Page 371 - Handbook of Biomechatronics

Current Advances in the Design of Retinal and Cortical Visual Prostheses 365

retina into the inner layers. The miniature implantable chip also includes an
internal digital imager (camera and processor), and photovoltaic power sup-
ply based on IR light. The IR laser light is delivered via a pair of accompa-
nying wireless and rechargeable glasses. The image capture and processing
functionalities embedded in the implanted chip permit the use of the natural
optical pathway, potentially eliminating the requirement for scanning of
the visual field through head movements. Preclinical trials done so far on
pigs have validated that the feasibility of the surgical approach (Yanovitz
et al., 2014). Histopathological evaluation demonstrates the needle elec-
trodes can be applied to anchor the implant to the retina, and that both
the electrodes and the tissue remained intact (Yanovitz et al., 2014;
Raz-Prag et al., 2014).

2.2 Subretinal Implants
Subretinal technologies rely on stimulation through either a standard elec-
trode array (similar to the epiretinal scheme) or an array of microphotodiodes
integrated with the implanted electrodes. The photodiodes transduce the
perceived luminescence into electrical signals. This strategy aims to achieve
a wire-free device that is directly stimulated by incident light, and to take
advantage of the biological amplification and modulation processing, that
occurs in the middle layers of the retina (Chow et al., 2010).
Optobionics Inc. were the first to develop a photovoltaic subretinal
device powered by incidence light, and implant it in humans. The ASR
consisted of 5000 silicon photodiodes coupled with IrOx electrodes
(Chow, 2013; Chow et al., 2004, 2010; Peachey and Chow, 1999). The
ASR was implanted in 42 patients with follow-up studies of up to 8 years.
The recipients reported improvement in VA, but it was not correlated with
the location of implant, suggesting that the neurons were not stimulated
through the array (Chow et al., 2004, 2010). This inability to elicit light-
mediated neuronal responses was probably because the output currents pro-
duced by the photodiodes were below the threshold for neuronal activation
(Palanker et al., 2005). The improvement in VA was attributed to a neuro-
tropic effect, slowing the progression of retinal degeneration (Chow, 2013).
To produce enough photocurrent to activate the retina, in the Alpha
IMS subretinal photovoltaic device, an amplifying circuit is connected to
each microphodiode (MPD), and to matching electrode [titanium nitride
(TiN)] (Zrenner et al., 2011; Stingl et al., 2013b, 2015; Schwahn et al.,
2001). The amplifier increases the output signal of the photodiode in order