Parylene and Sterilization

Parylene (XY) organic polymers are highly regarded through a wide range of industries – aerospace/defense, automotive, commercial, industrial, medical – for their utility as conformal coatings. Chemically inert, colorless and optically clear, Parylene coatings provide exceptional barrier protection, dielectric reliability, and insulation for printed circuit boards (PCBs) and similar electronic assemblies whose components must maintain performance through all operating conditions. Parylene conformal films safeguard function in the presence of biogases, biofluids, chemicals, moisture/mist, salt compounds, and temperature fluctuations.

Applied by a unique vapor deposition polymerization (VDP) process, Parylene is formed from in a gas phase process, allowing it to penetrate deep into substrate surfaces while simultaneously forming an effective outer layer of protective film. The result is a truly conformal, pinhole-free coating, absent of constituents that can otherwise be extracted, leached and outgassed. The physics of the VDP process results in a thin film that coats virtually any component configuration and provides a distinct advantage over liquid coatings such as acrylic, epoxy, silicone or urethane, applied by wet methods like brushing, immersion or spray.

The VDP process allows for the coating of small cracks, crevices, and openings along, within and under the assembly’s surface, reaching even hidden component areas, places where liquid coating materials – brushed, dipped or sprayed – cannot effectively approach.  The process requires no additional chemicals to complete, depositing uniform Parylene film thickness, even on irregular surfaces.

Sterilization Processes for Parylene Conformal Films      

Sterilization is intended to destroy all microbial contaminants on the surface of a medical device, and the process can be accomplished by a number of chemical or physical means. The challenge in sterilization is to render a surface sterile without degrading the function or useful life of either the sterilized item or its coating.

As with XY’s superior comparative worth in relation to liquid coatings, it also registers well for sterilization. Chief among the coating’s benefits is the ability to withstand common sterilization techniques — steam autoclave, electron beam (e-beam), ethylene oxide (EtO), gamma radiation and hydrogen peroxide (H₂O₂) plasma.

• Autoclave moist-heat sterilization devices subject XY-coated PCBs/implants to high-pressure saturated steam, measured at °C/pound or square inch, according to process time (minutes). These factors — pressure, temperature — are recorded throughout the entire automatically controlled/timed process, to assure appropriate component sterilization; autoclave procedures typically generate lower equipment/product damage.
• Electron beam sterilization’s shorter exposure time generates less breakdown and long-term aging for XY films, sterilizing low-density, uniformly coated devices quickly and effectively. E-beam also modifies polymers, improving the switching speed of semiconductors.
• A colorless liquid, ethylene oxide has a low boiling point of 10.8°C (55.44°F); rendered inflammable when mixed with CO2, sterilization relies on appropriately applied concentration – mg./lit. – in relation to time exposure, measured in hours.
• Gamma radiation is a cold sterilization method with considerable penetration power; lethal to DNA and other vital cell constituents, it requires little thermal energy, an advantage for sterilizing heat-sensitive materials/products.
• Compatible with most (>95%) medical devices and materials tested, hydrogen peroxide plasma sterilization is suitable for devices and materials — corrosion-susceptible metal alloys, electrical devices, some plastics — unable to tolerate high temperatures and humidity. H2O2 plasma generates free radicals hydroxyl and hydroperoxyl, which eliminate contaminant microorganisms.

The selection of the appropriate sterilization process is crucial to the success of the medical device. Selection criteria emphasize the type of device being sterilized, its purpose, and the variant of Parylene coating being used. The PVD process can be conducted on most vacuum-stable materials — ceramics, fabrics, granular materials, metals, paper, and plastics. SCS applications specialists can guide engineers through the selection of the most appropriate Parylene variant, taking into account the method of sterilization chosen.

• Parylene N, the most basic para-xylylene variants, provides excellent penetration. Unlike many other film materials, N withstands radiation sterilization via e-beam or gamma methods.  However, autoclave sterilization is less recommended, because of lower resistance to heat in the presence of oxygen.
• Parylene C maintains a higher moisture resistance, enhancing its value for biomedical applications. Parylene C maintains chemical structure under radiation sterilization via e-beam/gamma. It maintains a higher capability to withstand elevated temperatures and is more often recommended for devices that will undergo autoclave sterilization processes.
• Parylene HT^® has the highest thermal stability of the Parylene variants, sufficient to withstand high temperature environments of use as well as downstream process temperatures exceeding 350°C
• Parylene withstands all common sterilization methods, but matching the variant with sterilization process is can sometimes provide benefit.

For more details on this topic, contact SCS and request a copy of Assessing the Effects of Sterilization Methods on Parylene Coatings.