Physical properties and in vitro bioactivity of hierarchical porous silica–HAP composites

Abstract

In two recent papers (J. Andersson, S. Areva, B. Spliethoff and M. Lindén, Biomaterials, 2005, 26, 6827–6835 and J. Andersson, E. Johannessen, S. Areva, M. Järn and M. Lindén, J. Nanosci. Nanotechnol., 2006, 6, 2438–2444) we have presented new means of synthesizing silica–calcium phosphate (hydroxyapatite or tricalcium phosphate) composite materials, where the calcium phosphate is covered by a mesoporous layer of silica. These materials are bifunctional biomaterials, as they can be used both as drug carrier matrices and osteoconductive materials. Some of these materials, especially if synthesized according to a one-pot method, exhibit a very high in vitro bioactivity, and nucleate and grow calcium phosphate on their surfaces in less than 24 h if exposed to a simulated body fluid. In the present study, we have carried out a thorough characterization of the one-pot sol–gel derived composite materials by the means of solid state ²⁹Si magic angle spinning (MAS) NMR, ³¹P MAS NMR, ²³Na MAS NMR, and transfer of population in double resonance (TRAPDOR) NMR spectroscopy, scanning electron microscopy, and transmission electron microscopy. The aim of the study is to relate the material properties to the in vitro bioactivity. The reason for high bioactivity of the composites cannot be ascribed to the silica content, but primarily to the presence of a highly soluble second calcium phosphate phase, NaCaPO₄, co-existing with the hydroxyapatite in the hybrid material. Furthermore, the hydroxyapatite becomes increasingly calcium deficient with increasing silica content, which adds to increase the bioactivity. Also the overall crystallinity of the apatitic calcium phosphate phase could contribute to the bioactivity of the composites in vitro.