Biomedical Engineering Reference
In-Depth Information
FIGURE 7-44
SEM image of the
MPDA array
(Schubert 1999),
reproduced with
permission
As expected, in vivo implants of the MPDA were no more successful than the ASR in
generating neural responses. However, the group did show that the prosthesis using their
electrode materials was stable for up to 20 months (the duration of the longest test).
Retina Implant AG was founded to further develop this technology. An operational
implant has been produced that is about 3 mm in diameter and subtends a 12
◦
field of view.
It contains 1500 pixels, each consisting of two photocells, an amplifier, and a stimulating
electrode within an area of about 70
μ
m
×
70
μ
m.
The main difference between this and the MPDA array is that additional energy is
supplied to the stimulating electrodes through the integrated amplifiers. At present this
additional power is provided by an electrical connection to the prosthesis, but a high-
frequency electromagnetic link is in development to transfer power wirelessly.
7.9.5.3 Stanford University Retinal Implant
An interesting and apparently effective modification to the subretinal implants discussed
previously has been developed by researchers from the Hansen Experimental Physics Lab-
oratory and the Department of Neurobiology at Stanford University. Their first innovation
was to convert the visual image to a more intense infrared one that is projected into the eye,
as shown in Figure 7-45. This image projection device tracks the eye microsaccades es-
sential to proper perception and thus better reproduces what actually occurs in nature. The
second innovation is to introduce a subretinal array consisting of photodiode-electrode
pairs in which the electrodes are formed as pillars or pores onto which retinal cells are
encouraged to migrate. This cell migration results in cell-electrode distance of 7 to 10
m
after only 3 days, with the result that only moderate current densities are required to excite
the cell (Levy, 2005).
μ
7.9.5.4 Boston Retinal Implant Project
While neuro-ophthalmologist Joseph Rizzo III was researching retinal transplants to re-
store blind people's vision at the Massachusetts Eye and Ear Infirmary and the Boston
