Development of a new family of monolithic calcium (pyro)phosphate glasses by soft chemistry

The development of bioactive phosphate-based glasses is essential in biomaterials science, and especially for bone substitution applications. In this context, the preparation of amorphous calcium-phosphorus hydroxide/oxide monoliths at low temperature is a key challenge for being able to develop novel hybrid materials for these applications.

We herein report for the first time the synthesis and physical chemical characterisation of a novel family of pyrophosphate-based glasses (with the formula: {[(Ca²⁺)_1−x(H⁺/K⁺)_2x]₂[(P₂O₇⁴⁻)_1−y(PO₄³⁻)_4y/3]} n(H₂O)), which were prepared by soft chemistry using low temperatures (T < 70 °C) and water as a solvent. The effect of the initial Ca/Pyrophosphate ratio on the structure and morphology of these pyrophosphate glasses was investigated in detail. Depending on this ratio, a glass (mixed calcium pyro- and orthophosphate) or a glass-ceramic (Ca₁₀K₄(P₂O₇)₆·9H₂O crystals embedded in the amorphous phase) was obtained.

The proportion of the crystalline phase increased with an increase in the Ca/Pyrophosphate ratio in the batch solution. As expected for a glass, the formation of the glassy material was demonstrated not to be thermodynamically but rather kinetically driven, and the washing step was found to be crucial to prevent crystallisation.

The stability of the amorphous phase was discussed considering the structural degrees of freedom of pyrophosphate entities, ionic strength of the initial solution and the inhibitory effect of orthophosphate ions. Overall, this new strategy of preparation of monolithic calcium-(pyro)phosphate based glasses using soft chemistry in water is highly promising in view of preparing new functional organic-inorganic hybrids for bone substitution applications.

This article is currently listed as an Article in Press for Acta Biomaterialia.