Mancardi, Giulia; (2018) Computational Study of the Nucleation of Calcium Phosphate. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Hydroxyapatite (HA), the main mineral phase of mammalian tooth enamel and bone, originates in body fluids from amorphous calcium phosphate (ACP). The early stage of ACP formation from solution is the object of the investigations presented in this thesis. I have first performed ab initio molecular dynamics simulations to shed light on the structure and stability of the calcium phosphate (CaP) prenucleation clusters, [Ca(HPO4)3]4 – , in aqueous solution. The calcium is seven coordinated by two water molecules, two bidentate phosphates and one monodentate phosphate. Free energy profiles obtained using umbrella sampling simulations show that the complex with a Ca-to-P ratio of 1:3 is the most energetically favoured, and thermodynamically more stable than the free ions. In order to be able to study a larger system, I have employed shell-model molecular dynamics simulations to investigated the aggregation and clustering of calcium and phosphate ions in water. ACP presents short-range order in the form of small domains with size of 0.9 nm and chemical formula Ca9(PO4)6, known as Posner’s clusters. Calcium phosphate aggregates form in solution with compositions and Ca coordination that are similar to those found in Posner’s cluster, but the stoichiometry of these species is dependent on the ionic composition of the solution: calcium-deficient clusters in solutions with low Ca/P ratio; cluster containing protonated phosphate groups in neutral solutions; sodium ions partially substituting calcium in solutions containing a mixture of sodium and calcium ions. These Posner-like clusters can be connected by phosphate groups, which act as a bridge between their central calcium ions. The simulations of the aggregation in solution of calcium phosphate clusters are an unbiased and unequivocal validation of Posner’s model and reveal for the first time the structure and composition of the species that form during the early stages of ACP nucleation at a scale still inaccessible to experiment. Lastly, I have investigated the heterogeneous nucleation of CaP on a titanium implant. Titanium is commonly employed in orthopaedic and dental surgery due to its good mechanical properties. In order to promote the integration of the metallic implant with the biological tissues, titanium is passivated by a thin layer of oxide and covered by a bioactive material, normally HA; HA can originate on the oxide during a process called biomimetic deposition which consists in soaking the implant in simulated body fluids, that are supersaturated with respect to HA. This method allows to efficiently cover implants with complex shapes and to create a porous coating similar to the bone tissue. Here, I have used molecular dynamics and interatomic potentials to study the deposition of calcium and phosphate species on the titanium dioxide anatase. Different force fields developed for calcium phosphate, titanium dioxide and water were combined and the new parameters were benchmarked against DFT data. Calcium phosphate interaction with the (101) and (100) surfaces of anatase was successfully investigated and the force field here proposed can be used to study the nucleation of calcium phosphate on other titanium dioxide polymorphs and on common surface defects.

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