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Phosphate-based bioactive glasses (PBGs) dissolve harmlessly in the body with a dissolution rate which depends sensitively on composition. This makes them proposed vectors for e.g. drug delivery, or other applications where an active component needs to be delivered at a therapeutically appropriate rate. Molecular dynamics (MD) simulations provide atomic-level structural information about PBG compositions. We review recent work to show that MD is an excellent tool to unravel the connections between the PBG glass composition, its atomic structure, and its dissolution rate, which can help to optimise PBGs for specific medical applications.

Classical molecular dynamics (MD) simulations were used to study the structural changes in the surfaces of biocompatible phosphate glasses with compositions (P2O5)0.45(CaO)x(Na2O)0.55−x (x=30, 35, 40) to evaluate their effect on the solubility of the material. Direct comparison of the data for the three compositions highlighted the critical role that an enhancement in Na+ concentration plays in the hydrolysis of the material, which is responsible for the release of network components into solution. The calculations also confirm that the most soluble material is (P2O5)0.45(CaO)0.30(Na2O)0.25, has the lowest calcium coordination number, thereby causing fewer cross links to phosphate chains.

We have used classical molecular dynamics simulations to characterise the structure of three compositions of silver-containing phosphate glasses with 45mol% P2O5, 30mol% CaO, and varying amounts of Na2O and Ag2O. These compositions all have the same network connectivity, allowing us to highlight two other structural features which will affect the glass dissolution. Firstly, the number of different phosphate chains bonded to each modifier atom was computed and it was observed that silver and sodium bind to roughly the same number of phosphate chains, despite the differences in their local environments. Secondly, the clustering of modifier cations was characterised and shown to be enhanced at low concentrations of sodium and silver, but not to exist for calcium.

Classical molecular dynamics (MD) simulations were used to study the effects of water on the structural relaxation of the surfaces of bio-active phosphate-based glasses with compositions (P2O5)0.45(CaO)x(Na2O)0.55-x (x = 0.30, 0.35 and 0.40). Direct comparison of the data for the three compositions showed that surfaces with x = 0.30 experienced the highest calcium diffusion, as well as highest sodium concentration at the glass/water interface, confirming these systems as the most soluble of the three compositions studied. Our results also show the importance of surface hydroxylation in the simulation of these types of bio-glasses, which causes differential relaxation of the surfaces, leading to changes in network polymerization that modulate the diffusion of water and modifiers into and out of the glasses, with direct impact on dissolution.

Bioactive phosphate-based glasses (PBGs) have several possible biomedical applications because of the chemical reactions they undergo with their surroundings when implanted into the body. The dissolution rate of PBGs in physiological conditions is a crucial parameter for these applications, to ensure, e.g., delivery of drugs or nutrients to the body at the correct rate. While it has been well-known that increasing the CaO content of these glasses at the expense of Na2O slows the dissolution rate, this paper provides an atomistic explanation of this for the first time. In this work, molecular dynamics simulations of five ternary P2O5-CaO-Na2O glasses reveal the structural properties at the atomic level that enhance the durability of PBGs as more Ca is added: (i) Ca binds together more fragments of the phosphate glass network than Na, (ii) Ca binds together more PO4 tetrahedra than Na, and (iii) Ca has a lower concentration of intratetrahedral phosphate bonding than Na. This behavior is rooted in the calcium ion's higher charge and field strength. These results open the path to precise control and optimization of the PBG dissolution rate for specific biomedical applications.

Molecular dynamics simulations of phosphate-based glasses P(2)O(5)-CaO-Na(2)O have been carried out, using an interatomic force field that has been parameterized to reproduce the structural and mechanical properties of crystalline phosphorus pentoxide, o(')(P(2)O(5))(∞) orthorhombic phase. Polarization effects have been included through the shell-model potential and formal charges have been used to aid transferability. A modification to the DL_POLY code (version 2.20) was used to model the high temperature shell dynamics. Structural characterizations of three biomedically applicative molar compositions, (P(2)O(5))(0.45)(CaO)(x)(Na(2)O)(0.55-x) (x = 0.30, 0.35, and 0.40), have been undertaken. Good agreement with available experimental and ab initio data is obtained. The simulations show that, dependent on composition, the phosphorus atoms are primarily bonded to two or three oxygens that in turn bridge to neighbouring phosphorus atoms. Na(+) and Ca(2+) modifiers are found to occupy a pseudo-octahedral bonding environment with mean oxygen coordination numbers of 6.55 and 6.85, respectively, across all compositions studied.

The local and medium-range structural properties of phosphate-based melts and glasses have been characterized by means of first principles (density functional theory) and classical (shell-model) molecular dynamics simulations. The structure of glasses with biomedically active molecular compositions, (P2O5)0.45(CaO)x(Na2O)0.55−x (x = 0.30, 0.35 and 0.40), have been generated using first principles molecular dynamics simulations for the full melt-and-quench procedure and the changes in the structural properties as the 3000 K melt is cooled down to room temperature have been compared extensively with those of the final glasses. The melts are characterized by a significant fraction of threefold (P3c) and fivefold (P5c) phosphorus atoms, but structural defects rapidly decrease during the cooling phase and for temperatures lower than 1800 K the system is free of under- and over-coordinated species. The analysis of the structures of the glasses at 300 K shows a prevalence of the metaphosphate Q2 and pyrophosphate Q1 species, whereas the number of Q3 units, which constitute the three-dimensional phosphate network, significantly decreases with the increase of calcium content in the glass. The radial and angular distribution functions indicate that higher calcium concentration in the glass leads to an increase of the rigidity of the phosphate tetrahedral network, which has been explained in terms of the calcium's higher field strength compared to that of sodium. The structural characterization of the melts and glasses obtained from first principles simulations was used to assess and validate a recently developed interatomic shell-model forcefield for phosphate-based materials. For all three compositions, our potential model is in good agreement with the first principles data. In the glass network, the forcefield provides a very good description of the split between the shorter distances of phosphorus to non-bonded oxygen and the longer distances of the phosphorus to bonded oxygen; the phosphorus–phosphorus medium-range distribution; and the coordination environment around the Na and Ca glass modifiers. Moreover, the distribution of the Qn species in the melts and glasses is in excellent agreement with the values extracted from the first principles simulations. In contrast, simulations using standard rigid ion potentials do not provide a satisfactory description of the local short-range structure of phosphate-based glasses and are therefore less suitable to model this class of multicomponent amorphous system.

Phosphate-based bioactive glasses containing fluoride ions offer the potential of a biomaterial which combines the bioactive properties of the phosphate glass and the protection from dental caries by fluoride. We conduct accurate first-principles molecular dynamics simulations of two compositions of fluorinated phosphate-based glass to assess its suitability as a biomaterial. There is a substantial amount of F–P bonding and as a result the glass network will be structurally homogeneous on medium-range length scales, without the inhomogeneities which reduce the bioactivity of other fluorinated bioactive glasses. We observe a decrease in the network connectivity with increasing F content, caused by the replacement of bridging oxygen atoms by non-bridging fluorine atoms, but this decrease is small and can be opposed by an increase in the phosphate content. We conclude that the structural changes caused by the incorporation of fluoride into phosphate-based glasses will not adversely affect their bioactivity, suggesting that fluorinated phosphate glasses offer a superior alternative to their silicate-based counterparts.