Page 374 - Handbook of Biomechatronics

368 Lilach Bareket et al.

induced signals from the retina to the visual cortex was also demonstrated
with strip-shaped electrodes (300μm 750μm) implanted between the
sclera and the choroid of normally sighted rabbits by Zhou et al. (2008).
As expected, the stimulation threshold was higher compared with subretinal
or epiretinal stimulation. Optical coherence tomography (OCT) and fundus
imaging conducted for a duration of 16 months after the surgery, showed
that the implanted arrays maintained their position in the tissue, and there
was no sign for inflammation or structural damage (Zhou et al., 2008).
The BVA consortium is so far the only group to report a clinical study
with a suprachoroidal retinal prosthesis (Ayton et al., 2013). The surgical
procedure was based on the approach demonstrated by the Korean team
(Zhou et al., 2008) and was further developed using cadaver studies (feline
and human) as well as acute and chronic investigations in a feline model
(Shivdasani et al., 2010, 2012; Saunders et al., 2014; Villalobos et al.,
2012, 2013; Cicione et al., 2012). The implanted device was fabricated using
technology and materials similar to auditory brainstem implants and con-
sisted of 33 Pt electrodes (diameter of 400 or 600μm) embedded in a silicone
support and organized in a hexagonal pattern. Hexagonal layout is designed
to focus the electric field and reduce electrode cross talk as the active site is
surrounded by six shortened sites, collectively returning the current (Lovell
et al., 2005; Matteucci et al., 2013). Hexagonal stimulation with the
suprachoroidal prosthesis was clinically tested in a later study by the group
(Sinclair et al., 2016) and prior to that in preclinical studies (Wong et al.,
2009; Matteucci et al., 2013). In the study by Ayton et al. (2014), the outer
ring of the array was shortened to form a return electrode. Therefore, the
device had a total of 20 individually addressable stimulating sites, and 4
return electrodes: a shortened “U”-shaped including additional two embed-
ded in the array (diameter of 2000μm) and one implanted subcutaneously
behind the ear. The electrode array was wired to a percutaneous connector
behind the ear, to enable a connection to the external stimulator.
Three participants who were blind due to RP, were implanted with
the prototype prosthesis (Ayton et al., 2014; Sinclair et al., 2016). While
the appearance of phosphenes was markedly different between individuals
and between single electrodes in the same patient, all of the participants
reported reliable, retinotopically correlated perception of phosphenes,
and phosphene size and intensity were correlated with the stimulation
charge (Sinclair et al., 2016). Two of the patients were able to identify
the location of the active electrode in 57.2% and 23% of the trials by pointing