Thermal evolution of amorphous tricalcium phosphate

Abstract

Amorphous calcium phosphate (ACP) is one of the major components of biological mineralisation and plays a crucial role in several biomaterials. However, due to their instability, the thermochemical characteristics of ACP and of the derived metastable phases are still unknown. The thermal transformations affecting ACP, with a Ca/P atomic ratio close to 1.50, were studied in the range 373-1173 K. The samples were examined by X-ray diffraction, FTIR spectroscopy and BET measurements. The thermal behaviour was analysed by thermogravimetric and differential thermal analysis. The dissolution enthalpies of the products, in 9 wt% aqueous nitric acid solution, were determined using an isoperibolic calorimeter. The samples remained amorphous up to 773 K. The evolution of the dissolution enthalpy between 298 and 573 K was attributed to the irreversible loss of adsorbed water. The adsorption enthalpy at 298 K of liquid water on ACP was estimated at 212.3 kJ mol 21 . At 673 K, the variation of the dissolution enthalpy corresponded to the occurrence in the amorphous phase of a more stable ionic organisation. The nucleation of the crystalline phase was believed to begin at 773 K. At 873 K and above, the ACP crystallised mainly into metastable a-tricalcium phosphate (a-TCP). Though the enthalpy of dissolution of the formed a-TCP varied with temperature, it remained lower than that of the most stable tricalcium phosphate crystalline structure: b-TCP. This phase was formed at 1173 K. The thermal measurements confirm the high instability of ACP compared to other calcium phosphates, including apatites. These properties, plus the very high specific surface area, explain the very fast transformation of ACP into apatite in aqueous media and justify its use in new types of self-setting cements.

Preparation and characterization of novel biphasic calcium phosphate powders (α‐TCP/HA) derived from carbonated amorphous calcium phosphates

“…41 These may be attributed to the difference in the short range structure of ACP during precipitatation. The difference in aggregation/assembly of Ca polyhedron and P polyhedron or combination of structure cluster [(Ca 9 (PO 4 ) 6 or Ca 3 (PO 4 ) 2 )] in ACP lead to different short range structures.…”

Preparation, characterization and cytocompatibility of porous ACP/PLLA composites

“…Amorphous calcium phosphate (ACP) has a similar structural unit to that in apatite, 1 it can act as an important intermediate product for in vitro and in vivo apatite formation. 2,3 The disordered structure makes ACP have high reactivity with body fluid to cause large dissolubility and fast apatite reprecipitation.…”

Synthesis and characterization of nanosized calcium phosphates by flame spray pyrolysis, and their effect on osteogenic differentiation of stem cells

“…In aqueous environment, by high ionic interaction, ACPs show apatite-like recrystallization. The physiological pH induces this crystallization (Somrani et al 2003;Combes and Rey 2010). Crystalline calcium phosphate biomaterials show higher thermal stability than natural and amorphous ones.…”

“…The ACPSs showed better bioactivity than crystal calcium phosphates (Brunner et al 2007b;He et al 2013). According to the study, for the higher surface area, smaller particle-sized amorphous materials showed better osteogenic differentiation on seeded cells (Somrani et al 2003;Brunner et al 2007b).…”

“…ACPs have higher solubility in aqueous biological environment compared to crystalline ones resulting in high ionic concentration in the vicinity, initiating a recrystallization of apatite-like deposits. The physiological pH induces this crystallization (Somrani et al 2003;Combes and Rey 2010). This biomineralization is initiated by the electrostatic interactions between the calcium and phosphate ions in the body fluids and the surface functional groups of the biomaterial.…”