Page 386 - Handbook of Biomechatronics

380 Lilach Bareket et al.

impanation with the Argus II epiretinal device, and even up to 20/200 when
this device was coupled with an external optical magnification system (Sahel
et al., 2013), while VA of 20/546 was reported in one patient with a sub-
retinal implant (Stingl et al., 2015). While these results represent a great
promise for rehabilitation of impaired vision, none of these devices have
yet to achieve VA above 20/200, which is considered as legal blindness.
Further, more than 75% of patients who received retinal implants could
not achieve measurable VA (Stingl et al., 2015; Humayun et al., 2012;
Yue et al., 2015; Ha et al., 2016). Although several retinal devices have been
approved as safe for commercial use, avoiding health complications and
damage to the retina are still a major concern. Complications such as con-
junctival erosion, endophthalmitis, hypotony, retinal detachment, retinal
microaneurysms, gliosis, and retinal atrophy have been reported, and
removal of the device was required in several cases (Humayun et al.,
2012; Keser€u et al., 2009, 2012; Sailer et al., 2007; Menzel-Severing
et al., 2012; Butterwick et al., 2009). Improvements in the VA performance
achievable by visual implants and in their biocompatibility and long-term
reliability are highly desirable. In particular, high-resolution stimulation
through small electrodes with safe and focused electrical fields, minimizing
tissue damage, and facilitating long-term operation. In this section, we will
discuss important engineering considerations that need to be taken into
account in the development of retinal and cortical prostheses, including:
the placement and fixation of the electrode array in the proximity of the
tissue, electrode size, and materials and parameters for stimulation.

5.1 Placement and Fixation of Retinal and Cortical
Electrode Arrays
The anatomic location of the retinal array has implications on the complex-
ity of the surgery, risk for injury and retinal detachment, long-term mechan-
ical stability, threshold for electrical stimulation, as well as spatial resolution
and visual perception.
In epiretinal prostheses, the electrode array is placed on the surface of the
retina (Fig. 2B). The close positioning of the electrodes to the target cells, the
RGCs, potentially improves the efficiency of electrical stimulation. How-
ever, in this configuration, there is a potential of stimulating the bundle
of neural extensions passing through the center of the retina introduces
nonspecific stimulation pathways that may disrupt the accuracy of electrical
activation (Humayun et al., 2003; Rizzo et al., 2003). While insertion of the
implant via the vitreous chamber (between the lens and the retina), makes