Wireless power transfer and data communication for low-power micro electronic devices deeply implanted within the human body

Recent advances in low-power electronics, wireless communication, and bioengineering have produced many forms of implantable and migrating medical devices (a "migrating device" travels within the body along a specific route). Among them, smart implants and ingestible electronic pills that perform diagnostic functions inside the human body play increasing roles in modern medicine. Continuous access to physiological and biological information in real time disrupts the traditional symptom-based diagnostic approach, and ushers in a new era of technology- and evidence-based medicine. Despite the high potential impacts of diagnostic implants and migrating devices, numerous problems in both technical and biological domains must be solved. This talk focuses on several key problems common to these classes of devices: How to make them small while provide them with sufficient power and wireless communication functions when they are deeply located within the body? There are many compelling reasons to miniaturize these devices. First, smaller devices in centimeter or millimeter scales are easier to deliver in vivo using non-invasive (e.g., swallowing) or minimally invasive means, which in turn reduce medical care cost and complication rates. Second, smaller devices can be placed within less accessible compartments inside organs or tubular structures (e.g., a digestive tract) without interfering with the body's normal biologic functions. Third, smaller devices improve biocompatibility and immune tolerance due to a smaller biological footprint. Despite these advantages, it remains technical challenge to construct such devices. For over two decades, our laboratory has been investigating the power and communication problems in miniature implants and migrating devices. While the power supply for a migrating device can be a battery if the lifetime of the device is short, a depleted battery within a permanent or long-term implant must be replaced surgically which is painful and expensive. To solve the power supply problem, we have studied three alternatives to the battery power. Firstly, the body's own energy source is harvested using a biofuel cell taking advantage of the electrochemical activity of biological cells [1]. Secondly, electrical current is delivered to an implant through biological tissues using the volume conduction property of the human body [1]. Thirdly, wireless power transfer is used in which a power-carrying radio-frequency wave (between megahertz and gigahertz) is transmitted from the outside to the inside of the human body [2]. To minimize device size and implement the data communication function, we have investigated a dual-functional design in which the wireless power delivery link and the data communication link are shared. Our design supports both downlink and uplink communication. While the downlink is relatively easy to implement using a power delivery signal with modulated amplitude, the uplink from the inside device to an outside unit is difficult, especially when the device is deeply located within the body. We must overcome the problems of limited power source, small transceiver antenna and long communication distance through layers of tissues. Our solution is to use a special tapped coil and a simple passive circuit to produce a strong pulsed magnetic field (PMF) for signal transmission within the body [2]. The strong PMF enables a data uplink from the implant to outside world in a high instantaneous transmission wattage through the shared power delivery channel, whereas a low average power consumption is maintained. Our experimental result showed that, when compared to the existing load shift keying (LSK) method, our method provides extraordinary improvements, with over 10 times increase in emitted signal strength and over 15 times increase in received signal strength.