High-Resolution Opto-Electronic Retinal Prosthesis: Physical Limitations and Design - Artificial Sight: Basic Research, Biomedical Engineering, and Clinical Advances - page 261

100

10

r = 50 µ m

1

r = 15 µm

r = 5 µm

0.1

1 10 100 1000

distance from surface, µ m

Figure 14.2. Threshold current (in A) required for generation of a 5mV cross-membrane
voltage drop on a proximal surface of a large cell, plotted as a function of distance
between the cell and the electrode surface. Calculated for electrodes of 5, 15, and 50m
in radius.

From the experimental values of the threshold current required for retinal neural
cell stimulation - I th = 04-3A (at pulse duration 1ms)[24] measured with a

disk electrode of r 0 = 625m in diameter in a very close proximity to cells

Y ≈ 0 and having a remote return electrode d >> r 0 - we can deduce the

U mem = 16-12mV. The current required for depolarization of the

cell membrane by U mem rapidly increases with the separation of a cell from
the electrodes Y, asymptotically reaching quadratic dependence, as shown in
Figure 14.2. If this separation varies in different parts of the array, the stimulation
threshold (and the visual outcome) will vary accordingly. Close and stable
proximity of cells to the electrodes is thus an important issue in the design of a
high-resolution retinal implant .

I th · g/4r 0 =

Tissue Heating
Electric current in a conductive medium generates Joule heat. Since the threshold
voltage and current strongly depend on distance between electrodes and cells
so will the heating. For single pulses at the threshold of neural stimulation this
temperature rise is negligible - below 10 − 2 C. However, with application of
repetitive pulses to a large array of electrodes heat might accumulate, and thus
a regime of chronic stimulation requires careful consideration. Average power