Dependence of antimicrobial properties on site-selective arrangement and concentration of bioactive Cu2+ ions in tricalcium phosphate

Abstract

Cu-doped solid solutions Ca10.5–xCux(PO4)7 with β-Ca3(PO4)2 (β-TCP) structure were synthesized using high-temperature solid-state synthesis. Correctly characterizing the location of Cu2+ in the structure is important for a correct understanding of the biological properties formation. The limit of Ca10.5–xCux(PO4)7 solid solutions without impurity phases was found at x = 1.5. The crystal-chemical analysis reveals that the Cu2+ ions cannot be located in the M4 site of the β-TCP-type structure due to the very distorted environment. The Cu2+ ions preferably occupy the M5 site (x ≤ 1.00) and then located in the M3 and M2 sites (x > 1.00). The chemical composition, local environment, and structural features of Ca10.5–xCux(PO4)7 phosphates were investigated by a complex of methods. The antibacterial properties were studied on one fungus (C. albicans) and four bacteria (E. coli, E. faecalis S. aureus and P. aeruginosa). Biocompatibility tests were performed on human bone marrow mesenchymal stem cells (hMSCs). The biological response of the Cu2+ strongly depends on the occupied ion position and its concentration. The concentration-dependent behavior of the bacteria growth inhibition was observed, and the sample with x = 1.33 exhibits the highest activity between single-phase samples. Further increasing of bacteria growth inhibition is attributed to Ca3Cu3(PO4)4 impurity phase. Also, antimicrobial properties depend on the Cu2+ location in the β-TCP crystal structure in accordance with ion release behavior. The Cu2+ location in the large M3 polyhedra leads to a noticeable increase in ion release. Biocompatibility assays on hMSCs showed the absence of cytotoxicity in direct and indirect contact for single-phase samples. In the biphasic sample the presence of impurity Ca3Cu3(PO4)4 phosphate at 2 wt% causes a cytotoxicity effect. Thus, phase purity plays a critical role in understanding the regularities of bioactive properties formation. This work can be an initial basis for further investigations of the site-selective arrangement of bioactive ions, such as Cu2+ and other similar ions, in the β-TCP structure to enhance biological potential.