The effects of substrate roughness and associated surface properties on the biocompatibility of diamond-like carbon films

Diamond-like carbon (DLC) films have many excellent properties such as high hardness, low friction coefficient, chemical inertness, and biocompatibility. Their good biocompatibility is a result of their chemical composition containing only carbon and hydrogen which are biologically compatible. Previous studies on biocompatibility of DLC films focus primarily on morphological issues like roughness and wettability. However, we believe that the stress and the associated surface properties in the films are equally important. In exploring these effects, the silicon substrates were roughened with 30 wt.% KOH in different etching time. The substrate roughness was increased by longer etching time. Three different types of samples - silicon without DLC film, smooth silicon with DLC thin film and rough silicon with DLC - were prepared by culturing the human endothelial cell ECV304 for 72 h. The DLC films were examined by AFM, Raman spectroscopy, XPS, sessile drop and surfcorder to study the morphology, chemical bonding, wettability and residual stress. For DLC surface properties, as the substrate roughness increases, the residual stress increases and the sp3/sp2 ratio decreases. Meanwhile, the surface became more hydrophobic. The stress caused by the substrate roughness seems to be correlated with the surface properties of DLC. As for the ECV304 cell culture, the viability for bare silicon without DLC is almost zero. This indicates that silicon is not biocompatible for ECV 304. The cell number of ECV in DLC/rough substrate is less than that in DLC/smooth substrate in a 72-hour cell culture. This implies that a DLC/Si whose surface properties are associated with higher stress is adverse to ECV cell culture.