Bioceramic powders for bone regeneration modified by high-pressure CO2 process

Non-stoichiometric nanocrystalline apatites present enhanced bioactivity compared to stoichiometric hydroxyapatite. The purpose of this work was to modify the calcium phosphates (CaP) generally used to prepare bioactive ceramics in the aim of obtaining a biomimetic apatite powder. Hydroxyapatite (HA) powder, amorphous tricalcium phosphate (amTCP) powder and a blend of these two were modified by means of an innovative, simple, “green” carbonation process, involving water and high-pressure CO2 (80 bar). This process induced a modification of the CaP, which is sensitive to the environment in which it is located and, in particular, to the pH variations that occur during the treatment phase (decrease in the pH) and during the degassing phase (return to neutral pH). FTIR and Raman spectroscopy, XRD and SEM analyses showed that, depending on the type of initial CaP powder, high-pressure CO2 treatment led to the formation of different types of calcium phosphate phases. This process allowed partial dissolution of the initial powder, mainly of TCP when present, and precipitation of a new CaP phase. HA and HA/amTCP powders were transformed into a mixture of OCP and immature carbonated apatite (PCCA) phases, including OCP maturation/transformation into PCCA. In the case of amTCP powder, a DCPD phase was also present due to the high TCP solubility and an earlier precipitation during the degassing step. This work shows the great potential of such an innovative low-temperature and high-pressure process to transform HA, HA/TCP and TCP powder into bioactive biphasic ceramics composed of OCP and PCCA similar to bone mineral.