Originally Published MDDI June 2003

Plastics

by Gregg Nighswonger

Device makers traditionally have had simple demands of the medical plastics they use. While clarity is foremost, durability and resistance to sterilization also remain key considerations. In addition to providing exceptional material properties, the current generation of advanced plastics and elastomers offers other benefits for medical manufacturers. Among these are improved hemocompatibility, enhanced radiation sterilization stability, and reduced allergy risks.

Microtagging
Adding a New Level of Security to Materials

With the shift of manufacturing to countries outside of the United States, counterfeiting of parts has become a growing problem. To ensure that parts are made with the materials or resins specified, RTP Co. (Winona, MN) is offering a traceable, anticounterfeiting technology called microtagging. The process entails adding minute particles, called microtags, to thermoplastic compounds. 

“The microtag particle is essentially a unique numeric code sequence supplied in a microscopic multicolor layered format,” says Stuart Swain, RTP color product manager. Anticounterfeiting methods can be formulated to be seen by the naked eye, or to be invisible. “In the case of microtags, a simple nondestructive test involving ultraviolet light and a 100¥-power magnifying glass is all that is needed for a customer to test the parts and find out if they contain the correct resin, compound, or alloy—or if they have been counterfeited,” Swain adds. 

Microtags can be added to resins during compounding or supplied in concentrate form for use at the molding press. Standard particle sizes range from 600 µm down to 20 µm. Swain explains that, “We can actually code it down to the point where the company can have a corporate code, they can have a distributor code, and it can work all the way down to the channels of distribution. So when a part comes in, they know who made it and how it was distributed.” He adds, “There are 32 million different codes that can be used in this microtagging technology.”

Swain says medical companies are interested in the smaller size ranges for molding natural clear or colored material. He explains that, “You put these things under black light and they fluoresce green, like the glow-in-the-dark green, or phosphor green. That's what they look like under a black light.” Viewed under a 100¥-power microscope, the color-coded sandwich can verify or authenticate that it's the manufacturer's product. “Or,” he adds, “it can be for verification that they used the correct polymer.” 

Thermosets
Increasing Radiation Sterilization Stability

For manufacturers of traditional plastic devices such as implants, flexible catheters, and pacemaker leads, cross-linked polyethylene is nothing new. Zylon Polymers (Chestnut Ridge, NY), however, is cross-linking engineered thermoplastics—TPU, Hydrel, Pebax, and others—to create Irracure, which acts like rubber but is less difficult to process. 

According to Alan Zamore, Zylon's president, thermosets such as Irracure offer the advantages of rubber with the properties of a cross-linked polyethylene: higher temperature stability, enhanced solvent resistance, increased strength, better compression set, and decreased stretchability. Rubber, Zamore says, is “more archaic, more cumbersome, more difficult to form into various shapes.” He believes Irracure is the ideal material for angioplasty balloons, spinal implants, and pacemaker leads. 

“What prompted Zylon to develop this thermoset were requests from various companies for improvements to current materials. Developing Irracure was one method short of creating an entirely new material,” Zamore says. “Without creating an all-new material, we needed to improve upon cross-linked polyethylene's properties.” He also says using Irracure and other thermosets isn't expensive for large-volume production. “There are a number of service centers that have set up ways to perform cross-linking for manufacturers,” he adds. “They're glad to do it.” 

Antithrombogenic Coatings
Enhancing Blood Compatibility

Blood's tendency to coagulate upon contact with foreign objects creates a special challenge for manufacturers of devices used in such procedures as open-heart surgery, dialysis, and catheter insertion. As a result, tubing, catheters, and similar devices have traditionally been treated with anticlotting coatings. Research and development at Vestolit GmbH & Co. KG (Marl, Germany) and Teknor Apex (Pawtucket, RI), however, is proving that new vinyl compounds made from special copolymers may exhibit antithrombogenic properties equal to or greater than those of coated materials. 

The science involves incorporating functional copolymer groups directly and permanently into PVC's “backbone,” according to Teknor Apex, which has purchased exclusive, worldwide rights to the novel material developed by Vestolit. Unlike standard antithrombogenic coatings, Vestolit resins are nonextractable. The bioactive components of the copolymers simulate the antithrombogenic “messages” generated in the lining of blood vessels by heparin, a naturally occurring substance. 

Bertram M. Lederer, senior vice president of Teknor Apex, is certain the compounds will provide “substantial cost-performance advantages” over coatings as well, according to a company press release. Peter Galland, the company's industry manager, explains that the coatings currently used are “extremely expensive, perhaps an order of magnitude greater than the anticipated cost of this new invention.” He adds that since the new product need not be present throughout the entire wall of conventionally extruded medical tubing, the functional copolymer layer can be coextruded along the inner surface of the tubing only. “Since coextrusion has become a relatively routine processing technique,” he says, “the material savings will more than pay for the processing modification.”

The motivation for successfully applying these new vinyl compounds to medical devices is clear. The handling of blood outside the body “has always been a problem for the medical profession,” says Galland. He adds that it's a problem that adding heparin to the patient's blood and coating devices with special substances has not entirely solved. 

According to Teknor Apex, the goal of the professor who first created the resin sold later to Vestolit was to produce a “nonextractable material that blood could not distinguish from a venal or arterial wall.” Teknor Apex plans to spend the next two years developing and marketing Vestolit's resins. 

Dryfilm Lubricant 
Improving Mold Release Capabilities

Effective January 1 of this year, the Environmental Protection Agency's (EPA's) Clean Air Act requires that HCFC 141 B, a carrier agent in PTFE suspensions, be replaced with a chemical that does not contribute to ozone-layer depletion or smog. To that end, Micro Care Corp. (New Britain, CT) is using DuPont's Dryfilm lubricant to produce its VC1 and XC2 dry lubricants and mold releases for makers of finished surgical products and their components. 

In compliance with the Clean Air Act, DuPont has replaced the HCFC 141 B in its Dryfilm with HFC 4310 mee, an environmentally friendly chemical that serves as both the carrier and synthesizer of the PTFE particles in the suspension. Micro Care “blends down” concentrated Dryfilm to the correct ratio, according to vice president of operations Jay Tourigny, and then modifies it for device makers with an additional carrier fluid called HFC 365. That added carrier keeps the final product plastic-safe and nonflammable. These are both important considerations for device manufacturers looking to coat surgical instruments.

“Our customers are makers of surgical devices,” or parts of them, says Tourigny. “Also, our users are people who are molding plastic parts that go into surgical applications.  So, our product is used to lubricate either the end device, or the plastic parts that are made––with a fairly high 
degree of precision––for surgical applications.” He adds, “The beauty of what we're doing is that we can give you a more uniform coating at a lower cost, with universal plastics compatibility. Because the PTFE particles are more uniform in size, you can cut the amount that you need 
[in your suspension] by 50%. You can use less and still achieve the lubricating characteristics you're looking for.” 

Tourigny maintains that his company's VC1 and XC2 products reflect advances on a number of different fronts. “We're taking something that, at one point, was environmentally unfriendly, and now we've got something with full EPA approvals that works very well,” he says. 

Curing Process for Synthetic Polyisoprene Latex
Dip Molding Process Reduces Latex Allergy Risks

Apex Medical Technologies Inc. (San Diego), which specializes in nonlatex elastomeric thin film dip molding, has developed a curing process for synthetic polyisoprene latex that provides several notable benefits to medical device makers. According to Mark McGlothlin, company president, “Conventional methods use accelerators, rubber accelerators.” He says the new process substitutes organic peroxides that don't rely on any accelerators and thus avoid the allergies usually associated with the accelerators. “Ours differs in that we really can't form nitrosamine if we don't have anything there to turn into a nitrosamine,” says McGlothlin. “The second thing is that we don't have accelerators so we don't get into the type-IV allergens at all. And we have rather clean breakdown products from the curing system. That's the main difference. We have a cleaner system. Our cytotoxicity results are much better; we always score what is called zero zone of life, which is a very hard thing to do in rubber articles.” 

He believes there are advantages for both manufacturers and end-users. “For the end-user it's more of a safety or toxicity issue. There are so many of these allergy issues around, so the end-user should benefit from it. From the manufacturer's perspective, it should lower, in my opinion, the amount of liability that the manufacturer takes on. They would be providing the state-of-the-art, best-known technology to reduce the risk of latex allergy. So they're doing all in their power to reduce or eliminate it. In times past, this option was not available, and now it is.” 

McGlothlin adds that another advantage would be clarity. “If someone wants a clear part, there's a much higher probability of getting a clear part from our curing system versus the traditional. This is because we have so few ingredients that go in on the curing, they don't obscure the transparency of an article. On the other hand, conventional curing systems have things such as sulfur and zinc oxide, and these accelerators and other sorts of things that can go in and obscure the product substantially.”

The firm has also been developing silicone latex products that can compete effectively with injection-molded parts. Says McGlothlin, “We're still working at it. It's a, shall we call it a futuristic application, that's in development. We're not anticipating a product launch anytime soon. But, if it were successful, then it would compete with injection molding and would probably reduce the cost of tooling to make silicone parts in smaller quantities, or larger quantities.”

Topas Cyclic Olefin Copolymer
Device Material Offers Clarity of Polycarbonate

Developed by Ticona (Summit, NJ), Topas cyclic olefin copolymer (COC) combines optical transparency, high moisture barrier, and greater fracture resistance than glass. These qualities make it well suited as a replacement for glass in medical and diagnostic devices, as well as in drug packaging and delivery products. In April, to promote the development of use of the material in medical applications, Ticona has joined forces with contract manufacturer The MedTech Group Inc. (South Plainfield, NJ).

According to Gil Reich, vice president of sales and marketing for The MedTech Group, the collaboration was prompted “by synergies of the two companies. There was a situation where their Topas material is targeted for medical and pharmaceutical use. And all of our injection molding business is dedicated to medical products.” Reich adds, “We have a lot of experience in the fabrication of tools as well as injection molding of various syringe components, like barrels and plungers and such. And one of the applications for this Topas material is for syringe barrels that are almost as clear as glass. So this kind of synergy made sense.”

Hedden Miller, Ticona marketing specialist, healthcare applications, explains that Topas is “an amorphous resin—the first one that Ticona has offered—and it has clarity equal to polycarbonate.” He adds that the material's development was driven by two factors. “One is the fact that we see Topas as having a large fit generically in a lot of different markets, but medical is one that it has tremendous potential in,” says Miller. “That's why it's nice to have it as an addition to our product line.” 

Copyright ©2003 Medical Device & Diagnostic Industry