# Metal Release and Cell Viability of 316L Stainless Steel Sputter-Coated with N-Doped a-C:H Coatings
## 1 Introduction  
Any metallic alloy ultimately corrodes in vivo [1; 2], namely inside the human mouth. However, the first-generation biomaterials are still widely used in applications that requiredemanding mechanical properties, such as orthodontics. Stainless steel (SS), titanium (Ti) and its alloys, and cobalt (Co)-chromium (Cr) alloys are typical representative biometals in the referred medical field, providing the required load to induce tooth movement for correcting malocclusions [3; 4].  
Despite the alloys' biocompatibility--basic criteria for any material to be considered a biomaterial--concerns have been intensified regarding the effects of the intraoral environment in promoting metallic corrosion, which is ultimately accompanied by the release of metallic ions. Some of those ions--such as Ni(II)--cause allergic reactions and can be toxic to cells and tissues [2; 5; 6]. Therefore, further research is required to functionalize these bioalloys, while taking advantage of the already suitable bulk mechanical properties. Among the several surface treatments, coating deposition techniques, and classes of materials reported in the literature--for medical and biotechnological applications and orthodontics in particular [4; 7]--the potential of the amorphous C-based coatings deposited by sputtering must be highlighted.  
Physical vapor deposition (PVD) methods--including sputtering--present several advantages for demanding functional medical applications: it enables the deposition from monolithic to composite materials and equilibrium to metastable structures; it yields high deposition rates; and sputtered coatings exhibit better adhesion compared to others deposited by other methods, namely high purity, coforendliness, uniformity, and high reproducibility [8; 9]. Moreover, H-free and hydrogenated amorphous carbon coatings (a-C and a-C:H, respectively), including the so-called Diamond-Like Carbon (DLC), can be deposited by sputtering and are well-established due to their chemical inertness, and overall biocompatibility [10].  
Following the same approach that has long been used for the development of protective coatings in the mechanical and tribological fields, doping of DLCs with metals and nonmetals has been a common method for bioapplications [11; 12]. Moreover, for certain doping concentration ranges, the amorphous structure of DLC coatings can be preserved, therefore avoiding certain defects of the crystalline networks, such as grain boundaries, that are favored sites for phase precipitation and chemical reactions--such as corrosion. This has been explored recently by the authors, namely by selecting a less used element--nitrogen (N)--as a dopant of the a-C:H coatings produced by sputtering. The findings concerning the bacterial adhesion inhibition corroborate that the best N doping level was 6 at.% [13].  
The main objective of the current research is to understand whether the reactively sputtered a-C:H:N coatings can fulfill the trinomial saliva physicochemical parameters --living tissues--plaque-forming micro-organisms, that characterize the complex oral cavity environment. Aiming at orthodontics, it is intended to complete the study of the C-H-N system onto medical-grade 316L SS, preserving the N concentration up to 10 at.%. For that purpose, in vitro aggressive parameters were selected to better mimic the human mouth. The purpose of this article is twofold: (i) to assess the corrosion behavior of both uncoated and DLC-coated 316L SS substrates; and (ii) to evaluate the resulting in vitro cytotoxicity of their respective eluates in artificial saliva, using monocultures of macrophages and fibroblasts and their coculture.