Mg/Cu co-substituted hydroxyapatite – Biocompatibility, mechanical properties and antimicrobial activity

The aim of this study was to improve the mechanical properties and to optimize antimicrobial activity of hydroxyapatite (HAP) by simultaneous doping with Mg and Cu ions in order to obtain material that would be able to assist in the bone/tooth healing process, prevent post-implementation infections and provide satisfying values of hardness and fracture toughness for biomedical application. Ion doping was done during the hydrothermal synthesis of HAP powders, whereby the content of Mg ions in the starting solution was varied between 1-20 mol. % with regard to Ca ions, while the amount of Cu ions was kept constant at 0.4 mol. %. The green compacts were sintered for 2 h at temperatures ranging 750–1200 °C depending on the Mg content, chosen in agreement with dilatometry results. Presence of Mg ions was found to favour transition from HAP to β−tricalcium phosphate phase (β−TCP), which enabled formation of biphasic HAP/β−TCP and pure β−TCP phase at 160 °C during hydrothermal synthesis. In vitro investigation of antimicrobial activity against Escherichia coli, Staphylococcus aureus and Enterococcus faecalis showed satisfactory antimicrobial activity. MTT assay performed on MRC-5 and L929 cell lines showed excellent cytocompatibility and cell proliferation. Maximum hardness by Vickers and fracture toughness values, 4.96 GPa and 1.75 MPa m1/2 respectively, were obtained upon addition of 5 mol. % Mg, as a consequence of the lowest grain size and porosity, as well as the highest densification rate. This is, to the best of our knowledge, the highest fracture toughness for HAP or β-TCP ceramics reported thus far.