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Calcium carbonates and in particular amorphous calcium carbonate (ACC) are of great interest for bone substitution, due to their excellent biocompatibility and ability to release some of the bone mineral constitutive ions. Despite thorough investigations on the ACC precipitation mechanism in the presence of a wide variety of additives to stabilise it, their influence on the composition and structuration of the resulting ACC is rarely finely characterised. In this article, pyrophosphate (Py) was studied as an additive, due to its calcite inhibitor role and the good bioactive properties it demonstrated in vitro and in vivo. A series of pyrophosphate-stabilised ACC (PyACC) powders were synthesised by co-precipitation in water with various amounts of Py, and the structure, composition and multiscale organisation of the resulting compounds were thoroughly characterised. XRD, FTIR spectroscopy and FEG-SEM showed a reduction of the amount and size of calcite crystallites when increasing the Py content, crystallites co-existing with an increasing quantity of the amorphous phase, up to obtaining fully amorphous materials above a certain Py amount. Beyond the stabilising role of Py, and considering SAXS, PDF and solid-state NMR analyses, we proposed an experimental model for PyACC based on a vaterite-like (or proto-vaterite) cluster of 1 nm surrounded by a 2 nm-outer shell containing pyrophosphate ions and pseudo-structural water stabilising the cluster. This comprehensive study and the resulting model are predominant prerequisites to understand the underlying mechanisms of PyACC evolution and could contribute to the rational design of tunable amorphous and bioinspired inorganic bone substitute materials. [ABSTRACT FROM AUTHOR]

α-Canaphite (CaNa2P2O7·4H2O) is a layered calcium disodium pyrophosphate tetrahydrate phase of significant geological and potential biological interest. This study overcomes the lack of a reliable protocol to synthesize pure α-canaphite by using a novel simple and reproducible approach of double decomposition in solution at room temperature. The pure α-canaphite is then characterized from the atomic to the macroscopic level using a multitool and multiscale advanced characterization strategy, providing for the first time full resolution of the α-canaphite monoclinic structure, including the hydrogen bonding network. Synchrotron X-ray diffraction and neutron diffraction are combined with multinuclear solid state NMR experimental data and computational modeling via DFT/GIPAW calculations. Among the main characteristics of the α-canaphite structure are some strong hydrogen bonds and one of the four water molecules showing a different coordination scheme. This peculiar water molecule could be the last to leave the collapsed structure on heating, leading eventually to anhydrous α-CaNa2P2O7 and could also be involved in the internal hydrolysis of pyrophosphate ions as it is the closest water molecule to the pyrophosphate ions. Relating such detailed structural data on α-canaphite to its physico-chemical properties is of major interest considering the possible roles of canaphite for biomedical applications. The vibrational spectra of α-canaphite (deuterated or not) are analyzed and Raman spectroscopy appears to be a promising tool for the identification/diagnosis of such microcrystals in vitro, in vivo or ex vivo. [ABSTRACT FROM AUTHOR]

The development of amorphous phosphate-based materials is of major interest in the field of biomaterials science, and especially for bone substitution applications. In this context, we herein report the synthesis of gel-derived hydrated amorphous calcium/sodium ortho/pyrophosphate materials at ambient temperature and in water. For the first time, such materials have been obtained in a large range of tunable orthophosphate/pyrophosphate molar ratios. Multi-scale characterization was carried out thanks to various techniques, including advanced multinuclear solid state NMR. It allowed the quantification of each ionic/molecular species leading to a general formula for these materials: [(Ca2+ y Na+ z H+ 3+ x -2y-z)(PO 4 3−) 1-x (P 2 O 7 4−) x ](H 2 O) u. Beyond this formula, the analyses suggest that these amorphous solids are formed by the aggregation of colloids and that surface water and sodium could play a role in the cohesion of the whole material. Although the full comprehension of mechanisms of formation and structure is still to be investigated in detail, the straightforward synthesis of these new amorphous materials opens up many perspectives in the field of materials for bone substitution and regeneration. The metastability of amorphous phosphate-based materials with various chain length often improves their (bio)chemical reactivity. However, the control of the ratio of the different phosphate entities has not been yet described especially for small ions (pyrophosphate/orthophosphate) and using soft chemistry, whereas it opens the way for the tuning of enzyme- and/or pH-driven degradation and biological properties. Our study focuses on elaboration of amorphous gel-derived hydrated calcium/sodium ortho/pyrophosphate solids at 70 °C with a large range of orthophosphate/pyrophosphate ratios. Multi-scale characterization was carried out using various techniques such as advanced multinuclear SSNMR (31P, 23Na, 1H, 43Ca). Analyses suggest that these solids are formed by colloids aggregation and that the location of mobile water and sodium could play a role in the material cohesion. Image, graphical abstract [ABSTRACT FROM AUTHOR]

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: {[(Ca 2+ ) 1−x (H + /K + ) 2x ] 2 [(P 2 O 7 4− ) 1−y (PO 4 3− ) 4y/3 ]} n(H 2 O)), 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 (Ca 10 K 4 (P 2 O 7 ) 6 ·9H 2 O 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. Statement of Significance Phosphate-based glasses have gradually emerged as a potential alternative to silicate bioactive glasses in order to induce different biological mechanisms of degradation. The synthesis of such monolithic glasses at low temperature is a key step to allow new inorganic glass compositions to be reached and hybrid materials to be prepared. Although sol-gel and coacervate methods (respectively orthophosphate and metaphosphate precursors) have already been described to prepare such glasses, the use of toxic solvents and/or the final temperature treatment associated to these processes could limit the use of these materials for biomedical applications and/or the further development of hybrids. It is shown here that pyrophosphate precursors are an alternative strategy to obtain monolithic calcium (pyro)phosphate glasses under soft conditions (water solvent, 70 °C). [ABSTRACT FROM AUTHOR]