Fluropolymers have found markets in demanding applications, ranging from the extremes of space flight to deep oil exploration. In such applications, dielectric properties, superior chemical resistance and a broad continuous-service temperature range are critical. Fluoropolymers also excel in a range of other properties. Many of these properties are important in medical device manufacturing applications and vary depending on how the material is being used. In the medical device market, fluoropolymers have proven themselves in exacting applications and have been used in devices that treat coronary angioplasty as well as in permanently implanted vascular grafts.
In the medical device market, the use of fluoropolymers centers on two key properties: lubricity and biocompatibility. Fluoropolymers exhibit very good lubricity compared with other plastics. PTFE is the most lubricious polymer available, with a coefficient of friction (COF) of 0.1, followed by fluorinated ethylene propylene (FEP) with 0.2. These two polymers represent the vast majority of all fluoropolymer tubing used in medical devices.
Fluoropolymers, especially PTFE, excel in biocompatability and have a long history of in vivo use. Medical-grade fluoropolymers should meet USP Class VI and ISO 10993 testing requirements. The unique properties of fluoropolymers have made them the material of choice in the design of many minimally invasive medical devices. Their properties enhance the development and commercialization of next-generation devices. The continued development of fluoropolymer extrusion technology has enabled further refinement of catheter technology (eg. Neurological catheters) while developing new and innovative surgical devices. Precision microminature extrusions, such as very-thin-wall tubing, can be used in such devices because of their ultrathin walls and precise tolerances. Another example is multilumen tubing. Its multiple passages allow surgeons to conduct multiple procedures using the same catheter.
Refining tolerances and extrusion capabilities are important because catheter designers are concerned with tolerance stacking, where the production of materials on the high side of the tolerance range can reduce the workable inner diameter of the catheter.
A few well-established applications that highlight the key properties of fluoropolymers are
Guiding Catheter
Used to deliver coronary stents and other devices, the guiding catheter ia at the core of most guiding catheters is a PTFE inner liner. The superior lubricity of PTFE has made it the material of choice for this application, with the lowest dynamic COF of any polymer. During the construction of a guiding catheter, PTFE is chemically etched onto the tube's outer diameter. This process allows for the bonding of materials to the outer diameter of the liner. The bonding is accomplished by using an FEP heat-shrinkable fusing sleeve. The high shrink temperature of the FEP (350ÞF) is above the melting point of the Pebax inner layers. The FEP provides sufficient constrictive hoop strength to fuse the polymers together as the heat shrink recovers. The FEP can then be removed from the device, leaving a smooth outer jacket. Lubricity is so critical that even FEP (the second most lubricious material available) has not proven successful as a catheter liner.
PTFE Introducer
Developed and patented in the 1970s., the PTFE introducer utilizes a little-known property of PTFE that it can be processed in a manner that allows for molecular orientation of the material, allowing the PTFE tube to be split and torn longitudinally. The tubing must first be scored precisely at the edges, enabling the tear to be easily facilitated by hand along the longitudinal grain of the molecules. During use, a surgeon can remove a PTFE introducer from a patient while the primary device remains in place.
Multilumen Catheter
The multiple passages of today's fluoropolymer catheters enable surgeons to perform a series of procedures without the need to remove one catheter and insert another. The demands of the medical device community will continue to expand as it is challenged to reach previously inaccessible areas of the vasculature.
New materials also present interesting opportunities for advances in catheter designs and will allow polymer processors to continue to refine their capabilities. The continuous innovation of new polymer blends and extrusion technologies make these materials the ideal starting point for innovative designs.