## 4 Conclusions  
This study demonstrated the successful development of a sustainable and multifunctional coating system for Ti-6 Al-4V substrates, based on a thermally oxidized TiO\({}_{2}\) interlayer and a hydroxyapatite (HAp) top layer synthesized via a green route using _Hylocereus undatus_ extract. Structural and compositional analyses confirmed the effective formation of both layers, with the TiO\({}_{2}\) film enabling uniform and adherent electrophoretic deposition of the HAp coating. The bioactivity of the coatings was evidenced by the formation of apatite-like structures after immersion in simulated body fluid (SBF), confirming their osteoconductive potential. The antibacterial properties of the coated surfaces were assessed against _Staphylococcus aureus_ using SEM imaging and CFU quantification. The HAp-coated surface exhibited the most pronounced antibacterial effect, suggesting a combination of multiple mechanisms. These results support the hypothesis that the antibacterial efficacy of the HAp-coated surface is not solely attributable to its composition or surface topography but is also amplified by the presence of phytochemicals derived from the \(H\). _undatus_ extract. Together, these findings highlight the potential of this environmentally friendly approach to produce implant coatings that effectively promote bioactivity and reduce bacterial colonization, representing a promising solution for next-generation orthopedic and dental implants.  
Figure 16: Top-view SEM micrographs of _Staphylococcus aureus_ populations after 24 h of incubation (**a**) with _Hylocereus undatus_ (dragon fruit) cactus extract and (**b**) with TSB medium (control). Insets display the antibacterial activity as determined by the inhibition halo test. Yellow circles indicate the sample area, while yellow dashed circles delineate the zones of inhibition.  
**Author Contributions:** Conceptualization, M.R., D.S. and S.D.; methodology, M.R., D.S. and S.D.; validation, M.R., D.S. and S.D.; formal analysis, M.R., D.S. and S.D.; investigation, M.R., D.S., Z.B., P.F. and S.D.; resources, P.F. and S.C.; data curation, M.R. and D.S.; writing--original draft preparation, M.R. and D.S.; writing--review and editing, M.R., D.S., Z.B., R.A., P.F., S.C. and S.D.; visualization, M.R., D.S. and S.D.; supervision, Z.B., S.C. and S.D.; project administration, R.A. and S.C.; funding acquisition, S.C. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research is sponsored by national funds through FCT--Fundacao para a Ciencia e Tecnologia, under projects UIDB/00285--Centre for Mechanical Engineering, Materials and Processes and LA/P/0112/2020. This research was also supported by project DRIVOLUTION--Transition to the Factory of the Future (7141-02/C05-i01.02/2022.PC644913740-00000022-23), which was financed by the PRR--Recovery and Resilience Plan--and by the Next Generation EU European Funds, following NOTICE No. 02/C05-i01/2022.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The raw data supporting the conclusions of this article will be made available by the authors on request.  
**Conflicts of Interest:** The authors declare no conflict of interest.