An overview on diamond-like carbon coatings in medical applications

• Overview on diamond-like carbon (DLC) coatings in medical applications
• DLC shows no significant wear up to 101 million articulations on a spine simulator.
• Stress corrosion cracking in the interface material can cause slow delamination.
• Proteins can trigger crevice corrosion and cause implant failure after some years.
• Simulator testing of implants will not accelerate corrosion driven delamination.

This overview article on diamond-like carbon (DLC) coatings in medical applications covers the interaction of cells and tissue with DLC and alloyed DLC to generate desired cell reactions as well as the release of toxic elements. The promising in vitro results of DLC to prevent thrombus formation in vascular applications are summarized and the problems of transferring these results to in vivo applications are described. Since DLC shows extremely low wear in technical applications, a desired implementation is also to build wear particle free, articulating joint replacements by coating the bearing surfaces. Several in vivo studies using DLC coatings on articulating joints resulted in a failure due to partial coating delamination some years after implantation. It will be shown that this delayed delamination was caused by crevice corrosion (CC) of the adhesion-promoting interlayer and the reason why different corrosion processes may prevent an easy transfer of a successful technical application of DLC onto an implant will be discussed. The main issue for successful, long-term, in vivo applications of DLC coatings on implants is to predict the in vivo survival time and especially the long-term adhesion stability of the coating. The formation at the interface of a few atomic layers of reaction products, usually a metal-carbide, will be addressed. Furthermore, any contamination from residual gas or any cross contamination will result in a different reactively formed interface material with different properties. Delamination can then occur by a slowly advancing crack in this thin carbidic layer governed by the laws of stress corrosion cracking (SCC). It will be shown that if a stable coating adhesion is obtained, DLC coated articulating implants show basically no wear of the coating up to 101 million articulations on a simulator corresponding to about 101 years of in vivo articulation.