LAUSR

Abstract Calcium phosphate-based bioactive materials have attracted considerable attention in recent years to repair bone defects. In particular, poorly crystalline apatites (PCA) are successfully used for bone repair and regeneration, enhancing the osteointegration and osteoconduction properties. Our study aims to better understand the mechanism of apatite formation on the poorly crystallized apatite powders. For this purpose, two PCA samples were synthesized in the same conditions varying the maturation time. The most noticeable difference between both samples is that the immature specimen (PCA2h) is richer in both water and HPO 4 2 − labile species than the mature specimen (PCA2m). It has the highest surface area and negatively charged surface too. They were soaked in a simulated body fluid (SBF) for different periods of time at 37 °C, leading to the neoformation of bone-like apatite layer on their surfaces due to their bioactivity. The surface of all samples were investigated using scanning electron microscopy (SEM) coupled with energy-dispersive X-ray (EDX) spectroscopy, zeta-potential measurements and BET analysis. FT-IR spectroscopy, XRD, and TG-DTA were carried out to characterize bulk samples. For further investigation of bone-like apatite formation, thermodynamic calculations of Ca–P precipitation were performed. Experimental and theoretical data showed that the bone-like apatite formation mechanism on both samples is dominated by three stages. Only on PCA2h, a fast precipitation of positively Ca-rich amorphous phase (ACP) occurs at first time (prenucleation stage). It is converted into Ca-poor ACP by uptaking anions from surrounding media, to promote the formation of apatite nuclei during the second stage (nucleation stage). Then, nascent apatite crystals grow into bone like-apatite coating by poorly crystallized BCO3-apatite which is the most CaP phase thermodynamically stable in SBF (growing and maturation stage). While the bone like-apatite formation process on the mature PCA particles include similar events without Ca-rich ACP deposit. Thus, the formation of the Ca-poor becomes directly at the 1st stage, followed by apatite nucleation (2nd stage), promoting the growth and maturation of BCO3-apatite crystals (3rd stage). The main difference between the events associated to bone-like formation process on both PCA specimens consists of that the Ca2+-consumed by the PCA2h is higher than by PCA2m in the earliest stage, enhancing bioactivity of PCA-based materials. This is due to the discrepancy existing between negative surface charges of both specimens. Our results highlight the fact that in the waterlogged conditions of the hydrated shells surrounding the immature PCA nanocrystals, a high density of labile orthophosphate anions leads to enrich a PCA-liquid solution interface in Ca2+ ions, momentarily stabilizing them into a positively charged amorphous complex likely strongly solvated. Then, this interface is on its way to becoming more attractive for free orthophosphate anions, enhancing a bioactivity to improve osteointegration. The composition of the hydrated shells coating of the biomimetic apatite crystal in both labile species and hydration water seems to govern their bioactivity via surface charge potential and hydrophilic capacity. These data allow a better understanding of the bioactive behaviour of biomaterials involving biomimetic PCA phases for tissue engineering.