Medical Bionics - Theses
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ItemSafety of a suprachoroidal retinal prosthesis( 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 (ClinicalTrials.gov, NCT01603576). Furthermore, this thesis has important implications on safe suprachoroidal retinal prosthesis design and makes significant contributions towards our understanding of stimulation safety.