Delbert E. Day

Bioactive glass in tissue engineering

This review focuses on recent advances in the development and use of bioactive glass for tissue engineering applications. Despite its inherent brittleness, bioactive glass has several appealing characteristics as a scaffold material for bone tissue engineering. New bioactive glasses based on borate and borosilicate compositions have shown the ability to enhance new bone formation when compared to silicate bioactive glass. Borate-based bioactive glasses also have controllable degradation rates, so the degradation of the bioactive glass implant can be more closely matched to the rate of new bone formation. Bioactive glasses can be doped with trace quantities of elements such as Cu, Zn and Sr, which are known to be beneficial for healthy bone growth. In addition to the new bioactive glasses, recent advances in biomaterials processing have resulted in the creation of scaffold architectures with a range of mechanical properties suitable for the substitution of loaded as well as non-loaded bone. While bioactive glass has been extensively investigated for bone repair, there has been relatively little research on the application of bioactive glass to the repair of soft tissues. However, recent work has shown the ability of bioactive glass to promote angiogenesis, which is critical to numerous applications in tissue regeneration, such as neovascularization for bone regeneration and the healing of soft tissue wounds. Bioactive glass has also been shown to enhance neocartilage formation during in vitro culture of chondrocyte-seeded hydrogels, and to serve as a subchondral substrate for tissue-engineered osteochondral constructs. Methods used to manipulate the structure and performance of bioactive glass in these tissue engineering applications are analyzed.

Mixed alkali glasses — Their properties and uses

Abstract The mixed alkali effect in glass refers to the large, orders of magnitude, changes in many properties when a second alkali oxide is added. Properties most affected are those associated with alkali ion movement such as electrical conductivity and loss, alkali diffusion, internal friction, viscosity and chemical durability. The compositional dependence of these properties is briefly reviewed and examples are given of the relevance of the mixed alkali effect to the manufacture of commercial glasses. Also reviewed are various theories for this scientifically interesting effect.

Chemically durable iron phosphate glass wasteforms

Up to 40 wt% of a simulated high level waste, whose major components were 54.6 wt% Na2O, 14.9 wt% P2O5 and 8.3 wt% Fe2O3, was successfully vitrified into iron phosphate wasteforms whose chemical durability was equivalent to that of borosilicate glass wasteforms. Because of their high fluidity, the iron phosphate wasteforms could be melted in as little as 30 min at temperatures between 1015°C and 1200°C. The addition of 3–7 wt% CaF2 to the batch decreased the melting time and temperature, by as much as 100°C, and improved the chemical durability, especially for crystallized iron phosphate wasteforms. Iron phosphate wasteforms are concluded to be a practical alternative for vitrifying those nuclear wastes not well suited for borosilicate glasses.

Properties and Structure of Sodium-iron Phosphate Glasses

Abstract Selected properties of phosphate glasses, containing from 14 to 43 mol% Fe2O3 and up to 13 mol% Na2O, have been measured. With increasing Fe2O3 and Na2O content, the density and dilatometric softening temperature increased, whereas, the thermal expansion coefficient and dissolution rate in water or saline at 90°C decreased. Glasses containing more than 25 mol% Fe2O3 had an exceedingly good chemical durability. Their dissolution rate at 90°C in distilled water or in saline solution was up to 100 times lower than that of window glass. Mossbauer and X-ray photoelectron spectroscopy indicate that iron(II) and iron(III) were both present in the glasses and the chemical durability improved with increasing iron(III) concentration. The outstanding chemical durability of these glasses was attributed to the replacement of POP bonds by more chemically resistant POFe(II) and POFe(III) bonds.

Mechanical and in vitro performance of 13–93 bioactive glass scaffolds prepared by a polymer foam replication technique

A polymer foam replication technique was used to prepare porous scaffolds of 13-93 bioactive glass with a microstructure similar to that of human trabecular bone. The scaffolds, with a porosity of 85+/-2% and pore size of 100-500 microm, had a compressive strength of 11+/-1 MPa, and an elastic modulus of 3.0+/-0.5 GPa, approximately equal to the highest values reported for human trabecular bone. The strength was also considerably higher than the values reported for polymeric, bioactive glass-ceramic and hydroxyapatite constructs prepared by the same technique and with the equivalent level of porosity. The in vitro bioactivity of the scaffolds was observed by the conversion of the glass surface to a nanostructured hydroxyapatite layer within 7 days in simulated body fluid at 37 degrees C. Protein and MTT assays of in vitro cell cultures showed an excellent ability of the scaffolds to support the proliferation of MC3T3-E1 preosteoblastic cells, both on the surface and in the interior of the porous constructs. Scanning electron microscopy showed cells with a closely adhering, well-spread morphology and a continuous increase in cell density on the scaffolds during 6 days of culture. The results indicate that the 13-93 bioactive glass scaffolds could be applied to bone repair and regeneration.

Structural features of iron phosphate glasses

The structures and valence states of iron ions in several iron phosphate glasses with batch compositions similar to 40Fe2O3-60P2O5 (mol%) have been investigated using Mossbauer spectroscopy, X-ray absorption fine-structure spectroscopy (XAFS), X-ray photoelectron spectroscopy (XPS), differential thermal (DTA) and thermo-gravimetric (TGA) analysis and X-ray and neutron diffraction. Mossbauer spectra show that a redox equilibria corresponding to an Fe(II)/[Fe(II) + Fe(III)] ratio of 0.2–0.4 is reached under processing conditions described in this paper. Even though the valence state of iron ions in the glass appears to be insensitive to the oxygen content in the melting atmosphere, the Fe(II) content can be increased within the observed range of redox equilibria by increasing the partial pressure of a reducing gas in the melting atmosphere. Large amounts of Fe(II), Fe(II)/[Fe(II) + Fe(III)] ≥ 0.4, appear to be detrimental to the glass-forming ability of the iron phosphate melts. The local structure of the iron phosphate glasses appears to be related to the short range structure of crystalline Fe3(P2O7)2 which consists of a network of (Fe3O12)−16 clusters. These clusters consist of one iron(II) ion and two iron(III) ions in sixfold coordination with near-neighbor oxygen ions. The (Fe3O12)−16 clusters are interconnected via (P2O7)−4 groups. Compared to other phosphate glasses, the proposed structure for iron phosphate glasses contain a smaller number of POP bonds, a feature which is believed to be responsible for the unusually good chemical durability of iron phosphate glasses.

Iron Redox Equilibrium, Structure and Properties of Zinc Iron Phosphate Glasses

Abstract Iron redox equilibrium, structure and properties were investigated for 40Fe2O3–60P2O5 (mol%) glasses melted at different temperatures. The Fe2+/(Fe2++Fe3+) ratio increased from 17% to 50% as the melting temperature changed from 1150°C to 1400°C. The equilibrium constant, K, for the reaction of Fe3+ being reduced to Fe2+ varied with temperature as lnK=9.40–1.58×104/T. The Raman and infrared spectra indicated that the basic iron pyrophosphate structure of the 40Fe2O3–60P2O5 (mol%) glasses did not change as the Fe2+/(Fe2++Fe3+) ratio changed. All of the properties did not change to any major degree with increasing the melting temperature. The molar volume decreased while the density increased with increasing Fe2+/(Fe2++Fe3+) ratio. It was found by DTA and XRD that two phases, Fe3(P2O7)2 and Fe4(P2O7)3, crystallized from the glass when the glass was heated in nitrogen. The crystallization behavior suggested that the amount of the crystal, Fe3(P2O7)2, may increase with increasing Fe2+/(Fe2++Fe3+) ratio, which supported the opinion that there are some structural similarities between the iron phosphate glass and the crystalline Fe3(P2O7)2 in terms of the iron coordination number and bonding of the phosphate groups. The decrease in dc resistivity and increase in dielectric constant and dielectric loss tangent, which occurred with increasing the Fe2+/(Fe2++Fe3+) ratio, were attributed to the increase of the electronic hopping from Fe2+ ions to Fe3+ ions.

Chemical Durability and Structure of Zinc-iron Phosphate Glasses

Abstract The chemical durability of zinc–iron phosphate glasses with the general composition (40−x)ZnO–xFe2O3–60P2O5 has been measured. The chemical durability and density of these glasses increase with increasing Fe2O3 content. Glasses containing more than 30 mol% Fe2O3 had an excellent chemical durability. The dissolution rate (DR), calculated from the weight loss in distilled water at 90 °C for up to 32 days, was ∼ 10 −9 g / cm 2 / min which is 100 times lower than that of window glass and 300 times lower than that of a barium ferro, aluminoborate glass. The structure and valence states of the iron ions in these glasses were investigated using Mossbauer spectroscopy, X-ray diffraction, infrared spectroscopy and differential thermal analysis. X-ray diffraction indicates that the local structure of the zinc–iron phosphate is related to the short range structures of crystalline Zn2P2O7, Fe3(P2O7)2 and Fe(PO3)3. Both Fe(II) and Fe(III) ions are present in all of these glasses. The presence of an Fe–O–P related band in the infrared (IR) spectra of the glasses containing more than 30 mol% Fe2O3 is consistent with their excellent chemical durability.

Mechanisms for alkali leaching in mized-NaK silicate glasses

Abstract The kinetics of alkali removal from (1-X) Na2O·XK2O·3SiO2 glasses were studied using pH stat titration techniques, solution analyses, and elemental depth profiling by Rutherford backscattering spectrometry and elastic recoil detection. In the first stage of leaching ( t 1 2 kinetics), the interdiffusion coefficients measured for the exchange of alkali cations by H3O+ are orders of magnitude greater than the alkali diffusion coefficients in bulk glass and show no evidence of the mixed-alkali effect. At longer times, the rate of alkali removal becomes constant with time, but selective alkali leaching is still observed rather than uniform dissolution. These results support a model where the rate determining step for alkali leaching is the rate at which molecular water diffuses into the glass.

Effect of Melting Temperature and Time on Iron Valence and Crystallization of Iron Phosphate Glasses

The effect of melting temperature and time on iron valence, dissolution rate (DR) in deionized water, and crystallization of iron phosphate glasses was investigated using a 40Fe2O3–60P2O5, mol%, batch composition. The concentration of Fe2+ ions in these glasses increased from 17% to 57% as melting temperature increased from 1150°C to 1450°C, but remained nearly constant at about 20% for melting times longer than 1 h at 1200°C. Measurements by differential thermal analysis (DTA) combined with X-ray diffraction (XRD) and thermogravimetric analysis (TGA) showed that these glasses crystallized to Fe3(P2O7)2 and Fe4(P2O7)3 when heated in nitrogen between 600°C and 820°C, but with continued heating in air at 820°C the Fe3(P2O7)2 changed to Fe(PO4), which produced a weight gain in the sample associated with the oxidation of Fe2+ to Fe3+ ions. The DR (in deionized water) of these glasses was generally very low (∼10−9 g cm−2 min−1) and nearly independent of the relative concentration of Fe2+ or Fe3+ ions, but decreased with total iron content.