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|Preparation and Characterization of SiO2-CaO-MgO-K2O Glasses as Biomaterials
|Glasses and Glass Ceramics;In-Vitro Bio Activity;Antimicrobial;SBF;Structural property
|Glass and glass-ceramics are very important class of materials. These materials have wide range of applications in nonlinear optical devices, sealants, bioactive materials etc. Bioactivity basically refers to the bone binding capacity of the materials characterized by in-vitro formation of interfacial layer of hydroxyapatite (HAp) or carbonated hydroxyl apatite (HCA) when soaked in stimulated body fluids (SBF). Bioactivity is directly dependent on structural linkage of elements with each other in multi-component system. Glasses convert to glass-ceramics on controlled heat-treatment due to growth of some crystalline phases within the glassy matrix. The volume fraction of the crystalline phase(s) should be limited when better bioactivity along with reasonable mechanical strength and chemical durability is sought. The crystallization kinetics of the glasses using different mathematical models also gives insight of the structure of the glasses. Present thesis describes the preparation of glasses in the system SiO2-K2O-CaO-MgO with variable amount of MgO/CaO. As-quenched glasses have been characterized for their physical, thermal, structural and optical properties. The as-prepared and heat-treated glasses/glass-ceramics were soaked in SBF to check their in-vitro bioactivity. The prominent surface modification/formation could be better observed in pellets than powder form and it also leads to better mechanical compliance. The surface modifications of the samples can also be checked by UV-Visible spectroscopy due to change in absorption of light. However, this technique only gives some qualitative results in terms of disorder induced during surface reactions. Local change in the structure can most suitably be seen by FTIR spectroscopy. The soaked samples have been characterized using various techniques with respect to (w.r.t.) to the unsoaked samples to observe any the formation of HCA layer. The thesis is divided into five chapters with a list of cited references at the end of each chapter. Chapter 1 introduces glasses and glass-ceramics as biomaterials with a brief account of history and development of biomaterials. The biomaterials have been classified according to their response towards living tissues in physiological environments. The requirement of bioactivity followed by mechanism of formation of apatite layer on the sample surface in SBF has been dealt in detail. This chapter also describes the inspiration in choosing the glass compositions. The merits and demerits of using glass- ceramics in place of glasses have been highlighted. A brief account of the thermal stability conditions to optimize the heat-treatment parameters has been elaborated. The justification and theoretical outcome of characterization techniques has also been provided. The interaction of the implants with the body parts along with possible threat of failure of implant is discussed along with some notes on possible remedy. Chapter 2 reviews the literature related to the bioactive silicate glasses progressed since the development of 45S5 Bioglass ®. The bioactive properties which are directly linked directly to structural properties further depend on compositions of the glasses. The known properties of various oxides and their role in glass formation provide the basis of selection of present compositions. MgO behaves differently at different concentrations in the glass network. It mainly modifies the glass network. However, sometimes MgO can act as glass former by forming tetrahedron units (MgO4)2-. Glass properties can also be tailored by controlled heat-treatment to make the glass suitable for applications where better mechanical properties are required. The thermal stability of the glasses can be checked by various mathematical models, which are used to obtain various parameters, such as activation energy for glass transition, crystallization, inflection point, type of crystallization etc. These parameters are further used to study the crystallization kinetics of the glasses and their effect on the bioactivity. Chapter 3 provides the information on the raw materials used, methods of sample preparation and their processing along with the basics of each technique used to characterize these samples. Glasses were prepared by melting the hand milled mixture of raw chemicals by subsequent quenching in air between thick copper plates. Density of as-prepared samples was measured by Archimedes principle. The physical parameters such as molar volume, excess volume, oxygen packing density etc. were derived theoretically using the measured values of density. The structural properties of the as-prepared glasses were analyzed by the X-rays diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. Optical properties of the samples were investigated using UV-visible spectroscopic data. The micro hardness of the samples was measured using micro hardness tester. Thermal parameters have been calculated by differential thermal analyser (DTA). Dielectric measurements were carried out on Solartron Impedance Analyzer (SI 1260) within temperature and frequency ranges 100-400°C and 100-1MHz, respectively. The glass pellets were given controlled heat-treatment to make glass or glass-ceramics. The bioactivity of these samples was checked through in-vitro tests by soaking the samples into SBF for different time durations. During soaking, the pH and weight of the samples were measured. The samples after soaking in SBF were characterized by XRD, FTIR, UV-Visible spectroscopy, scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) and microwave plasma atomic emission spectroscopy (MP-AES).The implant materials are often prone to infection. Therefore, microbial limit test (MLT) has been done. Additionally, the viable microbial count under favorable conditions of microbial growth has been done to check the antimicrobial effect of the glasses. Chapter 4 is the interpretation of the data obtained from the various characterization techniques discussed in the previous chapter. Density being an additive property decreased on replacing CaO by MgO. XRD patterns confirmed the amorphous nature of the synthesized glasses. The major silica bands along with contributions from other oxide were identified and labeled with FTIR and Raman spectroscopy. The ratio of areas under the Raman bands within 950-1000 cm-1 and 1050-1100 cm-1 gives the approximate ratio of NBO/BO for a glass and is proportional to the degree of polymerization (Q3/Q2 ratio). This ratio is highest for glass containing 25 mol% MgO and 10 mol% CaO. The optical band gap of glasses gradually increased from 3.42 eV to 3.65 eV with an increase of MgO content and Urbach energy lies between 0.11 eV to 0.18 eV. The microhardness is in the range of 464-502 HV. The thermal analysis yields two major Tc and Tg corresponding to separated phases which followed the usual behavior with respect to heating rate. However, non-linearity in the properties arises due to the concentration dependent role of MgO. This type of behavior could be related to the mixed alkaline earth metal oxides effect. Crystallization kinetics analysis indicates that glass having CaO/MgO ratio ~1.33 has better polymerization among the present glasses. The highest dielectric permittivity is observed ~22 at room temperature and 100 Hz for the same glass. Higher MgO containing glasses showed reluctant behavior towards crystallization. The crystalline peaks become more prominent after soaking in SBF due to preferential loss of amorphous phase over the crystalline phase in the SBF. After soaking, the pH change of the SBF and leaching of ions as measured through MP-AES were consistent with the sequential series of changes occurring on the surface of glasses as explained in literature. Moreover, many weak bands/shoulders appeared in the FTIR spectra. An extra band at ~876 appears in most bioactive glass originating from ⱱ2 frequency of carbonate ions formed on the glass. FTIR bands due to NBOs are lost after the glass pellets were soaked. IR spectra of the soaked samples similar to that of as-quenched glasses. The optical band gap of the glasses increased after soaking which is an indicative of the reduction in NBOs caused by leaching of alkali and alkaline earth metal ions from glass without formation of any significant crystalline layer on the surface. Heterogeneous distribution of flakes has adhered to the glass surface. The MP-AES data also supported the EDS data since the percentage of these elements increased in SBF after glass soaking. The powder glasses exhibited antimicrobial effect. Chapter 5 presents the major conclusions drawn from the results discussed in Chapter 4. The higher MgO contained glasses show better polymerization (cross-linking) and have compact glass network as compared to higher CaO contained glasses. The CaO/MgO ratio influences the thermal stability, which leads to affect the dielectric properties. At CaO/MgO~1.33 most polymerized structure and thermally most stable glass is achieved. The microhardness the glass pellets were found closer to the human bone i.e. 45-60 HV after reaction with SBF. The crystalline peaks are absent in all the glasses even after soaking in SBF solution. However, XRD halo of the samples becomes broader after soaking in SBF. The SEM micrographs clearly showed the adherence of new entities on the glass/glass-ceramic surfaces in both powder form as well as pellet form. The EDS data also suggests increase in elemental composition of Ca, P, O etc. which are suggestive of formation of bioactive layers at the surface. The present glasses/glass-ceramics with good mechanical, bioactive and antimicrobial properties are promising candidates for bone engineering applications. These samples must be tested via more realistic in-vivo experiments to explore their candidature as biomaterials.
|Doctor of Philosophy
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