Date of Award

2011

Document Type

Thesis

Degree Name

Master of Science in Material Science Engineering (MSMatSE)

Department

Materials Science

First Advisor

Dr. Yaser E. Greith

Second Advisor

Dr. Abdel-Hamid I. Mourad

Third Advisor

Dr. Nayef Ghasem

Abstract

Hard tissues are natural composites of inorganic and organic components. The inorganic component is a basic Calcium phosphate; known as hydroxyapatite (HAp) while the organic is a natural polymer; known as collagen. The interlocking between the collagen nanofibers and the Hap nanocrystals growing on them offers the known unique mechanical stability of hard tissues. In case of partial fractures; bone or dental cements are often used. Cements such as polymethylmethacrylate (PMMA) has been long used in this regard. However, this polymer is classified as a bio inert material that can be accepted by the human body without having a positive interaction; bioactivity, with the surrounding tissues. Therefore, attempts have been done to introduce more bioactive bone/dental cements to replace PMMA.

Gypsum has been always considered a bioresorbable material and has wide scope of biomedical applications. In fact, gypsum is one of the first known biomaterials to be introduced to augment broken hard tissues. However, gypsum as a Calcium sulfate is characterized by its fast resorption after implantation, which may lead to limited stability of the cement after use. In addition, its chemical composition is different from that of the mineral components in hard tissues; HAp. Due to these concerns, a bioactive Calcium silicate; known as wollastonite, has been thought to be added to gypsum to introduce the aspect of bioactivity. Wollastonite is relatively more stable than gypsum in the body and is known to bond to the surrounding bone tissue through the formation of HAp-like layers on its surface after implantation.

The current study investigates the formation of a composite of gypsum and wollastonite at various proportions of the latter. . Three types of wollastonites; varying in their degree of crystallinity and aspect ratios of their crystals, were used in the study. Wollastonite was added in weight percentages of 1, 5, and 10%. Powder mixtures of a gypsum precursor; Plaster of Paris (POP) and wollastonite were well blended prior to reactions with water. Gypsum was formed in these composites through the hydration of POP and the introduction of 1.5 H2) to POP to form gypsum; CaSO4.2H2O. The effect of using as-received wollastonites fibers or those treated in acidic media on the setting reactions, phase composition, morphology, mechanical properties and the preliminary in vitro performance of the produced composites were studied. Phase composition was investigated by X-ray diffraction (XRD), infrared spectroscopy (FT-IR), and thermogravimetric analysis (TGA), while morphology was investigated by Scanning Electron Microscopy (SEM). Both tensile and compressive strengths of the composites were measured. Preliminary in vitro performance tests were carried out in simulated body fluids and were followed by studying the variations of concentrations of certain ions in the solutions as a result of soaking various composites for up to 14 days in these solutions. Variation in the morphology of the SBF-treated composites was followed by SEM.

Results showed an overall enhancement in the bioactivity of the composites as a result of the addition of wollastonites; both as-received and acid-treated. A slight decrease in the mechanical properties of the composites was observed with the addition of wollastonite fibers. Phase composition of the composites indicated the complete conversion of POP into gypsum in the presence of wollastonite without an interference of the latter in the setting reactions. The advantages of the currently studied composites are, therefore, the enhancement of the bioactivity and the ability to control the biodegradation of the composite by controlling the proportion of POP in the original powder mixture of the reactants.

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