Graeme Clark Collection
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ItemChronic monopolar high rate simulation of the auditory nerve: physiological and histopathological effects(Kugler Publications, 2001)There is clinical interest in the development of high rate speech processing strategies, since there are indications that these might enhance speech perception due to an improved representation of the rapid variations in amplitude of speech. Significant improvement in speech perception using high rate stimulation has been demonstrated in cochlear implant recipients. However, it is important that the long-term safety of high rate stimulation is clearly established prior to its general clinical application. This is especially important, since acute animal studies have shown that high rate stimulation can induce a reduction in the excitability of the auditory nerve. This was also associated with an increase in both threshold and latency of the electrically evoked auditory brainstem response (EABR). However, while a chronic stimulation study indicated that monopolar electrical stimulation of the auditory nerve at rates of 1000 pulses per second (pps)/channel (three channels) had no adverse effects on the spiral ganglion cell density (SGCO),5 there is limited data concerning higher rates. In the present study, we evaluated the electrophysiological and histopathological effects of chronic monopolar electrical stimulation of the auditory nerve using considerably higher stimulus rates than have been used in previous studies.
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ItemA prototype micro-machined thin-film electrode array for cochlear implants( 2001)Development of a micromachined electrode array for cochlear implant application is presented. The device is constructed from a silicon substrate with sputtered platinum electrodes and connection tracks. Electrochemical impedance spectroscopy (EIS) is used to study the properties of the electrode, and to identify potential problems caused by the micromachining process and materials. A variety of insulators are studied and a two-part epoxy is identified as an adequate insulator for operation under harsh electrochemical testing conditions. The semiconducting silicon substrate is found to contribute to the total impedance of the device at high frequencies due to the thin insulating oxide between the substrate and conducting tracks. This is a potential problem for micromachined electrodes operating under high frequencies or using square stimulating pulses. The charge-delivery properties are studied using electrochemical impedance spectroscopy. It is found that platinum sputtered under particular conditions results in excellent surface conditions for optimum charge-delivery.
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ItemPeak-Splitting in the Response of the Leaky Integrate-and-Fire Neuron Model to Low-Frequency Periodic Inputs(Monash University Press, 2001)
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ItemPotential applications of a small and high surface area platinum electrode as an implanted impedance bio-sensor or recording electrode.( 2001)A small Platinum (Pt) electrode (geometric area: -0.43 mm2) was treated in an electrochemical etching process, to produce a highly porous columnar thin layer (-600 nm) on the surface of the electrode. The modified Pt electrode (Pt-p) showed similar electrical properties to a platinum-black electrode but with high mechanical integrity. Previous studies of chronic stimulation had also shown good biocompatibility and surface stability over several months implantation. This paper discusses the potential applications of the modified electrode as an implanted bio-sensor: (1) as a recording electrode compared to an untreated Pt electrode. (2) as a probe in detecting electrical characteristics of living biological material adjacent to the electrode in vivo, which may correlate to inflammation or trauma repair. Results of electrochemical impedance spectroscopy (BIS) revealed much lower electrode interface polarisation impedance, reduced overall electrode impedance, and a largely constant impedance above 100 Hz for the Pt-p electrode compared with untreated Pt electrodes. This provides a platform for recording biological events with low noise interference. Results of A.C. impedance spectroscopy of the high surface area electrode only reflect changes in the surrounding biological environment in the frequency range (1 kHz to 100kHz), interference from electrode polarisation impedance can be neglected. The results imply that the surface-modified electrode is a good candidate for application to implantable biosensors for detecting bio-electric events. The modification procedure and its high surface area concept could have application to a smart MEMS device or microelectrode.
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ItemFactors determining and limiting the impedance behaviour of implanted bio-electrodes.( 2001)Impedance-frequency characteristics of several types of bio-electrodes, platinum (Pt), modified Pt, iridium (Ir), and iridium oxides, are presented in this paper. The study aimed at investigating the effects of bio-electrode array design and biological environments on the impedance behavior. Electrochemical impedance spectra were measured in physiological saline, and additional data were obtained from in vivo animal studies using implanted electrodes. The frequency spectrum can be approximately divided into three regions, in which different factors are dominant. At 1 kHz to 100 kHz or higher, impedance is mainly determined by the electrode geometric area and biological materials adjacent to the electrode. The impedance of a micromachined thin, film connector track could contribute in this region. At the low frequency region of 1 Hz (or lower) to 100 Hz, electrode material, the electrode real surface area and electrode potential playa dominant role in the impedance. There is a mix of these factors in the middle frequency region, 100 Hz to 1 kHz. However, the boundaries of the three regions are not fixed, but rather shift depending on the individual electrode. In the case of microelectrodes, the boundaries move towards high frequencies. Results showed that the effect of material selection and surface modification on impedance was more pronounced in the case of smaller electrodes. or when relatively low frequencies were used. The responses of living tissue to implants resulted in changes in the biological environment near the implanted electrodes and this led to a large increase in impedance at high frequencies. The impedance-frequency characteristic provides a guideline for a bio-electrode array design to meet a particular bio-medical application, and also an evaluation method for bio-electrode arrays.
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ItemAdvances in computational modelling of cochlear implant physiology and perception(IOS Press, 2001)Models of cochlear implant physiology and perception have historically utilized deterministic descriptions of auditory-nerve (AN) responses to electrical stimulation, which ignore stochastic activity present in the response. Physiological models of AN responses have been developed that do incorporate stochastic activity [8][13][14][27][38][39], but the consequences of stochastic activity for the perception of cochlear implant stimulation have not been investigated until recently [3]. Such an investigation is prompted by inaccuracies in predicting cochlear implant perception by deterministic models. For example, studies of single-fiber responses, where only an arbitrary deterministic measure of threshold is recorded, do not accurately predict perceptual threshold versus phase duration (strength-duration) curves for sinusoidal stimulation [24] or for pulsatile stimulation [25][26]. Furthermore, strength-duration curves of cochlear implant users are not well predicted by deterministic Hodgkin Huxley type models [25] [30].However, the complexity of previous stochastic physiological models has made the computation of responses for large numbers of fibers both laborious and time-consuming. Furthermore, the parameters of these models are often not easily matched to the fiber characteristics of the auditory nerve in humans or other mammals. This has prompted us to develop a simpler and more computationally efficient model of electrical stimulation of the auditory nerve [1][2][4] which is capable of direct and rapid prediction of perceptual data[3]
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ItemPhase-contrast radiography: a new x-ray technique for cochlear implant research(Moduzzi Editore, 2000)This study examines the application of a new x-ray modality, phase contrast radiography, in temporal bone (TB) imaging. Preliminary results from TB samples have shown that phase-contrast imaging offers greater contrast for edge-type features and weakly absorbing soft-tissue resulting in improved visualization of anatomic details of inner ear and microelectrode structures. This is potentially valuable in the development of new electrode arrays for cochlear implants.
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ItemIntracochlear damage following insertion of the Nucleus 22 standard electrode array: a post mortem study of 14 implant patients( 2000)The insertion of an intracochlear electrode array may cause trauma to cochlear structures which can result in degeneration of neural elements, jeopardizing the potential benefits of electrical stimulation. Safety studies for the assessment of trauma associated with the Nucleus 22 standard electrode array involved animal experiments as well as insertion studies in post mortem temporal bones. However, there are only few histological studies of temporal bones from deceased cochlear implant patients. A review of our temporal bone collection of implantees originating from a variety of centres has been conducted to evaluate the effects of electrode insertion trauma associated with the Nucleus 22 standard array.