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Guide wires with a highly lubricious coating are an essential staple of many interventional procedures. In the operating room (OR), you can observe guide wires undergoing multiple passes, constant rotational forces, insertions, and extractions. Just how a design team tests a guide wire coating is essential to predicting in vivo dynamics during a cardio- or neuro-vascular procedure.

Pinch and Rotational Testing Considerations

Coatings vendors and client device manufacturers all have some form of frictional testing regimes. Pinch testing is the most common, but techniques vary greatly and need inspection for relevance. Coating chemistries commonly in use today are either soap-like in action (such as polyvinylpyrrolidone or PVP) or water-trapping hydrogels. Hydrogels may be achieved thru the fixation of long chain poly-saccharides, such as hyaluronic acid (HA) to the surface of a wire.

Due to the specific nature of a substrate, Nitinol, Pebax®, silicones, metals, urethanes, and similar materials, coating chemistry may indicate advantages of PVP over hydrogels, or vice versa. Today, most coatings offer excellent lubricity. Cost and ease of manufacturing are also contributors to a coating selection. From a performance point of view, getting close to an apple-to-apple comparison is often problematic due to test parameter variance.

The selection of de-ionized water (DIW) or phosphate buffered saline (PBS) is the most important determinant of test results. Simply stated, hydrogels with an HA-based coating will demonstrate excellent lubricity and durability in PBS but show higher coefficients of friction in DIW medium. Exactly the opposite is true for PVP-based coatings. In DIW, PVP coatings will perform well but exhibit degradation in PBS. Adding heparin to either medium is not viewed to impact coating performance. Rather, heparin serves a biologic function to inhibit clot formation.

In both cases of PBS and DIW tests, the pads or point of friction should be immersed in the solution to avoid variability inherent to ambient air temperature and humidity. The pad composition used to create friction is important, as is the load in grams exerted on a pad. DuPont Delrin® and silicone are common pad materials with the latter used for softer materials, such as Pebax. Dramatically different results can be achieved on the same substrate with the same coating using pads of different materials. If you are not rough on your coating test, you may not achieve the gold standard of a calcified femoral artery.

One way to be rough but consistent is to choose a pinch load, commonly 470 grams or above, that stresses the coating. Establish the number of passes that mimics actual use. Some firms use one pass, others use 30 passes with attention paid to the change from the first to the 30th pass. Anatomical models are helpful as an option for tortuous path testing but consistency is unlikely to be achieved as models differ widely. Similar rules apply to rotational testing. Design teams should think of the user and environment as to what is the appropriate medium and test surface to prove performance.

Beyond Pinch Testing: Dehydration and Re-Hydration

During an aneurysm repair, the surgeons will commonly have a bowl of sterile saline and multiple gauze pads right next to the guide wire control mechanism. Hydration is essential to coating performance and surgeons are wise to constantly hydrate and rehydrate a guide wire during the course of a procedure. This process is necessary because, regardless of the coating chemistry, the OR remains a low humidity environment and a real risk of the coating drying out exists. A dry coating exhibits no lubricity.

  • Variables exist that may be addressed in design control. Here are some questions to ask:
  • What is the maximum amount of time the wire will be exposed to air following extraction and before re-insertion?
  • What fluid is the surgeon most commonly going to use for hydration? Deionized water, phosphate buffered saline, or heparinized saline? What do the instructions for use for the wire recommend?
  • Will the action of rubbing the wetted gauze over the guide wire scrape off coating or impair performance?
  • In tests, how long has the guide wire coating demonstrated lubricity after exposure to OR air?
  • Have you tested lubricity after a dry out cycle and re-hydration?
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