Sic Parvis Magna: Manganese-Substituted Tricalcium Phosphate and Its Biophysical Properties

Abstract

© 2019 American Chemical Society. Succeeding in the substitution of pharmaceutical compounds with ions deliverable with the use of resorbable biomaterials could have far-reaching benefits for medicine and economy. Calcium phosphates are known as excellent accommodators of foreign ions. Manganese, the fifth most abundant metal on Earth was studied here as an ionic dopant in β-tricalcium phosphate (β-TCP) ceramics. β-TCP containing different amounts of Mn2+ ions per MnxCa3-x(PO4)2 formula (x = 0, 0.001, 0.01, and 0.1) was investigated for a range of physicochemical and biological properties. The results suggested the role of Mn2+ as a structure booster, not breaker. Mn2+ ions increased the size of coherent X-ray scattering regions averaged across all crystallographic directions and also lowered the temperature of transformation of the hydroxyapatite precursor to β-TCP. The particle size increased fivefold, from 20 to 100 nm, in the 650-750 °C region, indicating that the reaction of formation of β-TCP was accompanied by a considerable degree of grain growth. The splitting of the antisymmetric stretching mode of the phosphate tetrahedron occurred proportionally to the Mn2+ content in the material, while electron paramagnetic resonance spectra suggested that Mn2+ might substitute for three out of five possible calcium ion positions in the unit cell of β-TCP. The biological effects of Mn-free β-TCP and Mn-doped β-TCP were selective: Moderately proliferative to mammalian cells, moderately inhibitory to bacteria, and insignificant to fungi. Unlike pure β-TCP, β-TCP doped with the highest concentration of Mn2+ ions significantly inhibited the growth of all bacterial species tested: Staphylococcus aureus, Salmonella typhi, Escherichia coli, Pseudomonas aeruginosa, and Enterococcus faecalis. The overall effect against the Gram-positive bacteria was more intense than against the Gram-negative microorganisms. Meanwhile, β-TCP alone had an augmentative effect of the viability of adipose-derived mesenchymal stem cells (ADMSCs) and the addition of Mn2+ tended to reduce the extent of this augmentative effect, but without imparting any toxicity. For all Mn-doped β-TCP concentrations except the highest, the cell viability after 72 h incubation was significantly higher than that of the negative control. Assays evaluating the effect of Mn2+-containing β-TCP formulations on the differentiation of ADMSCs into three different lineages-osteogenic, adipogenic, and chondrogenic-demonstrated no inhibitory or adverse effects compared to pure β-TCP and powder-free positive controls. Still, β-TCP delivering the lowest amount of Mn2+ seemed most effective in sustaining the differentiation process toward all three phenotypes, indicating that the dose of Mn2+ in β-TCP need not be excessive to be effective.