Poly (trivinyl-trimethyl-cyclotrisiloxane) or polyV3D3 is a promising insulating thin film known for its potential application in neural probe fabrication. However, its time-consuming synthesis rate renders it impractical for manufacturing standards. Previously, the growth mechanism of polyV3D3 was shown to be affected by significant steric barriers. This article describes the synthesis of a copolymer of polyV3D3 via initiated chemical vapor deposition (iCVD) using V3D3 as the monomer, hexavinyl disiloxane (HVDS) as a spacer, and tert-butyl peroxide (TBP) as the initiator to obtain nearly a 4-fold increase in deposition rate. The film formation kinetics is limited by the adsorption of the reactive species on the surface of the substrate with an activation energy of −41.5 kJ/mol with respect to substrate temperature. The films deposited are insoluble in polar and non polar solvents due to their extremely crosslinked structure. They have excellent adhesion to silicon substrates and their adhesion properties are retained after soaking in a variety of solvents. Spectroscopic evidence shows that the films do not vary in structure after boiling in DI water for 1 hour, illustrating hydrolytic stability. PolyV3D3-HVDS has a bulk resistivity of 5.6 (±1) × 1014 Ω-cm, which is comparable to that of parylene-C; the insulating thin film currently used in neuroprosthetic devices.

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.

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