Abstract
Hydroxyapatite (Ca10(PO4)6(OH)2 ; HAP) is a major component of mammalian hard tissues and one of the components of pathological and undesirable concretion, such as urolithiasis. In the present review, the interaction of biologically significant ions with HAP is discussed from the view point of physical and colloid chemistry. It was found that the concurrent adsorption of the lattice ions of HAP (OH-and Ca2+, for example) onto the surface of HAP was similar to the crystal growth of HAP. Pyrophosphate ion (PPi) showed specific behaviors, such as a retarding effect on the crystal growth of HAP and a dispersing effect on the HAP secondary particles, owing to the adsorption of the terminal phosphate group of PPi to the surface of HAP. PPi was hydrolyzed to orthophosphate ion in the presence of Ca2+ and/or on the surface of HAP. These effects are important to understand the inhibiting effect of PPi against biological calcification and calculus formation. When organic ions, such as dodecyl sulfate ion and chondroitin sulfate ion, were adsorbed on HAP, some phosphate ions were released from the surface of HAP by a mechanism of ion-exchange. It was shown that this phenomenon is of great importance in connection with the binding affinity of biopolymers in the connective tissues to the biological HAP of the hard tissues and to the synthesized HAP of the artificial bones.
