Numerical Estimation of Local Specific Absorption Rate and Temperature Elevation in a Human Head Model Implanted with Visual Prosthesis System

Abstract

This paper describes local specific absorption rate (SAR) and temperature elevation estimation in a human head model implanted with wireless power transfer technology (WPT) -based vision prosthesis system, as an example of the medical application of WPT and the safety issue of WPT-related electromagnetic field (EMF). A high-resolution numerical model of a human head and an internally implanted vision prosthesis component including WPT coil was constructed. Using finite-difference time-domain analysis, the electromagnetic field distribution inside the human head was calculated and exposure levels to ensure human safety were obtained. Implantable medical devices such as implantable pacemakers and capsule endoscopes are an important application of WPT. Traditionally, these devices use rechargeable or disposable batteries that constitute a source of constraints including large device size and limited operation life. WPT is now considered a key solution to these problems. WPT-based implantable devices are expected to offer patients better mobility and improved quality of life. Using inductive coupling to link the external power source to the implanted device is a sensible choice when considering short-range wireless power transfer for biomedical implants. However, EMF emitted by wireless devices is a subject in the radio radiation protection guidelines on human exposure to EMF. In Japan, Radio Radiation Protection Guidelines for Human Exposure to Electromagnetic Fields has been published. However, these guidelines provide no quantitative discussions about their relevance in patients with WPT-based active implantable devices. It is very important to estimate the amount and nature of EMF that users are likely to be exposed. Given the progress in biomedical technologies, the number of such users will rapidly increase in the near future. Based on precise numerical simulation, we showed that the estimated SAR and temperature elevation in the human head model implanted with a WPT-based vision prosthesis system were within the safety limits described in the guidelines.