## 3 Results and Discussion
### Oxide Cross-Section Evaluation
#### 3.2.2 Cross-Sectional Oxide Composition Analyses  
Figure 7 shows representative EDS line scans across the cross-sections of the NOJ CP and NOJ MP oxides. The elemental distributions across the cross-sectional thickness are divided into two subgroups. Titanium (Ti) and oxygen (O) distributions are provided in the left column, and the calcium (Ca), phosphorus (P), and magnesium (Mg) oxide dopant uptake levels are provided in the right column. Each line scan begins inside the titanium substrate to the left side of the image and progresses gradually through the oxide-substrate interface, the inner oxide, and then the outer oxide. The cross-sectional composition profiles agree with, and further explain, the bi-layered structures shown and discussed in the oxide thickness measurement section. A titanium dioxide-rich inner layer is evident in both oxides adjacent to the titanium substrate. Previously, in a sol-gel coating study, the presence of a titanium dioxide interlayer between the hydroxyapatite coating and the titanium substrate was reported to improve the substrate-coating adhesion strength [42]. In contrast, the outer layer composition in both oxides revealed a prominent uptake of Ca and P dopants. The outer layer of the NOJ MP oxide also showed a significant uptake of Mg. Another valid point that is well illustrated in the cross-sectional line is the large differences shown in outer layer oxide thickness values between the NOJ CP and NOJ MP oxides. The Figure 8 bar chart shows the EDS-derived elemental quantification of the outermost two microns of each oxide cross-section. Since the outer oxides are the surfaces that would be in direct contact with bodily fluids and tissues, it is of great interest to understand the compositions of these oxides. A summary of the Ca, P, and Mg dopant uptake levels in this outermost oxide is compiled in Figure 8A. The corresponding outermost surface Ca/P ratios for each oxide are provided in Figure 8B. The NOJ CP oxide revealed an outermost surface Ca/P ratio of 1.5 \(\pm\) 0.0, while the NOJ MP oxide showed a Ca/P ratio of 1.2 \(\pm\) 0.1. These outermost surface Ca/P ratios of the NOJ MP oxide were found to be significantly lower than the NOJ CP oxide (\(p\) < 0.0001). Comparing these Ca/P ratio values back to the outer layer line scans shown in Figure 7 reveals another interesting trend. Over approximately the first 30 \(\upmu\)m of the outer oxide layer (inner portion of the outer oxide layer), the NOJ CP and NOJ MP oxides showed similar Ca/P ratios. However, the Ca/P ratio for the NOJ CP oxide was then shown to increase through the outer portion  
Figure 6: Oxide cross-sectional thickness values for each oxide. (**A**) Inner layer oxide thickness values, (**B**) Outer layer oxide thickness values, and (**C**) Total oxide thickness values. The NOJ CP oxides showed a significantly (\(p\) < 0.0001) thicker inner oxide layer, averaging 1.7 \(\pm\) 0.3 \(\upmu\)m, compared to the NOJ MP oxides with an average inner oxide thickness of 1.5 \(\pm\) 0.2 \(\upmu\)m. The NOJ CP oxides exhibited a significantly (\(p\) < 0.0001) thicker outer oxide layer, averaging 77.7 \(\pm\) 27.8 \(\upmu\)m, compared to the NOJ MP oxides, showing an average outer oxide thickness of 34.8 \(\pm\) 13.3 \(\upmu\)m. The thicker outer oxide layer also contributed to a significantly thicker total oxide for the NOJ CP oxide (78.5 \(\pm\) 27.8 \(\upmu\)m), as compared to the NOJ MP oxide (35.5 \(\pm\) 13.3 \(\upmu\)m) (\(p\) < 0.0001). \(\cdot\cdot\cdot\cdot\cdot\cdot\) indicates (\(p\) < 0.0001). of the outer oxide layer. Thus, the increased thickness in the NOJ CP outer layer oxide was also shown to increase the Ca/P oxide dopant ratio closer to the known 1.67 value for stoichiometric apatite. It was also interesting to note that these outermost oxide Ca/P ratios were significantly lower (\(p<0.0001\)) than the corresponding surface-derived EDS Ca/P ratios, shown in Figure 3, for the respective oxides.