Influence of titanium ion doping on structural changes in porous hydroxyapatite ceramics

Abstract

Biocompatible titanium (Ti) is commonly used as an implant material. Hydroxyapatite (HAp, Ca10(PO4)6(OH)2) is a biomaterial that is similarly applied in the medical field owing to its biocompatibility. HAp is used as an alternative material for bones and teeth; for such purpose, the pores should have diameters of at least 200 μm. However, no studies have reported porous Ti-doped HAp. In this study, we fabricated five types of porous Ti-doped HAp ceramics using different amounts of 30% titanium sulfate solution (0.1, 0.2, 0.3, 0.4, and 0.5 g) as additive Ti ions in the chemical route. In general, the lattice parameters of Ti-doped HAp decrease because Ti ions are substituted for Ca ions but have a smaller ionic radius. However, the lattice parameters a and c actually increased in the porous Ti-doped HAp ceramics produced in this study, and only decreased in sample fabricated with 0.5 g of Ti solution. This change in structure was attributed to dehydroxylation of OH ions from the HAp lattice. Ti ions inserted into OH channels parallel to the c-axis in HAp lattices generate O–Ti–O bonds. Consequently, the lattice parameter c increased. The interaction between the dislocated OH ions and protonated HPO4 ions in the HAp lattices increased the lattice parameter a. The formation of O–Ti–O bonds did not maintain the charge balance, owing to the tetravalent Ti ions. However, the development of the c-plane in the HAp lattices increased as the amount of added Ti increased. Consequently, the growth of the negatively charged c-plane maintained the charge balance by compensating for the extra charges of the Ti ions.