Controlling biodegradation and corrosion of Mg and Mg alloys with interfacially engineered hybrid coating

<p>Metallic implants are engineered biomaterials developed to provide internal support to hard biological tissues. Several metallic biomaterials have been employed in various forms over the last few decades, of which magnesium (Mg) and its alloys are promising next-generation metallic bio-implant materials owing to their biocompatibility, biodegradability, bioresorbability and mechanical properties similar to that of human bone. Despite their advantageous properties for bio-implant applications, the clinical applications of Mg and its alloys are hindered or limited to the short term because of their high corrosion rate in the physiological environment. In contact with biological solutions, Mg and its alloys undergo corrosion causing the evolution of Mg2+ ion and H2 gas, which increase the osmolality and pH of adjacent environments, and also form gas cavities at tissue-implant interface, thereby leading to rapid loss of mechanical integrity and biological performance of implants hampering tissue and mineral regeneration process. Therefore, there is a dire need to address this issue in order to expand the application of Mg and its alloys in the field of biomedical engineering. Surface modification techniques can effectively reduce the corrosion and degradation rate of Mg and its alloys by forming a physical or chemical barrier layer against corrosion, thereby improving their long-term in vivo stability and performance. However, single layer or double layer inorganic or organic coatings on Mg and its alloys have been extensively studied rather than hybrid coatings to improve the surface properties of the substrate; where, layer by layer hybrid coatings using an electrochemical process and spin coating process is still demanding. In this research, the hypothesis is that by applying a novel interfacial engineering approach of fabricating layer by layer hybrid coatings (comprising bioinorganic and bioorganic layers), the surface chemistry and mechanical properties of Mg-based implants can be gradually tuned to enhance their resistance against corrosion and biodegradation in the biological environment. This PhD research work focuses on the development and understanding of novel bioinorganic-bioorganic hybrid coatings for effective protection of Mg and its alloys (WE43 and ZK60) against corrosion and biodegradation and aims to provide a path for future optimization of hybrid coatings for bio-implant applications.</p> <p>This thesis consists of seven chapters, including four experimental chapters, and includes a combination of published articles and submitted manuscripts (under peer-review) according to the thesis guidelines of the RMIT University. The contents of the chapters are briefly as follows:</p> <p>Chapter 1 provides a brief introduction about metallic biomaterials, the importance and advancements of Mg and its alloys in the field of biomedical engineering, current challenges for their clinical applications, and an overview of different approaches investigated/reported in the literature for addressing the limitations. The chapter also summarizes existing literature gaps, scope, research questions, motivation and rationale, and specific research objectives of this research.</p> <p>Chapter 2 describes the materials, substrate and sample preparation procedures, and working principle of characterization techniques used in this research project. The characterization techniques include scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), surface profilometry, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Vickers' microhardness testing, universal tensile testing, contact angle measurement, linear sweep voltammetry (potentiodynamic polarization) and electrochemical impedance spectroscopy (EIS).</p> <p>In Chapter 3, the effects of surface modification of Mg and WE43 Mg-alloy by applying an interfacial engineering approach of first introducing surface microroughness, followed by fabrication of a double-layer bioinorganic Ca-P coating was investigated. The surface microroughness was created by anodization technique and the first Ca-P (dicalcium phosphate dehydrate, DCPD) layer by cathodic electrodeposition. The DCPD layer was then converted into a relatively more crystalline and stable hydroxyapatite (HA) layer by alkali treatment. The fabricated HA layer significantly increased the surface microroughness, adhesion strength, microhardness and protection efficiency in modified simulated body fluid (compared to DCPD coated and bare Mg and WE43 samples), thereby effectively reducing the corrosion rate.</p> <p>In Chapter 4, the effects of anodization conditions on surface microroughness of Mg substrate, followed by fabrication of bioorganic protein coating was investigated. The anodization process parameters were first optimized to obtain the maximum possible surface microroughness. Biocompatible silk fibroin (SF) protein coating was fabricated on anodized Mg surface by spin coating. The fabricated SF layer on anodized Mg exhibited high adhesion strength and relatively low microhardness and significantly decreased the surface microroughness and corrosion rate (compared to HA coating described in the previous chapter); whereas, increased the cell viability and protection efficiency in modified simulated body fluid.</p> <p>In Chapter 5, a novel bioinorganic-bioorganic hybrid coating, harnessing the physicochemical property advantages of previously investigated Ca-P and protein coatings, was developed and investigated for their surface and corrosion resistance properties. The purpose of fabricating layer by layer hybrid coating was to enhance further corrosion resistance coupled with the synergistic effect of the underlying HA layer hindering the ingress of aggressive ions and the top hydrophobic SF-based coating preventing the ingress of corrosive solution. The fabricated hybrid coating significantly improved microhardness, adhesion strength, and surface hydrophilicity; and exhibited excellent protection efficiency against corrosion and weight loss in simulated biological solutions, demonstrating two orders of magnitude decrease in corrosion rate. In addition, the hybrid coating improved the biocompatibility and cell viability, with fibroblast cells exhibiting elongated morphology on coated samples as compared to round shape on bare samples.</p> <p>In Chapter 6, a proof of concept study was conducted in order to demonstrate the applicability of the above developed coating system to a wider range of Mg alloys, and to better understand their influence on surface properties and corrosion resistance on different Mg-based substrates. The HA-SF hybrid coating was fabricated on ZK60 Mg alloy by following the previously established steps and investigated for their surface property change and corrosion resistance. As expected, the fabricated sample exhibited improved surface properties; however, with substrate specific property differences compared to WE43; and also demonstrated two orders of magnitude decrease in corrosion rate.</p> <p>Chapter 7 summarizes the major findings/outcomes of this research and provides key conclusions, future perspectives and recommendations. In summary, the research presented here will contribute to the advancement of the interfacial engineering approach through the fabrication of anticorrosive and bioactive hybrid coating on Mg alloys. The ease of fabrication will potentially serve as a platform for future investigations of hybrid coatings for implant applications.</p>