Physicochemical and cytological properties of poorly crystalline calcium-deficient hydroxyapatite with different Ca/P ratios

Compared to highly crystalline hydroxyapatite (HA), poorly crystalline hydroxyapatite has been proved to have better bioactivity and degradability, owing to its similar crystallographic structure to natural bone. However, there are few systematic comparative studies on poorly crystalline HA with different Ca/P ratios. In this work, poorly crystalline HA with different Ca/P molar ratios (1.50, 1.55, 1.60 and 1.67) was prepared by chemical precipitation method. The effects of Ca/P ratio on its structure, composition, morphology, surface properties, protein adsorption behaviors, ion adsorption and release abilities, cytological properties were systematically investigated. Results showed that the prepared HA was poorly crystalline, nano-sized and with the gradient change of Ca/P ratios. Protein adsorption capacity and P release of calcium deficient HA (CDHA) were effectively improved by reducing the Ca/P ratio, but CDHA with lower Ca/P ratio would also adsorb more Ca ions in the culture medium, which jointly affected the cytological properties of CDHA. In vitro cell experiments indicated that when mouse bone mesenchymal stem cells were co-cultured with CDHA with a Ca/P ratio of 1.55, their proliferation, ALP activity and osteogenesis-related genes expression were the strongest. This study provides a theoretical support and potential for further improving the biological performance of poorly crystalline CDHA materials.