Patent Publication Number: US-10770923-B2

Title: Systems and methods for elastic wireless power transmission devices

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to provisional application Ser. No. 62/613,584, filed Jan. 4, 2018, which is incorporated herein in its entirety. 
    
    
     INCORPORATION BY REFERENCE 
     All publications and patent applications mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated by reference. 
     FIELD 
     This disclosure relates generally to methods and systems for transmitting and receiving power wirelessly, and in particular, an elastic wireless power transmission device wearable by a patient. 
     BACKGROUND 
     Powered devices need to have a mechanism to supply power to the operative parts. Typically systems use a physical power cable to transfer energy over a distance. There has been a continuing need for systems that can transmit power efficiently over a distance without physical structures bridging the physical gap. 
     Systems and methods that supply power without electrical wiring are sometimes referred to as wireless energy transmission (WET). Wireless energy transmission greatly expands the types of applications for electrically powered devices. One such example is the field of implantable medical devices. Implantable medical devices typically require an internal power source able to supply adequate power for the reasonable lifetime of the device or an electrical cable that traverses the skin. Typically an internal power source (e.g., a battery) is feasible for only low power devices like sensors. Likewise, a transcutaneous power cable significantly affects quality of life (QoL), infection risk, and product life, among many drawbacks. 
     More recently there has been an emphasis on systems that supply power to an implanted device without using transcutaneous wiring. This is sometimes referred to as a Transcutaneous Energy Transfer System (TETS). Frequently, energy transfer is accomplished using two magnetically coupled coils set up like a transformer so power is transferred magnetically across the skin from a transmitter coil to a receiver coil. Conventional systems are relatively sensitive to variations in position and alignment of the coils. In order to provide constant and adequate power, the two coils need to be physically close together and well aligned. 
     A transmitter coil in a TETS may be relatively heavy, and supporting the weight of a transmitter coil while keeping the transmitter coil aligned with the receiver coil may be relatively difficult, especially when attempting to maintain patient comfort. Further, the transmitter coil may vibrate or migrate over time, impacting power transfer efficiency. In addition, transmitter coils may generate significant amounts of heat. 
     SUMMARY OF THE DISCLOSURE 
     In one embodiment, a wireless power transmission device wearable by a subject is provided. The wireless power transmission device includes a power conditioner including a first end and an opposite second end, and a band. The band includes a first end fixedly coupled to the power conditioner first end, a second end fixedly coupled to the power conditioner second end, a body extending between the band first end and the band second end, the body including at least one elastic segment, and a plurality of conductive wires extending along the body to form a plurality of conductive loops, at least one of the plurality of conductive wires electrically coupled to the power conditioner. 
     In another embodiment, a wireless power transfer system is provided. The wireless power transfer system includes a wireless power receiver configured to be implanted within a subject, and a wireless power transmission device wearable by the subject, the wireless power transmission device configured to wirelessly transmit power to the wireless power receiver. The wireless power transmission device includes a power conditioner including a first end and an opposite second end, and a band. The band includes a first end fixedly coupled to the power conditioner first end, a second end fixedly coupled to the power conditioner second end, a body extending between the band first end and the band second end, the body including at least one elastic segment, and a plurality of conductive wires extending along the body to form a plurality of conductive loops, at least one of the plurality of conductive wires electrically coupled to the power conditioner. 
     In yet another embodiment, a method of operating a wireless power transfer system is provided. The method includes implanting a wireless power receiver in a subject, and positioning a wireless power transmission device on the subject, the wireless power transmission device including a power conditioner including a first end and an opposite second end, and a band including a first end fixedly coupled to the power conditioner first end, a second end fixedly coupled to the power conditioner second end, a body extending between the band first end and the band second end, the body including at least one elastic segment, and a plurality of conductive wires extending along the body to form a plurality of conductive loops, at least one of the plurality of conductive wires electrically coupled to the power conditioner. The method further includes supplying current to the at least one of the plurality of conductive wires using the power conditioner to wirelessly transfer power from the wireless power transmission device to the wireless power receiver. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features of the disclosure are set forth with particularity in the claims that follow. A better understanding of the features and advantages of the present disclosure invention will be obtained by reference to the following detailed description that sets forth illustrative embodiments, in which the principles of the invention are utilized, and the accompanying drawings of which: 
         FIG. 1  illustrates a basic wireless power transfer system. 
         FIG. 2  illustrates magnetic coupling between a pair of coils. 
         FIGS. 3A and 3B  illustrate the effect of coil alignment on the coupling coefficient. 
         FIG. 4  illustrates a patient placing an external coil to wirelessly transmit power to an implanted coil. 
         FIG. 5  illustrates an external wireless power transmission device worn by a patient. 
         FIG. 6  illustrates the wireless power transmission device shown in  FIG. 5 . 
         FIG. 7  illustrates a flow chart of one embodiment of a method for operating a wireless power transfer system. 
     
    
    
     DETAILED DESCRIPTION 
     In the description that follows, like components have been given the same reference numerals, regardless of whether they are shown in different embodiments. To illustrate an embodiment(s) of the present disclosure in a clear and concise manner, the drawings may not necessarily be to scale and certain features may be shown in somewhat schematic form. Features that are described and/or illustrated with respect to one embodiment may be used in the same way or in a similar way in one or more other embodiments and/or in combination with or instead of the features of the other embodiments. 
     The systems and methods in certain embodiments include a wireless power transmission device wearable by a subject. The wireless power transmission device includes a power conditioner fixedly coupled to a band. That is, the power conditioner is not selectively detachable from the band. The band includes a body having at least one elastic segment, and a plurality of conductive wires extending along the body. At least one elastic segment allows the wireless power transmission device to be stretched (e.g., when a subject is putting on, removing, or adjusting the wireless power transmission device). 
     Wireless Power Transmission System 
     Power may be transmitted wirelessly by magnetic induction. In various embodiments, the transmitter and receiver are closely coupled. 
     In some cases “closely coupled” or “close coupling” refers to a system that requires the coils to be very near each other in order to operate. In some cases “loosely coupled” or “loose coupling” refers to a system configured to operate when the coils have a significant spatial and/or axial separation, and in some cases up to distance equal to or less than the diameter of the larger of the coils. In some cases, “loosely coupled” or “loose coupling” refers a system that is relatively insensitive to changes in physical separation and/or orientation of the receiver and transmitter. 
     In various embodiments, the transmitter and receiver are non-resonant coils. For example, a change in current in one coil induces a changing magnetic field. The second coil within the magnetic field picks up the magnetic flux, which in turn induces a current in the second coil. An example of a closely coupled system with non-resonant coils is described in International Pub. No. WO2000/074747, incorporated herein for all purposes by reference. A conventional transformer is another example of a closely coupled, non-resonant system. In various embodiments, the transmitter and receiver are resonant coils. For example, one or both of the coils is connected to a tuning capacitor or other means for controlling the frequency in the respective coil. An example of closely coupled system with resonant coils is described in International Pub. Nos. WO2001/037926; WO2012/087807; WO2012/087811; WO2012/087816; WO2012/087819; WO2010/030378; and WO2012/056365, and U.S. Pub. No. 2003/0171792, incorporated herein for all purposes by reference. 
     In various embodiments, the transmitter and receiver are loosely coupled. For example, the transmitter can resonate to propagate magnetic flux that is picked up by the receiver at relatively great distances. In some cases energy can be transmitted over several meters. In a loosely coupled system power transfer may not necessarily depend on a critical distance. Rather, the system may be able to accommodate changes to the coupling coefficient between the transmitter and receiver. An example of a loosely coupled system is described in International Pub. No. WO2012/045050, incorporated herein for all purposes by reference. 
     Power may be transmitted wirelessly by radiating energy. In various embodiments, the system comprises antennas. The antennas may be resonant or non-resonant. For example, non-resonant antennas may radiate electromagnetic waves to create a field. The field can be near field or far field. The field can be directional. Generally far field has greater range but a lower power transfer rate. An example of such a system for radiating energy with resonators is described in International Pub. No. WO2010/089354, incorporated herein for all purposes by reference. An example of such a non-resonant system is described in International Pub. No. WO2009/018271, incorporated herein for all purposes by reference. Instead of antenna, the system may comprise a high energy light source such as a laser. The system can be configured so photons carry electromagnetic energy in a spatially restricted, direct, coherent path from a transmission point to a receiving point. An example of such a system is described in International Pub. No. WO2010/089354, incorporated herein for all purposes by reference. 
     Power may also be transmitted by taking advantage of the material or medium through which the energy passes. For example, volume conduction involves transmitting electrical energy through tissue between a transmitting point and a receiving point. An example of such a system is described in International Pub. No. WO2008/066941, incorporated herein for all purposes by reference. 
     Power may also be transferred using a capacitor charging technique. The system can be resonant or non-resonant. Exemplars of capacitor charging for wireless energy transfer are described in International Pub. No. WO2012/056365, incorporated herein for all purposes by reference. 
     The system in accordance with various aspects of the disclosure will now be described in connection with a system for wireless energy transfer by magnetic induction. The exemplary system utilizes resonant power transfer. The system works by transmitting power between the two inductively coupled coils. In contrast to a transformer, however, the exemplary coils are not coupled together closely. A transformer generally requires the coils to be aligned and positioned directly adjacent each other. The exemplary system accommodates looser coupling of the coils. 
     While described in terms of one receiver coil and one transmitter coil, one will appreciate from the description herein that the system may use two or more receiver coils and two or more transmitter coils. For example, the transmitter may be configured with two coils—a first coil to resonate flux and a second coil to excite the first coil. One will further appreciate from the description herein that usage of “resonator” and “coil” may be used somewhat interchangeably. In various respects, “resonator” refers to a coil and a capacitor connected together. 
     In general, most of the flux from the transmitter coil does not reach the receiver coil. The amount of flux generated by the transmitter coil that reaches the receiver coil is described by “k” and is referred to as the “coupling coefficient.” In general, the alignment is adequate if the resulting coupling coefficient k exceeds some minimum threshold k min . It is preferable if the alignment results in a coupling coefficient close to an optimum value k opt . The optimum value k opt  is always larger than the minimum value k min , and smaller than or equal to a maximum coupling coefficient value k max . In some embodiments, k min  is about 0.01, and k max  is about 0.2. 
     In various embodiments, the coils are physically separated. In various embodiments, the separation is greater than a thickness of the receiver coil. In various embodiments, the separation distance is equal to or less than the diameter of the larger of the receiver and transmitter coil. 
     Because most of the flux does not reach the receiver, the transmitter coil must generate a much larger field than what is coupled to the receiver. In various embodiments, this is accomplished by configuring the transmitter with a large number of amp-turns in the coil. 
     Since only the flux coupled to the receiver gets coupled to a real load, most of the energy in the field is reactive. The current in the coil can be sustained with a capacitor connected to the coil to create a resonator. The power source thus only needs to supply the energy absorbed by the receiver. The resonant capacitor maintains the excess flux that is not coupled to the receiver. 
     In various embodiments, the impedance of the receiver is matched to the transmitter. This allows efficient transfer of energy out of the receiver. In this case the receiver coil may not need to have a resonant capacitor. 
     Turning now to  FIG. 1 , a simplified circuit for wireless energy transmission is shown. The exemplary system shows a series connection, but the system can be connected as either series or parallel on either the transmitter or receiver side. 
     The exemplary transmitter includes a coil Lx connected to a power source Vs by a capacitor Cx. The exemplary receiver includes a coil Ly connected to a load by a capacitor Cy. Capacitor Cx may be configured to make Lx resonate at a desired frequency. Capacitance Cx of the transmitter coil may be defined by its geometry. Inductors Lx and Ly are connected by coupling coefficient k. Mxy is the mutual inductance between the two coils. The mutual inductance, Mxy, is related to coupling coefficient, k.
 
 Mxy=k√{square root over (Lx·Ly)} 
 
     In the exemplary system a power source Vs can be in series with a transmitter coil Lx so it may have to carry all the reactive current. This puts a larger burden on the current rating of the power source and any resistance in the source will add to losses. 
     The exemplary system includes a receiver configured to receive energy wirelessly transmitted by the transmitter. The exemplary receiver is connected to a load. The receiver and load may be connected electrically with a controllable switch. 
     In various embodiments, the receiver includes a circuit element configured to be connected or disconnected from the receiver coil by an electronically controllable switch. The electrical coupling can include both a serial and parallel arrangement. The circuit element can include a resistor, capacitor, inductor, lengths of an antenna structure, or combinations thereof. The system can be configured such that power is transmitted by the transmitter and can be received by the receiver in predetermined time increments. 
     In various embodiments, the transmitter coil and/or the receiver coil is a substantially two-dimensional structure. In various embodiments, the transmitter coil may be coupled to a transmitter impedance-matching structure. Similarly, the receiver coil may be coupled to a receiver impedance-matching structure. Examples of suitable impedance-matching structures include, but are not limited to, a coil, a loop, a transformer, and/or any impedance-matching network. An impedance-matching network may include inductors or capacitors configured to connect a signal source to the resonator structure. 
     In various embodiments, the transmitter is controlled by a controller (as shown in  FIG. 1 ) and driving circuit. The controller and/or driving circuit may include a directional coupler, a signal generator, and/or an amplifier. The controller may be configured to adjust the transmitter frequency or amplifier gain to compensate for changes to the coupling between the receiver and transmitter. 
     In various embodiments, the transmitter coil is connected to an impedance-matched coil loop. The loop is connected to a power source and is configured to excite the transmitter coil. The first coil loop may have finite output impedance. A signal generator output may be amplified and fed to the transmitter coil. In use power is transferred magnetically between the first coil loop and the main transmitter coil, which in turns transmits flux to the receiver. Energy received by the receiver coil is delivered by Ohmic connection to the load. 
     One of the challenges to a practical circuit is how to get energy in and out of the resonators. Simply putting the power source and load in series or parallel with the resonators is difficult because of the voltage and current required. In various embodiments, the system is configured to achieve an approximate energy balance by analyzing the system characteristics, estimating voltages and currents involved, and controlling circuit elements to deliver the power needed by the receiver. 
     In an exemplary embodiment, the system load power, P L , is assumed to be 15 Watts and the operating frequency, f, is 250 kHz. Then, for each cycle the load removes a certain amount of energy from the resonance: 
     
       
         
           
             
               
                 
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     It has been found that the energy in the receiver resonance is typically several times larger than the energy removed by the load for operative, implantable medical devices. In various embodiments, the system assumes a ratio 7:1 for energy at the receiver versus the load removed. Under this assumption, the instantaneous energy in the exemplary receiver resonance is 420 μJ. 
     The exemplary circuit was analyzed and the self inductance of the receiver coil was found to be 60 uH. From the energy and the inductance, the voltage and current in the resonator could be calculated. 
     
       
         
           
             
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     The voltage and current can be traded off against each other. The inductor may couple the same amount of flux regardless of the number of turns. The Amp-turns of the coil needs to stay the same in this example, so more turns means the current is reduced. The coil voltage, however, will need to increase. Likewise, the voltage can be reduced at the expense of a higher current. The transmitter coil needs to have much more flux. The transmitter flux is related to the receiver flux by the coupling coefficient. Accordingly, the energy in the field from the transmitter coil is scaled by k. 
     
       
         
           
             
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     For the same circuit the self inductance of the transmitter coil was 146 uH as mentioned above. This results in: 
     
       
         
           
             
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     One can appreciate from this example, the competing factors and how to balance voltage, current, and inductance to suit the circumstance and achieve the desired outcome. Like the receiver, the voltage and current can be traded off against each other. In this example, the voltages and currents in the system are relatively high. One can adjust the tuning to lower the voltage and/or current at the receiver if the load is lower. 
     Estimation of Coupling Coefficient and Mutual Inductance 
     As explained above, the coupling coefficient, k, may be useful for a number of reasons. In one example, the coupling coefficient can be used to understand the arrangement of the coils relative to each other so tuning adjustments can be made to ensure adequate performance. If the receiver coil moves away from the transmitter coil, the mutual inductance will decrease, and all other conditions being equal, less power will be transferred. In various embodiments, the system is configured to make tuning adjustments to compensate for the drop in coupling efficiency. 
     The exemplary system described above often has imperfect information. For various reasons as would be understood by one of skill in the art, the system does not collect data for all parameters. Moreover, because of the physical gap between coils and without an external means of communications between the two resonators, the transmitter may have information that the receiver does not have and vice versa. These limitations make it difficult to directly measure and derive the coupling coefficient, k, in real time. 
     Described below are several principles for estimating the coupling coefficient, k, for two coils of a given geometry. The approaches may make use of techniques such as Biot-Savart calculations or finite element methods. Certain assumptions and generalizations, based on how the coils interact in specific orientations, are made for the sake of simplicity of understanding. From an electric circuit point of view, all the physical geometry permutations can generally lead to the coupling coefficient. 
     If two coils are arranged so they are in the same plane, with one coil circumscribing the other, then the coupling coefficient can be estimated to be roughly proportional to the ratio of the area of the two coils. This assumes the flux generated by coil  1  is roughly uniform over the area it encloses as shown in  FIG. 2 . 
     If the coils are out of alignment such that the coils are at a relative angle, the coupling coefficient will decrease. The amount of the decrease is estimated to be about equal to the cosine of the angle as shown in  FIG. 3A . If the coils are orthogonal to each other such that theta (θ) is 90 degrees, the flux will not be received by the receiver and the coupling coefficient will be zero. 
     If the coils are arraigned such that half the flux from one coil is in one direction and the other half is in the other direction, the flux cancels out and the coupling coefficient is zero, as shown in  FIG. 3B . 
     A final principle relies on symmetry of the coils. The coupling coefficient and mutual inductance from one coil to the other is assumed to be the same regardless of which coil is being energized.
 
M xy =M yx  
 
     As described above, a typical TET system can be subdivided into two parts, the transmitter and the receiver. Control and tuning may or may not operate on the two parts independently. For example, as shown in  FIG. 1 , the transmitter or the receiver or both may include a controller. The goal of this invention is to minimize the effect of relative spatial position and orientation on the magnetic field power transfer rate between a transmitter and a receiver. 
       FIG. 4  illustrates one embodiment of a patient  400  using an external coil  402  (a transmitter) to wirelessly transmit power to an implanted coil  404  (a receiver). The implanted coil  404  uses the received power to power an implanted device  406 . For example, the implanted device  406  may include a pacemaker or heart pump. 
     In one embodiment, the external coil  402  is communicatively coupled to a computing device  410 , for example, via wired or wireless connection, such that external coil  402  may receive signals from and transmit signals to the computing device  410 . In some embodiments, the computing device  410  is a power source for the external coil  402 . In other embodiments, the external coil  402  is coupled to an alternative power supply (not shown). The computing device  410  includes a processor  412  in communication with a memory  414 . In some embodiments, executable instructions are stored in the memory  414 . 
     The processor  412  may include one or more processing units (e.g., in a multi-core configuration). Further, the processor  412  may be implemented using one or more heterogeneous processor systems in which a main processor is present with secondary processors on a single chip. In another illustrative example, the processor  412  may be a symmetric multi-processor system containing multiple processors of the same type. Further, the processor  412  may be implemented using any suitable programmable circuit including one or more systems and microcontrollers, microprocessors, reduced instruction set circuits (RISC), application specific integrated circuits (ASIC), programmable logic circuits, field programmable gate arrays (FPGA), and any other circuit capable of executing the functions described herein. 
     In the illustrated embodiment, the memory  414  is one or more devices that enable information such as executable instructions and/or other data to be stored and retrieved. The memory  414  may include one or more computer readable media, such as, without limitation, dynamic random access memory (DRAM), read-only memory (ROM), electrically erasable programmable read-only memory (EEPROM), static random access memory (SRAM), a solid state disk, and/or a hard disk. The memory  414  may be configured to store, without limitation, application source code, application object code, source code portions of interest, object code portions of interest, configuration data, execution events and/or any other type of data. 
     The computing device  410  further includes a user interface (UI)  416 . The UI  416  presents information to a user (e.g., patient  400 ). For example, the UI  416  may include a display adapter (not shown) that may be coupled to a display device, such as a cathode ray tube (CRT), a liquid crystal display (LCD), an organic LED (OLED) display, and/or an “electronic ink” display. In some embodiments, the UI  416  includes one or more display devices. Further, in some embodiments, presentation interface may not generate visual content, but may be limited to generating audible and/or computer-generated spoken-word content. In the example embodiment, the UI  416  displays one or more representations designed to aid the patient  400  in placing the external coil  402  such that the coupling between the external coil  402  and the implanted coil  404  is optimal. In some embodiments, computing device  410  may be a wearable device. For example, in one embodiment, computing device  410  is a wrist watch, and UI  416  is displayed on the wrist watch. 
       FIG. 5  is a schematic diagram illustrating one embodiment of an external wireless power transmission device  500  worn by patient  400 . Wireless power transmission device  500  may serve as, for example, external coil  402  (shown in  FIG. 4 ).  FIG. 6  is a schematic diagram of wireless power transmission device  500 . Wireless power transmission device  500  is capable of wirelessly coupling with and transmitting power to an implanted coil, or implanted wireless power receiver device (not shown in  FIGS. 4 and 5 ), such as implanted coil  404  (shown in  FIG. 4 ). 
     As shown in  FIGS. 5 and 6 , wireless power transmission device  500  is a generally annular device that may be worn by patient  400  proximate a waist of the patient  400 . Accordingly, wireless power transmission device  500  may be worn by patient  400  similar to a belt. 
     In one embodiment, wireless power transmission device  500  includes a power conditioner  502  and a band  504 . Power conditioner  502  and band  504  form a continuous, closed loop. Specifically, a first end  506  of power conditioner  502  is coupled to a first end  508  of band  504 , and a second end  510  of power conditioner  502  is coupled to a second end  512  of band  504 . Band  504  includes a body  520  that extends between band first end  508  and band second end  512 . Further, body  520  includes a first edge  522 , and an opposite second edge  524 . Body  520  further includes an inner surface  525  and an opposite outer surface  526 . Inner and outer surfaces  525  and  526  each extend between first and second edges  522  and  524 . When wireless power transmission device  500  is worn by patient  400 , inner surface  525  faces inwards towards patient  400  and generally contacts patient  400 . Further, when power transmission device  500  is worn by patient  400 , outer surface  526  faces outwards, away from patient  400 . In some embodiments, wireless power transmission device  500  is stretchable (as described below), and has an unstretched length in a range from approximately 50 cm to 200 cm. Further, wireless power transmission device  500  may have a height (i.e., a distance between first and second edges  522  and  524 ) in a range from approximately 18 mm to 100 mm, and a thickness in a range from approximately 3 mm to 1 cm. Wireless power transmission device  500  may be stretchable up to 100% (i.e., doubling the length of wireless power transmission device  500 ), and more particularly, may be stretchable between approximately 20% and 50%. Alternatively, wireless power transmission device  500  may have any suitable dimensions and/or strechability. 
     As shown in  FIG. 6 , band  504  includes a plurality of conductive loops  530 , or turns. In  FIG. 6 , three conductive loops  530  are shown for clarity, but those of skill in the art will appreciate that band  504  may include any suitable number of conductive loops  530 . For example, band  504  may include at least six conductive loops  530 . Each conductive loop  530  is formed by a conductive wire  532 , and a single conductive wire  532  may form multiple loops  530 . At least some of conductive wires  532  are electrically coupled to power conditioner  502 , such that power conditioner  502  is capable of supplying a current to each electrically coupled conductive wire  532 . For example, in one embodiment, a first conductive wire  532  forms an exciter coil and a second conductive wire  532  forms a resonator coil. Exciters and resonators are described in U.S. Pat. No. 9,287,040, which is incorporated herein for all purposes by reference. 
     In this embodiment, the first conductive wire  532  forms at least six conductive loops  530  that constitute the exciter coil, and is electrically coupled at either end to power conditioner  502  (i.e., the first conductive wire  532  bypasses the current supplying components of power conditioner  502  five times). Further, the second conductive wire  532  forms at least six conductive loops  530  that constitute the resonator coil, and is not electrically coupled to power conditioner  502  (i.e., the second conductive wire  532  bypasses the current supplying components of power conditioner  502  every time). Although portions of first and second conductive wires  532  bypass the current supplying components of power conditioner  502 , as described above, those portions of first and second conductive wires  532  may still be physically located within a housing of power conditioner  502 . 
     Conductive wires  532  may be, for example, Litz wire including copper cables. Alternatively, conductive wires  532  may be any suitable conductive material. In one embodiment, conductive wires  532  are embedded in body  520 . Alternatively, conductive wires  532  may be coupled to inner surface  525  and/or outer surface  526  of body  520 . 
     Band  504  includes at least one elastic segment  540  and at least one inelastic segment  542  in one embodiment. For example, in the embodiment shown in  FIG. 6 , band  504  includes one elastic segment  540  and two inelastic segments  542 . Specifically, a first inelastic segment  544  extends between power conditioner first end  506  and elastic segment  540 , and a second inelastic segment  546  extends between elastic segment  540  and power conditioner second end  510 . In other embodiments, band  504  may include any suitable number and arrangement of elastic and inelastic segments  540  and  542 . For example, band  504  may include a i) single elastic segment  540  and a single inelastic segment  542 , ii) a plurality of elastic segments  540  and a single inelastic segment  542 , or ii) a plurality of elastic segments  540  and a plurality of inelastic segments  542 . As another example, in some embodiments, the entire band  504  is a single elastic segment  540  (i.e., there are no inelastic segments  542 ). 
     Elastic segments  540  are generally stretchable, or flexible, and inelastic segments  542  are generally stiff, or inflexible. For example, portions of body  520  corresponding to elastic segments  540  may be made of cross-linked polyethylene, foam material, knitted nylons, polyesters, ariaprene, and/or spandex, and portions of body  520  corresponding to inelastic segments  542  may be made of nylon and/or leather encased in a biothane webbing. In some embodiment, portions of body  520  corresponding to inelastic segments  542  include, for example, stiff plastic plates and/or non-conductive non-magnetic wires (e.g., Kevlar and/or Vectran fibers) embedded therein to improve stiffness and to prevent patient  400  from folding or twisting band  504 . Further, in some embodiments, elastic segments  540  may also include relatively stiff plastic plates (e.g., segments with a cross-sectional aspect ratio from approximately 10:1 to 20:1) and/or non-conductive non-magnetic wires (e.g., Kevlar and/or Vectran fibers) to prevent excessive stretching, allow bending, and provide stiffness against torsion or twisting. In some embodiments, the relatively stiff plastic plates may partially overlap in the unstretched state. Alternatively, portions of body  520  corresponding to elastic and inelastic segments  540  and  542  may be made of any suitable material. 
     The one or more elastic segments  540  of band  504  allow patient  400  to put on, remove, or adjust a position of wireless power transmission device  500  relatively easily. Specifically, elastic segments  540  allow band  504  to stretch, from a relaxed state to an expanded state, when a sufficient force is applied. Once the force is removed, band  504  returns to the relaxed state. For example, patient  400  may stretch band  504  to put on wireless power transmission device  500  (e.g., by lowering the band over their head or raising the band around their legs), to remove wireless power transmission device  500  (e.g., by raising the band over their head or lowering the band around their legs), or to adjust a current position of wireless power transmission device  500 . Further, using one or more elastic segments  540  enables wireless power transmission device  500  to fit patients  400  of different sizes. For example, wireless power transmission device  500  may fit both a larger patient  400  in the expanded state and a smaller patient  400  in the relaxed state. 
     Because of one or more elastic segments  540 , patient  400  is able to put on, remove, or adjust wireless power transmission device  500  without detaching power conditioner  502  and band  504  from one another. Specifically, in one embodiment, power conditioner  502  and band  504  are fixedly attached (i.e., power conditioner  502  and band  504  are not readily detachable from one another). If power conditioner  502  and band  504  were detachably coupled to one another (e.g., with band  504  selectively engageable with power conditioner  502 ), it would generally require a plurality of electrical connectors to electrically couple wires  532  to power conditioner  502 . These electrical connectors would introduce significant resistance into conductive loops  530 , which negatively impacts the ability of wireless power transmission device  500  to transfer power. Accordingly, embodiments in which power conditioner  502  is fixedly coupled to band  504  provide substantial advantages over embodiments in power conditioner  502  and band  504  are selectively detachable from one another. 
     As shown in  FIG. 6 , each conductive wire  532  extends generally linearly through inelastic segments  542 . In contrast, in elastic segment  540 , each conductive wire  532  is arranged in a modulating pattern when elastic segment  540  is in the relaxed state. When elastic segment  540  transitions from the relaxed state to the expanded state, wires  532  in elastic segments “straighten out”, and more closely resemble a linear shape. When elastic segment  540  is fully stretched (i.e., in a fully expanded state), wires  532  extend substantially linearly through elastic segment  540 . In the embodiment shown in  FIG. 6 , the modulating pattern of each conductive wire  532  is a zigzag pattern. Alternatively, other modulating patterns may be used. For example, wires  532  may be arranged in a sinusoidal wave pattern in some embodiments. Notably, the modulating pattern of wires in elastic segment  540  has been shown to have a relatively small impact on the power transmission capabilities of wireless power transmission device  500 . 
     Wireless power transmission device  500  defines a longitudinal axis  550 . To improve power transfer between wireless power transmission device  500  and a wireless power receiver device implanted in patient  400  (such as implanted coil  404  shown in  FIG. 4 ), wireless power transmission device  500  should generally be positioned such that the wireless power receiver device is substantially surrounded by wireless power transmission device  500 . Further, longitudinal axis  550  of wireless power transmission device  500  should generally be parallel to a longitudinal axis similarly defined for the wireless power receiver device. In some embodiments, the wireless power receiver device may encircle a rib cage or lung of patient  400 . Alternatively, the wireless power receiver device may be implanted within patient  400  at any suitable location. 
     To facilitate aligning wireless power transmission device  500  with a wireless power receiver device implanted in patient  400 , in some embodiments, UI  416  (shown in  FIG. 4 ) displays one or more representations designed to aid patient  400  in positioning power transmission device  500 , For example UI  416  may operate in accordance with any of the embodiments described in U.S. patent application Ser. No. 15/709,743, filed Sep. 20, 2017, entitled “SYSTEMS AND METHODS FOR LOCATING IMPLANTED WIRELESS POWER TRANSMISSION DEVICES” which is incorporated herein by reference in its entirety. 
     In some embodiments, to improve wireless power transfer, a ferrite material (not shown) may be coupled to inner surface  525  and/or outer surface  526  of body  520 . The ferrite material external magnetic and/or electrical fields from interfering with operation of wireless power transmission device  500 . The ferrite material may include, for example, an array of flexible or inflexible ferrite tiles. 
     Because wireless power transmission device  500  wraps around patient  400 , wireless power transmission device  500  experiences relatively little vibration of migration, improving power transfer. Further, because of the relatively large circumscribed area, any vibration or migration that does occur has relatively little impact. In addition, the relatively large exposed area of wireless power transmission device  500  (i.e., outer surface  526 ) facilitates dissipating heat generated by wireless power transmission device  500 . 
       FIG. 7  illustrates a flow chart of one embodiment of a method  700  for operating a wireless power transfer system. Method  700  includes implanting  702  a wireless power receiver, such as implanted coil  404  (shown in  FIG. 4 ) in a subject. Method  700  further includes positioning  704  a wireless power transmission device, such as wireless power transmission device  500  (shown in  FIGS. 5 and 6 ), on the subject. The wireless power transmission device includes a power conditioner and a band. The band includes a body including at least one elastic segment, and a plurality of conductive wires extending along the body, the plurality of conductive wires electrically coupled to the power conditioner. Further, method  700  includes supplying  706  current to the plurality of conductive wires using the power conditioner to wirelessly transfer power from the wireless power transmission device to the wireless power receiver. 
     Accordingly, using the systems and methods described herein, a wireless power transmission device may be worn, removed, or adjusted by a subject relatively easily. Because the wireless power transmission device includes at least one elastic segment, the wireless power transmission device is stretchable, and does not require any type of detachable coupling mechanism. This allows for easier positioning of the wireless power transmission device, and eliminates the need for components that would otherwise increase resistances in the wireless power transmission device. 
     The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the Claims appended hereto and their equivalents.