Capacitive Electrode for Biomedical Use

At present, metal electrodes are employed as biomedical sensing electrodes, but these pose many problems, such as noise voltages and polarization potentials. An insulator also can be used as an electrode so that biomedical signals can be sensed by its capacitive coupling with the skin. In this paper, barium-titanate ceramics are used for insulator materials, because the electrodes can be made thick and mechanically strong by virtue of their large dielectric constant. The noise voltage characteristics of this barium-titanate ceramics electrode were examined specifically and it was found that this electrode was noise-free from the beginning of installation. This result stems from the fact that this capacitive type electrode does not employ the conduction mechanism caused arising from the electrochemical reaction, the instability of which is the noise source of the metal electrode. These capacitive electrodes are applied to EEG and ECG as electrodes. To decrease electrical artifacts, this electrode has been made active type that has FET impedance converter incorporated directly within the electrode. The ECG electrode does not require an electrolytic paste because of its high-input impedance. Pasteless operation is suitable for long term application such as patient monitoring in an intensive care unit. When this electrode is mechanically stressed, artifact voltage is generated. This artifact voltage is due to the piezoelectric effect of the barium-titanate ceramics. For this reason, the large mechanical stress such as tapping the electrode with finger should be avoided in the use of this electrode. However, it will be possible to decrease this voltage, if the dielectric material that has no piezoelectric effect is used. Moreover, this barium-titanate ceramics electrode protects the patient from electrical shock caused by dc current.