**5. Conclusion**

From the results obtained here, it is clear that the RF magnetron sputtering has the capability to be utilised to sputter deposit, in a one-step (direct) process, homogeneous, and amorphous CaP surfaces that consist of a porous micro-/nano-structured lattice network containing appreciable levels of both Ca and P at low deposition powers (150W). It was realised that the manipulation of the deposition time had a significant influence on the chemical and physical properties of the resultant thin films, particularly in relation to the surface topography, pore area, pore diameter, and number of pores. The Ca/P of the resultant modification was low; too low in fact to coincide with HA or other bioactive

CaP phases at ~0.85. It was postulated that this was due to the bombardment of the sputtered species coming into the contact with the polymeric material, causing species to embed into the PEEK substrate, which created a matrix interlayer between the CaP material and the PEEK substrate. Further to this, the build-up of a negative charge on the insulation PEEK surface prevented re-sputtering of the P species on the surface of the surface, as would normally be observed. It was indicated that the initial 60 min of sputter deposition could be an erosion/deposition process with the capability of causing physical and chemical alterations to the surface of the underlying polymeric substrate. The next step in this work would be to investigate the mechanical properties and the dissolution behaviour of the surfaces in physiological conditions, and to determine their in vitro potential using osteoblasts.

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

**Funding:** This research was funded by the Department for the Economy (Northern Ireland) and the Meehan Family Scholarship, which facilitated part of the ToF-SIMS experiments.

**Acknowledgments:** The authors would like to acknowledge the support of Invibio Ltd., (Thornton Cleveleys, UK), particularly Mark Brady, who kindly provided PEEK-OPTIMA ™ LT1 samples for these experiments. The authors would also like to acknowledge David Scurr and Matthew Piggott at the Interface and Surface Analysis Centre, University of Nottingham for their assistance with the ToFSIMS experiments.

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