# Surface Characteristics and Cell Adhesion Behaviors of the Anodized Biomedical Stainless Steel
## 3 Results and Discussion  
Figure 2 presents the FE-SEM micrographs of the unmodified 316L BSS specimen and 316L BSS specimens following various anodized voltages for 5 min. Figure 2a is the 316L BSS surface without any modification. No porosity feature can be found on the specimen surface, while a porous structure was observed on the 316L BSS surface following various modified voltages, as shown in Figure 2b through Figure 2d. It was found that dual porous (micro/nanoporous) microstructures were formed on the 316L BSS surface when the anodized voltage increased up to 5 V. This feature can also be observed in the modified surface at 8 V for 5 min. The dual porous structure was related to the anodic voltage. The improvement in implant performance is due to the thick oxide layer and porosity because it enhances the adhesion between the artificial bone and genuine bone. In addition, the metallic ion can be restrained by thick oxide film. A similar porous structure can also be found following dipping in the solution of NaOH at high temperature for a long duration [23; 24]. Figure 3 shows the XPS spectrum of the 1s orbital of oxygen atom (O 1s), which was taken from the 316L BSS specimen with anodization at 5 V for 5 min. Chemical bonding states relating to O are detected during anodization. After anodizing, the position of O 1s shifted significantly, corresponding to O 1s at 531 eV. The emission peak at ~577.5 eV is attributed to the presence of O atoms or molecules in the interstitial sites. As the voltage raised to 8 V, the emission peak at ~577.5 eV is dominated in the spectrum of O 1 s. The findings revealed that some of O atoms did not form covalent and/or ionic bonds with Cr atoms following the anodization. Some inserted O atoms segregate as functional impurities in the grain boundaries and interstitial sites of the oxide film. It is indicated that enhancement of Cr-O bonding is formed via anodization following various voltages. Accordingly, it reveals that the Cr oxides (Cr\({}_{2}\)O\({}_{3}\)) were formed on the anodized 316L BSS surface with a higher voltage above 5 V. Similar results were investigated by previous reports [25; 26]. Figure 2: FE-SEM micrographs of the unmodified 316L biomedical stainless steel (BSS) specimen and 316L BSS following various anodized voltages for 5 min: (**a**) unmodified, (**b**) anodized at 3 V, (**c**) anodized at 5 V, and (**d**) anodized at 8 V. (Microores and nanopores are indicated by white and black arrows, respectively). Figure 4 depicts TEM micrographs of the unmodified 316L BSS and anodized 316L BSS specimens. Figure 4a is a bright-field image of the unmodified 316L BSS, which was taken from the austenite (\(\gamma\)) matrix in the [001] zone under two-beam condition, showing the grain structure within the \(\gamma\) matrix. No other intermetallic compounds or precipitates were detected in the matrix. Figure 4b shows a bright-field micrograph of the anodized 316L BSS specimen at 5 V for 5 min. The figure indicates the formation of different microstructures (as indicated by A and B) in the matrix. The absence of diffraction points from area A reveal the formation of an amorphous structure on the oxidized surface (Figure 4c). From the area B, the pattern with a diffraction ring feature replaces the non-diffraction spots pattern (Figure 4d). It proves that area B belongs to a nanocrystalline structure. Based on the reflection spots corresponding to camera length and \(d\)-spacings, the precipitate with a nanocrystalline structure was determined to be Cr\({}_{2}\)O\({}_{3}\). The microstructural characteristics can also be found in the anodized specimen at 8 V for 5 min. Therefore, the oxide layer microstructure of the anodized 316L BSS with a treatment voltage above 5 V was amorphous \(\gamma\)-phase containing nano-Cr\({}_{2}\)O\({}_{3}\). Typically, the formation of an amorphous microstructure must have a high cooling rate [27]. Phase transition during supercooling resulted in the variation of microstructures in the oxide layer.