Interfacing Microelectronics and the Human Visual System - Biomimetics: Biologically Inspired Technologies - page 437

(a)
Figure 17.3 Qualitative image of the current spread in the frog retina due to (a) coaxial electrodes and (b) disc
electrodes. Current density values range from white (max) to black (zero).
The wireless link causes electromagnetic power deposition in the head and eye tissues, which
could lead to indirect thermal rise in the tissue, known to be the dominant physiological hazard due
to power deposition in human tissues (Adair and Petersen, 2002). Moreover, the implanted
electronic IC chips will dissipate power in the form of heat, which will directly lead to the thermal
elevation in the surrounding tissues. It is therefore necessary to quantify these thermal effects in
order to determine the safe limits of operation of the prosthetic system.
The temperature rise in the head and eye tissues due to the operation of the prosthesis can be
experimentally determined with in vivo experiments or computationally evaluated by means of a
computer code for the solution of the bio-heat equation. Preliminary computational predictions
have been performed to evaluate the thermal influence of a dual-unit epiretinal prosthesis system on
the human head and eye tissues and, therefore, provide a quantitative measure of the temperature
rise in human body as a result of the operation of an implantable neurostimulator. As an example of
typical methods and results, the following paragraphs and subsections provide a brief account of the
methods and model used in such bio-engineering computations.
To quantify the thermal impact of the dual-unit epiretinal prosthesis system, the bio-heat
equation can be numerically discretized both spatially and temporally using the well-known
finite-difference time domain (FDTD) method (Sullivan, 2000; Wang and Fujiwara, 1999). In
this example, the computational prediction was performed on a very high-resolution anatomically
accurate three-dimensional human head model obtained from the National Library of Medicine
(The National Library of Medicine, The Visible Human Project, 2000). For the computational
study, the different tissues in the head model were modeled by their dielectric and thermal
properties (DeMarco et al., 2003). Figure 17.4 shows the head model, which was utilized in the
computational domain to evaluate the natural steady state (or basal, initial) temperature distribution
in the model (due to the internal tissue metabolism with no implanted heat sources).