**4. Conclusions**

An aqueous sol-gel method was used for the synthesis of calcium hydroxyapatite (Ca10(PO4)6(OH)2; CHA) thin films on medical grade stainless steel substrates with transverse and longitudinal patterned roughness. Each layer in the preparation of CHA multilayers (15, 20, 25, and 30) was separately annealed at 850 ◦C in air. According to the XRD analyses results, the most crystalline CHA sample was obtained with 25 spin-coating and annealing layers. The FTIR and Raman spectroscopy results were in good agreemen<sup>t</sup> with the XRD analysis data, confirming the formation of good quality CHA coatings on the rough surface of stainless steel. The formation of CHA coatings was also confirmed by XPS analysis. The chemical bonding such as P–O and Ca–O were noticed in the CHA thin films using XPS measurements. The samples with 25 layers had the lowest friction under both 1 and 5 N loads. The contact angle of coatings (113◦–116◦) showed the formation of hydrophobic layers of CHA. CHA coated samples were immersed in simulated body fluid (SBF) for 2, 3, and 4 weeks and formed amorphous calcium phosphate (ACP). However, the microstructure of SBF soaked CHA samples was not influenced by immersion time or by the number of layers on the substrate, despite the phase composition and crystallinity differences. All CHA coatings after soaking in SBF were more hydrophilic compared to the initial samples, which could improve ossteointegration and promote bone

cell proliferation for a better bone-implant connection. ACP is biocompatible and a precursor for CHA, indicating this coating material may be suitable to promote cell proliferation and ALP activity.

**Author Contributions:** Formal Analysis, V.J., A.Z. and A.K..; Investigation, V.J., S.S., S.W.C., R.J., A.S. and T.M.; Resources, T.C.K.Y., K.I. and R.R.; Data Curation, V.J.; Writing—Original Draft Preparation, V.J.; Writing—Review and Editing, A.K.; Visualization, V.J.; Supervision, S.W.C. and A.K.

**Funding:** This research was funded by The Japan Society for the Promotion of Science (JSPS). Fellow's ID No.: L12546.

**Conflicts of Interest:** The authors declare no conflict of interest.
