Fundamental study of the design and development of iron based alloys for biodegradable implant device applications

Biodegradable metals have been widely studied in recent years as potential biomedical implant materials which exhibit higher mechanical properties than degradable polymers while corroding over time to alleviate known complications such as stress-shielding, infection, permanent physical irritation and secondary removal/revision surgeries that is inherent to permanent and bio-inert metallic biomaterials. Specifically, iron (Fe), magnesium (Mg), and their alloys are the two main classes of metals that have been considered as promising candidates for degradable load and non-load bearing bone implants, cardiovascular stents, and other implantable medical devices. However, most of the research on Fe and Fe based alloys have reported very slow degradation rate in physiological environments while Mg and Mg based alloys under rapid degradation. Hence, controlling the degradation rates of these metals has been a key challenge limiting the technological development and implementation of these systems requiring more fundamental studies to be conducted. There is therefore the need to modify the chemical composition and microstructural characteristics through the appropriate alloying techniques with suitable alloying elements. Some studies have been conducted to overcome these limitations but the degradation rates and cytocompatibility of the two main classes of metals are still far from levels necessary for implementation in clinical applications. An alternative approach is therefore needed to develp metallic materials with improved degradation behavior wihle maintaining ithe desired biocompatibility. Although Fe and Mg have been studied separately as degradable implants, there are no reports on the Fe-Mg binary alloy system for use as biodegradable metallic materilas because of the thermodynamic immiscibility of Fe and Mg under ambient conditions. In this dissertation, novel non-equilibrium alloys, particularly in the Fe-Mg binary system with additional elements, have been proposed and studied for their desirable corrosion and cytocompatible properties. The first part of this dissertation focuses on the formation of amorphous alloys in the Fe-Mg binary alloy system through the process of high energy mechanical alloying (HEMA) followed by pulsed laser deposition (PLD) in order to overcome the limitations of the immiscibility of Fe-Mg. In the second part of this dissertation, an alloy development strategy to accomplish high degradation rates, antiferromagnetic behavior and good cytocompatibility is presented. Thus, manganese (Mn), calcium (Ca) and zirconium (Zr) were selected and added as alloying elements, which include the following two aspects: (i) Electrochemical considerations to increase the corrosion rates by electrochemical modification of the Fe matrix; (ii) addition of Mn to introduce anti-ferromagnetic characteristics to the alloy for exploring biological applications. Powders and thin layers were characterized for their composition/structure and evaluated potential for biomedical applications using preliminary in vitro cytocompatibility and corrosion experiments. For Fe-Mg binary alloy, uniform corrosion of Fe70Mg30 amorphous thin layer was observed and the corrosion current density value was approximately 8 fold higher than pure Fe. The direct and indirect cytotoxicity results indicated that Fe70Mg30 amorphous thin layer has no cytotoxicity to MC3T3-E1, hMSCs, HUVECs and NIH3T3 cell lines. Similarly, addition of Ca, Zr and Mn indicate good cytocompatibility with suitable modifications in the corrosion rates