Fluoropolymers are commonly used for the construction of medical catheters. Polytetrafluoroethylene (PTFE) is the material of choice for low friction, inner liners of guide catheters used in cardiology and neurology. Fluorinated ethylene propylene (FEP) is used for vascular access sheaths and shrink tubing.
Although used in state-of-the-art medical devices, these materials are not new. The fluoropolymer industry began with the discovery of PTFE by DuPont in 1938 (tradename Teflon®). The chemical structure of PTFE is similar to that of polyethylene, except that the hydrogen atoms is replaced by significantly larger fluorine atoms. This results in a compact molecular structure that was highly crystalline (up to 98%) and very dense, resulting in one of the heaviest polymers.
Today, PTFE is typically sold in granular, fine powder and water-based dispersion forms. Granular PTFE is mainly used for compression molding, isostatic molding and ram extrusion. Fine PTFE powders can be processed into thin wall sections by paste extrusion (e.g., tubing) or used as additives to increase wear resistance or frictional property of other materials. PTFE dispersions are used for coatings and film casting.
Guide catheter liners require extremely low frictional properties to allow for easy passage of guidewires, balloon catheters and other devices. PTFE offers the lowest frictional properties of all commercially available polymers. PTFE also offers excellent chemical resistance, high thermal stability and a very low dielectric constant (i.e., very good electrical insulation). Limitations of PTFE include low tensile strength, wear resistance, creep resistance and radiation resistance.
Early on, DuPont and others recognized the shortcomings of PFTE and began research on copolymerization of tetrafluoroethylene (TFE) with other monomers. The first of these to be commercialized by DuPont in 1960 was FEP (fluorinated ethylene propylene). FEP is produced by copolymerization of TFE and hexafluoropropylene (HFP). The crystallinity of FEP is about 70%, and its melting temperature is in the range of 260–280 °C, depending on the HFP content. The molecular weight of FEP is much lower than that of PTFE, which results in a much lower melt viscosity and better processability.
FEP can be processed by the conventional polymer processing techniques such as injection molding, extrusion and film casting. FEP offers similar chemical resistance, weather resistance, flame resistance, radiation resistance, and the electrical properties to PTFE. FEP is also easier to surface modify to increase wettability and adhesive bonding or printing. FEP has better impact strength and wear resistance, yet slightly higher frictional properties and lower resistance to thermal stress cracking than PTFE.
Ease of processing without substantial attrition in low friction performance, and improved mechanical properties, is why FEP is commonly used for extruded (tube) sheaths on vascular introducer sets. These sheaths are the conduit through which all vascular devices are passed during arterial diagnostic and interventional procedures. These same properties, coupled with high temperature resistance, is why FEP is commonly used for heat shrink tubes. Thse tubes provide temporary protection over the union of catheter shaft segments during assembly and heat welding.
Source: Teng, H. ‘Overview of the Development of the Fluoropolymer Industry’. Applied Science. May 2012. p 496-512.