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Current Advances in the Design of Retinal and Cortical Visual Prostheses 367

of high-intensity laser pulses. In addition, the illumination intensity for neu-
2
ronal stimulation is 0.55–10mW/mm , which is more than 100 times
higher than bright daylight conditions (Lorach et al., 2015b).
The BRI device does not attempt to drive stimulation through micro-
photodiode array (MPDA) but rather applies electrical signals (Rizzo, 2011).
Prototypes with 15 electrodes that were implanted in 7 minipigs continued
to function for up to 1 year (Shire et al., 2009; Kelly et al., 2011, 2013). The
group specifically focuses on developing technology for wireless transmis-
sion to achieve 256 individually controlled channels, as well as hermetic tita-
nium casing for the extrascleral electronics, and ceramic feedthroughs for
the cables (Kelly et al., 2011, 2013). The latest version of the BRIP device
(256 channels) is currently undergoing preclinical tests.

2.3 Suprachoroidal Implants
While in the subretinal and epiretinal approaches the electrodes are in direct
contact with the neurons in the retina, it has been demonstrated that posi-
tioning of the array at a distance to the neurons, in the suprachoroidal space
or in the sclera (Fig. 2B), can also be applied to evoke visual sensation. The
idea of placing a retinal stimulator between the choroidal and the scleral tis-
sues was first tested by Tassicker (1956). The choroid contains several layers
of connective tissue and blood vessels, and the sclera contains mainly colla-
gen and elastic fibers protecting the eye. Following Tassicker’s study, the
surgical procedure was further developed by several research groups
(Sakaguchi et al., 2004a; Ayton et al., 2014; Zhou et al., 2008). The array
is inserted through an incision in the sclera with the electrodes facing toward
the choroid and the neurons beyond, and then stabilized with sutures (no
metal tacks are required). The return electrode can be placed in the front
of the eye (vitreous cavity or cornea), integrated into the electrode array,
or subcutaneously implanted behind the ear (Villalobos et al., 2012;
Ayton et al., 2014; Saunders et al., 2014).
Preclinical investigations of suprachoroidal implantation were con-
ducted by several groups (Sakaguchi et al., 2004a; Zhou et al., 2008;
Saunders et al., 2014; Shivdasani et al., 2010, 2012). In 2004, researchers
from Osaka University reported implantation of an eight-electrode array
in the suprachoroidal space of the eyes of Albino rabbits, and demonstrated
that stimulation of the retina evoked neuronal activity measured from the
visual cortex (Sakaguchi et al., 2004a). A histological study further revealed
the choroidal and scleral tissues became separated. Propagation of electrically