402a–402c. In some embodiments, the therapy generation circuitry 410 delivers pacing stimulation in the form of electrical pulses. The therapy generation circuitry 410 can include charging circuitry, and one or more charge storage devices (e.g., capacitors). Optionally, the therapy generation circuitry 410 can include switches and / or other circuitry to control when the charge storage devices are discharged to the electrodes 402a–402c. The switch circuitry 406 as controlled by the processing circuitry 414 can direct electrical stimulation signals from the therapy generation circuitry 410 to a selected combination of the electrodes 402a–402c having selected polarities, e.g., to selectively deliver pacing pulses to the RA, RV, LV, and / or the interventricular septum of the heart. For example, in order to pace one or both of the ventricles, the switch circuitry 406 can electrically couple the first electrode 402a (e.g., which contacts wall tissue of a ventricle or the intraventricular septum) to the therapy generation circuitry 410 as a cathode, and to one or both of the second electrode 402b or the third electrode 402c to the therapy generation circuitry 410 as an anode. As another example, in order to pace the RA, the switch circuitry 406 can couple the second electrode 402b (e.g., which contacts the RA endocardium) to the therapy generation circuitry 410 as a cathode, and to one or both of the first electrode 402a or the third electrode 402c to the therapy generation circuitry 410 as an anode. The processing circuitry 414 can include one or more processors, such as a microprocessor, a controller, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or equivalent discrete or analog logic circuitry. In some embodiments, the processing circuitry 414 can include multiple components, such as any combination of one or more microprocessors, controllers, DSPs, ASICs, and / or FPGAs, as well as other discrete or integrated logic circuitry. The functions attributed to the processing circuitry 414 herein may be embodied as software, firmware, hardware, or any combination thereof. Atty Ref. No. A0010209WO01 The processing circuitry 414 can control the therapy generation circuitry 410 to deliver stimulation therapy to a patient’s heart according to therapy parameters, which can be stored in the memory 418. For example, the processing circuitry 414 can control the therapy generation circuitry 410 to deliver electrical pulses with the amplitudes, pulse widths, rates, frequencies, and / or electrode polarities specified by the therapy parameters. In this manner, the therapy generation circuitry 410 can deliver pacing pulses to the heart via one or more of the electrodes 402a–402c. The device 400 can use any combination of the electrodes 402a–402cto deliver therapy and / or detect electrical signals from the patient. The memory 418 (e.g., a data storage device or other non-transitory medium) can store computer-readable instructions that, when executed by the processing circuitry 414, cause the device 400 to perform the various operations described herein. The memory 418 can include any volatile, non-volatile, magnetic, optical, or electrical media, such as a random-access memory (RAM), read only memory (ROM), non-volatile RAM (NVRAM), electrically-erasable programmable ROM (EEPROM), flash memory, or any other digital or analog media. The sensor(s) 412 can include one or more sensing elements that transduce patient physiological activity to an electrical signal to sense values of a respective patient parameter. Sensor(s) 412 can include one or more motion sensors, optical sensors, chemical sensors, temperature sensors, pressure sensors, and / or any other types of sensors. The sensor(s) 412 can output patient parameter values to the processing circuitry 414 that can be used as feedback to control sensing and / or delivery of therapy by the device 400. For example, the sensor(s) 412 can include at least one motion sensor, such as one or more inertial measurement units (IMUs), accelerometers, gyroscopes, electrical or magnetic field sensors, and / or other devices capable of detecting motion and / or the position of the device 400. The motion of the device 400 detected by the motion sensor may be indicative of cardiac events (e.g., paced activation of the ventricles), blood flow through the heart, patient posture, patient activity, and / or noise. The processing circuitry 414 can control and / or monitor the motion data produced by the motion sensor to identify one or more features of the cardiac contraction within the signal (e.g., on a beat-by-beat basis or otherwise) to facilitate delivery of therapy (e.g., delivery of ventricular pacing pulses in an atrial-synchronized manner). Optionally, the processing circuitry 414 can use the motion Atty Ref. No. A0010209WO01 data to detect a current activity level of the patient, which can be used for rate-responsive pacing of the patient’s heart. The communication circuitry 416 is configured to allow the device 400 to wirelessly communicate with another device, such as a device external to the patient’s body (e.g., the external device 110 of FIG. 1) and / or another device under the control of the processing circuitry 414. For instance, the processing circuitry 414 can receive updates to operational parameters from the other device, and / or can provide collected data, (e.g., sensed heart activity and / or other patient parameters) to the other device via the communication circuitry 416. The communication circuitry 416 can use radiofrequency (RF) communication techniques (e.g., via an antenna) and / or any other suitable communication modality. The power source 420 delivers operating power to various components of the device 400. The power source 420 can include one or more batteries, each of which can independently be rechargeable or non-rechargeable. Recharging of the power source 420 can be accomplished through proximal inductive interaction between an external charger and an inductive charging coil within the device 400. Alternatively or in combination, recharging of the power source 420 can be accomplished using an energy harvesting mechanism 422 of the device 400. Additional details of energy harvesting mechanisms and associated methods are provided in Section II below. The components of the device 400 illustrated in FIG. 4 can be modified in many different ways. For example, any of the components shown in FIG.4 can be combined with each other, e.g., the switch circuitry 406 can be incorporated into the sensing circuitry 408 and / or the therapy generation circuitry 410. Any of the components shown in FIG. 4 can be divided into smaller subcomponents. Some of the components in FIG.4 are optional and may be omitted (e.g., the switch circuitry 406 and / or sensor(s) 412). The device 400 can also include additional components not shown in FIG.4. For example, the device 400 can include power management circuitry coupled to the power source 420 to allow the processing circuitry 414 to monitor the status of the power source 420 (e.g., charge level, charging rate, net power into and / or out of the power source 420, remaining battery life). The components of the device 400 shown in FIG. 4 represent functionality that can be included in any of the devices of the present technology. The components Atty Ref. No. A0010209WO01 illustrated in FIG.4 can include any discrete and / or integrated electronic circuit components that implement analog and / or digital circuits capable of producing the functions attributed to the components herein. For example, the components can include analog circuits, such as amplification circuits, filtering circuits, and / or other signal conditioning circuits. The components can also include digital circuits, such as combinational or sequential logic circuits, memory devices, and the like. The functions attributed to the components of FIG. 4 may be embodied as one or more processors, hardware, firmware, software, or any combination thereof. The depiction of different features as separate blocks in FIG. 4 is intended to highlight different functional aspects, and does not necessarily imply that such components must be realized by separate hardware or software components. Rather, functionality associated with one or more components may be performed by separate hardware or software components, or integrated within common or separate hardware or software components. For example, although illustrated as separate functional components in FIG. 4, some or all of the functionality attributed to the switch circuitry 406, sensing circuitry 408, therapy generation circuitry 410, sensor(s) 412, and / or communication circuitry 416 can alternatively or additionally be implemented by the processing circuitry 414, or vice-versa. II. Energy Harvesting Mechanisms In some embodiments, the present technology provides implantable devices that include an energy harvesting mechanism (also known as an “energy harvester” or “harvester”). The power capacity of a power source of an implantable device may be limited due to size constraints, such as if the device is implanted within a small space within the patient’s body (e.g., within a single heart chamber) and / or to avoid the device interfering with normal physiological function, as well as safety considerations. To prolong the usable life of such implantable devices, an energy harvesting mechanism can be used to generate energy in situ to recharge the power source. FIGS. 5A–5C illustrate a device 500 including an energy harvesting mechanism 502, in accordance with embodiments of the present technology. Specifically, FIG.5A is a perspective view of the device 500, FIG.5B is a side cross-sectional view of the device 500, and FIG. 5C is closeup perspective view of a portion of the device 500. Referring first to FIGS.5A and 5B together, the device 500 can be an implantable device, Atty Ref. No. A0010209WO01 such as a pacing device configured to monitor activity of a patient’s heart and provide electrical stimulation to the heart. In such embodiments, the device 500 can include any of the features of the devices described above in connection with FIGS.1–4 (e.g., electrodes, fixation mechanism, circuitry, and / or other electronic components). In other embodiments, however, the device 500 can be a different type of implantable medical device. The device 500 includes a housing 504 having an elongate shape extending between a distal end 506 and a proximal end 508 (the housing 504 is depicted as being transparent in FIG.5A merely for illustrative purposes) to define a longitudinal axis A. The housing 504 includes an interior cavity 510 containing the energy harvesting mechanism 502 and other components of the device 500, such as a power source 512 and an electronics assembly 514 (shown schematically). When the device 500 is implanted in a patient’s body, the energy harvesting mechanism 502 generates energy from physiological motion. For example, in some embodiments, the device 500 is configured to be implanted within a heart chamber of the patient and generates energy from cardiac motion (e.g., motion of the heart wall to which the device 500 is affixed) and / or blood flow through the heart chamber. The energy produced by the energy harvesting mechanism 502 can be used to charge the power source 512, which in turn powers the operation of the device 500. In some embodiments, the energy harvesting mechanism 502 includes a plurality of piezoelectric elements that convert mechanical energy into electrical energy via the piezoelectric effect, such as a first piezoelectric element 516a and a second piezoelectric element 516b (collectively, “piezoelectric elements 516”). Although the illustrated embodiment shows two piezoelectric elements 516, in other embodiments, the device 500 can include a different number of piezoelectric elements 516, as described further below. Each of the piezoelectric elements 516 can be made partially or entirely out of a piezoelectric material, such as a piezoelectric ceramic (e.g., lead zirconate titanate (PZT)), a single crystal piezoelectric material (e.g., lead magnesium niobate-lead titanate (PMN-PT)), a piezoelectric polymer (e.g., polyvinylidene difluoride (PVDF)), or a piezoelectric composite (e.g., a piezoelectric ceramic embedded in a polymer matrix, such as a macro fiber composite (MFC)). In some embodiments, some or all of the piezoelectric elements 516 are piezoelectric bimorphs (e.g., piezoelectric bimorph beams) including two active layers of a piezoelectric material (e.g., an upper layer and a lower layer). Optionally, a piezoelectric bimorph can include a passive layer made out of a non-piezoelectric material (e.g., a Atty Ref. No. A0010209WO01 polymer or a metal) that serves as a support for the active layers. In other embodiments, however, some or all of the piezoelectric elements 516 can be a piezoelectric unimorph having a single active layer. For example, some or all of the piezoelectric elements 516 can include a piezoelectric MFC formed from a sheet of piezoelectric fibers embedded in an adhesive (e.g., epoxy). The fibers can be aligned with (e.g., parallel to) each other and with the longitudinal axis A of the device 500. A piezoelectric MFC may be more flexible than other types of piezoelectric materials and thus may have a lower resonant frequency, which may be more favorable for harvesting from physiological motion. In other embodiments, however, some or all of the piezoelectric elements 516 can be formed from other types of piezoelectric materials. The piezoelectric elements 516 can have any suitable geometry. Although the piezoelectric elements 516 are depicted as having a flattened, generally rectangular shape with a uniform width and thickness, in other embodiments, the piezoelectric elements 516 can have a different shape, such as a tapered shape with a varying width and / or thickness. In some embodiments, each of the piezoelectric elements 516 has a flexible elongate body (e.g., a beam, plate, shaft, rod, fiber) extending from a respective first (e.g., distal) end portion to a respective second (e.g., proximal) end portion to define respective longitudinal axes. For instance, as best seen in FIG.5B, the first piezoelectric element 516a can have a first end portion 518a and a second end portion 520a, and the second piezoelectric element 516b can have a first end portion 518b and a second end portion 520b. In some embodiments, the first piezoelectric element 516a has the same or similar characteristics as the second piezoelectric element 516b. For instance, the first piezoelectric element 516a and the second piezoelectric element 516b can be made of the same materials, can have the same geometry (e.g., length, width, thickness), and / or can have the same mechanical properties (e.g., stiffness). In other embodiments, however, the first piezoelectric element 516a can have different characteristics than the second piezoelectric element 516b. For example, the first piezoelectric element 516a can be made from a different material, can have a different geometry, and / or can have different mechanical properties than the second piezoelectric element 516b. Optionally, the first piezoelectric element 516a can have a greater thickness and / or stiffness than the second piezoelectric Atty Ref. No. A0010209WO01 element 516b, or vice-versa. The characteristics of the piezoelectric elements 516 can be selected to provide a desired resonant frequency for the energy harvesting mechanism 502, as discussed further below. The piezoelectric elements 516 can be arranged in a parallel or generally parallel configuration in which the first end portions 518a, 518b of the piezoelectric elements 516 are located near the distal end 506 of the housing 504, and the second end portions 520a, 520b of the piezoelectric elements 516 are located near the proximal end 508 of the housing 504. The piezoelectric elements 516 can be arranged so that their respective longitudinal axes are aligned with (e.g., parallel to or within 10°, 5°, 2°, or 1° of being parallel to) each other and / or to the longitudinal axis A of the housing 504. In other embodiments, however, the piezoelectric elements 516 can be oriented differently with respect to the housing 504. In some embodiments, the piezoelectric elements 516 are each coupled to a support 522 and to a harvester mass 524. For instance, the first end portions 518a, 518b of the piezoelectric elements 516 can be coupled to the support 522, and the second end portions 520a, 520b of the piezoelectric elements 516 can be coupled to the harvester mass 524. The couplings between the piezoelectric elements 516, the support 522, and the harvester mass 524 can include any suitable combination of movable couplings and fixed couplings. A movable coupling (e.g., a slidable coupling) can allow for relative movement of the coupled parts, such that the coupled parts can move relative to each other at and / or near the location of the movable coupling (e.g., with respect to up to three degrees of freedom in translation and / or up to three degrees of freedom in rotation). A fixed coupling (also referred to herein as a rigid coupling) can inhibit relative movement of the coupled parts, such that the coupled parts are in a fixed and / or rigid spatial relationship with each other at and / or near the location of the fixed coupling. In some embodiments, the first piezoelectric element 516a is fixedly coupled to both the support 522 and the harvester mass 524, while the second piezoelectric element 516b can include at least one movable coupling. For instance, the second piezoelectric element 516b can be fixedly coupled to the support 522 and movably coupled to the harvester mass 524, can be movably coupled to the support 522 and fixedly coupled to the harvester mass 524, or can be movably coupled to both the support 522 and the harvester Atty Ref. No. A0010209WO01 mass 524. This configuration can be used to tune the overall stiffness of the energy harvesting mechanism 502 and, thus, the resonant frequency of the energy harvesting mechanism 502, as described in further detail below. The support 522 can be part of the housing 504 or part of another component that is in a fixed spatial configuration relative to the housing 504. For example, as shown in FIG. 5B, the support 522 can be a bulkhead, interior wall, frame, bracket, etc., within the housing 504. The support 522 can be positioned between the electronics assembly 514 and the power source 512 (e.g., proximal to the electronics assembly 514 and distal to the power source 512). Alternatively, the support 522 can be at a different location, such as proximal to the power source 512 or distal to the electronics assembly 514. In some embodiments, the piezoelectric elements 516 are electrically coupled in parallel to the power source 512 via electrical interconnections (e.g., wires) extending through and / or along the support 522. In other embodiments, the support 522 can be part of the power source 512, or can be part of any other suitable component of the device 500. In some embodiments, the first piezoelectric element 516a is fixedly coupled to the support 522, e.g., using bonding, adhesives, fasteners, interference fit, and / or other suitable rigid attachment techniques. The support 522 can include a first recess 526a (e.g., a slot, opening, groove, cavity) configured to receive the first end portion 518a of the first piezoelectric element 516a. The first recess 526a can be sufficiently small so that the first end portion 518a of the first piezoelectric element 516a is spatially fixed within the first recess 526a. For instance, the height and width of the first recess 526a can be the same or similar as (e.g., within 10% of) the height and width of the first end portion 518a of the first piezoelectric element 516a. Although the first recess 526a is depicted as extending through the entire thickness of the support 522, in other embodiments, the first recess 526a can extend through less than the entire thickness of the support 522 (e.g., the first recess 526a can be formed in the proximal surface of the support 522 and can terminate before the distal surface of the support 522). Alternatively, the first recess 526a can be omitted and the first piezoelectric element 516a can instead be coupled to the proximal surface of the support 522. The second piezoelectric element 516b can also be fixedly coupled to the support 522, e.g., using bonding, adhesives, fasteners, interference fit, and / or other suitable Atty Ref. No. A0010209WO01 rigid attachment techniques. The support 522 can include a second recess 526b (e.g., a slot, opening, groove, cavity) configured to receive the first end portion 518b of the second piezoelectric element 516b. The second recess 526b can be sufficiently small so that the first end portion 518b of the second piezoelectric element 516b is spatially fixed within the second recess 526b. For instance, the height and width of the second recess 526b can be the same or similar as (e.g., within 10% of) the height and width of the first end portion 518b of the second piezoelectric element 516b. Although the second recess 526b is depicted as extending through the entire thickness of the support 522, in other embodiments, the second recess 526b can extend through less than the entire thickness of the support 522 (e.g., the second recess 526b can be formed in the proximal surface of the support 522 and can terminate before the distal surface of the support 522). Moreover, although the second recess 526b is depicted as having the same shape and size as the first recess 526a, in other embodiments, the second recess 526b can have a different shape and / or size than the first recess 526a. Alternatively, the second recess 526b can be omitted and the second piezoelectric element 516b can instead be coupled to the proximal surface of the support 522. Optionally, the second piezoelectric element 516b can instead be movably coupled to the support 522. For example, the second piezoelectric element 516b can be movable in a longitudinal direction with respect to the support 522 (e.g., parallel to the longitudinal axis A of the housing 504), such as toward the distal end 506 of the housing 504 and / or toward the proximal end 508 of the housing 504. Alternatively or in combination, the second piezoelectric element 516b can be movable in other directions relative to the support 522, such as laterally, rotationally, etc. Optionally, a lubricious material (e.g., a polymeric bearing, coating, or surface treatment) can be positioned within the second recess 526b to reduce friction and facilitate movement (e.g., sliding) of the second piezoelectric element 516b. In embodiments where the second piezoelectric element 516b is movably coupled to the support 522, the device 500 can include another support distal to the support 522 to provide additional stability to the second piezoelectric element 516b, as described further below with respect to FIG.6. In some embodiments, the dimensions of the second recess 526b can be larger than the corresponding dimensions of the first end portion 518b of the second piezoelectric element 516b so that the first end portion 518b of the second piezoelectric Atty Ref. No. A0010209WO01 element 516b fits into but can still move within the second recess 526b. For instance, the height of the second recess 526b can be greater than the height of the first end portion 518b of the second piezoelectric element 516b (e.g., by 10%, 15%, 20%, 25%, or more) to provide sufficient clearance for the second piezoelectric element 516b to slide relative to the support 522. The movement of the second piezoelectric element 516b can still be constrained to some extent, e.g., the dimensions of the second recess 526b can be sufficiently small so that the first end portion 518b of the second piezoelectric element 516b cannot move entirely out of the second recess 526b when the second piezoelectric element 516b slides longitudinally. Additionally, the second piezoelectric element 516b can be movable in certain directions (e.g., longitudinally), but constrained and / or fixed in other directions (e.g., laterally). Referring next to FIGS.5B and 5C together, the piezoelectric elements 516 can be coupled to the harvester mass 524 (also known as a “proof mass” or “inertial mass”). In some embodiments, the first piezoelectric element 516a is fixedly coupled to the harvester mass 524, e.g., using bonding, adhesives, fasteners, interference fit, and / or other suitable rigid attachment techniques. The harvester mass 524 can include a first recess 528a (e.g., a slot, opening, groove, cavity) configured to receive the second end portion 520a of the first piezoelectric element 516a. The first recess 528a can be sufficiently small so that the second end portion 520a of the first piezoelectric element 516a is spatially fixed within the first recess 528a. For instance, the height and width of the first recess 528a can be the same or similar as (e.g., within 10% of) the height and width of the second end portion 520a of the first piezoelectric element 516a. Although the first recess 528a is depicted as extending through the entire thickness of the harvester mass 524, in other embodiments, the first recess 528a can extend through less than the entire thickness of the harvester mass 524 (e.g., the first recess 528a can be formed in the distal surface of the harvester mass 524 and can terminate before the proximal surface of the harvester mass 524). Alternatively, the first recess 528a can be omitted and the first piezoelectric element 516a can instead be coupled to the distal surface of the harvester mass 524. The second piezoelectric element 516b can be movably coupled to the harvester mass 524. For example, the second piezoelectric element 516b can be movable in a longitudinal direction with respect to the harvester mass 524 (e.g., parallel to the longitudinal axis A of the housing 504), such as toward the distal end 506 of the housing Atty Ref. No. A0010209WO01 504 and / or toward the proximal end 508 of the housing 504. Alternatively or in combination, the second piezoelectric element 516b can be movable in other directions relative to the harvester mass 524, such as laterally, rotationally, etc. The harvester mass 524 can include a second recess 528b (e.g., a slot, opening, groove, cavity) configured to receive the second end portion 520b of the second piezoelectric element 516b and allow the second piezoelectric element 516b to move relative to the harvester mass 524. Optionally, a lubricious material (e.g., a polymeric bearing, coating, or surface treatment) can be positioned within the second recess 528b to reduce friction and facilitate movement (e.g., sliding) of the second piezoelectric element 516b. The dimensions of the second recess 528b can be larger than the corresponding dimensions of the second end portion 520b of the second piezoelectric element 516b so that the second end portion 520b of the second piezoelectric element 516b fits into but can still move within the second recess 528b. For instance, the height of the second recess 528b can be greater than the height of the second end portion 520b of the second piezoelectric element 516b (e.g., by 10%, 15%, 20%, 25%, or more) to provide sufficient clearance for the second piezoelectric element 516b to slide relative to the harvester mass 524. The movement of the second piezoelectric element 516b can still be constrained, e.g., the dimensions of the second recess 528b can be sufficiently small so that the second end portion 520b of the second piezoelectric element 516b cannot move entirely out of the second recess 528b when the second piezoelectric element 516b slides longitudinally. Additionally, the second piezoelectric element 516b can be movable in certain directions (e.g., longitudinally), but constrained and / or fixed in other directions (e.g., laterally). In some embodiments, no more than 30%, 25%, 20%, 15%, 10%, or 5% of the total length of the second piezoelectric element 516b is received within the second recess 528b when the second piezoelectric element 516b is in a resting (e.g., undeformed) configuration. Alternatively or in combination, the length of the second end portion 520b of the second piezoelectric element 516b received within the second recess 528b when the second piezoelectric element 516b is in the resting configuration can be less than or equal to 5 mm, 4 mm, 3 mm, 2 mm, or 1 mm. Although the second recess 528b is depicted as extending through the entire thickness of the harvester mass 524, in other embodiments, the second recess 528b can extend through less than the entire thickness of the harvester mass Atty Ref. No. A0010209WO01 524 (e.g., the second recess 528b can be formed in the distal surface of the harvester mass 524 and can terminate before the proximal surface of the harvester mass 524). Moreover, although the second recess 528b is depicted as having the same shape and size as the first recess 528a, in other embodiments, the second recess 528b can have a different shape and / or size than the first recess 528a. Optionally, the second piezoelectric element 516b can be fixedly coupled to the harvester mass 524, e.g., using bonding, adhesives, fasteners, interference fit, and / or other suitable rigid attachment techniques. The second recess 528b can be sufficiently small so that the second end portion 520b of the second piezoelectric element 516b is spatially fixed within the second recess 528b. For instance, the height and width of the second recess 528b can be the same or similar as (e.g., within 10% of) the height and width of the second end portion 520b of the second piezoelectric element 516b. Alternatively, the second recess 528b can be omitted and the second piezoelectric element 516b can instead be coupled to the distal surface of the harvester mass 524. Referring to FIGS.5A–5C together, due to the inertia of the harvester mass 524, when the device 500 is subjected to external forces from physiological motion, the harvester mass 524 can cause displacement of the second end portions 520a, 520b of the piezoelectric elements 516 relative to the housing 504 and to the first end portions 518a, 518b of the piezoelectric elements 516, and thus cause elastic deformation of the piezoelectric elements 516. For instance, one or both of the piezoelectric elements 516 can be deformed from a resting, straightened configuration (shown in FIGS. 5A and 5B) to a bent configuration (e.g., an upwardly bent configuration or a downwardly bent configuration). The resulting mechanical strain in one or both of the piezoelectric elements 516 can produce an electrical current that can be used to charge the power source 512. In some embodiments, the movable couplings at the first end portion 518b and / or the second end portion 520b of the second piezoelectric element 516b cause the second piezoelectric element 516b to move (e.g., slide) relative to the support 522 and / or the harvester mass 524, respectively, when a force is applied to the second piezoelectric element 516b by the harvester mass 524. The movable coupling(s) can reduce the effective overall stiffness of the energy harvesting mechanism 502 (e.g., compared to a configuration in which the second piezoelectric element 516b is fixedly coupled to both the support 522 Atty Ref. No. A0010209WO01 and the harvester mass 524). Stated differently, the movable coupling(s) can reduce the resistance of the second piezoelectric element 516b to the applied force from the harvester mass 524. This configuration can decrease the overall resonant frequency of the energy harvesting mechanism 502 to a range suitable for efficient harvesting from physiological motion. For example, the resonant frequency of the energy harvesting mechanism 502 can be within a range from 1 Hz to 50 Hz, 1 Hz to 30 Hz, 1 Hz to 20 Hz, 1 Hz to 10 Hz, 5 Hz to 10 Hz, 5 Hz to 15 Hz, 10 Hz to 20 Hz, 10 Hz to 15 Hz, 10 Hz to 30 Hz, 15 Hz to 20 Hz, 15 Hz to 25 Hz, 20 Hz to 30 Hz, 20 Hz to 25 Hz, or 25 Hz to 30 Hz. The power source 512 can include one or more rechargeable batteries that are electrically coupled to the energy harvesting mechanism 502 to store the energy produced by the energy harvesting mechanism 502. As best seen in FIG. 5A, the power source 512 can be configured as a hollow (e.g., tubular, annular) structure that surrounds at least a portion of the energy harvesting mechanism 502. For instance, an intermediate portion of the piezoelectric elements 516 between the first end portions 518a, 518b and the second end portions 520a, 520b can be at least partially received within the lumen 530. Optionally, the first end portions 518a, 518b can also be at least partially received within the lumen 530. In some embodiments, at least 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, of the total length of each piezoelectric element 516 is positioned within the lumen 530, and / or no more than 50%, 45%, 40%, 35%, 30%, 25%, 20%, 15%, 10%, 5% of the total length of each piezoelectric element 516 is positioned within the lumen 530. In other embodiments, however, the power source 512 can have a different shape (e.g., a solid shape without any lumen) and / or can be positioned differently with respect to the energy harvesting mechanism 502 (e.g., the piezoelectric elements 516 can be entirely outside of the power source 512). The power source 512 is electrically coupled to the electronics assembly 514 to power the operation thereof (the electronics assembly 514 is shown schematically in FIG. 5B). The electronics assembly 514 can include the electronic components of the device 500, such as any of the components described above with respect to FIG.4 (e.g., switch circuitry 406, sensing circuitry 408, therapy generation circuitry 410, sensors 412, processing circuitry 414, communication circuitry 416, and / or memory 418). The electronics assembly 514 can also include includes power conditioning circuitry, which can be electrically coupled to and interposed between the energy harvesting mechanism 502 and the power Atty Ref. No. A0010209WO01 source 512. The power conditioning circuitry can be configured to perform operations such as rectification, filtering, voltage regulation, etc., of the electrical signal produced by the energy harvesting mechanism 502, before transmission to the power source 512. Optionally, the electronics assembly 514 can also include components (e.g., processing circuitry 414 and / or other circuitry) that perform power management functions, such as monitoring the status of the power source 512 (e.g., the charge level of the power source 512; whether the charge level is increasing, decreasing, or constant; the net current and / or power into the power source 512) and / or monitoring the power output of the energy harvesting mechanism 502 (e.g., amount of current and / or power produced by the energy harvesting mechanism 502), power consumption of the electronics assembly 514, etc. FIGS. 6–10 illustrate additional examples of devices including energy harvesting mechanisms, in accordance with embodiments of the present technology. The devices of FIGS. 6–10 can be generally similar to the device 500 of FIGS. 5A–5C. Accordingly, like numbers (e.g., piezoelectric elements 616 versus piezoelectric elements 516) are used to identify similar or identical components across FIGS. 5A–10, and the following discussion of the embodiments of FIGS. 6–10 will focus on those features that differ from the embodiments described with respect to FIGS. 5A–5C. Any of the features of the embodiments of FIGS.6–10 can be combined with each other and / or with any of the features of the device 500 of FIGS.5A–5C. FIG. 6 is a partially schematic side cross-sectional view of a device 600 including an energy harvesting mechanism 602, in accordance with embodiments of the present technology (certain components of the device 600 are omitted in FIG.6 merely for purposes of simplicity, such as the housing, power source, and electronics assembly). As shown in FIG.6, the device 600 can include a first piezoelectric element 616a and a second piezoelectric element 616b (collectively, “piezoelectric elements 616”) coupled to a first support 632, a second support 634, and a harvester mass 624. In some embodiments, the first piezoelectric element 616a is fixedly coupled to the harvester mass 624 and to one or both of the first and second supports 632, 634, while the second piezoelectric element 616b is movably coupled to two or more of the harvester mass 624, first support 632, or second support 634. This configuration can be used to tune the overall stiffness and resonant frequency of the energy harvesting mechanism 602, as described in further detail below. The use of two supports 632, 634 may provide additional mechanical stability to the Atty Ref. No. A0010209WO01 piezoelectric elements 616, particularly in embodiments where the second piezoelectric element 616b is movably coupled to the first support 632 and / or the second support 634, since the energy harvesting mechanism 602 may otherwise experience large bending stresses at or near the fixed distal end of the first piezoelectric element 616a. The first support 632 and second support 634 can each be part of a housing of the device 600 or part of another component that is in a fixed spatial configuration relative to the housing. For example, the first support 632 and second support 634 can each independently be a bulkhead, interior wall, frame, bracket, etc., within the housing. As shown in FIG. 6, the first support 632 can be located closer to the distal end 606 of the device 600 than the second support 634, such that the first support 632 is distal to the second support 634, the second support 634 is distal to the harvester mass 624, and the harvester mass 624 is located near the proximal end 608 of the device 600. The first support 632 and second support 634 can be positioned at any suitable location in the device 600. In some embodiments, for example, the first support 632 is positioned similarly to the support 522 in FIGS.5A–5C, e.g., the first support 632 can be positioned between a power source and an electronics assembly of the device 600 (not shown), and the second support 634 can be positioned proximal to the power source. In other embodiments, the second support 634 can be positioned similarly to the support 522 in FIGS.5A–5C, e.g., the second support 634 can be positioned between the power source and electronics assembly, and the first support 632 can be positioned distal to the electronics assembly. The piezoelectric elements 616 can each respectively include a first end portion near the distal end 606 of the device 600, a second end portion near the proximal end 608 of the device 600, and an intermediate portion between the first and second end portions. For example, as shown in FIG. 6, the first piezoelectric element 616a includes a first end portion 618a, a second end portion 620a, and an intermediate portion 636a; and the second piezoelectric element 616b includes a first end portion 618b, a second end portion 620b, and an intermediate portion 636b. The first end portions 618a, 618b of the piezoelectric elements 616 can be coupled to the first support 632, the second end portions 620a, 620b of the piezoelectric elements 616 can be coupled to the harvester mass 624, and the intermediate portions 636a, 636b of the piezoelectric elements 616 can be coupled to the second support 634. The couplings between the piezoelectric elements 616, the first support Atty Ref. No. A0010209WO01 632, the second support 634, and the harvester mass 624 can include any suitable combination of movable couplings and fixed couplings. In some embodiments, the first piezoelectric element 616a is fixedly coupled to the first support 632, e.g., using rigid attachment techniques as described herein. The first support 632 can include a first recess 638a (e.g., same or similar to the first recess 526a of FIG.5B) configured to receive the first end portion 618a of the first piezoelectric element 616a. Alternatively, the first recess 638a can be omitted and the first piezoelectric element 616a can instead be coupled to the proximal surface of the first support 632. The coupling between the first piezoelectric element 616a and the first support 632 can be the same or generally similar to the coupling between the first piezoelectric element 516a and the support 522 of FIG.5B. The second piezoelectric element 616b can also be fixedly coupled to the first support 632, e.g., using rigid attachment techniques as described herein or can be movably (e.g., slidably) coupled to the first support 632. The first support 632 can include a second recess 638b (e.g., same or similar to the second recess 526b of FIG.5B) configured to receive the first end portion 618b of the second piezoelectric element 616b. The coupling between the second piezoelectric element 616b and the first support 632 can be the same or generally similar to the coupling between the second piezoelectric element 516b and the support 522 of FIG.5B. In some embodiments, the first piezoelectric element 616a is fixedly coupled to the harvester mass 624 e.g., using rigid attachment techniques as described herein. The harvester mass 624 can include a first recess 628a (e.g., same or similar to the first recess 528a of FIGS. 5B and 5C) configured to receive the second end portion 620a of the first piezoelectric element 616a. Alternatively, the first recess 628a can be omitted and the first piezoelectric element 616a can instead be coupled to the distal surface of the harvester mass 624. The coupling between the first piezoelectric element 616a and the harvester mass 624 can be the same or generally similar to the coupling between the first piezoelectric element 516a and the harvester mass 524 of FIGS.5A–5C. The second piezoelectric element 616b can be movably (e.g., slidably) coupled to the harvester mass 624 or can be fixedly (e.g., rigidly) coupled to the harvester mass 624. The harvester mass 624 can include a second recess 628b (e.g., same or similar Atty Ref. No. A0010209WO01 to the second recess 528b of FIGS.5B and 5C) configured to receive the second end portion 620b of the second piezoelectric element 616b. Alternatively, the second recess 628b can be omitted and the second piezoelectric element 616b can instead be coupled to the distal surface of the harvester mass 624. The coupling between the first piezoelectric element 616a and the harvester mass 624 can be the same or generally similar to the coupling between the second piezoelectric element 516b and the harvester mass 524 of FIGS.5A–5C. In some embodiments, the first piezoelectric element 616a is movably coupled to the second support 634. The second support 634 can include a first recess 640a (e.g., a slot, opening, groove, cavity) that extends through the entire thickness of the second support 634. The first recess 640a can be configured to receive the intermediate portion 636a of the first piezoelectric element 616a and allow the first piezoelectric element 616a to move in a longitudinal direction with respect to the first recess 640a. Alternatively or in combination, the first piezoelectric element 616a can be movable in other directions relative to the second support 634, such as laterally, rotationally, etc. In such embodiments, the dimensions of the first recess 640a can be larger than the corresponding dimensions of the intermediate portion 636a of the first piezoelectric element 616a so that the intermediate portion 636a of the first piezoelectric element 616a fits into but can still move within the first recess 640a. For instance, the height of the first recess 640a can be greater than the height of the intermediate portion 636a of the first piezoelectric element 616a (e.g., by 10%, 15%, 20%, 25%, or more) to provide sufficient clearance for the first piezoelectric element 616a to slide relative to the second support 634. Optionally, a lubricious material can be positioned within the first recess 640a as described herein to reduce friction and facilitate movement of the first piezoelectric element 616a. Optionally, the first piezoelectric element 616a can instead be fixedly coupled to the second support 634, e.g., using rigid attachment techniques as described herein. In some embodiments, the first recess 640a is sufficiently small so that the intermediate portion 636a of the first piezoelectric element 616a is spatially fixed within the first recess 640a. For instance, the height and width of the first recess 640a can be the same or similar as (e.g., within 10% of) the height and width of the intermediate portion 636a of the first piezoelectric element 616a. Atty Ref. No. A0010209WO01 The second piezoelectric element 616b can be movably coupled to the second support 634. The second support 634 can include a second recess 640b that extends through the entire thickness of the second support 634. The second support 634 can be configured to receive the intermediate portion 636b of the second piezoelectric element 616b and allow the second piezoelectric element 616b to move in a longitudinal direction with respect to the second recess 640b. Alternatively or in combination, the second piezoelectric element 616b can be movable in other directions relative to the second support 634, such as laterally, rotationally, etc. In such embodiments, the dimensions of the second recess 640b can be larger than the corresponding dimensions of the intermediate portion 636b of the second piezoelectric element 616b so that the intermediate portion 636b of the second piezoelectric element 616b fits into but can still move within the second recess 640b. For instance, the height of the second recess 640b can be greater than the height of the intermediate portion 636b of the second piezoelectric element 616b (e.g., by 10%, 15%, 20%, 25%, or more) to provide sufficient clearance for the second piezoelectric element 616b to slide relative to the second support 634. Optionally, a lubricious material can be positioned within the second recess 640b as described herein to reduce friction and facilitate movement of the second piezoelectric element 616b. Moreover, although the second recess 636b is depicted as having the same shape and size as the first recess 636a, in other embodiments, the second recess 636b can have a different shape and / or size than the first recess 636a. In some embodiments, the movable couplings at the first end portion 618b, the intermediate portion 636b, and / or the second end portion 620b of the second piezoelectric element 616b cause the second piezoelectric element 616b to move (e.g., slide) relative to the first support 632, the second support 634, and / or the harvester mass 624, respectively, when a force is applied to the second piezoelectric element 616b by the harvester mass 624. The movable coupling(s) can reduce the effective overall stiffness of the energy harvesting mechanism 602 (e.g., compared to a configuration in which the second piezoelectric element 616b is fixedly coupled to the first support 632, the second support 634, and the harvester mass 624). This configuration can decrease the overall resonant frequency of the energy harvesting mechanism 602 to a range suitable for efficient harvesting from physiological motion as described herein, while enhancing the mechanical stability of the energy harvesting mechanism 602. Atty Ref. No. A0010209WO01 FIGS. 7A and 7B illustrate a device 700 including an energy harvesting mechanism 702, in accordance with embodiments of the present technology. Specifically, FIG.7A is a side cross-sectional view of the device 700 and FIG.7B is a partially schematic side cross-sectional view of the device 700 (certain components of the device 700 are omitted in FIG. 7B merely for purposes of simplicity, such as the housing, power source, and electronics assembly). The device 700 can be generally similar to the device 500 of FIGS.5A–5C, except that the device 700 includes three piezoelectric elements 716a–c (collectively, “piezoelectric elements 716”). Each of the piezoelectric elements 716 can be made partially or entirely out of any suitable piezoelectric material and / or can have any suitable geometry, as described herein. In some embodiments, some or all of the piezoelectric elements 716 have the same characteristics (e.g., with respect to materials, geometry, and / or mechanical properties), while in other embodiments, some or all of the piezoelectric elements 716 can have the different characteristics (e.g., with respect to materials, geometry, and / or mechanical properties). The characteristics of the piezoelectric elements 716 can be selected to provide a desired resonant frequency for the energy harvesting mechanism 702. The first piezoelectric element 716a can have a first end portion 718a and a second end portion 720a, the second piezoelectric element 716b can have a first end portion 718b and a second end portion 720b, and the third piezoelectric element 716c can have a first end portion 718c and a second end portion 720c. The piezoelectric elements 716 can be arranged in a parallel or generally parallel configuration in which the first end portions 718a–718c of the piezoelectric elements 716 are located near the distal end 706 of the housing 704, and the second end portions 720a–720c of the piezoelectric elements 716 are located near the proximal end 708 of the housing 704. In some embodiments, the first piezoelectric element 716a is interposed between the second piezoelectric element 716b and the third piezoelectric element 716c (e.g., the second piezoelectric element 716b can be positioned above the first piezoelectric element 716a and the third piezoelectric element 716c can be positioned below the first piezoelectric element 716a). In other embodiments, however, the piezoelectric elements 716 can be oriented differently with respect to each other and / or to the housing 704 (e.g., some or all of the piezoelectric elements 716 can face different directions, as discussed further below). Atty Ref. No. A0010209WO01 In some embodiments, the piezoelectric elements 716 are each coupled to a support 722 and to a harvester mass 724. For instance, the first end portions 718a–718c of the piezoelectric elements 716 can be coupled to the support 722, and the second end portions 720a–720c of the piezoelectric elements 716 can be coupled to the harvester mass 724. The couplings between the piezoelectric elements 716, the support 722, and the harvester mass 724 can include any suitable combination of movable couplings and fixed couplings. For example, in some embodiments, the first piezoelectric element 716a is fixedly coupled to both the support 722 and the harvester mass 724, while the second piezoelectric element 716b is movably coupled to the support 722 and / or the harvester mass 724. The couplings between the first piezoelectric element 716a, second piezoelectric element 716b, support 722, and harvester mass 724 can be the same or similar to the couplings between the corresponding components in FIGS.5A–5C. The third piezoelectric element 716c can be fixedly coupled to both the support 722 and the harvester mass 724 (e.g., similar to the first piezoelectric element 716a), or can include at least one movable coupling (e.g., similar to the second piezoelectric element 716b). This configuration can be used to tune the overall stiffness of the energy harvesting mechanism 702 and, thus, the resonant frequency of the energy harvesting mechanism 702, as described in further detail below. For example, the third piezoelectric element 716c can be fixedly coupled to the support 722 using rigid attachment techniques, as described herein. The support 722 can include a third recess 726c (e.g., similar to the first recess 726a) configured to receive the first end portion 718c of the third piezoelectric element 716c. The third recess 726c can be sufficiently small so that the first end portion 718c of the third piezoelectric element 716c is spatially fixed within the third recess 726c. Although the third recess 726c is depicted as having the same shape and size as the first and second recesses 726a, 726b, in other embodiments, the third recess 726c can have a different shape and / or size than the first and / or second recesses 726a, 726b. Alternatively, the third recess 726c can be omitted and the third piezoelectric element 716c can instead be coupled to the proximal surface of the support 722. Atty Ref. No. A0010209WO01 Optionally, the third piezoelectric element 716c can instead be movably coupled to the support 722. In such embodiments, the dimensions of the third recess 726c can be larger than the corresponding dimensions of the first end portion 718c of the third piezoelectric element 716c so that the first end portion 718c of the third piezoelectric element 716c fits into but can still move within the third recess 726c. Movement of the third piezoelectric element 716c can still be constrained, e.g., the dimensions of the third recess 726c can be sufficiently small so that the first end portion 718c of the third piezoelectric element 716c cannot move entirely out of the third recess 726c when the third piezoelectric element 716c slides longitudinally. Optionally, a lubricious material can be positioned within the third recess 726c as described herein to reduce friction and facilitate movement (e.g., sliding) of the third piezoelectric element 716c. In some embodiments, the third piezoelectric element 716c is movably coupled to the harvester mass 724. For example, the third piezoelectric element 716c can be movable in a longitudinal direction with respect to the harvester mass 724 (e.g., parallel to the longitudinal axis A2of the housing 704), such as toward the distal end 706 of the housing 704 and / or toward the proximal end 708 of the housing 704. The harvester mass 724 can include a third recess 728c (e.g., similar to the second recess 728b) configured to receive the second end portion 720c of the third piezoelectric element 716c and allow the third piezoelectric element 716c to move relative to the harvester mass 724. Optionally, a lubricious material can be positioned within the third recess 728c as described herein to reduce friction and facilitate movement (e.g., sliding) of the third piezoelectric element 716c. The dimensions of the third recess 728c can be larger than the corresponding dimensions of the second end portion 720c of the third piezoelectric element 716c so that the second end portion 720c of the third piezoelectric element 716c fits into but can still move within the third recess 728c. The movement of the third piezoelectric element 716c can still be constrained, e.g., the dimensions of the third recess 728c can be sufficiently small so that the second end portion 720c of the third piezoelectric element 716c cannot move entirely out of the third recess 728c when the third piezoelectric element 716c slides longitudinally. Although the third recess 728c is depicted as having the same shape and size as the second recess 728b, in other embodiments, the third recess 728c can have a different shape and / or size than the second recess 728b. Atty Ref. No. A0010209WO01 Optionally, the third piezoelectric element 716c can be fixedly coupled to the harvester mass 724, e.g., using rigid attachment techniques as described herein. In such embodiments, the third recess 728c can be sufficiently small so that the second end portion 720c of the third piezoelectric element 716c is spatially fixed within the third recess 728c. For instance, the height and width of the third recess 728c can be the same or similar as (e.g., within 10% of) the height and width of the second end portion 720c of the third piezoelectric element 716c. Alternatively, the third recess 728c can be omitted and the third piezoelectric element 716c can instead be coupled to the distal surface of the harvester mass 724. The movable coupling(s) can reduce the effective overall stiffness of the energy harvesting mechanism 702 (e.g., compared to a configuration in which the piezoelectric elements 716 are all fixedly coupled to both the support 722 and the harvester mass 724). Stated differently, the movable coupling(s) and orientation of fixed / movable piezoelectric elements can reduce the resistance of the movable piezoelectric elements to the applied force from the harvester mass 724. This configuration can decrease the overall resonant frequency of the energy harvesting mechanism 702 to a range suitable for efficient harvesting from physiological motion. For example, the resonant frequency of the energy harvesting mechanism 702 can be within a range from 1 Hz to 50 Hz, 1 Hz to 30 Hz, 1 Hz to 20 Hz, 1 Hz to 10 Hz, 5 Hz to 10 Hz, 5 Hz to 15 Hz, 10 Hz to 20 Hz, 10 Hz to 15 Hz, 10 Hz to 30 Hz, 15 Hz to 20 Hz, 15 Hz to 25 Hz, 20 Hz to 30 Hz, 20 Hz to 25 Hz, or 25 Hz to 30 Hz. The combination of fixed couplings and movable couplings of the piezoelectric elements 716 can be selected to modify the effective overall stiffness and resonant frequency of the energy harvesting mechanism 702. For example, the energy harvesting mechanism 702 may be stiffer and thus have a higher resonant frequency if two of the piezoelectric elements 716 are “fixed,” e.g., if the first piezoelectric element 716a and the third piezoelectric element 716b are both fixedly coupled to the support 722 and to the harvester mass 724. Conversely, the energy harvesting mechanism 702 can be more flexible and thus have a lower resonant frequency if two of the piezoelectric elements 716 are “movable,” e.g., the second piezoelectric element 716b and the third piezoelectric element 716 each include at least one movable coupling to the support 722 or the harvester mass 724. Atty Ref. No. A0010209WO01 The characteristics of the energy harvesting mechanism 702 can also be tuned by altering the arrangement of the piezoelectric elements 716. For example, in embodiments where the energy harvesting mechanism 702 includes a single fixed piezoelectric element (e.g., the first piezoelectric element 716a), the fixed piezoelectric element can be the middle piezoelectric element (e.g., as shown in FIGS.7A and 7B), while the movable piezoelectric elements (e.g., the second and third piezoelectric elements 716b, 716c) can be positioned on opposite sides of the fixed piezoelectric element. Alternatively, the fixed piezoelectric element can be the upper piezoelectric element and the movable piezoelectric elements can both be below the fixed piezoelectric element, or the fixed piezoelectric element can be the lower piezoelectric element and the movable piezoelectric elements can both be above the fixed piezoelectric element. As another example, in embodiments where the energy harvesting mechanism 702 includes a single movable piezoelectric element (e.g., the second piezoelectric element 716b), the movable piezoelectric element can be the upper piezoelectric element (e.g., as shown in FIGS. 7A and 7B), while the fixed piezoelectric elements (e.g., the first and third piezoelectric elements 716a, 716c) can be positioned below the movable piezoelectric element. Alternatively, the movable piezoelectric element can be the lower piezoelectric element and the movable piezoelectric elements can both be above the fixed piezoelectric element, or the movable piezoelectric element can be the middle piezoelectric element and the fixed piezoelectric elements can be on opposite sides of the movable piezoelectric element. FIGS. 8A and 8B illustrate a device 800 including an energy harvesting mechanism 802, in accordance with embodiments of the present technology. Specifically, FIG.8A is a partially schematic side cross-sectional view of the device 800, and FIG.8B is a partially schematic end cross-sectional view of the device 800. The device 800 can be generally similar to the device 700 of FIGS.7A and 7B, except that the device 800 includes a roller assembly 842 (certain components of the device 800 are omitted in FIGS.8A and 8B merely for purposes of simplicity, such as the housing, power source, and electronics assembly). The energy harvesting mechanism 802 includes three piezoelectric elements 816a–816c (collectively “piezoelectric elements 816”). As described herein, the first Atty Ref. No. A0010209WO01 piezoelectric element 816a can be a fixed piezoelectric element that is fixedly coupled to the support 822 and the harvester mass 824, the second piezoelectric element 816b can be a movable piezoelectric element that is movably coupled to the support 822 and / or the harvester mass 824, and the third piezoelectric element 816c can be a fixed piezoelectric element or a movable piezoelectric element. The roller assembly 842 can be configured to hold the piezoelectric elements 816 together so that the movable piezoelectric element(s) follow the motion of the fixed piezoelectric element(s), while still permitting longitudinal movement (e.g., sliding) of the movable piezoelectric element(s). This configuration can ensure that each of the piezoelectric elements 816 is sufficiently deflected to generate power and improve harvesting efficiency. As shown in FIGS. 8A and 8B, the roller assembly 842 can be positioned proximal to the support 822 and distal to the harvester mass 824 such that the piezoelectric elements 816 extend through the roller assembly 842. The spacing between the support 822, the roller assembly 842, and the harvester mass 824, can be varied as desired. In some embodiments, the roller assembly 842 can be positioned closer to the support 822 than the harvester mass 824. Alternatively, the roller assembly 842 can be positioned closer to the harvester mass 824 than the support 822. The roller assembly 842 can be aligned with the support 822. For example, the vertical axis of the roller assembly 842 can be parallel to the vertical axis of the support 822. In some embodiments, the roller assembly 842 includes a frame 844 coupled to a plurality of rollers 846a–846d (collectively, “rollers 846”). Each roller 846 can be interposed between at least one piezoelectric element 816 and / or part of the frame 844. For example, the first roller 846a can be positioned between the frame 844 and the second piezoelectric element 816b. The second roller 846b can be positioned between the second piezoelectric element 816b and the first piezoelectric element 816a. The third roller 846c can be positioned between the first piezoelectric element 816a and the third piezoelectric element 816c. The fourth roller 846d can be positioned between the third piezoelectric element 816c and the frame 844. Although the roller assembly 842 is depicted as including four rollers 846, in other embodiments, the roller assembly 842 can include a different number of rollers 846 (e.g., one, two, three, five, or more rollers 846), depending on the number of piezoelectric elements 816. Atty Ref. No. A0010209WO01 The rollers 846 can have any geometry and spatial configuration allowing for movement and support of the piezoelectric elements 816. For example, the rollers 846 can permit longitudinal movement of one or more of the piezoelectric elements 816 (e.g., the movable piezoelectric element(s)), while constraining vertical movement of the piezoelectric elements 816 relative to each other. In the illustrated embodiment, the rollers 846 are cylindrical so that the rollers 846 rotate as the one or more of the piezoelectric elements 816 slide relative to the support 822, harvester mass 824, and / or to each other. The spacing between adjacent rollers 846 can be sufficiently small so that each roller 846 remains in contact with the corresponding piezoelectric element(s) 816 even when the piezoelectric elements 816 are deflected by the harvester mass 824. Optionally, the rollers 846 can be or include a lubricious material (e.g., a polymeric bearing, coating, or surface treatment) to reduce friction and facilitate movement of the piezoelectric elements 816. The rollers 846 can be mounted in the frame 844, which can be any structure suitable for supporting the rollers 846. Although the frame 844 is depicted in FIG. 8B as having a square shape, in other embodiments, the frame 844 can have a different geometry (e.g., rectangular, circular, oval). As best seen in FIG.8B, the rollers 844 can be coupled to the frame 844 at their respective ends. The coupling between the rollers 844 and frame 844 can be rotary couplings so that the rollers 844 can each rotate independently relative to the frame 844. In the embodiment of FIGS.8A and 8B, the frame 844 is not directly coupled to any other part of the device 800 besides the rollers 846, such that the roller assembly 842 can move vertically as the piezoelectric elements 816 are deflected upward and / or downward during operation. The roller assembly 842 can also move in a longitudinal direction, e.g., toward the support 822 and / or toward the harvester mass 824. In other embodiments, however, the frame 844 can be directly coupled to another part of the device 800 besides the rollers 846 to constrain the movement of the roller assembly 842, as described in further detail below with respect to FIGS.9A and 9B. FIG. 9A is a partially schematic illustration of a device 900a including an energy harvesting mechanism 902, in accordance with embodiments of the present technology. The device 900a is generally similar to the device 800 of FIGS. 8A and 8B, except that the roller assembly 942 of the device 900a is coupled to the housing 904 of the Atty Ref. No. A0010209WO01 device 900a via one or more connectors 948 (shown schematically). The connectors 948 can be wires, rods, beams, spacers, standoffs, links, or any other suitable structures that mechanically couple the roller assembly 942 to the housing 904 to constrain the movement of the roller assembly 942. As shown in FIG.9A, each connector 948 can include a first end coupled to the frame 944 and a second end coupled to the housing 904, in accordance with any of attachment techniques described herein. The connectors 948 can constrain or prevent movement of the roller assembly 942 in a longitudinal direction (e.g., toward the support 922 and / or the harvester mass 924) but can permit at least some movement in a vertical direction to accommodate the upward and / or downward deflection of the piezoelectric elements 916. For example, at least part of the connectors 948 can be a flexible and / or compressible structure (e.g., a spring or other component made of a flexible and / or compressible material) to allow for vertical movement of the roller assembly 942. In some embodiments, when the piezoelectric elements 916 are deflected upward, the upper connector 948 is compressed while the lower connector 948 is stretched; and when the piezoelectric elements 916 are deflected downward, the upper connector 948 is stretched while the lower connector 948 is compressed. FIG. 9B is a partially schematic illustration of a device 900b including an energy harvesting mechanism 902, in accordance with embodiments of the present technology. The device 900 is generally similar to the device 800 of FIGS. 8A and 8B, except that the roller assembly 942 of the device 900b is coupled to the support 922 via one or more connectors 950 (shown schematically). The connectors 950 can be wires, rods, beams, spacers, standoffs, links, or any other suitable structures that mechanically couple the roller assembly 942 to the support 922 to constrain the movement of the roller assembly 942. As shown in FIG.9B, each connector 950 can include a first end coupled to the frame 944 and a second end coupled to the support 922, in accordance with any of attachment techniques described herein. The connectors 950 can constrain or prevent movement of the roller assembly 942 in a longitudinal direction (e.g., toward the support 922 and / or the harvester mass 924) but can permit at least some movement in a vertical direction to accommodate the upward and / or downward deflection of the piezoelectric elements 916. For example, at least part of the connectors 950 can be a flexible structure (e.g., spring or other component made of a flexible material) to allow for vertical movement of the roller assembly 942. In some embodiments, when the piezoelectric elements 916 are deflected Atty Ref. No. A0010209WO01 upward, the connectors 950 also bend upward; and when the piezoelectric elements 916 are deflected downward, the connectors 950 also bend downward. Although FIGS. 5A–9B illustrate energy harvesting mechanisms in which the piezoelectric elements all face the same direction, any of the energy harvesting mechanisms described herein can be modified to include one or more piezoelectric elements that face different directions. This configuration can be advantageous for reducing the directional dependency of energy harvesting. In some embodiments, each piezoelectric element is characterized by a principal bending direction which corresponds to the direction of input motion that maximizes the power output of the piezoelectric element and / or minimizes the amount of force to deflect the piezoelectric element. For example, if the piezoelectric element is a flattened beam, the principal bending direction can be the direction orthogonal to the plane of the beam. When the direction of input motion is offset from the principal bending direction, the power output of the piezoelectric element can be reduced and / or more force may be required to deflect the piezoelectric element. In some instances, the power output can be minimal when the direction of input motion is orthogonal to the principal bending direction. Accordingly, it can be advantageous to arrange the piezoelectric elements to have different principal bending directions so that regardless of the direction of input motion, at least one piezoelectric element is sufficiently deflected to provide at least a minimum level of power output. For example, FIG.10 is a partially schematic end view of three piezoelectric elements 1016a–c (collectively, “piezoelectric elements 1016”) with different principal bending directions, in accordance with embodiments of the present technology. The piezoelectric elements 1016a–c can include any suitable combination of fixed and movable piezoelectric elements, as described herein. As shown in FIG.10, the piezoelectric elements 1016 can each have a flattened body (e.g., a beam, strip, film, plate) that defines a respective plane, e.g., the first piezoelectric element 1016a defines a first plane 1052a, the second piezoelectric element 1016b defines a second plane 1052b, and the third piezoelectric element 1016c defines a third plane 1052c. Accordingly, the principal bending direction of the first piezoelectric element 1016a can be a first direction 1054a that is orthogonal to the first plane 1052a, the principal bending direction of the second piezoelectric element 1016b can be a second direction 1054b that is orthogonal to the second plane 1052b, and the Atty Ref. No. A0010209WO01 principal bending direction of the third piezoelectric element 1016c can be a third direction 1054c that is orthogonal to the third plane 1052c. In the illustrated embodiment, the piezoelectric elements 1016 face different directions, such that the principal bending directions of the piezoelectric elements 1016 differ from each other, thus allowing for efficient energy harvesting from different directions of input motion. For instance, the piezoelectric elements 1016 can have different orientations so that the first plane 1052a, second plane 1052b, and third plane 1052c are offset from each other, such as by an angle greater than or equal to 60°, 70°, 80°, 90°, 100°, 110°, 120°, 130°, 140°, 150, 160°, or 170°. Accordingly, the first direction 1054a, second direction 1054b, and third direction 1054c are also offset from each other, such as by an angle greater than or equal to 5°, 10°, 20°, 30°, 40°, 50°, 60°, 70°, 80°, 90°, 100°, 110°, or 120°. In the illustrated embodiment, the piezoelectric elements 1016 are arranged in a radially symmetric configuration, such that the angle between neighboring planes is 60° and the principal bending directions are offset by 120°. In other embodiments, however, the piezoelectric elements 1016 can be arranged differently, such as in a non-radially symmetric configuration, an overlapping configuration, etc. The energy harvesting mechanisms herein can include any suitable number of piezoelectric elements that face different directions and / or have different principal bending directions, such as two, three four, five, six, seven, nine, 10, 20, or more piezoelectric elements. The arrangement of the piezoelectric elements can vary depending on the number of elements, e.g., two piezoelectric elements can be arranged in a cross configuration, three piezoelectric elements can be arranged in a triangular configuration, four piezoelectric elements can be arranged in a square configuration, five piezoelectric elements can be arranged in a pentagon configuration, and so on. Additional examples of configurations for energy harvesting mechanisms including piezoelectric elements with different principal bending directions are described in U.S. Provisional Application No. 63 / 496,471, filed April 17, 2023, the disclosure of which is incorporated herein by reference in its entirety. Although FIGS.5A–10 illustrate energy harvesting mechanisms with two or three piezoelectric elements, respectively, the techniques described herein can be applied to energy harvesting mechanisms with any suitable number of piezoelectric elements (e.g., Atty Ref. No. A0010209WO01 four, five, six, seven, eight, nine, 10, 15, 20, or more piezoelectric elements). An energy harvesting mechanism can include any suitable combination of fixed and movable piezoelectric elements, such as one, two, three, four, five, or more fixed piezoelectric elements; and / or one, two three, four, five, or more movable piezoelectric elements. The number and arrangement of fixed and movable piezoelectric elements can be selected to produce a desired overall stiffness and resonant frequency of the energy harvesting mechanism. Examples The following examples are included to further describe some aspects of the present technology, and should not be used to limit the scope of the technology. Example 1. A device comprising: a housing configured to be implanted within a patient; a power source positioned within the housing; and an energy harvesting mechanism positioned within the housing and configured to charge the power source, wherein the energy harvesting mechanism comprises: a first piezoelectric element including a first portion fixedly coupled to a support within the housing and a second portion fixedly coupled to a harvester mass, and a second piezoelectric element including a first portion coupled to the support and a second portion coupled to the harvester mass, wherein: (a) the first portion of the second piezoelectric element is slidably coupled to the support, (b) the second portion of the second piezoelectric element is slidably coupled to the harvester mass, or (c) both (a) and (b). Example 2. The device of Example 1, wherein the second portion of the second piezoelectric element is slidably coupled to the harvester mass. Example 3. The device of Example 2, wherein the harvester mass comprises a recess formed therein, and the second portion of the second piezoelectric element is received within the recess. Atty Ref. No. A0010209WO01 Example 4. The device of Example 3, further comprising a lubricious material within the recess. Example 5. The device of any one of Examples 1 to 4, wherein the first portion of the second piezoelectric element is fixedly coupled to the support. Example 6. The device of any one of Examples 1 to 4, wherein the first portion of the second piezoelectric element is slidably coupled to the support. Example 7. The device of any one of Examples 1 to 6, wherein: the first piezoelectric element comprises a third portion coupled to a second support within the housing, and the second piezoelectric element comprises a third portion coupled to the second support. Example 8. The device of Example 7, wherein the second support is positioned between the first support and the harvester mass, or wherein the first support is positioned between the second support and the harvester mass. Example 9. The device of any one of Examples 1 to 8, wherein the first and second piezoelectric elements each comprise a longitudinal axis aligned with a longitudinal axis of the housing. Example 10. The device of any one of Examples 1 to 9, wherein the first and second piezoelectric elements comprise different materials. Example 11. The device of any one of Examples 1 to 10, further comprising a third piezoelectric element including a first portion coupled to the support and a second portion coupled to the harvester mass. Example 12. The device of Example 11, wherein the first, second, and third piezoelectric elements face different directions. Example 13. A device comprising: a housing configured to be implanted within a patient; a power source positioned within the housing; and an energy harvester positioned within the housing and configured to charge the power source, wherein the energy harvester comprises: a harvester mass, Atty Ref. No. A0010209WO01 a first piezoelectric member rigidly coupled to a support within the housing and to a harvester mass, and a second piezoelectric member coupled to the support and to the harvester mass, wherein the second piezoelectric member is movably coupled to one or more of the support or the harvester mass. Example 14. The device of Example 13, wherein the second piezoelectric member is movably coupled to the harvester mass. Example 15. The device of Example 14, wherein the harvester mass comprises a slot formed therein, and the second piezoelectric member extends partially into the slot. Example 16. The device of Example 15, wherein the second piezoelectric member is movable in a longitudinal direction within the slot. Example 17. The device of Example 15 or 16, further comprising a polymeric bearing positioned within the slot. Example 18. The device of any one of Examples 13 to 17, wherein the second piezoelectric member is rigidly coupled to the support. Example 19. The device of any one of Examples 13 to 17, wherein the second piezoelectric member is movably coupled to the support. Example 20. The device of any one of Examples 13 to 19, wherein: the first piezoelectric member is coupled to a second support within the housing, and the second piezoelectric member is coupled to the second support. Example 21. The device of Example 20, wherein the second support is configured to constrain movement of the second piezoelectric member relative to the first piezoelectric member. Example 22. The device of any one of Examples 13 to 21, wherein the first and second piezoelectric members are electrically coupled to the power source in parallel. Example 23. The device of any one of Examples 13 to 22, wherein the first and second piezoelectric members comprise different stiffnesses. Atty Ref. No. A0010209WO01 Example 24. The device of any one of Examples 13 to 23, further comprising one or more additional piezoelectric members coupled to the support and to the harvester mass. Conclusion Although many of the embodiments are described above with respect to systems, devices, and methods for cardiac pacing, the technology is applicable to other applications and / or other approaches, such as other therapies involving implantable devices. Moreover, other embodiments in addition to those described herein are within the scope of the technology. Additionally, several other embodiments of the technology can have different configurations, components, or procedures than those described herein. A person of ordinary skill in the art, therefore, will accordingly understand that the technology can have other embodiments with additional elements, or the technology can have other embodiments without several of the features shown and described above with reference to FIGS.1–10. The embodiments of the present technology can be implemented, at least in part, in hardware, software, firmware or any combination thereof. For example, various embodiments can be implemented within one or more processors, including one or more microprocessors, DSPs, ASICs, FPGAs, or any other equivalent integrated or discrete logic circuitry, as well as any combinations of such components, embodied in programmers (e.g., physician or patient programmers), stimulators, or other devices. The terms “processor” or “processing circuitry” may generally refer to any of the foregoing logic circuitry, alone or in combination with other logic circuitry, or any other equivalent circuitry. The various processes described herein can be partially or fully implemented using program code including instructions executable by one or more processors of a computing system for implementing specific logical functions or steps in the process. The program code can be stored on any type of computer-readable medium, such as a storage device including a disk or hard drive. Computer-readable media containing code, or portions of code, can include any appropriate media known in the art, such as non-transitory computer-readable storage media. Computer-readable media can include volatile and non- volatile, removable and non-removable media implemented in any method or technology for storage and / or transmission of information, including, but not limited to, random-access Atty Ref. No. A0010209WO01 memory (RAM), read-only memory (ROM), electrically erasable programmable read-only memory (EEPROM), flash memory, or other memory technology; compact disc read-only memory (CD-ROM), digital video disc (DVD), or other optical storage; magnetic cassettes, magnetic tape, magnetic disk storage, or other magnetic storage devices; solid state drives (SSD) or other solid state storage devices; or any other medium which can be used to store the desired information and which can be accessed by a system device. The descriptions of embodiments of the technology are not intended to be exhaustive or to limit the technology to the precise form disclosed above. Where the context permits, singular or plural terms may also include the plural or singular term, respectively. Although specific embodiments of, and examples for, the technology are described above for illustrative purposes, various equivalent modifications are possible within the scope of the technology, as those skilled in the relevant art will recognize. For example, while steps are presented in a given order, alternative embodiments may perform steps in a different order. The various embodiments described herein may also be combined to provide further embodiments. As used herein, the terms “generally,” “substantially,” “about,” and similar terms are used as terms of approximation and not as terms of degree, and are intended to account for the inherent variations in measured or calculated values that would be recognized by those of ordinary skill in the art. Moreover, unless the word “or” is expressly limited to mean only a single item exclusive from the other items in reference to a list of two or more items, then the use of “or” in such a list is to be interpreted as including (a) any single item in the list, (b) all of the items in the list, or (c) any combination of the items in the list. As used herein, the phrase “and / or” as in “A and / or B” refers to A alone, B alone, and A and B. Additionally, the term “comprising” is used throughout to mean including at least the recited feature(s) such that any greater number of the same feature and / or additional types of other features are not precluded. To the extent any materials incorporated herein by reference conflict with the present disclosure, the present disclosure controls. It will also be appreciated that specific embodiments have been described herein for purposes of illustration, but that various modifications may be made without Atty Ref. No. A0010209WO01 deviating from the technology. Further, while advantages associated with certain embodiments of the technology have been described in the context of those embodiments, other embodiments may also exhibit such advantages, and not all embodiments need necessarily exhibit such advantages to fall within the scope of the technology. Accordingly, the disclosure and associated technology can encompass other embodiments not expressly shown or described herein. The following examples are a non-limiting list of clauses in accordance with one or more techniques of this disclosure. Example 1. A device comprising: a housing configured to be implanted within a patient; a power source positioned within the housing; and an energy harvesting mechanism positioned within the housing and configured to charge the power source, wherein the energy harvesting mechanism comprises: a first piezoelectric element including a first portion fixedly coupled to a support within the housing and a second portion fixedly coupled to a harvester mass, and a second piezoelectric element including a first portion coupled to the support and a second portion coupled to the harvester mass, wherein: (a) the first portion of the second piezoelectric element is slidably coupled to the support, (b) the second portion of the second piezoelectric element is slidably coupled to the harvester mass, or (c) both (a) and (b). Example 2. The device of Example 1, wherein the second portion of the second piezoelectric element is slidably coupled to the harvester mass. Example 3. The device of Example 2, wherein the harvester mass comprises a recess formed therein, and the second portion of the second piezoelectric element is received within the recess. Example 4. The device of Example 3, further comprising a lubricious material within the recess. Example 5. The device of any one of Examples 1 to 4, wherein the first portion of the second piezoelectric element is fixedly coupled to the support. Example 6. The device of any one of Examples 1 to 4, wherein the first portion of the second piezoelectric element is slidably coupled to the support. Atty Ref. No. A0010209WO01 Example 7. The device of any one of Examples 1 to 6, wherein: the first piezoelectric element comprises a third portion coupled to a second support within the housing, and the second piezoelectric element comprises a third portion coupled to the second support. Example 8. The device of Example 7, wherein the second support is positioned between the first support and the harvester mass, or wherein the first support is positioned between the second support and the harvester mass. Example 9. The device of any one of Examples 1 to 8, wherein the first and second piezoelectric elements each comprise a longitudinal axis aligned with a longitudinal axis of the housing. Example 10. The device of any one of Example s 1 to 9, wherein the first and second piezoelectric elements comprise different materials. Example 11. The device of any one of Examples 1 to 10, further comprising a third piezoelectric element including a first portion coupled to the support and a second portion coupled to the harvester mass. Example 12. The device of Example 11, wherein the first, second, and third piezoelectric elements face different directions. Example 13. A device comprising: a housing configured to be implanted within a patient; a power source positioned within the housing; and an energy harvester positioned within the housing and configured to charge the power source, wherein the energy harvester comprises: a harvester mass, a first piezoelectric member rigidly coupled to a support within the housing and to a harvester mass, and a second piezoelectric member coupled to the support and to the harvester mass, wherein the second piezoelectric member is movably coupled to one or more of the support or the harvester mass. Example 14. The device of Example 13, wherein the second piezoelectric member is movably coupled to the harvester mass Example 15. The device of Example 14, wherein the harvester mass comprises a slot formed therein, and the second piezoelectric member extends partially into the slot. Atty Ref. No. A0010209WO01 Example 16. The device of Example 15, wherein the second piezoelectric member is movable in a longitudinal direction within the slot. Example 17. The device of Example 15 or 16, further comprising a polymeric bearing positioned within the slot. Example 18. The device of any one of Examples 13 to 17, wherein the second piezoelectric member is rigidly coupled to the support. Example 19. The device of any one of Examples 13 to 17, wherein the second piezoelectric member is movably coupled to the support. Example 20. The device of any one of Examples 13 to 19, wherein: the first piezoelectric member is coupled to a second support within the housing, and the second piezoelectric member is coupled to the second support. Example 21. The device of Example 20, wherein the second support is configured to constrain movement of the second piezoelectric member relative to the first piezoelectric member. Example 22. The device of any one of Examples 13 to 21, wherein the first and second piezoelectric members are electrically coupled to the power source in parallel. Example 23. The device of any one of Examples 13 to 22, wherein the first and second piezoelectric members comprise different stiffnesses. Example 24. The device of any one of Examples 13 to 23, further comprising one or more additional piezoelectric members coupled to the support and to the harvester mass.

Claims

Atty Ref. No. A0010209WO01 CLAIMS 1. A device comprising: a housing configured to be implanted within a patient; a power source positioned within the housing; and an energy harvesting mechanism positioned within the housing and configured to charge the power source, wherein the energy harvesting mechanism comprises: a first piezoelectric element including a first portion fixedly coupled to a support within the housing and a second portion fixedly coupled to a harvester mass, and a second piezoelectric element including a first portion coupled to the support and a second portion coupled to the harvester mass, wherein: (a) the first portion of the second piezoelectric element is slidably coupled to the support, (b) the second portion of the second piezoelectric element is slidably coupled to the harvester mass, or (c) both (a) and (b).

2. The device of claim 1, wherein the second portion of the second piezoelectric element is slidably coupled to the harvester mass.

3. The device of claim 2, wherein the harvester mass comprises a recess formed therein, and the second portion of the second piezoelectric element is received within the recess.

4. The device of claim 3, wherein the second portion of the second piezoelectric element is movable in a longitudinal direction within the recess.

5. The device of claim 3 or 4, further comprising a lubricious material within the recess.

6. The device of any one of claims 1 to 5, wherein the first portion of theAtty Ref. No. A0010209WO01 second piezoelectric element is fixedly coupled to the support.

7. The device of any one of claims 1 to 5, wherein the first portion of the second piezoelectric element is slidably coupled to the support.

8. The device of any one of claims 1 to 7, wherein: the first piezoelectric element comprises a third portion coupled to a second support within the housing, and the second piezoelectric element comprises a third portion coupled to the second support.

9. The device of claim 8, wherein the second support is positioned between the first support and the harvester mass, or wherein the first support is positioned between the second support and the harvester mass.

10. The device of any one of claims 1 to 9, wherein the first and second piezoelectric elements each comprise a longitudinal axis aligned with a longitudinal axis of the housing.

11. The device of any one of claims 1 to 10, wherein the first and second piezoelectric elements are electrically coupled to the power source in parallel.

12. The device of any one of claims 1 to 11, wherein the first and second piezoelectric elements comprise different materials.

13. The device of any one of claims 1 to 12, wherein the first and second piezoelectric elements comprise different stiffnesses.

14. The device of any one of claims 1 to 13, further comprising a third piezoelectric element including a first portion coupled to the support and a second portion coupled to the harvester mass.Atty Ref. No. A0010209WO01 15. The device of claim 14, wherein the first, second, and third piezoelectric elements face different directions.

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