A comparison of the tissue response to chronically implanted Parylene-C-coated and uncoated planar silicon microelectrode arrays in rat cortex

In this study we employed a quantitative immunohistochemical approach to compare the brain tissue response to planar silicon microelectrode arrays that were conformally coated with Parylene-C to uncoated controls at 2, 4, and 12 weeks following implantation into the cortex of adult male Sprague-Dawley rats. We did not find any difference in the relative intensity or the spatial distribution of neuronal or glial markers over the indwelling period, even though Parylene-C-coated substrates supported significantly less cell attachment, indicating that the foreign body response to planar silicon microelectrode arrays has little to do with the composition or decomposition of the silicon electrode. Moreover, our results suggest that changes in microelectrode surface chemistry do not have a strong influence on the cytoarchitectural changes that accompany the brain foreign body response to planar silicon microelectrode arrays. Our quantitative comparison over the indwelling period does not support progressive increases in astrocyte encapsulation and/or progressive neuronal loss in the recording zone as dominant failure mechanisms of the type of chronic recording device. Finally, we found evidence of two potentially new failure mechanisms that were associated with CD68 immunoreactivity including demyelination of adjacent neurons and BBB breakdown surrounding implanted electrodes at long indwelling times.