Pasteless Biopotential Electrodes

Op-amp voltage followers are often used to buffer signals detected from biopotential sources with intrinsically high input impedance. One such application is detecting biopotential signals through capacitive bioelectrodes. One area in which these electrodes are particularly useful is in the measurement and analysis of biopotentials in humans subjected to conditions similar to those existing during flight. Knowledge regarding physiological reactions to flight maneuvers has resulted in the development of devices capable of predicting, detecting, and preventing certain conditions that might endanger the lives of crew members. For example, the detection of gravitationally induced loss of consciousness (loss of consciousness caused by extreme ^-forces during sharp high-speed flight maneuvers in war planes) may save many pilots and their aircraft by allowing an onboard computer to take over the controls while the aviator regains consciousness [Whinnery et al., 1987]. Gz+-induced loss of consciousness (GLOC) detection is achieved through the analysis of various biosignals, the most important of which is the electroencephalogram (EEG).

Another new application is the use of the electrocardiography (ECG) signal to synchronize the inflation and deflation of pressure suits adaptively to gain an increase in the level of gravitational accelerations that an airman is capable of tolerating. Additional applications, such as the use of the processed electromyography (EMG) signal as a measure of muscle fatigue and pain as well as an analysis of eye blinks and eyeball movement through the detection of biopotentials around the eye as a measure of pilot alertness, constitute the promise of added safety in air operations.

One problem in making these techniques practical is that most electrodes used for the detection of bioelectric signals require skin preparation to decrease the electrical impedance of the skin-electrode interface. This preparation often involves shaving, scrubbing the skin, and applying an electrolyte paste: actions unacceptable as part of routine preflight procedures. In addition, the electrical interface characteristics deteriorate during long-term use of these electrodes as a result of skin reactions and electrolyte drying. Dry or pasteless electrodes can be used to get around the constraints of electrolyte-interface electrodes. Pasteless electrodes incorporate a bare or dielectric-coated metal plate, in direct contact with the skin, to form a very high impedance interface. By using an integral high-input-impedance amplifier, it is possible to record a signal through the capacitive or resistive interface.

Figure 1.10 presents the constitutive elements of a capacitive pasteless bioelectrode. In it, a highly dielectric material is used to form a capacitive interface between the skin and a conductive plate electrode. Ideally, this dielectric layer has infinite leakage resistance, but in reality this resistance is finite and decreases as the dielectric deteriorates. Signals presented to the buffer stage result from capacitive coupling of biopotentials to the network formed by series resistor R1 and the input impedance Zin of the buffer amplifier. In addition, circuitry that is often used to protect the buffer stage from ESD further attenuates available signals. Shielding is usually provided in the enclosure of a bioelectrode assembly to protect it from interfering noise. The signal at the output of the buffer amplifier has low impedance and can be relayed to remotely placed processing apparatus without attenuation. External power must be supplied for operation of the active buffer circuitry.

A dielectric substance is used in capacitive biopotential electrodes to form a capacitor between the skin and the recording surface. Thin layers of aluminum anodization, pyre varnish, silicon dioxide, and other dielectrics have been used in these electrodes. For example, 17.5-^m (0.7-mil) film is easily prepared by anodic treatment, resulting in electrode plates that have a dc resistance greater than 1 GO and a capacitance of 5000 pF at

Figure 1.10 Block diagram of a typical capacitive active bioelectrode. A highly dielectric material is used to form a capacitive interface between the skin and a conductive plate electrode. Signals presented to the buffer stage result from capacitive coupling of biopotentials to the network formed by series resistor R1 and the input impedance Zin of the buffer amplifier. (Reprinted from Prutchi and Sagi-Dolev [1993], with permission from the Aerospace Medical Association.)

Figure 1.10 Block diagram of a typical capacitive active bioelectrode. A highly dielectric material is used to form a capacitive interface between the skin and a conductive plate electrode. Signals presented to the buffer stage result from capacitive coupling of biopotentials to the network formed by series resistor R1 and the input impedance Zin of the buffer amplifier. (Reprinted from Prutchi and Sagi-Dolev [1993], with permission from the Aerospace Medical Association.)

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