School of Physics - Theses
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ItemEngineering Approaches to Active Brazing of Diamond Medical Implant Components( 2023-12)This research aims to develop techniques for creating robust, hermetic joints and encapsulated structures using gold (Au) braze alloys and ceramic substrates for electronics and implantable medical devices. A major challenge is the poor wetting and adhesion typically observed between Au brazes and ceramics due to interfacial reactions and surface oxides. Without suitable adhesion layers, weak boundary joints prone to failure may result. We investigate processing methods involving thin film deposition, adhesion layer metals, brazing parameters, and joint analysis to overcome these adhesion issues with Au brazing on ceramics. A priority application is creating long-lasting, leak-proof packages for electronics in medical implants, which must meet strict encapsulation requirements for human use. Diamond demonstrates exceptional implantation lifetimes owing to outstanding biostability and biocompatibility but cannot be welded. One alternative is active braze alloys, containing carbide-forming elements that chemically bond to the diamond upon melting. However, biocompatible gold active braze alloys (Au-ABA) exhibit very poor wetting on diamond. I demonstrate that molybdenum (Mo) and niobium (Nb) interlayers significantly improve Au-ABA wetting, enabling excellent penetration into surface features. Optimal recipes for interlayer fabrication are determined, facilitating complex microstructures, hermetic electrical feedthroughs, and robust bonds. In laser-grooved diamond circuit boards, residual metal contamination resistant to etching and polishing was found to electrically short features after depositing Mo/Nb adhesion layers and Au-ABA brazing. Here, I reported an optimised reactive ion etching (RIE) process to remove this contamination and isolate fine circuit board features. Elemental and electrical testing verifies complete contaminant removal and restoration of electrically insulating diamond surface properties after RIE.
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ItemThe characterisation and implementation of nitrogen-doped ultrananocrystalline diamond photoelectrodes for biological applications( 2023-01)The development of electrodes for the electrical modulation of cell activity has become a research topic of increasing interest, due to an ever-growing list of applications. These include deep brain stimulation for neurodegenerative disorders such as Parkinson’s and Alzheimer’s disease; implantable bionic devices such as the Cochlear ear implant and bionic eye; and promoting cell growth for wound healing or tissue engineering. Electrical stimulation is conventionally achieved by delivering current from an external power source through metallic electrodes to the target biological tissue. However, these types of electrodes have drawbacks. For instance, they typically require cumbersome and invasive wiring, while the stimulating spatial resolution is limited by current spreading. One attractive alternative technique is light-based stimulation, which offers wireless and scalable targeting of cells with superior spatial precision. In particular, light-sensitive electrode materials can be used to transduce light into an electrical stimulus to excite nearby cells. Nitrogen-doped ultrananocrystalline diamond (N-UNCD) is a promising material for this technique, possessing a highly attractive combination of properties including high chemical inertness, durability, conductivity, and biocompatibility. In addition, it has also been shown to produce a photoresponse using near-infrared light, which offers greater optical penetration in biological tissue than shorter wavelengths. In this thesis, we optimise and characterise N-UNCD electrodes for light-based cell stimulation, and then demonstrate their use in the modulation of stem cell cultures. To achieve this, we tested the effect of different chemical surface treatments on the stimulating performance of the N-UNCD electrodes, measured by the capacitance and photoresponse. Of these treatment methods, we found that annealing N-UNCD in oxygen ambient produced dramatically improved properties, with the capacitance and photoresponse reaching values of 28.5 +/- 0.3 mF cm-2 and 3.75 +/- 0.05 uA W-1, respectively. This translates to an enhancement of up to 17 times compared with the previously used oxygen plasma treatment. We then investigated the reason for this enhancement, finding that it was due to a combination of factors. These included conductive grain boundary etching, and the chemical functional groups on the electrode surface changing the alignment of band-edge positions with respect to energy levels of redox species in solution. These properties were incorporated into a compact computational model which provides an overarching description of the photoelectrochemical data from the N-UNCD electrodes, from experiments including cyclic voltammetry, electrochemical impedance spectroscopy, and photocurrent measurements. Following this, we conducted a theoretical evaluation of the stimulating efficacy of the optimised electrodes, concluding that the photocurrent dynamics are best suited for long-term photomodulation applications rather than the short-term high intensity stimulation of neurons. Therefore, we decided to test the electrodes for long-term light-based stimulation of human mesenchymal stem cells (hMSCs). We found that the N-UNCD material itself promotes the process of osteogenesis (bone cell development) in the stem cells, which may be due to its extremely hard and nanotextured surface. Moreover, initial short term (6 hours) pulsed illumination of hMSCs cultured on the N-UNCD electrodes produced significant long term increases in cell proliferation and differentiation over 21 days. These results may assist in the development of stem cell techniques for tissue engineering, including for orthopaedic implants and bone healing therapies.