ASK THE EXPERT
Plastics and Elastomers
Lou Johnson serves as medical plastics sales specialist for Modern Plastics , located in Bridgeport, CT. He has been in plastics distribution and manufacturing for 20 years. Modern Plastics, an ISO 9001:2000 certified company, is a plastics distributor of high-performance plastic stock shapes that include sheet, rod, tube, and film, along with resin products such as Radel R polyphenylsulfone (PPSU) and Ultem polyetherimide ( PEI). Modern Plastics' Medical Plastic Division was launched several years ago and has quickly become a leading supplier of medical-grade plastics. Its purpose is to provide fulfillment solutions to an industry that historically has purchased directly from manufacturers.
MPMN: What are the most significant trends emerging in the manufacture of plastics and elastomers?
Johnson: That's a great question. So much is happening, at such an accelerated rate, that it can be difficult to keep track of all the changes and innovations occurring in our industry. But one that especially comes to mind, one big change that I think will have a huge impact, is the development of self-reinforcing plastics (SRPs). Currently, to modify a plastic's behavior, additives like glass or carbon are introduced during the compounding process. A common example is strength--often a plastic will have, say for instance, a weight advantage over metal in a device. If the plastic is not strong enough on its own to meet the load that will be placed on it in service, filler is added. That can cause almost as many problems as it solves in some cases. For instance, the filler can bring the weight of the device back up fairly close to what it would be if it were metal, making the switch pointless. SRPs solve that by eliminating the need to add filler to achieve superior properties. A good example is a resin called Parmax made by Solvay Advanced Polymers, and converted into an extruded product called Tecamax by Ensinger-Hyde. This material is being used in aircraft, semiconductor, and medical applications. For instance, it is being looked at extensively as a replacement for aluminum in external fixation devices--it is strong, lightweight, and radiolucent. Another area of great promise is the development of biodegradable plastics.
MPMN: What characteristics of plastics and elastomers make them desirable for medical use?
Johnson: Well, some of the attributes we described above with these plastics have a tendency to be lighter in weight than other materials; many grades are easily sterilized by most of the currently popular methods, some through many cycles; some grades of plastics are highly resistant to a broad range of chemicals; they are noncorrosive; many plastics are relatively easy to machine; and perhaps most significant of all, plastics can be custom-designed to fit a specific application to a greater degree than any other raw material I'm aware of.
MPMN: Patient exposure to DEHP through plastic-based medical devices such as tubing and transfusion bags has been a cause for concern. How are plastics manufacturers responding to this controversy?
Johnson: Exposure to DEHP--Di(2-ethyhexyl)phthalate--is indeed a concern. As you know, various studies have suggested a possible link between PVC-based devices like the ones you've mentioned and adverse health effects on humans. The industry is responding by: 1) advocating and encouraging further scientific research to make sure as many facts can be uncovered as possible. For instance, an abstract published by the FDA in April2006 suggested DEHP can be converted into MEHP, its monoester, in ways we didn't previously understand. So the recommendation was a reassessment of tolerable intake for DEHP and MEHP. 2) The industry is aggressively seeking alternative materials that don't require DEHP as a plasticizer such as EVA, silicones, polyethylene, or polyurethane.
MPMN: How has the growth of implantable devices affected the industry?
Johnson: It has made the industry realize how important it is to make sure each partner in the supply chain, from resin manufacturer, to raw materials distributor, to machining house, to molder, all understand and conform 100% to the required standards of traceability, core competence, and accountability. We realized this perhaps a little earlier on than most, and we have the advantage of already having a culture in place that has been conducive to this type of accountability for a long time. We were the first distributor in the country to become ISO certified. We sell materials like UHMW and PEEK in grades for implant, which a short time ago would never have been allowed to go through distribution as the manufacturers simply couldn't trust them to follow the protocols involved and have the right mindset. So I think it is affecting the industry by rewarding the companies who understand the critical issues involved, and forcing the rest to reevaluate the way they do things or simply more frequently decide not to participate in implantables.
MPMN: Materials such as plastics and elastomers are not typically viewed as environmentally friendly. What is being done in the industry in terms of waste reduction and biodegradability?
Johnson: This is a question I am asked regularly! The answer, at least as I see it, comes in two parts: first, the perception that many have about plastics not playing a part in a planned sustainable world view is misguided. Many grades of plastic, as is now becoming common knowledge, are completely recyclable. What isn't as widely known, or publicized, is that the process producing many plastic resins and products are actually more environmentally friendly than producing a similar product in metal. It takes more energy and resources to produce, say, steel, than a lot of polymers. So I think it's unfair and unproductive to cast too wide a negative connotation on the "green-ness" of plastics. Having said that, the industry certainly is keenly aware of the global issues all manufacturing sectors are facing. Ongoing research into improving manufacturing methods, biodegradable plastics, and more recyclables are all part of the tapestry of solutions our industry is working hard on to make sure we all keep our planet as healthy as possible.
MPMN: What effect can sterilization have on plastics and elastomers? How can it alter the performance or stability of materials?Johnson : The right choice of polymers is critical here. Our Medical Plastics division spends much of its time working with design engineers and other key people on product development advising on the best choice of materials based on their specific application. Sterilization is one of the areas we especially focus on. Many failures in a product trial or first run that are attributed to design or machining flaws may actually be caused by poor material selection. Example: a designer knows she can use a plastic referred to as acetal copolymer for medical applications. She has successfully used it before, knows it meets her regulatory requirements such as USP Class VI, and so uses it for a new application. Then, after the parts have been made and are in the field for a short time, they start to crack and discolor. Is the resin bad? Was the plastic manufactured incorrectly? Were they machined wrong? Is there too much stress on the part? The answer is probably none of the above. Upon investigating, we find out that the part gets repeated autoclaving. Acetal starts to degrade after multiple cycles, so it was doomed to fail from the start. Other materials that cycle higher, such as Ultem PEI or Radel R PPSU may have been better choices. There are a wide range of materials that can exhibit various degrees of stability and resistance to sterilization methods. What we try to do is correlate those properties with other desired features such as strength, chemical resistance, temperature requirements, cosmetic preferences, and budgetary restraints to arrive at the best plastic to meet those needs.
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