Gilbert Bonel

New concepts in the composition, crystallization and growth of the mineral component of calcified tissues

Abstract Several difficulties arise when studying the mineral component of calcified tissues: this material is complex, due to the large number of atomic components; it is poorly crystallized, heterogeneous, and varies with different factors (animal species, kind of bone, age, sampling zone, etc.); it is strongly linked to the organic component (collagen, etc.), and today no available technique allows a complete separation of these two components without alteration of one of the other. Research on synthetic materials allows the elaboration of some models to account, at least partially, for the nature and properties of the calcified-tissue mineral component. So, glycine fixation by apatite constitutes the first model of the collagen-apatite bond. The introduction of carbonate ions into the apatitic lattice can take place in two kinds of site, and under different forms. The replacement of PO3-4 ions by HPO2-4 can also be observed. The properties of phosphates depend on the presence of these various substituents, and therefore such substitutions can play an important role in phosphate behaviour in biological media. The study of the hydrolysis and crystallization of amorphous phosphate into apatite leads to new conceptions relative to the possible existence of an amorphous “phase” in calcified tissues. The conversion of amorphous phosphates to crystalline apatite is dependent on numerous ions (Mg2+, P2O4-7, CO32-, etc.). Studies on synthetic materials can be regarded as a basis for the further study of calcified tissues, partic ularly to determine their constitution and properties. Besides, such studies enable the synthesis of materials, for implants, very similar to calcified tissues.

Apatitic Calcium Orthophosphates and Related Compounds for Biomaterials Preparation

The authors show that to obtain well chemically defined apatitic bioceramics and to know the possible transformations of this material during sintering, it is necessary to prepare a good starting material. Moreover, they show that it is possible to prepare a new organic-inorganic phosphate compound. The precipitation of apatite in an aqueous medium at boiling temperature was studied using the methodology of experimental design. Independent variables were the volume of NH4OH in phosphate solution, the volume of NH4OH in calcium solution, and the time of precipitation; the response was the atomic Ca/P ratio of the obtained precipitate. A continuous variation of this ratio from 1.63 to 1.73 is observed. Implications of this result to the preparation of pure HA: Ca10(PO4)6(OH)2 is given. Moreover, when Ca/P greater than 1.67, HA reacts with Ca(OH)2 (after heating at 1000 degrees C in air for some days) to give rise to a single phase described as a modified HA (MHA), a Ca/P ratio of 1.75, an a value of 9.373 +/- 0.002 A, and a c value of 6.884 +/- 0.002 A. The reactivity (time versus temperature) of the MHA is described. If the precipitation of the calcium phosphate is realized at 37 degrees C in a water-ethanol medium in the presence of A2EP, a new apatite, chemically bonded to the organic molecule by pooling phosphate groups, is obtained.