Browsing by Author "Maxim, Vanessa"

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    In Vivo Feasibility of Epiretinal Stimulation Using Ultrananocrystalline Diamond 1 Electrodes
    (IOP Publishing, 2020-07) Shivdasani, Mohit; Evans, Mihailo; Burns, Owen; Yeoh, Jonathon; Allen, Penelope; Nayagam, David; Villalobos, Joel; Abbott, Carla; Luu, Chi; Opie, Nicholas; Sabu, Anu; Saunders, Alexia; McPhedran, Michelle; Cardamone, Lisa; McGowan, Ceara; Maxim, Vanessa; Williams, Richard; Fox, Kate; Cicione, Rosemary; Garrett, David; Ahnood, Arman; Ganesan, Kumaravelu; Meffin, Hamish; Burkitt, Anthony; Prawer, Steven; Williams, Chris; Shepherd, Robert
    PURPOSE: Due to their increased proximity to retinal ganglion cells (RGCs), epiretinal visual prostheses present the opportunity for eliciting phosphenes with low thresholds through direct RGC activation. This study characterised the in vivo performance of a novel prototype monolithic epiretinal prosthesis, containing Nitrogen incorporated ultrananocrystalline (N-UNCD) diamond electrodes. METHODS: A prototype implant containing up to twenty-five 120×120 µm N-UNCD electrodes was implanted into 16 anaesthetised cats and attached to the retina either using a single tack or via magnetic coupling with a suprachoroidally placed magnet. Multiunit responses to retinal stimulation using charge-balanced biphasic current pulses were recorded acutely in the visual cortex using a multichannel planar array. Several stimulus parameters were varied including; the stimulating electrode, stimulus polarity, phase duration, return configuration and the number of electrodes stimulated simultaneously. RESULTS: The rigid nature of the device and its form factor necessitated complex surgical procedures. Surgeries were considered successful in 10/16 animals and cortical responses to single electrode stimulation obtained in 8 animals. Clinical imaging and histological outcomes showed severe retinal trauma caused by the device in-situ in many instances. Cortical measures were found to significantly depend on the surgical outcomes of individual experiments, phase duration, return configuration and the number of electrodes stimulated simultaneously, but not stimulus polarity. Cortical thresholds were also found to increase over time within an experiment. CONCLUSIONS: The study successfully demonstrated that an epiretinal prosthesis containing diamond electrodes could produce cortical activity with high precision, albeit only in a small number of cases. Both surgical approaches were highly challenging in terms of reliable and consistent attachment to and stabilisation against the retina, and often resulted in severe retinal trauma. There are key challenges (device form factor and attachment technique) to be resolved for such a device to progress towards clinical application, as current surgical techniques are unable to address these issues.
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    Ring and peg electrodes for minimally-Invasive and long-term sub-scalp EEG recordings
    (Elsevier, Ltd., 2017-06) Benovitski, Yuri; Lai, Alan; McGowan, Ceara; Burns, Owen; Maxim, Vanessa; Nayagam, David; Millard, Rodney; Rathbone, Graeme; le Chevoir, M.A.; Williams, R.A.; Grayden, David; May, C.N.; Murphy, M.; D'Souza, Wendyl; Cook, Mark; Williams, Chris
    OBJECTIVE: Minimally-invasive approaches are needed for long-term reliable Electroencephalography (EEG) recordings to assist with epilepsy diagnosis, investigation and more naturalistic monitoring. This study compared three methods for long-term implantation of sub-scalp EEG electrodes. METHODS: Three types of electrodes (disk, ring, and peg) were fabricated from biocompatible materials and implanted under the scalp in five ambulatory ewes for 3months. Disk electrodes were inserted into sub-pericranial pockets. Ring electrodes were tunneled under the scalp. Peg electrodes were inserted into the skull, close to the dura. EEG was continuously monitored wirelessly. High resolution CT imaging, histopathology, and impedance measurements were used to assess the status of the electrodes at the end of the study. RESULTS: EEG amplitude was larger in the peg compared with the disk and ring electrodes (p<0.05). Similarly, chewing artifacts were lower in the peg electrodes (p<0.05). Electrode impedance increased after long-term implantation particularly for those within the bone (p<0.01). Micro-CT scans indicated that all electrodes stayed within the sub-scalp layers. All pegs remained within the burr holes as implanted with no evidence of extrusion. Eight of 10 disks partially eroded into the bone by 1.0mm from the surface of the skull. The ring arrays remained within the sub-scalp layers close to implantation site. Histology revealed that the electrodes were encapsulated in a thin fibrous tissue adjacent to the pericranium. Overlying this was a loose connective layer and scalp. Erosion into the bone occurred under the rim of the sub-pericranial disk electrodes. CONCLUSIONS: The results indicate that the peg electrodes provided high quality EEG, mechanical stability, and lower chewing artifact. Whereas, ring electrode arrays tunneled under the scalp enable minimal surgical techniques to be used for implantation and removal.