Takayasu Goto

Transition of octacalcium phosphate to hydroxyapatite in solution at pH 7.4 and 37°C

Transition of octacalcium phosphate (OCP) to apatite was studied under the condition where Ca2+ ions were continuously supplied to the reacting solution at pH 7.4 and at 37°C. OCP crystals were grown and subsequently converted into apatite by discontinuing of Ca addition. The rectangular (100) blades of OCP crystals developed notches on their short edges in the early stage of transition. The depth of the notches increased along the c-axis direction with the progress of the reaction, giving rise to a slit-like texture. The direction of the slit formation seemed to relate to the spatial configuration of H2O molecules in the OCP lattice, which are released during the transition from OCP to apatite.

Fluoride Analysis of Apatite Crystals with a Central Planar OCP Inclusion: Concerning the Role of F− Ions on Apatite/OCP/Apatite Structure Formation

Abstract. To study the roles of F− ions in the formation of apatite crystals embedding octacalcium phosphate (OCP) lamella in the center of apatite (Ap), a range of the Ap/OCP/Ap lamellar-mixed crystals were synthesized under various concentrations of fluoride ion (F−) from 0.1–1.0 ppm at pH 6.5 and 37°C. The products were analyzed for the F− incorporation, F− distribution, and the amount of OCP and Ap by chemical analysis, X-ray diffraction (XRD), electron probe microanalysis (EPMA), and nuclear magnetic resonance (NMR) techniques. The F− content and the amount of apatite in the crystalline product increased with an increase in the F− concentration in solution, whereas the amount of OCP and the yield of total product decreased. EPMA indicated that F− ions are distributed in the crystals almost homogeneously. The combined analysis suggested that a low-substituted fluoridated hydroxyapatite (FHAp) grew on a small amount of F−-containing OCP or on a surface-reaction layer of OCP, which has accumulated a small amount of F−. The roles of F− ions were hypothesized as the reduction of the growth rate and/or the critical thickness in the a*-axis direction of OCP, the enhancement of hydrolysis of OCP, and the activation of the growth of FHAp, resulting in thinner OCP lamella and thicker apatite lamella in the a*-axis direction with an increase in F− concentration.

Effects of CO2-3 ion on the formation of octacalcium phosphate at pH 7.4 and 37°C

The effects of carbonate (CO2-3) ion on the formation of octacalcium phosphate (OCP) were studied at pH 7.4 and 37°C. The reaction was carried out in solutions with various amounts of KHCO3 (0–20mM) and with an initial solution Ca/P ratio of 0.075–0.25. CO2-3 ion changes the OCP/apatite ratio of the product, depending upon the Ca/P ratio of the solution, that is, with the increase in CO3 concentration, the amount of OCP increases and then decreases, while the amount of apatite decreases and then increases: when the Ca/P of the solution is high, a smaller amount of CO2-3 ion disturbs the crystal growth of OCP. The morphology of the products is plate-like under various amounts of CO3, but the thickness decreases and the crystal size becomes small with increasing CO3 concentration. These results indicate that CO2-3 ion significantly affects the product and morphology of final crystals. Possible roles of CO2-3 ion in the biological crystallization are discussed.

Precipitation of octacalcium phosphate at 37°C and at pH 7.4: in relation to enamel formation

Abstract Octacalcium phosphate (OCP) has been proposed to be an important precursor of enamel apatite, but the question remains whether OCP crystals grow in the physiological milieu or not. In this study, the conditions under which OCP precipitates in preference to hydroxyapatite (HAp) were studied at 37°C and pH 7.40 in connection with the effects of the degree of supersaturation with OCP, initial Ca/P of solution, and stirring. Reactions were carried out both in a pH-stat system and in a buffer solution system. In the pH-stat system, OCP precipitated preferentially from a solution with small Ca/P, when Ca solution was supplied during reaction. Without Ca solution supply, OCP was not obtained. In the stationary buffer solution, OCP precipitated over a wide range of Ca/P (0.025–1.67). The thermodynamical analysis of the precipitation condition indicates that OCP crystals can grow in a basic solution at 37°C, so long as the driving force to precipitate OCP is larger than that to form HAp.

Sintering mechanism of hydroxyapatite by addition of lithium phosphate

The sintering mechanism of hydroxyapatite (HAp) by addition of lithium phosphate (Li3PO4) has been investigated. Using the X-ray diffraction method, HAp was confirmed to decompose into β-Ca3(PO4)2 (β-TCP) by addition of Li3PO4. The measurement of shrinkage rate by the isothermal firing made it clear that the densification process at the initial stage of sintering took place in the presence of liquid phase. Furthermore, the examination of the phase diagram on the binary system β-TCP-Li3PO4 revealed that there was an eutectic point at 1010°C in the composition of 60 wt% Li3PO4. From these evidences, we concluded that β-TCP produced by the decomposition of a part of HAp has formed the liquid phase by reacting with Li3PO4 above 1010°C, and that this liquid phase has largely promoted the densification by the rearrangement of HAp particles at the initial stage of sintering.

Thermal decomposition ofLingula shell apatite

SummaryLingula shell is composed of apatite with a preferred orientation. The shell apatites ofLingula unguis(Lu) andLingula shantoungensis(Ls) were characterized and compared with apatite of human tooth enamel. Insight into theLingula apatite was studied by following the change of lattice parameter, transformation to β-tricalcium phosphate (β-TCP), and the loss and change of CO3, OH, and H2O after heating up to 1,000°C in air and N2 for 24 hours. The OH stretching band was not observed in unheated apatites and in apatites heated in dried N2.Lu andLs apatite produced 26 and 17 wt% of β-TCP at 700°C, respectively. Fifty to 60% of H2O was lost at 200°C, being accompanied by a drastic contraction of the a- and c-axis and a drastic decrease in the crystallinity. These results indicate that (1)Lu andLs shell apatite is CO3 containing F+Cl-apatite, and (2) the structural H2O of theLingula apatite is loosely bounded such that they are lost at lower temperature than tooth enamel.