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Marianna E. Ivanova, Boris K. Baziyan, Andrey N. Serkov; Main Biocompatibility Factors of Microelectrode Array (MEA) Implantation in Cortical Visual Prosthesis (CVP). Invest. Ophthalmol. Vis. Sci. 2012;53(14):318.
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Concept of CVP is well-understood, however, brain-computer interface still requires improvement before it can be used in humans. To date there are 6 important biocompatibility factors in the production of MEA and implantation: electrode material & size, electrode coating, number of electrodes, stimulus intensity, interelectrode distance (IED) and distance to blood vessels. In this work we investigated factors of proximity to the blood vessels and influence of the interelectrode distance on biocompatibility and work stability of implanted coated MEA for CVP in rodent model.
12 male wistar rats were divided into 4 equal groups, based on the electrode material (stainless steel, nichrome, Tungsten or Molybdenum). MEAs of 2x3 or 2x5 100μm-diameter electrodes, with IEDs of 300, 500 and 700μm, and 2-μm thick silicon carbide (SiC) or 15-μm varnish coating, were implanted into the 4th layer of visual cortex of the rats. Follow-up period was 6 months. Tissue damage was evaluated by the extent of necrosis, fibrotic incapsulation thicknesses, and densities of macrophage accumulation.
The extent of the necrosis was the same for all types of electrode material and coating, ranging from 7.0 to 17.2 µm. Incapsulation thickness was statistically greater (p less than 0.05) in the group with 300-μm IED (4.1±0.6 μm) than in the 500-μm (2.9±0.7 μm) or 700-μm (2.0±0.6 μm) IED groups. In three cases of 300-μm IED, the incapsulation and macrophage accumulation were so profound that they enveloped two or more electrodes at once, thus 300-μm distance appeared to be insufficient for the successful long-term biocompatible MEA implantation. 75% of the electrodes were at the distance of ≥300μm from the nearest blood vessel, which did not affect the biocompatibility. Whereas the rest 25% of the electrodes were very close to small blood vessels. If a blood vessel was at the distance of ≤50μm, an intensive local inflammatory response was observed: thickness of fibrotic incapsulation reached 70μm, and macrophage density reached 5000 cells/mm2.
Implantation of MEA in avascular cortex zone is extremely important for the biocompatibility and long-term stability of the whole complex. Optimal interelectrode distance for the electrode diameters of 100μm is 500μm or greater.
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