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ASAIO Journal 2014

Technological innovation of a smaller, single moving part has an advantage over earlier large pulsatile ventricular assist devices (VADs) prone to mechanical failure. Drivelines limit the potential for extended patient survival durations with newer pumps and act as source for infection, increased mor-bidity, rehospitalizations, and reduced quality of life. The Free-range Resonant Electrical Energy Delivery (FREE-D) wireless power system uses magnetically coupled resonators to efficiently transfer power. We demonstrate the efficiency over distance of this system. The experimental setup consists of an radiofrequency amplifier and control board which drives the transmit resonator coil, and a receiver unit consisting of a resonant coil attached to a radiofrequency rectifier and power management module. The power management module supplies power to the axial pump, which was set at 9,600 rpm. To achieve a seamless wireless delivery in any room size, we introduced a third relay coil. This relay coil can be installed throughout a room, whereas a single relay coil could be built into a jacket worn by the patient, which would always be within range of the receive coil implanted in the patient’s body. The power was delivered over a meter distance without interruptions or fluctuations with coil, rectifier, and regula-tor efficiency more than 80% and overall system efficiency of 61%. The axial pump worked well throughout the 8 hours of continuous operation. Having same setup on the opposite

side can double the distance. A tether-free operation of a VAD can be achieved by FREE-D system in room-size distances. It has the potential to make the VAD therapy more acceptable from the patient perspective. ASAIO Journal 2014; 60:31–37.

Key Words: ventricular assist device, wireless power, drive-line, infection, cardiovascular implants

Left ventricular assist devices (LVADs) have evolved rapidly in the last 2 decades. The improved miniaturization technology has made creating pump pockets obsolete.1–3 Full advantage of these improvements cannot be realized yet by essentially unchanged peripherals that include the percutaneous driveline for data retrieval and power supply. Infection and cumbersome battery attachments are the most frequently cited concerns associated with current drivelines4–6 and may have implication when considering this technology for a less ill population.

The Free-range Resonant Electrical Energy Delivery (FREE-D) system uses resonant coupling technology to provide wireless power to an implanted receiving device. The idea of magnetic resonance originated from the work of Nikola Tesla7 in the early 20th century. Magnetic resonance as a means for wireless power transfer has recently been revived and applied to mod-ern applications including cellphone charging and electric vehicle charging.7,8 The FREE-D wireless power system uses an automatic tuning (auto-tuning) and impedance matching algo-rithm that adapts to variations in distance and misalignment between the transmit and receive resonators. We present our preliminary in vitro results of powering an LVAD pump using the FREE-D system.

Materials and Methods

Principle of Operation of FREE-D System

Two resonators efficiently exchange energy by sharing mag-netic fields that oscillate at a specific resonant frequency. A power amplifier delivers an alternating current (AC) signal to the transmit resonator consisting of a single-turn drive loop and a multiturn coil. Current flowing through the transmit coil induces a magnetic field that is emitted in all directions. When a second resonator also consisting of a multiturn coil and a single-turn receive loop is brought within range of the first, the magnetic field induces electrical current in the receive coil. Additional rectification circuitry connected to the receive loop converts the AC signal into a direct current (DC) voltage

Biomedical Engineering

Innovative Free-Range Resonant Electrical Energy Delivery System (FREE-D System) for a Ventricular Assist

Device Using Wireless Power

Benjamin H. Waters,* josHua r. smitH,*† and Pramod Bonde‡§

Copyright © 2013 by the American Society for Artificial Internal Organs

DOI: 10.1097/MAT.0000000000000029

From the *Department of Electrical Engineering, University of Wash-ington, Seattle, Washington; †Department of Computer Science and Engineering, University of Washington, Seattle, Washington; ‡Bonde Artificial Heart Laboratory, Center for Advanced Heart Failure and Transplantation, Yale School of Medicine, New Haven, Connecticut; and §Section of Cardiac Surgery, Center for Advanced Heart Failure and Transplantation, Yale School of Medicine, New Haven, Connecticut.

Submitted for consideration August 2013; accepted for publication in revised form October 2013.

The authors hold the original patent(s) on this invention. This phase of work was not supported by any governmental or nongovernmental funding.

This work was presented at the 57th Annual Conference of the ASAIO in Washington DC, June 2011. Willem J Kolff/Don B Olsen award recipient.

Disclosure: The authors have no conflicts of interest to report.Reprint Requests: Pramod Bonde, Mechanical Circulatory Sup-

port, Section of Cardiac Surgery, Center for Advanced Heart Failure and Transplantation, Yale School of Medicine, 330 Cedar Street, 204 Boardman, New Haven, CT 06520.

32 WATERS et Al.

compatible with the ventricular assist device (VAD) system controller. An equivalent circuit diagram of the FREE-D system is illustrated in Figure 1. The parasitic resistance (R), capaci-tance (C), and inductance (L) of each element are shown.

Magnetic fields emitted by the FREE-D resonators can per-meate through solid objects such as walls, household furniture, and human tissues and can pass around highly conductive objects such as metallic furnishings and other electronic devices. Auto-tuning makes it is possible for efficient wireless power transfer to occur through all of these interfering medi-ums. However, if either resonator is completely enclosed in metal, a Faraday cage will form around the resonator and the electromagnetic fields will not permeate through the conduc-tive medium.

The key feature that distinguishes the FREE-D wireless power system from previous inductive coupling technology is the use of strongly coupled resonators combined with an auto-tuning scheme that keeps the system operating at maxi-mum efficiency.9–11 Because the distance between the transmit and receive resonators increases, there is a region where con-stant, maximum efficiency can be achieved if the ideal operat-ing frequency or impedance matching condition is selected. This constant efficiency region can be seen in Figure 2. In this region, the ideal operating frequency changes as the resonator coupling changes. The resonator coupling is inversely propor-tional to the distance between the transmit and receive resona-tors. The auto-tuning scheme adapts to variations in distance and orientation between the two resonators by dynamically selecting the ideal operating frequency to ensure that the sys-tem operates at maximum efficiency. For two equally sized res-onators, the constant efficiency region occurs for a separation

distance of approximately zero to two coil radii between the transmit and receive resonators. Beyond the constant efficiency region, efficiency decreases by approximately 1/r3 dependence with increasing distance.

The wireless power transfer range can be extended by intro-ducing an additional coil (relay resonator) between the transmit and receive resonators. Relay resonators introduce additional resonant modes to the two modes associated with the typical two-resonator system. When the receive resonator is outside the working range of the transmit resonator, a relay resonator can be placed within range of the transmitter to extend the magnetic field out to the receiver. Multiple relay resonators can be used to extend the overall working range even further. However, each additional relay resonator slightly reduces the overall efficiency of wireless power transfer because of para-sitic loss factors.

Safety Considerations

To investigate the safety concerns of the FREE-D system, a preliminary study with the Foundation for Research on Infor-mation Technologies in Societies (IT’IS) has been conducted.12 This study used anatomical models of a virtual family to model the peak specific absorption rate (SAR) levels of the magneti-cally coupled resonators around humans. The study concluded that the highest SAR levels are reached for a coronal orientation between the coil and the human model. The maximum allow-able SAR limitations depend primarily on the amount of trans-mitted power; however, the system operating frequency, coil orientation, size, and geometry also affect the SAR limitations of the system. Because a typical VAD today consumes between 5 and 25 W of power, it is critical to maximize wireless power

Figure 1. Schematic diagram of the transmit and receive magnetically coupled resonators. R, L, and C represent the parasitic resistance, inductance, and capacitance, respectively, of the individual resonators.

FREE-D WIRELESS POWER SYSTEM FOR A VAD 33

transfer efficiency so that the transmitted power is minimized and SAR limitations are not realized.

Experimental Design

The goal of the first experimental implementation was to extend the working range of efficient wireless power transfer from the previous work.9–11 The large transmit resonator con-sisted of a single-turn drive loop (31 cm diameter) and a mul-titurn spiral coil (59 cm). A relay resonator (59 cm) extends the wireless power transfer to a receive coil (28 cm) and the receive loop (9.5 cm) located 1 m away from the transmit resonator. The experimental relay resonator configuration can be seen in Figure 3. An radiofrequency (RF)–DC recti-fier mounted on a clamp converts the oscillating RF signal at 7.65 MHz to DC power, and a DC–DC voltage regulator maintains a constant 13.1 V output voltage supplied to the VentrAssist (VentraCor Ltd., Brisbane, Queensland, Australia) centrifugal VAD system controller.13 The pump speed of the centrifugal VAD was manually increased by 200 rpm incre-ments from 1,800 to 3,000 rpm over 2 weeks time duration.

The goal of the second experimental implementation was to maximize the efficiency of the wireless power transfer system and monitor the temperature of the receive coil for a vest-worn coil configuration. In this experiment, a Heart-Mate II axial pump LVAD (Thoratec, Pleasanton, CA) was used.14 The VAD was set to a fixed pump speed of 9,200 rpm (approximately 8 W). A power amplifier delivers a 13.56 MHz RF signal to the transmit resonator using the FREE-D auto-tuning algorithm. The transmit resonator in this experi-ment consisted of a single-turn drive loop (8 cm) and a mul-titurn transmit coil (15 cm) that wirelessly powered a receive coil (6.5 cm) and receive loop (4 cm) embedded in a bio-compatible polydimethylsiloxane (PDMS) substrate. The test

configuration, receive circuit, and receive coil are all shown in Figure 4. The receiver printed circuit board including the integrated VAD motor controller is contained within the ABS plastic enclosure.

Portable FREE-D System

Portable transmitters for high-power devices, specifically implanted VADs, have very unique design requirements. They must be small enough to be comfortably worn by the patient in a pouch or vest, battery powered, and highly efficient to maxi-mize the time that a single battery can power the transmitter, and must have auto-tuning capabilities to adapt to interfering objects and coil misalignments. The FREE-D battery-powered portable controller consists of a microcontroller unit, signal generator, and power amplifier mounted on a printed circuit board with a heat sync for temperature control. The portable system shown in Figure 5 delivers RF energy at 13.56 MHz to a transmit resonator (6 cm in diameter) mounted on an exte-rior vest. An equivalently sized receive resonator connected to the receiver circuit and VAD motor controller receives the wireless power signal and powers the axial pump VAD. Figure 5, A–C show the wirelessly powered VAD for perpen-dicular, parallel, and side-by-side coil configurations, respec-tively. Figure 5D shows an artist rendering of the FREE-D system vision: relay coils installed throughout the room and directly on the patient’s vest capable of delivering wireless power the implanted receiver.

Results

The FREE-D system powered the centrifugal pump without any interruptions or failure of the wireless power system for the full 14 days of time duration. As the pump speed increased,

Figure 2. Three-dimensional plot of the simulated efficiency as a function of frequency and the resonator coupling coefficient. The coupling coefficient is inversely proportional to the distance between the transmit and receive resonators.

34 WATERS et Al.

the pump power consumption also increased. The FREE-D auto-tuning algorithm automatically detected a change in the load power and increased the transmit power to maintain a

seamless power delivery to the VAD. The VAD power con-sumption, resonator, rectifier, and system efficiencies are plot-ted against time in Figure 6.

Figure 4. Picture of the second experimental configuration. From right to left: transmit coil, receive coil encapsulated in polydimethylsilox-ane, receiver circuit, and integrated left ventricular assist device (LVAD) motor controller inside case, and commercially available HeartMate II axial pump system. Inset in the figure is a close-up photograph of the 6.5 cm receive coil and receive circuit.

Figure 3. Picture of the first experimental configuration. From right to left: transmit drive loop, transmit coil, relay resonator, receive coil, and receive loop. In the foreground is the RF-DC rectification circuitry, the VentrAssist centrifugal pump, and its system controller. LVAD, left ventricular assist device.

FREE-D WIRELESS POWER SYSTEM FOR A VAD 35

Although the resonator efficiency was above 90% for the full 2 weeks time period, the rectifier efficiency dictated the system efficiency ranging from 40% to 55%. The rectifier efficiency was not inherently optimized for each power level because the sys-tem was designed for stability for the purposes of this 14 days experiment. However, at each pump speed and power consump-tion level, the system efficiency still allowed for a transmit power that will remain below the maximum power at which the SAR limitations are realized at the operating frequency of 7.65 MHz.12

In the second experiment, the axial pump VAD was powered for the entire duration of the 8 hours test. For a fixed 10 cm gap between transmit and receive coils, the full system efficiency, rectifier efficiency, coil efficiency, and receive coil tempera-ture are plotted against time in Figure 7. With a fixed coil–coil

distance of 10 cm, the resonator efficiency was 92.3% for the entire duration of the experiment. The rectifier efficiency ranged from 77% to 82% with slight fluctuations caused by variations in load current from the LVAD. This experiment was optimized for system efficiency, whereas the previous experi-ment was optimized for stability over the longer time duration of the experiment. In future revisions of the hardware, the peak system efficiency can be increased using adaptive impedance matching.15 The coil temperature was measured with a Fluke 287 digital multimeter and thermocouple probe. The initial receive coil temperature was 22°C with a room temperature of 25.1°C. After an initial increase, the coil temperature sta-bilized at approximately 27°C for the majority of the 8 hours time duration.

Figure 5. Demonstration of the battery-powered portable transmitter system used to wirelessly power a commercially available axial left ventricular assist device (LVAD) using two equivalently sized transmit and receive coils in a (A) perpendicular configuration, (B) parallel configuration, and (C) side-by-side configuration. D: A rendering of the Free-range Resonant Electrical Energy Delivery system vision for a wireless, in-home ventricular assist device system.

Figure 6. Plot of the ventricular assist device (VAD) power con-sumption and measured efficiency across each component of the Free-range Resonant Electrical Energy Delivery relay resonator sys-tem including the resonator, rectifier, and the overall system effi-ciency for the entire 2 weeks experiment shown in Figure 3.

Figure 7. Plot of the receive coil temperature and measured effi-ciency across each component of the Free-range Resonant Electri-cal Energy Delivery system including the resonator, rectifier, and the overall system efficiency for the 8 hours experimental configuration in Figure 4.

36 WATERS et Al.

Discussion

Ventricular assist device therapy has radically improved with smaller device sizes, better reliability, and lower morbidity over earlier larger pulsatile pumps. However, the mandatory need for transcutaneous drivelines undermines the full poten-tial of these newer VADs. Infection, traumatic damage, and rehospitalizations are considered an inevitable consequence of the driveline.

The rate of infection in VAD patients4–6,16,17 is relatively high in comparison with other metallic implants such as cardiovascular implants (<2%), prosthetic valve endocardi-tis (2–4%),18 and orthopedic implants, such as hip and knee arthroplasty less than 1%.19 VADs differ from other metal-lic implants in a unique way: the presence of a percutane-ous driveline communicating with the exterior environment. Recognition of this problem has led some devices to incor-porate postauricular pedestal for percutaneous power deliv-ery (Jarvikheart Inc., New York, NY). As we move toward even longer durations of VAD support,2,3 the risk of exit site infection (ESI) continues to increase temporally, hampering quality of life and leading to repeated hospitalizations for antibiotic treatment or surgical interventions.4–6,16,17 In rare instances, ESI has even been the primary cause for a com-plete pump exchange.20

Previous attempts have been made to wirelessly power VADs using Transcutaneous Energy Transfer Systems (TETSs).21 Transcutaneous Energy Transfer System uses induc-tive coupling techniques to transfer power between coils on the inner and outer surfaces of the skin. Transcutane-ous Energy Transfer System has been tested in two systems (Arrow LionHeart and AbioCor TAH).21 The clinical and laboratory experiments have demonstrated several draw-backs with the current TETS technology. Restrictions on mis-alignment between the transmit and receive coils and the necessity for a close separation distance between the coils limit the practicality of TETS. The system efficiency reduces significantly beyond 10 mm separation.22,23 The proximity limitation requires that the receiving coil be implanted just under the skin and the external transmitting coil be secured in a single position on the skin surface with adhesives. For angular misalignments or excessive separation between the coils, the transmitter will attempt to supply more power to account for the reduced efficiency.

The FREE-D system vision consists of installing multiple transmit resonators throughout a room so that the AC mains and additional transmit circuitry would be concealed inside walls and beneath floors. Several freestanding relay resonators would also be situated throughout a room so that the receive resonator always will be within the working range of at least one relay resonator. A smaller relay resonator would be worn in an external vest. The implanted receive coil will be situated at a fixed distance away from the external vest coil to ensure seamless energy transfer to the VAD. Therefore, the patient will be free to maneuver throughout their home while receiving wireless power from the nearest transmit or relay resonator. For an additional level of security, the received wireless power will simultaneously charge an implantable battery, which can directly power the VAD in the event of a wireless power fail-ure. Alternatively, if the patient needs to leave the home, a por-table external battery will provide power to the external vest

coil, which will continue to wirelessly power the implanted receive resonator.

Certain challenges lie ahead as we incorporate this tech-nology with the existing VADs available for clinical use. One of these challenges, the development of a portable transmitter system capable of delivering consistent and dependable wire-less power to the implanted receiver under varying conditions and pumps speeds is already achieved and presented here. The presented system can be incorporated with any type of VAD: pulsatile or continuous flow, centrifugal, or axial pump. Other challenges that are part of ongoing work include the miniaturization of the pump controller and developing wire-less data communication to transfer pump parameters for in vivo use.24,25 Wireless communication inside the human body is restricted to certain protocols and operating frequencies. A common protocol used for pacemakers in particular is the Medical Implant Communication Service (MICS) specifica-tion for use in the 402–405 MHz frequency band. However, it remains a challenge to implement miniature antennas that communicate at high data rates within the specifications of the wireless communication protocol. Our current focus is to integrate the presented portable FREE-D system with adap-tive impedance matching and wireless data communica-tion in accordance with existing regulatory standards to an implanted receiver.

In summary, we have demonstrated that a tether-free opera-tion of a VAD can be achieved by the FREE-D system in room-size environments. The FREE-D system has the potential to improve VAD patient’s quality of life and make the VAD ther-apy more acceptable from the patient perspective by avoiding percutaneous drivelines.

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