Formation of hydroxyapatite layer on graphite sheet immersed in calcium phosphate solution by microwave heating

Abstract

The conditions for the formation of hydroxyapatite [Ca₁₀(PO₄)₆(OH)₂; HAp] layer on the graphite sheet were examined via the processes of (i) the acid treatment of conc. nitric and sulfuric acids at 50°C for 24 h for the formation of carboxyl group, and (ii) the external heating (90°C, 1 h) and/or microwave heating (100°C, 5 min) of the acid-treated graphite sheet in the “apatite-dissolved solution” prepared by bubbling CO₂ gas into the HAp-dispersed water. The combination of external and microwave heating was effective in the formation of large amount of calcium phosphates, because the seed crystals of apatite formed by the evaporation of solvent from the apatite-dissolved solution (i.e., external heating) contributed to enhancing the formation of calcium phosphate layer through the microwave heating. The calcium phosphate layer formed by stacking the plate-like particles possessed the typical thickness of 4.9 µm, and was identified as HAp with Ca/P ratio of 1.72 or carbonate-containing HAp. Based upon the graphite sheet with HAp layer implanted into femur and tibia of the Japanese white rabbit for 4 months, the formation of calcified bone at the interfaces was found to show the excellent biocompatibility.