Implantable Devices for a Retinal Prosthesis: Design and Electromagnetic and Thermal Effects

Abstract

A retinal prosthesis, wherein electrical stimulation is provided to the retina of a person inflicted with outer retinal degenerative diseases such as Retinitis Pigmentosa and Age-related Macular degeneration, has been clinically tested and has succeeded in providing limited vision; such as shape recognition. It is hoped that an increased electrode count will improve visual acuity.

The retinal prosthesis considered in this work is a dual-unit system with an external (outside the human body) unit and an internal (inside the body) unit with a wireless link for power and data transfer between them. Such a system poses possible health risks due to the incident electromagnetic energy of the wireless link and the power dissipated by the internal components, particularly the processing chip which drives the electrodes responsible for eliciting a neural response from the retina. Tissue damage via heating is one the primary concerns for such a system making it necessary to obtain via simulation and in-vivo and in-vitro experiments, accurate estimates of thermal elevation due to the operation of the such devices.

In this work, numerical methods have been developed to compute temperature increases and electromagnetic effects due to the prosthesis components in anatomically correct human head models. The explicit and the Alternating-Direction Implicit (ADI) Finite-Difference Time-Domain (FDTD) have been used. Further, a hybrid explicit-ADI method was developed for the heat equation which provided simulation speedup of more than 10x over conventional methods for the models considered.

FDTD methods were employed to compute the induced current densities and Specific Absorption Rate (SAR) in the human head due the inductive link comprising the primary coil (external) and a secondary coil (internal). Different orientations of the primary coil were considered in a frequency range of 1 MHz-20 MHz to provide guidelines for choosing eventual frequency and power parameters to conform to international safety standards. A novel displacement field excitation method was used for the spiral primary coil and verified with analytical results.

In an effort to reduce the size of the internal unit and to allow integration of a patch antenna (for a separate data link), and the active devices on a single substrate, a 3-D trench inductor geometry was investigated. To enable patterning of structured surface, a custom experimental setup was designed and maintained to process a positive tone PEPR2400 electro-depositable photoresist.