In order to develop new bioactive calcium phosphate (CaP) materials to repair bone defects, it is important to ensure these materials more closely mimic the non-stoichiometric nature of biological hydroxyapatite (HA). Typically, biological HA combines various CaP phases with different impurity ions, which substitute within the HA lattice, including strontium (Sr 2+), zinc (Zn 2+), magnesium (Mg 2+), carbonate (CO 3 2-) and fluoride (F-), but to name a few. In addition to this biological HA have dimensions in the nanometre (nm) range, usually 60 nm in length by 5-20 nm wide. Both the effects of ion substitution and the nano-size crystals are seen as important factors for enhancing their potential biofunctionality. The driving hypothesis was to successfully synthesise nanoscale hydroxyapatite (nHA), co-substituted with strontium (Sr 2+) and zinc (Zn 2+) ions in varying concentrations using an aqueous precipitation method and to understand their chemical and physical properties. The materials were characterised using Fourier Transform Infrared Spectroscopy (FTIR), X-Ray Diffraction (XRD), X-Ray Photoelectron Spectroscopy (XPS) and Transmission Electron Microscopy (TEM) techniques. The FTIR, XRD and XPS results confirmed that the nHA was successfully co-substituted with Sr 2+ and Zn 2+ , replacing Ca 2+ within the nHA lattice at varying concentrations. The FTIR results confirmed that all of the samples were carbonated, with a significant loss of hydroxylation as a consequence of the incorporation of Sr 2+ and Zn 2+ into the nHA lattice. The TEM results showed that each sample produced was nano-sized, with the Sr/Zn-10% nHA having the smallest sized crystals approximately 17.6 ± 3.3 nm long and 10.2 ± 1.4 nm wide. None of the materials synthesised here in this study contained any other impurity CaP phases. Therefore, this study has shown that co-substituted nHA can be prepared, and that the degree of substitution (and the substituting ion) can have a profound effect on the attendant materials' properties.

Control of the corrosion that occurs in magnesium alloys in vivo is a significant challenge for their use as resorbable orthopaedic implants. In this work, we report on the provision of bioactive calcium phosphate (CaP) coatings on magnesium alloys that can delay substrate corrosion while offering an attendant physiochemical environment with properties known to promote an osteoinductive response in vivo. RF magnetron sputtering from hydroxyapatite (HA) powder targets has been employed to create CaP coatings on AZ31 magnesium alloy substrates. Coatings of 70 and 210 nm thickness were achieved via regulation of sputtering parameters, in particular deposition time. XPS and ToF-SIMS were used to investigate the chemistry of the coating alloy interface and also to confirm composition and thickness. The Ca/P atomic ratio of the coatings was determined by EDX to be 1.54. μCT analysis showed a substrate volume loss after 14 days exposure to SBF of 5.89 ± 3.15 mm 3 for the un-coated alloy while the presence of the ~70 nm CaP coating reduced this to 3.42 ± 0.48 mm 3 and the ~210 nm coating to 0.30 ± 0.28 mm 3. The corrosion rates were calculated to be 1.74 ± 0.06 mmpy for the AZ31 control; 1.57 ± 0.09 mmpy for the ~70 nm CaP coated alloy and 1.01 ± 0.07 mmpy for the ~210 nm thick CaP coating. These data confirm that CaP coating

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