Converting from metal to plastic has been taking place for decades in many different industries, including automotive, mobile electronics, plumbing, sporting goods, and more. And while plastic offers many advantages over metal, the main driver for such conversion has been reducing costs. The increasing success of plastic in these other industries encouraged the healthcare industry and medical device manufacturers to further explore converting from metal to plastic conversion when they have been under are facing strong pressure to reduce costs reduction pressure. Additionally, the use of plastics—and more particularly high-performance plastics—enables medical device OEMs to address other issues, such as prevention of infectious diseases and practitioner comfort and ergonomics. Plastics can also aid in product differentiation and improved logistics. In the area of oOrthopedics, instruments handles, broaches, and other devices have already been converted from metal to plastic since for the last several years now.

Defining precisely how much savings will be achieved when converting from metal to plastic is extremely difficult, because the cost per part is a direct function of the design of the part, the volume of the part, the raw material, the number of manufacturing parts, and the manufacturing method. But it is possible to derive some general guidelines, particularly when moving from metal machining to injection molding of high-performance plastics. Every $100 per part in metal machining can be translated to $20 per part in injection molding of plastic. This difference becomes even more important as the number of parts produced per year increases.

It is important to highlight that in addition to reducing costs, the use of high- performance plastic rather than metal results in other advantages such as the reduction of secondary operations during the part production process, such as polishing and deburring. Secondary operations are expensive and time-consuming for metals, but are minimal for injection-molded plastic parts. Switching to plastics reduces weight by about 80% when compared with surgical stainless steel. Reduced weight is key to improving practitioner comfort because it reduces fatigue during surgical procedures. Moreover, with plastic it is much easier to design instruments that are more ergonomic and to produce devices that facilitate precision during surgical procedures. Though there are some general rules to respect when designing parts that will be injection molded, plastic enables the designer to decide where to place the material in the part, with instrument grips serving as an example. Also, unlike metal, plastic lets designers avoid increasing the overall weight of the parts.

Plastics provide an opportunity for medical device OEMs to differentiate their products in ways that metal simply cannot. For example, plastic provides integral and bright colors. Examples include aiding in quick identification of instruments, to identify orthopedic implant trial sizes, and to foster corporate identification. Some plastics such as polyphenylsulfone (PPSU) are particularly useful for the medical device industry because the colors remain unchanged after repeated sterilization cycles, and unlike some coated or painted metal parts, PPSU parts conserve their color even if scratched during use.

The pressure to reduce costs affects not just parts costs but also how logistics are managed in the healthcare industry. The increasing the number of surgeries per operating room is part of an effort to improve efficiency and reduce costs. So today, the speed at which a clinical team can determine whether a certain set of instruments is sterilized and ready for the next procedure is critical and is part of the bigger strategy of planning the sequence of sterilization of all the instrumentation at the hospital. Currently, most sterilization cases and trays are made of aluminum or Radel PPSU, but only plastic cases can integrate radio-frequency identification (RFID) technology for improved logistics. Aluminum creates interference that impedes the system and prevents it from working properly. Using RFID technology with Radel instead of aluminum is already in use in other industries, such as aircraft catering trolleys, with success and proven cost reduction.

Metal to plastic conversion: Where do we start?

When converting healthcare parts from metal to plastic, three key considerations need to be taken into account regarding material selection:

• Biocompatibility.

• Function of the device.

• Performance over time.

Biocompatibility. It is essential to define the biocompatibility requirements of the part. It is crucial to understand whether the part will be in contact with the patient’s body fluids or tissues, and if so, to define the duration of the contact. The healthcare industry recognizes three different durations of contact: less 24 hours, between 24 hours and 30 days, and more than 30 days. Each situation—whether a part has no patient contact or contact for any of the three durations—has different biocompatibility requirements, and thus only a selected group of materials can be adapted for use in each situation. It is essential to confirm with the plastic material supplier that a particular material is available for the required duration of contact in a given application.

Functionality. When approaching the subject of device functionality, it is important to remember that metal and plastics have different properties and require different designs to achieve the same functionality. Industries that are used to working with metals have a tendency to ask for a plastic material that will perform like metal in the same part configuration, without considering that metal is often overspecified, providing much more in terms of performance characteristics than are actually needed for the part to function properly. When converting a component from metal to plastic, it is important to define the function of the part. In other words, what do you want the part to do?

The retractor shown in Figure 1 illustrates functionality. Defining the function of the device here requires defining the range of loads that will be applied to the end of the retractor, along with range of deformations that would be acceptable for the user. Since metal and plastics have different modulus and bend differently it is clear that in order to obtain a plastic retractor that is equivalent to that of a metal one, the design must be modified to compensate those material property differences because copying a metal design for a plastic part does not work.

When defining the functionality of the device, it becomes evident that plastic can often enable much greater design freedom. With plastic, it is often possible to integrate functions, consolidate several parts into one, and even improve ergonomics or human factors by adding soft touch or improve grip through overmolding technology.

Performance over time. A third key consideration requires understanding the life cycle of the part. In this case, it is important to remember that all the plastics offer different? performance characteristics and as such can affect the performance of the device under different conditions. For example, it is necessary to establish whether the part will be sterilized or not, how many times it will be sterilized during its life, and which sterilization method will be used. For reusable instruments or devices, one of the most common sterilization methods is steam sterilization, which implies contact with steam at 134°C for 18 minutes. This repeated sterilization process is fatal for some plastic materials, but will not affect others such as polyphenylsulfone (PPSU), polyaryetherketone (PAEK), and polyetheretherketone (PEEK), even after 1,000 cycles of sterilization.

An illustration of this phenomenon is shown in Figure 2. The impact resistance of three different materials—polycarbonate (PC), polyetherimide (PEI), and PPSU—is being measured after increasing number of cycles of steam sterilization. All three materials show virtually the same initial impact strength; however, PC shows a reduction of 50% of its impact properties only after 25 steam sterilization cycles. PEI shows a similar reduction after 200 cycles, whereas PPSU basically retains its properties beyond the needed 250 cycles all the way up to 1,000 steam sterilization cycles. We can conclude that for three initial and apparently equivalent material candidates for a certain part, only PPSU could be adapted if the life cycle of the part requires more than 250 steam sterilization cycles.

Along with the performance-over-time consideration, it is important to look beyond the data sheet of a material. While this is an important step in the process, many factors can and will affect the performance of the part, and these will not be represented in a data sheet. Data sheet values are obtained using standardized specimens and methods (at room temperature and where time is equal to 0). When selecting a material, the actual conditions and life cycle of the part must be considered. In the healthcare industry, parts are often subjected to elevated temperatures during steam sterilization process, aggressive chemical environment during cleaning and disinfecting process, repeated exposures, etc. In addition, the plastic material’s properties may not be equivalent in all the areas of the part, depending on the design, geometry, location of the injection gate, and so on. These parameters will not necessarily appear in a data sheet; however, the raw material supplier will most likely be able to provide such information.

Conclusion

This article provides some general guidelines and considerations for material selection when converting a previously metal medical part or component to plastic. Each part, application, instrument, and device presents its own particular requirements.

To ensure a smooth and successful conversion from metal to plastic, consider involving a high-performance plastic supplier early in your project. It is important to become familiar with the selected material and to understand the impact of that selection on design, tooling, and processing specific to each plastic material. These parameters need to be considered in very early stages of the process. When talking to a high-performance plastic supplier, ensure that it can demonstrate experience in both the healthcare industry and in metal-to-plastic conversion.