Jean-Claude Heughebaert

A CALCIUM HYDROXYAPATITE PRECIPITATED FROM AN AQUEOUS-SOLUTION - AN INTERNATIONAL MULTIMETHOD ANALYSIS

Abstract In this paper a pure calcium hydroxyapatite prepared from aqueous solutions at low temperature, was analysed by a large number of techniques in six Institutes. The techniques employed (frequently in more than one laboratory) were X-ray diffraction, IR analysis, BET measurement, chemical analysis, differential thermal analysis, magic angle spinning NMR, TEM, size distribution measurements, crystal growth and crystal dissolution measurements. Several data were obtained not only at room temperature but in the range up to 900 °C. The results show that the apatite is very pure. The impurities CO 3 and NO 3 are both below 0.1%. The important ion HPO 2- 4 is present in amounts of ≈ 2 mol% of total phosphate and covers most likely the outer surface of the crystals. Five different techniques were employed to determine the HPO 2- 4 content. H 2 O is present in adsorbed form in about 2.0 wt%. The Ca/P ratio is between 1.63 and 1.66. The OH - content is about 10% by weight lower than the stoichiometric value. Crystal sizes have mean values of 33 nm width and 133 nm length. BET surface is 37 m 2 g -1 . X-ray powder diffraction yields a and c lattice parameters of 9.428 and 6.882 A at RT and after a brief heating to 900 °C in N 2 . If the apatite is heated at 850 °C for 18 h in air, partial decomposition takes place to form α and β Ca 3 (PO 4 ) 2 . 1 H MAS-NMR shows that the H 2 O is present in a very mobile form and is presumably adsorbed in several monolayers. The absolute quantitation of the structural hydroxyl content by 1 H MAS-NMR reveals a slight deficiency, as do IR results. Kinetic data suggest that the dissolution mechanism is controlled by polynuclear surface nucleation catalysed by hydrogen ions, the interfacial surface free energy σ being 50 mJ m -2 at pH = 7. Growth kinetics analysed for a polynuclear mechanism lead to the value σ = 87 mJ m -2 at pH = 7.

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.

The growth of nonstoichiometric apatite from aqueous solution at 37°C: I. Methology and growth at pH 7.4

Abstract The apatites found in vivo are invariably nonstoichiometric. It has been shown that such phases may also be formed by spontaneous precipitation at lower temperatures and pH. When carbonate is excluded, the stoichiometry of thse phases can be closely approximated by the formula Ca 10−u H u ( PO 4 ) 6 ( OH ) 2−u , in which 0 ⩽ u ⩽ 2. A modification of the constant solution composition method has been used to assess the stoichiometry of phases growth at 37°C in solutions, undersaturated with respect to octacalcium phosphate, of ionic strength 0.100 mole liter −1 (potassium nitrate), pH 7.40, and with total molar concentrations T Ca and T P of 4.0 × 10 −4 and 2.4 × 10 −4 mole liter −1 , respectively. The titrants used to replenish these solutions during crystallization experiments contained calcium and phosphate concentrations calculated for precipitated phases having nonstoichiometric coefficients, u , between 0.00 and 1.60. When the titrant stoichiometry did not match that of the precipitating phase, systematic variations in the analyzed solution concentrations of calcium and phosphate were used to determine the composition of the grown phase. In verification experiments with corrected titrant concentrations, the constancy of the analyzed calcium and phosphate concentrations indicated that the solid phase stoichiometry was consistent with a value, u = 0.57. The use of different apatite seed under the same experimental conditions has confirmed this precipitate stoichiometry.

Apatite Chemistry in Biomaterial Preparation, Shaping and Biological Behaviour

ABSTRACT The characteristics of the wet chemistry of apatites are discussed with a special emphasis on maturation processes, and their relationship with non-stoichiometry and ionic substitution. The thermal behaviour of stoichiometric and non-stoichiometric apatites is then presented including different aspects of intracrystalline reaction, restructuration process and high temperature reversible and irreversible decomposition. Finally, the surface reactivity of apatite, the surface composition and the adsorption behaviour are examined. Constant reference is made to bone mineral and biomaterials and some aspects of their biological behaviour are explained. Apatite is the most common crystalline form of calcium phosphate and is largely used in biological applications. This text is intended to give a general picture of apatite chemistry and how it relates to the problem of biomaterials and biological calcium phosphates.

The growth of whitlockite

Abstract The crystallization of whitlockite Ca18Mg2H2(PO4)14 from metastable supersaturated solutions following seeding with well characterized whitlockite has been studied at 37°C using the constant composition method. The results confirm that at a pH of 6.00, it is possible to precipitate whitlockite from a magnesium-calcium phosphate medium.

Calcium phosphate precipitation in the 60–80°C range

Abstract In the 60–80°C temperature range and in acidic medium, the precipitation of either octacalcium phosphate (OCP) or dicalcium phosphate dihydrate (DCPD) or dicalcium phosphate anhydrous (DCPA) onto DCPA seeds occurs. High temperature and low pH favor the precipitation of DCPA. A plot of temperature versus pH is given to summarize the results. The kinetics of DCPA growth onto DCPA seeds were studied by constant composition method. At 70°C the rate of growth is small and the order of reaction is 4, contrasting to 80°C where the rate of growth is higher and the order of reaction is 2. At 70°C the edge free energy is 1.7×10 -11 and 2.0×10 -11 J m -1 at pH = 4.3, respectively. The study of the nucleation of OCP onto DCPA seeds allows us to calculate the surface energy of OCP (51–59 mJ m -2 at 60–80°C).

Precipitation of stoichiometric apatitic tricalcium phosphate prepared by a continuous process

Various stages of the continuous precipitation of a stoichiometric apatitic tricalcium phosphate (Ca : P =1.50) have been observed with increasing duration of precipitation, particularly during the first 3 h necessary for the reactor to reach a steady state. The results show that an amorphous Ca2+-deficient phosphate close to the composition of octacalcium phosphate is formed in the early minutes of precipitation, then, after hydrolysis, it is transformed into an apatitic phase which leads to a stoichiometric apatitic tricalcium phosphate.

Solid state31NMR studies of the conversion of amorphous tricalcium phosphate to apatitic tricalcium phosphate

SummaryThe hydrolytic conversion of a solid amorphous calcium phosphate of empirical formula Ca9(PO4)6 to a poorly crystalline apatitic phase, under conditions where Ca2+ and PO43− were conserved, was studied by means of solid-state magic-angle sample spinning31P-NMR (nuclear magnetic resonance). Results showed a gradual decrease in hydrated amorphous calcium phosphate and the formation of two new PO43−-containing components: an apatitic component similar to poorly crystalline hydroxyapatite and a protonated PO43−, probably HPO42− in a dicalcium phosphate dihydrate (DCPD) brushite-like configuration. This latter component resembles the brushite-like HPO42− component previously observed by31P-NMR in apatitic calcium phosphates of biological origin. Results were consistent with previous studies by Heughebaert and Montel [18] of the kinetics of the conversion of amorphous calcium phosphate to hydroxyapatite under the same conditions.

Statistical analysis of apatitic tricalcium phosphate preparation

Apatitic tricalcium phosphate was obtained by a continuous (3–5 kg per 24 h) process using the conventional double decomposition method between an aqueous calcium nitrate solution, Ca(NO3)2, and an aqueous ammonium phosphate solution, (NH4)2HPO4. To check the effect of certain variables on the reaction, a fractional central composite design was set up taking six variables into account: pH, (Ca/P)reagents, concentration of the calcium nitrate solution ([Ca2+]), temperature (T), duration of precipitation (R) and speed of stirring (S). The limiting factors of precipitation for apatitic tricalcium phosphate are discussed.