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Natural coating improves performance of medical devices.

Medical devices placed in the body must not only perform their specified function, but must do so without causing any other complications. Traditionally, devices such as catheters, balloons, and guidewires have been coated with synthetic hydrophilic coatings (UV-cured polyurethanes, polyvinylpyrrolidone, PVP) that help reduce cellular attachment and blood clotting and, when used for extended periods of time, aid in preventing the immune system from recognizing the device as a foreign body. These coatings can, however, have adhesion and durability issues. Hydrophilic coatings based on hyaluronan, a naturally occurring polysaccharide that is present in all mammals, have been developed. These offer greater adhesion strength, durability, and actual measured coefficients of friction, thus improving the overall performance of medical devices.

Commercially available hyaluronan is either isolated from mammalian tissues or produced in cultured streptococci with average molecular weights ranging from one to six million. The biopolymer is a glycosaminoglycan and is comprised of repeating disaccharide units that contain a glucuronic acid and N-acetyglucosamine units. Other examples of glycosaminoglycans in the human body include chondroitin sulfates, ketatan sulfate, heparin, heparan sulfate, and dermatan sulfate. Hyaluranon's function is to help form the extracellular matrix that supports and protects cells. The material used in coatings for medical devices is chemically modified to ensure good adhesion to the substrate surface as well as durability. Both in vitro and in vivo tests have shown that these modifications do not significantly affect the biocompatibility of the coating.


In fact, hyaluronan coatings outperform PVP in terms of lubricity, durability, and biocompatibility, according to Joshua Simon, senior product manager for Biocoat, Inc., manufacturer of Hydak[R] immobilized hyaluronan coatings for medical device applications. Hydak coated surfaces show significantly reduced numbers of adsorbed platelets, proteins, and bacteria. The coatings are also naturally hydrophilic and become exceptionally lubricious when wet, reducing risk of trauma and tissue abrasion. In addition, they can be applied as very thin flexible films. Furthermore, the reactive carboxylate, hydroxyl, and acetamido groups in the polymer chain allow for customization of coating properties and inclusion of bioactive substances.

Two coats are applied in the Hydak coating system. The basecoat is a proprietary synthetic acrylic copolymer. Both solvent-based and waterborne options are available. Two different solvent-based coatings have different curing profiles and are ideal for use on polymeric substrates. The aqueous basecoat is ideal for use on stainless steel devices and substrates that may be soluble in organic solvents. These basecoats are designed such that they form covalent bonds with the hyaluronan topcoat, ensuring excellent intercoat adhesion. "Because we use a two-coat system, the quality and performance of the hyaluronan topcoat is always consistent, regardless of the different substrates used for the medical device," Simon observes.


In a typical coating process, the device is cleaned with solvent then dip-coated with the basecoat and allowed to dry (usually 10-20 minutes at 60[degrees]C). The hyaluronan topcoat is then applied, also by dip coating, and the coating system cured for one to two hours at 60-70 [degrees]C. A final wash removes unreacted hyaluronan and other additives. "Hydak coating application requires no special equipment and is generally much easier and more cost-effective than application of traditional UV-cured coatings," says Simon. Other coating processes include spraying, injecting, and wiping.

As with the basecoats, a choice of hyaluronan topcoats is available to suit the needs of different device applications. Crosslinked coatings find use on devices for urology, cardiology, endoscopy, and other applications where wear resistance (because of high surface-to-surface abrasion or multiple passes) combined with hydrophilicity and lubricity are required. Non-crosslinked Hydak coatings may be applied to single-use devices for quick procedures in ophthalmology, urology, cardiology, endoscopy, and other fields that involve little surface-to-surface abrasion but require a very slippery surface.

Biocoat also prepares customized formulations for specific applications. These topcoat systems are based on aqueous solutions of sodium hyaluronate with an added surfactant plus other proprietary additives or through modification of the sodium hyaluronate itself, which enables crosslinking. The covalent bond between the topcoat and the basecoat is formed by reaction of the functional groups in the hyaluronate molecule with functional groups in the acrylate resin of the basecoat.

It is important to note that the total thickness of the two coating layers is typically 2 to 5 pm when dry. When exposed to body fluids, they absorb water and swell to approximately 10 urn. They also become extremely hydrophilic and highly lubricious, making these coatings very slippery. Because the coatings are so thin, they remain very flexible and are suitable for use on devices that must pass through tortuous paths, according to Simon. "Using devices with hyaluronan coatings, physicians have the ability to steer and manipulate vascular devices more easily, minimizing trauma and reducing tissue abrasion. These coatings are also quite durable and can be used for procedures that last several hours (crosslinked topcoat) without loss of effectiveness," he explains.

Hydrophilic coatings used for medical device manufacture are very specialized. Each device that is coated required development of a custom coating and coating process to ensure excellent adhesion and performance for the given application, according to Simon. "Hydrophilic coatings, especially as used in the medical field, are not a commodity. They are an advanced, specialized, heavily regulated part of the overall GMP processes developed for making medical devices," he observes. "Coating suppliers must not only have extensive knowledge of coating chemistry, but must also understand the potential biological interactions of the coatings they offer."
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Publication:JCT CoatingsTech
Date:Nov 1, 2010
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