Ruben Kotoka

The Effects of Static and Dynamic Loading on Biodegradable Magnesium Pins In Vitro and In Vivo

Here we systematically assess the degradation of biodegradable magnesium pins (as-drawn pure Mg, as-cast Mg-Zn-Mn, and extruded Mg-Zn-Mn) in a bioreactor applying cyclical loading and simulated body fluid (SBF) perfusion. Cyclical mechanical loading and interstitial flow accelerated the overall corrosion rate, leading to loss of mechanical strength. When compared to the in vivo degradation (degradation rate, product formation, uniform or localized pitting, and stress distribution) of the same materials in mouse subcutaneous and dog tibia implant models, we demonstrate that the in vitro model facilitates the analysis of the complex degradation behavior of Mg-based alloys in vivo. This study progresses the development of a suitable in vitro model to examine the effects of mechanical stress and interstitial flow on biodegradable implant materials.

Novel Application of Optical Density Technique to Evaluation of Corrosion Behavior of Metallic Thin Films

Bulk nanostructured metallic alloys as well as coatings are being research closely for biomedical applications, especially for implants. Corrosion behavior is of particular interest, given the aggressive environment of body fluids that these metals experience. While weight loss measurement and electrochemical analysis are the two commonly used techniques for corrosion behavior evaluation, both have their own limitations with respect to characterizing the corrosion of films and coatings under a micron in thickness. This paper reports on a novel application of the optical density technique as a relatively faster yet accurate method to evaluate the corrosion of thin metallic films. With this technique, the survivability and/or resorption time of a pure metallic coating is evaluated by monitoring the changes in optical density (directly related to reduction in film thickness, and thereby to weight loss) of thin film immersed in a corrosive media. In our experiments, Mg, Cu and Ag thin films of a wide range of thicknesses were sputter-deposited on glass substrates and the corrosion kinetics of these metals in a simulated body fluid was studied at room temperature. The results were utilized to develop a resorption time model for the coating, which can be used to predict the survivability of these metallic thin films.Copyright

Physical and Structural Properties of Pulsed-DC Sputtered Al2O3, MgO and ZrO2 Coating for Mg Corrosion Control

Magnesium has attracted a lot of attention over the last few decades, due to its light weight and potential use as biomaterial. However, the poor corrosion resistance of magnesium restricts its practical use for application where exposure to aggressive aqueous media is unavoidable. This paper describes the growth, characterization and corrosion analyses of Al2O3, MgO and ZrO2 coatings for slowing down the degradation of Mg. Different thicknesses of Al2O3, MgO and ZrO2 were deposited on pure magnesium (99.95%) substrates using pulsed-DC magnetron sputtering process. The phase composition and microstructure analyses were performed using X-ray diffraction (XRD) and scanning electron microscopy (SEM) respectively. The corrosion protection behavior of the Al2O3, MgO and ZrO2 coated samples were evaluated using electrochemical measurements as such open-circuit potential, potentiodynamics and EIS. The corrosion test were performed in 0.9 wt% saline solution. The results showed that the Al2O3-coated Mg alloy exhibit a much superior stability and lower corrosion rate. The surface analyses of the corroded samples showed that the coating improved the corrosion resistance, whereas the bare Mg suffered severe localized corrosion. Adhesion assay was performed on the Al2O3, MgO and ZrO2 coatings to determine their biocompatibility. The results showed significant cell attachment to the coating compare to the control (bare glass).Copyright

In-Situ AFM Corrosion Study of Ti and Mg Thin Films

Atomic force microscopy (AFM) is powerful technique to study surface properties and processes in the μm- and nm-range. In-situ liquid AFM measurements were conducted to evaluate the surface degradation of two biocompatible metals: Ti as the most bio-inert (bio-stable) and Mg as the promising biodegradable metal. The pure Mg and Ti thin films were deposited using DC magnetron sputtering on glass substrates. The corrosion resistances of the samples were evaluated in in pure water and phosphate buffered saline (PBS). The results showed exponential decay of film thickness in water.Copyright

Structural and Mechanical Properties of Mg/MgO and Mg/Al2O3 Nanolaminate Coating for Implant Applications

Nanostructured magnesium coatings have the potential of enhancing the integration of implant to bone tissues due to their ability to regulate the functions of integrin, which modulates cell proliferation and differentiation. However, they are soft, ductile and have low wear resistance. These limitations prevent the practical use of Mg coatings for this application. However, application of Mg thin films in the form of nanolaminate coatings has been found to improve hardness as well as wear properties. In this study, Mg/MgO and Mg/Al2O3 nanolaminates with bilayer thicknesses (Λ) 10, 20, 40, 100, 200, 1000 nm were deposited on glass substrate using the reactive pulsed DC magnetron sputtering process. The Mg/MgO nanolaminates were developed from an Mg target. Λ was controlled by the duration of oxygen flow during the sputtering process. Values of Λ were obtained from low angle-XRD. We found that the rate of MgO deposition significantly depends on water vapor content in the chamber and that a partial base pressure of water below 10−8 Torr is required to achieve repeatable results. Structure and properties of multilayered coatings were studied by X-ray diffractometry, nanoindentation, SEM and AFM. At Λ and respectively, while at higher Λ, other orientations are present in the XRD patterns. The nanoindentation results showed slightly higher hardness of Mg/MgO and Mg/Al2O3 nanolaminate coatings compared to that of pure Mg. Nanolaminates have high ductility compared to MgO and Al2O3. Nanolaminate coatings at Λ < 100 nm exhibit an improvement in the mechanical properties due to the presence of interfaces which act as barrier to dislocation movement.Copyright

Mechanical Characteristics of an Anodized Magnesium Alloy for Biodegradable Implants

In recent years, magnesium (Mg) alloys have emerged as possible biodegradable implant materials; however the degradation rate of Mg occurs at a higher rate than tolerable for the human body. Plasma electrolytic oxidation (PEO) has been used in the past as a useful surface treatment technique to improve the anti-corrosion properties of Mg alloys by forming protective coatings. This present work focuses on the effect of electrolyte solution on the corrosion, microstructural, and nanomechanical behavior of PEO coatings for possible use in biodegradable implants. The experimental parameters applied during PEO process did influence the structure, thickness, and morphology of the coating. Microstructural characterization of the coating was carried out by X-ray diffraction (XRD), scanning electron microscopy (SEM) followed by image analysis and energy dispersive spectroscopy (EDX). Further, nanoindentation was employed to evaluate nanohardness and Young’s modulus of the PEO coating. The results show beneficial effects of the PEO coating to enhance the corrosion resistance of the uncoated AZ31 magnesium alloy. The XRD pattern shows that the components of the film vary based on electrolyte solution. The film composition does affect the nanomechanical behavior.© 2013 ASME