Dual-setting brushite-silica gel cements – Variations of the monomer precursors and tuning of the setting parametersPart of:
Calcium phosphate cements (CPC) in general set via a dissolution–precipitation reaction of the cement raw materials. A further modification by adding monomers to the aqueous cement phase results in the formation of a second interpenetrating network with a strong impact on cement properties. This was recently shown for cement modification with tetraethyl orthosilicate (TEOS) as precursor . Here, this approach is further explored by using silica precursors with a higher density of alkoxy groups to increase network density in the final gel. The precursors were initially hydrolysed under acidic conditions and the combined with a brushite forming cement powder. Due to the increase in pH of the sol during cement setting leads to a simultaneous formation of cement matrix and silica hydrogel. Silica sols were obtained by adding a mixture of precursors (X = (100- 60%) TEOS and (0-40%) 1,2-Bis(triethoxysilyl)ethane or 1,8-Bis(triethoxysilyl)octane) to water and 0.1 M HCl solution. The influence of the water to alkoxy-group ratio (R) was also analyzed. Composite cements were produced by mixing the sol with cement raw powder, consisting of ß-tricalcium phosphate and anhydrous monocalciumphosphate in an equimolar ratio and the addition of 1 wt% citric acid (retarding agent). The results demonstrated an increase of mechanical performance by using a mixture of precursors in the composite compared to the pure TEOS reference. The porosity characteristics of the silica–brushite networks showed a bimodal pore size distribution in the set matrices with nanosized pores originating from the silica matrix and µm pores from the cement. The latter is thought to have a strong effect on drug release capability by retarding drug diffusion from the cement matrix.
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