Study of modern mobile radiofrequency systems implanted into the human body

Sofia Bakogianni, October 2018

Wirelessly-linked implantable medical devices (IMDs) are attracting great scientific interest for in vivo monitoring, measuring and simulating vital signs of the human body within the framework of patient prevention, diagnosis and therapy. Medical telemetry constitutes the substantial processes that allow the bidirectional data transfer between implantable medical devices and exterior monitoring systems. Upon long-term implanted apparatuses and the necessity of power autonomy, the issue of wireless charging arises, as well. The present PhD Thesis deals with the critical design challenges of novel miniature and efficient implantable antennas for wireless medical telemetry and charging applications. Initially, the design of implantable antennas is studied in order to implement planar space-filling structures of small physical size and wireless medical telemetry operation in the Medical Device Radiocommunication Services (MedRadio, 401-406 MHz). Based on a novel parametric model of a dipole antenna, a computational study of the performance properties of implanted antennas is conducted as a function of: (a) critical design parameters and (b) embedded conditions (biological tissue, implanted depth and multi-layered/anatomical structures). The goal is to investigate the important design issues of planar space-filling antennas of extremely small size. Furthermore, the performance properties of planar implantable dipole antennas as a function of the physical length are examined for wireless medical telemetry applications. Given the numerical results, a novel implantable antenna of miniature physical size and wide bandwidth is developed for MedRadio operation. The fabrication and experimental testing of the suggested antenna is, also, carried out within an experimental tissue phantom in order to validate the designed dipole structure of small dimensions. A multi-level design analysis of electrically small antennas is, further, conducted upon three fundamental axes: a) geometry, b) impedance matching, and c) quality factor Q for wireless telemetry applications within MedRadio band. The goal is to produce a thorough source of antenna design considerations for optimum implantable medical devices in terms of size, efficiency and robustness. The thesis is completed with the study, fabrication and experimental testing of a novel radiofrequency medical system (rectenna) that comprises of a novel dual-frequency patch antenna and a rectifier circuit for wireless medical telemetry (MedRadio band) and charging (ISM band). With the aim of enhancing the total system efficiency, a novel technique of increasing the available input power of the implanted antenna is presented. Then, rectifying circuits are studied in order to achieve effective conversion of the electromagnetic energy into DC power. Finally, the fabrication and experimental testing of the implanted rectenna are carried out by integrating the rectifier into the dual-frequency implantable antenna.