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The aim of this study was to propose and validate a new unified method for testing dissolution rates of bioactive glasses and their variants, and the formation of calcium phosphate layer formation on their surface, which is an indicator of bioactivity. At present, comparison in the literature is difficult as many groups use different testing protocols. An ISO standard covers the use of simulated body fluid on

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Bioactive glasses represent important biomaterials being investigated for the repair and reconstruction of diseased bone tissues, since they exhibit outstanding bonding properties to human bone. In the present study bioactive glass (type 45S5) nanoparticles (nBG) with a mean particle size in the range of 20-60 nm, synthesized by flame spray synthesis, are investigated in relation to in vitro bioreactivity in simulated body fluid (SBF) and response to osteoblast cells. The structure and kinetics of hydroxyapatite formation in SBF were investigated using transmission electron microscopy (TEM), X-ray diffraction (XRD) and Fourier-Transform Infrared Spectroscopy (FT-IR) revealing a very rapid transformation (after 1d) of nBG to nanocrystalline bone-like carbonated HAp. Additionally, calcite is formed after 1d of SBF immersion due to the high surface reactivity of the nBG particles. In the initial state nBG particles were found to exhibit chain-like porous agglomerates of amorphous nature which are transformed upon immersion in SBF into compact agglomerates covered by hydroxyapatite with a reduced size of the primary nanoparticles. In vitro studies revealed high cytocompatibility of nBG with human osteoblast cells, indicated through high lactatedehydrogenase (LDH) and mitochondrial activity as well as alkaline phosphatase activity. Hence, this study contributes to the understanding of the structure and bioactivity of bioactive glass (type 45S5) nanoparticles, providing insights to the phenomena occurring at the nanoscale after immersion in SBF. The results are relevant in relation to the understanding of the nanoparticles' bioreactivity required for applications in bone tissue engineering.