A medical device designer’s guide to PEEK performance
Shawn Shorrock, global business manager, healthcare, Solvay Specialty Polymers talks PEEK in the med-tech space.
Specifying plastics for medical devices can be a major challenge for designers used to working with metals. Yet high-performance polymers can offer the same level of strength and rigidity as metals at ambient temperature, along with additional advantages. Polyetheretherketone (PEEK), a resin commonly fabricated via conventional melt processing methods, delivers particularly advanced performance in high-end applications, including aerospace, chemical processing and increasingly in medical devices.
PEEK offers several benefits over metals commonly specified for medical device designs, including lower weight, enhanced aesthetics, more favorable ergonomics, improved costs and easy colorability – which enables devices that are easily and quickly identified in the operating room.
PEEK has also shown it is an effective alternative to metals for implantable medical applications, where targeted grades deliver enhanced chemical inertness and biocompatibility for intracorporeal devices, such as surgical implants or components of surgical or dental devices. Implant-quality grades of PEEK, such as Solvay’s Zeniva ZA-500 and ZA-600, conform to the requirements of ASTM F2026 Standard for PEEK polymers targeting surgical implant applications. While these high-performance materials share fundamental properties with conventional PEEK grades, they are manufactured in compliance within the relevant guidelines of ISO 13485 and under the applicable requirements of current Good Manufacturing Practices. Solvay’s Zeniva PEEK finds application in numerous implantable devices and in the development of new applications cleared by the FDA as well as by other regulatory bodies globally.
To take full advantage of PEEK’s capabilities and benefits, it is helpful for designers to better understand the three interrelated factors that influence PEEK’s performance. These include the material’s molecular structure, its processing and its unique material properties. Understanding these relationships can lead to the design of more innovative, functional and reliable medical devices and implants.
PEEK structure
Generally, the polymerisation process through which PEEK is synthesised results in a molecular structure consisting of repeating units of aromatic rings linked by ether, ether and ketone. This aromatic backbone, along with PEEK’s crystallinity and lack of hydrolysable groups, helps explain its excellent chemical resistance, its ability to withstand sterilisation and its excellent resistance to in vivo degradation.
The regular, linear structure of the PEEK backbone is rigid and crystallisable, which makes the bulk material strong and stiff. PEEK also delivers low density vs. metals, leading to high specific strength. The level of strength and stiffness depend on the level of crystallinity, whereas the amorphous region determines ductility and fatigue responses. In medical implants, the typical crystallinity is between 30 and 35%, depending on the melt fabrication process and post-fabrication thermal treatments.
PEEK processing
PEEK’s physical properties, such as crystallinity, strength, stiffness and ductility, can be modified by controlling the bulk resin’s average molecular size, often described as molecular weight distribution and associated molecular weight averages. Smaller molecules correspond to lower viscosity and higher flow. For example, lower average molecular weight and a distribution of molecules that is shifted toward smaller sizes can result in a two- to three- times reduction in viscosity vs. standard grades.
PEEK’s molecular size is determined by its flow grade, and the resin’s manufacturing technology. Although commercial resins within a certain flow grade may all share a similar average molecular size, closer examination may reveal some grades offer a narrower distribution biased toward larger molecules. Such grades have been shown to deliver improved ductility, durability, notch sensitivity and fatigue resistance than comparable resins. Grades favouring slightly larger average molecular sizes also crystallise more slowly at a given temperature. These are relevant considerations for designers expanding existing product lines or innovating new applications.
PEEK exhibits the same basic crystallization behavior when it cools to the solid state, generating the same crystal structure and essentially the same semi-crystalline morphology overall. So, designers may select flow grades based on their intended fabrication method. Certain processing conditions and post-processing thermal treatments can offer minor differences in physical properties. But on a practical level, matching the flow characteristics of different PEEK grades is sufficient to ensure similar physical properties.
In addition to influencing the average molecular size within a PEEK resin, manufacturing technology can also introduce measureable differences in colour. There is no known direct correlation between colour and performance of PEEK. However, colour measurements actually reflect structural information, and offer a practical means to distinguish grades from different suppliers. For example, neat PEEK resins made with Solvay’s technology consistently exhibit greater luminosity (or brightness) and appear less yellow than corresponding competitive grades.
PEEK properties
PEEK’s chemistry, structure and semi-crystalline morphology form the foundation for its excellent combination of mechanical and chemical properties, as well as its superb biocompatibility. The material also performs well in applications requiring good impact strength. As with most high-performance polymers, PEEK is notch sensitive, exhibiting lower impact strength in parts with sharper notches, such as sharp corners, threads and similar features. As it does with other properties, manufacturing technology influences notch sensitivity. In some cases, such as Solvay’s specific nucleophilic polymerisation process, fabrication of the polymer can enable improvements of 10 to 20% in notched Izod impact strength in some grades.
PEEK offers high stiffness. This does not vary appreciably across different grades from different suppliers for short-term elastic properties. All materials exhibit PEEK’s characteristic elastic response at low strains, followed by yield and necking before failure. Higher flow grades crystallise more readily, but fracture at lower elongations compared to standard flow materials.
Manufacturing technology can impart more notable disparities in the comparative tensile stress-strain curves for different materials, where a substantial difference in elongation at failure becomes evident. Solvay’s manufacturing technology, for example, produces PEEK resins that exhibit greater ductility and durability, as measured by the area under the stress strain curve. Medical devices manufacturers may design products to perform within PEEK’s elastic regime, where manufacturing technology has no discernable effect. However, by choosing grades with greater ductility, they can enhance the long-term durability and fatigue properties of parts targeting more demanding applications.
Conclusion
The growth opportunity for medical-grade polymers is outpacing the growth rate of medical devices overall, thanks in part to the increasing selection of high-performance polymers over conventional metal solutions. PEEK resins, in particular, offer unique benefits over metal, including lower weight, enhanced aesthetics, more favorable ergonomics and improved costs. But medical device manufacturers can optimize their designs further by working with PEEK suppliers that can couple their material’s outstanding performance with detailed processing guidance, technical support and regulatory support.
Manufacturers should also seek PEEK suppliers that offer well-developed 510(k) master access files for their resins to facilitate clearance from the U.S. Food & Drug Administration (FDA). This path can help accelerate time to market for a new device, and greatly improve its competitive edge.