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).
PEKK (polyetherketoneketone) is a higher strength variant of PEEK in the PAEK family of materials. PEKK is semi-crystalline like PEEK but is unique in that it comes in amorphous-only grades. PEKK is slightly stiffer and harder than PEEK but notably has higher tensile and compression strengths and lower elongation than PEEK. PEKK's higher compression strength makes it better suited for structural bone implants as well as better pushability of catheters over longer distances.
PEKK is unique in that in it has received FDA approval for an additive manufacturing process for custom bone implants (Oxford Performance Materials 2017). This process requires special licensing and is proprietary to the material manufacturer. Permanent implant grades of PEKK are available for implant applications but require an intensive approval process for their use and come with extremely high costs. PEKK can withstand multiple cycles of all sterilization processes, making it suitable for one-time use or use in durable, multiuse devices.
Due to the uniqueness of PEKK, it has limited color options and any compounding operations are best performed by the material manufacturer. PEKK is considerably more expensive than PEEK and compounds and permanent implant grades can have even higher associated costs, forcing it to be used for niche applications that only PEKK is suitable for.
FEP (fluorinated ethylene propylene) is a fully fluorinated fluoropolymer, which is a thermoplastic similar to PTFE. PFA is another, less common, fluoropolymer that has almost identical physical, thermal, and chemical properties to PTFE but is a thermoplastic. FEP is a well-known and widely utilized polymer in the medical industry, predominantly as catheter liners and in the form of heat shrink tubing used in the lay-up process of catheter manufacturing.
FEP is a semicrystalline material but is unique among semicrystalline materials because it is clear. It is the clearest polymer with 96 percent UV transmittance. FEP's other most well-known property is its very low coefficient of friction, second only to PTFE and PFA. FEP is inert and has very low extractables, making it a good candidate for caustic chemical and drug transmission. All the materials discussed here are considered rigid materials and are the hardest and stiffest polymers, but FEP differs in that it is the softest and most flexible material on this list.
FEP can withstand most sterilization methods but has lower resistance to gamma sterilization. The raw FEP is not a very expensive material, compared with some others, but it can have more than double the density of other thermoplastics, meaning that almost twice as much weight of material is used to make the same volume of finished extrusions compared to other polymers adding to the raw material costs of a particular project. FEP is also very sensitive to shear, sometimes limiting output because of that. FEP is very corrosive to process. It not only requires specialized machinery for its high processing heats, but every surface that comes in contact with the molten material needs to be made of a high nickel alloy to prevent corrosion of those surfaces. FEP can also have dangerous off-gassing that can lead to “fluoropolymer flu” in operators, so adequate ventilation is a must.
LCP (liquid crystal polymer) is a semicrystalline polymer based on polyester chemistry with highly ordered molecules in the flow direction, which can be seen with the naked eye in some cases. Due to LCP's crystallinity and highly ordered molecules, it has extreme properties such as high tensile and flexural strength, extreme temperature and chemical resistance for a polymer, and ultra-low tensile elongation (less than 5 Percent). These extreme properties, for a polymer, position LCP as a closer mechanical replacement to stainless steel in some applications, notably as an MRI-compatible braiding filament. LCP's properties also make it suitable for replacing stainless steel in access devices and other similar applications. Most commonly available grades have some sort of physical reinforcement in them like minerals or carbon or glass fibers, further enhancing their strength.
These extreme properties come with some trade-offs. While LCP has highly ordered molecules in the flow direction, tubing can have radial homogeneity issues causing tubing to seem fragile. This, in conjunction with some grades having physical reinforcements, does make tubing extremely stiff but it can flex very little before it breaks, and the homogeneity issues can cause parts shatter or shred. LCP is predominantly used worldwide in high-voltage electrical applications and is just recently finding applications in the medical industry, so there are very limited medically approved material grades. LCP has limited color options due to its fillers and is a rather expensive material. It has a very high melt flow rate, which can be extremely challenging for extrusion, and the filled grades require specialized tooling to handle the fillers.
Extrusion of medical components out of ultra-engineering polymers is a relatively new type of application area in the medical industry. Ultra-engineering polymers have added a level of performance that wasn't possible until somewhat recently. Navigating the properties and differences between ultra-engineering polymers requires those in development and specification roles to gain a new knowledge set.
The goal of this article was to provide a high-level overview of these materials to quickly help designers and engineers gain awareness of the many ultra-engineering polymers that are available. Knowledge of and familiarity with these materials will enable designers and engineers to specify the ideal ultra-engineering polymer for their extruded medical applications and will open a brave new world of medical devices. This article reviewed PEEK, PAEK, PEKK, FEP, and LCP. Part 2 will look at amorphous polymers, including PPSU, PSU, PESU, and PEI.
This article was written by Jonathan Jurgaitis, Senior Extrusion Engineer for Apollo Medical Extrusion Technologies, Sandy, UT. For more information, Click Here .