Date of Award

2017-01-01

Degree Name

Doctor of Philosophy

Department

Material Science and Engineering

Advisor(s)

Devesh Misra

Abstract

Magnesium alloys are considered to be the next generation of biomaterials because of their ability to degrade in the physiological environment. We elucidate here the impact of multiaxial forging of Mg-2Zn-2Gd alloy on grain refinement to sub-micron regime and relate the structure to mechanical properties and biological functionality. As-cast and annealed samples were multiaxial forged (MAF) for a total number of two passes with a true strain of ~2/pass. Considering that the microstructure governs the biological response of materials, we studied the constituents of the microstructure in conjunction with the mechanical behavior. The antimicrobial behavior in a Mg-2Zn-2Gd alloy with different grain size in the range of ~44 µm to ~710 nm was studied by seeding. Surface energy and contact angle measurements using goniometer and wettability were assessed with water, SBF, n-Hexane, and DMEM. The structure-property relationship in Mg-2Zn-2Gd alloy to maintaining mechanical integrity during degradation was studied by seeding Escherichia coli (E. coli). Furthermore, we studied the effect of degradation behavior in the presence and absence of cells. This was followed by the study of bioactivity in terms of phases present on the surface and degradation products in simulated body fluid (SBF). Magnesium coated with apatite using a biomimetic approach were placed in a 24-well culture plate with α-MEM media and the degradation behavior was studied in the absence and presence of cells (seeding density: 10,000 cells/cm2). The change in pH was monitored at regular intervals. Cell attachment was studied by seeding the cells for 4h and cell viability was studied by seeding the cells for up to 1, 3, and 7 days.

The study underscores that the fine-grained alloys exhibited superior mechanical properties, antimicrobial resistance, and cell attachment. The degradation rate was also least for fine-grained alloy. The higher surface energy of ultrafine-grained Mg-2Zn-2Gd alloy led to the release of more Mg+2 ions at an early stage, which consequently increased the pH of the fluid in the vicinity of the implant, therefore producing an unfavorable environment for the survival of bacteria. This led to damage of bacterial cell walls and reducing their adhesion. Furthermore, a significant degree of apatite formation was an indication of high bioactivity and cell attachment along with controlled degradation in the ultrafine-grained alloy. Thus, the reduction in grain size significantly improved load bearing capacity and biological functionality of Mg-2Zn-2Gd alloy.

Language

en

Provenance

Received from ProQuest

File Size

158 pages

File Format

application/pdf

Rights Holder

Pramanshu Trivedi

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