Retinal Prosthesis Dissertation - Student Home Pages

5.8 Stimulator Retinal Interface
In the sub_retinal approach the electrode array is positioned, replacing damaged
photoreceptors, such that the electrodes impinge into the INL and stimulate bipolar
cells. In the epiretinal approach the electrode array is tacked onto the innermost layer
of the retina and the electrodes stimulate the axons or soma of the ganglion cells; the
stimulator circuitry is atop of the electrode array. The distinctions between these two
approaches were being formed in the 1990’s when several major projects in the
USA, Germany and elsewhere were on-going.
Early electrode array formulations [120, 206] were housed on thin flexible polyimide
strips; e.g. 4µm thick, on which at one end was housed the stimulator circuitry and at
the other the electrodes to be held to the retina (≈250µm thick). In 1999 [207]
Humayun et al, in their investigation of planar disc electrodes of > 125µm noted a
dramatic increase in current requirement, when the distance between a stimulating
electrode and the retina was more than 0.5mm (500µm); thus highlighting the
importance of this particular parameter.
5.8.1 Factors affecting current requirement for stimulation
Electrode positioning; in terms of distance [52] from electrode to point of
stimulation e.g. 30µm [49] , electrode geometry; in terms of size and shape,
electrode material in terms of biocompatibility and charge delivery capability.
Electrode arrays will also take into consideration number of electrodes, electrode
spacing and number of electrodes actually to be or being used. In the case of a
sub_retinal array [122, 174] typically the number of electrodes will match up to the
number of photodiodes as each photodiode will be connected to the electrode
designed ostensibly to stimulate bipolar cells. In the case of an epiretinal array it was
realised a decade ago (2002) [174] that to achieve safe current density with smaller
electrodes it would be necessary to use penetrating electrodes rather than planar
electrodes; as this geometry would allow a reduced distance to target cells that are
embedded in ganglion cell layers of 20-40µm within nerve fibre layers of 20-200µm.
Electrode materials used for stimulation are titanium nitride, platinum, and iridium
oxide [208], which have good charge carrying capacity.
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