Ultrasonically powered implantable medical devices for neural stimulation
Keywords: Electrical Engineering.

Miniature "electroceuticals" - therapeutic devices that can stimulate the body's nervous system to treat diseases - have the potential to revolutionize medicine. In particular, implantable medical devices (IMDs) for peripheral nerve stimulation (PNS) have shown great promise for the treatment of an array of conditions such as chronic pain, inflammatory diseases (e.g., rheumatoid arthritis), metabolic diseases (e.g., diabetes), digestive disorders (e.g., gastroesophageal reflux disease) and overactive bladder. However, existing PNS IMDs are bulky, invasive and have several other limitations due to the use of batteries and wired interfaces. Miniaturization of these devices to millimeter dimensions is essential for overcoming these limitations and necessitates wireless power and data transfer. While there have been several efforts at miniaturizing wireless PNS IMDs in the past decades, there is a major unmet need for a PNS IMD that is not only miniaturized to millimeter dimensions, but also includes all complex functions required for nerve stimulation and can operate wirelessly at large tissue depths (> 10 cm). Maximizing the ratio of tissue depth to IMD volume (depth-to-volume ratio), is a fundamental challenge for wirelessly powered IMDs. Complex functions for PNS include precise or programmable stimulation of the nerve, robust wireless data links between an external device and the IMD, and biocompatible packaging of the IMD. The focus of this research was on addressing these challenges and, thereby, solving one of the major unmet needs in the field of electroceuticals. We first present a comprehensive system-level analysis of a wireless power transfer system to understand fundamental trade-offs, and derive design insights, for the far-field powering of mm-sized devices. Using these insights, we motivate the promise of ultrasound (US) for powering next-generation IMDs and demonstrate the first proof-of-concept mm-sized IMD based on US power transfer and a hybrid bi-directional data link. This IMD has final dimensions of 2.4 mm x 4 mm x 7.8 mm and was demonstrated to operate at a tissue depth of 3 cm, while achieving a maximum DC load power of 100 uW at just ~5% of the FDA diagnostic ultrasound intensity limit. The US-powered IMD platform was further enhanced to include all complex functions required for PNS, and to enable IMD operation at tissue depths greater than 10 cm. Key design techniques developed in this work include co-design of the US receiver and power recovery circuits for enabling stimulation with a high compliance voltage, design of a robust US data downlink to precisely program stimulation parameters, and charge balance for safe stimulation. The PNS IMD was integrated into final dimensions of 2 mm x 3 mm x 6.5 mm (39 mm3, 78 mg), and achieved a depth-to-volume ratio > 10x that of state-of-the-art devices. Finally, we discuss biocompatible packaging and in vivo experiments of the PNS IMD, thus, demonstrating complete validation of its functionality. Through this research, we demonstrate that US has significant advantages for powering miniature PNS IMDs at large tissue depths. System and circuit design techniques presented in this thesis are expected to play a role in bringing miniature electroceuticals into mainstream medicine in the future.