Isaac Abrahams

Development of soluble glasses for biomedical use Part I: in vitro solubility measurement.

In this study soluble glasses have been developed for biomedical applications containing P2O5 as a network former and CaO and Na2O as modifiers. This study shows that as expected, the glasses have an inverse exponential relationship between solubility and CaO content. Furthermore, there is evidence for compositional related non-linearity in the dissolution of the glasses with time. This is thought to be due to either layer formation on the glass surface hindering ion diffusion, ion exchange process or change of ionic strength of the solution. Bioactivity of these glasses is indicated by the formation of a brushite precipitate, a precursor to apatite formation. Further evidence for bioactivity is also presented in the second part of this paper.

Investigation of thermal parameters and crytallisation in a ternary CaO-Na2O-P2O5-based glass system.

This paper presents the results of a study of the thermal properties of a range of phosphate-based glasses in the system CaO-Na2O-P2O5. The glasses had a fixed P2O5 content of 45 mol% and the CaO:Na2O ratio was varied. The glasses were produced and ground and their thermal properties measured using a Setaram differential thermal analyser (DTA). Initial data showed that for the high Na2O containing glass, NaPO3 forms and a second unidentified phase that is probably calcium rich. In the middle compositional region, multiple phases precipitated out and were identified as NaPO3 and Na4Ca(PO3)6. For the high CaO content glasses, Na4Ca(PO3)6 was the only phase formed. Further studies were carried out to examine whether the phases were bulk or surface nucleating, by grinding the samples to different particle sizes. From this data, it is evident that the NaPO3 forms via a bulk nucleation mechanism and that Na4Ca(PO3)6 forms via surface nucleation. The factors controlling this precipitation process are discussed and ionic radius as a controlling factor is hypothesised.

Investigation of the solubility and ion release in the glass system K2O–Na2O–CaO–P2O5

Glasses from the quaternary glass system K2O-Na2O-CaO-P2O5 were produced by standard glass forming techniques. The compositions were limited by fixing the P2O5 at 45 mol%, fixing the CaO content at either 20, 24 or 28 mol%. The K2O and Na2O made up the residual varying from 0 to 25 mol% K2O. General trends showed that with increasing CaO content, the glasses showed a decrease in solubility as expected. For a single system of fixed CaO content, with increasing K2O content, there was an increase in solubility. This was seen at all three CaO contents. All the glasses showed an initial increase in pH followed by a gradual decrease with time and this was accounted for by the initial release of Na+ ions into solution. For the ion release curves, for all fixed CaO contents, the glass with 0 mol% K2O showed the lowest Ca2+ release. This was accounted for as being due to the low solubility compared to the K2O containing glasses. The Na+ release appeared anomalous, as it was higher than all the K2O containing glasses. Even though the glasses with 0 mol% K2O showed the lowest solubility, the amount of Na+ contained in the glass was high, hence the high levels of release.

Poly(butylcyanoacrylate) nanoparticles for topical delivery of 5-fluorouracil

Poly(butylcyanoacrylate) nanoparticles (PBCN) as a drug carrier of 5-fluorouracil (5FU) intended for topical treatment of skin lesia were investigated. The presence of 5FU (as saline solution, pH 10-11) in the polymerization medium affected the polymerization as well as the nanoparticle formation by influencing the initiation of the polymerization reaction. 5FU acted as an initiator in the anionic polymerization of n-butylcyanoacrylate monomer through its nucleophilic nitrogen centers. The results obtained by GPC, 1H NMR, and X-ray diffraction allude to a possible mechanism of cytostatic immobilization in the polymer matrix, with evidence for both free and bound forms of the drug.

Lithium ion conductivity and thermal behaviour of glasses and crystallised glasses in the system Li2O–Al2O3–TiO2–P2O5

Abstract The effects of Al 2 O 3 doping on ionic conductivity and glass stability against crystallisation in the glass system Li 2 O–TiO 2 –P 2 O 5 have been investigated using differential scanning calorimetry and ac impedance spectroscopy. Glass samples of the general molar composition (50+ x )Li 2 O: x Al 2 O 3 :(10−2 x )TiO 2 :40P 2 O 5 (0≤ x ≤4.5) were prepared by air quenching of melts. The highest conductivity at 350°C ( σ 350 =2.00×10 −2 S cm −1 ) and lowest activation energy, E a =0.50 eV, were exhibited by glasses at the x =0.5 composition. This composition also shows the greatest stability against crystallisation ( T c − T g =146°C). The electrical behaviour of the cast, powdered and crystallised glasses was examined. At x =0.5 the conductivity of the crystallised cast glass at room temperature exceeded that of the parent glass. The structural role of Al in the glasses is discussed with respect to variation in the network modifier to network former ratio on substitution for Ti.

Defect chemistry of the BIMEVOXes

Since their discovery in 1988, the BIMEVOXes have been the subject of significant research due to their high oxide ion conductivity at relatively low temperatures. The development of these materials is briefly reviewed. The defect structure of the BIMEVOXes is discussed and used to construct general defect equations for solid solution formation. Two limiting models are proposed by which solid solution formation can occur. In the Equatorial Vacancy (EV) model, vacancies are located exclusively in bridging sites in the vanadate layer. In contrast, the Apical Vacancy (AV) model assumes vacancies are located exclusively in non-bridging apical sites in the vanadate layer. The general defect equations can be used to predict theoretical solid solution limits for all types of substitutions for vanadium in Bi4V2O11−δ. These limits are found to vary not only with the charge of the dopant ion, but also with the coordination number of the metal dopant. In most cases it is found that the EV model yields theoretical solid solution limits close to those observed. The EV model is also used to present a mechanism for ionic conduction in BIMEVOXes, which involves movement of equatorial oxide ions/vacancies between vanadium octahedra and tetrahedra with the formation of a five-coordinate vanadium intermediate.

Defect structure of quenched γ-BICOVOX by combined X-ray and neutron powder diffraction

Abstract A detailed investigation of the defect structure of the Co doped BIMEVOX solid electrolyte, Bi 2 V 1 − x Co x O 5.5 − 3x 2 ( x = 0.1 and x = 0.2), quenched from high temperature, has been carried out using X-ray and neutron powder diffraction data measured at room temperature. The structure is built up from alternating layers of [Bi 2 O 2 ] n 2 n+ and [ V 1 − x Co x O 3.5 − 3x 2 ] n 2n− with disorder limited to the vanadate layer. The ideal V/Co co-ordination is octahedral with corner sharing of equatorial oxygens. The refinements show that the true structure is distorted, with disorder in both apical and equatorial oxygens and oxygen vacancies concentrated in the equatorial positions. Detailed analysis of the oxygen site occupancies reveals two main types of V/Co co-ordination viz. distorted octahedral and distorted tetrahedral. The majority of the sites in both compositions are tetrahedral.

A model for the mechanism of low temperature ionic conduction in divalent-substituted γ-BIMEVOXes

Abstract A model for the conduction mechanism in divalent-substituted γ-BIMEVOXes, of general formula Bi2MxV1−xO5.5−(3x/2)−δ (M=divalent cation), is proposed. This model is based on the defect structure derived from combined X-ray and neutron powder diffraction studies, which show both tetrahedral and octahedral vanadium polyhedra in the defect vanadate layer. The proposed defect structure can be used to accurately predict solid solution limits in the divalent-substituted BIMEVOXes. The proposed conduction mechanism involves positional exchange of oxide ions and vacancies in the equatorial plane of the vanadate layer and results in the effective two-dimensional movement of the vanadium polyhedra through the structure. Three main features are identified in the structure of the γ-BIMEVOXes which are fundamental to the exhibition of high oxide ion conductivity shown in these materials. These are, firstly, the polarizibilty and arrangement of the Bi 6s2 lone pairs with respect to vacancies in the vanadate layer; secondly, the well-known variable co-ordination of V in oxide systems; and thirdly, the large vacancy concentration in the vanadate layer.

Defect structure and ionic conductivity as a function of thermal history in BIMGVOX solid electrolytes

The defect structure of the oxide ion conducting solid electrolyte, Mg substituted Bi4V2O11−δ (BIMGVOX), was examined by high-resolution powder neutron diffraction. A detailed explanation of interpretation of the defect structure is presented. The general formula for the BIMGVOX solid solutions Bi2V1−xMgxO5.5−3x/2 assumes complete oxidation of vanadium to VV. Analysis of the neutron diffraction data reveals the defect structure and indicates that there is, in fact, partial reduction of vanadium to VIV. The extent of reduction is dependent on thermal history, with high temperature quenched samples showing a greater degree of reduction than exponentially slow cooled samples. This is correlated with differences in electrical behaviour at low and high temperatures. Differences in ionic conductivity and activation energies between samples with different thermal histories are explained in terms of the balance between charge carrier concentration and the extent of defect trapping effects.

Structural and electrical characterisation of BINIVOX

Electrical conductivity behaviour and unit cell parameter variation have been investigated in the double substituted BIMEVOX system Bi2V0.9CoyCu0.1-yO5.35, 0<y<0.1 by a.c. impedance spectroscopy and X-ray powder diffraction. Variation of the unit cell volume indicates a microdomain structure for the material with Co rich and Cu rich regions. Both low and high temperature conductivities increase with increasing Co content. However, the overall conductivities appear to be lower than the two end members, BICOVOX and BICUVOX, which suggests that some type of defect trapping may be present in these double substituted systems.