Retinal Prosthesis Dissertation - Student Home Pages

6.2.2 Envisaged retinal array
This is the nature of the connection between the stimulator and the retinal array. The
optic disc is an oval of 1.76mm by 1.92mm giving an area of 2.66mm 2
[211]. There
needs to be an assurance for a colour orientated prosthesis that only one axon will be
targeted. Given 50% of the ganglion cells within the optic disc are concerned with
colour signals originating from the fovea [212] then this implies 1.33mm 2 housing
600, 000 fibres will have fibre areas of ≈2.2µm 2 and diameters of 1.68µm. The
overlap between a 2µm diameter electrode and a 1.68µm fibre is 0.32 i.e. the
encroachment upon neighbouring fibres will be 0.16µm, however as supporting
tissue (0.34µm) is present before the axon we have a 0.17µm `safe’ zone before the
neighbouring axon is targeted, figure 54.
Figure 54 Axon clearance
The stimulation site of a 2µm electrode is therefore well able to stimulate the 1µm
axon of the RGC. As each electrode has an associated pitch of 50µm (0.05mm) the
obtainable electrode density can be estimated by producing a square to cover the
same area (1.33mm 2 ) meaning a side of ≈1.15mm. Then the number of pitches to
110 of 200