Page 392 - Handbook of Biomechatronics

386 Lilach Bareket et al.

for improved electrical activation of neurons, and were demonstrated for
activation of retinal neurons. These materials offer enhanced
electrochemical properties and superior neuron-electrode mechanical
attachment through unique surface topography and charge injection
mechanism.
A different approach employs optical activation of retinal neurons
through photoactive interfaces, offering a new rout for wire-free, self-
powering autonomous retinal prostheses (Antognazza et al., 2015;
Bareket-Keren and Hanein, 2014). Several investigations have proposed
photovoltaic polymers (Antognazza et al., 2012, 2016; Ghezzi et al.,
2013; Gautam et al., 2014; Feyen et al., 2016), quantum dot (QD) films
(Pappas et al., 2007; Molokanova et al., 2008; Bareket et al., 2014), and
QDs directly interfacing the cell membrane (Winter et al., 2001). Bareket
et al. (2014) demonstrated photostimulation of light insensitive retina
explants with ambient light intensity using a nanomaterial film. In the com-
posite film of semiconductor nanorods and CNTs (SCNR-CNT), absor-
bance of light was followed by charge separation at the NR-CNT
interface, that activated the RGCs. Maya-Vetencourt et al. (2017) showed
a fully organic retinal prosthesis restoring vision to a rat model of RP. The
device is made of poly(3-hexylthiophene) (P3HT) and poly(3,4-
ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) deposited
on a silk fibroin substrate. When light is absorbed by the organic polymer,
it results in a mobile excited state (excitons) that enable stimulation of nearby
neurons. The researchers demonstrated recovery of cortical responses and
visually driven behavior (Maya-Vetencourt et al., 2017).
For cortical stimulation, a novel method of magnetic cortical stimulation
was developed by Fried and coworkers (Lee et al., 2016). This approach is
based on the concept that magnetic fields are not affected by encapsulating
glial scarring, which impairs the functionality of conventional electrodes.
The absence of an electrode-tissue interface opens the possibility of
improved device longevity and patient safety. Micro-coils (50 100μm)
were able to activate neurons in the fifth layer of the visual cortex of mouse
brain (Lee et al., 2016). Alternatively placing miniature field generators on
the surface of the brain can be applied as a method of delivering focused
magnetic fields. This would be less invasive than intracortical electrodes
but also less precise (Lewis et al., 2016b).
These investigations are exciting breakthroughs describing powerful
enabling tools in neuroprostheses in general and in artificial vision in partic-
ular. However, these systems are in early stages of development, and