Modeling and Techniques for in vivo Wireless Power Transfer and Localization

This thesis investigates low-complexity models and optimization strategies for electromagnetic (EM) propagation, localization, and wireless power transfer (WPT) in in-vivo applications, addressing the challenges posed by the complex and variable environment of the human body. The work focuses on developing computationally efficient solutions that maintain high accuracy and are practical for use in real-world scenarios, guided by five research questions. The first research question explores whether a low-complexity propagation model can accurately represent the human body. A mathematical model based on a Hertzian dipole is proposed with a stratified planar medium, chosen for its suitability to model electrically small in vivo antennas. This model achieves accuracy within 10% error tolerance at over 70% of observation points when compared to full-wave simulations using CST Microwave Studio Suite. Additionally, it is computationally efficient, completing simulations of 7400 observation points in 6 minutes on a consumer laptop, compared to 85 minutes using CST on high-performance hardware. The independence of the model from mesh-based calculations and low computational demand make it feasible for real-time applications such as online localization. The second research question examines the accuracy of this low-complexity model in localization applications. The planar propagation model is validated against a human digital phantom, with comparisons made at four locations within the stomach and small intestine. In three of these locations, the localization error is approximately 1 cm. The model performs best in regions with flat tissue structures, supporting the use of planar approximations. A proposed localization method based on the difference between measured and modeled electric fields demonstrates the feasibility of achieving centimeter-level accuracy for in vivo antennas. The third research question addresses the optimization of rectifiers and rectennas to support efficient WPT. An analytical model for rectifiers simplifies the analysis of various topologies. Closed-form solutions for optimal loading conditions in the low-power region accelerate system design. For rectennas, a novel time-domain simulation method replaces the low-pass filter section of a rectenna with an equivalent DC source, reducing simulation time by over tenfold with respect to Harmonic Balance simulations while maintaining accuracy. This approach is scalable to the rectifier topology and facilitates rapid parameter sweeping and optimization. The fourth research question focuses on predicting and optimizing WPT conditions within variable human body forms. The link budget under safety regulation and propagation loss constraints is analyzed using the planar human body model and clinical data on tissue variability. The 915 MHz band is identified as the most suitable ISM band due to its high RF link budget and bandwidth. The available bandwidth is leveraged by an adaptive multisine waveform design, enhancing the harvested power by 1.4 dB with respect to the single-tone waveform, and significantly improves the rectifier's resilience against load variation without adding circuit complexity to the rectifier. The fifth research question compares RF-based and inductive WPT systems. A proposed inductive system using tri-polar transmitter and orthogonally oriented receiver coils achieves localization accuracy with a root mean square error (RMSE) of 1.17 cm. Additionally, the inductive link exhibits predictable behavior, as the body is transparent to magnetic fields, whereas RF links are more sensitive to environmental variations. The inductive system achieves comparable WPT link efficiency to RF methods, demonstrating its potential for reliable in vivo applications.