Are You Ready for the Metal-to-Plastic Conversion?

Metals have been used in the healthcare industry for decades to manufacture a range of instruments and implantable medical devices. But because they offer cost-cutting potential and distinct manufacturing advantages, plastics are gradually encroaching on the domain long held by metals. In the following Q&A, Dane Waund, global healthcare market manager at Solvay Specialty Polymers (Alpharetta, GA), probes some of questions and challenges that manufacturers face when they consider embarking on the metal-to-plastic conversion.

MPNN: Please go into the physical and other properties that plastics need to be able to compete with metals and to make them suitable for medical devices, particularly load-bearing applications.

Waund: While the properties that plastics must offer in order to be compatible with medical devices is very dependent on the application, some of the main challenges facing the shift to the use of polymers include achieving a combination of stiffness, strength, and biocompatibility. It is necessary to ensure that additives or chemicals used to achieve the necessary physical properties must be suitable from a biological and safety point of view. In other words, a balance must be struck.

The move to replace the use of metals is progressing in the medical device sphere. Solvay has been involved with metal replacement for more than 20 years, starting with the automotive industry. As we transitioned into healthcare, low-risk, low-performance applications were among the first to undergo the transition away from metals as companies began to explore the plastics and performance continuum. Now we are seeing conversions in more demanding applications.

MPNN: What are the advantages of converting from metal to plastics?

Waund: For instrumentation, there are typically several drivers, one of which is cost pressure. Both single- and repeat-use devices often favor either an all-plastic or a hybrid construction that combines both metal and plastic in order to reduce costs. In addition, plastics offer design freedom. In some cases, plastic designs can be achieved at a much more reasonable cost than can metal designs, especially in the case of higher-throughput devices that are used once and then disposed of. Such 'single-use solutions' enable manufacturers to achieve economies of scale by manufacturing large quantities of a given product from plastics. For example, in production quantities of 3000 to 10,000 devices per year, it is frequently possible to manufacture high-performance plastic instruments at a lower cost model than metal instruments.

Another advantage of transitioning from metals to plastics for single-use applications is that plastic-based medical devices can contribute to achieving higher sterility in healthcare settings. Used only once, some plastic medical instruments have been shown to help reduce hospital-acquired infection rates because they do not have to be cleaned and sterilized after each use. This is of particular importance in the case of complicated, cannulated instruments with tubular forms and shapes that feature complex geometries and crevices. Such devices are difficult to sanitize, much less to clean and sterilize.

A third advantage of plastic-based medical devices is ergonomics. By incorporating plastics into a design, if not converting completely, manufacturers can make their products easier to use. Plastics can be used to create handles or to make devices that combine both hard and soft surfaces. They can also be colorized to denote different sizes or other identifying characteristics. Furthermore, because plastics provide lower thermal conductivity, they are not cold to the touch, benefiting the patient in terms of comfort.

MPNN: Solvay was involved in a study of high-performance polymers for use in surgical retractors used to perform hip replacements. Typically, such devices are made of metal. What were the results of this study?

Waund: The healthcare industry remains very conservative and very metal centric. We wanted to share a robust process and our experiences with OEMs about what a metal-to-plastic conversion could look like, trying to remove some of the mystery out of the process for companies that have not done this before. Our point was to show that in supply chains already available to OEMS, a lot of know-how is being developed to help smooth the transition to plastic.

First, we had to define the problem, ensuring from the outset that we had a solid set of requirements--or what some companies call critical to quality. According to this notion, a problem well formulated is already half solved. A common mistake that companies make in considering the use of new materials is not understanding exactly the function of the medical device in question. Some metal-based orthopedic devices have been used for so many years that specifications for them no longer exist. For example, it is difficult to know how much force or deflection some devices are designed to withstand or even what they're supposed to do. One of the best practices that we discussed was recharacterizing the performance of the incumbents, or predicate products, in order to establish new specifications based on the capabilities of the metal device.

Second, we discussed what a new plastic-based device could look like. In terms of conceptual design, this was the stage for thinking big. If a company employs a bunch of engineers that design metal instruments, their plastic designs, conceptually, may look a lot like a colored metal ones. In designing a plastic surgical retractor, we therefore talked about looking for external influences to provide a bigger-picture input. In doing so, it was important to incorporate people into the design process that have experience with plastics. Designing with plastics is not necessarily harder than designing with metals, but it is different. Therefore, we wanted to ensure that we could work with partners that have a plastics-oriented skill set.

The third step involved materials selection. For engineers with metals backgrounds, we introduced the concept of a plastics pyramid, with commodity materials at the bottom, engineering materials in the middle, and specialty materials at the top. Based on how we defined the problem in step one, we climbed the plastics pyramid until we found a number of suitable candidates that could fulfill such requirements as mechanical performance and chemical resistance.

We divided the materials selection step into three parts. The first included the materials' performance requirements. The second was biocompatibility. The third was the product lifecycle--in other words, determining whether the device will be for single or repeat use. In some cases, materials lose such mechanical properties as flexibility and stiffness over the lifetime of the device as they undergo repeated cleanings. If an instrument has a projected lifetime of 1000 uses and must undergo repeated sterilization cycles, the use data should indicate how the material will perform on its 1000th use.

Fourth, we produced several mechanical engineering designs of the surgical retractor using performance-simulation tools that are available for both metals and plastics. Basically a healthcare crowbar, the retractor was subjected to high force levels. The materials we chose for our simulations, Solvay's AvaSpire polyaryletherketone for repeat-use devices and Ixef polyarylamide for single-use devices, exhibited strength numbers approaching those of aluminum and zinc but provided significantly lower rigidity. To compensate for this weakness, we performed finite element analysis and experimented with different geometries, shapes, and lengths. Our industrial designers also simulated different ergonomic handle designs because they have an effect on the retractor's mechanical performance. An advantage of this iterative approach is that many design considerations can be addressed using simulations, avoiding the need to make prototypes.

The fifth step involved manufacturing simulations. A typical dilemma facing production processes in the healthcare sphere is deciding whether to machine or injection mold a part. Traditionally, machining is seen as suitable for producing small lots, while injection molding is chosen for larger quantities. Historically, the cutoff point between small and large lots has been about 10,000 parts per year. However, as companies try to take greater advantage of plastic molding, that cutoff point is decreasing. For example, inserts and insert cavities feature geometries that are common to families of medical devices that are available in different sizes. Another strategy takes advantage of the fact that most sizes fall in the middle of a bell curve, while less popular, very small, and very large sizes fall on the left and right sides of the curve. The objective is to injection mold the parts in the center of the curve and machine the outliers for very small and very large patients.

The sixth step was prototyping. At this point, we supplied the customer and opinion influencers such as doctors with finished prototypes. We discussed the five or six relevant prototyping methods and simulations that are suitable for use with plastics and why we preferred one to the others. Some methods provide real mechanical data, while some are not suitable for large or small parts. Some are quite fast, while some take a long time. And of course, some are more expensive, while others are more cost-effective. But at the end of the day, our objective was to produce prototype parts that could truly simulate the key properties of interest.

The last step was final validation. At the beginning, we had established a requirement profile based on the incumbent device. At the end, we ran the same battery of tests based on whether the device was a plastic or metal-plastic hybrid design.

By following these steps, using simulation methods, and relying on the experiences our customers have acquired with their supply chains, we have succeeded in illustrating a more efficient and effective process than existed five or 10 years ago. This robust process is suitable for replacing metal with plastic for many medical device applications.

MPNN: What are the future prospects for metal-to-plastic conversion in the medical device industry?

Waund: A lot depends on factors that are not directly relevant to performance. Shifting from a reusable-device to a single-use model is certainly going to contribute to replacing metal. Whether single-use instrumentation goes 'big' or whether its adoption is less common will definitely influence the pace at which plastics will replace metals. But this isn't really a design or performance criterion. Some of the factors that will influence this conversion are how patients get billed, how services will be delivered, how companies hold inventory, and the lifecycles of plastics versus metals. And while there is increasing surgeon and industry acceptance of plastic-based devices, many surgeons are perceived to still like working with heavy metal instruments and don't want to change, independent of the types of surgeries they may perform. This factor will surely play a role in the metal-to-plastic conversion.

Bob Michaels is senior technical editor at UBM Canon.

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