Page 372 - Handbook of Biomechatronics

366 Lilach Bareket et al.

to send high enough current to the electrode to stimulate the nearby
neurons. There are 1500 stimulating elements in the Alpha-IMS, arranged
in a 38 by 40 layout. The distance between the electrodes is 700μm,
corresponding to a FOV 15degrees (diagonally). The investigational version
of the implant includes a percutaneous cable connected to an external bat-
tery and additional 16 electrodes used to study pure electrical stimulation in
addition to the photovoltaic scheme. In 3 out of the 11 patients implanted
with this investigational device direct photostimulation of the retina was
achieved (Zrenner et al., 2011). Patients were able to recognize unknown
bright objects on a dark background without training, and one of them
was even able to read letters (Zrenner et al., 2011). So far 29 people were
implanted with the commercial version (Retina Implant GmBH) of this
device which is wireless and does not contain the additional 16 electrodes
designed to test electrical stimulation (Wilke et al., 2011; Stingl et al.,
2013a,b). In a year postimplantation report, the majority of the patients
described improvement in perception of light (25 out of 29), in recognition
(shape, details) and localization of objects (21 out of 29) (Stingl et al., 2013a,
2015). The best VA demonstrated with the Alpha IMS was 20/200 and
20/546 in VGA and Landolt-C optotypes acuity (LCA) tests, respectively
(Stingl et al., 2013a, 2015). These long-term studies also revealed two
sources for potential device failure: breaks in the intraorbital cable probably
due to mechanical stress induced by eye movement, and corrosion of the
electronics seal indicating need to improve hermeticity (Stingl et al., 2015).
Subretinal technologies that are currently at preclinical experimental
stage are the PRIMA vision restoration system and the device developed
by the BRIP. In the PRIMA vision restoration system, several near infrared
(NIR; 880–915nm) light activated photodetectors are connected to the
same stimulating electrode (Palanker et al., 2005; Mathieson et al., 2012).
Prototype arrays with 37 stimulating elements each composed of an IrOx
electrode (diameter of 70 or 140μm) connected to two or three NIR pho-
todiodes, and to a return electrode were realized. The purpose for an indi-
vidual return is to focus the electric field and reduce electrode crosstalk
(Mathieson et al., 2012; Lorach et al., 2015b). Incoming light reaching
the device is transduced into high-intensity NIR laser pulses that are pro-
jected onto the retina, to activate the photodiodes. Optical activation of ret-
inal cells was so far validated in rats with retinal degeneration by recording of
visually evoked potentials (VEPs) from the cortex (Lorach et al., 2015a).
This scheme allow for direct optical powering of the implant at the expense
of increased complexity and power required for production and projection