Artificial Human vision - KSOS

426 Kerala Journal of Ophthalmology Vol. XXI, No. 4
out with a new generation device having 49 electrodes.
This trial named as the ‘Europe trial’ is going on in
different universities in Europe including Hamburg,
Parris, Austria, London. Reports of the out come can
be expected by early next year.
The project named EPI RET3 of Prof Walter et al. at
University of Aachen reported results of a 25-electrode
epiretinal array implanted for 4 weeks in 6 blind
subjects 26 (Fig. 4).
Fig.4. Camera chip embedded in goggle, epiretinal chip in
position with stimulator in the posterior chamber.
(image courtesy,IMI, Dr Hornig.)
(ii) Subretinal Prostheses
In the subretinal approach a micro photodiode array is
implanted between the bipolar cell layer and the retinal
pigment epithelium, either through an abexterno
(scleral incision) or abinterno approach (through the
vitreous cavity and retina).
This was first described by Alan and Vincent Chow of
Optobionics Corp, who believed that a subretinal
implant could function as a simple solar cell without
the need for a power or input source of any type 27,28,29 .
Their Artificial Silicon Retina (ASR) Microchip is
powered entirely by light entering the eye, without
batteries or other ancillary devices. Two millimeters in
diameter, the ASR contained approximately 5000
microelectrode-tipped micro photodiodes which
convert incident light into electrical signals similar to
those normally produced by the retina’s own
photoreceptors. These electrical impulses, in turn,
stimulate any viable retinal neurons, which then process
and send these signals to the visual processing centers
in the brain via the optic nerve.This chip was implanted
in 6 patients, with a follow-up of 6 to 18 months and
reported gains in visual function in all patients as well
as unexpected improvements in retinal areas distal to
the implantation site. They hypothesized this as an
effect due to the neuro modulation of the existing
neurons due to the electrical activation.
But later works demonstrated that the idea behind this
simple approach is not feasible because it lacks a source
of viable power 30 . Gabel et al showed that cortical
activation secondary to retinal stimulation with such a
device required brightness comparable to 2 to 3 times
sunlight levels (energy) 31 . Simple photodiodes will also
not produce charge balanced pulses, which are the
safest form of electrical stimulation of nerve tissue. 32
Across time, pulses that are not charge balanced will
lead to dissolution of metal with toxicity to neural tissue
and loss of electrode function. Methods to amplify these
signals and produce charge balanced pulses are
proposed but these add significantcomplexity 31 .
In fact, Chow et al too have abandoned the notion that
their ASR Microchip is efficacious as a prosthetic device
and later thought that the low levels of current delivered
from the implant, although insufficient to electrically
activate any remaining retinal neurons in a retina with
damaged photoreceptors, may act as therapeutic as well
as neuro protective to otherwise dying retinal
photoreceptors. Hence, it is thought that this type of
an implant works through a “growth factor” that then
rescues the remaining photoreceptors. Thus, some of
the researchers claimed that this device is not a true
retinal prosthesis but should be best classified as a
therapeutic device.Studies by Pardue et al were on to
determine whether these effects are indeed neuro
protective as well as if they are persisting and
reproducible 33, 34 . In addition, studies are also ongoing
to determine whether an electronically inactive implant
can have similar effects.
In Germany another design for a subretinal implant
has been under development since 1996 by a
consortium of research universities under the guidance
of Eberhart Zrenner. They have demonstrated in various
animal models with comparable retinal degenerations
that subretinal stimulation elicits neuronal activity in
retinal ganglion cells. Also they were successful in
defining parameters necessary for successful electric
stimulation and then incorporated these data into the
development of their photodiode arrays 35,36 .
Having identified that the subretinal approach to a
retinal prosthesis is not practical without an additional