Graeme Clark Collection
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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.