What You Need to Know about Medtech Coatings

Coatings are a crucial component of many medtech technologies, enabling medical devices to be inserted into tight body spaces. On Wednesday, October 29, Keith Edwards, president and CEO of Biocoat Inc. (Horsham, PA), will speak to a range of issues in a presentation at MD&M Minneapolis titled "Design and Manufacturing Considerations for Medical Device Coatings." In the following Q&A, Edwards provides a foretaste of the topics he will address in his talk.

MPMN: Please give examples of cases where a coating is necessary or beneficial on a medical device or component. What devices can benefit from a coating?

Edwards: In the coronary, cardiovascular, neurovascular, peripheral vascular, urologic, and ophthalmic areas, there is a very large demand for hydrophilic coatings. Coatings are an enabling technology that allows such medical devices as catheters and guidewires to perform their functions in the body.

Without a coating, a catheter would not be able to traverse the tortuous paths of the blood vessels, nor would a guidewire be able to reach from way down in the femoral artery all the way up to a remote portion of the brain to remove a thrombus. It would be very difficult to insert an intraocular lens through a very tiny opening in the eye unless the delivery mechanism had a very slick hydrophilic coating. And in the evolving field of female health, when a surgeon wishes to perform breast reconstructive cosmetic surgery after cancer tissue has been removed, the breast implant is inserted through a funnel that has a coating on it.

Thus, the insertion of catheters, guidewires, intraocular lens insertion cartridges, and implantable devices requires the use of coatings.

MPMN: Which factors must manufacturers keep in mind when weighing a coating's performance attributes?

Edwards: A coating's performance attributes are a key but not the sole issue facing coating technologies. Most manufacturers of hydrophilic coatings make a very, very slick coating. Thus, a coating's performance in a simple application is pretty well known, and almost any coating can be used. However, if a device is to be inserted into a diabetic patient with heavily calcified arteries or will have to wend its way through a tortuous path, as in a coronary wire procedure, the device must have a coating that's more than slick. It must also be able to survive multiple passes--ins, outs, adjustments, or rotations--during the course of the procedure. Thus, a performance attribute is not just how slick or lubricious a coating is but also how durable it is over the intended period of use.

Beyond performance is the question of biocompatibility. You want to make sure that a coating passes all the various cytotoxicity tests commonly requested by FDA and that it has a very low particle count so that only a small amount of the material is left behind once the device enters and exits the body.

MPMN: Please go into the role of polyvinylpyrrolidone (PVP) and hyaluronic acid (HA) in medical device coatings. What are the differences, synergies, and test results between the two materials?

Edwards: PVP and HA are two entirely different technologies. Most medical device coatings available today are based on PVP, a well-known biocompatible material with a long, established history as a blood substitute and blood thinner. During World War II, it was used as a filler to keep hearts pumping. Moreover, it's known to be biocompatible. PVP's chemistry makes it behave somewhat like soap. Soap is very slick, but over a period of time, it rubs off and loses its slickness.

HA-based coatings, on the other hand, form more of a hydrogel-type surface. This material is a long-chain polysaccharide that traps water and hence makes a very durable, pliable coating offering advantages in certain cases.

The process used to apply both of these coatings is as different as their chemistries. Typically, PVP-based coatings involve an ultraviolet-light cure, while HA-based coatings involve a thermal cure in ovens. Both materials offer comparable synergies, providing very slick coatings. HA coatings, however, may have a slight durability edge over PVP coatings, but it really depends on the application.

While both materials produce very slick coatings, the procedures used to test them are quite different. If you use the same procedure in each case, you will get very different results under one change in medium.

Typically, all of Biocoat's clients perform some form of pinch, rotational, or anatomical-model testing. The bottom line is that a PVP coating performs very well in water, while a HA coating performs very well in phosphate-buffered saline, which mimics the properties of blood. When you switch the two, the test results are still OK, but there's definitely a distinction. PVP does well in water, while it does not do well in saline. HA, on the other hand, does not do well in water, while it does very well in saline. Thus, design engineers need to understand this difference.

It's easy to confirm a process going forward with a PVP coating if the device will be used in a short procedure involving only one or two passes--a quick in and out without too many readjustments. In such cases, PVP is just as good as HA. But when the device will be used to perform a more demanding procedure in which multiple passes are required or in which there are really tight, tortuous paths to navigate, HA is probably a better choice.

MPMN: What are some of the future prospects for coating technologies? What is ahead in coating R&D and innovation?

Edwards: Here's the long and short of it. I travel and visit clients--including engineers and vice presidents of engineering and R&D--and I hear loud and clear that it's all about the cost. Our innovation activities at Biocoat--and certainly at our peer companies as well--are really geared toward shortening the coating process for manufacturing throughput, extending the shelf life of reagents so that materials don't expire, and coming up with cheaper raw materials while still complying with FDA's carefully selected biocompatibility guidelines. That's a tough one to sell.

On the innovation side, a movement is slowly beginning to develop devices that can be E-beam or gamma sterilized. Typically, a PVP or HA coating does not perform well when subject to these sterilization techniques. Thus, improvements in this area are absolutely necessary and exciting going ahead. And quite frankly, if the coatings community is able to address E-beam, gamma, and perhaps even autoclave sterilization, the whole field of orthopedics will open up as a good market for lubricious, hydrophilic coatings.

MPMN: Which types of materials are being developed that can withstand these sterilization methods?

Edwards: Materials under development tend to be very specific polymers with chemistries that can withstand radiation or E-beams but also provide a strong bond to the medical device surface--be it a guide wire or a catheter. That's a tough chemistry to crack. It requires a lot of trial and error and a good amount of theoretical polymer chemistry as well.

MPMN: Is Biocoat working to develop such materials?

Edwards: Yes, that's correct.

Bob Michaels is senior technical editor at UBM Canon.

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