Many studies of calcium phosphate precipitation have been made using relaxation techniques in which the concentrations of the lattice ions are allowed to decrease as equilibrium is approached. Since the nature of the phases that form depend markedly on the solution composition, this decrease can lead to concomitant phase transformations during the crystallization experiments. The results of the present constant composition (CC) studies show that defect apatites may be formed under conditions of sustained supersaturation with a non-stoichiometric coefficient dependent on the pH of the growth medium. An important factor in analyzing these experiments is the initial surface modification and ion-exchange processes involving H + and Ca2 + ions after inoculation of the supersaturated solutions. Thereafter, active growth sites may be eliminated as the crystals undergo lattice perfection. Transformation of dicalcium phosphate dihydrate to octacalcium phosphate, involving dissolution and subsequent nucleation and growth of the new phase, is also influenced by surface roughening of the initial phase. Typical inhibitors that reduce the rate of growth of seed crystals in supersaturated solutions may actually induce the nucleation of calcium phosphate phases when immobilized on inert surfaces. This may be a factor in the modulation of crystal growth in many biological systems.
The formation, remineralization, and dissolution of hard tissues such as bones and teeth are very complicated processes not only because of the presence of multiple components in the solution media but also because of the numerous calcium phosphate phases that may be involved in the reactions. Nevertheless, the mechanism of mineralization and demineralization may be studied in the laboratory if model systems are suitably chosen. It is often assumed that the thermodynamically most stable hydroxyapatite (Ca5(P04)3 OH; HAP) is a suitable prototype for biological minerals, but it is now generally accepted that other phases such as amorphous calcium phosphate (Ca9(PO4I6; ACP), dicalcium phosphate dihydrate (CaHP04-2H20; DCPD) and octacalcium phosphate (Ca4H6(P0&; OCP) as well as defect apatites may participate. The resurgence of interest in the precipitation of calcium phosphates is due not only to their involvement in these biological mineralization reactions but also to involvement in areas such as the removal of phosphate from wastewater; the fate of elements such as aluminum, iron, and other heavy metals in the formation of lake and ocean sediments; and in industry, where the production of calcium phosphate scale on metal surfaces is a continuing problem. In this paper, we will be concerned with the mechanism of precipitation and transformation of calcium phosphates under conditions close to those in vivo and the nucleation of these phases at organic surfaces-natural, such as collagen, and synthetic, such as immobilized protein films.
Many kinetic studies of calcium phosphate precipitation have been made in mineralizing solutions considerably more supersaturated than typical biological media. Under these conditions, the formation of apatite 0 1989 ALAN R. LISS, INC.