Modeling the water-bioglass interface by ab initio molecular dynamics simulations

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'.

Behaviors of sodium and calcium ions at the borosilicate glass–water interface: Gaining new insights through an ab initio molecular dynamics study

We study reactivity and leaching at the calcium sodium borosilicate (CNBS)–water interface by means of a Car–Parrinello ab initio molecular dynamics simulation over a simulation time of 100 ps. With an emphasis on the comparison between the behaviors of Ca2+ and Na+ cations at the CNBS glass–water interface, different mechanism events during the trajectory are revealed, discussed, and correlated with other density functional theory calculations. We show that Na+ ions can be released in solution, while Ca2+ cannot leave the surface of CNBS glass. This release is correlated with the vacancy energy of Ca2+ and Na+ cations. Here, we found that the CNBS structure with the Na+ cation vacancy is energetically more favorable than the structure with the Ca2+ cation vacancy. The calcium adsorption site has been shown to have a greater affinity for water than can be found in the case of the sodium site, demonstrating that affinity may not be considered a major factor controlling the release of cations from the glass to the solution.

Structure and dissolution of silicophosphate glass

The solubility of P2O5–SiO2–Na2O–CaO glasses was suppressed by the coexistence of CaO and Na2O, attributed to the delocalization of the electron distribution of P in QP3 units coordinated to the six-fold-coordinated Si.

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.

Diffusion of protons and sodium ions in silicophosphate glasses: insight based on first-principles molecular dynamic simulations

We propose a microscopic diffusion mechanism of protons and Na+ ions in phosphate glasses using first-principles molecular dynamic simulations. Protons hop and are chemisorbed onto non-bridging oxygen (NBO) of nearby PO4 tetrahedra through hydrogen bonds. The subsequent behavior depends on the morphology of the PO4 tetrahedra (QnP values). When a proton is adsorbed onto the NBO of a Q3P unit, it is desorbed on a short time scale of within 10 fs and re-adsorbed onto the NBO that was previously adsorbed. However, when a proton is adsorbed onto the NBO of a Q2P unit, another proton coordinated before adsorption is desorbed in a chain, resulting in the diffusion of protons. When a Na+ ion is present in the vicinity, the adsorption of a proton onto a Q2P unit leads to a decrease in the electrostatic interaction between Na+ and O- ions and induces the diffusion of Na+ ions. We conclude that the difference in the morphology of PO4 tetrahedra greatly affects the diffusion of protons and Na+ ions.

Interfacial Adhesion of Polylactic Acid on Cellulose Surface: A Molecular Dynamics Study

Interfacial bonding and adhesion mechanisms are important in determining the final properties of the polymer composite. Molecular dynamics (MD) simulations have been used to characterize the interfacial structure and adhesion behavior of crystalline cellulose planes in contact with polylactic acid. The structure of the PLA at the interface exhibits a shape that can accommodate the structure of the cellulose surface. The adhesion between the PLA and the cellulose surface is affected by the polarity of the functional groups and the surface roughness. The improved adhesion is primarily due to hydrogen bonds formed between the cellulose and PLA molecular chains. Cellulose planes with higher molecular protrusions and greater surface roughness produce stronger adhesion to PLA due to enhanced hydrogen bonding. This study provides a basic insight into the interfacial mechanisms of PLA and cellulose surfaces at the molecular level.

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.

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.

Dynamic behaviour of the silica-water-bio electrical double layer in the presence of a divalent electrolyte

Molecular dynamics simulation of the electric double layer at the silica-water-bio interface in mixed electrolyte. Water orientation and charge distribution showed a significant effect on the electrostatics at the interface.

Antibacterial and osteo-stimulatory effects of a borate-based glass series doped with strontium ions

This work considered the effect of both increasing additions of Strontium (Sr²⁺) and incubation time on solubility and both antibacterial and osteo-stimulatory effects of a series of glasses based on the B₂O₃-P₂O₅-CaCO₃-Na₂CO₃-TiO₂-SrCO₃ series. The amorphous nature of all the glasses was confirmed by X-ray diffraction. Discs of each glass were immersed in de-ionized water for 1, 7 and 30 days, and the water extracts were used for ion release profiles, pH measurements and cytotoxicity testing. Atomic absorption spectroscopy was employed to detect the release of Na⁺, Ca²⁺ and Sr²⁺ ions from the glasses with respect to maturation, which indicated that the addition of Sr²⁺ retarded solubility of the glass series. This effect was also confirmed by weight loss analysis through comparing the initial weight of glass discs before and after periods of incubation. The incorporation of Sr²⁺ in the glasses did not influence the pH of the water extracts when the glasses were stored for up to 30 days. Cytotoxicity testing with an osteoblastic cell line (MC3T3-E1) indicated that glasses with the higher (20 mol% and 25 mol%) Sr²⁺ incorporation promoted proliferation of osteoblast cells, while the glasses with lower Sr²⁺ contents inhibited cell growth. The glass series, except for Ly-B5 (which contained the highest Sr²⁺ incorporation; 25 mol%), were bacteriostatic against S. aureus in the short term (1-7 days) as a result of the dissolution products released.

Vibrational Sum Frequency Generation Spectroscopy Study of Hydrous Species in Soda Lime Silica Float Glass

It is generally accepted that the mechanical properties of soda lime silica (SLS) glass can be affected by the interaction between sodium ions and hydrous species (silanol groups and water molecules) in its surface region. While the amount of these hydrous species can be estimated from hydrogen profiles and infrared spectroscopy, their chemical environment in the glass network is still not well understood. This work employed vibrational sum frequency generation (SFG) spectroscopy to investigate the chemical environment of hydrous species in the surface region of SLS float glass. SLS float glass shows sharp peaks in the OH stretching vibration region in SFG spectra, while the OH stretch peaks of glasses that do not have leachable sodium ions and the OH peaks of water molecules in condensed phases are normally broad due to fast hydrogen bonding dynamics. The hydrous species responsible for the sharp SFG peaks for the SLS float glass were found to be thermodynamically more stable than physisorbed water molecules, did not exchange with D2O, and were associated with the sodium concentration gradient in the dealkalized subsurface region. These results suggested that the hydrous species reside in static solvation shells defined by the silicate network with relatively slow hydrogen bonding dynamics, compared to physisorbed water layers on top of the glass surface. A putative radial distribution of the hydrous species within the SLS glass network was estimated based on the OH SFG spectral features, which could be compared with theoretical distributions calculated from computational simulations.

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.

Acid-base dissociation mechanisms and energetics at the silica-water interface: An activationless process

HYPOTHESIS

Silanol groups at the silica-water interface determine not only the surface charge, but also have an important role in the binding of ions and biomolecules. As the pH is increased above pH 2, the silica surface develops a net negative charge primarily due to deprotonation of the silanol group. An improved understanding of the energetics and mechanisms of this fundamentally important process would further understanding of the relevant dynamics.

SIMULATIONS

Density Functional Theory ab initio molecular dynamics and geometry optimisations were used to investigate the mechanisms of surface neutralisation and charging in the presence of OH(-) and H3O(+) respectively. This charging mechanism has received little attention in the literature.

FINDINGS

The protonation or deprotonation of isolated silanols in the presence of H3O(+) or OH(-), respectively, was shown to be a highly rapid, exothermic reaction with no significant activation energy. This process occurred via a concerted motion of the protons through 'water wires'. Geometry optimisations of large water clusters at the silica surface demonstrated proton transfer to the surface occurring via the rarely discussed 'proton holes' mechanism. This indicates that surface protonation is possible even when the hydronium ion is distant (at least 4 water molecules separation) from the surface.

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.

Strong electric fields at a prototypical oxide/water interface probed by ab initio molecular dynamics: MgO(001)

We report a density-functional theory (DFT)-based study of the interface of bulk water with a prototypical oxide surface, MgO(001), and focus our study on the often-overlooked surface electric field.

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.

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.

The amorphous silica-liquid water interface studied by ab initio molecular dynamics (AIMD): local organization in global disorder

The structural organization of water at a model of amorphous silica-liquid water interface is investigated by ab initio molecular dynamics (AIMD) simulations at room temperature. The amorphous surface is constructed with isolated, H-bonded vicinal and geminal silanols. In the absence of water, the silanols have orientations that depend on the local surface topology (i.e. presence of concave and convex zones). However, in the presence of liquid water, only the strong inter-silanol H-bonds are maintained, whereas the weaker ones are replaced by H-bonds formed with interfacial water molecules. All silanols are found to act as H-bond donors to water. The vicinal silanols are simultaneously found to be H-bond acceptors from water. The geminal pairs are also characterized by the formation of water H-bonded rings, which could provide special pathways for proton transfer(s) at the interface. The first water layer above the surface is overall rather disordered, with three main domains of orientations of the water molecules. We discuss the similarities and differences in the structural organization of the interfacial water layer at the surface of the amorphous silica and at the surface of the crystalline (0 0 0 1) quartz surface.

Aqueous solutions: state of the art in ab initio molecular dynamics

The simulation of liquids by ab initio molecular dynamics (AIMD) has been a subject of intense activity over the last two decades. The significant increase in computational resources as well as the development of new and efficient algorithms has elevated this method to the status of a standard quantum mechanical tool that is used by both experimentalists and theoreticians. As AIMD computes the electronic structure from first principles, it is free of ad hoc parametrizations and has thus been applied to a large variety of physical and chemical problems. In particular, AIMD has provided microscopic insight into the structural and dynamical properties of aqueous solutions which are often challenging to probe experimentally. In this review, after a brief theoretical description of the Born-Oppenheimer and Car-Parrinello molecular dynamics formalisms, we show how AIMD has enhanced our understanding of the properties of liquid water and its constituent ions: the proton and the hydroxide ion. Thereafter, a broad overview of the application of AIMD to other aqueous systems, such as solvated organic molecules and inorganic ions, is presented. We also briefly describe the latest theoretical developments made in AIMD, such as methods for enhanced sampling and the inclusion of nuclear quantum effects.

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.

Short Term Culture of Human Mesenchymal Stem Cells with Commercial Osteoconductive Carriers Provides Unique Insights into Biocompatibility

For spinal fusions and the treatment of non-union fractures, biological substrates, scaffolds, or carriers often are applied as a graft to support regeneration of bone. The selection of an appropriate material critically influences cellular function and, ultimately, patient outcomes. Human bone marrow mesenchymal stem cells (BMSCs) are regarded as a critical component of bone healing. However, the interactions of BMSCs and commercial bone matrices are poorly reported. BMSCs were cultured with several commercially available bone substrates (allograft, demineralized bone matrix (DBM), collagen, and various forms of calcium phosphates) for 48 h to understand their response to graft materials during surgical preparation and the first days following implantation (cell retention, gene expression, pH). At 30 and 60 min, bone chips and inorganic substrates supported significantly more cell retention than other materials, while collagen-containing materials became soluble and lost their structure. At 48 h, cells bound to β-tricalcium phosphate-hydroxyapatite (βTCP-HA) and porous hydroxyapatite (HA) granules exhibited osteogenic gene expression statistically similar to bone chips. Through 24 h, the DBM strip and βTCP-collagen became mildly acidic (pH 7.1–7.3), while the DBM poloxamer-putties demonstrated acidity (pH < 5) and the bioglass-containing carrier became basic (pH > 10). The dissolution of DBM and collagen led to a loss of cells, while excessive pH changes potentially diminish cell viability and metabolism. Extracts from DBM-poloxamers induced osteogenic gene expression at 48 h. This study highlights the role that biochemical and structural properties of biomaterials play in cellular function, potentially enhancing or diminishing the efficacy of the overall therapy.

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).

Surface transformations of Bioglass 45S5 during scaffold synthesis for bone tissue engineering

In physiological fluid, a layer of hydroxycarbonate apatite, similar to bone mineral, develops on the surface of Bioglass 45S5. Collagen from the surrounding tissue is adsorbed on this layer that attracts osteoblasts, and favors bone regrowth. Bioglass is therefore an osteoinductive material. Still, due to its brittleness, the glass alone cannot be used to heal large bone defects. To overcome this issue, Bioglass is used to form a composite scaffold with poly(D,L-lactide) (PDLLA), a biodegradable polymer. The goal of this work is to understand Bioglass reactivity throughout scaffold fabrication via a low-temperature route, the solvent casting and particulate leaching technique. Changes in Bioglass (especially its surface) are susceptible to occur both while in contact with the processing fluids and potentially through a reaction with the surrounding polymeric matrix. Here we analyzed the surface changes of three different Bioglass samples: (i) as-received, (ii) treated in solutions that parallel those used in scaffold fabrication, and (iii) extracted from the scaffolds. We showed that extracted, just like treated, Bioglass deviates from the as-received, but to a larger extent. X-ray photoelectron and infrared spectroscopy support the theory that Bioglass surface was modified not just through contact with the solutions in scaffold fabrication, but upon an interaction with the polymeric matrix. The polymer network slows down the Na(+)/H(+) exchange between Bioglass and water used to leach salt particles to create pores within the scaffold. Changes in surface properties affect the bioactivity of Bioglass and thus of the composite scaffolds, and are therefore critical to identify.

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.

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.

The initial stages of bioglass dissolution: a Car–Parrinello molecular-dynamics study of the glass–water interface

The initial dissolution stages following implantation of a biomaterial in a physiological environment are critical for its bioactive properties. Car–Parrinello molecular-dynamics (CPMD) simulations of the interface between the 45S5 bioglass surface and liquid water have been carried out to investigate these processes. The analysis of a 40 ps CPMD trajectory has highlighted the potential mechanism of Na + /H + exchange, leading to formation of surface silanols through water dissociation. Moreover, by comparing the properties of water layers arranged at different distances from the glass surface, we discuss the way in which the particular structure and composition of the bioglass surface affects the hydrogen-bond network and orientation of water in its close proximity.

Surface signatures of bioactivity: MD simulations of 45S and 65S silicate glasses

The surface of a bioactive (45S) and a bioinactive (65S) glass composition has been modeled using shell-model classical molecular dynamics simulations. Direct comparison of the two structures allowed us to identify the potential role of specific surface features in the processes leading to integration of a bioglass implant with the host tissues, focusing in particular on the initial dissolution of the glass network. The simulations highlight the critical role of network fragmentation and sodium enrichment of the surface in determining the rapid hydrolysis and release of silica fragments in solution, characteristic of highly bioactive compositions. On the other hand, no correlation has been found between the surface density of small (two- and three-membered) rings and bioactivity, thus suggesting that additional factors need to be taken into account to fully understand the role of these sites in the mechanism leading to calcium phosphate deposition on the glass surface.