Computational Modeling of Electromagnetic and Thermal Effects for a Dual-Unit Retinal Prosthesis: Inductive Telemetry, Temperature Increase, and Current Densities in the Retina - Artificial Sight: Basic Research, Biomedical Engineering, and Clinical Advances - page 300

It should be noted that the method could incorporate arbitrary lumped
impedances at any point of the impedance mesh to account for contact capac-
itances or any other effects that can be described by means of localized
impedances. Furthermore, the method can be characterized by a “multi-
resolution” grid, where the size of the cells is chosen to fit the model to
be simulated with fine resolution in regions of interest and relatively coarse
resolution elsewhere [34].

Computational Challenges
As mentioned earlier, the size of the computational space can easily become
too large to be handled with uniform cells. An approach to processing large
models is to use a multi-resolution impedance method [33], where larger
voxels are used in homogeneous regions of the model, and small voxels are
used in regions with material boundaries, where detail is needed. In retinal
models, the use of multi-resolution models can reduce the number of voxels
from 30% to over 80% depending on the level of detail in the model.
Building an optimized multi-resolution model is a complex process. Figure 15.16
shows a simplified way of creating a multi-resolution model from a uniformly
discretized model. As electric properties change at material boundaries, it
is important to keep small voxels at the boundaries to minimize numerical
errors.
To process larger models, there are additional techniques that can be used.
In some cases simulations can be performed in 2D instead of 3D. While those
results are often qualitative in nature, preliminary results from simulations
performed by our group indicate that some configurations allow the scaling of
data obtained using 2D simulations into 3D values with an acceptable error
margin. In particular, current density values taken far from the current return
and close to the symmetry plane of the electrode array, in configurations with
electrode arrays larger than 4

×

4, can be scaled well from 2D to 3D.

Figure 15.16. Procedure to obtain a multi-resolution model.