Biocompatibility of Penetrating Recording Electrode Arrays Implanted Chronically in the Feline Visual Cortex

Abstract

Abstract: : Purpose: In order to comprehensively evaluate the long–term neurocompatibility of penetrating multielectrode arrays, commercially–available devices (Cyberkinetics^TM) were implanted into the primary visual cortex of adult cats. These devices were implanted to compare cortical responses to natural visual stimuli with electrical stimuli from an epiretinal electrode array. Methods: Each array consisted of platinum electrodes 1.0mm long, 400µm apart mounted on a silicon platform. Each electrode had a diameter of 80µm at its base and 1–3µm at the tip, and an active region 35–75µm long. Arrays were surgically implanted into area 17 using a pneumatic inserter; the dura mater was replaced with Goretex^TM and the bone flap secured with titanium straps and screws. Neurophysiological recordings were collected from many electrodes for several weeks. 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 and serial 3–D reconstructions of the implantation site were made to identify each electrode track. Results: After several weeks the signal–to–noise ratio began to decline, and at ninety days post–implantation useful recordings could not be obtained from the majority of the electrodes. Grossly, each array was encapsulated with a thick layer of fibrous tissue on its external surface; a thinner layer of leptomeningeal tissue was present between the array and the pial surface, and in the depths of remote sulci. A robust astrocytic and microglial reaction was present at each site of entry into the cortex and as a sheath along the path of each electrode. Nevertheless, reactive astrocytes and microglia were not found in the vicinity of the electrode tips in layer IV. In the suprasylvian gyrus, where connecting wires coursed and created a depression over the surface, activated microglia were found in layers I and II. Perivascular cuffing was observed occasionally in cortical gray and white matter. Residual hemorrhage was also present along some electrode tracks. Neurons were intact, but mild axonal degeneration was present in layer VI and white matter. Conclusions: These results suggest that chronic penetrating multi–electrode arrays can elicit persistent inflammation and mild axonal damage.

Keywords: visual cortex • immunohistochemistry • inflammation