This biocompatible conformal-coating material protects medical devices against fluids, chemicals, and stray electrical charges
Many medical devices and their electronics need protection from moisture, chemical contamination, electrical charges, and body fluids. Otherwise, patients and healthcare providers may be put at risk. One way biomedical engineers provide this protection is by encapsulating devices in a conformal coating, one made of a dielectric, or poor conductor of electricity, such as silicone, acrylic, urethane, or epoxy. But one of the best materials for this purpose is parylene.
Parylene is the generic name for a series of organic polymers — poly(para-xylylene) polymers — used as coatings. They are polycrystalline and linear in nature, optically clear, and colorless. Parylene coatings have useful dielectric and barrier properties and are chemically inert. Three different types give engineers a range of dielectric and other properties from which to choose. The coatings contain no fillers, stabilizers, solvents, catalysts, or plasticizers, so they are not subject to any leaching, outgassing, or extraction issues.
Parylene coatings are also compatible and stable in the presence of bodily fluids and tissues, critical factors in the medical-device industry.
Parylene provides dryfilm lubricity with coefficients of friction similar to that of PTFE (Teflon), and dielectric strengths up to 7,000 V at a mil (25 microns) of coating thickness. No other material can be applied as thinly as parylene and provide the same levels of protection.
Parylene withstands all common sterilization methods — steam, ethylene oxide, electron beam, hydrogen peroxide plasma, and gamma radiation. It can be applied to most vacuum-stable materials, including plastics, metals, ceramics, fabrics, paper, and even granular materials. For example, parylene coatings could be applied to microspheres or moisture-absorbent powders.
Parylene can be selectively removed with plasma, lasers, or strong abrasion, for instance, to repair devices. Parylene is not soluble in harsh detergents and chemicals; in fact, it protects components from such chemicals. Parylene is not a “hard” coating, so excessive abrasion will remove it. However, most components coated with parylene do not abrade or rub against other parts. If an application does include abrasive contact, it is not a good candidate for parylene.
Parylene coatings are applied using vapor- deposition polymerization (VDP) in a vacuum chamber at room temperature. Film deposition actually takes place on the molecular level, with the coating literally growing one molecule at a time. This lets parylene penetrate and coat small cracks, crevices, and openings, and protect even hidden surfaces in areas where other coating methods such as sprays and brushes cannot reach. Coating thickness is uniform, even on irregular surfaces. And VDP is a clean, self-contained process that uses no additional chemicals.
Parylene is deposited as a vapor, so it surrounds the target and perfectly follows its contours, literally encapsulating it. Parylene coatings are ultrathin and pinhole-free.
The only raw material used in the coating process is known as dimer. Technicians place the powdered double-molecule dimer into the vaporizing chamber at one end of the coating machine. The dimer is heated, sublimating it directly to a vapor, and then heated again until the dimer cracks into a monomeric vapor. This vapor flows into an ambient-temperature deposition chamber kept at a medium vacuum (0.1 torr) where it spontaneously polymerizes onto all surfaces, forming an ultrathin, uniform film. No curing or additional steps are required.
The size of the coating chamber may be an issue if products are too large to fit inside. For example, medical wire on a reel that needs to be coated as one continuous piece may not be suitable for parylene. However, if wires are precut to various lengths, hundreds of pieces might fit into one chamber.
Because there is never a liquid phase in VDP, there is no meniscus or pooling. There is also no bridging or blocking of small openings, which can happen when applying a liquid coating.
The thickness of a parylene coating can range from 500 Å to 75 microns, so it does not significantly change the coated device’s dimensions or mass. In many medical devices, such as intraocular and cochlear implants, maintaining minimal dimension and mass are critical to the device’s performance.
An added benefit of parylene is its ability to strengthen delicate wire bonds by an estimated factor of 10.
The preparation and coating processes vary from device to device. Typical turn times are five to 10 business days, but that can be negotiated. Times may be extended if parts require extra inspection, pretreatment, or masking and demasking.
Many medical-device manufacturers send parts to coating-service providers due to the art and complexity of parylene coating process. Also, medical-device manufacturers typically do not want to become experts in a coating process they may use on only one or two product lines. Some device manufacturers do, however, purchase VDP equipment and bring the process in-house.
The parylene family includes several members. Parylene N, for example, is nonchlorinated poly(paraxylylene) that has a low dissipation factor, high dielectric strength, and a dielectric constant that doesn’t vary with the frequency of the electrical current. Parylene N also performs well when it comes to penetrating and coating into a device’s small crevices and spaces.
Parylene C is produced from the same dimer used to make Parylene N, but it is modified by a chlorine atom attached to the molecule’s benzene ring. It has a useful combination of electrical and physical properties, plus a low permeability to moisture, fluids, and corrosive gases. Its ability to provide pinholefree conformal barriers makes it the coating of choice for many critical medical electronic assemblies.
Parylene HT is the newest commercially available parylene. It carries fluorine atoms on the benzene ring instead of hydrogen atoms. It has the lowest dielectric constant and dissipation factor of all the parylenes, as well as the highest continuous service temperature (350°C). It also maintains its properties despite exposure to UV light. The other two parylenes are susceptible to damage by UV light.
All three parylene formulations are biocompatible and biostable, as confirmed by ISO-10993 and USP Class VI biological evaluations.
As noted, parylene coatings protect devices from moisture, biofluids, and biogases that can cause assemblies to fail prematurely. This protection extends product life, prevents costly repairs and, most importantly, reduces the risk of failure.
Parylene has also been helpful in tackling challenges raised by new regulations. Metallic whiskers, for example, are one of the unintended by-products of removing lead from solder as part of RoHS regulations. These whiskers can lead to reliability problems for electronic assemblies. Parylene coatings suppress the formation of metallic whiskers.
Another benefit is parylene’s dry-film lubricity, which makes it an ideal release agent for molds. Being solid and inert, parylene leaves no residue to contaminate molded products. And parylene’s lubricity extends the life of forming tools such as wire mandrels by eliminating flaking and delamination.
The cost of coating a product with parylene depends on several factors, including:
• Complexity of the item being coated. Do one or more areas need to be masked so that parylene does not coat them?
• How thick a coating is needed? This depends on the coating’s intended function. Will it be used to protect electronics, add lubricity, be a tie-layer for other coatings, or be an elution-control layer for drugs?
• What type of parylene is required? N, C, or parylene HT?
• How many parts are to be coated at one time? It is obviously less expensive to coat hundreds of parts in a large chamber than 10 or 20 parts in a smaller chamber.
While some elastomeric O-rings can be coated for less than a penny each, a single, large, complex, military circuit board can cost hundreds of dollars to coat. In general, parylene is competitive with other coatings given the right production volumes, complexity, and other variables. And although it may be more costly than some other coatings, rylene may be the only option for the protection needed by a given device.
For the sophisticated microdevices being developed for medical implants, parylene provides longterm biocompatibility and device protection. The available parylene formulations, coupled with newly developed adhesion-enhancement technologies, let parylene coating perform on medical-device components, circuits, and equipment, regardless of their size, configuration, or material.
A short history of parylene
In 1947, Michael Szwarc was pursuing his academic career in physical chemistry at the Univ. of Manchester, England. His interest in the strength of individual chemical bonds led him to investigate a class of aliphatic carbon-hydrogen bonds in which the carbon was directly attached to a benzene ring. While doing so, he heated gases of the simplest compounds having both benzene and carbon — toluene and the xylenes — to high temperatures. He monitored both the decomposition products and rates of decomposition as a function of temperature.
With p-xylene only, a tan-colored deposit formed in the cooler reaches of his glassware. The material has been described as a thin, imsy, tube-shaped mass, “the skin of a small snake.”
Szwarc correctly deduced that this lm had been formed by polymerizing reaction products of the p-xylene, called p-xylylene. He also noticed the new polymer’s physical properties and chemical inertness. This serendipitous polymerization was the world’s rst vapor deposited poly(paraxylyene). Today its purer colorless form is called parylene N.
A few years later, William Franklin Gorham at Union Carbide Corp. continued the research on parylene. By 1967, this work led to the availability of a new polymeric coating. “Parylenes” was the term used to describe both a new family of polymers and the vacuum-deposition process for applying them. In fact, Union Carbide developed over 20 types of parylene, but only three were deemed commercially viable.