Plasma-assisted deposition of nitrogen-doped amorphous carbon films onto polytetrafluoroethylene for biomedical applications

With growing demand for cardiovascular implants, improving the performance of artificial blood-contacting devices is a task that deserves close attention. Current prostheses made of fluorocarbon polymers such as expanded polytetrafluoroethylene (ePTFE) suffer from early thrombosis and require periodic replacement. A great number of attempts have already been made to improve blood compatibility of artificial surfaces, but only few of them found commercial implementation. One of the surfaces under intensive research for cardiovascular use is amorphous carbon-based coatings produced by means of the plasma-assisted deposition. However, this class of coatings can be produced using various techniques leading to a number of coatings with different properties. Carbon coatings produced in different plasmas may be of hard diamond-like type or soft graphite-like type, doping with different elements also changes the surface structure and properties. Taking this into account, the search for blood-compatible coating requires the understanding of surface composition and structure and its influence on blood-compatibility. This work attempts to advance our knowledge of this field. Here, commercial PTFE thin film was used as a working material, which composition corresponds to the composition of modern ePTFE vascular grafts and which compatibility with blood we tried to improve by deposition of nitrogenated amorphous carbon (a-CN) coatings in the plasma. Biocompatibility was assessed by a number of tests including the interaction with whole blood and various cells such as platelets, endothelial cells, neutrophils, and fibroblasts. Most of tests showed the blood compatibility of coated surface is better than that of untreated PTFE. Physico-chemical and morphological properties of coated surfaces were studied in parallel using x-ray photoemission spectroscopy (XPS), electron energy loss spectroscopy (EELS), x-ray absorption spectroscopy (XAS), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM). Some correlation between the structure of coatings and blood compatibility was inferred. It was found that at first nitrogen incorporation into amorphous carbon film stimulates blood compatibility. However, when nitrogen fraction increases over 23-25 %, no further improvement but reduction of blood compatibility was observed. Conclusion is made that for best biomedical performance, nitrogen percentage in a-CN coatings must be adjusted to the optimum value