Localization and Stimulation Techniques for Implantable Medical Electronics

Implantable medical devices (IMDs) are emerging as one of the keystones of tomorrow’s medical technology. Although they have enabled a revolution in medicine, from research to diagnosis to treatment, most of today’s devices have critical limitations. They are bulky, have low resolution, and, in some cases, are limited to basic functionality. Miniaturization of IMDs will have an enormous impact not only on the technology itself and the medical procedures they enable, but also on the lives of patients, who will be more comfortable, have greater confidence in their medical treatments, and enjoy an overall improvement in their quality of life. The path towards miniaturized bioelectronic devices requires a reevaluation of existing paradigms to reach a seamless integration of electronics and biology. Miniaturization of medical electronics then involves an exploration of advanced integrated circuit processes and novel circuit and system level architectures. In this dissertation, we provide an overview of implantable medical devices and present novel circuit and system level techniques for the miniaturization of medical electronics. The function of wireless miniaturized medical devices such as capsule endoscopes, biosensors, and drug delivery systems depends critically on their location inside the body. However, existing electromagnetic, acoustic, and imaging-based methods for localizing and communicating with such devices with spatial selectivity are limited by the physical properties of tissue or imaging modality performance. In the first part of this dissertation, we introduce a new approach for microscale device localization by embodying the principles of nuclear magnetic resonance in a silicon integrated circuit. By analogy to the behavior of nuclear spins, we engineer miniaturized RF transmitters that encode their location in space by shifting their output frequency in proportion to the local magnetic field. The application of external field gradients then allows each device’s location to be determined precisely from the frequency of its signal. We demonstrate the core capabilities of these devices, which we call addressable transmitters operated as magnetic spins (ATOMS), in an integrated circuit smaller than 0.7 mm^3, manufactured through a standard 180 nm complementary metal-oxide-semiconductor (CMOS) process. We show that ATOMS are capable of sub-millimeter localization in vitro and in vivo. As a technology that is inherently robust to tissue properties and scalable to multiple devices, ATOMS localization provides an enabling capability for the development of microscale devices to monitor and treat disease. In neuroprosthetics, retinal prostheses aim to restore vision in patients suffering from advanced stages of retinal degeneration (e.g., retinitis pigmentosa) by bypassing the damaged photoreceptors and directly stimulating the remaining healthy neurons. In the second part of this dissertation, we describe a fully intraocular self-calibrating epiretinal prosthesis that reduces area and power consumption, and increases the functionality and resolution of traditional implementations. We introduce a novel novel digital calibration technique that matches the biphasic stimulation currents of each channel independently while sharing the calibration circuitry among every 4 channels. The system-on-chip presents dual-band telemetry for power and data with on-chip rectifier and clock recovery. These techniques reduce the number of off-chip components and achieve a power conversion efficiency >80% and supporting data rates up to 20 Mb/s. The system occupies an area of 4.5 x 3.1 mm2 and is implemented in 65 nm CMOS . It features 512 independent channels with a pixel size of 0.0169 mm2 and arbitrary waveform generation per channel. The chip is integrated with flexible MEMS origami coils and parylene substrate to provide a fully intraocular implant.