Nanoscale Chains Control the Solubility of Phosphate Glasses for Biomedical Applications

Development of a ReaxFF reactive force field for ternary phosphate-based bioactive glasses

Phosphate-based glasses (PBGs) in the CaO-Na2O-P2O5 system have diverse applications as biomaterials due to their unique dissolution properties. A reactive force field (ReaxFF) has been developed to simulate these materials at the atomic level. The ReaxFF parameters of PBGs, including the interaction between phosphorus and calcium atoms, have been developed using a published code based on genetic algorithms. The training data, including the atomic charges, atomic forces, bond and angle parameters, and different differential energies, are chosen and measured from static quantum-mechanical calculations and ab initio molecular dynamics of PBGs. We did different short- and medium-range structural analyses on the bulk simulated PBGs with different compositions to validate the developed potential. Radial and angular distribution functions and coordination numbers of network formers and modifiers, as well as the network connectivity of the glass, are in agreement with experimental and previous simulations using both shell-model classical force fields and ab initio simulated data; for example, the coordination number of phosphorus is 4.0. This successful development of ReaxFF parameters being able to describe the bulk PBGs enables us to work on the dissolution behavior of the glasses, including the interaction of water molecules with PBGs, in future works.

Unravelling the dissolution mechanism of polyphosphate glasses by 31P NMR spectroscopy: ligand competition and reactivity of intermediate complexes

Phosphate glass dissolution can be tailored via compositional and subsequent structural changes, which is of interest for biomedical applications such as therapeutic ion delivery. Here, solid-state ³¹P nuclear magnetic resonance characterisation of 45P₂O₅-xCaO - (55 -x)Na₂O glasses was correlated with dissolution studies using time-dependent liquid ³¹P NMR spectroscopy and quantitative chemical analysis. Glasses dissolved congruently in aqueous media, and the first dissolution stage was the hydration of phosphate chains. In deionised water and Tris buffer (pH₀ 7.4 or 7.9), trimetaphosphate rings and orthophosphates were the predominant species in solution, indicating relatively fast degradation. By contrast, long phosphate chains were identified in EDTA (pH₀ 10.0). Besides pH differences, coordination of phosphate species by metal cations appears to play a catalytic role in the hydrolysis mechanism via turning phosphorus atoms into suitable electrophiles for the subsequent nucleophilic attack by water. Hydrolysis rates were proportional to phosphate complex stability, with stronger complexes for chains than for rings. A competition between solvent and phosphate species for the metal ion occurred in the order EDTA > Tris > deionised water.

Understanding the structural drivers governing glass-water interactions in borosilicate based model bioactive glasses

The past decade has witnessed a significant upsurge in the development of borate and borosilicate based resorbable bioactive glasses owing to their faster degradation rate in comparison to their silicate counterparts. However, due to our lack of understanding about the fundamental science governing the aqueous corrosion of these glasses, most of the borate/borosilicate based bioactive glasses reported in the literature have been designed by "trial-and-error" approach. With an ever-increasing demand for their application in treating a broad spectrum of non-skeletal health problems, it is becoming increasingly difficult to design advanced glass formulations using the same conventional approach. Therefore, a paradigm shift from the "trial-and-error" approach to "materials-by-design" approach is required to develop new-generations of bioactive glasses with controlled release of functional ions tailored for specific patients and disease states, whereby material functions and properties can be predicted from first principles. Realizing this goal, however, requires a thorough understanding of the complex sequence of reactions that control the dissolution kinetics of bioactive glasses and the structural drivers that govern them. While there is a considerable amount of literature published on chemical dissolution behavior and apatite-forming ability of potentially bioactive glasses, the majority of this literature has been produced on silicate glass chemistries using different experimental and measurement protocols. It follows that inter-comparison of different datasets reveals inconsistencies between experimental groups. There are also some major experimental challenges or choices that need to be carefully navigated to unearth the mechanisms governing the chemical degradation behavior and kinetics of boron-containing bioactive glasses, and to accurately determine the composition-structure-property relationships. In order to address these challenges, a simplified borosilicate based model melt-quenched bioactive glass system has been studied to depict the impact of thermal history on its molecular structure and dissolution behavior in water. It has been shown that the methodology of quenching of the glass melt impacts the dissolution rate of the studied glasses by 1.5×-3× depending on the changes induced in their molecular structure due to variation in thermal history. Further, a recommendation has been made to study dissolution behavior of bioactive glasses using surface area of the sample - to - volume of solution (SA/V) approach instead of the currently followed mass of sample - to - volume of solution approach. The structural and chemical dissolution data obtained from bioactive glasses following the approach presented in this paper can be used to develop the structural descriptors and potential energy functions over a broad range of bioactive glass compositions.

STATEMENT OF SIGNIFICANCE

Realizing the goal of designing third generation bioactive glasses requires a thorough understanding of the complex sequence of reactions that control their rate of degradation (in physiological fluids) and the structural drivers that control them. In this article, we have highlighted some major experimental challenges and choices that need to be carefully navigated in order to unearth the mechanisms governing the chemical dissolution behavior of borosilicate based bioactive glasses. The proposed experimental approach allows us to gain a new level of conceptual understanding about the composition-structure-property relationships in these glass systems, which can be applied to attain a significant leap in designing borosilicate based bioactive glasses with controlled dissolution rates tailored for specific patient and disease states.

Structural and physico-chemical analysis of calcium/strontium substituted, near-invert phosphate based glasses for biomedical applications

Neutron diffraction, ²³Na and ³¹P NMR, and FTIR spectroscopy have been used to investigate the structural effects of substituting CaO with SrO in a 40P₂O₅·(16-x)CaO·20Na₂O·24MgO·xSrO glass, where x is 0, 4, 8, 12 and 16mol%. The ³¹P solid-state NMR results showed similar amounts of Q¹ and Q² units for all of the multicomponent glasses investigated, showing that the substitution of Sr for Ca has no effect on the phosphate network. The M-O coordinations (M=Mg, Ca, Sr, Na) were determined for binary alkali and alkaline earth metaphosphates using neutron diffraction and broad asymmetric distributions of bond length were observed, with coordination numbers that were smaller and bond lengths that were shorter than in corresponding crystals. The Mg-O coordination number was determined most reliably as 5.0(2). The neutron diffraction results for the multicomponent glasses are consistent with a structural model in which the coordination of Ca, Sr and Na is the same as in the binary metaphosphate glass, whereas there is a definite shift of Mg-O bonds to longer distance. There is also a small but consistent increase in the Mg-O coordination number and the width of the distribution of Mg-O bond lengths, as Sr substitutes for Ca. Functional properties, including glass transition temperatures, thermal processing windows, dissolution rates and ion release profiles were also investigated. Dissolution studies showed a decrease in dissolution rate with initial addition of 4mol% SrO, but further addition of SrO showed little change. The ion release profiles followed a similar trend to the observed dissolution rates. The limited changes in structure and dissolution rates observed for substitution of Ca with Sr in these fixed 40mol% P₂O₅ glasses were attributed to their similarities in terms of ionic size and charge.

STATEMENT OF SIGNIFICANCE

Phosphate based glasses are extremely well suited for the delivery of therapeutic ions in biomedical applications, and in particular strontium plays an important role in the treatment of osteoporosis. We show firstly that the substitution of strontium for calcium in bioactive phosphate glasses can be used to control the dissolution rate of the glass, and hence the rate at which therapeutic ions are delivered. We then go on to examine in detail the influence of Sr/Ca substitution on the atomic sites in the glass, using advanced structural probes, especially neutron diffraction. The environments of most cations in the glass are unaffected by the substitution, with the exception of Mg, which becomes more disordered.

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 ∼2.44-2.49 Å, and the coordination number is 7.5-7.8. The Q n 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 (Christie et al. in 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.

Structures and properties of phosphate-based bioactive glasses from computer simulation: a review

Computer simulations have enabled breakthroughs in understanding the connections between the atomic structure and properties of bioactive phosphate glasses.

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 ∼13 for Mg, ∼11 for Zn and ∼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.

Degradation and Characterization of Resorbable Phosphate-Based Glass Thin-Film Coatings Applied by Radio-Frequency Magnetron Sputtering

Quinternary phosphate-based glasses of up to 2.67 μm, deposited by radio-frequency magnetron sputtering, were degraded in distilled water and phosphate-buffered saline (PBS) to investigate their degradation characteristics. Magnetron-sputtered coatings have been structurally compared to their compositionally equivalent melt-quenched bulk glass counterparts. The coatings were found to have structurally variable surfaces to melt-quenched glass such that the respective bridging oxygen to nonbridging oxygen bonds were 34.2% to 65.8% versus 20.5% to 79.5%, forming metaphosphate (PO3)(-) (Q(2)) versus less soluble (P2O7)(4-) (Q(1)) and (PO4)(3-) (Q(0)), respectively. This factor led to highly soluble coatings, exhibiting a t(1/2) degradation dependence in the first 2 h in distilled water, followed by a more characteristic linear profile because the subsequent layers were less soluble. Degradation was observed to preferentially occur, forming voids characteristic of pitting corrosion, which was confirmed by the use of a focused ion beam. Coating degradation in PBS precipitated a (PO3)(-) metaphosphate, an X-ray amorphous layer, which remained adherent to the substrate and seemingly formed a protective diffusion barrier, which inhibited further coating degradation. The implications are that while compositionally similar, sputter-deposited coatings and melt-quenched glasses are structurally dissimilar, most notably, with regard to the surface layer. This factor has been attributed to surface etching of the as-deposited coating layer during deposition and variation in the thermal history between the processes of magnetron sputtering and melt quenching.

Structure-solubility relationships in fluoride-containing phosphate based bioactive glasses

Structural role of fluoride on chemical dissolution behavior of bioactive phosphate glasses has been studied.

On the structure of Ce-containing silicophosphate glasses: a core–shell molecular dynamics investigation

New Ce³⁺–O and Ce⁴⁺–O parameters for a force-field based on the core–shell model were developed and applied to get insights into the structure of five silicophosphate glasses with increasing Ce₂O₃ and P₂O₅ content.

On the structure of biomedical silver-doped phosphate-based glasses from molecular dynamics simulations

First-principles and classical molecular dynamics simulations have been carried out on undoped and silver-doped phosphate-based glasses with 50 mol% P₂O₅, 0–20 mol% Ag₂O, and varying amounts of Na₂O and CaO.