Precipitation behaviour of calcium phosphate

Thesis



Various aspects of the precipitation behaviour of calcium phosphate in aqueous media have been investigated using seeded growth in conjuction with constant-volume and constant-composition techniques under different physical and chemical conditions. In each case, precipitation was allowed to proceed for up to seven days. The solid precipitates thus obtained were characterised by means of scanning electron microscopy, powder X-ray diffractometry and wavelength dispersive spectroscopy.

During these precipitation experiments, the formation of the thermodynamically most stable and most supersaturated phase was invariably preceded by the appearance of less supersaturated precursor phase(s). These precursors subsequently underwent step-wise phase transformation into more stable phases. The preferred precursor and the rates of precipitation and phase transformation were dependent on the physical conditions and the chemical composition of the calcifying medium.

Under physiological conditions, precipitation experiments were also carried out with the addition of certain non-collagenous bone-specific bio-chemicals. Phosphoserine dramatically accelerated the precipitation of a large quantity of small plate-like crystals, while osteonectin and phosphatidylserine induced the formation of quasi-cubic crystals at a slow rate. Bone protein extract displayed the strongest inhibitory effect on calcification. Bovine serum albumin showed signs of being irreversibly adsorbed to the crystal surface, thereupon inducing a high degree of calcium deficiency in the precipitate stoichiometry.

Using a number of phosphorylated amino acids of different molecular masses, it was found that the processes of precipitation and phase transformation were facilitated by organic molecules whose phosphoryl functional groups were sterically accessible and highly electronegative. However, the acceleration brought about by the presence of a phosphorylated amino acid was maximised at an optimum concentration. The existence of such an optimum was very likely to be consequent of the competition for free calcium ions by the ongoing complexation and precipitation reactions.

A model has also been developed to describe and predict the precipitation behaviour of calcium phosphate. The model is based on the Avrami-Johnson-Mehl expression for threedimensional nucleation and growth processes. Appropriate modifications to the original equation have been made, in order to adapt to this multi-ionic aqueous system. The resulting model has been found to describe the actual precipitation process accurately. It has also been applied to systems in which organic additives were present, and has again furnished predictions closely resembling the behaviour as observed experimentally.

Authors: Wong, A

• Alternative title: a model for hard tissue mineralisation
• Publication date: 1993
• DOI: 10.5287/ora-qonyoj4rr
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Subjects: Calcium phosphate; Biomedical materials