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B. B. Thomas, A. Ray, L. L. Chan, M. S. Humayun, J. D. Weiland; Electrode Impedance Can Be a Reliable Measurement of Retinal-Electrode Proximity During Epiretinal Electrical Stimulation in Rats. Invest. Ophthalmol. Vis. Sci. 2007;48(13):2551.
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To investigate electrical impedance as a tool to assess the electrode-retina proximity during epiretinal electrical stimulation in rats.
A platinum-iridium stimulating electrode (75 µm diameter hemisphere) was advanced under control of a micromanipulator through the vitreous of the rat eye (n=4) while visualized through a surgical microscope. The impedance of the electrode was measured at various depths inside the vitreous and after the electrode visibly reaches close proximity with the epiretinal surface. The impedance was measured using a 20 mV, 100 kHz sine wave. Biphasic current pulses (1ms/phase, cathodic first, 1.0-200 µA) were delivered and responses were recorded from the superior colliculus (SC). The return electrode for impedance testing was a large needle electrode inserted under the skin over the nostril and the concentric bipolar electrode was used for current pulsing.
When the electrode tip was inside the vitreous but distant from the retina, the impedance was 8.11±0.39 kΩ. The electrode was advanced in 10-50 um steps. When the electrode was visibly near the retina, but before the electrode visibly deformed the retina (no direct contact), the impedance rapidly increased (13.41±0.61 kΩ, p<0.006, paired t-test). At high impedance location, the threshold value for SC evoked responses were lower (2.0-50 uA) compared to the low impedance location where threshold value for SC responses was greater than 70 uA
In the rat retina, considerable increase in the electrode impedance level could be measured after an apparent proximity was established with the epiretinal surface during which the threshold value for SC evoked responses were minimum. This suggests that the electrode impedance measurement can be a reliable tool to optimize the retinal-electrode contact during epiretinal electrical stimulation
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