The use of yttrium aluminosilicate (YAS) glasses as vectors for radiotherapy is critically affected by the glass durability in a physiological medium. To understand the relation between glass composition, structure, and durability at an atomistic level, we have carried out classical molecular dynamics (MD) simulations of two YAS compositions with different durability. The analysis of the MD trajectories shows that the lower durability at high Y2O3 concentration is due to the combined effect of lower connectivity of the glass network and reduced yttrium clustering. Increasing the yttrium content increased the coordination numbers of all atomic species, made possible a greater range of atomic environments, and reduced the network connectivity, particularly related to silicon. Aluminum ions show a strong tendency to self-aggregate, and can form additional Al−O−Al linkages to balance the reduced number of Si network-formers in the high Y2O3 composition: this leads to some very highly connected aluminum atoms, characterized by the appearance of large-n Q n (Al) species in the corresponding distribution. The presence of significant yttrium clustering only in the more durable, low Y2O3 composition denotes that clustering of modifier ions can further enhance the glass durability, in agreement with previous results for bioactive glasses. (Tilocca et al. Chem. Mater. 2007, 19, 95.)

The incorporation of yttrium in bioactive glasses (BGs) could lead to a new generation of radionuclide vectors for cancer therapy, with high biocompatibility, controlled biodegradability and the ability to enhance the growth of new healthy tissues after the treatment with radionuclides. It is essential to assess whether and to what extent yttrium incorporation affects the favourable properties of the BG matrix: ideally, one would like to combine the high surface reactivity typical of BGs with a slow release of radioactive yttrium. Molecular Dynamics simulations show that, compared to a BG composition with the same silica fraction, incorporation of yttrium results in two opposing effects on the glass durability: a more fragmented silicate network (leading to lower durability) and a stronger yttrium-mediated association between separate silicate fragments (leading to higher durability). The simulations also highlight a high site-selectivity and some clustering of yttrium cations, which are likely linked to the observed slow rate of yttrium released from related Y-BG compositions. Optimisation of yttrium BG compositions for radiotherapy applications thus depends on the delicate balance between these effects.

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.

Fluorinated bioactive glasses (FBGs) combine the antibacterial properties of fluorine with the biological activity of phosphosilicate glasses. Because their biomedical application depends on the release of fluorine, the detailed characterization of the fluorine environment in FBGs is the key to understand their properties. Car-Parrinello molecular dynamics (CPMD) simulations have been performed on a 45S5 Bioglass composition in which 10 mol % of the CaO has been replaced with CaF(2), and have allowed us to resolve some longstanding issues about the atomic structure of fluorinated bioglasses, with particular regard to the structural role of fluorine. F is coordinated almost entirely to the modifier ions Na and Ca, with a very small amount of residual Si-F bonds, whose fraction only becomes significant in the melt precursor. High temperature leads to Si-F bonds in both tetra- (SiO(3)F) and, less frequently, penta-coordinated (SiO(4)F and SiO(3)F(2)) complexes, showing that formation of these bonds through the expansion of the SiO(4) coordination shell is generally less favored. There is no evidence for preferential bonding of F to either modifier ion: almost all F atoms are coordinated to both calcium and sodium in a "mixed state", rather than exclusively to either, as had been conjectured. We discuss the consequences of these findings on the properties of fluorine-containing bioglasses.

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.

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The low solubility (high durability) of yttrium aluminosilicate (YAS) glass is one of its most important properties for use in in situ radiotherapy. Simple parameters, such as silica or yttria content or network connectivity, are not sufficient to rationalize the dependence of the solubility on the glass composition observed experimentally. We performed classical molecular dynamics (MD) simulations of eight different YAS glasses of known solubility and analyzed the MD trajectories to identify specific structural features that are correlated and can be used to predict the solubility. We show that the (Si-)O-Si coordination number CN(SiOSi), the yttrium-yttrium clustering ratio R(YY), and the number of intratetrahedral O-Si-O bonds per yttrium atom N(intra) can be combined into a single structural descriptor s = f(CN(SiOSi),R(YY),N(intra)) with a high correlation with the solubility. The parameter s can thus be calculated from MD simulations and used to predict the solubility of YAS compositions, allowing one to adjust them to the range required by radiotherapy applications. For instance, its trend shows that high-silica- and low-yttria-content YAS glasses should be sufficiently durable for the radiotherapy application, although additional clinical considerations may set a lower limit to the yttria content.

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.