Biomedical Engineering Reference
In-Depth Information
FIGURE 7-41
Common sites for
visual prostheses.
7.9.4 Worldwide Research Activity
A number of groups around the world are involved in research and development of neuro-
visual prostheses of various kinds, with new research institutes entering the field regularly.
Many of those now or recently active are listed in Table 7-2.
7.9.5 Subretinal Implants
A number of blinding disorders are primarily due to photoreceptor or outer retinal degen-
eration and destruction. These include but are not exclusive to diseases such as RP and
AMD. Most retinal implants are designed to provide some form of vision to this subset
of blind patients. Research groups have followed different paths to achieving this goal,
with the least invasive being self-powered arrays of phototransducting elements placed in
the subretinal space. In this configuration, each element comprises a semiconductor based
photodiode-electrode pair. Light striking the photodiode causes a current to flow between
its electrode and a reference electrode over the entire back of the array. The result is a
voltage gradient that is intended to stimulate the dendrites of the bipolar cells.
The success of this approach depends on three assumptions. The first is that the bipolar
cells of the damaged retina still operate in a physiologically normal manner. The second is
that the photodiodes can produce sufficient current under normal illumination conditions,
and the third is that the electrodes can be placed close enough to the bipolar cells for the
induced photocurrent to excite them (Finn and LoPresti, 2003). Whether these assumptions
are valid remains controversial; therefore, a number of alternatives have been considered,
which will be discussed later in this chapter.
7.9.5.1 Optobionics
The Optobionics Corporation is a private company started by Dr. Alan Chow and his
electrical engineer brother, Vincent Chow, in the 1990s. They developed the artificial
silicon retina (ASR) microchip, as described already, which was designed to stimulate
damaged retinal cells, allowing them to send visual signals again to the brain.
Initial tests involved a single element containing a photodiode and a large gold elec-
trode (36 mm
2
area) that was implanted in the eye of a rabbit. Illumination of the photo-
diode was registered by increased cortical activity with the activity levels being propor-
tional to the light intensity. With an electrode charge density of 100 nC/mm
2
, resulting
