Biocompatibility assessment of insulating silicone polymer coatings using an in vitro glial scar assay

Vapor-deposited silicone coatings are attractive candidates for providing insulation in neuroprosthetic devices owing to their excellent resistivity, adhesion, chemical inertness and flexibility. A biocompatibility assessment of these coatings is an essential part of the materials design process, but current techniques are limited to rudimentary cell viability assays or animal muscle implantation tests. This article describes how a recently developed in vitro model of glial scar formation can be utilized to assess the biocompatibility of vapor-deposited silicone coatings on micron-scale wires. A multi-cellular monolayer comprising mixed glial cells was obtained by culturing primary rat midbrain cells on poly(D-lysine)-coated well plates. Stainless steel microwires were coated with two novel insulating thin film silicone polymers, namely poly(trivinyltrimethylcyclotrisiloxane) (polyV(3)D(3)) and poly(trivinyltrimethylcyclotrisiloxane-hexavinyldisiloxane) (polyV(3)D(3)-HVDS) by initiated chemical vapor deposition (iCVD). The monolayer of midbrain cells was disrupted by placing segments of coated microwires into the culture followed by immunocytochemical analysis after 7 d of implantation. Microglial proximity to the microwires was observed to correlate with the amount of fibronectin adsorbed on the coating surface; polyV(3)D(3)-HVDS adsorbed the least amount of fibronectin compared to both stainless steel and polyV(3)D(3). Consequently, the relative number of microglia within 100 µm of the microwires was least on polyV(3)D(3)-HVDS coatings compared to steel and polyV(3)D(3). In addition, the astrocyte reactivity on polyV(3)D(3)-HVDS coatings was lower compared to stainless steel and polyV(3)D(3). The polyV(3)D(3)-HVDS coating was therefore deemed to be most biocompatible, least reactive and most preferable insulating coating for neural prosthetic devices.