Showing papers by "Iain R. Gibson published in 2007"

Synthesis and Phase Stability of Silicate-Substituted α-Tricalcium Phosphate

Abstract: Silicate-substituted calcium phosphates have been shown to result in enhanced biological performance compared to the corresponding, silicate-free, calcium phosphates. We have produced a range of silicate-substituted alpha-TCP compositions using two different synthesis methods and two different substitution mechanisms. Single phase compositions were only observed for a silicate substitution of 1.3 wt% by both solid state synthesis and aqueous precipitation synthesis, although the latter was the result of a design composition with a higher silicate substitution (3 wt%). The silicate substitution resulted in small changes in the unit cell parameters of the alpha-TCP. More importantly, this small level of silicate substitution had a strong effect on the thermal stability of the alpha-TCP phase, with the silicate substitution stabilising the alpha-polymorph to lower temperatures. This has an immediate advantage in that the quenching conditions are not as critical for the production of silicate-substituted alpha-TCP compositions compared to silicate-free alpha- TCP.

Synthesis of Novel High Silicate-Substituted Hydroxyapatite by Co-Substitution Mechanisms

Abstract: Silicate substituted hydroxyapatite bioceramics have been shown to enhance bone repair in vivo compared to hydroxyapatite (HA), although the amount of silicate ions that can be substituted alone into the hydroxyapatite structure is limited to approximately 5.2 wt%, or 1.6 wt% Si. This study describes the substitution of greater levels of silicate ions via co-substitution of silicate ions with trivalent yttrium ions, without resulting in the formation of any secondary phases. This substitution mechanism involves a coupled substitution of yttrium and silicate ions for calcium and phosphate ions, respectively, and enables a level of silicate substitution up to approximately 9 wt%. Two different substitution mechanisms result in subtle differences in the crystal structure. When the mechanism xY3+ + xSiO4 4- was used, a small decrease in the a-axis, but no change in the c-axis, of the unit cell compared to HA was observed. In contrast, when the mechanism x/2Y3+ + xSiO4 4- was used, a significant increase in the c-axis of the unit cell was observed, compared to HA. XRF analysis and FTIR spectroscopy supported the proposed substitution mechanisms. These novel substitution mechanisms not only enable greater levels of silicate-substitution in HA to be prepared, but also allow the production of compositions with the same level of silicate substitution, and with subtle differences in chemical structure.

Sol-Gel Synthesis and In Vitro Cell Compatibility Analysis of Silicate-Containing Biodegradable Hybrid Gels

Abstract: Novel hybrid gels in the system gelatin-GPTMS-TEOS were prepared via a sol-gel route, and their ability to release Si(IV) was examined using MG63 osteoblast-like cell culture. The amount of Si released and the release rate were controllable by changing the mixing ratio of GPTMS and TEOS. In addition, the hybrids had biocompatible surfaces. It is expected that the hybrids will be utilized for the investigation of the effect of Si on cell differentiation and tissue regeneration.

Comparison of Carbonate Hydroxyapatite with and without Sodium Co-Substitution

Abstract: Carbonate hydroxyapatite (CHA) bioceramics can be synthesised to contain sodium ions as a co-substituted ion, or as sodium-free compositions. It is unclear, however, which composition would produce the optimum biological response. The aim of this study was to find a reliable method to produce sodium co-substituted and sodium-free CHA compositions that would have the same level of carbonate substitution, and to characterise the effects of the two different substitutions on the structure of the CHA samples. After sintering at 900oC in a CO2 atmosphere, all samples contained approximately equal amounts of carbonate groups on the A- and B-sites, as observed by FTIR. The sample produced with NaHCO3 and the sodium-free sample (CHA1) have comparable carbonate contents, whereas the sample produced with Na2CO3 contains significantly more carbonate, probably due to the excess sodium ions allowing more carbonate co-substitution. The sodium-free CHA sample, however, has significantly smaller unit cell parameters compared to both sodium co-substituted CHA samples, and also to HA. This characterisation of the samples shows that the sodium-free CHA sample (CHA1) and the sample produced with NaHCO3 would provide CHA compositions for biological testing with similar carbonate contents and distributions, but with structural differences due to the sodium substitution.

Synthesis and Stability of Potassium/Carbonate Co-Substituted Hydroxyapatites

Abstract: A systematic study of the stability of potassium/carbonate co-substituted hydroxyapatite has been carried out, with samples synthesized by aqueous precipitation according to the chargebalanced mechanism: Ca10-xKx(PO4)6-x(CO3)x(OH)2 Samples up to x=10 were prepared and their stability determined by heating at a range of temperatures in both air and CO2 environments Results showed that whilst samples up to x=10 can be prepared phase-pure, the stability of these materials is strongly dependent on sintering temperature with the full range of compositions only being stable at 600°C in CO2 The c unit cell parameter increases linearly with x, and, for a fixed composition, decreases linearly with temperature indicating loss of carbonate from the A-site FTIR showed that samples contained carbonate at both A- and B-sites, and that carbonate content increased with x

Optimisation of the Aqueous Precipitation Synthesis of Silicate-Substituted Hydroxyapatite

Abstract: Silicon-substituted hydroxyapatite (SiHA) bioceramics are widely used as bone replacement materials. There are various synthesis methods used to produce SiHA samples using different sources of silicon. This study aims to investigate the role of tetraethyl orthosilicate (TEOS) as the silicon source in the precipitation reaction synthesis of silicate-substituted HA. Four different synthesis methods were studied by changing the order of addition of the TEOS solution during the precipitation reaction. XRD and QXRD were used to determine the phase purity of the prepared samples. FTIR and SSNMR were used to assess silicon/silicate substitution in the prepared materials. Of the initial four methods used, only one resulted in a sample that was phase pure. The other three syntheses, which produced biphasic compositions, were modified and a further single phase sample was prepared. Results showed that the final composition is strongly dependant on how and when the TEOS was added during the precipitation reaction.

Substituted calcium phosphate materials

Abstract: A process is disclosed for the preparation of a silicate and carbonate co-substituted calcium phosphate material. The process comprises the steps of: forming a silicon and optionally carbon-containing calcium phosphate precipitate by an aqueous precipitation method involving preparing an aqueous solution comprising phosphate ions, silicate ions, calcium ions and optionally carbonate ions, wherein the ratio of Ca/P and of Ca/(P+Si) in the solution is maintained above approximately 1.67; and heating the precipitate in an atmosphere comprising carbon and oxygen to form a silicate and carbonate co-substituted calcium phosphate material. The present invention also provides a synthetic carbonate and silicate co-substituted hydroxyapatite material, and process of formation, which may contain up to 2.86% by weight of silicon and up to 13% by weight of carbonate.