As modern medical devices push the limits of technology, parylene coatings are finding more places to assist evolving technologies. Parylene, a conformal polymer coating that’s biostable and biocompatible, is suited to multiple industries and applications because it’s implantable; has outstanding barrier protection and electrical insulation properties; and a unique deposition method.
The coating process
Parylene is applied using a three-stage, vapor-deposition process. It lets the material deposit molecule by molecule onto parts placed in a vacuum chamber. This creates an extremely conformal coating that evenly covers grooves, crevices, gaps, and even sharp points. Because the coating is applied molecule by molecule, its thickness can be controlled to the micron.
The process works this way:
Stage 1: Parts are fixtured into a vacuum coating chamber. The solid parylene dimer – in powder form – is placed inside the vaporizer, where it turns into a dimer gas.
Stage 2: The dimer gas then flows into the pyrolysis furnace, which heats the dimer gas and turns it into a monomer (single molecule) gas.
Stage 3: Finally, the monomer gas enters a room-temperature deposition chamber, which contains the fixtured parts, where the parylene deposits itself molecule by molecule onto everything in the chamber to create a thin and highly conformal coating.
Although the parylene process has several advantages over dip and spray coatings, there are process considerations. For example, the adhesion of parylene to the substrate is critically important in every application. To get the best adhesion, it’s important that parts are clean and free of oils and debris. With a clean substrate, additional measures such as liquid surface activators or plasma treatment can be used to improve the bond between parylene and the substrate.
Because parylene evenly deposits on every surface in the vacuum chamber, areas that must remain free of coating are masked, or the coating is removed afterwards. When coating removal is necessary, it’s typically done with a laser or via plasma ablation. During the device- and component-design stage, identifying the areas that must remain coating free can greatly improve downstream processing efficiency.
Inspection and testing
Quality attributes for parylene typically specify the coating thickness, area of coverage, visual, and adhesion-testing requirements. Thin films can be measured non-destructively using spectral reflectance directly on the parts, or by measuring witness coupons that were coated with the parts.
Parylene typically becomes a material of choice when an ultra-thin, pinhole-free coating is required for implantation, a dielectric barrier, a chemical and moisture barrier, or dry-film lubricity.
The two most commonly used variants of parylene are type “C” and type “N.” Depending on the application, the variant selected can optimize deposition time, crevice penetration, lubricity, dielectric strength, and barrier permeability properties.
Both parylene types are FDA-approved and have a USP XXII, Class VI biocompatibility rating, making them a perfect fit for medical device applications.
Designing with parylene
Because each parylene coating application is unique, it’s helpful to develop the coating process along with the medical device to help ensure that it’s scaleable and high-quality. Process design conversations will include discussions about the proper parylene thickness and type, allowable clearances, plus dielectric-protection and barrier protection requirements, and cost expectations. Working with an experienced, innovative parylene coating provider ensures the best solution. Lead time is also critical when selecting a parylene coating provider, because faster iterations help a design evolve quicker and reduce time to market.