Parylene and Sterilization
Posted by Sean Horn
Friday, August 30, 2019 8:00
@ 8:00 AM
Parylene (XY — poly(para-xylylene)) 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, linear/polycrystalline and optically clear, XY 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 chemical vapor deposition (CVD) process, parylene assumes a gaseous consistency, allowing it to seep deep within a substrate surface while simultaneously forming an effective outer layer of protective film. The result is a truly conformal, pinhole-free coating, absent extraction issues, leaching and outgassing. CVD synthesizes coating in-process, allowing the deposited film to:
- assume virtually any component configuration,
- a distinct advantage over liquid coatings such as acrylic, epoxy, silicone or urethane,
- applied by wet methods like brushing, immersion or spray.
CVD lets XY penetrate and coat 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.
Clean, self-contained CVD requires no additional chemicals to complete the process, depositing uniform XY film thickness, even on irregular surfaces. Vapor methods assure reliable coating deposition on an expanded range of devices and products, with longer-lasting security and performance, minimizing the need for repair and potential of assembly failure.
Sterilization Processes for Parylene Conformal Films
Sterilization procedures for industrial processes destroy, permanently deactivate or otherwise remove all lifeforms existing on or within products, many designated for biomedical use. The objective is to establishing exceptional product safety, eradicating all sources of existing or potential contamination. Sterile pharmaceutical products include medical implants of all kinds, which need to function without fail in often turbulent biomedical environments.
- Sterilization of implants – cannulae, cardiac assist devices (CADs), catheters/probes, electronic circuitry, needles, prosthetics, stents and similar devices — purges any existing contamination, improving options for patient health after placement in the body.
- XY conformal coatings protect the implant’s sterile environment within while generating reliable barrier protection that shields the device from the effects of biofluids/gases that can cause assemblies to fail prematurely.
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 assignment. Selection criteria emphasize the type of device being sterilized, its purpose, and the type of XY coating being used. CVD allows parylene application to most vacuum-stable materials — ceramics, fabrics, granular materials, metals, paper, plastics. Across type, XY varietals withstand the sterilization methods described above; different XY material types respond better to specific sterilization techniques.
- Highly elastic, N. the most basic para-xylylene type, provides excellent penetration of minute assembly compartments. 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.
- Type C is less elastic than N, but high moisture resistance enhances its value for biomedical applications. C maintains chemical structure under radiation sterilization via e-beam/gamma. Yet, C’s low T5 point (125°C — maximum operational surface temperature) is less than the temperature applied for most autoclave sterilization. XY-C also can anneal during high-temperature steam sterilization, increasing film crystallinity.
- Parylene F has high thermal stability, sufficient to withstand steam autoclave temperatures required for creating multi-use category smart catheters/probes — devices that offer lower cost and longer function, but require repeated sterilization cycles. While N and C adapt well to other sterilization methods, autoclave sterilization can reach temperatures that challenge their survival, we recommend parylene F in those situations.
Parylene withstands all common sterilization methods, but matching XY type with sterilization process is necessary. While autoclave is not recommended for N or C, radiation techniques are successful. H2O2 plasma sterilization treatment slightly alters C’s dielectric strength but does not for N. Radiation dosage and procedural duration always need to be monitored for optimal results. With high thermal stability, F better withstands autoclave temperatures recommended for sterilizing multi-use implants.