Medical implants (MIs) have been advocated as an effective-solution to numerous health-issues. Conventional MIs exploit near-field magnetic communication technologies and typically operate in low-RF frequencies, from 5 to 49 MHz, while their transmit power is in the order of tens of mW. As a result, they cannot support high-data-rates (in the order of Mb/s), which are required to achieve similar performance to the cochlea. Optical wireless communications (OWCs) are an important building-block to guarantee high-speed-connectivity between the external and internal units with low-energy-consumption. Optical-wireless MIs are envisioned to enable a vast variety of novel biomedical-applications, such as smart drug delivery and tissue recovery, as well as to improve the quality-of-experience and safety of the conventional ones, not only by targeting 10-100 times higher-data-rates, but also by combining them with reliability and compact designs. In this direction, the first chapter of the current thesis identifies OWCs as a promising technological solution to counterbalance the power, bandwidth, and reliability limitations of the radio frequency (RF) band. Specifically, it focuses on studying the feasibility and assessing the performance of OWCs that are characterized by partial transparency of skin at infrared wavelengths as well as the extremely high immunity to external interference. The analysis accounts for the particularities of the transdermal channel, namely extracting a fundamental design toolbox for transdermal OWC system models and architectures. Finally, it evaluates its performance in terms of the average signal-to-noise-ratio (SNR). The second chapter is devoted to the performance evaluation of transdermal OWCs. It introduces the concept of optical wireless cochlear implant (OWCI). The OWCI system is modeled with focus on its design parameters as well as the dynamic nature of their interactions. Next, its performance is evaluated with regard to the average SNR, ergodic capacity, spectral efficiency, and outage probability. This framework takes into account the technical characteristics of the optical link as well as the transdermal medium particularities; hence, it provides meaningful insights into the behavior of the considered setup. In the third chapter, the focus is turned towards the current state-of-the-art (SOTA) of cell stimulation techniques that constitute an integral part of the medical implants and greatly affect the end-to-end (E2E) system performance. Given the fact that the most promising ones are optical cell stimulation (OCS) and electrical cell stimulation (ECS) techniques, they are investigated from the engineering point of view with regard to their achievable stimulation amplitude, frequency, pulse width, latency and successful stimulation rate. The final chapter presents a complete system architecture that is termed all-optical cochlear implant (AOCI), which is fundamentally different from OWCI and its all-optical architecture creates many new challenges, especially regarding the optimization of the related parameters. In more detail, transdermal OWCs, optogenetic cell stimulation, as well as recent optoelectronics advances that enabled the creation of nano optical fibers capable of near lossless light delivery are combined in order to transform the SOTA cochlear implant into an all-optical system that is capable of achieving higher energy efficiency, signal quality, channel capacity, safety, stimulation accuracy and frequency.
S. E. Trevlakis, “Analysis and optimization of communications in biomedical systems,” Ph.D. dissertation, Dept. of Electrical and Computer Engineering, Aristotle University of Thessaloniki, Thessaloniki, 2022.