Page 375 - Handbook of Biomechatronics

Current Advances in the Design of Retinal and Cortical Visual Prostheses 369

the center of the perceived phosphine (randomized recognition task)
(Sinclair et al., 2016). While the calculated limit of acuity with 20 electrodes
and a distance of 1mm (FOV 17degrees diagonally) is 20/4242, the best VA
score reported was 20/8397 (LCA) in one out of the three patients (Ayton
et al., 2014). The biocompatibility of the device was investigated using OCT
scans during a year following the surgery. It was reported that the implant
remained mechanically stable, functional, and without signs for adverse
immune response, except for an infection at the percutaneous connector
which was treated with antibiotics (Ayton et al., 2014). The researchers
noticed that the electrode-retina distance increased with time especially
in one of the patients, resulting in an elevation of the stimulation threshold
(Ayton et al., 2014).
Two suprachoroidal systems that are currently under investigation at
preclinical stage include the Phoenix99 prototype developed by groups at
UNSW and the University of Sydney (Schuettler et al., 2005; Suaning
et al., 2014; Matteucci et al., 2013; Barriga-Rivera et al., 2016b), and a
LCP-based device developed by a group from Seoul National University
(South Korea) (Jeong et al., 2012, 2016).
During the fabrication of the Phoenix99, the entire electrode array
including electrodes and conducting tracks is patterned through laser micro-
machining of Pt foil on top of medical grade PDMS film (Schuettler et al.,
2005). The high-density Phoenix99 with 98 stimulation electrodes is
achieved by folding of the planar array into a multilayer architecture
(Suaning et al., 2007, 2014). An additional return electrode is located under
the hermetic capsule that contains the electronics. The main advantages of
this approach are fewer fabrication steps and the ability to readily implement
thick metal layers (tens of micrometers). Thick metallization layers contrib-
ute to improved mechanical strength and resistance to electrochemical cor-
rosion. Alternative methods such as photolithography and metal sputtering
do not permit deposition of thick metal layer (<hundreds of nanometer),
and may not be reliably reproduce (Schuettler et al., 2005). The disadvan-
tages with laser patterning, however, are limited precision (few tens of
micrometers) and reduced elasticity due to the use of thick metal foil.
The electrodes in this suprachoroidal implant are arranged in a hexagonal
pattern where each channel is individually connected to a stimulation circuit
and can serve as a stimulating element (Suaning et al., 2014). When hexag-
onal stimulation is applied, each hexagon cluster of electrodes contains one
electrodes serving as stimulating element surrounded by six electrodes