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?
Experiment and Results
In order to test the effect of dry time on performance of hydrogel coatings applied to guide wires, a series of experiments were conducted with the goal to be the determination of time post-hydration in PBS and DIW to coating failure, weight gain of a coating in both PBS and DIW, and performance post-failure upon rehydration. The design of the experiment served also to evaluate the differences, if any, in results depending on pad material or room temperature. The results presented here detail the characteristics of a coating most common to guide wires.
Figure 1 details the results of a guide wire hydrated in DIW for one minute followed by pinch testing using Delrin pads and silicone pads. The left scale is the pulling force in grams that is required to move the wire through the pad. A total of three inches of wire are recorded. Note humidity is a bit high at 66.5 percent to 67 percent. For both pad types, coating failure was observed after five minutes of air exposure. The silicone pads did demonstrate some additional coating degradation but not for the entire length observed (after four minutes of air exposure). In both cases, coating performance was fully restored after re-hydration.
Figure 2 uses a constant of Delrin pads with the variable being hydration in PBS or DIW. Weight gain on hydration appears equal at 0.007 grams. Note that the coating in PBS failed at three minutes, while the coating hydrated in DIW failed at five minutes of air exposure. Weight retention measured also shows a more rapid loss of hydration in the PBS situation.
In Figure 3, silicone pads were employed and results compared in PBS and DIW. The results show a different pattern than with the Delrin pads in Figure 2. In contrast, silicone pads show a coating failure in four minutes while in DIW, the failure time is between two to three minutes of air exposure. Again, post-failure, if the wire is re-hydrated, full lubricity returns.
Conclusions from the experiments to date indicate that there is a definitive time that a coating will dry out and lose lubricity—typically three to five minutes following first hydration. One can also surmise that regular hydration in the OR with either DIW or PBS accomplishes the goal of maintaining lubricity and, lastly, that lubricity may be regained after prolonged exposure to air upon a refreshed hydration. The experiment did not employ cotton gauze to hydrate, so no conclusion can be made about the abrasive effect of the gauze rubbing over the device surface. This variable is operator-dependent and difficult to standardize.
Coating performance characterization via pinch testing is a valuable indicator of in vivo experience. In terms of performance it is a difficult task to make one-to-one comparisons due to differing chemistries and pinch test parameters. Knowing the points of failure of a coating and how recovery of lubricity may be accomplished are truly beneficial data sets that guide techniques when an operation is extended unexpectedly. Design engineers should seek a firm understanding of both the biocompatibility of a coating as well as the limits of coating performance with equal vigor.
This article was written by Keith Edwards, President and CEO, Biocoat, Inc., Horsham, PA. For more information, Click Here .