Crystallized nano-sized alpha-tricalcium phosphate from amorphous calcium phosphate: microstructure, cementation and cell response

New insight on the conversion of amorphous calcium phosphate (ACP) to nano-sized alpha tricalcium phosphate (alpha-TCP) provides a faster pathway to calcium phosphate bone cements. In this work, synthesized ACP powders were treated with either water or ethanol, dried, crystallized between 700 and 800 degrees C, and then cooled at different cooling rates. Particle size was measured in a scanning electron microscope, but crystallite size calculated by Rietveld analysis. Phase composition and bonding in the crystallized powder was assessed by x-ray diffraction and Fourier-transform infrared spectroscopy. Results showed that 50 nm sized alpha-TCP formed after crystallization of lyophilized powders. Water treated ACP retained an unstable state that may allow ordering to nanoapatite, and further transition to beta-TCP after crystallization and subsequent decomposition. Powders treated with ethanol, favoured the formation of pure alpha-TCP. Faster cooling limited the growth of beta-TCP. Both the initial contact with water and the cooling rate after crystallization dictated beta-TCP formation. Nano-sized alpha-TCP reacted faster with water to an apatite bone cement than conventionally prepared alpha-TCP. Water treated and freeze-dried powders showed faster apatite cement formation compared to ethanol treated powders. Good biocompatibility was found in pure alpha-TCP nanoparticles made from ethanol treatment and with a larger crystallite size. This is the first report of pure alpha-TCP nanoparticles with a reactivity that has not required additional milling to cause cementation.