Polytetrafluoroethylene (Teflon)

Teflon is a hydrophobic polymer composed of a carbon chain saturated with fluorine atoms. The fluorine-carbon bond is largely ionic, producing a strong dipole. The dipole prevents Teflon from being susceptible to Van der Waals forces, so other materials will not stick to the surface.^[3] Teflon is commonly used to reduce friction in biomaterial applications such as in arterial grafts, catheters, and guide wire coatings.

Polyetheretherketone (PEEK)

Polyetheretherketone (PEEK) is a thermoplastic, semicrystalline polymer. The backbone consists of ether, ketone, and benzene groups

PEEK is a semicrystalline polymer composed of benzene, ketone, and ether groups. PEEK is known for having good physical properties including high wear resistance and low moisture absorption ^[4] and has been used for biomedical implants due to its relative inertness inside of the human body.

Surface Energy

The surface energy is equal to the sum of disrupted molecular bonds that occur at the interface between two different phases. Surface energy can be estimated by contact angle measurements using a version of the Young–Laplace equation:

${\displaystyle \gamma _{SV}-\gamma _{SL}=\gamma _{LV}cos\theta }$ ^[6]

Where ${\displaystyle \gamma _{SV}}$ is the surface tension at the interface of solid and vapor, ${\displaystyle \gamma _{SL}}$ is the surface tension at the interface of solid and liquid, and ${\displaystyle \gamma _{LV}}$ is the surface tension at the interface of liquid and vapor. Plasma modification techniques alter the surface of the material, and subsequently the surface energy. Changes in surface energy then alter the surface properties of the material.

Surface Functionalization

Surface modification techniques have been extensively researched for the application of adsorbing biological molecules. Surface functionalization can be performed by exposing surfaces to RF plasma. Many gases can be excited and used to functionalize surfaces for a wide variety of applications. Common techniques include using air plasma, oxygen plasma, and ammonia plasma as well as other exotic gases. Each gas can have varying effects on a substrate. These effects decay with time as reactions with molecules in air and contamination occur.

Argon plasma used for polymer surface functionalization prior to bonding.

Surface Cleaning

There are many examples of contamination of biomaterials that are specific to the preparation or manufacturing process. Additionally, nearly all surfaces are prone to contamination of organic impurities in the air. Contamination layers are usually limited to a monolayer or less of atoms and are thus only detectable by surface analysis techniques, such as XPS. It is unknown whether this sort of contamination is harmful, yet it is still regarded as contamination and will most certainly affect surface properties.

Glow discharge plasma treatment is a technique that is used for cleaning contamination from biomaterial surfaces. Plasma treatment has been used for various biological evaluation studies to increase the surface energy of biomaterial surfaces, as well as cleaning.^[9] Plasma treatment has also been proposed for sterilization of biomaterials for potential implants.^[10]

Schematic of cleaning of a polymer surface using glow plasma discharge. Note the removal of adsorbed molecules and presence of dangling bonds.

Adhesion of Coatings

In general, the lower the surface tension of a liquid coating, the easier it will be to form a satisfactory wet film from it. The difference between the surface tension of a coating and the surface energy of a solid substrate to which a coating is applied affects how the liquid coating flows out over the substrate. It also affects the strength of the adhesive bond between the substrate and the dry film. If for instance, the surface tension of the coating is higher than the surface tension of the substrate, then the coating will not spread out and form a film. As the surface tension of the substrate is increased, it will reach a point to where the coating will successfully wet the substrate but have poor adhesion. Continuous increase in the coating surface tension will result in better wetting in film formation and better dry film adhesion.^[11]

More specifically whether a liquid coating will spread across a solid substrate can be determined from the surface energies of the involved materials by using the following equation:

${\displaystyle S=\gamma _{SA}+(\gamma _{CA}-\gamma _{SC})}$^[12]

Where S is the coefficient of spreading, ${\displaystyle \gamma _{SA}}$ is the surface energy of the substrate in air, ${\displaystyle \gamma _{CA}}$ is the surface energy of the liquid coating in air and ${\displaystyle \gamma _{SC}}$ is the interfacial energy between the coating and the substrate. If S is positive the liquid will cover the surface and the coating will adhere well. If S is negative the coating will not completely cover the surface, producing poor adhesion.

Guide Wires

Guide wires are an example of an application for biomedical coatings. Guide wires are used in coronary angioplasty to correct the effects of coronary artery disease, a disease that allows plaque build up on the walls of the arteries. The guide wire is threaded up through the femoral artery to the obstruction. The guide wire guides the balloon catheter to the obstruction where the catheter is inflated to press the plaque against the arterial walls.^[14] Guide wires are commonly made from stainless steel or Nitinol and require polymer coatings as a surface modification to reduce friction in the arteries. The coating of the guide wire can affect the trackability, or the ability of the wire to move through the artery without kinking, the tactile feel, or the ability of the doctor to feel the guide wire's movements, and the thrombogenicity of the wire.