Fluoride-containing bioactive glass-ceramics

Fluorapatite glass-ceramics are osteoconductive, and glass-ceramics containing fluorapatite crystals in a bioactive silicate glass matrix can combine the benefits of fluorapatite with the bone-regenerative properties of bioactive glasses. High phosphate content (around 6 mol% P₂O₅) bioactive glasses (SiO₂–P₂O₅–CaO–Na₂O–CaF₂) were prepared by a melt-quench route. Structural investigation using density measurements and calculations confirmed the presence of phosphorus as orthophosphate. Upon heat treatment, the glasses crystallised to mixed sodium calcium fluoride orthophosphates (sodium-containing compositions) and fluorapatite (sodium-free composition). Fluoride suppressed spontaneous crystallisation, allowing formation of glass-ceramics by controlled crystallisation. A notable feature is that silicate network polymerisation and network connectivity did not change during crystallisation, resulting in orthophosphate and fluorapatite crystals embedded within a bioactive glass matrix. By keeping the phosphate content high and the sodium content low, fluorapatite glass-ceramics can be obtained, while not affecting the structure of the bioactive silicate glass phase.     

The influence of CaF2 content on the physical properties and apatite formation of bioactive glass coatings for dental implants

This report details the physical properties, bioactivity and biocompatibility of manufactured glasses containing a range of calcium fluoride (CaF₂) concentrations. Compositions were based on the following system: SiO₂, CaO, Na₂O, K₂O, P₂O₅, ZnO and MgO, and in total seven glasses were synthesized using a melt–quench route. The ratio of the base compounds was kept constant, but had increasing CaF₂ concentrations (0.00, 2.44, 4.77, 9.11, 10.33, 11.53 and 13.00 mol%). Glasses were characterized using X-ray diffraction (XRD), differential scanning calorimetry (DSC) and dilatometery. Density was quantified according to Archimedes method and apatite formation tested following immersion in simulated body fluid (SBF) and Tris-buffer solution. Glass coatings were prepared by enamelling technique using 10 mm in diameter pure titanium disks. XRD demonstrates that all glasses are amorphous and that the sintering window, glass transition and softening temperatures decrease with increasing CaF₂ content. In contrast, thermal expansion coefficient and glass density increase with CaF₂ content. After 1 week immersion in SBF and Tris, XRD and Fourier transform infrared (FTIR) spectroscopy showed that the surfaces of all glasses underwent structural changes with evidence of surface apatite formation. Fluoride-electrode analysis indicates that the amount of fluoride released was proportional to the original CaF₂ content. The survival and growth of osteoblasts on the surface of these glasses is consistent with biocompatible characteristics.     

Crystallization kinetics of fluoride nanocrystals in oxyfluoride glasses

Oxyfluoride glasses containing different alkaline earth fluoride (CaF₂, SrF₂, and BaF₂) were prepared and their crystallization behavior was analyzed using non-isothermal kinetics based upon differential scanning calorimetry (DSC) scan results. The glass containing CaF₂ showed the fastest kinetics for CaF₂ crystallization, while that containing BaF₂ showed the slowest kinetics for BaF₂ crystallization. On the other hand, all the oxyfluoride glasses showed very similar behavior in the crystallization of glass matrices. The difference in the crystallization behavior of the oxyfluoride glasses was discussed based upon the difference in the size of alkaline earth ions and the difference in the dissociation energy of alkaline earth-fluorine bonds.     

Optical properties of new low-phonon SnF2–PbF2–ZnF2 glasses

New fluoride glasses were developed in the SnF₂–PbF₂–ZnF₂ system. Additions of 5 mol% of CsCl or 6–15 mol% of AlF₃ were found to stabilize the glass formation. The IR absorption of the glasses and the optical properties of the europium ions doped in the glasses were compared with those of a fluorozirconate glass as a representative fluoride glass. The IR spectra showed that the new glasses have the peak of the phonon energy at 400 cm⁻¹, which is about 100 cm⁻¹ lower than that of the fluorozirconate glass. The results of the fluorescence measurement of the europium ions revealed that the multi-phonon relaxation rate in these glasses is smaller than that in the fluorozirconate glass.     

Structural analysis and devitrification of glasses based on the CaO-MgO-SiO2 system with B2O3, Na2O, CaF2 and P2O5 additives

Glasses, whose basic composition was based on the CaO-MgO-SiO₂ system and doped with B₂O₃, P₂O₅, Na₂O, and CaF₂, were prepared by melting at 1400 °C for 1 h. Raman and infrared (IR) spectroscopy revealed that the main structural units in the glass network were predominantly Q¹ and Q² silicate species. The presence of phosphate and borate units in the structure of the glasses was also evident in these spectra. X-ray analysis showed that the investigated glasses devitrified at 750 °C and higher temperatures. The crystalline phases of diopside and wollastonite dominated, but weak peaks, assigned to akermanite and fluorapatite, were also registered in the diffractograms. The presence of B₂O₃, Na₂O, and CaF₂ had a negligible influence on the assemblage of the crystallized phases, but it caused a reduction of crystallization temperature, comparing to similar glasses of the CaO-MgO-SiO₂ system.     

Structural characterization of ionomer glasses by multinuclear solid state MAS-NMR spectroscopy

In the present study we report the results of ²⁹Si, ²⁷Al, ³¹P and ¹⁹F magic angle spinning nuclear magnetic resonance spectroscopy (MAS-NMR) of 4.5SiO₂-3Al₂O₃-1.5P₂O₅-(5−z)CaO-zCaF₂ glasses with z = 0-3 to elucidate the effect of fluoride content on the glass structure. The ²⁹Si MAS-NMR spectra gave a chemical shift of about −90 ppm corresponding to Q³(3Al) and Q⁴(3Al). The ²⁷Al MAS-NMR showed a large broad central peak around 50 ppm, which is assigned to four-coordinated Al linked via oxygen to P. A shoulder around 30 ppm and a small peak at about 0 to −10 ppm appeared in the ²⁷Al MAS-NMR spectra of the glasses on increasing the fluoride content assigned to five-coordinated and six-coordinated Al species, respectively. The ³¹P MAS-NMR spectra indicated the presence of Al-O-P bonds. The ³¹P chemical shift decreased with increasing fluoride content as a result of calcium being complexed with fluoride. This resulted in a reduction of the number of available cations to charge balance non bridging oxygens in phosphorus and an increase in the number of Al-O-P bonds being formed, instead. The ¹⁹F spectra indicated the presence of Al-F-Ca(n) and F-Ca(n) species in all the glasses containing fluoride as well as an additional Si-F-Ca(n) species in the glasses with higher fluoride content.     

The influence of strontium substitution in fluorapatite glasses and glass-ceramics

Strontium is often substituted for calcium in order to confer radio-opacity in glasses used for dental cements, biocomposites and bioglass-ceramics. The present paper investigates the influence of substituting strontium for calcium in a glass of the following composition: 4.5SiO₂3Al₂O₃1.5P₂O₅3CaO2CaF₂, having a Ca:P ratio of 1.67 corresponding to calcium fluorapatite (Ca₅(PO₄)₃F). The glasses were characterized by magic angle spinning nuclear magnetic resonance (MAS-NMR), by differential scanning calorimetry (DSC) and X-ray powder diffraction (XRD). The ²⁹Si, ²⁷Al and ³¹P NMR spectra for the glasses with different strontium contents were identical. The ¹⁹F spectra indicated the presence of F-Ca(n) and Al-F-Ca(n) species in the calcium glasses and in the strontium glasses F-Sr(n) and Al-F-Sr(n). It can be concluded that strontium substitutes for calcium with little change in the glass structure as a result of their similar charge to size ratio. The low strontium glasses bulk nucleated to a calcium apatite phase. Intermediate strontium content glasses surface nucleated to a mixed calcium-strontium apatite and the fully strontium substituted glass to strontium fluorapatite.     

Dispersion and reflection spectra of fluorine and oxide glasses in the vacuum and extreme ultraviolet region

Reflection spectra of aluminium fluoride glass, zirconium fluoride galss, silicate galss, borate glass, and phosphate glass were measured in the range of 10 eV to 35 eV using synchrotron radiation.Two main bands at 10 eV and 22 eV were observed for fluoride glasses and oxide glasses. The band at 10 eV shifts to higher energy in the order of borate < phosphate ≈ silicate < aluminium fluoride ≈ zirconium fluoride glasses. The dispersion formula developed by Drude and Voigt [1] was applied to explain the observed dispersion value taking into account five oscillators. It was clarified that the reason for the high Abbe value of the fluoride glasses is mainly attributed to the shorter wavelength absorption due to bridging ions, γ₁. The higher abnormal partial dispersion is attributed to the lower oscillator strenght at the infrared reflection band, N₄f₄. The reflection spectra of oxide glasses due to bridging ions (λ₁) shifts to the higher energies in the order of borate < phosphate < silicate glasses, which coincides with the dispersion behavior except for silicate glass which shows a higher energy band. The high dispersion behavior of silicate glass will be attributed to the higher intensity absorptions below 10 eV due to non-bridging ions.     

Solubility of glasses in the system P2O5-CaO-MgO-Na2O-TiO2: Experimental and modeling using artificial neural networks

Phosphate glasses in the system P₂O₅-CaO-MgO-Na₂O-TiO₂ for use as degradable implant materials were produced. In order to classify their solubility behavior, dissolution experiments were performed in deionized water for 60 min at 98 °C. Resulting solutions were analyzed using ICP-OES. In addition, pH measurements were carried out in physiological NaCl solution. With increasing phosphorus oxide content, the glasses showed a higher solubility and gave lower pH values in aqueous solution. This was caused by changes in the glass structure, as long phosphate chains are more susceptible to hydration than smaller phosphate groups. These changes in glass structure were followed by ³¹P MAS-NMR experiments. Increasing sodium oxide concentrations in exchange for calcium or magnesium oxide also increased the glass solubility by disrupting ionic cross links between chains. By contrast, addition of titania made the glasses more stable towards dissolution by cross linking smaller phosphate groups. The aim of this study was to find a relationship between glass composition and solubility behavior. As classical linear methods of data analysis were unsuitable due to the complexity of the relationship, preliminary artificial neural networks analyses were performed and were found to be an interesting tool for modeling the solubility behavior of phosphate glasses.     

Improvement of the chemical resistance of zirconium fluoride glasses coated with a Tiron® modified tin oxide layer prepared by the sol–gel process

In order to improve the chemical resistance of zirconium fluoride glass a protective transparent SnO₂ layer was deposited by the sol–gel dip-coating process in the presence of Tiron^® as particle surface modifier agent. After water immersion for different periods of time, both coated and non-coated fluoride glasses were analyzed by scanning electron microscopy, mass loss evaluation, infrared spectroscopy and X-ray photoelectron spectroscopy. In contrast to the effects occurring for non-coated glass, where the surface undergoes a rapid selective dissolution of the most soluble species, the results for the SnO₂-coated glass showed that the filling of the film nanopores by dissolved glass material results in a hermetic barrier protecting the glass surface. The selective glass dissolution was confirmed by liquid chromatography measurements of the etching solution after each exposure time.     

Physicochemical studies of phosphate based P2O5-Na2O-CaO-TiO2 glasses for biomedical applications

Glasses P₂O₅-Na₂O-CaO-TiO₂ with different TiO₂ contents and fixed P₂O₅ (45 wt%) and CaO (24 wt%) have been prepared employing the normal melting and annealing technique. Measurements such as ultrasonic velocity, attenuation, solubility and pH have been carried out in all the compositions of the glasses. It is interesting to note that the above measured ultrasonic parameters exhibit an abnormal behavior (minimum) at 0.5 wt% of TiO₂ content, beyond which an increase in these parameters with increasing TiO₂ content is observed. The maximum pH values and Ca²⁺ ion release have been observed for the TiO₂ free glass those compositions with and the low TiO₂(?1.0 wt%) content. As the content of the TiO₂ increases, the solubility of the glasses decreases. The observed weight loss reveals two stages of phosphate dissolution kinetics i.e. the first stage, in which the weight loss is proportional to t^1/2, and a second stage in which a linear behavior is observed.     

Formation and optical properties of (R2O or R′O)Nb2O5Ga2O3 glasses

Glass-forming tendencies of melts in the systems (alkali oxide or alkaline earth oxide)-Nb₂O₅Ga₂O₃ were examined by an ordinary crucible-melting technique. The glass-forming tendency increased with increasing radius of alkali or alkaline earth ion in the respective groups. Clear glasses were obtained on a practically useful scale in the systems (K₂O or Cs₂O)Nb₂O₅Ga₂O₃ and (SrO or BaO)Nb₂O₅Ga₂O₃. The infrared absorption spectra indicated that the Ga³⁺ ions in the glasses are tetrahedrally coordinated with oxygen ions. The glasses showed high optical transmissions from the ultraviolet region of 0.3 μm in wavelength to the infrared region of 7 μm, except for a region near 3 μm. The absorption near 3 μm, which is attributed to OH vibration, could be eliminated by replacing part of the carbonate in the raw materials with a fluoride and melting the mixture of raw materials in a dry N₂ gas atmosphere. The glass-forming tendencies of the melts and the optical transmissions of the glasses were discussed in terms of the glass structure.     

Water penetration mechanisms in nuclear glasses by X-ray and neutron reflectometry

To determine the water diffusion at the early stage of the alteration, X-ray and neutron reflectometry have been performed on altered simplified glasses and the SON68 glass (an inactive R7T7-type French nuclear glass). For the first experiment, the simplified and SON68 glasses were altered at pH 3 and pH 6 and characterized by X-ray reflectometry as a function of the alteration duration. The evolutions of the electron density profile obtained from the reflectivity curves simulations have allowed the determination of the layers compositions. At the beginning of the alteration and for pH 3, the altered surface layer is constituted of a dealkalized zone. Upon alteration progress, the water diffuses inside the layer and hydrolyzes the Si–O–B bonds. For the second experiment, glasses were altered in D₂O (pD 3) and analyzed in D₂O saturated cell. After a D₂O/H₂O substitution, the samples were characterized one more time in H₂O saturated cell. The evolution of the scattering length density shows that in the first stage of the alteration, the layer is constituted of two parts: a dealkalized glass and a dealkalized and boron depleted glass where water has diffused. According to the glass composition and after few hours of alteration, this dealkalized glass part can disappear.     

Influence of the partial substitution of CaO with MgO on the thermal properties and in vitro reactivity of the bioactive glass S53P4

The influence of up to 4 mol% substitution of MgO for CaO on the properties of the bioactive glass S53P4 was studied. Thermal analysis, hot stage microscopy and X-ray diffractometry were utilized to measure the thermal properties and the crystallization characteristics of the glasses. The in-vitro bioactivity was measured by immersing the glasses for 4 h to one week in simulated body fluid. The formation of silica rich and hydroxyapatite layers was assessed from FTIR spectra analysis and SEM images of the glasses surface. Increasing substitution of MgO for CaO decreased the glass transition, the onset and endset of melting and the fusion temperatures. The activation energies for glass transition and crystallization also decreased from (790 ± 30) to (407 ± 30) kJ/mol and from (283 ± 30) to (145 ± 30) kJ/mol, respectively, indicating a decrease in bond length and an increase in bond strength with progressive MgO at the expense of CaO. All glasses dissolved identically in SBF during the first 24 h of immersion with subsequent formation of hydroxyapatite at the grain surfaces. The thickness of the surface layers decreased with increasing MgO content. For longer duration of immersion, the glasses with the highest MgO contents exhibited a slower reaction tendency, with simulated body fluid, than the Mg-free glass. These changes in the glass structure and in-vitro properties may be of interest for products from bioactive glasses with large surface area to volume ratio.     

Glass-forming ability and crystallization of bulk metallic glass (HfxZr1−x)52.5Cu17.9Ni14.6Al10Ti5

We have produced a series of bulk metallic glasses of composition (Hf_xZr_1−x)_52.5Cu_17.9Ni_14.6Al₁₀Ti₅ (with x=0-1) by an arc melting/suction casting method. The density of these alloys increases by nearly 67% with increasing Hf content from 6.65 g/cm³ (x=0) to 11.09 g/cm³ (x=1). Over the same composition range the glass-forming ability decreases, as demonstrated by the size of the largest amorphous ingots that can be cast without crystallization. Although both the glass transition temperature and the melting temperature increase linearly with increasing Hf content, the reduced glass transition temperature (T_g/T_m) decreases, from 0.64 (x=0) to 0.62 (x=1), which suggests that the `confusion principle' correlating increased glass-forming ability with increased number of components, does not apply in this case due to the chemical similarity between Zr and Hf. A different crystallization behavior is observed for Zr-based and Hf-based glasses. The final crystalline phases are CuZr₂ and Zr₂Ni for Zr-based alloys, and Al₁₆Hf₆Ni₇ and CuHf₂ for Hf-based alloys.     

X-ray absorption near edge structure analysis of valence state and coordination geometry of Ti ions in borosilicate glasses

The effects of borosilicate glass composition and melting condition on valence state and coordination geometry of polyvalent Ti ions have been studied by X-ray absorption experiments. The results indicate that most of Ti³⁺ ions are oxidized into Ti⁴⁺ ions when Ti₂O₃ is doped into sodium borosilicate glasses. When TiO₂ is doped into sodium borosilicate glasses, almost all of titanium ions are in the form of Ti⁴⁺. Doping both graphite and titanium oxide simultaneously in borosilicate glasses favors the formation of small amounts of Ti³⁺ ions. Ti⁴⁺ ions occupy both fivefold and sixfold coordinated sites in borosilicate glasses with non-bridging oxygen (NBS1), mainly fivefold coordinated sites. For borosilicate glasses without non-bridging oxygen (NBS2), Ti⁴⁺ ions are in both fourfold and fivefold coordinated sites. Using reducing agents during melting procedure favors the formation of fourfold coordinated Ti⁴⁺ ions in NBS2.     

Synthesis and characterization of calcium phosphate based bioactive quaternary P2O5-CaO-Na2O-K2O glasses

Calcium phosphate based bioactive quaternary glass systems P₂O₅-CaO-Na₂O-K₂O were prepared by melt growth technique. Glasses were prepared in five different compositions by fixing P₂O₅ at 47 mol% and CaO at 30.5 mol% and by varying the K₂O and Na₂O concentrations. The structural properties of the glasses are analyzed using X-ray diffraction (XRD) studies and scanning electron microscopy (SEM) studies; and the composition of the glasses are studied using energy dispersive X-ray spectrum (EDS). The microhardness of the glass systems are studied by Vickers hardness measurements and the bioactivity of the glasses are studied using in vitro study. The thermal properties have been examined by means of thermogravimetric analysis (TGA) and differential thermal analysis (DTA). The ultrasonic velocity measurements show that the addition of K₂O contents produces non-bridging oxygen ion and hence weaken of the glass structure. The weakening of the glass structure reduces the ultrasonic velocity and hence an increase in attenuation.     

Properties and structure of binary tin phosphate glasses

The properties and structures of binary xSnO*(100 − x)P₂O₅ (50 ≤ x ≤ 70 mol%) glasses were evaluated. The glass transition temperatures (T_g), determined by differential thermal analysis (DTA), range from 246 to 264 °C, for glasses prepared under identical conditions. The refractive index (n_D) increases from 1.701 to 1.833 as x increases from 50 to 70, and the Abbe number (ν_D) decreases from 29.1 to 20.4 over the same range. Infrared spectroscopy was used to estimate water contents in the glasses, which decreased with an increase in SnO content, from about 1570 ppm OH for x = 50 to about 50 ppm OH for x = 70, for glasses quenched from melts held at 1000 °C for 15 min. Residual water affects thermal properties, like T_g, and variations in water contents due to differences in melt processing explain the wide variety of glass properties reported in the literature. Raman spectroscopy indicates that progressively shorter phosphate chains are present in the structures of the binary Sn-phosphate glasses with increasing SnO contents.     

Bi precipitates in Na2O–B2O3 glasses

Bi particles of different sizes were produced in Na₂O–B₂O₃ glasses by melt quenching and heat treatment technique. Melting temperature of Bi particles was measured by differential scanning calorimetry and X-ray diffraction. Measured melting temperatures of Bi particles are lower than bulk Bi melting temperature. Results of transmission electron microscopy were analyzed for the dependence of melting temperature on particle radius. The pressure and surface energy effect on melting temperature is estimated. The melting behavior of Bi particles in Na₂O–B₂O₃ glasses depends on the difference in the interfacial energies between the solid particle/glass and liquid particle/glass, and liquid particle/glass, σ_sm−σ_lm, which is estimated to be 255×10⁻³ J m⁻².     

The role of MgO on thermal properties,structure and bioactivity of bioactive glass coating for dental implants

The successful application of bioactive glasses as a coating for titanium implants to improve osseointegration is dependent on achieving a thermal expansion coefficient (TEC) match between the two phases. Many studies have indicated that magnesium affects the thermal and structural properties of a glass. However, its effect on bioactivity of bioactive glasses is still under debate. In this study, we characterize the effect of magnesium on the thermal properties, structure and bioactivity of glasses containing MgO. Seven glasses with different MgO concentrations have been synthesized by melt-derived technique. All these glasses contain SiO₂, CaO, MgO, ZnO, Na₂O, K₂O and P₂O₅ and have been characterized by X-ray diffraction (XRD), differential scanning calirometry (DSC) and dilatometry. Additionally, the oxygen density and bulk density have been investigated. The bioactivity of the MgO glass series was evaluated in simulated body fluid (SBF) and Tris-buffer solution. The results indicate that magnesium suppresses crystallization and decreases TEC, glass transition temperature (T_g) and softening (T_s) temperature. Oxygen density increases proportionally with MgO content; whereas glass density decreases. After different time points, XRD and Fourier transform infra-red spectroscopy (FTIR) show that magnesium does not inhibit the apatite forming ability of the glasses, but retards the time of apatite deposition. The glass coatings of this series exhibit clear clinical application for use as a functional barrier: osteoblastic attachment and growth was poor, however, fibroblastic biocompatibility was good.