Calcium phosphate cements are applied for bone repair due to their excellent biocompatibility. Hydration of α-tricalcium phosphate (α-TCP) results in formation of calcium-deficient hydroxyapatite (CDHA) according to (1) .
3 α-Ca3(PO4)2 + H2O → Ca9HPO4(PO4)5OH (1)
With respect to minimal-invasive applications, cement pastes need to be well injectable. Sodium phytate (IP6), Na12C6H6O24P6, is a promising additive to improve the injectability of cement pastes . Hence its concentration-dependent effect on paste injectability was evaluated in this study. Furthermore, the influence of any additives on the reaction mechanism needs to be well understood for targeted development of cement formulations. The reaction mechanism was therefore thoroughly investigated by isothermal calorimetry and in-situ XRD. Mixtures of α-TCP and CDHA in a weight ratio of 9:1 were used as cement powder. Sodium phytate was added in concentrations ranging from 0.25 to 1 wt.% related to the powder content. A 0.2 M Na2HPO4 aqueous solution was used as mixing liquid, with a liquid to powder ratio of 0.3 ml/g. Measurements were performed at 37 °C.
Injectability was proven to be significantly improved by sodium phytate addition. Calorimetry measurements indicated an increasing retarding effect of sodium phytate with increasing concentration. By addition of 1 wt.% sodium phytate, even an induction period of about 10 h occurred. In-situ XRD measurements indicated CDHA formation in all samples as expected, which was slowed down with increasing sodium phytate concentration. For samples containing sodium phytate, additional formation of a few wt.% of octacalcium phosphate was observed. An increase of sodium phytate content only resulted in a very slight decrease of CDHA crystallite size.
While sodium phytate was proven to be very effective in increasing the injectability of apatite cement, its intense retarding effect on cement hydration needs to be considered when developing cement formulations for medical application.
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