Optimization of Parylene Coatings for Implantable Neural Devices

Parylene C is a widely utilized biocompatible thin-film polymer coating for implantable devices, specifically neural implants, due to its ability to cover uniformly complex shapes, protect the implants against corrosive body fluids such as CSF (cerebrospinal fluid), electrical insulation, chemical stability, and durability. However, weak adhesion to the various types of device substrates remains a significant challenge for this promising coating material. Here, we will present the impact of substrate material selection,silanization (as adhesion promoter), different deposition parameters (such as silane concentration) and post-deposition annealing on overall characteristics of the coating layer (such as thickness and adhesion strengths). First, silicon wafers were coated with different thicknesses of Parylene C using SCS coating machine, followed by annealing in Ar atmosphere at temperatures of 150, 200, and 250°C. Additionally, the effect of thickness (1 and 3μm), and silane A-174 concentration (0.5, 1, and 1.5%) on adhesion strength were investigated. To assess long-term adhesion of Parylene, samples were submerged in an in-vitro accelerated aging bath consisting of PBS at 87°C for 7 days, and the properties were compared before and after the aging, to estimate the coating’s stability in body environment after implantation. Coating thickness and molecular structure were investigated using Ellipsometry and FT-IR, respectively. Adhesion strength was determined using ASTM D3359 tape test. Our initial results indicated that 1μm Parylene coating with 1.5 vol% silane as adhesion promoter, and annealed at 150°C can potentially result in optimal adhesion for silicon substrates. We are pursuing follow up studies on silicon and other types of substrates (such as polyimide, which is a common flexible substrate for a variety of brain and neural implants) to further validate these observations. We envision that our results are critical to address unmet challenges for prolonging the lifetime of the implantable neural devices.