Nanostructured zirconia via anodization for applications as coatings and potential bio-medical implant modifications

Commonly used implant materials are metals, metal alloys, and more recently ceramics. Modern medical approaches aim towards the development of tailor-made materials, capable of high mechanical and physio-chemical functionalities for enhanced health response and an improvement in overall patient comfort. Functionality may be added as a result of physical, chemical, and/or structural modifications. One such strategy involves using nanotechnology. Surfaces in the submicron range primarily benefit by maximizing the surface area to volume ratio. This can be achieved via top-down and bottom-up fabrication strategies. A combined effect of these synthesis routes, as with electrochemical anodization, can transform conventional bulk materials like metal monoliths, subsequently into nanostructures. This is observed, as the anodization process accommodates a simultaneous bottom-up (oxide growth) and top-down (nanostructure formation). This dissertation explores the design of such homogeneous nanostructured metal-oxides (MOs) via electrochemical anodization.
MOs fall under the category of ceramics. They are increasingly used as biomaterials, are commonly used for dental/orthopaedic applications, and are readily transformed into nanostructured materials via electrochemical anodization. It is a versatile and cost-effective fabrication technique that has high transferability and scale-up possibilities. During this work, we develop and optimize the fabrication of metal-oxide nanotubes on zirconium (Zr) metal via electrochemical synthesis and shed light on the role of zirconia (ZrO2) nanotube (ZrNT) morphology for subsequent surface modifications. The role of surface morphology and different structural changes are compared to determine the influence on the extent of achievable surface functionality. Herein, functionality is determined by variability in the hydrophobic effect and stability of the coatings resulting from organic molecule modification of the nanotubular structures via the formation of self-assembled monolayers (SAM). This work also offers insights on the role of SAM facilitated hydrophobic-effect as a result of application technique, i.e., immersion in bulk solution, aka bulk immersion (BI), and micro-contact printing (μCP), predominantly while using modifications of phosphonic acid carbohydrate molecules. These modified ZrNTs were further evaluated along their tube length in the depth-profiling mode using time-of-flight secondary ions mass spectrometry (ToF-SIMS). Using the depth profile mode, ToF-SIMS analysis was a successful tool, capable of ascertaining the presence of targeted molecules at various depths inside the nanotubes. These ZrNTs were extensively characterized using; Scanning Electron Microscopy (SEM) for morphological insights into nanotube geometries, X-ray diffraction (XRD) for the effects of heat treatment on oxide crystallinity, X-ray photoelectron Spectroscopy (XPS) for surface chemistry of chemically modified oxide nanomaterials. These modified ZrNTs, were also subjected to several applications ranging from drug-reservoirs to superhydrophobic optical coatings. As drug-eluting surfaces, an Ultraviolet-Visible Photospectrometer (UV-Vis Spectrometer) was used for time-dependent drug release monitoring and rapid confirmation of the superhydrophobic extent of SAM-modified surfaces was investigated via Water Contact Angle (WCA) measurements.
The results provide a proof-of-concept to develop multi-depth and multi-functional modifications within nanotubes. Herein, the nanotube walls were effectively functionalized at different depths via facile wet-chemistry and soft-lithography techniques devoid of clean-room fabrication. Additionally, these nanotube reservoirs when evaluated for volumetric storage via simulated dye-release behaviour offer insights into developing potential drug-eluting surfaces for controlled-release applications. In addition to acting as repositories, nanostructure architecture promotes surface texturization in the micro-nanometer scale, which reportedly improves biointegration due to structurally mimicking the extracellular matrix (ECM) and surface functionality as a result of superior adhesion and enhanced reactivity. Therefore, it would be highly interesting to modify biomaterial surfaces with such nanoarchitecture to elicit advantageous responses. Bearing this in consideration and the resulting robustness of the ZrNT
layers, a facile strategy to seamlessly transfer such ZrNTs on bulk ceramics has also been explored within the scope of this dissertation. These ZrNT coatings are reported to self-adhere to a contacting substrate with a high coefficient of friction (CoF), that avoids slippage and promotes anchorage.
Ultimately, the individual research aspects of this work can propose strategies for designing hybrid materials, bearing material chemistry and structure in hindsight, and further modification. The protocols described in this thesis, build up on well-established manufacturing principles and have high translation ability for real-world applications. The overall outcome of several of the results in this work can easily be categorized in TRL (3-4), as is presented in the proof-of-concept. In areas such as superhydrophobic self-cleaning photovoltaic coatings and nanostructured implant surfaces. Especially for biomedical implants, a potential for triggered release and static drug (multi-molecule) eluting surfaces have strong practical implications.