Adaptive Wireless Power for Ventricular Assist Devices
Waters, Benjamin Herron
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Every device needs power to function. Whether that device is a cell-phone, laptop, wearable device, or even a robot, if the power source dies then the device is unusable until charged up again. In our world today, this is something we are all too familiar with: the battery dies and the device is off until it is physically plugged in again. As irritating as this can be for our consumer devices today, what if our lives depended on that device staying on? Consider an implanted medical device for example. The battery powering that device simply cannot die. If it does, the consequences are far more severe than being unable to use a cell-phone for a few hours. How can we make sure that all devices - industrial, consumer, and especially medical devices - always have a reliable power supply in any environment - in our homes, outside, underwater, in a factory or even inside the body? The challenging aspect about reliability is that it depends highly on the environment in which the device is used. If a device is solar powered, it will work great in Australia, but not so well in cloudy Seattle. Or if a device is powered by harvesting ambient energy, that might work great in a big city, but not so well in a remote town. For a device implanted inside the body, how does one ensure that the power supply is always reliable? These power questions are among the most important questions to ask when designing any device. Finding answers to these questions has driven my research in wireless power systems. The wireless power system presented in this dissertation is adaptive. Adaptive wireless power means that the distance and alignment between the transmitter and the receiver can change without impacting the efficiency of the system. Existing wireless charging solutions require the device to be placed directly on a charging pad. If the device is not perfectly aligned or if it moves at all, the wireless charging stops. An application that not only benefits from having wireless charging, but inherently needs adaptive wireless charging is a ventricular assist device (VAD). VADs are implanted medical devices for end-stage heart failure patients that pump blood throughout the body when the heart is unable to do so. For patients with end-stage heart failure, VADs are the only option for treatment other than a full heart transplant. However, heart transplants are not available to all patients, and there is a long wait list for a transplant: the number of heart failure patients is increasing while the number of available donor hearts continues to decline. VADs are unique implanted devices because they require a lot of power to pump blood through the body. To power these devices, large batteries are worn on a belt outside the body. A thick power cable protrudes through the patients stomach and powers the implanted VAD. This power cable can become infected at the site where it penetrates through the skin. Which is why patients with this device are told that they can never take a full shower. Imagine not being able to shower for the rest of one's life! So far, that' s what every one of the nearly 20,000 VAD patients have been told. This dissertation presents an adaptive wireless power system for ventricular assist devices. This system is called the Free-range Resonant Electrical Energy Delivery (FREE-D) system. A thorough analysis shows the techniques used to design each component in the FREE-D system, including the coils used for wireless power transfer, the transmitter circuit, and the receiver circuit. Next, three different techniques used for adaptive tuning are presented including frequency tuning, adaptive impedance matching, and power tracking. A control algorithm implements all three of these techniques simultaneously, for maximum flexibility and optimal efficiency at any distance and orientation between the coils. In-vivo experiments are presented that demonstrate the viability of the FREE-D system in eight separate acute animal trials. Finally, additional capabilities of the adaptive wireless power system are shown using transmit coil arrays and phased-array wireless power transfer systems. These capabilities demonstrate that the FREE-D system may be able to deliver efficient wireless power to a device anywhere inside an entire room.
- Electrical engineering