Table 1.

Most common doping ions and their influence on biological properties of hydroxyapatite. The ionic radii (Å) of stoichiometric hydroxyapatite ions are Ca²⁺ 0.99; PO₄³⁻ 2.38; OH⁻ 1.40.

Doped Cations	Ionic Radius (Å)	Main Properties	Reference
Mg2+	0.69	enhanced biodegradability improved biocompatibility osteogenic antibacterial	[12,13,14,15,16]
Sr2+	1.13	improved biocompatibility osteogenic inhibit osteoclast activity improve mechanical properties	[17,18,19,20,21,22,23,24]
Fe2+/Fe3+	0.84/0.66	drug targeting bioseparation hyperthermia therapy growth of osteoblast	[25,26,27,41,42]
Zn2+	0.74	osteogenic promotes osteoblast proliferation cell growth and differentiation antibacterial	[28,29,30]
Ag+	1.28	antibacterial	[31,32]
Doped Anions	Ionic Radius (Å)	Main Properties	Reference
CO32−	1.78	enhanced solubility enhanced biodegradability improved biocompatibility osteoconductivity increased collagen production	[10,33,34,43]
SiO44−	2.40	biomineralisation osteogenic increased bioactivity growth of osteoblast enhanced cell proliferation more efficient differentiation	[39,40,44,45]
F−	1.19	biomineralisation osteogenic less soluble in acidic solutions influences cell behaviour and responsiveness antibacterial	[27,28,29]