Compositional dependence of bioactivity of glasses in the system CaO-SiO2-Al2O3: itsin vitro evaluation

In order to investigate fundamentally the effect of Al₂O₃ on the bioactivity of glasses and glass-ceramics, the compositional dependence of bioactivity of glasses in the system CaO-SiO₂-Al₂O₃ was studiedin vitro. It is already known that the essential condition for glasses and glass-ceramics to bond to living bone is the formation of an apatite layer on their surfaces in the body, and that the surface apatite layer can be reproduced even in an acellular simulated body fluid which has almost equal ion concentrations to those of the human blood plasma. In the present study, bioactivity of the glasses was evaluated by examining apatite formation on their surfaces in the simulated body fluid with thin-film X-ray diffraction, Fourier transform infrared reflection spectroscopy and scanning electron microscopic observation. Only CaO-SiO₂-Al₂O₃ glasses containing Al₂O₃ less than 1.5 mol % formed the surface apatite as well as Al₂O₃-free CaO-SiO₂ glasses, but CaO-SiO₂-Al₂O₃ containing Al₂O₃ more than 1.7 mol % did not form it as well as an SiO₂-free CaO-Al₂O₃ glass. This indicates that only a small amount of addition of Al₂O₃ to glass compositions suppresses the bioactivity of glasses and glass-ceramics by suppressing apatite formation on their surfaces in the body.     

Bioactivity of CaO·SiO2-based glasses:in vitro evaluation

In order to fundamentally study compositional dependence of bioactivity of glasses, both the surface structural changes of P₂O₅-free CaO · SiO₂ glass due to exposure to a simulated body fluid and the effects of adding a third component, such as Na₂O, MgO, B₂O₃, Fe₂O₃, P₂O₅ and CaF₂, were investigated. An acellular aqueous solution which had almost equal ion concentrations to those of the human blood plasma was used as the simulated body fluid. The surface structure changes were examined by electronprobe X-ray microanalysis, thin-film X-ray diffraction (XRD) and Fourier transform infrared (FTIR) reflection spectroscopy. It was found that even P₂O₅-free CaO · SiO₂ glass forms an apatite layer on its surface in the simulated body fluid, and that the rate of formation of the surface apatite layer is increased with the addition of Na₂O and P₂O₅ while it decreased with the addition of MgO, B₂O₃, CaF₂ and Fe₂O₃. This indicates that even P₂O₅-free CaO · SiO₂ glass can bond to living bone, forming the surface apatite layer in the body and that its bioactivity is increased with the addition of Na₂O and P₂O₅ while it is decreased with MgO, B₂O₃, CaF₂ and Fe₂O₃. It is speculated that a glass of the composition CaO · SiO₂ 100, Fe₂O₃ 3 in weight ratio does not bond to living bone.On leave from: Sumitomo Metal Industries, Ltd, Otemachi, Tokyo 100, Japan.     

Reaction mechanism of the hydrothermally treated CaO-SiO2-Al2O3 and CaO-SiO2-Al2O3-CaSO4 systems

Mixtures of CaO, amorphous SiO₂ and kaolinite in the presence or absence of SO ₄ ^2– ions (added as CaSO₄·2H₂O) were treated in suspension under hydrothermal conditions at temperatures of 80–200 °C. Kaolinite was added to replace 3 and 10% of the total weight of the dry mix with the overall CaO/SiO₂ mole ratio being 0.83. The hydration products were investigated by XRD, IR spectroscopy and DTA techniques in order to elucidate their phase compositions. The results indicate that the presence of SO ₄ ^2– ions leads to a marked increase in the reaction rate at all temperatures investigated. In the C-S-A system, the detected hydration products are calcium silicate hydrate which is then transformed into 1.13 nm aluminium-substituted tobermorite and -C₂SH by increasing the autoclaving temperature. In the C-S-A-C¯s system the hydrated products are calcium silicate hydrate, ettringite and monosulpho-alumirtate. On increasing the hydrothermal temperature they decompose, recrystallize and 1.13 nm aluminium-substituted tobermorite, -C₂SH and anhydrite II are formed. In both systems the excess Al₂O₃ appeared as a hydrogarnet phase, C₃ASH₄.Cement Notations used are C CaO - S SiO₂ - A Al₂O₃ - H H₂O - CH Ca(OH)₂ - C¯s CaSO₄     

Assessment of in vitro bioactivity of SiO2-BaO-ZnO-B2O3-Al2O3 glasses: An optico-analytical approach

Bioactive glasses are an important subclass of biomaterials. The bioactivity of a glass depends on its initial constituents and their respective amounts. In the present investigation, five barium-zinc-borosilicate glass samples have been studied by varying Al₂O₃ mol% to check their bioactivity. The optical and bioactive properties of pristine glasses are compared with glasses soaked in Simulated Body Fluid (SBF) for 10 and 30 days using pH measurement, Ultraviolet–visible-Near Infrared-red (UV–vis–NIR), Fourier Transform Infra-Red (FTIR) spectroscopy, X-ray diffraction and Scanning Electron Microscopy (SEM) techniques. Although calcium is not present as an initial constituent in glass composition, yet bioactivity is observed in some glass samples after dipping them in SBF.     

Effect of iron state on crystallization and dissolution in Fe2O3-CaO-SiO2 glasses

The possibility of iron-containing glasses as thermoseeds for hyperthermia of bone tumor was reported previously. There is, however, no report about the effect of iron state on the crystallization of magnetite and the resultant properties. The iron states were determined by Mo¨ssbauer spectroscopy in Fe₂O₃-CaO-SiO₂ system. It was found that the higher CaO content interrupts the crystallization of magnetite crystallites as well as the oxidation of iron, that is, the transformation from Fe³⁺ to Fe²⁺. A sample containing large amounts of Fe²⁺ showed the faster increment of temperature when the alternating magnetic field was applied. In order to use the thermoseed for a hyperthermia, we can say that the composition with low CaO content is most useful.     

Mixed alkali effect in Li2O–Na2O–B2O3 glasses containing Fe2O3—An EPR and optical absorption study

This paper reports the interesting results on mixed alkali effect (MAE) in xLi₂O–(30-x)Na₂O–69.5B₂O₃ (5 ≤ x ≤ 28) glasses containing Fe₂O₃ studied by electron paramagnetic resonance (EPR) and optical absorption techniques. The EPR spectra in these glasses exhibit three resonance signals at g = 7.60, 4.20 and 2.02. The resonance signal at g = 7.60 has been attributed to Fe³⁺ ions in axial symmetry sites whereas the resonance signal at g = 4.20 is due to isolated Fe³⁺ ions in rhombic symmetry site. The resonance signal at g = 2.02 is due to Fe³⁺ ions coupled by exchange interaction. It is interesting to observe that the number of spins participating in resonance (N) and its paramagnetic susceptibility (χ) exhibits the mixed alkali effect with composition. The present study also gives an indication that the size of alkali ions we choose in mixed alkali glasses is also an important contributing factor in showing the mixed alkali effect. It is observed that the variation of N with temperature obeys Boltzmann law. A linear relationship is observed between 1/χ and T in accordance with Curie–Weiss law. The paramagnetic Curie temperature (θ_p) is negative for the investigated sample, which suggests that the iron ions are antiferromagnetically coupled by negative super exchange interactions at very low temperatures. The optical absorption spectra exhibit only one weak band corresponding to the transition ⁶A_1g(S) → ⁴A_1g(G); ⁴E_g(G) at 446 nm which is a characteristic of Fe³⁺ ions in octahedral symmetry.     

Preparation of CaO-SiO2 glasses by the gel method

The formation of CaO-SiO₂ glasses, which could not have been prepared with the conventional glass-melting technique because of the presence of the stable liquid-liquid immiscibility zone, was investigated by the gel method. The monolithic transparent glasses of CaO·9SiO₂ and CaO·4SiO₂ composition were obtained by slowly heating to 800° C the transparent gels formed by hydrolysis with atmospheric moisture. In this binary system, the promotion of hydrolysis by the addition of water was not so favourable, because it caused the precipitation of the hydrolysed product, i.e. Ca(OH)₂, and therefore the gels obtained were translucent or opaque. The values of true density and refractive index measured for the transparent glasses were comparable with those calculated from the corresponding compositions. Crystallization occured during heat-treatment above 800° C and wollastonite was precipitated.     

Thermal expansion of silico-phosphate glasses with small additions of Fe2O3: EPR and dilatometric studies

Electron paramagnetic resonance (EPR) and dilatometric measurements in phosphosilicate glass have been made in order to elucidate an anomalous trend of the linear thermal expansion coefficientβ when Fe₂O₃ was added to the composition. The g≃2 and g_eff≃4.27 EPR lines were attributed, respectively, to undistorted and octahedrally coordinated Fe³⁺ ions, and to Fe³⁺ ions in low symmetry (rhombic) sites. The Fe³⁺ ion distribution in the network at different temperatures has been explained by a model of chemical insertion and it has been proved that the Fe³⁺ ions have two kinds of role as network modifiers. A relationship has also been found betweenβ and the amounts of Fe₂O₃ added in the range 0 to 12 wt %. It is with deep regret that we note the death of Professor G. L. Del Nero.     

Evaluation of CaO-SiO2-P2O5-Na2O-Fe2O3 bioglass-ceramics for hyperthermia application

Magnetic bioglass ceramics (MBC) are being considered for use as thermoseeds in hyperthermia treatment of cancer. While the bioactivity in MBCs is attributed to the formation of the bone minerals such as crystalline apatite, wollastonite, etc. in a physiological environment, the magnetic property arises from the magnetite [Fe3O4] present in these implant materials. A new set of bioglasses with compositions 41CaO x (52-x)SiO2 x 4P2O5 x xFe2O3 x 3Na2O (2 < or = x < or = 10 mol% Fe2O3) have been prepared by melt quenching method. The as-quenched glasses were then heat treated at 1050 degrees C for 3 h to obtain the glass-ceramics. The structure and microstructure of the samples were characterized using X-ray diffraction and microscopy techniques. X-ray diffraction data revealed the presence of magnetite in the heat treated samples with x > or = 2 mol% Fe2O3. Room temperature magnetic property of the heat treated samples was investigated using a Vibrating Sample Magnetometer. Field scans up to 20 kOe revealed that the glass ceramic samples had a high saturation magnetization and low coercivity. Room temperature hysteresis cycles were also recorded at 500 Oe to ascertain the magnetic properties at clinically amenable field strengths. The area under the magnetic hysteresis loop is a measure of the heat generated by the MBC. The coercivity of the samples is another important factor for hyperthermia applications. The area under the loop increases with an increase in Fe2O3 molar concentration and the. coercivity decreases with an increase in Fe2O3 molar concentration The evolution of magnetic properties in these MBCs as a function of Fe2O3 molar concentration is discussed and correlated with the amount of magnetite present in them.     

The relative influences of Al2O3 and Fe2O3 on the chemical durability of silicate glasses at different pH values

The influence of the pH of aqueous solutions on the chemical durability of two glasses in the system Na₂O-FeO-Fe₂O₃-SiO₂ has been compared with that of two glasses in the system Na₂O-MgO-Al₂O₃-SiO₂ where equimolar substitution of MgO for FeO and Al₂O₃ for Fe₂O₃ have been made. The presence of Fe₂O₃, and particularly of Al₂O₃, in glass reduces its alkali extraction in the pH range 4 to 9. The extraction results have been discussed and interpreted in the light of the available standard thermochemical data for the component oxides of these glasses.     

Correlation of electrical properties and internal friction in mixed conduction glasses containing ionic and electronic conduction (1) Fe2O3-Na2O-P2O5 glasses

The electrical properties and internal friction in (40–x)Fe₂O₃·xNa₂0.60P₂O₅ glasses were measured. Two or three peak on internal friction were observed in the temperature range of –100 to 300° C at a frequency of about 1 Hz. The peak area of internal friction could be explained quantitatively by the additivity law of diffusion of Na⁺ ion and hopping of electrons which are carriers similar to those of dielectric loss. Activation energy, peak temperature of dielectric loss and internal friction showed almost the same value. Both relaxation phenomena have the same mechanism which is due to the diffusion of Na⁺ion and the hopping of electrons between Fe²⁺ Fe³⁺. The high-temperature peak is assumed to result from the interaction between protons or alkali ions and non-bridging oxygen.     

Crystallization and properties of CaO-P2O2-B2O3 glasses

Glasses were prepared with compositions (50–0.5 x) CaO.(50–0.5 x) P₂O₅ · x B₂O₃ with B₂O₃ contents (x) from 0 to 45 mol%. The glass transformation temperature (T _g), dilatational softening temperature (T _D) and Vickers hardness (H _V) initially increased with x, but showed maxima at about x=20 for T _g and T _D and at about x=35 for H _V. The thermal expansion coefficient decreased with x, levelling off at about 35 mol% B₂O₃. The maximum tendency to crystallize occurred at around 25 mol% B₂O₃. Volume nucleation (and hence glass-ceramic formation) and surface nucleation were obtained for x between 15 and 25 mol%. The first phase to appear was BPO₄, which was probably homogeneously nucleated. Subsequently the 4CaO · P₂O₅ phase was heterogeneously nucleated on the BPO₄. For 10 x 35 only surface nucleation was observed. The kinetics of nucleation were investigated in the 20 mol% B₂O₃ glass. The changes in properties and crystallisation behaviour with B₂O₃ content were related to short-range structural information. Infrared spectra and literature data indicated a threedimensional network of B-O-B and B-O-P linkages in the glasses.