When it comes to orthopedic material selection, the question often involves choosing which metal one wants to use. There are good reasons metal is indicated within the sector as it provides many of the properties beneficial to the devices used both inside and outside the body.
Polymers, on the other hand, are much more often associated with an array of healthcare applications, but within the orthopedic space, companies may be hesitant to select them. However, these materials provide unique advantages that make them an attractive alternative to metal. Without sacrificing attributes similar to metal, polymers bring with them favorable cost and repeatability elements.
In order to provide more direction and insight on how to determine which material to use for an orthopedic application and when, Kenny Freitag, commercial director of specialty molding at Spectrum Plastics Group, and Jim Hicks, application development and processing engineer at Solvay Specialty Polymers, responded to several questions in the following presentation.
Sean Fenske: Why might an orthopedic device maker consider going from metal to polymer with a component? What are the advantages? What key features are most important?
Kenny Freitag: Polymers offer several advantages over metal in the production of medical components. On the manufacturing side, you can consolidate components, which can eliminate potential costly machining operations such as welding, polishing, and passivation. This can provide a lower overall cost to the finished device. For the end user, polymer offers the potential for lighter weight components with a more ergonomic design. Thus, reducing fatigue and improving clinician comfort.
The use of polymers also provides improved visibility for imaging in minimally invasive surgeries. Coloration of the polymer also allows for product identification, such as trial sizes or key touch, as well as engagement points for better usability.
Fenske: Before choosing to replace a metal component with a polymer version, what considerations should an OEM keep in mind?
Jim Hicks: When evaluating the move to a polymer solution from a historically metal design, an OEM needs to consider what the overall manufacturing volume will be, not just annually but over the expected product lifetime. When exploring injection molding as the manufacturing route, the overall product lifetime is significant as tooling requirements are the biggest upfront capital cost and tend to be the biggest constraint. Taking the long view on total lifetime production needs can offset the cost of the tooling expense and puts the part cost in a more favorable position. Consolidation of parts into a few or even a single molded component can significantly improve overall part cost and reduce complexity, not just in manufacturing but in final part validations. Also, consider that even when combining metal and polymer into a single design, the OEM can look at a simpler design for a metal component to be captured by the polymer structure, thus reducing the need for detailed metal fabrication steps. Finally, the use of advanced and high-performance polymers, such as those offered by Solvay, allows a range of performance with material properties specific to the need of the final medical device. This allows polymeric options for high strength to high impact in both implantable and non-implantable devices and instrumentation.
Fenske: Why should an orthopedic device manufacturer consider the use of polymer versus metal for a component?
Freitag: One of the areas that clearly is an advantage for polymers in orthopedic devices is in minimally invasive surgeries. The ability to offer a radio-translucent device with the stiffness of metal allows imaging of the body without the image artifacts due to metal, and with a device that offers the same functional performance of the metal component. Spectrum has also seen an increase in the development and use of single procedure/patient instrumentation. These are complete kits containing implants and instrumentation that come pre-sterilized, and ready to use. The kits eliminate the cost of cleaning and sterilization validation work and lower the patient risk to HAIs (healthcare-associated infections). In addition, these single procedure/patient kits allow for more intricate designs and functionality due to the elimination of cleaning after use. The use of high strength polymers from Solvay have replaced entire metal surgical sets, lowering the overall cost of the instrumentation without sacrificing performance or patient safety.
Fenske: When choosing to go from metal to polymer for an orthopedic device component, what should be considered related to materials and manufacturing?
Hicks: When looking at material options, the first question to be answered is whether the device is meant to be reusable or for a single procedure. When considering reusable material options, we must consider each of the following: the mode of sterilization by steam or low temperature gas; compatibility with hospital cleaning and disinfection chemicals; and retention of physical properties in those aggressive environments. The polymeric material options for those conditions are limited but Solvay provides multiple choices that still allow a tailoring of performance for the end use device. Polymers typically can offer either high strength or high impact performance, but the design of the part will also play a critical part in performance. Although you can copy a metal design one to one, it is best to understand the limitations of any material and adjust the design to take into account the material properties and what the device is meant to do in use.
One important point to consider is while polymers can simulate many functions of metal, they cannot do everything, and we typically suggest combining the best of both worlds by using metals where they are required and allowing polymers to do the rest. An example of this is a cutting instrument. Although you can cut with an all-polymer blade, holding a sharp edge is nearly impossible. However, if you support a simple metal blade with a strong and rigid polymer, you can create a low cost, always sharp instrument specifically for single-procedure instrumentation.
When considering more complex designs with polymers or polymer/metal hybrids, it is critical to get the designers, material experts, and manufacturers involved at the beginning. Utilizing experienced processors and contract manufacturers such as Spectrum, the project can be viewed from a practical and, more importantly, manufacturable point of view. DFM (Design for Manufacturing) is a process that requires the experience and expertise of a team that understands the material, processing, and assembly methods. This overall process will benefit from partners that understand not just processing but post-processing needs such as welding, product marking and packaging. This expert collaboration can reduce the development and product validation time for the overall components.
Fenske: What considerations should be taken into account when looking for polymeric solutions?
Freitag: When a company comes to Spectrum looking for a medical device solution, we dive into the overall requirements the customer is looking for and this starts with the device use—whether it’s inside or outside of the body. Our engineering team (design, process, and quality) will engage at a highly technical level and work closely with material suppliers on materials makeup and properties (for example, bio comp performance, etc.).
When looking at material options for orthopedics, we have to consider polymers that are strongly supported by their supplier or manufacturer. Solvay has been producing some of its healthcare-grade polymers for over 50 years. These chemistries are well known and used widely in various medical devices. A supplier should have a team dedicated to the healthcare industry that understands and keeps ahead of changing regulatory and industry needs. Solvay has a family of polymers that are well documented with the FDA and continue to be supported, not only in regard to the polymer but also the additives, such as colorants used. Understanding these material requirements and staying current with regulatory needs helps to minimize the risk an orthopedic OEM may take when using a new material for a device. Whether the device is implantable or used to install the implantable device, understanding and providing well-documented polymeric material options is critical to the device company.
The manufacturing requirements and environment are critical, whether being a reusable device or single use. Acquiring ISO certifications such as 9001:2015 and 13485:2016 are a prerequisite. FDA filings are becoming required at the manufacturing level. So being FDA registered is a must in today’s times. It’s the supporting documents and procedures tied to these certifications and registrations that make you a world-class manufacturer and what customers expect from an injection molding device company. When you add the requirement to source or validate a metal component and marry the metal to a polymer; it involves another level of complexity in manufacturing and certifying that finished component. Since we are not making this product once, we need repeatability and to maintain the same quality requirements over and over. As the supply chain is becoming more complex, the quality system and processes need to tie everything together.
Fenske: In making this material transition, how important is it to keep in close communication with the material supplier? What role does the material supplier play versus the manufacturer?
Hicks: Many of the orthopedic customers we work with are extremely good at designing and manufacturing with metal. However, they are less comfortable designing with polymers. Involving an experienced material supplier up front can help to avoid design mistakes with polymers. Many of the properties of polymers are measured in the same manner as metal, so understanding the numbers is not that difficult, but translating those inherent properties into a good polymer design may not be as straight forward—especially if you are looking at filled or modified polymer grades. Polymer suppliers should be able to assist the designer to adjust the design to get the most out of the recommended grades. Tools such as mold filling analysis can help to simulate the processing of the part and, when combined with a computer FEA analysis, can help to identify weak spots or areas of concern.
However, a material is only as good as the processing and manufacturing supplier and involving both the material experts and the manufacturing experts as early as possible will greatly increase the success of the design and the quality of the manufacturing. For example, we always strongly recommend that, during the development and validation process, polymer parts are checked for key properties such as crystallinity when using semi-crystalline polymers such as polyarylamides (PARA) and polyetherehterketones (PEEK). For amorphous polymers such as polysulfones (PSU) and polyphenylsulfones (PPSU), we want to look at residual stress, which can lead to chemical stress cracking during use in the field. Checking these polymer properties during validation are as important to the final part design as physical dimensions.
Fenske: Do you have any additional comments you’d like to share based on any of the topics we discussed or something you’d like to tell orthopedic device manufacturers?
Freitag: Our customer base consists of startup companies through to large OEMs looking for guidance and knowledge in polymers or plastic design. They have a great design concept with how they intend their device to work but, in many cases, limited experience of the design requirements needed for injection molding the component. In either scenario, it is essential to engage with the manufacturer, polymer supplier, and, in this case, the metal fabricator as early as possible in the design phase of any program. Spectrum and Solvay have a great reputation with our knowledge in combining metal with plastic and we also work with the best metal suppliers. We collaborate with all suppliers and bring their resident experts together to work on the program.
Designing with polymers is not difficult, it’s just different. The range of polymers available today is broad. However, when it gets into the demanding and higher risk environment of orthopedic devices, the choices are smaller but often still unfamiliar to many design engineers. Good polymer suppliers will have dedicated resources for areas such as healthcare to provide that support and expertise to the medical device maker. Utilizing their resources and knowledge will help minimize development time and increase the success of producing a high-quality polymer component.
One item a customer generally underestimates is the overall cost to develop a new device. There is all the upfront industrial engineering work that usually occurs before engaging an injection molder or material supplier. Next, moldability and polymer options are reviewed and you start getting into modeling. Then, 3D printing of samples and/or rapid prototyping for proof of concept are generally required to keep the project moving forward. At this point, you are ready to develop the commercial production system, which requires injection molds, metal tooling, and validation requirements for these processes. This area tends to bring the biggest cost and it usually brings sticker shock; a customer needs to have an accurate estimate on these costs so they can incorporate that into their long-term component cost to depreciate the expense. We work with our customers up front, so they have the understanding of the economics. Early engagement is critical for a customer to provide comprehensive insight on what to expect moving forward through design and development to complete commercialization.
Hicks: A final comment is related to sustainability. Spectrum and Solvay are concerned about the impact of polymers in the area of single procedure instrumentation. LCA (life cycle analysis) studies have shown the use of single procedure instrumentation does not present a significant increase in resources utilized as compared to reusable instrumentation. This can vary from region to region, but when combined with a targeted effort to recover and reprocess the polymers used, the recycled components can offer an attractive source of high-quality polymers for use in other applications. Currently, the industry needs to assist in optimizing the design and recovery of polymer materials in this market, but activity is underway in Europe and the U.S. to develop a closed-loop process. Efforts are also being made in recycling metal from the polymer in a polymer/metal hybrid device; this starts in the design as part of the DFM process. Current medical device design is incorporating more sustainable measures with the intent of recycling as much of the device as possible.
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