Biomaterials exhibit various degrees of compatibility with the harsh environment within a living organism. They need to be nonreactive chemically and physically with the body, as well as integrate when deposited into tissue. The extent of compatibility varies based on the application and material required. Often modifications to the surface of a biomaterial system are required to maximize performance. The surface can be modified in many ways, including plasma modification and applying coatings to the substrate. Surface modifications can be used to affect surface energy, adhesion, biocompatibility, chemical inertness, lubricity, sterility, asepsis, thrombogenicity, susceptibility to corrosion, degradation, and hydrophilicity.
Background of Polymer Biomaterials
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. Teflon is commonly used to reduce friction in biomaterial applications such as in arterial grafts, catheters, and guide wire coatings.
Polyetheretherketone (PEEK)
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 and has been used for biomedical implants due to its relative inertness inside of the human body.
Plasma modification of biomaterials
Plasma modification is one way to alter the surface of biomaterials to enhance their properties. During plasma modification techniques, the surface is subjected to high levels of excited gases that alter the surface of the material. Plasma's are generally generated with a radio frequency (RF) field. Additional methods include applying a large (~1KV) DC voltage across electrodes engulfed in a gas. The plasma is then used to expose the biomaterial surface, which can break or form chemical bonds. This is the result of physical collisions or chemical reactions of the excited gas molecules with the surface. This changes the surface chemistry and therefore surface energy of the material which affects the adhesion, biocompatibility, chemical inertness, lubricity, and sterilization of the material. The table below shows several biomaterial applications of plasma treatments. Abbreviations used in table: PC: polycarbonate, PS: polystyrene, PP: polypropylene, PET: poly (ethylene terephthalate), PTFE: polytetrafluoroethylene, UHMWPE: ultra high molecular weight PE, SiR: silicone rubber
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:
Where is the surface tension at the interface of solid and vapor, is the surface tension at the interface of solid and liquid, and 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.
Plasma Treatment to Reduce Thrombogenesis
Ammonia plasma treatment can be used to attach amine functional groups. These functional groups lock on to anticoagulants like Heparin decreasing thrombogenicity.
Covalent Immobilization by Gas Plasma RF Glow Discharge
Polysaccharides have been used as thin film coatings for biomaterial surfaces. Polysaccharides are extremely hydrophilic and will have small contact angles. They can be used for a wide range of applications due to their wide range of compositions. They can be used to reduce the adsorption of proteins to biomaterial surfaces. Additionally, they can be used as receptor sites, targeting specific biomolecules. This can be used to activate specific biological responses.