Dr B P van der Wal, Merit Medical Coatings and Charles Fields, Whitford Corporation, discuss the use of PTFE coatings in intravascular medical devices.

There has been much discussion about PTFE coatings in the vascular medical device market. In November, 2015 the FDA issued a safety communication referencing the effects of coating delamination on intravascular devices on the heels of several reported incidences. For many years, PTFE coatings have been a silent partner, reducing friction every day in catheterisation procedures. They have become a ubiquitous, commodity finish on standard catheter components.

Continual regulatory pressure in the chemical industry is always forcing change. Readers may recall that PFOA (Perfluorooctanic acid) was regulated out of the fluoropolymer manufacturing stream. This change had an effect on both coating performance and application requirements. This is not a new phenomenon. Coating manufacturers are constantly forced to deal with functional raw material changes including solvents, pigments, and resins. This does not mix well with the medical device market. While the medical device industry has processes in place to deal with material change, it tends to be a time-consuming and often expensive undertaking to implement. These changes appear to be increasing as the global requirements for greener and safer products become a more serious driver. Current raw materials being evaluated include Chrome 6 (hexavalent chromium) and NMP (N-methyl-2-pyrrolidone). These are key materials in the manufacture of many of fluoropolymer (PTFE) coatings used on catheter components. Vascular medical device manufacturers will need to prepare to accommodate these coating changes to ensure the coating processes meet their end-use requirements.

Merit Medical Coatings, whose wire coating facility is located in Venlo, Netherlands, has been taking a pro-active approach addressing the regulatory pressures being placed upon the fluoropolymer industry. Dr B P van der Wal, senior engineer said: “The application of (PTFE) coatings to medical wires requires extensive expertise. The small size and flexibility of many medical devices make the coating process challenging and completely different from the coating process for flat and rigid substrates such as cookware. The significantly lower heat capacity for substrates such as wires necessitate completely different processing times and temperature.”

There are two different approaches to the coating application: Pre-coating and post-coating. In the pre-coating process, the coating is applied to the substrate before assembly.  In general, the pre-coating process is used for high-volume processing for which the substrate allows reel-to-reel processing. Pre-coating is a continuous process allowing for high coating speeds and a very consistent coating result for a full reel of material. For guidewires, this means the coating is applied to the wire in a reel-to-reel process before coiling. The coating therefore has to withstand all the forces used in the assembly process. For instance, the coating should not delaminate from the wire during coiling. This demands a lot from the coating because of the high forces applied to the wire and resultant deformation. Parameters such as the coating thickness are critical to make the coating flexible and strong enough.

In post-coating, the catheter wire assembly processes have already been completed. Post-coating is a batch process and is selected if the substrate does not allow reel-to-reel processing, such as a wire with a ground tip. Post-coating also gives more flexibility to only coat part of the substrate. By masking part of the substrate, marker bands can be left on the surface. The subsequent dry film does not have to withstand the high mechanical forces of coiling and other wire manufacturing processes. This does not make the coating application less demanding. With the pre-coating process coating adhesion failure would be observed during the assembly process. With the post-coating process coating adhesion failure would likely occur during patient contact if it is not caught during QA-inspection. This can be a dangerous situation placing very serious requirements upon the coating application processes. 

For fluoropolymer coatings, the non-stick property and good adhesion to the substrate contradict each other. Additionally, medical wires are typically constructed of reasonably inert metals, such as 304 stainless steel and Nitinol, further complicating the heating and pretreatment processes. Often multi-layer coatings are used to get a good adhesion to the substrate using a binder layer and a fluoropolymer rich top coat. Of course, this makes the coating application process more complex because multiple layers must be applied. There have been many new coatings developments in which one-coat systems have been developed giving the same effect: good adhesion to the substrate and a fluoropolymer rich topcoat. The coating process, however, hugely impacts the efficiency of this self-stratification process.

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