Key Questions to Ask When Ordering medical guidewires

Precision Coating Answers Your PTFE Guidewire Coating Questions

Dave DiBiasio, Precision Coating’s (Precision Coating) Vice President of Sales and Marketing, and Dick Buxton, Precision Coating’s Director of Process and Application Engineering answer more of your questions—today, they address coating guidewire. Precision Coating is recognized as the expert, worldwide, in coating guidewires. We’ve literally coated hundreds of millions of wires over the past 40 years. Precision Coating coats for all major CMOs and OEMs throughout the world, coating various platforms in eventual, minimally invasive platforms, wire products used in various markets and sectors, and other core surgical applications.

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Today we’re exploring some common questions customers ask us about coating guidewires and the substrates. We hope this information helps you to choose the right coating and expedite your platform to production as soon as possible.

Question: One of the most common questions that we receive from our customers is: when you’re coating discreet length wire, what is the maximum length and the minimum length that you can coat?

Answer: The real issue for us isn’t the wire length, but what is the maximum length coated area you’d like to have coated. We can coat a longer wire length than the actual coated length.  The coated length maximum is 320 centimeters, or about 10 feet. The remainder of the wire would be masked or suspended somehow on the fixture. Obviously, it would have to be somehow handled in a different way.

Question: That takes us to our next question, which is: how much holding area do you need to be able to coat the wire?

Answer: That goes hand-in-hand with some of the questions of masking.

How much of a masking tolerance will I get? In some cases, if I have a tight masking tolerance, it’s preferred to use a tape method; the masking tolerance in that case is plus or minus 60,000th, or one and a half millimeters. In that case, I prefer to have at least 50 millimeters, if I could (about two inches). I can live with less, but it also is dependent upon wire diameter and length.

If I go into the mechanical fixturing, it’s not such of a discrete length or cut; let’s say sharp. It’s more of a fade out of coating. The fade out could be as much as 200 thousands. Usually it’s a little less, about five millimeters. In that mechanical fixturing, I can hold a minimum of 200 thousands, but prefer a half an inch if I could have it.

Question: You mentioned masking, and are a lot of questions associated with that. You talked about fixtures and tape—sounds like there’s multiple ways to mask the product.

Answer: There are many ways. There are also ways we can add areas in the midsection of the wire that would be uncoated as well.

In either case, being taped or mechanical fixturing, we have ways of doing that both. And we have ways that we can coat wires with two different colors, and put micro bands, if you’d like. There’s a lot of flexibility in that regard.

Question: Do you have to buy specific fixturing to mask, or can you use existing fixturing and maybe a cap on tape?

Answer: Depending on your drawing requirements, we prefer almost always to use our standard fixturing methods. Volume might have more to do with a repeat customer producing a large monthly quantity, we might ask for an NRE charge where we would buy fixturing, or you would buy fixturing, so the racks are ready to go and readily usable.

Question: For our listeners, Precision Coating coats, about 89% of our product via automated application. What are the benefits of automated application?

Answer: One of the things we did is we replicated as much as we could, what the sprayer [person] did. I can’t imagine me being an old man, trying to keep up with that robot all day long—keeping the same arm speed, same distance. And just doing that every minute. Every day, the same method. I think it’d make anybody tired, holding tight tolerances, to boot. The benefit of automation, as well as the fact that as we’ve grown and learned more and more about the attributes of coating, is we can dial in and hold a better coating thickness, masking tolerances, and overall a better performing wire because of the automation process.

Question: Regarding fixturing, you prefer to use the existing fixturing, it tailors itself well to a lot of different applications that we have, but there are times where you may need specific fixturing that the customer may need to purchase?

Answer: True. And again, it’s based mainly on the drawing, application, wire diameter. There’s a lot that goes into it, and really, to have a drawing upfront to understand the customer’s requirement is always beneficial.

Question: You just mentioned wire diameter. One of the other questions we almost always seem to receive is: what’s the smallest diameter wire you can coat? Customers are using this in torturous vessels with a lot of turns and small diameter. What’s the smallest we can go?

Answer: We go down as small as five thousands in diameter. We have done some four thousands, but they’re more of a ground distal end on a larger diameter.

So, if my set up is an O 14 and then be ground as a fine distal down to a three and a half or a four; when we’re down around the 5,000, we’re probably at a length of anywhere from, let’s say four to 10 inches. The issue with the small diameter is it becomes more of a, let’s say, a bird’s nest when we start to handle large volumes of wires—it wants to intertangle. So it’s very important how we receive it, how the packaging is, what packaging is used, and how it’s set.

Question: There are various substrates that you can coat. What type of substrates do you work with?

Answer: All the substrates. We work with all the stainless-steel alloys, such as MP35N; we do nitinol, tungsten, and platinum, as well. We handle just about all.

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We hope we’ve answered question you may have in choosing the right coating for your medical guidewires.

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The MICRALOX® portfolio of chemistries offers superior patented aluminum oxide coatings with a microcrystalline barrier that revolutionizes aluminum anodizing with exceptional barrier properties and corrosion resistance over industry leading product life cycles.

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.

Conclusion

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  .