Cationic and anionic substitutions in hydroxyapatite

Abstract

Hydroxyapatite (HAp, Ca10(PO4)6(OH)2, Ca/P = 1.67) is widely employed in biomedical sector, particularly in dentistry and orthopedics, due to its chemical similarity to the mineral component of hard tissue. However, the biological apatite, whose bone and tooth mineral phases are composed, remarkably differs from stoichiometric HAp, being Ca deficient (Ca/P < 1.67), composed of small crystals and characterized by poor crystallinity and relatively high solubility. Furthermore, it consists in carbonated apatite characterized by the presence of various amounts of vicarious ions, either incorporated within the apatite lattice or just adsorbed on the crystal surface, including anionic (e.g., F−, Cl−, SiO44−, and CO32−) and/or cationic substitutions (e.g., Na+, Mg2+, K+, Sr2+, Zn2+, Ba2+, Al3+). Thus, the synthesis of hydroxyapatites partially substituted by these elements has attracted a lot of interest, in order to mimic and resemble the chemical composition of the bone mineral component. In fact, the ability to exchange ions in apatite structure allows to design, develop, and characterize new and better calcium phosphates for certain specific applications. This manuscript provides an overview about the majority of the investigated substitutions within the hydroxyapatite lattice, evidencing the influence of the different vicarious ions on the physical, microstructural, mechanical, and biological properties of the obtained HAps. In detail, after an introduction about the biological apatites and the stoichiometric hydroxyapatite structure, this chapter reports the synthesis and features of anionic and cationic substituted hydroxyapatites with an outline of the most important findings. Finally, the last session presents some considerations, concluding remarks, and future developments.