Soft diamond-like carbon coatings with superior biocompatibility for medical applications

Biomedical applications of diamond-like carbon (DLC) coatings are continuously increasing due to their superior mechanical properties, low friction coefficient, antiwear characteristics, and biocompatibility. The mechanical and tribological characteristics of DLC coatings have been comprehensively investigated on various substrate systems as a function of their deposition parameter dependant features for various biomaterial applications. However, the relationship between biocompatibility and resulting hardness of DLC coatings as a function of their bias voltage driven intrinsic features like sp2 and sp3 bonds remains largely unexplored. In this work, a series of DLC coatings are prepared as a function of varying bias voltage from 0 to -120 V, and characterised for their atomic structure, physical, and mechanical properties, and biocompatibility. The contact angle and surface roughness of the DLC coatings reduce while hardness increases from 7.8 to 20.3 GPa with increasing bias voltage from 0 to -120 V. A relatively soft DLC coating is shown to retain excellent biocompatibility which is approximately 38% higher than the harder DLC coatings following exposure of their leached extracts to L929 mouse fibroblast cells. This work demonstrates the complex interdependence of biocompatibility and hardness of DLC coatings and the outcomes will support correct material selection with an appropriate balance of these key properties for specific biomedical applications such as load-carrying and non-load carrying devices.