Sometimes all a medical device needs is a single coating that performs a single function. But many devices require coatings that have two or more applications. These multifunction coatings can consist of a single layer or multiple layers.

If a multilayer coating is chosen, a manufacturer must tackle a number of challenges involved in the creation of a complex coat. Fortunately, suppliers can often bring expertise and equipment to bear on these coatings to ensure that they function as intended on medical devices.

Multipurpose Parylene

Device manufacturers seeking multipurpose single-layer coatings may find what they need in the parylene family. Parylenes are polymer coatings designed to provide biocompatibility, lubricity, electrical insulation, and isolation from moisture and chemicals.

Parylenes are applied in a vapor form that penetrates holes and crevices in a substrate. With this penetrating ability, parylene can coat the inside and outside of a device simultaneously.

Of course, some medical device manufacturers don't want parylene to penetrate the interior of their device; these manufacturers must find a way to mask or plug any surface openings before the coating process begins. The alternative is removing the unwanted interior coating afterward. There are several different methods of removal, but the process is not an easy one, says Lonny Wolgemuth, medical market specialist for Specialty Coating Systems Inc. (SCS; Indianapolis).

Besides preventing or dealing with unwanted penetration, parylene users must make sure that the surface of the device is clean prior to coating. Otherwise, Wolgemuth says, surface contamination will be both visible and magnified due to parylene's transparency and index of refraction.

Parylene coating is done in a vacuum, so coated materials must be vacuum stable. Wolgemuth says that a parylene coating would not be suitable for materials that outgas in a vacuum because the outgassing would adversely affect the adhesion of the coating to the substrate.

The attachment of parylene to a substrate is a mechanical process rather than a chemical one, “so parylene likes a surface with some texture or topography to it,” Wolgemuth explains. Materials such as highly polished stainless steel “don't have that topography, so the parylene doesn't have anything to grip onto.”

Parylene can now be used in applications with extremely high temperatures. One type of parylene that has been on the market less than two years (offered by SCS under the brand name Parylene HT) can withstand extreme temperatures because its formulation contains fluorine. Parylenes can typically withstand continuous temperatures of about 100°C. But coatings containing fluorine in their formulations can perform at continuous temperatures of 350°C—they can even handle short-duration temperature spikes up to 450°C, Wolgemuth notes.

The HT version is also resistant to UV light, so it can be used on ocular implants and other applications that involve exposure to outdoor environments. Parylenes have not traditionally had properties that enable such exposure, Wolgemuth says.

Coatings with these properties also have a much lower coefficient of friction and better penetrating ability than other coatings. The former makes these parylene coatings suitable for coating components such as endoscopic port seals, while the latter enables the coatings to reach small crevices.

To make materials suitable for parylene coatings, their surfaces require what Wolgemuth calls “adhesion promotion.” This can be achieved using plasma etching, a process in which a smooth surface is bombarded with gas ions to coarsen the surface. Although oxygen, argon, helium, hydrogen, and nitrogen are among the different gases that can be used to etch a surface, the right choice depends on the material, says Marty Dues. He is corporate sales and marketing manager for Plasma Etch Inc. (Carson City, NV), which provides plasma etching equipment and services.

During the etching process, gas ions are excited into the plasma state by a radio-frequency generator. The gas stays in this state for a set amount of time, depending on a customer's requirements. Before beginning a job for a customer, Dues recommends running tests to determine the optimal etching time for that particular application.

Multiple Coating Layers

In some cases, the coating process doesn't end once a coating is applied to a medical product. A product may need a second coat. For example, manual handling of a product could inadvertently leave voids in the first coat. A second parylene coating could fill them, says Gustavo Arredondo, technical manager for Para Tech Coating Inc. (Aliso Viejo, CA), a provider of parylene coating services.

Arredondo notes that an extra coating layer could also be added to address surface contamination. When a coated surface is contaminated with something that cleaning solutions cannot remove, a second coating can be applied to hide the contamination. In addition, many medical instruments are masked during the coating process. As a result, overcoated masking debris may linger on an instrument after the coating process. A second parylene layer can be applied to the instrument after the debris is removed. Auxiliary coatings such as urethane or acrylic can be used to touch up the surface if the device will not be subjected to aggressive environments such as those produced by sterilization, Arredondo says. But he adds that the use of another coating material often isn't considered if parylene was the only coating on the product during its validation process. Other coatings could be added, but the product would have to be retested.

Parylene can also be used with other coatings when the final assembly of a device involves putting a coated component inside a housing. The space between the component surface and the inside wall of the housing is sometimes filled with another coating material. In the medical industry, common filler materials include silicone, acrylic, and urethane, Arredondo says.

Parylene also plays an important role when used with other polymer coatings on drug-eluting stents. Many drug-carrying polymers do not adhere very well to the metals used to make stents. Before applying these polymers, Wolgemuth says, some manufacturers apply a coat of parylene, which adheres both to the stent and the drug-carrying layer. “Parylene is inert, so it remains dormant in this case,” he says. “It's a silent partner on a drug-eluting stent.”

But Ih-Houng Loh maintains that parylene does not adhere well to stents. He is vice president of business development for AST Products Inc. (Billerica, MA), which makes coatings for the medical device industry. According to Loh, a much more effective adhesion option is a coating called a plasma foundation or plasma polymer. Like parylene materials, plasma polymer coatings are applied in a vapor deposition process. But unlike parylenes, whose mechanical bonds provide only “weak adhesion,” plasma polymer coatings form strong chemical bonds with stent substrates, Loh says.

Stent coatings can include more than one layer for reasons other than improved adhesion (see Figure 1). SurModics Inc. (Eden Prairie, MN) offers a coating combination that delivers drugs and helps incorporate stents into blood vessel walls. This combination consists of two separate coating layers. For drug delivery, SurModics makes a variety of polymer systems. Some are individual polymers and others are blends of various different polymers. The choice of polymers for a particular application depends on the type of drug and whether a biodegradable polymer is required, says Aron Anderson, SurModics' chief scientific officer.

A family of products that SurModics refers to as “prohealing” coatings aids stent incorporation. These coatings facilitate healing. They consist of biological molecules such as proteins and peptides that are chemically linked to the surface of a drug-delivery coating.

Making a Decision

Manufacturers that need multifunction coatings for their products can apply a single coating layer with more than one property or multiple coating layers that offer different individual properties. To produce a single coating with multiple desired properties, various coating formulations can be mixed together and then applied as a single coating.

How do manufacturers decide whether to mix coating formulations or apply multiple coating layers? In most cases, Loh says, mixing is the better choice because it is easier and less expensive than laying down separate layers. Mixing also eliminates concerns about whether multiple layers will adhere to each other.

But mixing coating formulations is not always possible. Loh points out that there may be a chemical incompatibility that could prevent the mixing of two coating materials. Also, the performance of the coating components might be different—and perhaps better—if they are applied in separate layers. The right approach must be determined on a case-by-case basis, Anderson says.

Adhesion Concerns

If device manufacturers decide that a multilayer coating is the best approach, it is important to make sure that the coating layers can adhere to each other. To ensure adequate layer-to-layer adhesion, manufacturers and their coating suppliers can take a number of steps. The most obvious is the choice of coating materials. Normally, Loh says, a coating consists of a base polymer and an active compound that provides the functional property. When more than one of these coatings must be added to a device, manufacturers should try to maximize adhesion between the layers by using formulations with the same base polymer.

If for some reason the application requires two coatings with different base polymers, adhesion is improved when the polymer in the top coating layer penetrates the bottom layer, forming what Loh calls an “interpenetrating network.” According to Loh, creating this network depends on the amount of time the top coating remains in liquid form before turning into a solid. This so-called wetting time must be long enough for the top-layer polymer to mix with the bottom layer, he explains.

Adhesion between two coating layers can also be improved by plasma etching the surface of the first coating, Arredondo says. After plasma etching, manufacturers or their coating suppliers can further promote adhesion by repeating the process they use to pretreat the device surface in preparation for the first coating layer.

For example, a silane primer can be applied to the first coating surface before the second layer is laid down. Once the primer is applied, manufacturers have 48 hours to overlay the second coating. The sooner the second coating is applied, the stronger the adhesion will be, according to Arredondo. He recommends minimizing the time between the application of the primer and the second coating layer.

Even if the adhesion between coating layers is initially adequate, it may deteriorate when the coating is exposed to the conditions of an application. “Will you still have adhesion after the third sterilization process?” Arredondo asks.

To answer questions like this, device manufacturers can use tests that replicate application conditions such as the temperature and moisture oscillations that devices will experience in use. Manufacturers can run these tests to failure in order to see how many cycles the coating adhesion can withstand, Arredondo says.

In many cases, solvents are used in the application of coatings. Solvents sometimes reduce the adhesion of one coating layer to another. A solvent used to apply the top layer could also cause the coating layer beneath it to swell or partially dissolve. Bearing these challenges in mind, it is important to use solvents that won't negatively affect either the coating layers themselves or their adhesion, Anderson says.

Another potential concern when using more than one coating layer is the order in which the layers are applied to the product. Will the top coat mask the bottom layer and negate its properties? To address such concerns, Anderson says, manufacturers can measure the properties of applied coatings to assess how the processing approach has affected the performance of the coating.

SurModics uses a number of tools to study coatings. One is a scanning confocal Raman microscope, which “allows us to chemically and physically analyze a coating without having to process it in any way,” Anderson says. The microscope provides an image of multiingredient coatings with a spatial resolution of about 0.5 µm. It can determine coating thickness and uniformity, as well as the degree of mixing or segregation of the ingredients.

“Are the chemical components of a coating mixing the way we think? Are they layered the way we think? We can sort these things out with this instrument,” Anderson says. “It's very powerful and simple to use.”

Conclusion

Device manufacturers can find a variety of coatings that serve multiple purposes. When medical devices require more than one coating, or even a single coating with several distinct properties, manufacturers have to consider several key factors. They should determine the characteristics needed for the coating and select the material with the appropriate properties. This includes determining whether the chemical components of a coating mix and layer as expected. They must also determine which techniques, such as plasma etching or the use of solvents, will yield the best results when the coating is applied. Considering these factors will help medical devices function as intended.

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