Cleanrooms helping to advance medical device coating technology
09/01/2004
BY HANK HOGAN
Indianapolis, Ind.—Lonny Wolgemuth, medical market manager for Specialty Coating Systems (www.scscookson.com), cites the movie Field of Dreams when he talks about his company's recently finished 1,700 square-foot cleanroom. The facility features a concentric layout, with an ISO Class 7 area enclosing an ISO Class 6 ring that surrounds a final ISO Class 5 core. It's a cleanroom, complete with controlled entrances, gowning areas, HEPA filters, and pressure differentials.
Wolgemuth likens the new facility, the most state-of-the-art of SCS' four cleanrooms, to the baseball field in the film. If the facility was built, the customers would come. Unlike the baseball field in the movie, however, this cleanroom is intended for parylene coating services and has more behind its construction than a whisper that only a few can hear.
"We saw the need to more precisely control the environment in which these fine wire geometry devices would be coated," says Wolgemuth. "So, in order to provide the control that we felt necessary to exceed the customer's expectations, we needed to have this particular facility."
Driven by miniaturization?
SCS is already coating customers' products in its new cleanroom. The move toward cleanrooms and contamination-controlled environments is something those in the parylene coating industry see as inevitable due to ongoing miniaturization, particularly of medical devices and hybrid electronics. But there's disagreement about the timing.
"Do I think there is a trend towards smaller and smaller circuitry, miniaturization of this technology that's going to drive parylene coaters toward cleanroom environments? Yes, I think there is some of that, but I think that we're not being driven to that by the end user at this point," says Bill Gleason, general manager of Para Tech Coating Inc. (Aliso Viejo, Calif.; www.parylene.com).
The Para Tech facility is an ISO Class 8 cleanroom, with some specialized laminar flow hood areas within it that are ISO Class 7. The company also takes steps to control contamination during surface cleaning through the circulation and filtering of an aqueous-based cleaning solution used to prep parts before deposition. Care is also taken during other processing and masking steps to minimize the addition of contaminants to products being coated.
SCS, Para Tech and others make and sell coater systems. The companies also provide parylene coating services. The technology involves depositing layers of parylene that range in thickness from tens or hundreds of angstroms, or fractions of a micron (µm), all the way up to 25 µm—or about a thousandth of an inch. The thickness of the deposited layer is determined by the applications, which include hybrid electronics, implantable medical devices such as pacemakers and stents, and microelectricalmechanical systems (MEMS).
In the medical field, parylene is sometimes used to serve as a bridge material. For example, a parylene coating may offer a way to create a drug-eluting stent, which combines a structure—often made of some kind of metal—with a measured drug release mechanism. The structure props open arteries while the drug prevents clogs and other problems. The drug delivery method can involve materials that don't bond well to metal. So, an intermediate layer, such as parylene, may be required to bridge the two dissimilar materials.
Drug-eluting stents can be tricky to manufacture but are proving popular nonetheless. According to Kalorama Information, a publishing division of MarketResearch.com, the market in the United States for drug-eluting stents was $683 million in 2003 and is forecast to grow to $1.23 billion by 2008 (see Figure 1). It's partly because of this surge in sales that medical device coatings are generating additional interest.
 Figure 1: Forecast of U.S. market for drug-eluting stents (2001???2010) |
Parylene is also used in medical devices because it's biocompatible and thus protects devices from the body—and vice versa. For that reason, parylene may be found in various implants, such as pacemakers.
"We use parylene coating at times, but only on an as-needed basis with patients who for some reason cannot tolerate the standard titanium shield," says Scott Papillon, a spokesman for implantable medical device maker Medtronic Inc. (Minneapolis, Minn.; www.medtronic.com). The use of such a coating, he adds, is fairly rare.
Papillon notes, however, that implantable devices are shrinking in size. Implantable cardioverter defibrillators, for example, were more than 100 cubic centimeters when they were introduced in the mid-1990s. Five years ago, Medtronic's products were about 50 cubic centimeters. Today, the company's latest offerings come in at about 36 cubic centimeters. Pacemakers haven't gone through as drastic a volume reduction, but they, too, have slimmed down over the years.
As a result, device dimensions are smaller and associated coating challenges bigger.
Uniformity matters
The need to uniformly coat small structures is another reason why parylene and other conformal coatings are increasingly used. The parylene deposition process begins when a powder is sublimated under vacuum. The resulting vapor is then converted into individual units (monomers) thanks to a 680°C vacuum treatment. In the next step of the process, the monomer accumulates one molecule at a time on surfaces in the deposition chamber, which is at room temperature and a moderate vacuum. Thanks to the nature of the process, the resulting parylene polymer build-up doesn't discriminate in its deposition.
"I've casually referred to parylene as an equal opportunity coating," says SCS' Wolgemuth. "That is to say, it coats all surfaces equally. Unless you actively mask something to keep the parylene off, it will be coated."
Thus, parylene coating will handle deep holes and tall structures. There's no need to worry about voids appearing in the film due to gravity and related effects. On the other hand, the process is slower than a simple dip in a liquid. Such a dousing can take a few seconds. Wolgemuth estimates that putting down a parylene layer can take several hours, with the actual time dependent on the layer thickness. If film uniformity is important, however, Wolgemuth says the wait will be worthwhile.
According to Wolgemuth, SCS has developed a parylene coating that can withstand temperatures as high as 450°C in the short term and 350°C for long-term applications. That's in contrast with other forms of the coating, which typically have problems when much above 100°C.
Being able to survive an elevated temperature is important in implantable medical devices, which have to be sterilized before going into a patient.
As for other contamination concerns, SCS contends there's a need to make sure that the various masking and processing steps taken before deposition don't generate particulate and other problems that can be trapped by the parylene film. That's one reason why SCS has pursued a cleanroom approach. Much of the fixturing of a product to be coated in the new cleanroom will be done in the ISO Class 5 and 6 areas.
But some in the industry say that parylene's ability to produce a conformal film actually works somewhat against this cleanroom need. As Para Tech's Gleason notes, it only takes a thin polymer layer to trap any particles. Those particles are then encapsulated and effectively rendered harmless.
Thus, Gleason points out, depositing a film after system maintenance ensures that any contaminants left over from the cleaning process can't come into contact with product during subsequent deposition cycles. That's one reason he doesn't see a big need at present to surround parylene deposition systems with high-quality cleanrooms. Such precautionary coatings along with the use of a vacuum, Gleason contends, mean that the chamber "… is very, very clean."