## 3 Results
### Characterization of Coatings
Figure 9: SEM and EDS elemental mapping of the HAp coating: (**a**) SEM image (scale bar: 8 \(\upmu\)m); (**b**) SEM image (scale bar: 6 \(\upmu\)m); (**c**) EDS map showing the distribution of Ca; (**d**) EDS map showing the distribution of P (scale bar for c and d: 6 \(\upmu\)m). Studies suggest that electric and electromagnetic energy can be applied exogenously to accelerate bone formation and healing, largely due to surface charges and electrical conductivity that promote bone cell differentiation and proliferation. To gain deeper insight into these effects, one of the most comprehensive approaches is to examine the electrical properties of biomaterials through impedance spectroscopy. In this context, the electrical properties of the modified surfaces were also analyzed using this technique [49; 50; 51]. The complex impedance \(Z^{*}\) is a frequency-dependent parameter that characterizes the electrical response of a material and is defined as \(Z^{*}=Z^{\prime}+|Z^{\prime\prime}|\), where \(Z^{\prime}\) is the real part (resistive component) and \(Z^{\prime\prime}\) is the imaginary part (reactive or capacitive component). The impedance magnitude, \(|Z|\), commonly used for comparative analysis, is derived from these components as \(|Z|=(Z^{\prime 2}+Z^{\prime\prime 2})^{1/2}\)[52; 53]. In this study, the measured values of \(Z^{\prime\prime}\) and \(Z^{\prime\prime}\) were used to calculate and plot \(|Z|\) to evaluate the electrical properties of the different surface modifications. Figure 11 presents the impedance magnitude \(|Z|\) as a function of the frequency for the Ti-6 Al-4V substrate, the thermally oxidized substrate (referred to as TiO\({}_{2}\) pre-coating), and the hydroxyapatite (HAp)-coated sample. At room temperature, both the bare alloy and the TiO\({}_{2}\) pre-coating exhibit perceptibly lower impedance in the low-frequency region when compared to the HAp-coated surface. This behavior is expected, as in ambient air, impedance is primarily governed by the resistive and capacitive characteristics of surface layers. The hydroxyapatite layer, being more resistive and less conductive than the metallic substrate or thin oxide film, acts as an effective electrical barrier, increasing the overall impedance. In contrast, the bare and oxidized surfaces facilitate easier displacement or leakage currents, resulting in reduced impedance. Upon heating to 37 \({}^{\circ}\)C, the impedance of the HAp coating decreases, and the corresponding curve converges with those of the substrate and TiO\({}_{2}\) pre-coating, indicating enhanced charge transport due to thermally activated conductivity. This temperature-driven convergence is consistent with the behavior expected of thermally stimulated conduction mechanisms in solid-state systems. Figure 11: Frequency-dependent impedance magnitude \(|Z|\) for Ti-6 Al-4V substrate, TiO\({}_{2}\) pre-coating, and hydroxyapatite (HAp)-coated sample, measured in air. The main plot shows the data at room temperature and the inset presents the same measurements at 37 \({}^{\circ}\)C.  
At physiological temperature, the observed decrease in impedance of the HAp-coated sample--typically indicative of increased electrical conductivity--may enhance cellular metabolism. Conductive implants facilitate intercellular electrical signaling, thereby promoting essential cellular functions such as adhesion and migration, which are critical for effective tissue integration [54].