Graphite coatings for biomedical implants: A focus on anti-thrombosis and corrosion resistance properties

Stainless steel is the metal of choice for medical implants owing to excellent mechanical properties, hemocompatibility and flexibility; nevertheless, heavy metals like nickel, chromium and molybdenum are a point of concern because of their ability to evoke inflammatory and coagulation cascades resulting in implant failure. In present study, stainless steel substrate was coated with graphite through physical vapor deposition (PVD) to improve hemocompatibility, antithrombosis and anti-leaching properties. Both nano- and micro-meter thick coatings were optimized for the microstructure, roughness, and strength parameters. The hemocompatibility and antithrombosis properties of coating were evaluated through hemolysis and blood coagulation testing. Lastly, the corrosion potential of coating and material leaching from coated specimen was quantified by means of atomic absorption spectroscopy and its effects were evaluated on the material surface and simulated body fluids. A direct relationship among smoothness, strength, and coating thickness was observed. After the coating, the hydrophilic character of the specimen enhanced from 108° to 90.8° which increased the anticoagulation properties and less platelets adhered to the coated surface as compared to bare surface. The hemocompatibility evaluation demonstrated 1.09% hemolysis potential of coated specimen as compared to 1.8% of bare specimen. The rate of corrosion of coated specimen was found to far less than bare specimen (1.4 mpy vs 22 mpy). The leaching studies proved no signs of redox reaction and material release from coated specimen under accelerated aging conditions. In conclusion, the PVD based graphite promoted antithrombosis and anti-leaching properties making it a potential candidate for biomedical implant coatings.