Medical Bionics - Theses

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    Safety of a suprachoroidal retinal prosthesis
    LEUNG, RONALD ( 2014)
    Light is transformed into neural signals by the retina. Certain conditions, such as retinitis pigmentosa, can cause extensive degeneration of the outer retinal layers, resulting in profound vision impairment. Prosthetic devices have the potential to restore visual percepts in these patients by electrically stimulating the remaining retinal neurons. One such device, the suprachoroidal retinal prosthesis, is placed between the vascular choroid and sclera. It is currently under development and is the focus of this thesis. Safety is an important aspect of medical device design. This thesis focuses on several key aspects of suprachoroidal retinal prosthesis safety. Retinal prostheses must be designed to allow for the ability to safely remove and replace the device in the case of infection, device malfunction, or a device upgrade. This thesis explores the safety and feasibility of explanting or replacing prototype suprachoroidal electrode arrays using clinical and laboratory analysis techniques in a feline model. The results indicate that suprachoroidal electrode arrays can be safely removed or replaced with minimal damage to the retina and surrounding tissues. Furthermore, the device replacement procedures were not detrimental to the retinal response to electrical stimulation. However, careful wound closure was required to minimise post-operative complications. Another key requirement of electrically active neural prostheses is that they should not cause damage that could adversely affect the efficacy of the device. The safe stimulation levels of the platinum macroelectrodes used in the prototype suprachoroidal retinal prosthesis is unknown. In addition to the above, this thesis aims to determine the safe stimulation levels of platinum macroelectrodes using electrochemical methods in vitro and in vivo. Furthermore, methods to safely increase the safe stimulation limit by altering the stimulation waveform or by using nitrogen-doped ultra-nanocrystalline diamond as an electrode material are explored. The results indicate that electrochemically safe stimulation limits were lower than stimulation levels likely to cause histologically observable damage and that altering the stimulus waveform has the potential to increase electrochemically safe stimulation limits. Also, nitrogen-doped ultra-nanocrystalline diamond electrodes have a higher safe stimulation limit than platinum and showed no signs of degradation when stimulated in vitro. The results of this thesis have helped to ensure the safety of patients implanted with prototype suprachoroidal retinal prosthesis (, NCT01603576). Furthermore, this thesis has important implications on safe suprachoroidal retinal prosthesis design and makes significant contributions towards our understanding of stimulation safety.
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    Peripheral nerve stimulation for the treatment of chronic neuropathic pain
    SENN, PHILIPP ( 2014)
    Neuropathic pain is a chronic health condition with a severe impact on the quality of life of affected patients. The condition is often difficult to manage and refractory to traditional pain treatment strategies such as pharmacological management, physiotherapy and psychological therapy. Peripheral nerve stimulation has been proposed as an alternative treatment with numerous successful clinical reports. Nevertheless, the systematic understanding of the underlying mechanism of action is still limited. Efficacy studies in the form of randomised controlled trials have predominantly been conducted for occipital nerve stimulation to treat various headache conditions. Without trials of a wide range of neuropathic conditions, the commercial availability of approved medical devices is limited. The overall objective of this thesis was to advance towards the development of a peripheral nerve stimulation system for a small-scale clinical trial that will be used to gain a deeper understanding of the underlying mechanisms of pain modulation. Design features of electrode arrays and new stimulation strategies were tested in order to facilitate the development of advanced clinical peripheral nerve stimulation systems. The first part of the work consisted of the development of a small, wearable neural stimulator for the use in clinical trials. Chapter 2 presents the design and characterisation of the stimulator. It was shown that safe and efficacious neural activation could be achieved and the system will be suitable for use during a short-term clinical trial of electrode arrays with a percutaneous leadwire system. In the second part, a model electrode setup was used to investigate a bipolar stimulation strategy. Chapter 3 documents an electrophysiological study on the maximisation of the therapeutic window available for stimulation. An electrode screening strategy was developed in order to increase the efficiency of intra- and post-operative testing of stimulation arrays with a large number of electrode combinations. The third part of the work focussed on the development of single-source multipolar stimulation as a novel method to perform current focussing for increased selectivity of the neural activation. Chapter 4 presents the in vitro investigation that showed that a successful reduction of voltages at electrode sites other than the centre electrode was achieved when compared to monopolar stimulation. Furthermore, a significant improvement of the voltage reduction was also found compared to tripolar and common ground stimulation. The promising results from the in vitro tests were followed by an in vivo evaluation as presented in Chapter 5. However, the focussing effects found in vitro did not translate to functional benefits in vivo for the investigated setup. Rather, increased neural activation thresholds were found resulting in potentially higher power requirements for a clinical system. Monopolar stimulation was identified as the favourable mode under the tested conditions. In conclusion, the results of this thesis suggest that a safe and reliable, tailored electrode array in combination with a monopolar stimulation strategy forms a promising system in order to progress towards the overall objective, a short-term clinical trial. This will help to gain a deeper understanding of the underlying mechanism of action of peripheral nerve stimulation for the treatment of chronic neuropathic pain.