Incorporation of Linear Spacer Molecules in Vapor-Deposited Silicone Polymer Thin Films

Abstract

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.

Biocompatibility Assessment of Insulating Silicone Polymer Coatings Using an in vitro Glial Scar Assay

“…The iCVD silicone coatings deposited at their optimal conditions, shown in Table 1, were sufficiently crosslinked to prevent delamination or dissolution in polar and non-polar organic solvents. [11,15] Following chemical characterization via IR spectroscopy, monomer leaching from the deposited material was evaluated by immersing silicon wafers coated with both polymers in supplemented cell culture medium at 37 8C for 7 d. Following this immersion period, 5 mL of the medium was aspirated and subjected to FTIR spectroscopy following 3 and 7 d of immersion (results not shown). FTIR spectra obtained from both liquid samples did not show any peaks in the 1 010 to 1 020 cm À1 range, representative of trifunctional cyclic siloxane groups [32] present in the monomers of both silicone coatings (results not shown).…”

“…While probes are being engineered to mitigate the host tissue response, novel insulating materials that protect the electronic circuitry in neural implants are also being investigated to enhance the device performance in chronic settings. [11][12][13][14][15] The majority of the investigated insulating coatings are synthesized via chemical vapor deposition (CVD), primarily because the technique allows conformal coating of small, high-aspect-ratio three-dimensional substrates, such as neural probes. [16][17][18] Application of such insulating coatings conformally on miniature devices via conventional solution polymerization techniques is complicated due to dominant surface tension and capillary effects on the micron scale.…”

“…[18] Novel vapor-deposited silicones, such as polymers made from trivinyltrimethylcyclotrisiloxane (polyV 3 D 3 ) and trivinyltrimethylcyclotrisiloxane in combination with hexavinyldisiloxane (polyV 3 D 3 -HVDS) are attractive candidates as insulating coatings for neural probes owing to their robust cross-linked structure, excellent adhesion to silicon substrates, high resistivity and long-term stability under a saline environment. [11,15] Although physical, chemical and mechanical test methods for vapor-deposited polymers are very well-developed, biocompatibility assessment in research laboratories has been limited to rudimentary viability assays with immortalized cell lines. [15] Such tests are not physiologically relevant, since the cell lines utilized for biocompatibility assessments are not sensitive to pharmacological insults (such as monomer leaching) compared to primary cells.…”

Initiated-chemical vapor deposition of organosilicon layers: Monomer adsorption, bulk growth, and process window definition

“…29) and on the V 3 D 3 /HVDSO/d-TBPO system (Ref. 17) report on the existence of a pressure dependent substrate temperature threshold below which unreacted monomer or short oligomer units may be trapped in the growing layer and pumped away during the chamber evacuation and leaving, in this case, micrometric size disk-shaped holes in the layer surface 29 or even holes of visible size, 17 therefore, pointing out a bulk-related phenomenon. However, under our experimental conditions, those defects were not detected on the surface of the deposited layers even for the highest thickness loss recorded by in situ SE, as pointed out by the SEM image in Fig.…”

“…So far, in literature, quartz crystal microbalance (QCM) measurements have been used for the determination of the monomer surface concentration, [12][13][14] while the film growth development has been followed by means of laser interferometry. [15][16][17][18] During QCM measurements, the mass adsorption on the quartz crystal, generally gold coated, results in a frequency shift that can be directly related to the mass of the adsorbed monomer. [12][13][14] As only physisorption is considered to control the monomer/surface interaction, [12][13][14] at each value of P M /P sat , the amount of adsorbed monomer is taken as the average of the measured monomer surface concentration during the adsorption and desorption of the monomer from the surface of the QCM.…”