Electrical Stimulation of Mammalian Retinal Ganglion Cells Using Dense Arrays of Small-Diameter Electrodes - Artificial Sight: Basic Research, Biomedical Engineering, and Clinical Advances - page 334

(such as retinitis pigmentosa or age-related macular degeneration) contains
very few photoreceptors, but a substantial fraction of ganglion cells remain
intact [2, 3]. Epiretinal implants specifically target surviving ganglion cells by
positioning stimulating electrodes in close proximity to the inner surface of the
retina.
In spite of recent successes, the present-day implants are but a small step
toward restoring meaningful sight. Psychophysical studies indicate that foveal
implants which create useful vision must contain a minimum of about 600
electrodes [4]. To achieve this number or greater, electrodes must be tightly
packed, necessitating small stimulation sites. At present, the resolution is exceed-
ingly crude and the density of electrodes per implant area is low: a typical
epiretinal implant contains a few electrodes with diameters of several hundred
microns, spaced hundreds of microns apart [1].
Useful artificial vision will require implants with hundreds or thousands
of much smaller electrodes. Ideally, an advanced implant would devote one
electrode to every ganglion cell and each electrode would be similar in size
to the cell it is designed to stimulate (tens of microns). Little is known about
the parameters which would permit reliable retinal stimulation with electrodes
which approach cellular dimensions. When the electrode surface area is reduced,
current and charge densities increase drastically, and high charge densities are
known to cause tissue damage by electrochemical reactions [5-7].
A review of the pertinent literature reveals that the feasibility of stimu-
lation with arrays of small electrodes in mammalian tissue has not been
adequately tested. The majority of studies involving retinal stimulation have
used needle-shaped probes such as platinum wires or concentric microelec-
trodes [8-10]. Others have attempted to utilize smaller stimulating microprobes
with tip diameters of 25m or less [11-14]. The geometry of such probes
differs greatly from the planar disk electrode design developed for current
epiretinal implants. Stimulation is always limited to a single stimulation site,
prohibiting the study of stimulation using multiple electrodes, their interactions,
and crosstalk effects. The use of multi-electrode arrays for retinal stimulation
has been mainly limited to large electrodes with diameters between 100 and
1500m [15-18]. While multi-electrode arrays with smaller electrodes have
been utilized to selectively stimulate the axons of retinal ganglion cells [19, 20]
and to stimulate the retina in the subretinal space [21, 22], no study has reported
the use of electrodes with surface areas below 200m 2 to target mammalian

ganglion cells.
The goal of this study was to establish current and charge thresholds for
stimulation of rat, guinea pig, and primate retina using small electrodes with
surface areas of 50-100m 2 (corresponding to diameters of 8-12m). In this
manner, this study directly addresses the prevalent concerns about the usability
of small electrodes for retinal prosthetics. Our two-dimensional multi-electrode
arrays use planar disk microelectrodes very similar to those utilized in present
epiretinal prosthetics, but smaller by 1-2 orders of magnitude.