Biomedical Engineering Reference
In-Depth Information
FIGURE 7-46
The
Boston retinal
implant flexible
prosthesis showing
the telemetry and
power coil and
stimulator draped
over the eyeball.
confident that it will help with their first goal: improving blind people's quality of life by
allowing them to walk around unfamiliar areas more easily than they can with canes.
The design has a number of special features, including ultralow power consumption
and modularity that minimize that amount of hardware placed into the eye. This allows
for the use of a minimally invasive surgical method of implantation.
7.9.6 Epiretinal Implants
As an alternative to stimulating the bipolar cells, other researchers have proposed placing
the stimulating electrodes on the inner surface of the retina and stimulating the remaining
retinal ganglion cells. It has been shown that even at the terminal stages of the common
pathologies leading to blindness these cells mostly remain intact.
A number of groups have been pursuing similar approaches along these lines. An array
of surface electrodes is attached to the inner surface of the retina between the vitreous
humor and the inner limiting membrane. Driven by the signal processing electronics,
electrodes stimulate ganglion cells to produce a similar pattern of phosphenes.
In a clinical device, the signal processing will be placed within the eyeball with con-
trol and power supplied by an optical or RF link. The success of this process relies on
four assumptions. The first is that the retinal ganglion cells continue to function normally.
Second, the neural interface can be attached to the retina without damaging the under-
lying tissue any further. The third assumption is that useful percepts can be generated at
reasonable stimulation levels and, finally, that patterned percepts can be created (Finn and
LoPresti, 2003).
7.9.6.1 Harvard Medical School-MIT Collaboration
The Retinal Implant Project began in 1989 as a Massachusetts Eye and Ear Infirmary-
Harvard Medical School and MIT collaboration. The chip rests on the inside surface of
the retina, opposite the damaged rods and cones, and in contact with the neural ganglion
cells. Early research showed that individual percepts could be generated, but that, in most
cases, the pattern of the percept did not accurately follow the pattern of the electrode array.
In addition, the reliability of the percept was only 66% compared with 82% for the results
obtained from the prosthesis fitted to a sighted volunteer. Other problems included the
