Polyether ether ketone (PEEK) is finding growing use in the medical device sector, and is helping to fuel advances in cutting-edge heart valves, catheters, and surgical devices.
PEEK is a semi-crystalline high temperature thermoplastic that is growing in use for high performance medical tubing applications owing to its biocompatibility, excellent physical and electrical properties, chemical resistance, and ease of sterilization.
PEEK is used in minimally invasive devices including vascular applications where high pushability, torsion, and resistance to kinking are required while the catheter system is tracked through tortuous vascular pathways. For instance, thin-wall PEEK microtubing can be used as a component in a stiff proximal shaft of a stent/balloon delivery system. Large diameter PEEK tubing is used in the shaft of the delivery system for transfemoral heart valve implantation. Owing to its high temperature tolerance and dielectric strength, PEEK tubing is also commonly used in ablation catheters as an electrical insulator.
A novel development in minimally invasive procedures where large diameter PEEK tubing may be applied for new surgical instruments is "scar-free surgery," otherwise known as natural orifice translumenal endoscopic surgery (NOTES). NOTES surgery is performed on internal organs with an entry point through the mouth or other natural orifice, and then through an internal incision in the stomach wall. This procedure avoids external skin incisions and the associated pain, scars and potential wound infection along with faster recovery time and shorter hospital time.
PEEK tubing is also gaining favor as a replacement for metal tubing such as stainless in medical applications because it is lightweight and resistant to chemicals.For instance in analytical chemistry fluid applications, PEEK tubing is advantageous because it is inert to most solvents.
PEEK tubing has low surface energy and therefore it is difficult to print or bond to it. The surface of PEEK tubing can be activated "wetted" with plasma treatment in order for printing to adhere to the tubing. This process does not damage the tubing or discolor the surface appearance.
Markerbands printed on PEEK tubing. Printing by C.I. Medical.
PEEK tubing can be extruded into two forms: amorphous (transparent) and crystalline (opaque), which provides far superior physical properties. For instance, amorphous PEEK has up to 30% lower strength and modulus compared to crystalline PEEK. If the PEEK is opaque, then it is likely 25-35% crystallized and will have 90-95% of its optimal properties. Amorphous PEEK is used to make shrink tubing. When amorphous PEEK is exposed to heat it will crystallize and will shrink. Amorphous PEEK is also used for low temperature (<100°C) applications that require transparency and when more flexibility is required. Short of these three situations, most applications use crystallized PEEK.
One of the challenges of extruding high-tolerance crystalline PEEK medical tubing is related to downstream cooling and sizing. For most medical-grade thermoplastics, the tubing exits the die head and enters directly into a cooling or vacuum sizing tank where it is rapidly quenched. However, PEEK requires additional time in order to crystallize uniformly as it cools, giving it optimum physical properties, a smooth surface finish and an opaque appearance. Therefore, a gap with guide rollers between the die head and the entrance to the cooling/vacuum sizing tank is required to achieve a slower rate of cooling and a high degree of crystallization. For this reason, amorphous PEEK tubing is actually easier to extrude within really tight tolerance levels, but with less than optimum physical properties.PEEK tubing is often annealed for a certain time and temperature cycle after extrusion to relax the stresses from the extrusion process and to bump the crystallinity up to the maximum for enhanced physical properties.
PEEK polymers should ideally be processed on completely clean equipment. Ideally a modular style extruder should be used that allows for the screw, barrel, and feed insert of the extruder to be easily and quickly removed for unrestricted bare metal cleanout. Otherwise, a purging compound must be used to get PEEK off a screw and barrel.
0.75-in. high temperature conversion before (left) and after (right).
For processors that would like to enter the high performance PEEK extrusion market, they must use extruders that include high temperature capabilities required for processing PEEK. The extrusion processing temperatures of PEEK are much higher than medical-grade thermoplastics such as nylons, polyurethane, and polyolefins. The extruder machine for these materials includes cast aluminum barrel heaters that are rated for up to 600°F. The barrel set points for PEEK however are in the 725°F range and therefore the barrel heaters should be cast bronze and rated up to 900°F. PEEK melts at around 650°F. Unlike fluoropolymers that are corrosive to most metals, PEEK is not corrosive to extruders, so the barrel does not require special construction and the screw does not require special high cost nickel-based alloys such as Hastelloy or Inconel. As a result of the high operating temperatures, a special dual-layer barrel safety shroud should be used over the barrel to protect the operator from burns.
Of course the best situation for a processor that would like to start extruding PEEK would be to purchase a new modular style extruder that is designed with high-temperature capabilities. However, an alternative lower cost approach to help a processor get started in the PEEK extrusion game would be to retrofit an existing extruder that is designed for typical thermoplastic resins to a high temperature barrel heating and cooling package.
By retrofitting existing extruders with high-temperature capabilities, the core process remains unchanged; therefore existing extrusion processes do not need to be re-validated. The time consideration is also favorable when retrofitting existing extruders with high-temperature capabilities because in many cases it can be accomplished in the field, which also reduces downtime. Retrofitting a 1-in. (25-mm) extruder in the field with a high-temperature barrel heating, cooling system and safety shroud system can take place in as little as two days.
Once the extruder is retrofitted to process high temperature PEEK, it can then also be used to process other high temperature medical-grade thermoplastics such as Polysulfone (PSU), which is an amorphous polymer with processing temperatures in the 600 to 700°F range and Polyetherimide (PEI), an amorphous polymer with processing temperatures in the 620 to 670°F range.