The widely adopted trend for minimally invasive surgical procedures has catapulted innovations in material science technologies, specifically within the coatings realm. As the surgical sites become smaller and smaller, there is a corresponding increase in the engineering complexity and with that, ultimately, the material technology used to assist in those innovations.

During surgery, the body’s inherent defense mechanisms race to the site of the device insertion or implantation to attack and decimate the foreign substances. The use of coatings has become increasingly important to the medical device industry to promote a nonreactive relationship (biocompatibility) to mitigate the interaction of the device in contact with the surrounding tissue or body fluid. As a direct result, coating technologies have emerged as a prominent component used to enhance device characteristics. Examples of this are prevalent in drug-delivery coatings; antimicrobial coatings used to promote healing and decrease infection at the surgical or incision site; lubricious coatings for heightened ease of insertion and decreased friction in use; and improved biocompatibility to resist restenosis and thrombus formation associated with implantation or insertion.

Interest in coating technologies has greatly increased within the medical device industry through the influx of convergent technologies and the resulting combination products that incorporate a drug and/or biologic with the device, including antimicrobial catheters and drug-eluting stents (DES). As a result, the FDA Office of Combination Products was established in 2002. It has characterized devices to be limited contact (less than 24 hours), prolonged contact (24 hours to 30 days), or permanent contact (over 30 days). This interest in coatings has also escalated the value of early-stage discussions between medical device design engineers and material science companies so as to incorporate the device’s coating requirements into the initial design phase of the production.

There are many factors to consider when entering into a partnership with a material science or coatings company.

Determining your coating needs

What performance criteria are required for the coating? Just as different jobs require different levels of expertise, the performance characteristics for each coating application differs based upon the device requirements.

Biodurability and biocompatibility. Taking into consideration the length of time (limited, prolonged, or permanent) that the device is expected to perform in vivo or externally will directly influence the material selection indicative to the biodurability of the coating.

Surface cracking, commonly known as environmental stress cracking (ESC) has been attributed to biochemical and cellular interactions at the surface of the implanted material, causing polymer chain cleavage. These cracking effects are believed to be due to mechanical stress combined with the oxidizing actions of macrophages and giant cells. These cells form part of the body’s immune response and use enzymes and oxygen species (O2 and H2O2) to degrade foreign material.” Biodurable materials can be used to help eliminate surface microfissures resulting from ESC.

Lubricious andhydrophilic coatings. Many medical devices are manufactured using plastics, elastomers, ceramics, metals, or alloys, some of which have an inherently high coefficient of friction (COF). COF is the measurement of the friction between two surfaces—generally a number between 0 and 1—which indicates the ease with which a material can pass over a surface. With a higher COF, more force will be required to move across a surface. A low COF allows for ease of insertion and maneuverability within the body, and, therefore, is an essential characteristic in many devices.

Some hydrophobic coatings, such as PTFE, silicone fluids, silicone polymers, and oils, can be used to lower the COF. Hydrophobic coatings detract water and are generally used as protective coatings to curtail exposure of fluid to the device (e.g., a pacemaker contains electrical impulses that would fail if exposed to liquid). However, these materials may not be suitable for all applications, due to some potential ancillary characteristics such as component leaching, stiffness, biocompatibility, and tackiness.

An alternative is to use a hydrophilic material, which attracts, absorbs, and retains water. Hydrophilic coatings are widely used today in minimally invasive devices. Coatings of this type can be used to increase the functionality of the device through heightened lubricity, thus reducing friction against patient tissue and allowing for ease of insertion and maneuverability throughout the body. Many types of hydrophilic coatings used on medical devices can allow for varying ranges of frictional coefficient and selection is typically dependent on factors such as percentage of water uptake and linear expansion, chemical composition, and surface modification abilities as a result thereof.

Antimicrobial materials. The wide use of polymeric materials in medical devices has been associated with an increasing incidence of patient infections. This phenomenon is particularly common with indwelling catheters, especially when those catheters are used for extended periods of time. Antimicrobial coatings have, as a result, emerged as an alternative solution to requalification of a substrate material.

Alternatively, some manufacturing techniques and processes have been explored to provide a fully homogenous antimicrobial material, thus ensuring uniform dispersion of the antimicrobial agent throughout the finished polymer.

What coating process will be used? Many coating techniques are available industry-wide, and each is created with specific application requirements. With each application comes a different process in which to apply the coating to a substrate or device. The application technique is a critical step in ensuring lot-to-lot consistency and full coating adhesion. With each application comes a different process, each with inherent characteristics that should be coordinated with the coating goals so as to achieve the highest level of optimization.

Some of the application techniques utilized in medical coatings include, but are not limited to brush, dip, spray, UV, vacuum deposition, knife, etc. In order to properly choose the right application method for a chosen polymer, one must look into several different factors:

• The area of the device to be coated
• The substrate being coated
• Final coating thickness requirements
• Solvent package requirements
• Solution concentrations
• Sterilization
• Curing

Determine restrictions to coating alternatives. While establishing the coating’s performance criteria from the onset of the project helps to guide the material selection process, consideration should also be made to where the device will need to be coated.

In considering which type of coating will optimize a device’s performance, the surface geometry becomes a critical factor in determining the coating material and process. For example, the surface expansion of an outer lumen, while important, will not be as critical as maintaining a smaller surface expansion for the inner lumen. Or, if the device has a complex surface geometry, the coating process will be dictated by which method can apply a consistent and reproducible coating on the component.

Which substrate will need to be coated? Most coatings are tailored to meet the adhesion specifications for the material substrate. Typical substrates for medical devices include, but are not limited to, the following: polyurethane, polycarbonate, nitinol, nylon, Pebax, polytetrafluoroethylene (PTFE), polyetheretherketone (PEEK), polyvinylchloride (PVC), silicone, stainless steel, and many more. Before embarking on the coating process, steps must be taken to ensure that the device substrate is compatible with the coating material to ensure the coating will not leach, flake, or fall off the device.

In some instances, a thin layer of a bonding agent may be required to ensure proper coating adhesion. Acting as a primer, this bonding agent can adhere dissimilar or similar materials together, thereby opening up the options for selecting a coating material.

Coating thickness. Coating thickness is a critical part of the medical device coatings. The final dry thickness desired will be dependent on each individual application and the desired performance characteristics of the coating itself. The final dry coating thickness is relative to coating material, and the application method. Coating thickness also modifies coating flexibility and adhesion to the substrate.

For example, one of the main uses for a hydrophilic or lubricious coating is to allow the device to slide and maneuver with ease throughout the body. A thin coating, such as a few microns (one-millionth of a meter) to a few mils (a mil is 0.001 of an inch), is sufficient to accomplish this.

Conversely, a hydrophobic protective coating tends to be of a much higher film thickness than a lubricious or hydrophilic coating. The function of protective coatings is to shield the substrate or device from any foreign object or environmental condition that may attack it and influence the device functionality. Therefore, the coating in this case may be required to be thicker in the magnitude of a few mils to many millimeters.

Solvent packages. Typically, polyurethane materials can be manufactured into soluble form by dissolving the polymer into various solvent packages (solutions). In a solution, one replaces the polymer-polymer chemical bonds with polymer-solvent chemical bonds. These solutions can be made using a number of organic and inorganic solvent packages, such as dimethylacetate (DMAc), tetrahydrofuran (THF), an ethanol/water ratio, and a host of other solvent selections.

Typically, a higher durometer polymer is harder to dissolve. Usually a maximum solution concentration is between 10% and 20% solids, for thermoplastic polyurethanes or from 1% to 15% solids for hydrophilic materials.

Some medical polymers are manufactured and supplied as a liquid polymer. These materials are synthesized in solution to achieve specific molecular structure, solid concentrations, etc.

It is imperative to the success of the coating process for the substrate to maintain suitable cleaning preparation (ethylene oxide, gamma radiation, or beta radiation, for example) to remove any oils and contaminants to ensure proper adhesion and to prevent contamination of the coating.

What methods will be used to cure the coating? Many applied coatings cure by solvent evaporation. Some of the popular curing methods available are heat cured, a process using special oven equipment; air or ambient conditions, where the device is left to dry over time; and ultraviolet (UV) curing, in which a UV light is used to harden the coating. UV Curing can only be used on specialized UV coatings.

Cooperative development

While many of the above questions will dictate the materials, process, and capital equipment, the bottom line is the successful launch of the product and enabling the customer’s success.

Initial conversations when embarking upon a coatings project should include identification of each of the above requirements so as to ensure a clear cut and seamless transfer or scale-up of production requirements.

Whether the customer is prepared to bring the technology in-house or maintain the coatings process at the OEM or contract manufacturers, establish clear manufacturing guidelines with concise specifications for your coatings process early to ensure a smooth transfer of the coatings process.

Conclusion

Interest in coating technologies has increased with the advent of convergent technologies and the resulting combination products. But each case is unique and requires careful consideration of required performance characteristics, coating process, and partners and providers.