There has been a profound shift is taking place in the medical industry of more minimally invasive, quicker, and more-effective procedures. The goal of this shift is to minimize patient recovery times, reduce access incision sizes, and provide better patient outcomes through advanced medical procedures. This necessitates new medical devices that tend to be more demanding of their components than in devices past, which in turn requires medical devices and their components to use advanced polymers. Many of these advanced materials fall under the general description of high-heat polymers.
The high-heat polymers discussed in this article are referred to as ultra-engineering polymers to identify that they reside at the pinnacle of performance within the engineering polymer designation. There are a wide range of these ultra-engineering polymers, but many may be relatively unknown to device designers and engineers. Unfamiliarity with ultra-engineering polymers can prove challenging in specifying the ideal material for today's demanding and cutting-edge medical devices and components. The aim of this article is to share information about the variety of ultra-engineering polymers that are available for high-requirement medical applications, their potential applications, general properties, and their benefits and limitations.
Ultra-engineering polymers fall under the general classification of engineered polymers, yet they are at the pinnacle of performance for all thermoplastics. Ultra-engineering polymers bridge the performance gap between standard engineering polymers, such as nylon and polycarbonate, and metals, composite materials, and thermoset plastics like polyimide (see Figure 1). Their description of high-heat polymer indicates not only that these materials are processed at higher temperatures, typically between 600° and 750° F+, but also that they subsequently have high continuous operating temperatures, most well over 300° F.
Ultra-engineering polymers have very good chemical resistance, which makes them ideal for the hospital environment and the many harsh chemicals and drugs to which plastics can be exposed. The physical properties of ultra-engineering polymers also outperform all other standard engineered polymers in the areas of tensile strength, flexural strength, and impact resistance. In addition, these materials have good dielectric properties and have some level of inherent flame resistance without additives. All the materials discussed in this article have USP Class VI and ISO 10993 approvals; some have permanent implant approved grades as well as master file support.
All the materials listed here are suitable for the extrusion of profile shapes, multi-lumens, microbore tubing, large diameter tubing, thin wall tubing, rods, and filament. All of them can be compounded with additives, radiopacifiers, colors, and reinforcements but will have some limitations due to their high processing heats. Additionally, due to their high processing temperatures, tooling and processing equipment need to be specialized to withstand these high heats. Part 1 of this article focuses on PEEK, PAEK, PEKK, FEP, and LCP. Part 2, which will run in a future issue, will focus on amorphous polymers, including PPSU, PSU, PESU, and PEI, as well as some additional ultra-engineering polymers.
Currently the buzz material for high-requirement plastic medical applications is PEEK (polyetheretherketone). PEEK is the highest performing commercial polymer, so it is no wonder that it is specified in so many high-requirement applications. There are many aspects of PEEK that make it the ideal choice for some applications. PEEK is a semicrystalline material, which contributes to some of its best-in-class properties. Its properties are also optimized in its semicrystalline state. Semicrystalline polymers, like PEEK, have a portion of their molecules align and form crystals during proper processing.
PEEK has some of the highest tensile and flexural strength of all commercial, nonreinforced thermoplastics. This translates to great pushability and torque functionality in catheter components and access devices. PEEK also has low elongation and good compressive strength, as well as excellent chemical resistance to hospital solvents, wipe down chemicals (Solvay Specialty Polymers 2015), and harsh drugs. PEEK has good dielectric and insulative properties as well as a high continuous operating temperature of 465° F or more, which makes it a great choice for ablation procedures. Its low surface energy finish and hardness makes PEEK a good choice for atherectomy devices.
PEEK is inert and biostable, which can reduce the possibility of negative patient reactions. PEEK is unique in that it has similar physical properties and density to cortical bone (Solvay Specialty Polymers 2015). It also doesn't have degradation or sensitivity issues like that of titanium and has no reactivity to MRI procedures, which makes PEEK ideal for bone and dental permanent implants (Solvay Specialty Polymers 2015); permanent implant grades of PEEK are available for these types of applications but require an intensive approval process for their use and come with extremely high costs.
PEEK is also becoming a popular polymer for use in additive manufacturing or 3D printing, enabling the production of patient specific implants, devices, and tools among other custom devices. PEEK has excellent resistance to all major sterilization techniques in 100 cycles or more and 1,000+ cycles of steam sterilization (Solvay Specialty Polymers 2015). All these properties combined allow PEEK to occupy a unique position as a material suitable for one-time use devices but also for durable devices that will be reused multiple times. PEEK can be thermally formed for catheter applications as well as RF welded (Solvay Specialty Polymers 2016) providing for easier assembly and connection methods than stainless steel.
There is no such thing as a perfect or universal polymer, so PEEK does have some restrictions to keep in mind while considering it in a medical device. PEEK has high raw material costs, and creating a custom compound can at least double those raw material costs. PEEK has an opaque beige appearance, which may not be aesthetically pleasing for some users. This opaque beige color also can limit how well it accepts color, especially light colors, and how vibrant bright colors may appear. The high processing temperatures can also limit which colors and additives can be used because not all are thermally stable at those high temperatures. Prior to printing, PEEK requires special surface preparation, usually plasma. While PEEK is extremely strong, it is not as tough and ductile as other high-heat polymers.
There is another high-heat, semicrystalline polymer in the PEEK family called PAEK (polyaryletherketone). This material can be utilized as an alternative to PEEK with an approximate 20-30 percent raw material cost savings. PAEK has similar physical, thermal, and chemical properties (Solvay Specialty Polymers 2016) as PEEK but with improved toughness and ductility (Solvay Specialty Polymers 2015). PAEK also has similar restrictions to PEEK (see Figure 2).