Gautam Sarkar

Document Type

Thesis - University Access Only

Award Date

1998

Degree Name

Doctor of Philosophy (PhD)

Department / School

Chemistry and Biochemistry

Abstract

Bioactive materials such as Bioglass® 45S5 (45% SiO2, 24.5% Cao, 24.5% Na2O, and 6% P 20 5 by weight) are sodium-phosphosilicate glasses containing independent three-dimensional silicate and phosphate networks and Na+ and Ca2+ ions as modifying cations. Due to their bioactivity, thes materials are currently used as implants and for other surgical and clinical applications. The bioactivity of Bioglasses® is due to their unique capability to form chemical bonds to tissues through an octacalciumphosphate (OCP)and/ or hydroxyapatite-like (HA) "interfacial" matrix. The formation of OCP and/or HA is preceded by the formation of a silica-rich surface layer and the subsequent growth of an amorphous calcium phosphate (a-CP) layer. Structural characterization of a series of commercial and synthesized Bioglass materials 45S5 52S, 55S, 60S, and synthesized 17O-labelled "Bioglass Bioglass®, trademark, University of Florida, Gainesville, FL, 32611. materials 45S, 52S, 55S and 60S" have been obtained using solid-state singlepulse magic-angle spinning (SP/MAS) 170, 23Na, 29Si and 31P NMR. The 29Si SP/MAS NMR spectra of Bioglass® materials show the presence of Q2 and Q3 silicate network units in Bioglass® 45S5 and 52S; 0 2, Q3 and Q4 silicate units in Biogiass® 55S and Q3 and Q4 silicate units in Bioglass® 60S. The 31P SP/MAS NMR spectra show the presence of primarily orthophosphate (0°) and a small amount of pyrophosphate (01) units in these materials. Based on correlations between the 29Si SP/MAS and 31 P SP/MAS NMR isotropic chemical shifts (o) and the local silicon and phosphorus chemical environments, a representative structural model for the silicate networks and phosphate units in these Bioglass® materials have been proposed. The 170 SP/MAS NMR isotropic chemical shifts and estimates of the quadrupole coupling constants (Cq) [at fixed asymmetry parameter (rJ°> values of zero] have been obtained from solid-state spin-echo 170 SP/MAS NMR spectra of 17O-labelled "Bioglasses". The simulation results of these spectra reveal the presence of both bridging-oxygens (BO, i.e. :::Si-17O-Si=) and nonbridging oxygens [NBO, i.e. =Si-11O-Na+ and <=Si- 11O-) 2Ca2+] in the silicate networks in these materials. 170 SP/MAS NMR spectra of these "Bioglass materials" do not show any direct evidence for the presence of BO and NBO atoms in the phosphate units; however, they are expected to be present in small amounts. The 23Na SP/MAS NMR frequency shifts of these Bioglass® materials are consistent with the sodium ions in these materials being involved in 5 or 6-coordinate association with NBO atoms of both the silicate and the phosphate network units. In vitro reactions of Bioglass® 45S5, 60S and 77S powders have been used to study the "interfacial" surface chemistry of these materials in simulated body- fluid (SBF, Kyoto or K9 solution) and/or 17O-enriched tris-buffer solution. 29Si and 31 P SP/MAS NMR have been used to identify and quantify the extent of formation of surface silicate species [SiO3(OH), SiO/OH)2, SiO4] and follow the formation of surface phosphate species [POs(OH), HPOt and H2PO4l respectively, while cross-polarization magic-angle spinning (CP/MAS) 29Si and 31P NMR have provided information about low intensity NMR peaks due to various silicate- and phosphate-species present in the vicinity of associated protons on the surface of in vitro reacted Bioglass® materials. 170 SP/MAS and CP/MAS NMR have been used for the preliminary investigations of the "interfacial" chemistry of Bioglass® 45S5 and 77S following in vitro reaction in 17O-enriched tris-buffer solution. The solid-state NMR investigations of the "interfacial" surface reactions of Bioglass® materials are discussed in the context of the structure of these materials and the influence of this structure on the kinetics and the mechanism of their "interfacial" surface chemistry. The results from these investigations have significantly advanced our atomic-level understanding of the structure of Bioglass® materials and the important processes relevant to the bioactivity of these materials.

Library of Congress Subject Headings

Biomedical materials

Surface chemistry

Nuclear magnetic resonance spectroscopy

Format

application/pdf

Number of Pages

262

Publisher

South Dakota State University

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