Large-scale Integration–Based Stimulus Electrodes for Retinal Prosthesis - Artificial Sight: Basic Research, Biomedical Engineering, and Clinical Advances - page 154

Figure 8.2. Experimental results of the PFM image sensor (a) output pulse frequency
of the PFM photosensor as a function of input light intensity, (b) image captured by the
image sensor with 128 × 128 pixels.

obtained. The PFM image sensor used herein has 128 128 pixels. The captured

image with the fabricated sensor is also shown in Figure 8.2. The output pulse
frequency was converted into the voltage value outside the chip to display
the image.

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Modification of the PFM Photosensor for Retinal
Cell Stimulation
In this section, we discuss the modification of the PFM photosensor for
retinal cell stimulation. The reasons for modifying the PFM photosensor are as
follows [4]. First, the output from a PFM photosensor is in the form of a voltage
pulse waveform, whereas current output is preferable for injecting charges into
retinal cells constantly, even if the contact resistances between the electrodes and
the cells are changed. Second, biphasic output, i.e. positive and negative pulses,
is preferable for charge balance. Third, output frequency limitation is needed
because an excessively high frequency may cause damage to retinal cells. The
output pulse frequency of the original PFM, as shown in Figure 8.2, is generally
too high (approximately 1MHz) for stimulating retinal cells.