Characterization, physicochemical properties and biocompatibility of La-incorporated apatites

In this study, the physicochemical properties and biocompatibilities of La-containing apatites were intensively investigated together with their characterizations in terms of composition, structure, valent state and morphology using X-ray diffraction, Fourier-transform infrared spectra, X-ray photoelectron spectroscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy, respectively. The results indicate that the La³⁺ ion can be incorporated into the crystal lattice of hydroxyapatite resulting in the production of La-incorporated apatites (La_xCa_10-x(PO₄)₆(OH)_2+x-2yO_y□y-x (x ≤ 0.5, y < 1 + x/2) or La_xCa_10-x(PO₄)₆O_y□y-x (0.5 < x < 2, y = 1 + x/2)) by high-temperature solid phase synthesis. For La content <20%, the product is composed of the major phase, La_x-OAP, as well as a small amount of tricalcium phosphate, but for a La content of 20%, the product is pure La-incorporated oxyapatite with the formula La₂Ca₈(PO₄)₆O₂ (La₂-OAP, x = 2, y = 2). It is also found that the La content plays important roles in both the physicochemical properties and biocompatibilities of the La-incorporated apatites. In contrast to La-free apatite, La-incorporated apatites possess a series of attractive properties, including higher thermal stability, higher flexural strength, lower dissolution rate, larger alkaline phosphatase activity, preferable osteoblast morphology and comparable cytotoxicity. In particular, the sintered La-incorporated apatite block achieves a maximal flexure strength of 66.69 ± 0.98 MPa at 5% La content (confidence coefficient 0.95), increased 320% in comparison with the La-free apatite. The present study suggests that the La-incorporated apatite possesses application potential in developing a new type of bioactive coating material for metal implants and also as a promising La carrier for further exploring the beneficial functions of La in the human body.