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K.E. Brabant, J.P. McAllister, II, S.D. Elfar, N.P. Cottaris, G.W. Abrams, R. Iezzi; Biocompatibility of Acute and Chronic Penetrating Electrode Arrays Implanted in the Feline Visual Cortex . Invest. Ophthalmol. Vis. Sci. 2006;47(13):5056.
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The goal of this study was to evaluate the short– and long–term neurocompatibility of penetrating multielectrode arrays used to record cortical responses to visual stimuli.
Penetrating multielectrode arrays (CyberkineticsTM) were implanted acutely (3–4 days) and chronically (3–6 months) into the primary visual cortex of adult cats. Each array consisted of platinum electrodes 1.0mm long, diameters of 80µm at the base and 1–3µm at the tip, an active region 35–75µm long, and mounted 400µm apart on a silicon platform. Tissue underlying and adjacent to each implant was processed for light microscopy using Nissl staining, silver staining for degenerating axons and neurons, and immunohistochemistry for reactive astrocytes (GFAP) and microglia (IBA–1). Quantitative assessments for cellular damage and glial reactivity were performed using non–biased optical factionator methods..
Grossly, the chronically implanted arrays were encapsulated in a thick glial scar which became tethered to the skull over time causing increased cortical damage, and damage along the electrode tracks. Acutely implanted arrays did not appear to cause any surface damage but showed glial responses along the electrode tracks. The portion of the electrodes that elicited the most intense gliotic response corresponded to the thickest part of the shaft and the junction with the silicon backing. Although the gliotic response was not as severe near the tips of the electrodes in layer IV, some astrocytes and microglia were activated in this region. This response was observed in both acute and chronic preparations, suggesting that surgical procedures cause most of the gliosis. Neuronal degeneration was rare at the electrode tips and in the cortical white matter. In chronically implanted animals, the signal–to–noise ratio began to decline after several weeks, and in some cases useful recordings could not be obtained at ninety days post–implantation.
These results suggest that fibrous encapsulation and glial scarring are localized to the meninges and layers I–III of the cortex, but little neuronal damage ensues over protracted implantation periods. Superficial fibrosis can be so extensive that it probably causes the repositioning of electrode tips and thus might impair neuronal recording.
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