Biomedical Engineering Reference
In-Depth Information
Figure 8.10. Experimental results of evoking retinal cells with the PFM photosensor.
Response data (a) and firing rate as a function of input light intensity (b). Reprinted from
Ref. [11] with permission from Elsevier.
Implantation of LSI-based Retinal Prosthesis Devices
In the previous sections, we have demonstrated the possibility of application
of the PFM photosensor in subretinal stimulation. In this section, we focus on
the implantation of the LSI-based device into the eye. Using the packaging and
electrode technologies described in the previous sections, a thin Si dummy chip
with bump electrodes was implanted on the suprachoroid+ of a rabbit. A 3-mm-
wide and 4-mm-long chip was mounted on a polyimide substrate. Although the
chip was successfully implanted, such a thin LSI chip must be handled very
carefully. Another issue is how bending affects the device characteristics [5].
In order to overcome the issue of mechanical rigidity and realize a feasible
LSI-based device, we have proposed a device architecture consisting of small
microchips that work under a single set of control signals [8-10]. Figure 8.11
shows the concept of the proposed smart distributed stimulator. The array consists
of a number of LSI-based microchips, each of which is approximately 500-m
square in size. Each microchip has several Pt-/Au-stacked bump electrodes and is
covered by the process described in a previous section. The detailed fabrication
process is described in [10].
In order to implant the device smoothly, the thickness of the device must
be as thin as possible. The proposed device has a thickness of approximately
200m, which is acceptable. A microchip-based stimulator is fabricated in order
to demonstrate proof-of-concept. The microchips are dummy Si chips with Pt/Au
bumps. The entire image of the stimulator with Pt wires covered with silicone
tubing is shown in Figure 8.12a. The width of the stimulator is approximately
3mm.
The device was implanted into a pocket that was made in the sclera of a
rabbit eye to stimulate retinal cells by the STS method. Figure 8.12b shows the
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