Atomic structure of biodegradable Mg-based bulk metallic glass

We have used highly accurate first-principles molecular dynamics simulations to elucidate the structure of Mg60Zn35Ca5 and Mg72Zn23Ca5 bulk metallic glasses, which are candidate materials for biomedical implants; these two compositions exhibit different behaviours when implanted. The environments of each species are different, and average coordination numbers are [similar]13 for Mg, [similar]11 for Zn and [similar]18–19 for Ca. A wide range of local environments were found and icosahedral motifs, often seen in bulk metallic glasses, were among the most common for both Mg and Zn. Through the computation of a chemical short-range order parameter, a moderate avoidance of Zn–Zn bonding over Zn–Mg or Zn–Ca was observed. No statistically significant difference in structure was observed between the two compositions

Atomic structure and dissolution properties of yttrium-containing phosphate glasses

We have conducted classical molecular dynamics simulations of three compositions of yttrium-containing phosphate glasses, to study the atomic structure around yttrium, and understand how yttrium incorporation will affect the glass dissolution rate. The Y-O bond length is about 2.2 Å and the coordination number is 6.3. To avoid effects due to different network connectivities, our compositions were chosen to keep the Qn distribution and network connectivity roughly constant, which was confirmed through direct calculation. For these compositions, the structure of the phosphate network is comprised of finite-length chains of PO4 tetrahedra bound to the network modifiers. We showed that yttrium bonds to 4.2-4.3 of these chains, compared to 3.8 for calcium, and 3.1-3.2 for sodium. This implies that yttrium will bond more parts of the glass to the same place, and therefore, that yttrium incorporation will strengthen phosphate glass against dissolution, making it potentially suitable for radiotherapy applications where a durable glass is required

The role of fluoride in the nano-heterogeneity of bioactive glasses

Fluoride-containing bioactive phospho‐silicate glasses have recently attracted interest for dental applications, particularly as remineralising additives in dentifrices, and are potentially attractive for bone regeneration, particularly in patients suffering from osteoporosis. The incorporation of fluoride into phospho­‐silicate glasses is also attractive from a structural viewpoint: Fluoride complexes modifier ions rather than binding to the silicate network, and it thereby adds a significant ionic contribution to the average character of chemical bonds in the system. Molecular dynamics simulations have suggested that this also results in the formation of nano-­eterogeneities. In this paper, we review the current knowledge on the structural role of fluoride in bioactive glasses, with a particular focus on inhomogeneities on a nano-­‐scale

Effect of strontium inclusion on the bioactivity of phosphate-based glasses

We have conducted first-principles and classical molecular dynamics simulations of various compositions of strontium-containing phosphate glasses, to understand how strontium incorporation will change the glasses’ activity when implanted into the body (bioactivity). To perform the classical simulations, we have developed a new interatomic potential, which takes account of the polarizability of the oxygen ions. The Sr-O bond length is ∼ 2.44 − 2.49Å, and the coordination number is 7.5 – 7.8. The Qn distribution and network connectivity were roughly constant for these compositions. Sr bonds to a similar number of phosphate chains as Ca does; based on our previous work [J. K. Christie et al., J. Phys. Chem. B 117, 10652 (2013)], this implies that SrO ↔ CaO substitution will barely change the dissolution rate of these glasses, and that the bioactivity will remain essentially constant. Strontium could therefore be incorporated into phosphate glass for biomedical applications

Atomic-scale clustering inhibits the bioactivity of fluoridated phosphate glasses

© 2019 Adja B. R. Touré et al. Here, molecular dynamics simulations have been carried out on phosphate glasses to clarify the previously debated influence of fluoride on the bioactivity of these glasses. We developed a computationally advanced inter-atomic force field including polarisation effects of the fluorine and oxygen atoms. Structural characterisations of the simulated systems showed that fluoride ions exclusively bond to the calcium modifier cations creating clusters within the glass structure and therefore decreasing the bioactivity of fluoridated phosphate glasses, making them less suitable for biomedical applications

A new potential for radiation studies of borosilicate glass

Borosilicate glass containing 70 mol% SiO2 and 30 mol% B2O3 is investigated theoretically using fixed charge potentials. An existing potential parameterisation for borosilicate glass is found to give good agreement for the bond angle and bond length distributions compared to experimental values but the optimal density is 30% higher than experiment. Therefore the potential parameters are refitted to give an optimal density of 2.1 g=cm3, in line with experiment. To determine the optimal density, a series of random initial structures are quenched at a rate of 5 1012 K/s using constant volume molecular dynamics. An average of 10 such quenches is carried out for each fixed volume. For each quenched structure, the bond angles, bond lengths, mechanical properties and melting points are determined. The new parameterisation is found to give the density, bond angles, bond lengths and Young’s modulus comparable with experimental data, however, the melting points and Poisson’s ratio are higher than the reported experimental values. The displacement energy thresholds are computed to be similar to those determined with the earlier parameterisation, which is lower than those for ionic crystalline materials

Properties of water confined in hydroxyapatite nanopores as derived from molecular dynamics simulations

Bone tissue is characterized by nanopores inside the collagen-apatite matrix where fluid can exist and flow. The description of the fluid flow within the bone has however mostly relied on a macroscopic continuum mechanical treatment of the system, and, for this reason, the role of these nanopores has been largely overlooked. However, neglecting the nanoscopic behaviour of fluid within the bone volume could result in large errors in the overall description of the dynamics of fluid. In this work, we have investigated the nanoscopic origin of fluid motion by conducting atomistic molecular dynamics simulations of water confined between two parallel surfaces of hydroxyapatite (HAP), which is the main mineral phase of mammalian bone. The polarizable core–shell interatomic potential model used in this work to simulate the HAP–water system has been extensively assessed with respect to ab initio calculations and experimental data. The structural (pair distribution functions), dynamical (self-diffusion coefficients) and transport (shear viscosity coefficients) properties of confined water have been computed as a function of the size of the nanopore and the temperature of the system. Analysis of the results shows that the dynamical and transport properties of water are significantly affected by the confinement, which is explained in terms of the layering of water on the surface of HAP as a consequence of the molecular interactions between the water molecules and the calcium and phosphate ions at the surface. Using molecular dynamics simulations, we have also computed the slip length of water on the surface of HAP, the value of which has never been reported before

Review: Understanding the properties of amorphous materials with high-performance computing methods

Amorphous materials have no long-range order in their atomic structure. This makes much of the formalism for the study of crystalline materials irrelevant, and so elucidating their structure and properties is challenging. The use of computational methods is a powerful complement to experimental studies, and in this paper we review the use of high-performance computing methods in the simulation of amorphous materials. Five case studies are presented to showcase the wide range of materials and computational methods available to practitioners in this field.  This article is part of a discussion meeting issue 'Supercomputing simulations of advanced materials'.</p

Bioactive glasses

This chapter reviews the use of computer simulation of bioactive silicate-based glasses in order to understand how the glass composition and structure affects the bioactivity. Key to this is the successful modeling of the interatomic forces, which is often done through the use of classical empirical expressions (potentials) to allow for simulations larger in length and time than could be done from first principles. The inclusion of polarizability into these potentials is crucial to represent the Q n distribution and medium-range structure accurately, which both strongly affect the bioactivity of the glass. Results obtained with such potentials are in good agreement with experimental data and first-principles simulations. Silicon and phosphorous are network formers; sodium and calcium are network modifiers, which compete for nonbridging oxygen (NBO) atoms and the amounts of NBOs in the first coordination shell of each cation correlates with its field strength. Local chemical bonding can also drive clustering on larger length scales, which can lead to structural inhomogeneities which could affect the bioactivity. By appropriate fitting of the potentials, the effect on including other therapeutic ions can be considered, including, in some cases, ions which exist in more than one oxidation state in the glass. The changes in the structure present at the surface of the glass after implantation in the body are also accessible to computer simulation, although it can be difficult, using current techniques, to understand completely the chemical reactions between the glass and the surrounding environment. Future directions for research are also discussed.</p