Structural Effects of Phosphorus Inclusion in Bioactive Silicate Glasses

Clustering of fluoride and phosphate ions in bioactive glass from computer simulation

In order to understand the nature of ionic clustering in bioactive glass compositions, computer simulation was used to model four different compositions of bioactive glass with various amounts of flouride and phosphate. Fluoride ions were chemically bonded only to sodium and calcium, creating regions rich in fluoride and modifiers, and fluoride clustering was seen to be present in all compositions. The majority of phosphate groups are present as orthophosphate and phosphate clustering is also seen, and shown to be stronger in compositions with a lower phosphate content. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 1)'.

Tuning Network Connectivity of Silicate and Sodium Borosilicate Glasses by TiO2 for Enhanced Chemical Durability: Molecular Dynamics Simulation Investigations

Extensive molecular dynamics (MD) simulations were performed to disclose the beneficial aspects of TiO₂ doping in SiO₂ and sodium borosilicate (NBS) glasses. Significant amendment in short- and intermediate-range orders of glasses was captured by the radial distribution function, coordination number, bond/angle distribution profiles, structure factor, and probability of linking X-O-X' (where X and/or X' = Si, B, and Ti) structural motifs. Successively, the effect of microscopic structural modification on the macroscopic properties was analyzed in terms of mechanical strength, thermal stability, vibrational characteristics [(vibrational density of states (VDOS)], and chemical durability. The results show that Ti participates in the network chain in the form of TiO₆ and TiO₅ for the Ti-NBS glass whereas in the form of TiO₆, TiO₅, and TiO₄ for the binary TiO₂-SiO₂ glass. The presence of TiO₂ was found to strengthen the glass skeleton. However, the glass-transition temperature was also increased with Ti addition, which indicates increased hurdles during synthesis due to increased cross connections in the glass network with Ti doping. The computed results envisage enhanced chemical durability of Ti-added glasses. In addition, VDOS spectra showed network former-like characteristics of Ti in the glass network with significant contributions up to a vibration frequency of 800 cm^-1. The strong binding of Ti-O-connected Na⁺ in the glass skeleton prevents Na⁺ migration toward the interface or bulk aqueous phase, which contributes to improved chemical stability of Ti-containing glasses. During contact with water, Na⁺ were less likely to leach out from glass to the aqueous solution during Ti doping. In addition, the increased fraction of stable ring structures (5^m-7^m) for Ti-NBS glasses than bare NBS glasses also supports the increased leaching resistivity of Ti-added glasses. Essentially, the elucidation of macroscopic glass properties has been provided in terms of microscopic understanding. The present findings will incite further MD simulations and experiments to disclose more interesting microstructures and dynamics due to the presence of TiO₂ in glasses.

Design of Refining Slag Based on Structural Modifications Associated with the Boron Removal for SoG-Si

Solar grade silicon (SoG-Si) is the core material of solar cells. The removal of boron (B) has always been a challenge in the preparation of high purity Si. Slag refining has always been considered as one of the effective methods to remove B, but the design of refined slag has been limited by the cognition of the relationship between slag structure and impurity removal, and can only rely on the apparent basicity and oxygen potential adjustment of slag based on a large number of conditional experiments. In order to clarify the B removal mechanism of slag refining from Si, nuclear magnetic resonance (NMR) and Raman vibrational spectroscopy were used to investigate in detail the behavior and state of B and aluminum (Al) in the SiO₂–CaO–Al₂O₃–B₂O₃ slag. The role of the degree of B–Si cross linking on the B activity in slag was highlighted by comparing the partition ratio (L_B) between slag and Si. Q² structural unit of slag is an important site for capturing B. BO₄ (1B, 3Si) species is the main form of connection between B and silicate networks, which determines the activity of B in the slag. The addition of Al₂O₃ into SiO₂–CaO slag can change the relative fraction of Q² and BO₄ (1B, 3Si). Increasing Al₂O₃ content from 0 to about 20 wt% can lead to the overall increase of Q² population, and a tendency to decrease first and then increase of BO₄ (1B, 3Si) fraction under both basicity conditions (0.6 and 1.1). When Al₂O₃ content is less than 10 ± 1 wt%, the decrease of BO₄ (1B, 3Si) population plays a major role in deteriorating the connectivity between B and aluminosilicate network, which leads to a higher activity of B. When the Al₂O₃ content is greater than 10 ± 1 wt%, B is incorporated into the silicate network more easily due to the formation of more Q² and BO₄ (1B, 3Si), which contributes to a rapid decline in activity of B in slag.

Bioactive fluorescent hybrid microparticles as a stand-alone osteogenic differentiation inducer

Osteogenic differentiation of stem cells is one of the essential steps in bone regeneration. While supplementing exogenous factors using differentiation media is the established method to differentiate stem cells into osteoblasts on biomaterials, designing biomaterials that can act as a stand-alone differentiation inducer and promote bone regeneration is preferred for clinical translation. In this work, we report dexamethasone-loaded organic-inorganic hybrid microparticles synthesized from an intrinsically fluorescent poly (ester amide) and tertiary bioactive glass (PEA-BG) as a stand-alone osteogenic differentiation inducer. The mechanical properties data indicated that the compressive modulus of fluorescent hybrid microparticles could be modulated by its composition. The hybrid fluorescent microparticles supported the adhesion and proliferation of 10T1/2 ​cells in culture for up to seven days. Both pristine and dexamethasone-loaded PEA-BG microparticles were able to induce osteogenic differentiation of 10T1/2 ​cells in the absence of any media supplement, to a level even higher than standard osteogenic media, as evidenced by the expression of osteogenic markers on gene and protein levels and matrix mineralization. Taken together, the fluorescent PEA-BG hybrid microparticles have the potential to be used as a stand-alone biomaterial for osteogenic differentiation and bone regeneration.

In Vitro Dissolution of Na-Ca-P-Oxynitrides

Sodium-calcium-phosphate based oxynitride glasses and glass-ceramics doped with Mg, Si, and Nb were studied in vitro in simulated body fluid (SBF) under static conditions. The release of ions and pH changes up to 7 days of immersion were investigated. The nitrogen incorporation into phosphate glass matrix was found to notably influence in vitro dissolution only of homogenous glasses. Increasing the nitrogen content in the samples decreased the mean mass loss, while the niobate incorporation increased it. The correlation between the nitrogen content and increase in pH of SBF was also observed. The presence of phosphates crystallites was found to support the dissolution process at the beginning step (up to 3 days).

Nano-imaging confirms improved apatite precipitation for high phosphate/silicate ratio bioactive glasses

Bioactive glasses convert to a biomimetic apatite when in contact with physiological solutions; however, the number and type of phases precipitating depends on glass composition and reactivity. This process is typically followed by X-ray diffraction and infrared spectroscopy. Here, we visualise surface mineralisation in a series of sodium-free bioactive glasses, using transmission electron microscopy (TEM) with energy-dispersive X-ray spectroscopy (EDXS) and X-ray nano-computed tomography (nano-CT). In the glasses, the phosphate content was increased while adding stoichiometric amounts of calcium to maintain phosphate in an orthophosphate environment in the glass. Calcium fluoride was added to keep the melting temperature low. TEM brought to light the presence of phosphate clustering and nearly crystalline calcium fluoride environments in the glasses. A combination of analytical methods, including solid-state NMR, shows how with increasing phosphate content in the glass, precipitation of calcium fluoride during immersion is superseded by fluorapatite precipitation. Nano-CT gives insight into bioactive glass particle morphology after immersion, while TEM illustrates how compositional changes in the glass affect microstructure at a sub-micron to nanometre-level.

Composition-Structure-Solubility Relationships in Borosilicate Glasses: Toward a Rational Design of Bioactive Glasses with Controlled Dissolution Behavior

Owing to their fast but tunable degradation kinetics (in comparison to silicates) and excellent bioactivity, the past decade has witnessed an upsurge in the research interest of borate/borosilicate-based bioactive glasses for their potential use in a wide range of soft tissue regeneration applications. Nevertheless, most of these glasses have been developed using trial-and-error approaches wherein SiO₂ has been gradually replaced by B₂O₃. One major reason for using this empirical approach is the complexity of short-to-intermediate range structures of these glasses which greatly complicate the development of a thorough understanding of composition-structure-solubility relationships in these systems. Transitioning beyond the current style of composition design to a style that facilitates the development of bioactive glasses with controlled ion release tailored for specific patients/diseases requires a deeper understanding of the compositional/structural dependence of glass degradation behavior in vitro and in vivo. Accordingly, the present study aims to decipher the structural drivers controlling the dissolution kinetics and ion-release behavior of potentially bioactive glasses designed in the Na₂O-B₂O₃-P₂O₅-SiO₂ system across a broad compositional space in simulated body environments (pH = 7.4). By employing state-of-the-art spectroscopy-based characterization techniques, it has been shown that the degradation kinetics of borosilicate glasses depend on their R (Na₂O/B₂O₃) and K (SiO₂/B₂O₃) ratios, while the release of particular network-forming moieties from the glass into solution is strongly influenced by their role in-and effect on-the short-to-intermediate-range molecular structure. The current study aims to promote a rational design of borosilicate-based bioactive glasses, where a delicate balance between maximizing soft tissue regeneration and minimizing calcification and cytotoxicity can be achieved by tuning the release of ionic dissolution products (of controlled identity and abundance) from bioactive glasses into physiological media.

An experimental and theoretical approach on stability towards hydrolysis of triethyl phosphate and its effects on the microstructure of sol-gel-derived bioactive silicate glass

The sol-gel method is versatile and one of the well-established synthetic approaches for preparing bioactive glass with improved microstructure. In a successful approach, alkoxide precursors undergo rapid hydrolysis, followed by immediate condensation leading to the formation of three-dimensional gels. On the other hand, a slow kinetics rate for hydrolysis of one or more alkoxide precursors generates a mismatch in the progression of the consecutive reactions of the sol-gel process, which makes it difficult to form homogeneous multicomponent glass products. The amorphous phase separation (APS) into the gel is thermodynamically unstable and tends to transform into a crystalline form during the calcination step of xerogel. In the present study, we report a combined experimental and theoretical method to investigate the stability towards hydrolysis of triethyl phosphate (TEP) and its effects on the mechanism leading to phase separation in 58S bioactive glass obtained via sol-gel route. A multitechnical approach for the experimental characterization combined with calculations of functional density theory (DFT) suggest that TEP should not undergo hydrolysis by water under acidic conditions during the formation of the sol or even in the gel phase. The activation energy barrier (ΔG^‡) showed a height of about 20 kcal·mol^-1 for the three stages of hydrolysis and the reaction rates calculated for each stage of TEP hydrolysis were k_FHR = 7.0 × 10^-3s^-1, k_SHR = 6.8 × 10^-3s^-1 and k_THR = 3.5 × 10^-3s^-1. These results show that TEP remains in the non-hydrolyzed form segregated within the xerogel matrix until its thermal decomposition in the calcination step, when P species preferentially associate with calcium ions (labile species) and other phosphate groups present nearby, forming crystalline domains of calcium pyrophosphates permeated by the silica-rich glass matrix. Together, our data expand the knowledge about the synthesis by the sol-gel method of bioactive glass and establishes a mechanism that explains the role played by the precursor source of phosphorus (TEP) in the phase separation, an event commonly observed for these biomaterials.

Mesoporous bioactive glass composition effects on degradation and bioactivity

Mesoporous bioactive glasses (MBGs) are promising materials for regenerative medicine, due to their favorable properties including bioactivity and degradability. These key properties, but also their surface area, pore structure and pore volume are strongly dependent on synthesis parameters and glass stoichiometry. However, to date no systematic study on MBG properties covering a broad range of possible compositions exists.

Here, 24 MBG compositions in the SiO₂–CaO–P₂O₅ system were synthesized by varying SiO₂ (60–90 mol %), CaO and P₂O₅ content (both 0 to 40 mol-%), while other synthesis parameters were kept constant. Mesopore characteristics, degradability and bioactivity were analysed.

The results showed that, within the tested range of compositions, mesopore formation required a molar SiO₂ content above 60% but was independent of CaO and P₂O₅ content. While mesopore size did not depend on glass stoichiometry, mesopore arrangement was influenced by the SiO₂ content. Specific surface area and pore volume were slightly altered by the SiO₂ content. All materials were degradable; however, degradation as well as bioactivity, i.e. the ability to form a CaP mineral on the surface, depended on stoichiometry. Major differences were found in early surface reactions in simulated body fluid: where some MBGs induced direct hydroxyapatite crystallization, high release of calcium in others resulted in calcite formation.

In summary, degradation and bioactivity, both key parameters of MBGs, can be controlled by glass stoichiometry over a broad range while leaving the unique structural parameters of MBGs relatively unaffected. This allows targeted selection of material compositions for specific regenerative medicine applications.

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Mesoporous bioactive glasses can be obtained over a broad range of compositions.

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In the SiO₂/CaO/P₂O₅ system up to 15 mol-% P₂O₅ allow ordered porosity.

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In SiO₂/P₂O₅ glasses, up to 30 mol-% P₂O₅ are possible.

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Bioactivity and degradation can be tailored by controlling stoichiometry.

Bioactive Glass Nanoparticles for Tissue Regeneration

Sol-gel-derived bioactive glass nanoparticles have attracted special interest due to their potential as novel therapeutic and regenerative agents. Significant challenges are yet to be addressed. The fabrication of sol-gel-derived nanoparticles in binary and ternary systems with an actual composition that meets the nominal has to be achieved. This work addresses this challenge and delivers nanoparticles in a ternary system with tailored composition and particle size. It also studies how specific steps in the fabrication process can affect the incorporation of the metallic ions, nanoparticle size, and mesoporosity. Sol-gel-derived bioactive glass nanoparticles in the 62 SiO₂-34.5 CaO-3.2 P₂O₅ (mol %) system have been fabricated and characterized for their structural, morphological, and elemental characteristics using Fourier transform infrared spectroscopy, X-ray diffraction analysis, scanning electron microscopy associated with elemental analysis, transmission electron microscopy, and solid-state nuclear magnetic resonance. The fabricated nanoparticles were additionally observed to form the apatite phase when immersed in simulated body fluid. This work highlights the effect of the different processing variables, such as the nature of the solvent, the order in which reagents are added, stirring time, and the concentrations in the catalytic solution on the controlled incorporation of specific ions (e.g., P and Ca) in the nanoparticle network and particle size.

Recent advances and future perspectives of sol–gel derived porous bioactive glasses: a review

Sol–gel derived bioactive glasses have been extensively explored as a promising and highly porous scaffold materials for bone tissue regeneration applications owing to their exceptional osteoconductivity, osteostimulation and degradation rates.

Quantitative determination of the phosphorus environment in lithium aluminosilicate glasses using solid-state NMR techniques

The phosphorus environment is determined quantitatively using ³¹P Magic Angle Spinning NMR constrained by ³¹P–²⁷Al Multiple-Quantum Coherence-based NMR techniques.

Role of the Short Distance Order in Glass Reactivity

In 2005, our group described for the first time the structural characterization at the atomic scale of bioactive glasses and the influence of the glasses’ nanostructure in their reactivity in simulated body fluids. In that study, two bioactive sol-gel glasses with composition 80%SiO₂–20%CaO and 80%SiO₂–17%CaO–3%P₂O₅ (in mol-%) were characterized by High-Resolution Transmission Electron Microscopy (HRTEM). Such characterization revealed unknown features of the glasses’ structure at the local scale that allowed the understanding of their different in vitro behaviors as a consequence of the presence or absence of P₂O₅. Since then, the nanostructure of numerous bioactive glasses, including melt-prepared, sol-gel derived, and mesoporous glasses, was investigated by HRTEM, Nuclear Magnetic Resonance (NMR) spectroscopy, Molecular Dynamics (MD) simulations, and other experimental techniques. These studies have shown that although glasses are amorphous solids, a certain type of short distance order, which greatly influences the in vitro and in vivo reactivity, is always present. This paper reviews the most significant advances in the understanding of bioactive glasses that took place in the last years as a result of the growing knowledge of the glasses’ nanostructure.

Structure–composition trends in multicomponent borosilicate-based glasses deduced from molecular dynamics simulations with improved B–O and P–O force fields

Improved B–O and P–O force fields provide accurate molecular dynamics simulations of multicomponent glasses.

Experimental and Theoretical Investigation of the Structural Role of Titanium Oxide in CaO-P2O5-TiO2 Invert Glass

Understanding the structural role of TiO₂ in calcium phosphate invert glasses is key for developing a new glass design for biomedical applications. Experimental and computational analysis methods were used to investigate the impact of TiO₂ substitution in these glasses. Spectroscopic analyses indicated that titanium oxide exists as both TiO₄ and TiO₆ units, leading to the formation of Ti-O-P bonds, in spite of depolymerization of the phosphate chains. Classical molecular dynamics showed that the presence of TiO₂ influences the phosphate units and CaO polyhedral structures. The formation of the Ti-O-P bonds caused an increase in the network connectivity of the invert glasses, leading to the improvement of the glass forming ability and wettability. The addition of TiO₂ to calcium phosphate invert glasses led to the introduction of bioactivity.

Effective role of CaO/P2O5 ratio on SiO2-CaO-P2O5 glass system

In the present work, the effect of the CaO/P2O5 ratio on the composition of sol-gel synthesized 58SiO2-(19 - x)P2O5-(23 + x)CaO (x = 0, 5, 10 and 15 mol%) glass samples was studied. Further, the effect of NBO/BO ratio on hydroxy carbonated apatite layer (HCA) forming ability based on dissolution behavior in simulated body fluid (SBF) solution was also investigated. CaO/P2O5 ratios of synthesized glass samples were 1.2, 2, 3.6, and 9.5, respectively. NBO/BO ratios were obtained using Raman spectroscopic analysis as 0.58, 1.20, 1.46, and 1.78, respectively. All samples were soaked in the SBF solution for 7 days. The calculated weight losses of these samples were 58%, 64%, 83%, and 89% for corresponding NBO/BO ratios. The increase in CaO/P2O5 ratio increases the NBO/BO ratios. However, the increase in NBO/BO ratio increases HCA forming ability of SBF treated samples. The HCA crystalline layer formation was confirmed through X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), Raman and Infrared spectroscopic analysis. Higher CaO/P2O5 ratio favors the increase in HCA formation for SBF treated calcium phospho silicate glasses.

Bioactive Glasses with Low Ca/P Ratio and Enhanced Bioactivity

Three new silica-based glass formulations with low molar Ca/P ratio (2–3) have been synthesized. The thermal properties, the crystalline phases induced by thermal treatments and the sintering ability of each glass formulation have been investigated by simultaneous differential scanning calorimetry-thermogravimetric analysis (DSC-TG), X-ray diffraction (XRD) and hot stage microscopy (HSM). The glasses exhibited a good sintering behavior, with two samples achieving shrinkage of 85%–95% prior to crystallization. The bioactivity of the glasses in simulated body fluid (SBF) has been investigated by performing XRD and Fourier transform infrared spectroscopy (FTIR) on the samples prior and after immersion. The glasses with lower MgO contents were able to form a fully crystallized apatite layer after three days of immersion in simulated body fluid (SBF), while for the glass exhibiting a higher MgO content in its composition, the crystallization of the Ca–P layer was achieved after seven days. The conjugation of these properties opens new insights on the synthesis of highly bioactive and mechanically strong prosthetic materials.

Controlling the ion release from mixed alkali bioactive glasses by varying modifier ionic radii and molar volume

Modifier ionic radius controls ion release from bioactive phospho-silicate glasses via silicate network compactness.

Structure–composition relationships of bioactive borophosphosilicate glasses probed by multinuclear 11B, 29Si, and 31P solid state NMR

By combining ¹¹B, ²⁹Si, and ³¹P nuclear magnetic resonance (NMR) experimental results, we present the first comprehensive structural investigation of 15 borophosphosilicate (BPS) glasses of the Na₂O–CaO–B₂O₃–SiO₂–P₂O₅ system.

Na/Ca Intermixing around Silicate and Phosphate Groups in Bioactive Phosphosilicate Glasses Revealed by Heteronuclear Solid-State NMR and Molecular Dynamics Simulations

We characterize the intermixing of network-modifying Na(+)/Ca(2+) ions around the silicate (QSi(n)) and phosphate (QP(n)) tetrahedra in a series of 16 Na2O–CaO–SiO2–P2O5 glasses, whose P content and silicate network connectivity were varied independently. The set includes both bioactive and bioinactive compositions and also encompasses two soda-lime-silicate members devoid of P, as well as two CaO–SiO2 glasses and one Na2O–SiO2–P2O5 glass. The various Si/P↔Na/Ca contacts were probed by molecular dynamics (MD) simulations together with heteronuclear magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) experimentation utilizing (23)Na{(31)P} and (23)Na{(29)Si} REDOR, as well as (31)P{ (23)Na} and (29)Si{(23)Na} REAPDOR. We introduce an approach for quantifying the extent of Na(+)/Ca(2+) ordering around a given QP(n) or QSi(n) group, encoded by the preference factor 0⩽ PM ⩽ 1 conveying the relative weights of a random cation intermixing (PM = 0) and complete preference/ordering (PM = 1) for one of the species M, which represents either Na(+) or Ca(2+). The MD-derived preference factors reveal phosphate and silicate species surrounded by Na(+)/Ca(2+) ions intermixed nearly randomly (PM ≲ 0.15), except for the QSi(4) and QSi(1) groups, which manifest more significant cation ordering with preference for Na+ and Ca2+, respectively. The overall weak preferences are essentially independent of the Si and P contents of the glass, whereas PM primarily correlates with the total amount of network modifiers: as the latter is increased, the Na/Ca distribution around the {QP(0), QSi(1), QSi(2)} groups with preference for Ca2(+ )tend to randomize (i.e., PCa decreases), while the PNa-values grow slightly for the {QP(1), QSi(3), QSi(4)} species already preferring coordination of Na. The set of experimental preference factors {PCa} for the orthophosphate (QP(0)) groups extracted from (31)P{(23)Na} REAPDOR NMR-derived M2(P–Na) dipolar second moments agrees well with the MD-generated counterparts. Our results on the Na/Ca intermixing in soda-lime-silicate glasses are discussed in relation to previous reports, highlighting the dependence of the conclusion on the approach to data evaluation.

Bioactive glasses—structure and properties

Bioactive glasses were the first synthetic materials to show bonding to bone, and they are successfully used for bone regeneration. They can degrade in the body at a rate matching that of bone formation, and through a combination of apatite crystallization on their surface and ion release they stimulate bone cell proliferation, which results in the formation of new bone. Despite their excellent properties and although they have been in clinical use for nearly thirty years, their current range of clinical applications is still small. Latest research focuses on developing new compositions to address clinical needs, including glasses for treating osteoporosis, with antibacterial properties, or for the sintering of scaffolds with improved mechanical stability. This Review discusses how the glass structure controls the properties, and shows how a structure-based design may pave the way towards new bioactive glass implants for bone regeneration.

Understanding the composition-structure-bioactivity relationships in diopside (CaO·MgO·2SiO₂)-tricalcium phosphate (3CaO·P₂O₅) glass system

The present work is an amalgamation of computation and experimental approach to gain an insight into composition-structure-bioactivity relationships of alkali-free bioactive glasses in the CaO-MgO-SiO2-P2O5 system. The glasses have been designed in the diopside (CaO·MgO·2SiO2; Di)-tricalcium phosphate (3CaO·P2O5; TCP) binary join by varying the Di/TCP ratio. The melt-quenched glasses have been investigated for their structure by molecular dynamic (MD) simulations as well as by nuclear magnetic resonance spectroscopy (NMR). In all the investigated glasses silicate and phosphate components are dominated by Q(2) (Si) and Q(0) (P) species, respectively. The apatite forming ability of the glasses was investigated using X-ray diffraction (XRD), infrared spectroscopy after immersion of glass powders in simulated body fluid (SBF) for time durations varying between 1 h and 14 days, while their chemical degradation has been studied in Tris-HCl in accordance with ISO 10993-14. All the investigated glasses showed good bioactivity without any substantial variation. A significant statistical increase in metabolic activity of human mesenchymal stem cells (hMSCs) when compared to the control was observed for Di-60 and Di-70 glass compositions under both basal and osteogenic conditions.

Elasticity and structure of the compounds in the wollastonite (CaSiO3)-Na 2SiO 3 system: from amorphous to crystalline state

The elastic properties and structure of four potential bioactive compounds in the CaSiO3-Na2SiO3 system were characterized by Raman and Brillouin spectroscopy at ambient conditions. The increase of Na2O content in the Na2O-CaO-SiO2 glass with the same silica content and hence polymerization was found to lower the elastic moduli with accompanied decrease of Q(0) and Q(2) species, increase of Q(1) species and negligible change of Q(3) species, corresponding to a lower and higher equilibrium constant for the disproportional reactions [Formula: see text] and [Formula: see text] (without balance), respectively. The composition-dependent variation in the shear modulus (G) of the Na2O-CaO-SiO2 glass can be attributed to the concentration change of Q(2) and probably Q(4) species; while bulk modulus (K) ascribed to the cohesion factor. The elastic moduli of the corresponding crystalline phases in this system also lower with the increase of Na2O content following two general criteria as a function of Na2O/CaO molar ratio: (1) K decreases faster than G for both the amorphous and crystalline phases and (2) both K and G decreases faster for the crystals than the glasses.

Atomic-scale models of early-stage alkali depletion and SiO2-rich gel formation in bioactive glasses

Molecular dynamics simulations of Na⁺/H⁺-exchanged 45S5 Bioglass® reveal the co-existence of bonded and non-bonded hydroxyls, suggesting a direct mechanism for forming a silica-rich gel structure upon the initial ion exchange.

Cooling rate and size effects on the medium-range structure of multicomponent oxide glasses simulated by molecular dynamics

A set of molecular dynamics simulations were performed to investigate the effect of cooling rate and system size on the medium-range structure of melt-derived multicomponent silicate glasses, represented by the quaternary 45S5 Bioglass composition. Given the significant impact of the glass degradation on applications of these materials in biomedicine and nuclear waste disposal, bulk structural features which directly affect the glass dissolution process are of particular interest. Connectivity of the silicate matrix, ion clustering and nanosegregation, distribution of ring and chain structural patterns represent critical features in this context, which can be directly extracted from the models. A key issue is represented by the effect of the computational approach on the corresponding glass models, especially in light of recent indications questioning the suitability of conventional MD approaches (that is, involving melt-and-quench of systems containing ~10(3) atoms at cooling rates of 5-10 K/ps) when applied to model these glasses. The analysis presented here compares MD models obtained with conventional and nonconventional cooling rates and system sizes, highlighting the trend and range of convergence of specific structural features in the medium range. The present results show that time-consuming computational approaches involving much lower cooling rates and/or significantly larger system sizes are in most cases not necessary in order to obtain a reliable description of the medium-range structure of multicomponent glasses. We identify the convergence range for specific properties and use them to discuss models of several glass compositions for which a possible influence of cooling-rate or size effects had been previously hypothesized. The trends highlighted here represent an important reference to obtain reliable models of multicomponent glasses and extract converged medium-range structural features which affect the glass degradation and thus their application in different fields. In addition, as a first application of the present findings, the fully converged structure of the 45S5 glass was further analyzed to shed new light on several dissolution-related features whose interpretation has been rather controversial in the past.

Toward a rational design of bioactive glasses with optimal structural features: composition-structure correlations unveiled by solid-state NMR and MD simulations

The physiological responses of silicate-based bioactive glasses (BGs) are known to depend critically on both the P content (n_P) of the glass and its silicate network connectivity (N̅_BO^Si). However, while the bioactivity generally displays a nonmonotonic dependence on n_P itself, recent work suggest that it is merely the net orthophosphate content that directly links to the bioactivity. We exploit molecular dynamics (MD) simulations combined with ³¹P and ²⁹Si solid-state nuclear magnetic resonance (NMR) spectroscopy to explore the quantitative relationships between N̅_BO^Si, n_P, and the silicate and phosphate speciations in a series of Na₂O–CaO–SiO₂–P₂O₅ glasses spanning 2.1 ≤ N̅_BO^Si ≤ 2.9 and variable P₂O₅ contents up to 6.0 mol %. The fractional population of the orthophosphate groups remains independent of n_P at a fixed N̅_BO^Si-value, but is reduced slightly as N̅_BO^Si increases. Nevertheless, P remains predominantly as readily released orthophosphate ions, whose content may be altered essentially independently of the network connectivity, thereby offering a route to optimize the glass bioactivity. We discuss the observed composition-structure links in relation to known composition-bioactivity correlations, and define how Na₂O–CaO–SiO₂–P₂O₅ compositions exhibiting an optimal bioactivity can be designed by simultaneously altering three key parameters: the silicate network connectivity, the (ortho)phosphate content, and the n_Na/n_Ca molar ratio.

Probing the fate of interstitial water in bulk bioactive glass by ab initio simulations

The mechanism and effects of the interaction of a water molecule with different sites found in the bulk of 45S5 bioactive glass have been investigated through ab initio simulations.

Current challenges in atomistic simulations of glasses for biomedical applications

Atomic-scale simulations of bioglasses are being used to tackle several challenging aspects, such as new structural markers of bioactivity, ion migration and nanosized samples.

DFT modeling of 45S5 and 77S soda-lime phospho-silicate glass surfaces: clues on different bioactivity mechanism

The reactivity of bioglasses, which is related to the dissolution of cations and orthosilicate groups in the physiological fluid, strongly depends on the key structural features present at the glass surfaces. On the basis of the composition and the synthetic routes employed to make the glass, surfaces with very different characteristics and thus presenting different mechanisms of dissolution can be observed. In this paper, the surface structures of two very different bioglass compositions, namely 45S5 (46.1 SiO2, 24.4 Na2O, 26.9 CaO, and 2.6 P2O5 mol %) and 77S (80.0 SiO2, 16.0 CaO, and 4.0 P2O5 mol %), have been investigated by means of periodic DFT calculations based on a PBE functional and localized Gaussian basis set as encoded in the CRYSTAL code. Our calculations show that the two glass surfaces differ by the relative amount of key structural sites such as NBOs, exposed ions, orthosilicate units, and small rings. We have demonstrated how the number of these sites affects the surface stability and reactivity (bioactivity).

Dissolution patterns of biocompatible glasses in 2-amino-2-hydroxymethyl-propane-1,3-diol (Tris) buffer

A continuous flow measurement system with sensitive on-line ion analysis has been applied to study the initial dissolution behaviour of biocompatible glasses in Tris. Altogether 16 glasses with widely varying compositions were studied. The measurement system allowed for quantitative determination of the time-dependent rates of dissolution of sodium, potassium, calcium, magnesium, silicon and phosphorus during the first 10-15 min in contact with Tris solution. The dissolution rates of the different ions showed significant glass to glass variations, but all glasses studied showed one of four distinct dissolution patterns. The ion dissolution rates after an exposure of 1000 s, expressed as the normalized surface-specific mass loss rates, were compared with the in vitro and in vivo reactivity of the glasses as predicted by models in the literature. The results showed a clear correlation between the dissolution rates of the glasses in Tris and their reactivity as measured by other different methods. Consequently, the measured short-term dissolution patterns could be used to determine which glasses are suitable as bioactive, biodegradable, or inert biomaterials for medical devices.

Structure and biological activity of glasses and ceramics

Biomaterials for repairing and regenerating parts of the human body play a key role in contemporary medicine, and have an increasing impact in modern society. Given the importance of orthopaedic medicine (bone is the second most replaced organ after blood), bioactive glasses and ceramics represent a key reference to guide technological advances in this field. Their established role in current biomedical applications has already led many research groups worldwide to look into their structural properties, with a view to identifying the molecular basis of their biological activity. As the efforts directed towards this crucial and exciting direction continue to increase, it is now timely to review the situation, in order to guide future investigations on structure-bioactivity relationships. In this introductory article, the field is reviewed, to provide an appropriate context for the contributions to this Theme Issue.

Effects of CaO/P2O5 ratio on the structure and elastic properties of SiO2-CaO-Na2O-P2O5 bioglasses

The evolution of elastic properties and structure upon the change of CaO/P(2)O(5) ratio in SiO(2)-CaO-Na(2)O-P(2)O(5) glasses (45S5-derived and 55S4-derived) at ambient conditions has been studied by using both Brillouin and Raman spectroscopy coupled with X-ray diffraction. Under the same SiO(2)/Na(2)O ratio, it is found that a decrease in CaO/P(2)O(5) molar ratio has caused a more-polymerized silicate network via a net consumption of Q(0), Q(1), and Q(2) species yet enriching in Q(3) and Q(4) species. Brillouin experiments revealed that all the bulk, shear and Young's moduli of the glasses studied increases with the increase of CaO/P(2)O(5) molar ratio. The unexpected variation trend in shear modulus can be correlated to the contribution from cohesion, the less-polymerized phosphate Q species, and density. Compared to the 45S5-derived, the more-polymerized 55S4-deived glass has a lower bulk but slightly higher shear modulus at the given CaO/P(2)O(5) ratio.

Molecular dynamics methods for modeling complex interactions in biomaterials

The molecular dynamics method is a powerful computer simulation technique which provides access to the detailed time evolution (trajectory) of a system in specified conditions, such as a particular temperature or pressure. The full trajectory of the system can be analyzed using statistical mechanics tools to obtain thermodynamical quantities and dynamical properties; the mechanism of chemical reactions and other time-dependent processes, such as diffusion, can also be revealed in high detail. When applied to model extended and complex system such as biomaterials, MD simulations represent an invaluable tool to discover structure-activity relationships and rationalize biomedical applications.

Alkali-free bioactive glasses for bone tissue engineering: a preliminary investigation

An alkali-free series of bioactive glasses has been designed and developed in the glass system CaO-MgO-SiO(2)-P(2)O(5)-CaF(2) along the diopside (CaMgSi(2)O(6))-fluorapatite (Ca(5)(PO(4))(3)F)-tricalcium phosphate (3CaO·P(2)O(5)) join. The silicate network in all the investigated glasses is predominantly coordinated in Q(2) (Si) units, while phosphorus tends to remain in an orthophosphate (Q(0)) environment. The in vitro bioactivity analysis of glasses has been made by immersion of glass powders in simulated body fluid (SBF) while chemical degradation has been studied in Tris-HCl in accordance with ISO-10993-14. Some of the investigated glasses exhibit hydroxyapatite formation on their surface within 1-12 h of their immersion in SBF solution. The sintering and crystallization kinetics of glasses has been investigated by differential thermal analysis and hot-stage microscopy, respectively while the crystalline phase evolution in resultant glass-ceramics has been studied in the temperature range of 800-900°C using powder X-ray diffraction and scanning electron microscopy. The alkaline phosphatase activity and osteogenic differentiation for glasses have been studied in vitro on sintered glass powder compacts using rat bone marrow mesenchymal stem cells. The as-designed glasses are ideal candidates for their potential applications in bone tissue engineering in the form of bioactive glasses as well as glass/glass-ceramic scaffolds.

Predicting the glass transition temperature of bioactive glasses from their molecular chemical composition

A recently published paper (M.D. O'Donnell, Acta Biomaterialia 7 (2011) 2264-2269) suggests that it is possible to correlate the glass transition temperature (T(g)) of bioactive glasses with their molar composition, based on iterative least-squares fitting of published T(g) data. However, we show that the glass structure is an important parameter in determining T(g). Phase separation, local structural effects and components (intermediate oxides) which can switch their structural role in the glass network need to be taken into consideration, as they are likely to influence the glass transition temperature of bioactive glasses. Although the model suggested by O'Donnell works reasonably well for glasses within the composition range presented, it is oversimplified and fails for glasses outside certain compositional boundaries.

Bioactive glasses as potential radioisotope vectors for in situ cancer therapy: investigating the structural effects of yttrium

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.