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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 play a 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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