Guidewire PTFE Adhesion: The Aftermath of a Panic

EPA's decision to bar the compound PFOA from guidewire coatings sparked a crisis in the medical device industry. Here's how one company tackled the problem.

Bruce Nesbitt

In 2012, the EPA ordered that polytetrafluoroethylene (PTFE) coatings no longer be made with perfluorooctanoic acid (PFOA), a surfactant linked to increasing film adhesion, but is suspected of possibly being carcinogenic. This triggered a performance crisis as the PFOA-free PTFE coatings began to flake off guidewires in service. Green flakes of PTFE were found in saline tanks in operating rooms, and, startlingly, a few were found inside patients. Besides subpar adhesion, the coating proved to be sensitive to saline and high levels of humidity, both of which accelerated coating delamination and flaking.

The FDA issued a string of recalls of the guidewires--Class I recalls--sending the medical device industry scrambling for a new type of coating that would provide the tactile "feel" of PTFE to surgeons in service, but one that would not flake off. However, no coating ever matched the low friction of pure PTFE. So the question became how to keep the feel of a "pure" PTFE coating and regain the adhesion lost or attributed to removing the PFOA. After all, the "pure" PTFE coatings had reliable adhesion for more than 40 years while maintaining that silky, reliable feel.

A Step Change in Adhesion

In 2013, Surface Solutions Group of Chicago discovered a new approach to the adhesion problem of PTFE involving molecular-level attraction. They examined the relationship between the polymer molecules of the coating and the metal surface of the wire to see if they could induce an attraction between the two.

Up to now, the bond between a coating and any substrate--metal, glass, or plastic--has been mostly mechanical, with the wet coating filling the interstices on the substrate surface, then becoming mechanically locked in place when cured. See Figure 1.

For thousands of applications, this works fine, but for those in chemically active environments--including coated guidewires in saline--mechanical bonds can fail. PFOA-free coatings were quickly determined to be an example where the mechanical bonds fail.

SSG engineers looked at coating-to-metal bonding from the standpoint of the basic van der Waals forces, and discovered technology for boosting the attraction between the coating and guidewire substrate. Essentially, charges are biased to increase attraction at both the molecular and the atomic level.

In addition, new methods to eliminate all surface contamination of wire that has been drawn through sizing dies, were developed. Finally, adhesion promoters provide more grip with a mono-molecule layer to further increase adhesion between the ultra-purified wire surface and the enhanced formulation PTFE coatings.

Altogether, this multi-step process increased adhesion of PFOA-free coatings from inconsistent adhesion to 100% adhesion, even after the most severe saline testing.

The new process that increases PTFE coating adhesion is named FluoroBond PT.

Same, One after Another     

One of the primary reasons guidewires are coated with PTFE is to give surgeons consistent tactile "feedback" in vitro, enabling them to feel blockages and other obstructions. For this feedback to be consistent, the coating itself must be consistent.

To achieve absolute uniformity, SSG engineers developed a process "recipe" for their computer-controlled coating system that holds coating tolerances to a micron level, far better (closer) than even plating tolerances. In refining their formulation, the engineers developed coating techniques that apply specifically to guidewires. For instance, the SSG process includes absolute control of the environment in which coating takes place. Experiments showed that temperature and humidity must be held to exact limits, and that deviations from optimum conditions have a negative effect on adhesion. "We were surprised to learn that even the time interval between cleaning and coating needed to be controlled to improve adhesion," noted Bruce Nesbitt, chief engineer at the Chicago coater. "Now, we can apply the PTFE to a nominal thickness of 0.0002-0003 in. or 5-7 ? on a 300 mm long wire," says Mike Osterhout of SSG.

Proof Testing

Because of the critical ramifications of PTFE flaking off guidewires, SSG designed a sequence of QC tests for FluoroBond PT that would have, in earlier years, been thought of as overkill. Cross-hatch (with razors), tape adhesion tests, and abrasion tests were raised to new levels of stress. For instance, the tape that was normally used for tape pulls was changed to one that has 13 times more adhesive strength.

Quality testing involves two paths: First, coated samples are soaked in saline for 40 minutes at 140°F, then subjected to the cross-hatch and tape-pull tests. This simulates the worst case situation that the wires will encounter. Second, the full-length coated wires are visually inspected by the naked eye, Figure 2, and then under 40 × magnification, as shown in figure 3. At this level, any surface irregularities are clearly visible. Any evidence of voids is enough to reject them and quarantine the entire lot.

In Service

Early field trials indicated that FluoroBond PT was even more durable than expected. It proved to be impervious to saline and even tropical humidity--and to handling and shipping practices that could, by themselves, cause cracking in the original PFOA-free coatings. In many developing nations, transportation is based on bicycles or sometimes even on animals, and handling and storage can be primitive. Humidity and storage temperature controls can be nonexistent. A shipment of guidewires may sit for weeks in an uncontrolled warehouse where temperatures can exceed 130°F for weeks. 

No proof of performance is as strong as a product's service history. Guidewires pre-processed with FluoroBond PT combined with the enhanced PTFE formulations have proven to be the answer to the crisis caused by PFOA removal. Now, after a year and a half of service in locations all around the globe, FluoroBond PT has shown that it performs even better than expected.

In all this time--and in tens of thousands of medical procedures using PTFE coated guidewires--FluoroBond PT preprocessed stainless steel and even nitinol guidewires coated with the enhanced formulation of "pure" PTFE coatings have never delaminated in service.

Bruce Nesbitt is the "chief tinkerologist" at Surface Solutions LLC (Chicago).