Papers by Author: Robert V. Law

Abstract: Fluoride is an important mineral for hard tissues in the body and appropriate fluoride exposure and usage are beneficial to bone and tooth integrity. Fluoride increases both bone density and bone mass due to stimulation of bone formation and it is used as a treatment for osteoporosis. Bioactive glasses have the capacity to form an intimate bond with living bone tissue due to formation of a mixed hydroxycarbonate apatite layer (HCA) in vitro and in vivo. This makes fluoride-containing bioactive glasses attractive biomaterials. In order to design fluoride-containing bioactive glasses, we need to understand the role of fluorine within the glass structure. A series of bioactive glasses with increasing fluoride content was prepared by a melt-quench route. Characterisation included differential scanning calorimetry (DSC), density measurements, MASNMR spectroscopy and studies in simulated body fluid (SBF). DSC results showed a linear decrease in glass transition temperature (Tg) with increasing amounts of fluoride. Density of the glasses increased with increasing amounts of fluoride. This may indicate an expansion of the silicate glass network accompanying incorporation of CaF2. 19F MAS-NMR spectroscopy showed broad peaks at chemical shifts between -135 and -120 ppm. As sodium fluoride gives a chemical shift of -223 ppm and calcium fluoride of -108 ppm, this indicated possible formation of mixed calcium sodium fluoride species. HCA and calcium phosphate layers were found on the glasses after one week of immersion in SBF showing the bioactivity of the glass series.

Abstract: Fluorine containing calcium aluminosilicate glasses are widely used for a number of technological applications including dental cements, mould fluxes in steel making and in a variety of glass-ceramic systems. Despite of their importance these systems remain quite poorly understood with respect to their composition. To address this question a glass composition corresponding to the equimolar binary system anorthite−fluorite (Ca2Al2Si2O8−CaF2) was chosen as a base point for two series of compositions. One of the series is designed on the anorthite stoichiometry and considered as classically charge balanced. Another series starts from the fluorine free composition of the anorthite−lime (Ca2Al2Si2O8−CaO) stoichiometry and, therefore, is characterized by a disrupted network with at least one non-bridging oxygen (NBO) attached to silicon. A multinuclear 19F, 27Al, 29Si solid state NMR study of the glasses was undertaken. It is shown that in both series fluorine is predominantly coordinated by calcium, F−Ca(n), and in addition interacts with aluminium forming Al−F−Ca(n) complexes, where n denotes the number of first neighbouring calcium cations. Small amounts of high coordinated aluminium grows with increasing fluoride content in both glass series. However, the high coordinated aluminium may not be solely due to the formation of the Al−F−Ca(n) complexes. Glasses of the first series displayed systematic upfield shift of 29Si NMR resonance while substituting fluoride for oxide, starting from the fluorine free composition. This upfield shift is interpreted as the lack of cations in the network, due to formation of the F−Ca(n), which drives silicon network to polymerize toward a higher Qn structure. Contrary to the first series, the 29Si NMR resonance remains constant for fluorine containing compositions of the second series but differs downfield from the initial anorthite glass. The latter is explained by the excess of cations in the network due to addition of the fluorite resulting in formation of NBO on the silicon. Binding of fluorine with silicon is considered negligible in these systems. Thus, fluorine and calcium both define the degree of network polymerization and are considered as a cause for the changes in silicon and aluminium networks.

Abstract: Strontium is one of the most common substituents in apatite crystals. The presence and behavior of Sr in apatite-group phases are of considerable significance in biology. The present paper investigates the substitution of strontium for calcium in a glass-ceramic of the following composition 4.5SiO23Al2O31.5P2O54CaO1CaF2. The glasses were characterized using Differential Thermal Analysis (DTA), X-ray powder diffraction (XRD), neutron diffraction (ND) and 19F Resonance Magic Angle Spinning Nuclear Magnetic Resonance (MAS-NMR). The all calcium glass crystallized to calcium fluorapatite (Ca5(PO4)3F). Substituting strontium partially for calcium resulted in the formation of a mixed strontium/calcium fluorapatites. In contrast complete substitution resulted in the formation of strontium fluorapatite. MAS-NMR showed the the F to be present as F-Ca(3) representing a fluoride ion surrounded by three Ca2+ ions in the all calcium glass and was present as F-Sr(3) in the all strontium glass. In the mixed glasses fluorine was present as FCa( 3), F-Ca(2)Sr, F-CaSr(2) and F-Sr(3). Ca had a higher tendency to occupy the F-M(3) sites than Sr which may reflect the higher charge to size ratio of Ca2+ relative to Sr2+ and its greater affinity for F- ions.