Devices or materials that come into direct contact with human tissues during medical procedures must be biocompatible and resistant to corrosive body fluids, electrolytes, proteins, enzymes, and lipids. Devices such as catheters, cardiac pacemakers, needles, medical probes, stents, and cochlear and other implants must be biostable to avoid substrate degradation and loss of medical efficacy.

Materials that are not biostable must be protectively coated with a material and by a process that does not degrade mechanical tolerances or performance characteristics. Other reasons to consider protective conformal coatings include:

• Physical isolation from fluids, moisture, chemicals, and other substances

• Surface passivation

• Electrical insulation

• Tiedown of microscopic particles

• Friction reduction

The polymer known as Parylene is a good option for these applications because it is biocompatible and biostable. The transparent film is formed from a pure molecular precursor (a monomer gas), so the finished film has no contaminating inclusions. Parylene is a pure, crystal clear, polycrystalline, and amorphous linear polymer.

Traditional conformal coatings are solvent-based liquid resins such as epoxies, silicones, acrylics, and urethanes. Some liquid coatings are also available in solid forms without solvents. All of these materials have liquid properties such as pooling, bridging, and a meniscus that may make them unsuitable for medical-coating applications. What's more, liquid coatings may not meet ISO and FDA toxicity and biocompatibility requirements, and cannot be applied with precise process control available with Parylene.

Parylene characteristics

The transparent and flexible Parylene coating meets the requirements of USP Class VI plastics, and can be applied as a film in layers as thin as 0.1 mil to provide pinhole free and conformal coverage, even on complex surfaces.

Parylene is highly resistant to the potentially damaging effects of corrosive body fluids and also forms an effective barrier against passage of contaminants from the substrate to the body or surrounding environment. The coating is part of a family of thermoplastic polymers that forms on surfaces by a rarefied gas rather than being applied as a liquid by conventional methods. This application process has several big plusses. For example:

It is effective in thin layers. Thicknesses of 0.5 to 0.75 mil are ample protection for typical printed circuit assemblies or electronic components.

The application is stress-free thanks to a room-temperature polymerization that uses no solvents or additives. There is no outgassing and no differential-cure forces act on fragile components or coated substrates.

Parylene is inert and highly resistive to moisture, hydrocarbons, acids, and other agents.

Parylene is applied as a vapor rather than a liquid, making it highly conformal to the surfaces of coated assemblies. Encapsulation is complete and free of pinholes and coating gaps.

The protective film resists chemical attack from organic solvents, inorganic reagents, and acids, and has good adherence to plastics, ceramics, glasses, and metals. Coated devices may be sterilized with steam, ETO, paracetic acid, or gamma and e-beam radiation.

The bulk electrical properties of Parylene make it a good candidate for electrical and electronic coating applications. Low dielectric constant and dissipation factor properties yield low losses, and are unaffected by moisture absorption.

The polymer's high purity, low moisture absorption, and freedom from trace ionic impurities contribute to high bulk resistivity values. These polymers compare favorably in critical performance categories with other coating materials, even when applied in much thinner coatings.

Dry-film lubricity is also useful in some conformal coating applications. Parylene static and dynamic coefficients of friction are in the range of 0.15 to 0.33, making this coating only slightly less lubricious, or slippery, than PTFE.

And if surfaces such as circuit assemblies need repair, Parylene can be selectively removed from them. This feature is particularly useful on costly assemblies or devices that would otherwise have to be discarded. Several removal methods are available and depend on the nature of the assembly and the location that needs repair. The removal methods include micro-abrasion, heat softening, plasma etching, and laser etching. Repaired assemblies can be cleaned and recoated with Parylene before returning them to service.

How it's applied

Objects are prepared for Parylene coating by cleaning to remove oils and other surface contaminants. Pretreatment with a multi-molecular layer of organosilane adhesion promotion allows applying Parylene to almost any vacuum-stable material.

Some assemblies must be masked with tape or other materials to prevent the polymer from covering electrical contacts or other areas that must remain uncovered. Coated items are generally fixtured in the chamber during deposition, although small objects can be Parylene coated in a tumbling process.

Vacuum-deposited Parylene is applied in an evacuated chamber by a process called vapor deposition polymerization. The coating grows a molecule at a time as a conformal film simultaneously on all exposed surfaces, edges, and crevices.

The Parylene precursor, a granular white powder, is first vaporized at approximately 150°C and 1.0 torr. The resulting dimeric vapor is further heated in a pyrolysis chamber to about 680°C at 0.5 torr, producing the monomeric diradical para-xylylene.

Finally, the monomer vapor enters the ambient-temperature deposition chamber at just 0.1 torr vacuum where it spontaneously polymerizes on all surfaces in the chamber, producing a coating of high molecular weight. The mean-free path of monomer vapor molecules in the chamber is on the order of 0.1 cm, and all sides of the target are uniformly coated.

The coating grows from the substrate surface outward, and thus its thickness is controllable from as little as 500 Angstroms to over 75 microns (three mils). As a result, users get the required protection with minimal coating mass. There is no cure cycle, as polymerization is complete within the deposition chamber and the Parylene-coated device is ready for use.

After the Parylene-deposition cycle, objects are removed from fixtures, demasked, and inspected. Thickness is confirmed by measuring film on witness strips or slides that accompany each coating batch.

Forms of Parylene

Parylene polymer comes in Parylene C, Parylene N, Parylene D, and SCS Parylene HT.Each has properties that suit it to particular applications. For example, Parylene N and SCS Parylene HT have particularly high dielectric strength, and a dielectric constant independent of frequency. Because of its high molecular activity in the monomer state, SCS Parylene HT has the greatest penetrating power of the four, with an ability to coat deep recesses and blind holes. A low dissipation factor and low dielectric constant make SCS Parylene HT ideal for high frequency applications where the coating is in a direct radio-frequency field.

Parylene C has a chlorine atom on the benzene ring which modifies its electrical and physical properties, particularly enhancing its low permeability to moisture and corrosive gases. The Parylene C deposition rate is substantially faster than Parylene N, which reduces its crevice penetration.

Parylene HT, a fluorinated Parylene, has the highest degree of thermal stability of the four. It also has superior physical and electrical properties at high temperatures.

Medical applications

The material's dry-film lubricity (static and dynamic coefficients of friction for Parylenes span 0.15 to 0.33) is an important attribute for medical-coating applications. Sliding devices especially benefit from this property.

The Parylenes are not new to the medical-device industry, having played significant roles in protecting patients and devices for decades. Parylene coatings are found in pacemakers and implantable cardiac defibrillators (ICDs), coronary stents, cochlear and ocular implants, neurostimulators, RFID implants, and radiology dosimeters. They call also be found in transdermal drug-delivery devices, digital dental-imaging systems, electrical and RF surgical devices, and endoscopic devices. All these devices benefit from Parylene's biocompatibility, lubricity and barrier properties. Their combination of physical and chemical properties and good biocompatibility lets designers explore new applications in biosensors, biochips and other micro and nano-dimension medical devices.

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