A Review of DLC Coatings for Biological Applications Available to Purchase

Diamond-like carbon (DLC) is a class of materials with outstanding mechanical, tribological and biological properties. From in-vitro experiments, it is known that by incorporating other elements into the DLC film, the ratios of the different proteins adsorbed on the surface can be changed. These proteins will then subsequently control cell attachment, cell proliferation and cell differentiation. In a total hip joint replacement, the metallic femoral head, which slides against a polyethylene pan, causes polymeric wear debris. These wear particles may then trigger inflammatory reactions, resulting in osteolysis (bone resorption) and subsequent implant loosening. DLC has proven its outstanding tribological properties in many technical applications, mainly due to the build up of a transfer layer on the counterpart. DLC coated load bearing implants sliding against ultra high molecular weight polyethylene (UHMWPE) have been investigated. The different in-vitro experiments apparently showed contradicting results, mainly due to the different experimental setups and especially the different liquids used as lubricants. The synovial fluid present in a biological joint, contains large organic molecules which function as a boundary lubricants. Phospholipids, proteoglycans or proteins can be chemisorbed on the joint surfaces and trap water molecules, resulting in water acting as a viscose lubricant. When a DLC coated femoral head is tested against a polyethylene pan in a hip joint simulator, using synovial fluid as a lubricant, the build up of a transfer layer, protecting the softer counterpart (i.e. the polymer) does not seem to take place and the UHMWPE counterpart still shows wear. However, when DLC slides against DLC in medical applications, the build up of a transfer layer may not be a critical issue or is not drastically altered by the presence of proteins, and very low wear rates could be obtained in different in-vitro tests. Additionally, DLC coatings have an excellent haemocompatibility, which is expressed in a decreased thrombus formation. When exposed to blood, an increased ratio of albumin to fibrinogen adsorption, as well as decreased blood platelet activation is observed on coated surfaces. A few DLC coated cardiovascular implants such as artificial heart valves, blood pumps and stents are already commercially available. When coating a metal with DLC, good adhesion is obtained due to the about one nanometer thick metal-carbide reaction layer at the DLC/substrate interface. Upon implantation, it has to be guaranteed that this reaction layer is also chemically long-term stable under in-vivo conditions.