New generation silicate and carbonate co-substituted calcium phosphate synthetic bone substitutes : synthesis and characterisation

Synthesis of chemically modified or ion-substituted hydroxyapatite (HA) as a bioactive bone replacement material is of great interest since the main component of hard tissues in vertebrates is a substituted apatite. The most abundant species in natural bone mineral other than calcium and phosphate ions is carbonate ions. Carbonate ions can be substituted at 2 distinct sites in the HA lattice; the hydroxide (OH) site and the phosphate (PO4) site. Synthetic silicate substituted HA (SiHA) and carbonate-substituted HA (CHA) have each previously been shown to enhance new bone formation when compared to HA in vivo. The positive bioactive properties that result, individually, from carbonate or silicate substitution led to the hypothesis that simultaneous co-substitution of these two ions in to the HA lattice would lead to further improved bioactivity. A range of novel silicate and carbonate co-substituted HA (SiCHA) materials were prepared with silicate substitution of up to 12.5 wt% (3.8 wt% Si) and carbonate substitution up to 8.2 wt%. These compositions were characterised extensively, examining both their chemical and physical attributes. These compositions were single phase after sintering in a wet CO2 atmosphere to near-theoretical density, and contained carbonate ions on both hydroxide and phosphate sites. The grain sizes of these ceramic specimens of these compositions were intermediate between those of CHA and SiHA controls. Further controlled synthesis demonstrated that carbonate and silicate ions could be substituted in non-equimolar quantities, and increased amounts of carbonate could be substituted independently on to the hydroxide site of the SiCHA samples. The biological response to these materials was assessed by completing direct and indirect cell culture experiments using both the MG-63 osteoblast-like cell line and primary human osteoblast (pHOB) cells. The silicate and carbonate co-substituted materials and their dissolution products were not toxic to either of the cell lines, and cell proliferation was observed with all materials studied. The effect of the synthesis route on the purity of silicate-substituted hydroxyapatite (SiHA) was also assessed, with a particular focus on the role of tetraethyl orthosilicate (TEOS) as the source of silicate. The outcomes of this study showed that the final composition of SiHA was strongly dependent on how and when the tetraethyl orthosilicate (TEOS) solution was incorporated during the precipitation reaction.EThOS - Electronic Theses Online ServiceGBUnited Kingdo