PEEK tubing is used in minimally invasive devices, such as vascular applications, where the catheter system needs to pass through tortuous vascular pathways and be traced, so high pushability, torsional properties, and kink resistance are essential. For example, thin-walled PEEK microtubing can be used as a component in the rigid proximal shaft of a stent/balloon delivery system. Large diameter PEEK tubing is used in the shaft of delivery systems for trans-femoral heart valve implantation. Due to its high temperature tolerance and dielectric strength, PEEK tubing is also commonly used as an electrical insulator in ablation catheters.
A new development in minimally invasive surgery that allows the use of large-diameter PEEK tubing for new surgical instruments is "scarless surgery," also known as natural-transluminal endoscopic surgery (NOTES). Note that the procedure is performed by accessing the internal organs through the mouth or other natural cavity and then through an internal incision in the stomach wall. The procedure avoids external skin incisions and the associated pain, scarring, and potential for wound infection, and for the patient it also means a faster recovery time and shorter hospital stay.
PEEK tubing is also gaining popularity as an alternative to metal tubing, such as stainless steel, in medical applications because it is lightweight and resistant to chemicals. In analytical chemical fluid applications, for example, PEEK tubing is advantageous because it is inert to most solvents.
PEEK tubing can be extruded into two forms: amorphous (clear) and crystalline (opaque), which has superior physical properties. For example, the strength and modulus of amorphous PEEK is somewhat lower, possibly as much as 30% lower, compared to crystalline PEEK. If PEEK is opaque, it may be 25-35% crystalline and have 90-95% of the best properties. Amorphous PEEK is used to make shrink tubing, and when amorphous PEEK is heated, it crystallizes and shrinks. Amorphous PEEK can also be used in low temperature (<100°C) applications where transparency is required and greater flexibility is needed. In addition to the above, most other applications use crystalline PEEK.
One of the challenges of extruding highly resistant crystalline PEEK medical tubing relates to cooling during processing. For most medical-grade thermoplastics, the tubing leaves the die head and goes directly into a cooling or vacuum-setting tank, where it is quickly quenched. However, PEEK requires additional time to crystallize uniformly during cooling, giving it optimal physical properties, a smooth surface finish and an opaque appearance. Therefore, a gap with guide rolls between the die head and the inlet of the cooling/vacuum shaping tank is required to achieve a slower cooling rate and a high degree of crystallization. For this reason, amorphous PEEK tubing is actually easier to extrude within very tight tolerances, but the physical properties are relatively inferior.PEEK tubing is often annealed after extrusion for a period of time and temperature cycling to relieve stresses during the extrusion process and increase crystallinity to a maximum value to enhance physical properties.
Ideally, PEEK polymers should be processed on completely clean equipment. Ideally, a modular extruder should be used so that the extruder's screw, barrel and feed insert can be easily and quickly removed for unrestricted bare metal cleaning. Otherwise, the screw and barrel must be cleaned.
The extrusion processing temperature of PEEK is much higher than medical grade thermoplastics such as nylon, polyurethane and polyolefins. Unlike fluoropolymers, which are corrosive to most metals, PEEK is not corrosive to the extruder, so the barrels do not require special construction and the screws do not require special high-cost nickel-based alloys such as Hastelloy or Inconel. Due to the high working temperature, a special double barrel safety guard should be used on the barrel to prevent operator burns.
Of course, the best-case scenario for processors who want to start extruding PEEK is to purchase a new modular extruder with high-temperature capabilities. Another low-cost approach is to retrofit existing extruders for typical thermoplastic resins to high-temperature drum heating and cooling packages
By retrofitting existing extruders with high-temperature capabilities, the core process remains the same; therefore, there is no need to revalidate the existing extrusion process. Time considerations are also favorable when retrofitting an existing extruder with high-temperature capabilities because in many cases it can be done on-site, which also reduces downtime. Retrofitting a 1-inch (25-millimeter) extruder with high-temperature barrel heating, cooling systems, and safety enclosure systems in the field can be accomplished in as little as two days.
Once the extruder is converted to process high temperature PEEK, it can be used to process other high temperature medical grade thermoplastics such as polysulfone (PSU) and polyetherimide (PEI).