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Multinuclear solid state NMR of novel bioactive glass and nanocomposite tissue scaffolds

By Claudia Adriana Turdean-Ionescu


Sol-gel derived bioactive glasses are promising candidates for bone regeneration,\ud where bone is a natural nanocomposite of collagen (organic polymer) and hydroxyapatite\ud (inorganic mineral) with a complex hierarchical structure and excellent mechanical\ud properties. Solid-state NMR is a sensitive probe and offers atomic-level information on the\ud structure of sol-gel derived bioactive glasses. In this thesis, a multinuclear solid state NMR\ud approach, as part of an extensive study, has been applied to a key range of sol-gel derived\ud materials related to novel nanocomposites to act as tissue scaffolds.\ud The nanostructure evolution of sol-gel derived bioactive glasses 70S30C (70 mol%\ud SiO2 and 30 mol% CaO) was characterised by 29Si, 1H and 13C CP MAS NMR. Calcium was\ud found to be incorporated into the silica network during the stabilisation stage and to\ud increases its disorder. The inhomogeneity found within 70S30C bioactive glass monoliths\ud showed that the calcium concentration was higher in the outer region of the monolith caused\ud by the way calcium only enters into the structure after breakbown of the nitrate.\ud Trimethylsilylation reaction mechanisms used to tailor the nanoporosity of sol-gel derived\ud 70S30C bioactive glass was also studied. The 29Si NMR results showed that the modification\ud processes affected the atomic scale structure of the glass, such as Qn structure and network\ud connectivity. 1H and 13C NMR was used to follow the loss of hydroxyls and organic\ud groups directly.\ud The study was extended to 58S (60 mol% SiO2, 36 mol% CaO, 4 mol% P2O5)\ud systems and compared for two synthesis routes: inorganic and alkoxide. Via the inorganic\ud route high temperatures were needed for calcium incorporation, while via alkoxide route\ud calcium was found to be incorporated at low temperatures. Reactive surface Ca ions were\ud involved in the formation of different types of carbonates for the two routes. The addition of\ud P2O5 to the silica-calcium oxide system results in a scavenging of calcium ions by phosphate\ud groups to give orthophosphate and pyrophosphate units.\ud Solid-state NMR of new organic-inorganic hybrid scaffolds, class II, in the silicagelatin\ud and silica-calcium oxide-poly(γ-glutamic acid) (γ-PGA) systems indicates that 3-\ud glycidoxypropyltrimethoxysilane (GPTMS) provides a covalent link between the organic\ud and inorganic networks and increased the inorganic condensation. 1H-1H intra- and\ud intermolecular proximities have been identified using 1H DQ (double-quantum) CRAMPS\ud (combined rotation and multiple pulse spectroscopy) techniques. 13C NMR results indicate\ud that an efficient promotion of epoxide ring opening of GPTMS was reached by either gelatin\ud or γ-PGA. 43Ca NMR identified different calcium environments in the hybrid systems.\ud The last part of this thesis is focused on the comparison studies in the mechanism of\ud apatite growth on both melt-derived (Bioglass®) and sol-gel derived (TheraGlass®) bioactive\ud glass surfaces. By using a combination of 1H, 13C, 31P, 29Si and 23Na, using one and two\ud dimensional NMR spectroscopy, the inhibitive effects of serum proteins in the mechanism of\ud the apatite growth was revealed. The solid-state NMR experimental data support the\ud hydroxycarbonate apatite formation mechanism proposed by Hench. Apatite formation takes\ud place from the largely amorphous phosphate ions initially deposited on the glass surface.\ud Serum proteins adsorbed on the glass surface have been found to significantly inhibit the\ud apatite formation. Multiple sodium sites have been identified in Bioglass® composition with\ud the formation of a more ordered local structure on increasing immersion time

Topics: QC, R1
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