PTFE Guidewire Application Process Eliminates Delamination

The guidewire coating delamination problem reached catastrophic proportions late last year. In October, Medtronic—a major guidewire supplier—recalled over 84,000 units that had the potential for the PTFE coating to flake off. But Medtronic was not alone; other manufacturers have recalled guidewires, too.
 
These were FDA Class 1 recalls, meaning these guidewires and microguidewires represented serious health risks in which the applied coating—PTFE in this case—had the potential to delaminate or flake off. If this occurs before or during a medical procedure and even a microscopic flake enters the patient’s bloodstream, the results can be serious, including blood clots, stroke, heart attack, tissue necrosis, and even death. Between January 2014 and November 2015, the FDA received approximately 500 Medical Device Reports—including reports of nine deaths—attributed to PTFE coating delaminating from guidewires.
 

The medical ramifications to the patient and their family are undeniably devastating. For the guidewire manufacturer, the results can also be catastrophic. Based on other comparable recalls, the device manufacturer will take a substantial hit to their reputation and see their stock prices and market cap often falling 10 percent from a single incident.
 
Further, there is the matter of unresolved liability. Defending a lawsuit stemming from blood clots, stroke, or death resulting from a faulty guidewire would undoubtedly be a costly endeavor, and the damages available to the patients and their families can be significant. There is no set benchmark for the types of damages available in these cases, particularly when the damage done to a patient is open-ended and requires ongoing treatment or hospitalization, which may make early resolution more difficult. Litigants and the courts will invariably look to other defective medical products for guidance. “The lawsuits that stem from defective catheter guidewires may follow the pattern established for the IVC filters that were designed to block blood clots,” explained Rachel V. Rose, a trial lawyer in Houston who is experienced in medical litigation. “In the case of guidewires, look for ‘bellwether’ cases in different parts of the nation to set the standard for monetary awards.”
 
Background
Coating a guidewire with PTFE is necessary for its smooth operation in the peripheral, coronary, and neuro vasculature. The coating reduces friction and eliminates the potential for binding and kinking during a procedure and, from the physician’s standpoint, the motion of the guidewire needs to be smooth and unhesitating. The difference in the tactile feel of a guidewire without and with PTFE is dramatic.
 
The origin of the problem of PTFE flaking on guidewires appears to correspond with the Environmental Protection Agency’s mandate earlier in this decade to eliminate the surfactant—PFOA—from water-borne PTFE formulations because PFOA is a suspected carcinogen. For 50 years, pure PTFE, with the aid of PFOA, had been the gold standard coating for guidewires. It is believed that PFOA improved adhesion to smooth metal surfaces, including guidewires. Without it, the flaking problem struck with a vengeance, especially when guidewires were soaked before use in the ever-present saline used in medical procedures. Saline penetrates the porous coating and, if adhesion is marginal, causes it to bubble and delaminate. Reports from operating theaters indicated that guidewires coated with the new PFOA-free PTFE were visibly flaking after being placed in saline soaking tanks.
 
An initial “fix” for the delamination problem was to switch from pure PTFE to resin-bonded PTFE. These early formulations are based on particles of pure PTFE and other low-friction particles, suspended in a tough polymer resin. While adhesion was improved, friction became a problem. While the coefficient of friction of pure PTFE is as low as 0.02 (similar to ice), the original resin-bonded coatings often had between 1.5 to 4 times more friction. Over time, the friction of the resin-bonded coatings has been improved, but not to the level of pure PTFE. This means the smooth operating feel of the old coated guidewires was lost, and the tactile feedback sensed by a physician operator is heavier, hesitant, and jerky. Known as “stick-slip,” this phenomenon is the transition from static friction to dynamic friction. Some physicians reported that they could not differentiate between a vascular obstruction and a momentary resistance of the guidewire.
 
There is another potential problem with resin-bonded PTFE: an unknown shelf life. If the substitute coating is not completely cured, it contains solvents that could adversely affect packaging, while the original water-based coatings had no such issue. Given all of this information, it appears that PTFE would be the ideal coating for guidewires as long as the delamination problem could be resolved once and for all.
 
Not unexpectedly, the problem of flaking on guidewires has cast doubts in the minds of cardiologists when faced with using them. According to Sam Butman, a cardiologist in Verde Valley, Ariz., “…shearing off non-radio-opaque material is…an important issue. It makes one wonder about what we do not know at times [in a procedure].”
 
A Coating Solution
Astonishingly, none of the problems with PTFE-coated guidewires need to have happened—not the flaking, not the recalls, not the medical harm to patients, and not the pending lawsuits. The solution lies in how the coating is applied to guidewires, particularly the cleaning of the wire before coating. The following process has been used to coat over 33 million guidewires, with zero failures and no recalls.
 
By essentially following the concept of “best practices” for all phases of the coating process, reliable adhesion of PTFE can be achieved. This includes surface preparation, coating viscosity and solids content, humidity, airborne particulates, spray pressure, temperature, electrostatic voltage, spray pattern, coating line humidity, and line speed, among others.
 
Surface pretreatment: “Cleanliness is next to godliness” applies to the process of coating guidewires perhaps better than anywhere else. PFOA-free PTFE has surprisingly good adhesion—so long as there is hyper-clean metal to which it can adhere.
 
Machine variables: Air pressure, line speed, electrostatic voltage, and similar variables are all controlled by a robotic application system that holds them to extremely tight tolerances, on the order of ±1 micron per surface, even on a 2,800 mm (90+ feet long) guidewire.
 
Environmental variables: Temperature, humidity, and airborne particulates can all impact PTFE adhesion. A HEPA-cleaned atmospheric control system that ensures cleanroom status in the coating area is great insurance. All staging areas and coating theaters must be tested in accordance with ISO 14644-1 to be either Class 7 or 8 with regard to particulate counts at 0.5 and 5.0 microns per cubic meter of air tested.
 
The significance of this factor was best explained by Dave Willis, chairman of Whitford Worldwide, producer of XylaMed medical grade fluoropolymer coatings, “Having spent more than 50 years in nearly every aspect of the fluoropolymer business and its multitude of uses, this level of environmental control at Surface Solutions gives cleanliness a whole new meaning to me. Most application areas are always unkempt. This is cleanliness to a level I have never seen before.”
 
Material variables: Coating viscosity and solids content of the aqueous coating are preadjusted in and held in a consistently tight range from batch to batch. To further control viscosity, each coating room is controlled within 2 degrees F and 3 percent humidity.
 
Proof tests: Quality tests are performed on each lot of coated guidewire to ensure there is no delamination. This includes a microscopic visual inspection, saline soak (to the specifications of the wire OEM), and other tests to attempt to force the coating to peel.
 
PTFE coating delamination is an extremely serious issue, but following best practices while still working with PFOA-free PTFE can offer a solution that has resulted in zero Class 1 FDA recalls connected to the process.