Synthesis of hydroxyapatite in a continuous reactor: a review

We propose a scaled-up reactor for the synthesis of hydroxyapatite nanoparticles (nHAp) applicable for lecithin-modified precipitation. A novel 3D-printed BOX reactor allowed, confirmed by computational fluid dynamics analyses, maintaining or improving the hydrodynamic conditions corresponding to the conditions within single-channel millireactors described in our previous work. As confirmed by X-ray diffraction analysis, we obtained hydroxyapatites. Fourier-transform infrared spectroscopy showed the same characteristic functional groups in powders from single-channel millireactors and BOX reactors. Scanning transmission electron microscopy showed that regardless of lecithin modification, the hydroxyapatite powders from the BOX reactor have particles of the same size as those from the single-channel millireactors; the particle size does not exceed 30 nm. Moreover, nanoparticle tracking analysis and dynamic light scattering show that regardless of the selected reactor and the presence of surface modification, particles tend to agglomerate, which we confirmed in zeta potential measurements. The nHAp’ chemical and physical properties and the lecithin-modified nHAp obtained in the BOX reactor do not depend significantly on the reactants’ flow rate, so it is possible to obtain particles with a precipitate suspension productivity of up to 150 g/h. The presented method of hydroxyapatite wet precipitation scale-up is a step toward stable and efficient production of a restorative material for medical and dental applications.

This review highlights the reproducibility challenges related with HAp-based biomaterials production processes for 3D printing, with a focus on continuous production as an alternative to bridge the gap between research and industrial/clinical use.

Fabrication of calcium phosphates with controlled properties using a modular oscillatory flow reactor

Precipitation of hydroxyapatite nanoparticles in 3D-printed reactors