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Joseph Majdi, Saugandhika Minnikanti, Anant Agrawal, Nathalia Peixoto, Ethan Cohen; Determination of Electrode Proximity to the Retinal Surface from the Stimulus Pulse Waveform. Invest. Ophthalmol. Vis. Sci. 2013;54(15):1045.
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© ARVO (1962-2015); The Authors (2016-present)
To determine the proximity of an stimulus electrode to the retinal surface for effective activation of the underlying local retinal neurons by epiretinal prostheses.
Using a superfused rabbit eyecup preparation, we advanced a thick fluopolymer insulated (1.6mm) Pt electrode (0.25mm diameter, cut at a 37° angle) parallel to the retinal surface, while applying 0.95msec biphasic 10µA pulses and recorded the resulting voltage waveform. Using a micromanipulator, we advanced the electrode from 1000µm towards the retinal surface, while simultaneously monitoring the electrode’s charging characteristics and position using Fourier domain optical coherence tomography. A series of proximity experiments were performed by applying 10mV rms sine waves from frequencies of 1Hz to 100kHz in order to evaluate changes in the electrode impedance and phase angle.
We found that the voltage step at pulse onset increased as the electrode approached the retinal surface. The series resistance of electrical stimulation rose indirectly proportional to the electrode-retina proximity by 3300±680Ω with the 50% midpoint averaging 32µm from the surface and the charging characteristics fit closely to exponential decay for greater distances. The resistance increased an additional 1300±640Ω as the electrode depressed the retina 50µm against the choroid. A similar impedance relation was seen for retinal proximity with impedance spectroscopy. At 100kHz, the impedance modulus increased 4.6±0.9 fold (n=6) at the retinal surface compared to a distance of 1000µm. Interestingly, at 10kHz we observed a 25° difference in phase between the 1000µm and 0µm electrode configurations. This difference was significant and observed in all experiments we performed (n=6). These results point to signature impedance spectral features (modulus and phase) for proximity of electrode to the retinal surface.
We found several different methods to measure retinal proximity from an epiretinal stimulus electrode. An inverse relationship was found between the voltage step on the stimulus pulse and its proximity to the retinal surface and there were also differences using impedance spectroscopy.
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