**5. Conclusions**

The work performed in this study investigated HA and SiHA RF/Pulsed DC magnetron sputtered thin films as coatings for orthopaedic applications. As deposited HA thin films were found to be amorphous or nanocrystalline, however, upon annealing (600 ◦C for all samples, except SiHA3, which required 800 ◦C) recrystallised. Furthermore, the addition of silicon to HA thin films inhibited HA crystallite growth as demonstrated by crystallite sizes calculated from XRD line broadening.

Both EDX and XPS showed a reduction in Ca/P ratio with increasing silicon content due to possible creation of a P–Si–O chemical species during deposition, which allows P to reach the substrate more readily. After annealing, however, the Ca/P ratio increased with increasing temperature, likely due to the evaporation of volatile phosphate species, facilitated by silicon inclusion destabilising the HA structure. XPS demonstrated that deposited SiHA thin films contain polymerised silicate networks, transforming to monomeric states after annealing, suggesting SiO4 4− substitution in the HA lattice for PO4 3− chemical species.

The roughness was shown to be similar for all as deposited films, which were measured to be approximately 20 nm (Ra), similar to the CP-Ti substrate, which increased following annealing; this was inversely proportional to the silicon content, due to silicon inhibition of the HA crystallite growth and the rise of rutile grain growth. Silicon is also known to inhibit rutile growth, thus explaining the lowering in roughness with increasing silicon content. Water contact angle testing demonstrated silicon addition to the HA structure increased hydrophilicity with increasing silicon content.

Initial adhesion, proliferation and differentiation assays all suggested HOBs preferred HA to the SiHA surfaces, due to silicon doping destabilising the HA thin films, removing the protein conditioning layer essential for normal cell adhesion and growth. HOBs on the highest silicon doped HA thin films annealed at 600 ◦C showed some proliferation due to the stable CPTi substrate surface becoming available for protein mediated cell adhesion. After annealing at 700 ◦C, no significant difference (p > 0.05) was seen between the HA and the SiHA1 surfaces, suggesting enhanced cellular response due to crystallinity levels.

This study ultimately demonstrates that for higher (one of the highest tested in the literature), meta-stable doping levels of Si into the HA structure, cellular response is strongly linked to the crystallinity of the produced HA and SiHA films, the surface stability, as well as other properties, such as surface wettability, roughness, etc. Despite literature studies showing small doping levels of Si have a positive e ffect on cellular proliferation, this is not seen in higher-doped systems and, therefore, careful optimisation is required to glean appropriate properties.

**Author Contributions:** Conceptualization, S.C.C., G.S.W., C.A.S. and D.M.G.; methodology, S.C.C., G.S.W., C.A.S. and D.M.G.; validation, S.C.C., M.D.W., R.M.F., I.A., G.S.W., C.A.S. and D.M.G.; formal analysis, S.C.C., and M.D.W.; investigation, S.C.C.; resources, S.C.C., G.S.W., C.A.S. and D.M.G.; data curation, S.C.C. and M.D.W.; writing—original draft preparation, S.C.C., M.D.W., R.M.F., and D.M.G.; writing—review and editing, S.C.C., M.D.W., R.M.F., I.A., G.S.W., C.A.S. and D.M.G.; visualization, S.C.C., M.D.W., R.M.F., I.A., G.S.W., C.A.S. and D.M.G.; supervision, G.S.W., C.A.S. and D.M.G.; project administration, G.S.W., C.A.S. and D.M.G.; funding acquisition, G.S.W., C.A.S. and D.M.G. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported through funding from Teer Coatings Ltd.

**Acknowledgments:** The authors would like to thank all technical sta ff who aided in equipment operation, particularly Keith Dinsdale, George Anderson, Martin Roe, Nigel Neate, Julie Thornhill, Tom Buss, Rory Screaton, and Graham Malkinson.

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