Li, Jiangling
(2010).
Structural characterisation of apatite-like materials.
University of Birmingham.
M.Res.
Abstract
Hydroxyapatite (HA) is an important material for biomedical applications and has found application as bone substitute and implants coating due to the high similarity to the inorganic material of bone and tooth. Usually the HA lattice in living organism is substituted by different cations in order to help maintaining a normal metabolism. Once the type of substitution or substitution concentration is below or beyond the required level, severe problems may subsequently occur. Therefore, it is a great importance to understand the effect of different substitutions on the HA structure as well as the substitution levels in the HA structure. In this work, the effect of ionic substitutions of Zn\(^{2+}\), Mg\(^{2+}\), Sr\(^{2+}\), Si/ SiO\(_4\)\(^{4-}\), and CO\(_3\)\(^{2-}\), on the HA structure was studied. Emphasis was given to changes in the crystallite size and crystallinity of apatite samples produced by three different methods: chemical precipitation method, hydrothermal method, and bacterial biosynthesis. A general observation was that all samples contained a certain amount of CO\(_3\)\(^{2-}\), HPO\(_{4}\)\(^{2-}\), and in some cases residual nitrates depending on the method of synthesis. The presence of CO\(_3\)\(^{2-}\) is particularly interesting because it seems that CO\(_3\)\(^{2-}\) can easily be present in the apatite lattice replacing either OH\(^-\) and/or PO\(_4\)\(^{3-}\). XRD, FTIR, Raman and \(^{31}\)P MAS-NMR spectroscopy in Zn-HA (S7) and Si-HA (S8-S12) did not show strong evidence of Zn and Si substitution in the apatite lattice most likely because the concentration of Zn and Si was very small in the above mentioned samples. On the other hand, in the case of Mg substitution from 0.7 to 9.1 wt%, the information received by XRD, FTIR, Raman and 31P MAS-NMR spectroscopy was valuable suggesting that 0.7 wt% Mg substitution did not cause strong effect on the apatite structure but a phase transformation from HA to Mg-whitlockite was favoured at and above 1.6% of Mg substitution (S2-S4). Sr substitution was easier (S15-S19) and incorporation of Sr into the apatite lattice was complete at 100 mol% Sr substitution. Rietveld analysis confirmed that substitution of Sr for Ca in the apatite lattice resulted in an increase in the lattice parameters a and c. Interestingly small Sr-HA crystallite size was measured when Sr substitution reached 50 mol%, whereas the largest crystallite size was measured once the Sr substitution reached 100 mol%. In the case of CO\(_3\)\(^{2-}\) substituted HA samples (S13-S14), increasing CO\(_3\)\(^{2-}\) substitution from 0 to 21 wt% resulted in a decrease of the crystallite size from 42 to 16 nm, and formation of CaCO\(_3\) was observed in the case of S14, when CO\(_3\)\(^{2-}\) reached 21 wt% substitution. The bacterially derived biological HA (S20) contained both A- and B-type CO\(_3\)\(^{2-}\) due to the bio-mineralisation process during the biological synthesis. The biological HA exhibited the smallest crystallite size (8 nm) and lower degree of crystallinity compared to the other HA samples.
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