Page 367 - Handbook of Biomechatronics

Current Advances in the Design of Retinal and Cortical Visual Prostheses 361

head movements are used to scan the visual field (Ho et al., 2015). The
photodiode-based configuration potentially permits object tracking via
eye movements, as the light is transduced to electrical signals at the
implanted device. These signals are then processed, amplified, and delivered
to the coupled electrodes.
Four sites of implantation for the stimulation array have been clinically
investigated: epiretinal, subretinal, intrascleral, and suprachoroidal (Fig. 2B).
Epiretinal devices are placed between the vitreous humor and the inner lim-
iting membrane (ILM) to interact with the RGCs (Humayun et al., 2003;
Rizzo et al., 2003), while subretinal devices are inserted between the RPE
and the outer retina to stimulate the inner retinal neurons (primarily bipolar
cells, but also amacrine and horizontal cells) (Chow et al., 2004; Zrenner
et al., 2011). Suprachoroidal electrodes are maintained between the choroid
and scleral tissues (suprachoroidal) (Ayton et al., 2014; Zhou et al., 2008;
Wong et al., 2009) and intrascleral electrodes are embedded in the sclera
(Nakauchi et al., 2005; Fujikado et al., 2007)(Fig. 2B). While in the
suprachoroidal and intrascleral configurations the electrodes are distant from
the target cells, these strategies offer a substantially less invasive surgical pro-
cedure. An extraocluar retinal prosthesis (ERP) placed on the surface of the
sclera (episcleral) was also proposed by Chowdhury et al. (2005, 2008).
Epiretinal prostheses (Fig. 3A) that have been clinically tested include the
devices by Second Sight Medical Products Inc. (Argus I and Argus II; United
States) (da Cruz et al., 2016; Yue et al., 2015), EPIRET GmBH (EPIRET3;
Germany) (Klauke et al., 2011), and Intelligent Medical Implants GmBH
(IMI, Switzerland) (Keser€u et al., 2012), which later became part of Pixium
Vision SA (France) with the Intelligent Retinal Implant System (IRIS I and
IRIS II). Subretinal devices (Fig. 3B) include the artificial silicon retina
(ASR) developed by Optobionics (United States) (Chow et al., 2010),
and the Alpha IMS by Retina Implant AG (Germany) (Stingl et al.,
2015). Finally, the suprachoroidal (Fig. 3C) and intrasceral devices are clin-
ically examined by groups in Australia [Bionic Vision Australia (BVA)
consortium] (Ayton et al., 2014), and in Japan (Osaka University)
(Fujikado et al., 2016).
Retinal systems that have not yet been tested in humans include the sub-
retinal PRIMA vision restoration system (Pixium Vision) developed by
Palanker and coworkers (Stanford University, United States) (Lorach
et al., 2015a; Palanker et al., 2005), and the Boston Retinal Implant
(BRI) developed by the Boston Retinal Implant Project group (BRIP;
Boston, United States) (Rizzo, 2011). The Bioretina epiretinal device is