## 3 Results and Discussion  
### Morphological Aspects of the Surface  
This study investigates the surface modification of Ti-6 Al-4V samples machined using electrical discharge machining (EDM), aiming to evaluate their morphological, chemical, and mechanical characteristics for potential biomedical applications. Specifically, the morphology, chemical composition, and hardness of the recast layer were analyzed, along with the influence of two dielectric fluids and different electrode polarities. In addition, in vitro cytotoxicity analysis was performed to assess the biocompatibility of the modified surfaces. Considering the critical role of surface roughness (at the micrometric scale) and chemical composition in the performance of biomedical devices, two distinct dielectric fluids were selected for the EDM process. Beyond modifying surface topography, EDM can also enable ionic implantation, depending on the chemical nature of the dielectric fluid used [49; 57]. One significant chemical property of the surfaces in such biomedical devices is related to the control of the oxide layer formation [3; 58]. Thereby, a fluid-containing fluorine was selected due to its high electronegativity and pronounced corrosive effect, which makes it able to chemically etch the surface of titanium parts. Another selected fluid was composed of calcium and phosphorus ions, known for their role in skeletal composition and involvement in bone repair processes. Samples subjected to EDM, not only using dielectric fluids DF1-F but also with DF2-Ca, acquired a rough visual and tactile appearance on the surfaces exposed to electrical discharges in both polarities, as shown in the insets in Figure 1.  
The SEM micrograph in Figure 1A shows the surface topography of samples machined with DF1-F in negative polarity. The surface exhibits a microrelief formed by molten and re-solidified material, along with the presence of small pores. Samples processed with the same fluid but under positive polarity, as shown in Figure 1B, exhibited deep and wide cracks. The surface resembled a scaly structure, with features distributed across the image in a gradient of different sizes. This morphological pattern is commonly associated with materials that develop high residual stress during solidification, which can lead to surface cracking [59]. Additionally, distinct darkened regions were observed, which is consistent with the intense heat generated during discharge. This thermal energy may partially burn the tool, producing carbon compounds that can react with molten titanium and lead to the formation of metallic carbides [60]. Figure 1C,D show the surface morphologies of samples machined in DF2-Ca, a fluid formulated with calcium and phosphorus elements, using negative and positive polarities, respectively. In Figure 1C, the surface exhibits a broad region of molten and re-solidified material, with asymmetrical rippled structures and tiny open pores visible on the left side of the image. In the Figure 1D, the deposited layer consists of prominent islands distributed over the titanium surface, exhibiting a non-uniform morphology. These regions resemble discontinuous clusters of elongated structures, encrusted in a manner similar to splashes of viscous material approximately 50 \(\upmu\)m in width. The formation of such morphologies can be explained by the thermal and physicochemical mechanisms involved in the EDM process. In this process, electrical energy is converted into thermal energy due to the plasma channel formed between the two electrodes. The resulting temperature is sufficient to melt the surface of both the workpiece and the tool. At these extreme temperatures, the dielectric medium breaks down, altering the workpiece surface by incorporating dissociated dielectric materials. Additionally, the subsurface experiences a heat-affected zone that does not melt but is exposed to high temperatures [61; 62; 63; 64].