Formation and Transformation of Calcium Phosphate Phases under Biologically Relevant Conditions: Experiments and Modelling. Agnese Carino (1,2) , Christian Ludwig (1,2) , Antonio Cervellino (4) , Elisabeth Müller (3) , and Andrea Testino (1)* 1 Paul Scherrer Institut (PSI), ENE, CH-5232 Villigen PSI, Switzerland, 2 École Polytechnique Fédérale de Lausanne (EPFL), ENAC IIS GR-LUD, CH-1015 Lausanne, Switzerland, 3 Paul Scherrer Institut(PSI), BIO, CH-5232 Villigen PSI, Switzerland, and 4 Paul Scherrer Institut (PSI), SYN, CH-5232 Villigen PSI, Switzerland Abstract The experimental data on calcium phosphates formation were collected in dilute solution at constant pH (7.40) and temperature (37.0 C) at different levels of ionic strength (IS). The evolution of the solid phase formation is described in detail using a thermodynamic-kinetic model. The thermodynamic model takes into account all relevant chemical species as well as Posner’s clusters; the kinetic model, based on the discretized population balance approach, accounts for the solid formation from solution. The experimental data are consistent with an initial formation of dicalcium phosphate dihydrate (DCPD, brushite), which dominates the nucleation rate, and its rapid transformation into octacalcium phosphate (OCP) or hydroxyapatite (HA), which dominates the growth rate. Depending on the experimental conditions and, including the influence of the IS level, OCP may be further transformed into apatite. The classical nucleation theory is able to describe the experimental results very well and the solid phase growth is limited by the diffusion of Ca 2+ ions. The precipitation pathway described by a complete thermodynamic-kinetic model is expected to contribute to the understating of the in vivo osteogenesis. Keywords: calcium phosphates, precipitation kinetics, modelling, classical nucleation, diffusion-limited growth. This document is the accepted manuscript version of the following article: Carino, A., Ludwig, C., Cervellino, A., Müller, E., & Testino, A. (2018). Formation and transformation of calcium phosphate phases under biologically relevant conditions: Experiments and modelling. Acta Biomaterialia, 74, 478-488. https://doi.org/10.1016/j.actbio.2018.05.027 This manuscript version is made available under the CC-BY-NC-ND 4.0 license http:// creativecommons.org/licenses/by-nc-nd/4.0/