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606 LIN ET AL.

a multi-electrode array placed adjacent to the ret- the diffused and/or distorted visual percepts due to
ina to elicit visual percepts that the field of retinal undesired activation of the axons of passage (7). As
prostheses started to advance rapidly (7). Different an epiretinal implant, both Argus I and II require
retinal prostheses products and prototypes have surgical fixation of an electrode array to the retinal
since been developed and tested in humans, among surface with a retinal tack. The array is designed
which two devices have received regulatory approval to conform to the curvature of the inner retina to
for clinical use. Argus II epiretinal implant (Second maintain a consistent distance between the electrodes
Sight Medical Products, Sylmar, California) received and the retina for optimized stimulation.
both European Union approval (CE mark) and the The Argus systems contain a miniature camera
US Food and Drug Administration (FDA) market mounted on a pair of glasses, an external video pro-
approval in 2011 and 2013, respectively. Alpha-IMS cessing unit (VPU) worn by the user (Figure1), and
(Retina Implant AG, Germany) received CE mark extraocular electronics and an intraocular electrode
approval in 2013. array that are interconnected via a transscleral cable
Retinal degeneration that involves progressive (Figure 2). The camera captures visual scenes and
deterioration and loss of function of photorecep- sends the information to the VPU for advanced pix-
tors is a major cause of permanent vision loss (8,9). ilation and processing. The extraocular electronics,
Age-related macular degeneration (AMD) and RP are along with the receiver coil, converts the radio fre-
two of the more prevalent forms (10). AMD affects quency signals it receives wirelessly from the VPU to
30 to 50 million people globally and more than two the electrical pulses. Stimulation pulses proportional
million in the United States (11,12), and RP is esti- to the luminance of the pixelated images are subse-
mated to affect 1.5 million people in the world (13). quently delivered to the intraocular electrode array,
The etiology of AMD begins by primarily affecting which is attached to the retina.
cone photoreceptors in the macula. RP begins with
progressive degeneration of rod photoreceptors in the
peripheral retina. Preservation of the inner retina in
these photoreceptor degenerative diseases has been
widely reported (14,15), supporting the possibility
of vision restoration by establishing a stimulation
mechanism that bypasses the damaged photoreceptor
layer and interfaces with the surviving inner retinal
neurons that remain capable of neural signaling (7).
Retinal implants interface with the retina at dif-
ferent positions (16). For example, Argus I and II are Figure 1. External part of the Argus system (Image courtesy of
implanted epiretinally and alpha-IMS subretinally. Second Sight Medical Products, Inc.)
Epiretinal implantation has the following advantages.
First, the prosthesis contacts the retina on the inner
surface that is accessible from the vitreous cavity,
which reduces the risk of mechanical damage to the
retina. Second, besides choroidal perfusion, fluid in
the vitreous cavity serves as an additional heat sink
that enhances the removal of the heat generated by
the implant. Finally, the device directly stimulates
ganglion cells, thus being potentially useful in cases
of extended retinal degeneration where inner retina
circuitry is altered. The disadvantages of the epiretinal
prostheses include the difficulty of fixating the elec- Figure 2. Implant part of the Argus system (Image courtesy of
trode array uniformly onto the retina and potentially Second Sight Medical Products, Inc.)