Calcium phosphate precipitation under biomineralization conditions: Effect of microgravity

Structure and morphology of solid particles in precipitation from aqueous solutions correlate with formation (solution) conditions and thus reflect the growth mechanism. Calcium phosphate precipitation obtained from dilute aqueous solutions at room and body temperatures and pH5.5 - 7.5 were investigated by high-resolution transmission electron microscopy (HRTEM), transmission electron diffraction (selected-area diffraction and microdiffraction), scanning electron microscopy and X-ray diffraction, Supersaturated solutions of calcium phosphates were prepared by different methods of mixing of the stock solutions: diffusion-controlled mixing in space, convection-controlled mixing on earth and forced mixing on earth. A combination of HRTEM with image calculation, microdiffraction and image processing provided full information on the state of matter, local phase composition of crystals and atomic structure when small particles and small areas on large samples are considered. It was shown the strong dependence of these parameters on concentrations of the stock solutions, rate of solution mixing and duration of precipitation. Phase analysis by HRTEM and electron microdiffraction demonstrated two crystalline compounds in precipitation hydroxyapatite (HAP) and octacalcium phosphate (OCP) independent on the sizes of crystals and grains in crystals.