Antenna designs for communication between a wirelessly powered implant to an external device outside the body

FIELD

0001The field relates generally to resonant wireless power transfer systems, and more specifically to communication systems and methods for implantable resonant wireless power transfer systems.

BACKGROUND

0002Many types of devices require transmitting energy between locations. Recent advances have accelerated the pace of innovation for wireless energy transmission (WET) without the use of cords. An example of a system using wireless energy technology is a powered, implantable medical device.

0003Many implantable medical devices require electrical systems to power the implant. Typically, this is achieved using percutaneous wiring to connect a power source to the implant. More recently, there has been interest in development of Transcutaneous Energy Transfer (TET) systems, e.g., through an oscillating magnetic field, for powering implantable medical devices.

0004A TET system usually includes a number of components or systems. A conventional TET system is implemented with a transmitting coil and a receiving coil for transmitting energy across the skin layer. The system typically includes a controller for driving the transmitting coil and/or controlling the implanted electronics.

0005Typically, implantable medical devices, such as implanted sensors, require very little power to operate. With such low power levels (on the order of milliwatts), power transfer levels and efficiency can be lower. With higher power devices (e.g., on the order of watts and up to 15 W or more), efficient transfer of wireless power is extremely important. Additionally, positions within the body are limited that can accommodate larger implanted devices, some of which are deep below the skin surface. These implant locations require additional attention to position and orientation of both the transmit and receive coils, as well as techniques to improve and maximize transfer efficiency.

0006Previous TET systems for implantable medical devices required the implanted receiver coil to be positioned just under the skin, and typically include a mechanical feature to align the receive and transmit coils and keep them together. 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. TET systems can be designed for operation even while power is not being received by the receiver coil. In a typical configuration, solid-state electronics and a battery can power the implanted medical device when external power is interrupted or not available. In this case, it may be beneficial to provide a user interface or other electronic device to communicate information to the patient and/or caregiver regarding the implanted components. For example, a user interface may include alarms to notify the patient when the internal battery level is low.

0007Reliable communication between an implantable medical device, a user interface, and an external transmitter can be a challenge because of varying conditions and distances between the components of the TET system.

0008Radio signals have limitations when used for communication between implantable devices. Attenuation of radio signals by the human body is very large and can disrupt communication signals. Even under optimal circumstances, such as a shallow implant depth, a properly designed antenna, proper orientation of the implanted module, and a reliable radio link, attenuation can be on the order of 10 dB to 20 dB. For deeper implant depths, or if the implant rotates significantly from its intended position, attenuation may grow to 100 dB or more. This can lead to an unreliable or totally interrupted radio link with a high loss rate.

0009In-band communication has been used in implanted systems and comprises modulation of a receiver load that can be sensed by a transmitter. The problem with in-band communication is that it requires additional electronics in the resonant circuit, which lowers the power transfer efficiency and leads to additional heating of the receiver. Additionally, there is a fundamental design conflict between optimizing a resonant circuit to be power efficient and to transmit a meaningful amount of information. The former requires coils with a high quality factor while the latter prefers lower quality factors.

0010It is therefore desirable to provide a system in which the implant can communicate effectively with the user interface in the absence of the transmitter.

0011<patcit id="pcit0001" dnum="US2014265621A1"><text>US 2014/265621 A1</text></patcit> discloses systems and methods for wireless energy transfer. A transmitter unit has a transmitter resonator with a coil that is coupled to a power supply to wirelessly transmit power to a receiver unit. A receiver unit has a receiver resonator with a coil coupled to a device load. The receiver unit can include a ferrite enclosure to prevent transmission of magnetic flux into electronics of the receiver unit, and can include ferrite fins to increase a coupling between the transmitter resonator and the receiver resonator.

0012<nplcit id="ncit0001" npl-type="b"><text>QING X ET AL, "Segmented loop antenna for UHF near-field RFID applications", ELECTRONIC LET, THE INSTITUTION OF ENGINEERING AND TECHNOLOGY, (20090813), vol. 45, no. 17, doi:10.1049/EL:20090326, ISSN 1350-911X, pages 872 - 873</text></nplcit>, XP006033688 discloses a segmented loop antenna for ultra-high frequency (UHF) near-field radio frequency identification (RFID) applications. The segmented configuration makes the current along the loop remain in-phase even though the perimeter of the loop is comparable to the operating wavelength, so that a strong and uniform magnetic field is generated in the region surrounding the antenna. The antenna printed on a FR4 printed circuit board (PCB) with an overall size of 160 x 180 x 0.5 mm achieves good impedance matching and uniform magnetic field distribution over an operating bandwidth of 800-1040 MHz, which is desirable for UHF near-field RFID reader applications.

0013<patcit id="pcit0002" dnum="US5333095A"><text>US 5 333 095 A</text></patcit> discloses a feedthrough filter capacitor assembly for human implant.

0014<nplcit id="ncit0002" npl-type="s"><text>Kunpeng Wei ET AL: "Design of a Wideband Horizontally Polarized Omnidirectional Printed Loop Antenna", IEEE ANTENNA AND WIRELESS PROPAGATION LETTERS, vol. 11, 1 January 2012 (2012-01-01), pages 49-52, XP055572058, US ISSN: 1536-1225, DOI: 10.1109/LAWP.2012.2182670</text></nplcit>, discloses the design of a novel wideband horizontal polarized omnidirectional printed loop antenna.

BRIEF DESCRIPTION OF THE DRAWINGS

0015The 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: <ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">Fig. 1</figref> illustrates a basic wireless power transfer system.</li><li><figref idref="f0001">Fig. 2</figref> illustrates the flux generated by a pair of coils.</li><li><figref idref="f0002">Figs. 3A-3B</figref> illustrate the effect of coil alignment on the coupling coefficient.</li><li><figref idref="f0003">Fig. 4</figref> is one embodiment of a wireless power transfer system including a communications antenna on an implantable TETS receiver.</li><li><figref idref="f0004">Fig. 5</figref> shows one embodiment of a communications antenna for use with a wireless power transfer system.</li><li><figref idref="f0005">Fig. 6</figref> shows another embodiment of a communications antenna for use with a wireless power transfer system.</li><li><figref idref="f0006">Fig. 7</figref> shows yet another embodiment of a communications antenna for use with a wireless power transfer system.</li><li><figref idref="f0007">Fig. 8</figref> shows a cross-sectional view of a communications antenna for use with a wireless power transfer system.</li><li><figref idref="f0008">Fig. 9</figref> illustrates antenna match parameterized with increasing gap between conductive elements of a communications antenna.</li><li><figref idref="f0009">Fig. 10</figref> illustrates antenna match (reflection coefficient) parameterized with increasing gap.</li></ul>

SUMMARY OF THE DISCLOSURE

0016According to the present invention there is provided the wireless receiver of claim 1. Additional aspects of the invention are set out in the dependent claims.

DETAILED DESCRIPTION

0017In 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.

0018Various aspects of the invention are similar to those described in International Patent Pub. No. <patcit id="pcit0003" dnum="WO2012045050A"><text>WO2012045050</text></patcit>; <patcit id="pcit0004" dnum="US8140168B"><text>U.S. Patent Nos. 8,140,168</text></patcit>; <patcit id="pcit0005" dnum="US7865245B"><text>7,865,245</text></patcit>; <patcit id="pcit0006" dnum="US7774069B"><text>7,774,069</text></patcit>; <patcit id="pcit0007" dnum="US7711433B"><text>7,711,433</text></patcit>; <patcit id="pcit0008" dnum="US7650187B"><text>7,650,187</text></patcit>; <patcit id="pcit0009" dnum="US7571007B"><text>7,571,007</text></patcit>; <patcit id="pcit0010" dnum="US7741734B"><text>7,741,734</text></patcit>; <patcit id="pcit0011" dnum="US7825543B"><text>7,825,543</text></patcit>; <patcit id="pcit0012" dnum="US6591139B"><text>6,591,139</text></patcit>; <patcit id="pcit0013" dnum="US6553263B"><text>6,553,263</text></patcit>; and <patcit id="pcit0014" dnum="US5350413A"><text>5,350,413</text></patcit>; and <patcit id="pcit0015" dnum="US20100308939A"><text>U.S. Pub. Nos. 2010/0308939</text></patcit>;<patcit id="pcit0016" dnum="US2008027293A"><text> 2008/027293</text></patcit>; and <patcit id="pcit0017" dnum="US20100102639A"><text>2010/0102639</text></patcit>.

<u>Wireless Power Transmission System</u>

0019Power may be transmitted wirelessly by magnetic induction. In various embodiments, the transmitter and receiver are closely coupled.

0020In 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.

0021In 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.

0022In 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. <patcit id="pcit0018" dnum="WO2000074747A"><text>WO2000/074747</text></patcit>. 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. Examples of closely coupled system with resonant coils are described in International Pub. Nos. <patcit id="pcit0019" dnum="WO2001037926A"><text>WO2001/037926</text></patcit>; <patcit id="pcit0020" dnum="WO2012087807A"><text>WO2012/087807</text></patcit>; <patcit id="pcit0021" dnum="WO2012087811A"><text>WO2012/087811</text></patcit>; <patcit id="pcit0022" dnum="WO2012087816A"><text>WO2012/087816</text></patcit>; <patcit id="pcit0023" dnum="WO2012087819A"><text>WO2012/087819</text></patcit>; <patcit id="pcit0024" dnum="WO2010030378A"><text>WO2010/030378</text></patcit>; and <patcit id="pcit0025" dnum="WO2012056365A"><text>WO2012/056365</text></patcit>, <patcit id="pcit0026" dnum="US20030171792A"><text>U.S. Pub. No. 2003/0171792</text></patcit>, and <patcit id="pcit0027" dnum="US5350413A"><text>U.S. Patent No. 5,350,413</text></patcit> (now abandoned).

0023For given coil sizes and separations, coupling a given amount of power requires generating the same magnetic field strength for either inductive or resonant systems. This requires the same number of ampere-turns in the coils. In inductive systems, all the ampere-turns pass through the MOSFETs and generate power losses in their on-state resistance. In resonant systems, only the exciter ampere-turns pass through the MOSFETs, while the resonator ampere-turns do not. As a consequence, resonant systems will always have lower losses and higher efficiencies than inductive systems of the same dimensions and power through-put.

0024In 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. <patcit id="pcit0028" dnum="WO2000074747A"><text>WO2000/074747</text></patcit>. 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. <patcit id="pcit0029" dnum="WO2001037926A"><text>WO2001/037926</text></patcit>; <patcit id="pcit0030" dnum="WO2012087807A"><text>WO2012/087807</text></patcit>; <patcit id="pcit0031" dnum="WO2012087811A"><text>WO2012/087811</text></patcit>; <patcit id="pcit0032" dnum="WO2012087816A"><text>WO2012/087816</text></patcit>; <patcit id="pcit0033" dnum="WO2012087819A"><text>WO2012/087819</text></patcit>; <patcit id="pcit0034" dnum="WO2010030378A"><text>WO2010/030378</text></patcit>; and <patcit id="pcit0035" dnum="WO2012056365A"><text>WO2012/056365</text></patcit>, and <patcit id="pcit0036" dnum="US20030171792A"><text>U.S. Pub. No. 2003/0171792</text></patcit>, incorporated herein for all purposes by reference.

0025In 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. <patcit id="pcit0037" dnum="WO2012045050A"><text>WO2012/045050</text></patcit>.

0026Power may be transmitted wirelessly by radiating energy. In all 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. <patcit id="pcit0038" dnum="WO2010089354A"><text>WO2010/089354</text></patcit>. An example of such a non-resonant system is described in International Pub. No. <patcit id="pcit0039" dnum="WO2009018271A"><text>WO2009/018271</text></patcit>. In examples not part of the claimed invention, 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. <patcit id="pcit0040" dnum="WO2010089354A"><text>WO2010/089354</text></patcit>.

0027Power 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. <patcit id="pcit0041" dnum="WO2008066941A"><text>WO2008/066941</text></patcit>.

0028Power 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. <patcit id="pcit0042" dnum="WO2012056365A"><text>WO2012/056365</text></patcit>.

0029The 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.

0030While 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.

0031In accordance with various embodiments of this disclosure, the 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.

0032In 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.

0033In 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.

0034In 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."

0035In 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.

0036In 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.

0037Because 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.

0038Since 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.

0039In 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.

0040Turning now to <figref idref="f0001">Fig. 1</figref>, 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.

0041The 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.<maths id="math0001"><math display="inline"><mi mathvariant="italic">Mxy</mi><mo>=</mo><mi>k</mi><msqrt><mrow><mi mathvariant="italic">Lx</mi><mo>⋅</mo><mi mathvariant="italic">Ly</mi></mrow></msqrt></math><img file="EP3198677B1_D0001.tif" /></maths>

0042In 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.

0043The 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.

0044In 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.

0045In 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.

0046In 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.

0047In 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.

0048One 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.

0049In an exemplary embodiment, the system load power, P_L, is assumed to be 15 Watts and the operating frequency of the system, <b><i>f</i></b>, is 250 kHz. Then, for each cycle the load removes a certain amount of energy from the resonance:<maths id="math0002"><math display="block"><msub><mi>e</mi><mi>L</mi></msub><mo>=</mo><mfrac><msub><mi>P</mi><mi>L</mi></msub><mi>f</mi></mfrac><mo>=</mo><mn>60</mn><mi mathvariant="italic">μJ</mi></math><img file="EP3198677B1_D0002.tif" /></maths> Energy the load removes in one cycle

0050It 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.

0051The 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.<maths id="math0003"><math display="block"><msub><mi>e</mi><mi>y</mi></msub><mo>=</mo><mfrac><mn>1</mn><mn>2</mn></mfrac><msup><mi mathvariant="italic">Li</mi><mn>2</mn></msup></math><img file="EP3198677B1_D0003.tif" /></maths><maths id="math0004"><math display="block"><msub><mi>i</mi><mi>y</mi></msub><mo>=</mo><msqrt><mfrac><mrow><mn>2</mn><msub><mi>e</mi><mi>y</mi></msub></mrow><mi>L</mi></mfrac></msqrt><mo>=</mo><mn>3.74</mn><mi>A</mi><mspace width="1ex" /><mi mathvariant="italic">peak</mi></math><img file="EP3198677B1_D0004.tif" /></maths><maths id="math0005"><math display="block"><msub><mi>v</mi><mi>y</mi></msub><mo>=</mo><msub><mi mathvariant="italic">ωL</mi><mi>y</mi></msub><msub><mi>i</mi><mi>y</mi></msub><mo>=</mo><mn>352</mn><mi>V</mi><mspace width="1ex" /><mi mathvariant="italic">peak</mi></math><img file="EP3198677B1_D0005.tif" /></maths>

0052The 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.<maths id="math0006"><math display="block"><msub><mi>e</mi><mi>x</mi></msub><mo>=</mo><mfrac><msub><mi>e</mi><mi>y</mi></msub><mi>k</mi></mfrac></math><img file="EP3198677B1_D0006.tif" /></maths>

0053Given that k is 0.05:<maths id="math0007"><math display="block"><msub><mi>e</mi><mi>x</mi></msub><mo>=</mo><mfrac><mrow><mn>420</mn><mi mathvariant="italic">μJ</mi></mrow><mn>0.05</mn></mfrac><mo>=</mo><mn>8.4</mn><mi mathvariant="italic">mJ</mi></math><img file="EP3198677B1_D0007.tif" /></maths>

0054For the same circuit the self inductance of the transmitter coil was 146uH as mentioned above. This results in:<maths id="math0008"><math display="block"><msub><mi>i</mi><mi>x</mi></msub><mo>=</mo><msqrt><mfrac><mrow><mn>2</mn><msub><mi>e</mi><mi>x</mi></msub></mrow><mi>L</mi></mfrac></msqrt><mo>=</mo><mn>10.7</mn><mi>A</mi><mspace width="1ex" /><mi mathvariant="italic">peak</mi></math><img file="EP3198677B1_D0008.tif" /></maths><maths id="math0009"><math display="block"><msub><mi>v</mi><mi>x</mi></msub><mo>=</mo><msub><mi mathvariant="italic">ωL</mi><mi>x</mi></msub><msub><mi>i</mi><mi>x</mi></msub><mo>=</mo><mn>2460</mn><mi>V</mi><mspace width="1ex" /><mi mathvariant="italic">peak</mi></math><img file="EP3198677B1_D0009.tif" /></maths>

0055One 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.

<u>Estimation of Coupling Coefficient and Mutual Inductance</u>

0056As 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.

0057The 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.

0058Described 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.

0059If 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 <figref idref="f0001">Fig. 2</figref>.

0060If 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 <figref idref="f0002">Fig. 3A</figref>. 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.

0061If 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 <figref idref="f0002">Fig. 3B</figref>.

0062A 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.<maths id="math0010"><math display="block"><msub><mi>M</mi><mi mathvariant="italic">xy</mi></msub><mo>=</mo><msub><mi>M</mi><mi mathvariant="italic">yx</mi></msub></math><img file="EP3198677B1_D0010.tif" /></maths>

0063<figref idref="f0003">Fig. 4</figref> illustrates a wireless power transfer system comprising an implantable TETS receiver unit 400 implanted in an abdomen of a human patient. The receiver unit 400 can be coupled to a device load 402, such as an implantable medical device, e.g., an implantable LVAD or heart pump. The exemplary receiver unit 400 can include a receiver resonator coil and electronics configured to receive wireless energy from an external transmitter 401, which can include a power supply such as a pulse generator connected to a transmitter resonator coil. In one embodiment, the electronics and coils are implanted separately and connected by an implanted cable. In some embodiments, external controller 404 can be configured to communicate with the TETS receiver 400 and can be worn by the patient, such as on the patient's wrist. In other embodiments, the external controller can be an electronic computing device such as a personal computer, a tablet, smartphone, or laptop computer.

0064In all embodiments, the receiver unit 400 further includes a communications antenna 406 disposed along an outer periphery, the antenna being configured to send and receive communications data to and from other electronic devices inside and outside of the body. In one embodiment, the receiver unit further includes an internal rechargeable power source. In various embodiments, the receiver unit 400 of the TET system is configured as a single implanted device including the receive coil, antenna, power source, and associated circuitry. The receiver unit is configured so the implantable medical device can be plugged directly into the unit. The single housing configuration makes implantation easier and faster. Additionally, since there are less implants, and consequently less tunneling in the body and percutaneous defect sites, adverse event risks like bleeding and infection are reduced. One of skill will appreciate from the description herein that various internal components of the system can be bundled together or implanted separately. For example, the internal rechargeable power source can be implanted separately from the receiver unit and connected by a power cable. The antenna assembly, power source, and receive coil can all be configured in separate hermetically sealed housings. International Pub. No. <patcit id="pcit0043" dnum="WO2007053881A1"><text>WO2007/053881A1</text></patcit>, <patcit id="pcit0044" dnum="US20140005466A"><text>U.S. Pub. No. 2014/0005466</text></patcit>, and <patcit id="pcit0045" dnum="US8562508B"><text>U.S. Pat. No. 8,562,508</text></patcit>, disclose several exemplary configurations.

0065<figref idref="f0004">Fig. 5</figref> illustrates a top-down view of one embodiment of a communications antenna 506 configured to send and receive communications data from an implanted wireless powered device to external devices. The communications antenna 506 can be disposed on or in the receiver unit illustrated in <figref idref="f0003">Fig. 4</figref>. The communications antenna 506 can comprise at least one feedline 507 that connects the communications antenna to the radio transmitter and/or receiver, and a plurality of conductive elements 508a-h coupled to the feedline. While conductive elements 508a and 508d are physically connected to the feedline (e.g., with a wire or conductive connection), adjacent conductive elements in the communications antenna are not physically connected, but instead are separated by gaps 510a-p. The gaps can be, for example, air gaps between adjacent conductive elements. More specifically, adjacent conductive elements are not electrically connected to each other, such as with a conductive wire or trace, but instead are separated from each other by the gaps. In the illustrated embodiment, gap 510a separates conductive elements 508a and 508b, gap 510b separates conductive elements 508b and 508c, gap 510c separates conductive elements 508c and 508d, gap 510d separates conductive elements 508d and 508e, gap 510e separates conductive elements 508e and 508f, gap 510f separates conductive elements 508f and 508a. Similarly, gap 510g separates conductive elements 508a and 508g, gap 510h separates conductive elements 508b and 508g, gap 510i separates conductive elements 508c and 508g, gap 510j separates conductive elements 508d and 508g, gap 510k separates conductive elements 508d and 508h, gap 5101 separates conductive elements 508e and 508h, gap 510m separates conductive elements 508f and 508h, and gap 510n separates conductive elements 508a and 508h. Finally, gaps 510o and 510p separate conductive elements 508g and 508h on the perimeter of the communications antenna.

0066As is known in the art, currents can be induced in a loop of conductor or metal when exposed to a magnetic field. Therefore, a looped conductor placed in the TETS field of a magnetically coupled wireless power transfer system can result in potentially large currents being induced in the looped conductor by the TETS field. In this example, an antenna can be saturated by the wireless power transfer. To solve this problem, the communications antennas described herein can include gaps between adjacent conductive elements to minimize low-frequency coupling in the antenna. These antennas can be designed without any closed paths that can be traced to induce currents in the presence of a TETS field. In some embodiments, a wireless power transfer system (such as the system described in <figref idref="f0003">Fig. 4</figref>) can have a low operating frequency, such as an operating frequency of 250 kHz. By comparison, the communication antenna frequency of the exemplary system is sufficiently higher (e.g., over 100 MHz) such that the TET system does not interfere with the data communication system. Due to the induced power loss in the antenna, especially relative to the loop conductor described above, traditional patch antennas and any antenna with a ground plane would be unsuitable for this application in the presences of a TETS field. The gaps in the communications antenna, therefore, can be configured to prevent induced currents in the antenna during wireless power transfer at this operating frequency. In other words, the exemplary structure includes gaps designed to reduce or minimize low-frequency coupling. The location and spacing of these gaps can be modified and optimized to control the operation and resonant frequencies of the communications antenna relative to the TET system frequency. For example, the width and position of the traces can be changed (e.g. midway through signal flow) to improve performance and coupling at a first frequency range while minimizing coupling at a second frequency range.

0067In the embodiment of <figref idref="f0004">Fig. 5</figref>, the conductive elements can comprise a plurality of electrical paths, including an interior trace and an exterior trace. In some embodiments, these electrical paths can be tuned to the same resonant frequencies. In some embodiments, these electrical paths can be tuned to different resonant frequencies. The interior trace can comprise the electrical path from feedline 507 to conductive element 508a, to conductive element 508b, to conductive element 508c, to conductive element 508d, back to feedline 507. The interior trace can also comprise the electrical path from feedline 507 to conductive element 508d, to conductive element 508e, to conductive element 508f, to conductive element 508a, back to feedline 507. Still referring to <figref idref="f0004">Fig. 5</figref>, the exterior trace can comprise the electrical path from feedline 507 to conductive element 508a, to conductive element 508g along the perimeter of the antenna, to conductive element 508d, back to feedline 507. The exterior trace can also comprise the electrical path from feedline 507 to conductive element 508d, to conductive element 508h along the perimeter of the antenna, to conductive element 508a, back to feedline 507. The choice of interior vs. exterior path, or which path composed of a plurality of conductive elements, depends on the degree to which the path is tuned to resonance at the frequency in question.

0068As described above, the exemplary communications antenna 506 can comprise two paths, an exterior trace and an interior trace. Each of these paths can be tuned to two different resonant frequencies. If the antenna is driven at the lower of the two resonance frequencies, the RF energy tends to go around the outer path or exterior trace of the antenna. If the antenna is driven at the higher of the two resonance frequencies, the RF energy tends to go around the internal trace of the antenna. When the antenna is driven in the frequencies between two resonance frequencies, the RF energy radiates though both the interior and exterior traces of the antenna. The length of the interior and exterior traces is determined by the wavelength of the radio waves used in the antenna. Since the interior trace can be designed to radiate at a different resonant frequency than the exterior trace, the total length of each trace can be designed based on the needs of the antenna. In some embodiments, both the interior and exterior traces can be configured as quarter-wave antennas, half-wave antennas, 5/8 wave antennas, full-wave antennas, or any other appropriate wavelength. In one specific embodiment, the interior path can comprise a half-wave antenna and the exterior path can comprise a 3/2 wave antenna. In some embodiments, the electrical length of the interior trace can be different than the electrical length of the exterior trace. For example, the interior trace can comprise a half-wave antenna, and the exterior trace can comprise a full-wave antenna, or other appropriate combinations.

0069As shown in <figref idref="f0004">Fig. 5</figref>, communications antenna 506 can be mirrored along one or more lines of symmetry. Symmetrical designs tend to create symmetrical radiation patterns. In this embodiment, the top/bottom symmetry is strongly suggested by the symmetry of the feed points 607. In the illustrated embodiment, the top portion of the antenna (conductive elements 508h, 508f, 508a, 508b, and 508g) can mirror the bottom portion of the antenna (conductive elements 508h, 508e, 508d, 508c, and 508g). Similarly, in <figref idref="f0004">Fig. 5</figref>, the left portion of the antenna (conductive elements 508h, 508f, 508e, 508a, and 508d) can mirror the right portion of the antenna (conductive elements 508g, 508b, 508c, 508d, and 508a).

0070<figref idref="f0005">Fig. 6</figref> illustrates another embodiment of a communications antenna 606. The antenna 606 is similar to the antenna of <figref idref="f0004">Fig. 5</figref>, but with a slightly different arrangement of conductive elements. In <figref idref="f0005">Fig. 6</figref>, antenna 606 includes feedline 607, conductive elements 608a-f, and gaps 510a-j. Similar to the embodiment of <figref idref="f0004">Fig. 5</figref>, antenna 606 can also include an interior trace and an exterior trace. The electrical path along the interior trace can comprise, for example, a path from the feedline 607 to conductive elements 608a, 608b, 608c, back to feedline 607 (or reversed), or alternatively, a path from the feedline 607 to conductive elements 608c, 608d, 608a, back to feedline 607 (or reversed). Similarly, the electrical path along the exterior trace can comprise, for example, a path from the feedline 607 to conductive elements 608a, 608e, 608c, back to feedline 607 (or reversed), or alternatively, a path from the feedline 607 to conductive elements 608c, 608f, 608a, back to feedline 607 (or reversed). As described above, the interior and exterior traces of the antenna can be tuned to different resonant frequencies. Also shown in <figref idref="f0005">Fig. 6</figref>, the conductive elements can have a looping design wherein the individual conductive elements are bent or turned so as to arrive at the desired antenna length. It should be noted that even when the conductive elements are bent into a loop-like shape, the loop is not closed and includes a small gap so as to prevent the induction of low-frequency current in the elements. However, high-frequency (antenna frequency) currents must flow or the antenna doesn't radiate.

0071<figref idref="f0006">Fig. 7</figref> illustrates one embodiment in which one or more of the conductive elements 708a-d can include a meander or meandering design, in which the conductive element zig-zags in different directions to achieve the desired electrical length.

0072<figref idref="f0007">Fig. 8</figref> is a cross-sectional view of a communications antenna 806 mounted on a receiver unit 800 (such as the receiver unit of <figref idref="f0003">Fig. 4</figref>). The antenna 806 can comprise a plurality of conductive elements 808 which make up the resonators of the antenna. The exemplary antenna does not comprise a ground plane. The conductive elements can comprise any conductive material, such as aluminum, copper, silver, gold, or the like. The conductive elements can rest on an antenna body 812. The antenna body can comprise an insulator, such as ceramics, glasses, certain polymers, ceramic powders, glass fibers bonded with polymers, mineral or mineral oxide crystals (sapphire, diamond), or the like. In some embodiments, the antenna body can have a high dielectric constant configured to cause the antenna to appear electrically larger.

0073In the exemplary embodiment, the receiver unit 800 is a fully integrated antenna. A resonant structure is integrated into the antenna structure without the use of a ground plane. The exemplary receiver unit 800 includes a housing or can 814 comprising a biocompatible material such as titanium. The housing 814 can be hermetically sealed to contain the receiving and power management electronics of the receiver unit. In some embodiments, the housing 814 can be surrounded with a low-frequency ferrite material 816, which can be held in place over the housing 814 with an adhesive layer 818. The low-frequency ferrite material can be configured to prevent the magnetic fields of the wireless power system from exciting circulating currents in the housing. For example, the low-frequency ferrite material can be configured to block or diminish magnetic fields in the operating frequency of the wireless power system (e.g., frequencies in the range of 250 kHz).

0074A high-frequency ferrite material 820 can be disposed between the antenna body 812 and the low-frequency ferrite material 816. In some embodiments, the low-frequency ferrite material 816 can be extremely lossy at the frequencies used by the communications antenna. The high-frequency ferrite material can serve as a spacer to separate the antenna from the low-frequency ferrite material of the receiver unit, and can also serve to reflect the downward facing energy from the communications antenna back upwards towards the intended communications target. This high-frequency ferrite material can prevent the downward facing energy from being dissipated by the low-frequency ferrite material. In some embodiments, the high-frequency ferrite material can have a low permeability and permittivity. The entire assembly, including the antenna 806 and the receiver unit 800 can be encapsulated with an encapsulation layer 730, which can comprise a plastic, ceramic, or adhesive material.

0075A feedthrough subassembly comprising a conductive well 822, feedthrough ceramics 824, and feedthrough pins 826, is designed to behave as controlled impedance transmission line between the electronics of the receiver unit and the communications antenna. A clearance layer 828 can be included during manufacturing for mechanical tolerance, and can provide an empty space that is backfilled with an adhesive to ensure that all the components fit together properly.

0076In <figref idref="f0007">Fig. 8</figref>, the antenna and housing design can include an inductor configured to shunt any TETS energy picked up by the antenna away from the electronics inside the can. In some embodiments, there may be some finite pickup of the TETS signal by the antenna even with the gaps separating the conductive elements. The inductor value should be as large as possible to shunt away any energy picked up. In some embodiments, the self-resonant frequency of the inductor is 25% higher than the RF frequency of the antenna. In some embodiments, the self-resonant frequency of the inductor is at least 5% higher, at least 10% higher, at least 15% higher, or at least 20% higher than the RF frequency of the antenna. In some embodiments, the self-resonant frequency of the inductor is at least 50% higher than the RF frequency of the antenna. In some embodiments, the self-resonant frequency of the inductor is an order of magnitude higher than the RF frequency of the antenna.

0077<figref idref="f0008">Fig. 9</figref> illustrates antenna match (return loss) parameterized with increasing gap between the conductive elements of the antenna. The resonance frequency can be adjusted by varying the meander portion of the pattern, but typically multiple parameters should be adjusted in concert to tune performance. In one embodiment, a lower value of return loss is desired in the antenna design.

0078<figref idref="f0009">Fig. 10</figref> illustrates antenna match (reflection coefficient) parameterized with increasing gap. A value close to the center (0) is desired in the antenna design. This is the same data as shown in <figref idref="f0008">Fig. 9</figref>, but plotted as complex reflection.

0079Although described in some respects as a medical system, one will appreciate from the description herein that the principles can apply equally to other types of systems including, but not limited to, consumer electronics, automotive, phones and personal communication devices, gaming devices, and computers and peripherals.