Patent Publication Number: US-10770919-B2

Title: Wireless energy transfer system and wearables

Description:
This application claims is a continuation of U.S. patent application Ser. No. 15/252,981, filed Aug. 31, 2016, which claims the benefit of priority to U.S. Provisional Application No. 62/212,351, filed Aug. 31, 2015, the entire contents and disclosures of which are hereby incorporated by reference in their 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 
     Embodiments of the invention relate to wireless energy transfer, and more specifically, to the transfer of energy from a power source outside the body to an implanted medical device inside the body. 
     BACKGROUND 
     Various techniques have been developed to transfer energy wirelessly, and in some cases over long distances. Examples of such systems include U.S. Pat. Nos. 6,327,504; 6,772,011; 7,825,543; and 8,076,801 and U.S. Pub. Nos. 2010/0102639 and 2010/010909445, the entire contents of which are incorporated herein for all purposes by reference. 
     More recently, there has been development into powering an implanted device wirelessly with a Transcutaneous Energy Transfer (TET) system. Many implantable medical devices require power sources or electrical systems to power the implant. Typically this is achieved using transcutaneous wiring to connect a power source to the implant. TET systems are designed to replace or supplement the transcutaneous wires. 
     TET systems typically include a lot of hardware and components. One example of a TET system includes the transmission of energy from a transmit coil to a receive coil using an oscillating magnetic field. The TET system also includes a power supply (e.g., battery and/or power conditioner to connect to AC mains) and processing electronics (e.g., solid state electronics and a controller), and other components. It can be burdensome for a patient to carry all these components, in particular for life-saving devices which must be carried at all times. Furthermore, TET systems often require precise alignment of components. Accordingly, there is a need for improvements to peripherals for carrying the necessary system components. 
     There is also the need for improved utilization and positioning of TET components. Modern medical devices typically require maximal power efficiency. For example, pumps such as ventricular assist devices (VAD) require a relatively high level of sustained and continuous power. With the advances of medical technology, there are an increasing number of implanted medical devices which can benefit from improvements in wireless energy transmission. Improvements in power usage can translate to meaningful reductions in the form factor of the internal power storage (e.g., battery). Improvements in power transmission can also lead to improvements in operation. For example, a slight improvement in power efficiency can mean significant increases in run time on the battery thus improving patient quality of life (QoL). 
     TET systems by their nature are sensitive to changes in the system. Even small relative changes to the relative orientation between the transmit and receive coil—distance or angle—can lead to a dramatic increase or decrease in power transmission. Indeed, many modern TET systems can only withstand a separation distance on the order of millimeters and require the coils to be generally in desired alignment. Any deviations can drop the power transmission efficiency below acceptable levels. Some existing TET systems for implantable medical devices require the implanted receiver coil to be positioned just under the skin, and typically include a mechanical feature to maintain exact alignment between the receive and transmit coils. However, by implanting these devices directly under the skin, the size and power requirements of these implanted devices is limited if they are to be powered by a TET system. Moreover, many TET systems are system to changes even within an operational range. For example, if one coil is moving or vibrating rapidly with respect to the other coil the power efficiency will drop dramatically. 
     The lack of effective positioning systems means that many TET systems are designed for placement of the transmit and receive coils directly adjacent each other in the pectoral region. The pectoral region is known to be relatively stable during activity due to the minimal amount of soft tissue and fat. There is less variability from patient to patient. In part for this reason the pectoral region is a common placement for implantable cardioverter defibrillators (ICD). 
     Accordingly, there is a need for devices and methods for addressing these and other problems. There is a need for systems and methods that reduce the burden on the patient and improve power transmission. There is the need for improvements to wearable devices for use with wireless energy transfer systems, and in certain respects TET systems. 
     SUMMARY OF THE DISCLOSURE 
     The present invention relates to a wireless energy transfer system, and more particularly, to wearable devices and garments for a wireless energy transfer system. 
     One aspect of the invention relates to a garment to be worn by a patient requiring treatment, the garment comprising a body portion configured for wrapping around at least a thoracic region of a patient&#39;s body, at least one strap configured to drape over a shoulder of the patient, at least one pocket sized and shaped to receive a battery for powering an implantable medical device, and at least one pocket for maintaining a transmitter coil at a desired location relative to the patient&#39;s body. In various embodiments, the coil pocket is formed of a non-metal. The at least one coil pocket may be positionable on a plurality of positions of the body portion. The at least one battery pocket may be positionable on a plurality of positions of the body portion. 
     One aspect of the invention relates to an accessory carrier for a patient requiring treatment, comprising a carrier having an interior for holding components of a medical system, the carrier including a first clip for securing a battery and a divider for separating the battery from the remainder of the interior, a flap covering at least a portion of the carrier, a strap for draping the carrier over a patient&#39;s body, and a closure for sealing the flap against the carrier. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features of the invention are set forth with particularity in the claims that follow. A better understanding of the features and advantages of the present 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 energy transfer (WET) system. 
         FIG. 2  illustrates the flux generated by a pair of coils. 
         FIGS. 3A and 3B  illustrate the effect of coil alignment on the coupling coefficient. 
         FIG. 4  illustrates half of an exemplary two-coil system for WET having a transmitter and a receiver. 
         FIG. 5  is a basic implementation of a transcutaneous energy transfer (TET) system, illustrating operation with power from AC mains. 
         FIGS. 6A and 6B  show components of an exemplary TET system.  FIG. 6A  shows selected external components of an exemplary TET system including batteries and a coil for a VAD.  FIG. 6B  shows the basic components of a TETS system on a patient. 
         FIG. 7  is a basic configuration of the exemplary TET system, illustrating wireless energy transmission from an external coil to an implanted device. 
         FIGS. 8A and 8B  illustrate exemplary uses and orientations with the TET system of  FIGS. 5, 6A, and 6B . 
         FIG. 9  shows positioning of an external coil and internal battery in accordance with aspects of the invention. 
         FIGS. 10A and 10B  are illustrations of certain potential locations for placement of an external coil, and external peripherals in accordance with aspects of the invention.  FIG. 10A  is a front view of a patient.  FIG. 10B  is a back view of the patient. 
         FIG. 11A  is a front view of a wearable device for holding the external system components of a VAS and TET system in accordance with the inventions.  FIG. 11B  includes front, back, and side views of the device of  FIG. 11A .  FIG. 11C  is an enlarged view of a pocket in the device of  FIG. 11A .  FIG. 11D  is an enlarged view of the pocket of  FIG. 11C  in a partially open position. 
         FIGS. 12A and 12B  are perspective views of a wearable device similar to the device of  FIG. 11A , illustrating a removable battery pocket connected to a body portion by zippers. 
         FIG. 12C  is a rear view of a wearable device similar to  FIG. 12A , illustrating a battery pocket having flaps for attachment. 
         FIG. 13  is a front view of another wearable device similar to the device of  FIG. 11A . 
         FIG. 14  is a perspective view of a wearable device similar to the device of  FIG. 11A , illustrating a pocket and straps for fastening the pocket to a patient&#39;s abdominal area or waist. 
         FIGS. 15A to 15E  are several views of another wearable device similar to the device of  FIGS. 11A and 14 , the device formed as an abdominal band.  FIG. 15A  is a front view of a wearable device, illustrating the device opened into a flat position.  FIG. 15B  is a schematic view of the device of  FIG. 15A , illustrating the inside of the band.  FIGS. 15C, 15D, and 15E  are back, front, and side views of the band on a patient. 
         FIG. 16  is a front view of another wearable device similar to the device of  FIG. 15A . 
         FIGS. 17A to 17C  are several views of another wearable device similar to the device of  FIG. 15A , illustrating a modified band and connector to improve patient comfort and accommodate greater adjustments by the patient. 
         FIGS. 18A and 18B  are several views of another wearable device similar to the device of  FIG. 11A  except the device is configured to hold only the external batteries.  FIG. 18A  is a front view of the device.  FIG. 18B  is a perspective view of the device, illustrated with batteries in the pockets. 
         FIGS. 19A and 19B  are several views of another wearable device similar to the device of  FIGS. 6A, 11A, and 18A .  FIG. 19A  is a front perspective view of the device connected to a battery.  FIG. 19B  is a rear perspective view of the device, illustrated with a flap closure opened to allow removal of the battery. 
         FIGS. 20A and 20B  are several views of another wearable device similar to the device of  FIG. 19A .  FIG. 20A  is an exploded front view of the device including a belt, clip, and battery pocket.  FIG. 20B  is an enlarged view of the battery clip. 
         FIGS. 21A and 21B  are several views of a shoulder bag for use with a device similar to that of  FIG. 11A .  FIG. 21A  is a front view of the bag.  FIG. 21B  are perspective views of the bag, illustrating the bag in open and closed positions. 
         FIG. 22  is a front view of a device similar to that of  FIG. 21A , the device formed as a backpack. 
         FIGS. 23A-23C  illustrates a wearable device similar to the device of  FIG. 11A .  FIG. 23A  is a front perspective view, illustrating the device worn on a body.  FIG. 23B  is a front view, illustrating the device worn on a body.  FIG. 23C  is a front perspective view, illustrating the device in an open position and detailing a pocket on the inner side of the device. 
         FIGS. 24A-24D  illustrates a wearable device similar to the device of  FIG. 11A .  FIG. 24A  is a front perspective view, illustrating the device worn on a body.  FIG. 24B  is a front view, illustrating the device worn on a body.  FIG. 24C  is an enlarged detail view of the clip of the device of  FIG. 24B .  FIG. 24D  is an exploded view of the device of  FIG. 24A . 
         FIGS. 25A-25C  illustrates a wearable device similar to the device of  FIG. 11A .  FIGS. 25A and 25B  are front perspective views, illustrating the device worn on a body.  FIG. 25C  is an enlarged detail view of the battery case of the device of  FIG. 25A . 
         FIGS. 26A and 26B  illustrates a wearable device similar to the device of  FIG. 11A .  FIG. 26A  is a front perspective view, illustrating the device worn on a body with the coil carrier detached.  FIG. 26B  is a front view, illustrating the coil carrier attached to the main body portion. 
         FIG. 27  consists of several views of various wearable devices for use with the device of  FIGS. 26A and 26B . 
         FIG. 28  consists of several views of another wearable device similar to the device of  FIG. 11A . 
     
    
    
     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. 
       FIGS. 1-4  illustrate a basic wireless energy transmission (WET) system. The exemplary system is a configured to transmit energy wireless using resonant coils and an oscillating magnetic field. 
     Various aspects of the system are similar to those described in International Patent Pub. No. WO2013110602; WO2012045050; U.S. Pat. Nos. 8,562,508; 8,140,168; 7,865,245; 7,774,069; 7,711,433; 7,650,187; 7,571,007; 7,741,734; 7,825,543; 6,772,011; 6,591,139; 6,553,263; 6,327,504; and 5,350,413; and U.S. Pub. Nos. 2014/0028110; 2013/0127253; 2013/0007949; 2010/0308939; 2008/027293; 2007/0123948; 2010/0114143; and 2010/0102639, the entire contents of which patents and applications are incorporated herein for all purposes. 
     Although important aspects of the inventions are directed to peripherals and wearable devices for a WET system, the design of the devices are typically informed by the performance constraints of the underlying WET system. Accordingly, the basic principles of the WET system will be described below. 
     Wireless Energy Transfer 
     With reference to  FIG. 1 , 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. For example, in some exemplary cases the external coil is directly adjacent the skin and the internal coil must be implanted subcutaneously just below the external coil. 
     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 some cases, “loosely coupled” or “loose coupling” refers a highly resonant system and/or a system using strongly-coupled magnetic resonators. 
     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. Exemplars 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, U.S. Pub. No. 2003/0171792, and U.S. Pat. No. 5,350,413, incorporated herein for all purposes by reference. 
     In the following description and claims, the terms “coupled” along with its derivatives, may be used. It should be understood that the term “coupled” is used to indicate that two or more elements, which may or may not be in direct physical or electrical contact with each other, cooperate or interact with each other. The terms “energy transfer,” “power transfer,” and “power transmission,” and their derivatives, are used interchangeably and refers to the transmission of energy between two devices. 
     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 antennas, 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 invention 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, the system in accordance with various embodiments of this disclosure may include any system for wireless transmitting energy over a distance. The system generally includes one or more components for transmitting and receiving the energy. The energy may take various forms such as an electromagnetic field. 
     With reference to  FIGS. 1-4 , the exemplary system comprises one or more transmitters configured to transmit power wirelessly to one or more receivers. In various embodiments, the system includes a transmitter and more than one receiver in a multiplexed arrangement. A frequency generator may be electrically coupled to the transmitter to drive the transmitter to transmit power at a particular frequency or range of frequencies. The frequency generator can include a voltage controlled oscillator and one or more switchable arrays of capacitors, a voltage controlled oscillator and one or more varactors, a phase-locked-loop, a direct digital synthesizer, or combinations thereof. The transmitter can be configured to transmit power at multiple frequencies simultaneously. The frequency generator can include two or more phase-locked-loops electrically coupled to a common reference oscillator, two or more independent voltage controlled oscillators, or combinations thereof. The transmitter can be arranged to simultaneously delivery power to multiple receivers at a common frequency. 
     In various embodiments, the transmitter is configured to transmit a low power signal at a particular frequency. The transmitter may transmit the low power signal for a particular time and/or interval. In various embodiments, the transmitter is configured to transmit a high power signal wirelessly at a particular frequency. The transmitter may transmit the high power signal for a particular time and/or interval. 
     In various embodiments, the receiver includes a frequency selection mechanism electrically coupled to the receiver coil and arranged to allow the resonator to change a frequency or a range of frequencies that the receiver can receive. The frequency selection mechanism can include a switchable array of discrete capacitors, a variable capacitance, one or more inductors electrically coupled to the receiving antenna, additional turns of a coil of the receiving antenna, or combinations thereof. 
     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 referred to as the “coupling coefficient.” 
     In various embodiments, the system is configured to maintain a value of k in the range of between about 0.2 to about 0.01. In various embodiments, the system is configured to maintain a value of k of at least 0.01, at least 0.02, at least 0.03, at least 0.04, or at least 0.05. 
     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 in the exemplary system, 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. 
       FIG. 1  illustrates a simplified circuit for wireless energy transmission (WET). 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 the power source Vs is in series with the 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. In an exemplary embodiment, the load an operative element such as an implanted medical device. In various embodiments, the load is one of a rechargeable power source and an operative element. For example, the receiver may be connected to a DC bus which is in turn connected to various components requiring power. These components may include, but are not limited to, a power source (e.g., battery), an operative medical device, a telemetry system, and associated circuitry. 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 (not shown) 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, PL, is assumed to be 15 Watts and the operating frequency of the system, 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. 
     One can also appreciate from the above that the form factor and weights of the various WET components may depend to a large degree on the WET performance criteria. 
     Turning to  FIGS. 2, 3A, and 3B , the coupling coefficient and mutual inductance in view of the coil alignment will be explained. 
     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 ceteris paribus, 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. 
     U.S. Pub. No. 2014/0028110 to Petersen et al., incorporated herein for all purposes by reference, describes 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 arranged 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  
 
     In an exemplary embodiment, the WET system is a TET system for an implanted medical device such as a blood pump. Systems and methods are provided herein for wirelessly transmitting power from an external power transmitter to a separate power receiver. The TET systems described herein can be configured to wirelessly transmit power from a transmitter positioned outside of a human body to a receiver implanted within the body. The receiver can be coupled to circuitry and a power source to power and operate an implantable medical device coupled to the receiver. 
       FIG. 4  is an exemplary circuit diagram of half of an exemplary TET circuitry in accordance with the above. In practice the other half of the circuitry mirrors what is shown in  FIG. 4 . In some embodiments, the resonant systems described herein can operate at high voltages (possibly greater than 1000 Vac rms) to transmit the high power levels required by some implantable applications. For example, power levels of as high as approximately 10 W, 15 W, or more are typically required for a fully implanted LVAD system. In these embodiments, a voltage divider can be used in the TET system so that the load of the receiver resonator and power source of the transmitter resonator can operate at a lower voltage than other parts of the LVAD system. The voltage of the driving circuit and load can be dictated by a battery pack, which is typically in the range of 12-20 Vdc. 
     In one embodiment shown in  FIG. 4 , a TET system  100  comprises two resonant systems, a transmitter resonator  102  and a receiver resonator  104 . Each of the resonant systems can be connected to a voltage divider circuit. Transmitter resonator  102  includes an inductor Lx and a capacitor Cx 2  configured as a tank circuit. Receiver resonator  104  includes an inductor Ly and a capacitor Cy 2  configured as a tank circuit. In order to excite each resonant system an impedance matching circuit can connect the transmitter resonator to the power source and the receiver resonator to the load. This way the load and power source only have to supply the real part of the power, and the reactive part of the power is handled by the impedance matching circuit. 
     In  FIG. 4 , the impedance matching circuits can comprise voltage dividers formed from capacitors. Voltage divider  106  can be coupled to transmitter resonator  102  and can comprise capacitor Cx 1  and inductor Ls, coupled to voltage source Vs. Voltage divider  108  can be coupled to receiver resonator  104  and can comprise capacitor Cy 1  and inductor L 1 , coupled to the Load. An additional inductor may be needed in series with the source and load. In a practical circuit the source is most likely a FET push pull circuit operating as a square wave voltage source. The output of the voltage source should not be placed directly across a capacitor or there will be extremely large currents on the switching transitions. 
     Many drive circuits are possible in addition to the FET push pull circuit (class-D amplifier). These include variations on resonant power amplifiers (classes B, C, and E) or self resonant circuits such as a Royer oscillator. Linear amplifiers (classes A and A-B) will also work, but will have lower efficiency. 
     In a further embodiment, at least one of the resonators  502  and  512  is coupled to a tunable resistor or an array or network of resistive elements to tune a quality factor “Q” of the system to maximize a voltage gain at the receiver unit  511 . A tunable resistor is a circuit or component with a variable resistance value that can be changed in response to a control input. The control input may be voltage, current, or any other input that can cause the materials or circuit of the tunable resistor to change its resistance value. Similarly, an array or network of resistive elements can be configured to form different series and/or parallel arrangements of resistive elements to achieve an effective resistance value. For example, the resistive elements can be resistors, capacitors with effective resistance values, or a combination of both. In an embodiment, an array of resistive elements can be an array of capacitors, where the capacitors have the same capacitance values but different effective series resistance values. This allows the effective resistance to be adjusted while keeping the effective capacitance the same. 
     Turning to  FIG. 5 , an exemplary system  700  in accordance with the various inventions is shown. The exemplary system described has a relatively high quality factor. The quality factor “Q” describes the inverse power loss of the resonator. Hence, a larger Q means a lower power loss in the resonator and a higher energy transfer efficiency, resulting in a higher voltage gain at the receiver unit  711 . The quality factor “Q” of a resonant system that has a transmitter resonator  702  and a receiver resonator  712  can be described by the square root of the product of the quality factors of the two resonators  702  and  712 . In order to reduce the power loss in the system  700  to maximize the voltage gain at the receiver unit  711 , the quality factor “Q” of the system can be increased by increasing the quality factors of either or both resonators  702  and  712 . This can be achieved by tuning a tunable resistor or an array of resistive elements that is coupled to the coil of the respective resonator to modulate and to match the impedance seen by the resonator. Alternatively, a tunable or network of capacitive and/or inductive elements can be used to adjust the effective capacitance and/or the effective inductance to change the quality factor “Q.” 
     Exemplary Ventricular Assist System Using Transcutaneous Energy Transfer 
       FIG. 5  illustrates exemplary embodiments of a transcutaneous energy transfer system (TETS)  700  with a ventricular assist device (VAD)  750  according to embodiments of the present invention. A VAD is a mechanical circulatory device that is used to partially or completely replace the function of a failing heart. For patients suffering from congestive heart failure, the VAD is implanted into the patient for support of the natural pumping function of a heart requiring assistance. VADs are designed to assist either the right (RVAD) or left (LVAD) ventricle, or both at once (BiVAD). VADs can be designed with an axial flow, centrifugal flow, or mixed flow configuration. An impeller is typically suspended by a journal bearing such as a ball and cup, or by a combination of active and/or passive magnetic forces, or by a combination of passive magnetic forces and hydrodynamic forces. In other embodiments, the blood pump can be an artificial heart, which is designed to completely take over cardiac function and may require the removal of a patient&#39;s heart. 
     The exemplary VAD  750  includes a pump assembly  713  including a blood pump  714 , a rechargeable power storage device  716 , and a power receiver unit  711 , which are all implanted in the body. The rechargeable power storage device  716  may include two or more rechargeable batteries  715  to provide the VAD  750  with a backup battery in case the stored energy in the primary battery is depleted or if the primary battery fails otherwise. The rechargeable power storage device  716  can be implanted in a location away from the blood pump assembly  713 , for example, in the lower abdominal as shown in  FIG. 7A . The power receiving unit  711  includes a resonator  712  with a coil that is coupled to the power storage device  716 , which is the electrical load of the power receiver unit  711 . The receiver unit  711  is implanted in the lower abdominal area where there may be less spatial constraints on the size and shape of the receiver resonator  712  compared to the pocket in which the pump is positioned. The resonant frequency of the exemplary receiver resonator  712  can be in a range of 100 kHz to 10 MHz. In an exemplary embodiment, the resonant frequency of the receiver resonator  712  can be 100 kHz, 500 kHz, 1 MHz, or 10 MHz. In other embodiments, another resonant frequency that is safe for the human body can be used. 
     The TETS  700  also includes a power transmitter unit  701  that is external to the patient. The transmitter unit  701  includes a transmitter resonator  702  with a coil that is configured to be coupled to a power supply source  720  such as an electrical wall outlet. Additionally or alternatively, the transmitter unit  701  can be coupled to other power sources such as a battery that can be used to drive an electrical current through the coil of the transmitter resonator  702 . When the transmitter unit  701  is plugged into the electrical wall outlet  720 , an electrical current is generated in the coil of the transmitter resonator  702 . The exemplary resonant frequency of the transmitter resonator  702  can be in a range of 100 kHz to 10 MHz. In an exemplary embodiment, the resonant frequency of the transmitter resonator  702  can be 100 kHz, 500 kHz, 1 MHz, or 10 MHz. In other embodiments, another resonant frequency that is safe for the human body can be used. The transmitter resonator  702  as part of the transmitter unit  701  may be embedded in a stationary object such as a wall, a chair, a bed, or other fixtures such as a car seat or objects that do not move by themselves without external control or human assistance. The source of power for a stationary and embedded transmitter resonator is generally alternating current (AC) from an electric outlet, but can also be direct current (DC) from a battery source. Likewise, the power signal may need to be converted between AC and DC depending on the pump type. 
     The exemplary system  700  includes one or more non-planar resonators. In the illustrated embodiment, the transmitter resonator  702  is a planar resonator made of a planar wire loop. The receiver resonator  712  is a non-planar resonator. In particular, the exemplary resonator has a curvature and is formed in a frustoconical shape. One will appreciate from the description herein that the size and shape of the resonators may vary depending on the application. Factors affecting the coil geometry include, but are not limited to, the designed for coupling zone between the receiver and transmitter, expected movement between the receiver and transmitter during use, and configuration of peripherals (e.g., controller and batteries). 
     When the receiver unit  711  in the patient comes within a separation distance D of the transmitter unit  701 , the TETS  700  is able to wirelessly transfer energy from the transmitter unit  701  to the receiver unit  711  to recharge the power storage device  716  of the VAD  750 . In one embodiment, at a given separation distance D being in the range of 2.5 cm to 35 cm, the transmitter unit  701  is able to deliver power in the range of 5 W to 20 W to the receiver unit  711  to recharge the batteries  715  in the power storage device  716  of the VAD  750 . By using a non-planar coil in the receiver resonator  712  in the receiver unit  711 , the TETS  700  is able to achieve a power transfer between the transmitter coil in the transmitter resonator  702  and the receiver coil in the receiver resonator  712  at a given separation distance D that is at least 25% of a maximum achievable power transfer at that given separation distance D, regardless of the coils&#39; respective orientation to each other. For example, in one embodiment, the TETS  700  is able to transfer a maximum amount of 20 W at a distance D of 10 cm when the respective coils in the receiver and transmitter resonators  702  and  712  are in their ideal alignment. As the patient moves around and causes the coil in the receiver resonator  712  to be orientated at a different angle relative to the coil in the transmitter resonator  702  away from their ideal alignment, the TETS  700  is still able to transfer at least 5 W of power to the receiver unit  711  at the separation distance D of 10 cm. In other embodiments, a greater or lesser amount of power can be delivered over longer distances, for example, separation distances of 35 cm and beyond, by adjusting the size and geometries of the resonators. 
     The use of a non-planar resonator that spans a surface area occupying three spatial dimensions in the receiver unit  711  according to embodiments of the present invention has the advantage over conventional systems that uses only planar resonators, in that the non-planar receiver resonator  712  is able to couple with more magnetic flux generated from the transmitter unit  701  in a wider range of spatial orientations. Hence, at a given separation distance D within an operating range of the TETS  700 , there is no requirement that the resonators  702  and  712  have to be placed in a particular orientation with respect to each other in order to transfer a meaningful amount of power required to recharge the rechargeable storage device  716 . 
     As will be described below, in various embodiments the transmitter resonator  702  is part of a piece of wearable clothing such as a vest or a jacket, or other wearable accessories. In the case of a transmitter resonator that is embedded into a piece of clothing or object wearable by a person that moves with a person, the source of power may be a portable-sized rechargeable battery worn by the patient. The power source for the transmitter may also include AC mains and other power source configurations. Exemplars of various power source configurations for a VAD system employing TETS are disclosed in International Pub. No. WO2007/053881 to Glanzmann et al. and U.S. Pub. No. 2014/0005466 to Crosby et al., the entire contents of which are incorporated herein for all purposes by reference. 
     In various embodiments, the system is configured as a hybrid making use of TETS and a percutaneous driveline. Such a system is comparable to the TETS described above except a driveline for providing power and data through the skin is further provided. In various embodiments, the system makes use of a conventional driveline. In various embodiments, the system includes a driveline which can be detached for a period of time. An example of such a system is described in U.S. Pat. No. 8,562,508 to Dague et al., the entire contents of which are incorporated herein for all purposes for reference. Such a configuration provides additional flexibility and may also have reduced technical complexity. 
     One will appreciate from the description herein that these various configurations may affect the number and type of implantables and peripherals worn by the patient. Although the invention is described below in terms of TETS, one will appreciate that the principles can apply equally to other system configurations. 
       FIGS. 6A and 6B  illustrate some of the typical components for a TETS similar to TETS  700  described above. In particular,  FIG. 6A  shows a few of the external components for power transmission including a transmitter coil  30  powered by batteries  35 . The batteries  35  are electrically connected to the coil  30  by user interface (UI) hubs  32  having battery clips  37 . The batteries  35  are configured to plug into a corresponding portion of the exemplary battery clips  37  to form the connection to the UI hubs  32 . The exemplary system includes two UI hubs  32  for connecting to first and second batteries  35  (e.g., in a “chain” or “daisy-chain” configuration of the batteries  35 ). One will appreciate, that other configurations may be used to connect the batteries  35  to the coil  30 . For example, the &#39;881 publication to Glanzmann et al. incorporated above illustrates embodiments where the external VAD controller includes an integrated battery. The controller can be connected to external batteries, AC mains, or both. Thus, the controller can provide power to the coil using the controller-integrated battery, the external batteries, AC mains, or a combination of the same. 
     UI hubs  32  include a system computer (not shown) having a screen  33  viewable by patients. The screen  33  is configured to display the system status and other information related to the system and its operation. In an exemplary embodiment, the screen is removable from the UI hub  32 . 
       FIG. 6B  illustrates a TET system similar to TETS  700  described above. The system includes external components similar to those shown in  FIG. 6A  except that it includes a user interface configured as a device to be worn on the wrist. By contrast, the user interface is integrated into the battery hubs in  FIG. 6A . The exemplary UI is formed with straps similar to a wrist watch. The UI includes a display (e.g., a screen) and input device. The exemplary UI is configured to monitor the operation of the VAS and/or TET systems. For example, the UI can monitor operation of the internal components—receiver coil, batteries, and/or implantable medical device—and display information to the patient based on the monitored information. The exemplary UI is configured for two-way communication with the internal components. The exemplary UI is configured to program the internal components. For example, the UI may be used to update the firmware or change patient settings. In one embodiment, the internal controller or implantable medical device includes memory and the UI is configured to modify data stored in the memory. In the exemplary case of a VAS, the implantable medical device or controller includes memory for storing patient data and the external UI is used to change the patient settings. Otherwise the patient settings could not be modified without a surgical procedure. The UI can incorporate any of the above-described features in any combination whether the UI is configured as a device to be worn on the wrist ( FIG. 6B ), as a UI integrated into the battery hubs (i.e., a “UI Hub,” shown in  FIG. 6A ), and/or as any other wearable in any other configuration. In some embodiments, multiple UIs (e.g., one wrist device and one UI hub, two or more UI hubs, etc.) may have distributed control functions. For example, one UI may include an input device (e.g., one or more buttons) and the other may include an output device (e.g., a screen), or one UI may store patient settings and the other may be configured for two-way communication with the implanted components. 
       FIGS. 7, 8A, and 8B  illustrate the layout and an exemplary use of the TETS components. In  FIG. 7 , a transmitter coil  30  transmits power to a receiver unit  502  using magnetic resonance, inductance, or other techniques. The receiver unit  502  implanted in the body is connected to a load  504 , which may be a VAD or other implantable medical device. The receiver unit may include an internal controller, batteries, electronics, an antenna for data communication, and the like. Alternatively, any of the components may be segregated in different locations within the body. 
       FIGS. 8A and 8B  show an exemplary use of the TETS. In  FIGS. 8A and 8B , the transmitter resonator  702  as part of the transmitter unit  701  of the TETS  700  is embedded in a bed  800 . Implanted in a patient are a VAD  750  and a receiver unit  711  with a non-planar receiver resonator that is coupled to the rechargeable batteries of the VAD  750 . In the exemplary embodiment, the TETS  700  is configured to be “loosely coupled” whereby sufficient power is transmitted between the transmitter and receiver in a variety of orientations and distances. Hence, the TETS  700  is still able to transfer a sufficient amount of energy to recharge the rechargeable batteries of the VAD  750  whether the patient is lying flat on the patient&#39;s back on the bed as shown in  FIG. 8A  or is lying on the patient&#39;s side as shown in  FIG. 8B . 
     Turning to  FIGS. 9-10B , the external and internal coils may be positioned in a variety of locations. Generally the closer the external transmitter and internal receiver coils are positioned to each other, the better the coupling and power efficiency. One approach to the design of a TET system is to determine an optimal location for the implanted components including the receiver coil and let the receiver location inform the location of the external transmitter.  FIG. 9  illustrates a typical scenario whereby both coils are positioned about the lateral abdominal area. For example, the external coil is placed on the lower abdomen, to the right of midline/belly button. The internal battery is also placed on the lower abdomen, to the right of midline/belly button. The external coil must align with the placement of the internal battery, as described above, although the exact location of the external coil will vary slightly from patient to patient, as the location of the internal battery is determined by the surgeon. 
       FIGS. 10A and 10B  are front and back views, respectively, showing a variety of potential anatomical positions for placement of the various system external components. The components include, but are not limited to, the external transmitter coil and peripherals such as batteries, electronics, a user interface, and a controller. Because the location of some of these external components relate to the position certain internal components, the positions shown in  FIGS. 10A and 10B  also relate in certain respects to placement of internal components including, but not limited to, the internal receiver coil, battery, TET circuitry, antenna and data communication circuitry, and implantable medical device. 
     In an exemplary embodiment, the desired position is determined by a wearable device (e.g., jacket, shirt, or straps). The desired position of the coil component depends on the medical device, application, and intended use. For example, some positions are more desirable for holding relatively heavy components. Some positions are sensitive to prolonged pressure. Restrictive collars should be avoided, as should weight on the back of the neck. Any pressure on the shoulders should be balanced. Side areas may be desirable, but also require flexibility of components placed there, and components should not interfere with the swing of the arms. In addition, the lumbar area may be positive for many positions. The strategy for selecting a position of the external coil may also depend on the patient population. One position which works for a patient with a high body mass index (BMI) may be impractical for another patient with a low BMI. Body shape and gender may also be factors. It may be impractical to manufacture individually customized wearable solutions for each patient. Accordingly, one strategy may be to select a position that is desirable for large portion of the patient population and is acceptable for the rest of the population. Another strategy may be to provide a plurality of wearables each with different coil positions for partial customization. 
     As described above with respect to  FIGS. 1 to 4 , the position of the coil can be critical to performance, in particular energy transmission, and patient quality of life (QoL). With continued reference to  FIGS. 10A and 10B , in one embodiment the system is designed to maintain the coil on the abdomen. With continued reference to  FIGS. 10A and 10B , in one embodiment the system is designed to maintain the coil on the abdomen. In one embodiment, the system is designed to maintain the coil laterally on the abdomen. In one embodiment, the system is designed to maintain the coil on the lumbar region of the patient&#39;s back. 
       FIGS. 11A to 28  illustrate a variety of devices for carrying external components of exemplary system mechanical circulatory support (MCS) system, and in particular a ventricular assist system (VAS), making use of wireless energy transfer. Although described in terms of a VAS, one will appreciate that the same principles may be applied equally to other systems using percutaneous wire or wireless energy including, but not limited to, ICDs, neurostimulators, and other medical devices as well as phones, cameras, and other electronics. U.S. Pub. No. 2010/0122995 to Thomas et al., incorporated herein for all purposes by reference, describes wearables and other devices for peripherals with a conventional VAD system configuration. Various aspects of the invention are similar to those described by Thomas et al. 
     The wearable devices described herein address a number of system and patient needs including, but not limited to, positioning and alignment of the transmitter coil, patient comfort and allowing a wide range of motion, distributing and balancing the weight of the peripherals on the body to reduce strain, stability of components and security, flexibility of configurations, and providing the patient easy access to peripherals during use. For example, as explained above with respect to  FIGS. 2, 3A, and 3B , the relative positioning of the transmitter and receiver coils greatly influences the coupling efficiency and plays an important role in the overall effectiveness of the system. The wearable devices may also address other needs such as maintaining a separation distance between the skin and peripheral components which generate heat. For example, in various embodiments the wearable device is designed to separate heat generating components (e.g., the control electronics) from the patient&#39;s skin. In various respects, the embodiments described herein seek to optimize patient comfort, QoL, and usability, while also providing good system performance. 
       FIGS. 11A to 11D  illustrate a wearable device for holding the transmitter coil and other peripheral components in accordance with aspects of the invention. The wearable device in  FIG. 11A  is formed as a vest or shirt. The vest may be provided in different sizes (e.g., small, medium, and large). The exemplary vest is a compression shirt. In one embodiment, a section of the fabric at the breast region is configured to be very stretchy. In one embodiment, a section of the fabric at the breast region is configured to hang relatively loose to provide more comfort around a woman&#39;s bra sizes. 
     As shown in  FIGS. 11C and 11D , the exemplary wearable includes a pocket for holding the transmitter coil. The provision of a pocket provides more precision alignment of the coil relative to the patient&#39;s body, and in turn the internal components. The pocket includes a zip closure. The closure has two zip closures: one zipper on the side going down and one on the bottom going across. The zip closure allows the patient to easily open a full corner of the pocket and insert the coil. In various embodiments, the closure is a plastic zipper. The plastic zipper may reduce the risk of interference with the coil flux. 
     In various embodiments, the wearable&#39;s pocket is configured so the coil position is adjustable. The sides of the interior can be lined with a “grip” coating or material (e.g., silicone) to hold coil in correct position within the pocket. Padding may be provided for patient comfort. In addition, the pocket may be slightly oversized, so the patient can align it properly and the coil will not shift during wear. 
     With reference to  FIGS. 12A to 12C , the exemplary wearable may include a battery pocket on the back. The battery pocket is fully removable. The pocket is padded for patient comfort. The pocket is configured so the battery is accessible even while the patient is wearing it. 
     The exemplary battery pocket design is configured with flaps for attaching the pocket to the patient&#39;s body. In order to change the batteries, the patient releases the hook-and-loop fastener on the patient&#39;s left side and then swings the battery pocket around to the front thereby enabling the patient to change batteries. In one example, the batteries must be changed periodically so it is advantageous to provide easier battery exchange without requiring removal of the whole wearable. Additionally, a battery pocket zipper attachment allows the patient to remove the battery pocket to wear the shirt for holding the coil during sleep. 
       FIG. 13  shows another wearable device similar to the one shown in  FIG. 11A  (e.g., a vest). 
       FIG. 14  shows another wearable device formed as a band or belt for wrapping around the patient. The exemplary embodiment is a two-band design. The band is configured to fit a wide range of sizes from a waist size of about 24″ to about 40″. In various embodiments, the band is configured to fit waist sizes of at least 40″. A hook-and-loop fastener on each strap provides size adjustment. The coil is held within a pocket (mesh front) with a zipper closure. Vertical grommets on the pocket to support where the straps loop through the pocket portion. The wearable optionally includes a movable vertical support strap. Horizontal straps may be provided which slide into the vertical support. The wearable is configured with an “edge-less” fabric for patient comfort. For example, standard elastic straps may cause skin irritation in some patients. A set of interior of straps may be included. The straps may have a “non slip” grip coating or material (e.g., silicone) in the inside of the straps (and/or on the back of the pocket). 
       FIGS. 15A to 15E  show another wearable device formed as belt. The belt in  FIGS. 15A to 15E  is similar in some respects to the band shown in  FIG. 14 . The belt is configured to allow adjustment of a height of belt and contour of the shape. For example, the belt features even tapering around a middle section thereof. This allows optimization, for example, to eliminate fabric bunching at the back of belt. The coil pocket is provided with a zipper that has been minimized in size. The belt optionally includes a relatively more flexible “loop” section. An interior has a strip of “nonslip” grip coating or material. 
     Attention is now directed to  FIGS. 16 and 17A-17C  which show another wearable device. By contrast to the wearables described above, the illustrated embodiment is a wearable for holding the external coil. 
     The illustrated embodiment is formed as a full coil belt. The belt is formed of a breathable and/or perforated fabric (e.g., neoprene) to reduce heat. In one embodiment, the primary belt fabric has minimal edges (e.g., die cut or welded) to reduce any skin irritation a stitch might create. A secondary tension adjustment is formed in a manner similar to the vest of  FIG. 11A . 
       FIGS. 18A and 18B  show another wearable device formed as a battery belt. The belt is configured to maintain a large belt attachment point to distribute weight evenly. The belt is split into two or more sections to allow for pant belt loops if patient chooses to use his or her existing pant belt. In one embodiment, the belt is configured so to accommodate an existing pant belt. The exemplary belt is configured to allow adjustment for a waist between about 24″ to about 40″. 
     Turning to  FIGS. 19A and 19B , the exemplary belt has a battery clip pocket height relatively higher than the pockets described above. For example, the pocket height ends just below the UI hub. The increased height of the battery clip pocket allows for added security so the battery remains stably in the pocket. The battery pocket may also include a vertical slot for connecting a cable between batteries, coverable by a flap (e.g., with a hook-and-loop closure). 
     In one embodiment shown in  FIGS. 20A and 20B , a pivot or snap is provided between the battery clip pocket and the belt mounting system. The pivot allows the battery clips to rotate for added comfort and move to prevent breakage during use, e.g., when the patient is sitting in a chair. 
       FIGS. 21A, 21B, and 22  show another form of a wearable device. The device shown in  FIGS. 21A and 22B  is a battery bag having a front zipper. The device shown in  FIG. 22  is a backpack. 
       FIGS. 21A and 22B  show a battery bag with a front flap that opens to expose an interior. The interior of the bag includes storage compartments and dividers. An internal battery clip mounts so the battery clips are held in place when bag is opened. 
     The bag includes a padded shoulder strap for carrying. The bag further includes a waist strap stored in a strap storage pocket. The shoulder and waist straps are configured to distribute the weight of the batteries and other items in the bag to reduce strain on the patient. 
     The illustrated embodiment further includes a UI hub window with additional reinforcement. The exemplary hub window is formed as an opening in the bottom of the bag. 
       FIG. 22  shows another battery bag. The bag of  FIG. 22  is similar to the bag of  FIG. 21  except it is formed as a backpack. The bag optionally includes a chest strap to prevent the shoulder straps from slipping off chest. A cut-out window is provided for the UI hub. The bag may include a flap (with clear window) to fold over for security with a hook-and-loop closure. The flap accommodates access to the UI hub and provides visual access to the UI hub for patient during use. An internal divider is provided in the battery compartment. 
       FIGS. 23A to 28  show additional embodiments of wearable devices in accordance with the invention. 
       FIGS. 23A-23C  show a vest similar in various respects to the wearable device of  FIG. 11A . The vest includes a plurality of specific battery pockets to allow the user to choose which ones he or she prefers. The vest may include pocket on the front and/or on the back of the vest. The coil is stored in an internal pouch on the front right panel. A tension belt is hidden between two layers and is used to secure the coil in place. The tension belt may need to be secured (e.g., clipped) before the vest is closed (e.g., zipped). A strap on the user&#39;s left side is used to create tension on the coil belt. The wiring between the batteries and coil may be secured on the inside of the vest. The vest may be fabricated from a breathable and/or perforated fabric (e.g., mesh, neoprene) for breathability on skin contact. 
       FIGS. 24A-24D  show a wearable device similar in various respects to the devices of  FIG. 11A  and the belts described above. The device is formed as a waist belt with the addition of straps. The batteries are clipped to a rigid track which travels around the back and allows the batteries to be positioned where they are most convenient. The belt is adjusted similar to a corset with hooks for the overall sizing, but it can be quickly removed by unzipping the front. The coil sits in a semi rigid cradle and can be pushed up and down for vertical positioning. The cradle moves horizontally for lateral positioning to the left and right. The device is configured so a doctor can aid the user in adjusting the fit of the device on the patient&#39;s body, and thereafter the user may make finer, incremental adjustments. 
     In some embodiments, the garment is configured to provide a mounting track, as shown in  FIGS. 24A-24D . The mounting track provides a plurality of mounting points, and is integrally attached to the garment. The mounting track may substantially encompass the thoracic region of the wearer. For example, the mounting track may be positioned on the waist of the wearer. The mounting track is configured to provide at least two mounting positions, and to removably attach to at least one of, a battery, a coil, and/or a pocket (e.g., battery pocket, coil pocket.) The mounting positions may include, but are not limited to, Velcro®, adhesive, magnets, clips, hooks, and the like. In one example embodiment, the mounting positions removably attach to a coil pocket using Velcro®. In another example embodiment, the mounting track is configured to removably couple with a battery by providing a hook and/or a ridge to support the battery. (Velcro is a registered trademark of Velcro Industries BVBA, Deinze, Belgium) 
       FIGS. 25A-25C  show another wearable device similar to the device of  FIGS. 23A-24D . The wearable device is embodied as an undershirt, fabricated from a breathable and/or elastic material for comfort of the wearer. The device has a primary portion for wrapping around the waist or chest and a shoulder strap. The shoulder strap maintains the vertical height of the waist band. The shoulder strap also carries some of the weight to alleviate pressure on the waist. The device includes a vertical zipper in for facilitating putting on and taking off the device. 
     In some embodiments, the garment is configured to provide an adjustable housing, as shown in  FIGS. 25A and 25B . The adjustable housing is configured to removably attach at least one of, a battery, an external coil, and a pocket (e.g., coil pocket, battery pocket), to the garment. For example, one or more Velcro® straps may be provided to secure a coil to a garment, by securing the one or more straps after placing the coil under at least one strap. In another example, an elastic strap may be provided to secure a battery, by placing the battery under the midsection of a strap. The adjustable housing is further configured to allow the position of the battery, coil, and/or pocket, to be adjusted by the person wearing the garment. For example, the position may be adjusted with moderate force and may not require additional tools. 
     In some embodiments, the garment is configured to provide flaps attached at the waist of the garment, and that hang below the waist of the garment wearer, as shown in  FIGS. 25A and 25B . The flaps are configured to be removably attached to at least one of, a battery, a coil, and/or a pocket (e.g., battery pocket, coil pocket). In some embodiments, the flaps are configured to pivot at the waist of the garment. For example, the flaps may hang from the garment while the wearer is standing, and pivot to lay in the lap of the garment wearer after sitting. The battery, coil, and/or pocket may be removably attached to the flaps by at least one of, Velcro®, adhesive, magnets, and the like. In some embodiments, the batteries and/or coils may be stored in a fabric case with cable openings. 
       FIG. 26A  and  FIG. 26B  shows another device similar to the devices of  FIGS. 23A-25C  except the device is formed as a full vest/undershirt. In some embodiments, the shirt is substantially made of an elastic material, and/or includes tension straps to secure the garment to the wearer. The device is formed to hold snugly against the body so it can be easily fit under normal clothing. 
     In some embodiments, a dual lock strip is provided, as shown in  FIGS. 26A and 26B . The dual lock strip is configured to removably attach to both a pocket containing a coil and/or battery and a garment. The dual lock strip is configured to allow for the placement of the pocket (e.g., external coil pocket, battery pocket) to be adjusted. For example, the pocket may be removed from the dual lock strip and attached to a slightly lower position on the dual lock strip. Additionally or alternatively, the dual lock strip is configured to allow for adjustments to the placement of the dual lock strip on the garment. For example, the dual lock strip, which a pocket may be attached to, may be removed from a garment and attached on a higher portion. In some embodiments, the dual lock strip has a larger attachment surface than the pocket, to allow for the positioning of the pocket to be adjusted. In further embodiments, placement guides are provided on the dual lock strip to indicate proper placement of the dual lock strip on the garment and/or pocket. For example, the dual lock strip may include a visual marker indicating a corresponding location on the garment for proper placement. The dual lock strip may be removably coupled to the garment and/or pocket using one of, Velcro®, adhesive, magnets, and the like. 
     The exemplary vest does not include pockets for the coil or battery. Instead, illustrated embodiment is intended as a wearable system. A separate carrier is provided for holding the transmitter coil. The carrier is removable from the vest.  FIG. 27  shows additional accessories for carrying the batteries. For example, the batteries can be carried in a bag similar to the one shown in  FIG. 21B . Battery pockets may also be provided. In the illustrated embodiments, the pockets can be removably attached to the vest.  FIG. 28  shows an additional embodiment for holding the batteries. As shown in  FIG. 28 , the battery pocket and vest can be configured for connection with hook-and-loop fasteners. A portion of the vest includes hook fasteners, and an outer surface of the battery pocket includes loop fasteners or a felt material. Thus, unlike the embodiments above with discrete, predetermined battery locations, the battery pocket can be placed in a variety of customized locations. The exemplary battery pocket is relatively large and can accommodate two or more batteries. One will appreciate from the description herein that other configurations may be used to provide semi-customized configuration of the battery pockets. Additionally, the vest includes a large pocket on the inside of the vest where the external coil may be placed. 
     While the invention has been described in terms of several embodiments, those skilled in the art will recognize that the invention is not limited to the embodiments described, can be practiced with modification and alteration within the spirit and scope of the appended claims. The description is thus to be regarded as illustrative instead of limiting.