Stabilizing effect of ionic polymers on calcium phosphate mineralizing solutions: A kinetic study

Objectives: A significant amount of research has shown that in the presence of polymers, such as polyallylamine hydrochloride (PAH), amorphous calcium phosphate (ACP) can be stable in a supersaturated calcium phosphate solution for an appreciable period of time. However, the effect of varying the concentration of the polymer on the kinetics of ACP's transformation to hydroxyapatite (HA) has not been studied in detail. The aim of our study is therefore to study the effect of different concentrations of PAH on the stability of calcium phosphate solutions by developing a kinetic model to predict changes in ion concentrations.
Methods: Mineralizing solutions were prepared by mixing equal volumes of 9 mM CaCl₂ and 4.2 mM K₂HPO₄ solutions, supplemented with 0, 50, 100 and 200 µg/mL of PAH (MW 15 kDa). The mineralizing solutions were placed in an incubator kept at 37 °C. Changes in the concentration of Ca²⁺ were measured using a pH meter with a Ca²⁺ electrode. The turbidity of the solutions was measured using a spectrophotometer.
The overall reactions and kinetic equations of the different reactions were assumed to be Fig.1
The concentrations of Ca²⁺, PO₄^3-, Mg²⁺, ACP and HA were normalized by the initial Ca²⁺ concentration. The rate constants (K) were calibrated by fitting the numerical solutions to the measured changes in Ca²⁺ concentration. The predicted solution turbidity was then compared with the experimental results.
Results: The concentration of Ca²⁺ in the solutions dropped sharply at the beginning of mixing. After the initial drop, the Ca²⁺ concentration quickly returned to a higher value, at which it remained for a period of time before decreasing again. The higher the polymer concentration, the longer the period of stability. Numerical solutions of the calibrated rate equations correctly captured the changes in Ca²⁺ concentration (Fig. 2(a) and (b)). Fig. 3 shows that normolized and actual rate constants. The actual k₁, X and Y increased firstly, then decreased. And the actual k_-1, k₂ and k₃ decreased as the PAH concentration increased. The predicted turbidity of the solutions showed similar rates of increase as those measured (Fig. 2(c) and (d)).
Conclusions: A kinetic model has been developed successfully to simulate the concentration-dependent stabilizing effect of PAH on calcium phosphate mineralizing solutions.