Biometallic alloys thrive in fixed orthodontics for their unmatched balance of mechanical, corrosion resistance and biocompatibility properties. Nitinol, Ti-based alloys, and several stainless steel (SS) grades are widely used for manufacturing brackets, archwires, tubes and bands that will remain inside the oral cavity for approximately 2 years. However, intraoral environment is highly dynamic and aggressive for any biomaterial. Oscillations on pH, temperature and chemical composition are enhanced by multiple factors, including diet and hygiene, oral biofilm activity and even the time of the day. The in vivo degradation of metals and alloys is therefore inevitable, disrupting any passive film and releasing metallic ions into the oral cavity. Among those ions, some are toxic and can elicit hypersensitive reactions (allergies). Ni stands out as the International Agency for Research on Cancer (IARC) classifies Ni (II) and all Ni compounds as carcinogenic or potentially carcinogenic to humans [1].  
To overcome this issue, this study follows a solution proposed by Surface Engineering: protective coatings, which may increase corrosion resistance while maintaining the properties of the alloys’ “bulk”. Hydrogenated amorphous carbon (a-C:H) coatings were therefore deposited on the surface of 316L (AISI) SS grade by reactive magnetron sputtering. Variable Ar/CH₄ flow provided different H contents, while N2 inflow allowed doping with N. The coatings’ surface morphology and mechanical properties were then evaluated before and after a 30-day corrosion test in Fusayama-Meyer artificial saliva, while released Ni, Cr and Fe were quantified. Finally, cell viability was assessed in vitro with mono- and co-cultures of macrophages and fibroblasts. This study shows that a-C:H-based coatings are promising surface materials in orthodontics.