Crystallinity and morphology of hydroxyapatite alternation with carbonate co-precipitation, and its changing in biological response

Abstract

  The main inorganic component of vertebrate hard tissues is hydroxyapatite (HAp), a type of calcium phosphate. However, during the formation of HAp in hard tissues, carbonate is invariably present and influences crystal growth. This review summarizes previous findings on the effects of carbonate on HAp formation processes and its physical, chemical and biological properties. Carbonate ions substitute for phosphate or hydroxyl groups within the HAp crystal structure, which not only increases solubility but also reduces the crystal aspect ratio and size. Taking advantage of these effects allows the preparation of HAp pseudomorphs, which are soluble calcium phosphates. Furthermore, compared with pure HAp, carbonate-containing apatite exhibits higher bioreactivity and enhanced osteoconductivity. The underlying mechanisms cannot be explained solely by calcium ion release and therefore warrant further investigation.