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G. Y. McLean; Charge Injection Characteristics of Tethered Photodiode Arrays for Retinal Prostheses. Invest. Ophthalmol. Vis. Sci. 2007;48(13):2572.
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© ARVO (1962-2015); The Authors (2016-present)
We have previously presented preliminary characterization of ASR devices operating in photovoltaic mode. Here we investigate operation of the device in two alternative modes that both involve the application of bias potentials provided by an auxiliary source of power: forward-biased injection and reverse-biased injection.
We controlled the common terminal of ASR devices via a tether attached to their non-pixelated surfaces. We investigated both forward-biased and reverse-biased injection modes by applying repetitive pulsatile bias signals and measuring the charge injected at the pixelated electrode-electrolyte interface in PBS, under various illumination conditions.In the forward-biased injection mode, photo-induced charge was accumulated for most of each cycle. During the accumulation phase the diodes were reverse-biased and the electrode-electrolyte interface was gradually polarized at a rate proportional to the irradiance of the device. Accumulated charge was injected at the end of each cycle in a brief interval during which the diode junctions were forward-biased by a potential applied to the common terminal. We measured the charge that could be injected under forward-bias conditions by accumulating a known polarization and then applying an anodic potential excursion. We investigated the charge injection over the range of safe operation of the electrodes, and investigated the accumulation in the dark due to reverse leakage currents.In the reverse-biased injection mode, we investigated the charge that could be injected in a brief interval during which the diodes were reverse-biased and bright illumination was applied. For the remainder of each cycle the diodes were forward-biased to restore the electrodes to a quiescent potential. We investigated the maximum charge that could be injected over a range of illumination conditions and bias potentials.
Under both modes the peak current that can be injected exceeds the peak current that can be delivered under photovoltaic operation. The forward-biased mode provides high injection currents under relatively low illumination, whereas the reverse-biased mode presents the advantage that the stimulus current is more directly governed by the illumination conditions (since the diodes are operating essentially as current sources whose current is proportional to the illumination).
The use of pulsatile bias signals applied to the common terminal of ASR devices extends the range of stimulus currents sufficiently that tethered ASR devices might serve as a platform for retinal stimulation when operated in combination with a source of external infrared illumination.
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