| Summary: | Silicate substitution in hydroxyapatite (HA), with a substitution limit of ~5.2 wt%, has been shown previously to enhance the bioactivity of this material <i>in vivo</i>. The current study describes the substitution of greater levels of silicate <i>via </i>a coupled substitution of yttrium and silicate ions for calcium and phosphate ions, respectively. This was carried out using two different substitution mechanisms, and silicate levels of up to 11.25 wt% were incorporated; these compositions were characterised extensively. There was not a clear correlation between the surface properties of samples produced by the different mechanisms and the growth behaviour of MG-63 osteoblast-like cells, although there was a correlation with substitution mechanism. Following the success in increasing the level of silicate substitution in HA <i>via </i>the co-substitution of the Y<sup>3+</sup> ion, the synthesis of similar materials, but substituting another ion in place of Y<sup>3+</sup>, was attempted. Gadolinium was chosen for its similar size, but primarily because of its use in MRI contrast agents. Similar levels of silicate substitution to the Y<sup>3+</sup>/SiO<sub>4</sub><sup>4-</sup> co-substituted materials were achieved for both mechanisms. An MRI study showed improved contrast of the Gd<sup>3+</sup>/SiO<sub>4</sub><sup>4-</sup> co-substituted compositions compared to stoichiometric HA, which suggests that these new compositions may have clinical applications where bone repair and implant resorption could be monitored using MRI. Carbonate-substituted HA is the subject of the final part of this study. The effect of the synthesis route upon the resulting carbonate location within the lattice is examined. Since CHA can be synthesised to contain sodium ions as a co-substituted ion, or as a sodium-free composition, investigations were carried out to find a synthesis method for both types of composition.
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