It takes neurovascular catheters with micro-thin, biocompatible coatings to inject substances into delicate blood vessels such as those in the brain.

Surface coating is a finishing operation often required for disposable medical devices that will be used for tasks such as inserting tubes or catheters into blood vessels. The correct coating enhances lubricity, drug elution, biocompatibility, and anti-microbial properties. Examples of surface coatings with these useful properties come from our Hydak hyaluronan-based materials.

First isolated in 1934, hyaluronan is the human body's natural lubricant. It constitutes a major component in joint-lubricating sinovial fluid and is present in all body tissues. The material was originally isolated from rooster combs and human umbilical cords. Today, most of it comes from purified bacterial sources. Regardless of source, all hyaluronan is chemically identical.

Because hyaluronan is normally present in the blood and gets continuously degraded and renewed by normal biochemical pathways, it is an ideal substance to use for a lubricious coating. During use, any coating might shed particles, or non-bonded coating components might dissolve in blood. This can pose a serious health risk for patients when dissolved particles are not biocompatible.

In contrast, should hyaluronan release from the coating, the body's normal biochemistry easily accommodates it. Interestingly, numerous studies have shown that hyaluronan attached to a surface is not rapidly degraded by hyaluronidases, a family of enzymes that degrade hyaluronic acid. Hyaluronan is also hydrophilic (has an affinity for water).

Coating difficulties

It's wise to consider coatings early in the design cycle. Evaluate a coating's effects on the intended use of the device and its risks. For example, a non-coated device might require excessive force to push it through the vasculature and thus damage the blood vessels. Also evaluate the device's technical requirements as well as the need for a coating to realize a business opportunity. For example, reducing the incision length to place an intraocular lens (IOL) into the eye requires that the cartridge inserting the lens also be reduced in size. This is only possible if the forces required to insert the IOL can be reduced by coating internal areas of the cartridge.

Most disposable devices of the sort discussed here require a coating, unless the path-length traversed is relatively short and straight, and thrombus (clot) formation is unlikely. Design elements set in place early in the cycle, such as the basic choice of materials, can often strongly affect the ease and effectiveness of a coating process.

The surgeons are using disposable medical devices that have been coated with a Hydak hyaluronan-based coating from Biocoat to smoothly insert a tube into one of the patient’s blood vessels.

For instance, plastic substrates usually contain additives such as antioxidants, colorants, and lubricants that may bloom to the surface of the plastic. These can interfere with coating adhesion and performance, regardless of the type of coating selected. Similarly, metallic substrates can have surface oxide layers that are difficult to coat. Surface-preparation methods, including solvent washes and plasma treatment, can mitigate these adverse effects. However, consider preparation methods in relationship to their effects on the substrate and coating.

A two-layer coating

Hydak coatings are bilaminar, consisting of an acrylic base coat that adheres to the substrate and a formulated hyaluronan top coat that covalently bonds to the base coat. One advantage of a two-layer coating is the chemical reaction that binds the hydrophilic layer to the base coat does not vary with different substrates. Consequently, coating performance does not change.

Coatings can be applied by any conventional technology such as dipping or spraying. A layer's thickness depends on the coating formulation and how the material is applied. In general, base coats are about 1 to 5-µm thick and the dry hyaluronan top coat is about 0.05 to 0.5-µm thick. After hydration, depending on the dry coat thickness, the hyaluronan layer can swell to a thickness of up to about 2 to 10 µm. This action is what provides a durable, lubricious, and non-thrombogenic surface.