Bioelectrodes

INTRODUCTION
Biomedical electrodes are used in various forms in a wide range of biomedical applications, including: 1. The detection of bioelectric events such as the electrocardiogram (ECG).
2. The application of therapeutic impulses to the body [e.g., cardiac pacing and defibrillation and transcutaneous electrical nerve stimulation (TENS)].
3. The application of electrical potentials in order to facilitate the transdermal delivery of ionized molecules for local and systemic therapeutic effect (iontophoresis).
4. The alternating current (ac) impedance characterization of body tissues.
Good electrode design is not as simple and straightforward a matter as is often assumed, and all electrode designs are not equal in performance (1). One must, therefore, not simply choose an electrode with as conductive a metal plate as possible, which unfortunately, was and appears to still be the case in many designs. Probably due to this mistaken view, it would appear that the associated electronic systems are often first developed and the electrode design is left to the end, almost as an afterthought. If the clinician is to properly diagnose the patient's cardiac problem, for example, it is imperative that the measured biosignal is clear, undistorted, and artefict-free. Unfortunately, monitoring bioelectrodes, if they are not chosen correctly, give rise to significant problems that make biosignal analysis difficult, if not impossible. Similarly, stimulation electrodes must be well-chosen if they are to optimally supply the therapeutic waveforms without causing trauma to the patient.
Current or charge is carried by ions inside the patient's body and by electrons in the electronic device itself and in its leads. The ''charge-transfer'' mechanism between current/ charge carriers takes place at the electrode-patient interface and is of major importance in the design of an optimal electrode. Both the electrode-electrolyte interface and the skin under the electrode (collectively known as the contact) give rise to potentials and impedances that can distort the measured biosignal or adversely affect the electrotherapeutic procedure.
Implanted electrodes are generally made from inert or noble materials that do not react with surrounding tissues. Unfortunately, as a consequence, they tend to give rise to large interface impedances and unstable potentials. Implanted biosignal monitoring electrodes, in particular, require stable potentials and low interface impedances to minimize biosignal distortion and artifact problems. External biosignal-monitoring electrodes can generally use high electrical performance nonnoble materials such as silver-silver chloride without fear of biocompatibility problems (2). They do, however, have to address the additional and very significant problem of the skin with its sizeable impedance and unstable potential. Along with the desired biosignal, one amplifies the difference between the two contact potentials. If the contact potentials were identical (highly improbable), they would cancel each other out due to the use of a differential amplifier. If the potential mismatch were very large (several hundred mV), the amplifier would not be able to cope and would saturate. If the mismatch in contact potentials is small and stable, this mismatch will be amplified along with the biosignal, and the biosignal will appear shifted up or down on the oscilloscope screen or printout paper, which would generally not be a major problem as the additional voltage offset can be easily removed. What is a significant problem, however, is when the contact potentials fluctuate with time. Their mismatch, therefore, varies and the baseline of the biosignal is no longer constant, which leads to the problem termed baseline wonder or baseline drift, which makes analysis of some of the key features of the biosignal difficult. Filtering out of the drift is often not an option, as the filtering often also removes key components of the desired biosignal.
Large mismatched contact impedances can cause signal attenuation, filtering, distortion, and interference in biosignal monitoring. If contact impedances are significant compared with the input impedance of the amplifier, they can give rise to signal attenuation as a result of the voltage divider effect. Attenuation of the signal is not a major problem, after all, the amplifier is going to be used to amplify the signal by a factor of around 1000 (in the case of an ECG). A significant problem develops, however, because the contact impedance varies with frequency.
The frequency-dependence of the contact impedance is a consequence of the presence of parallel capacitances at the electrode-electrolyte interface or at the skin under the electrode. At very high frequencies, the contact impedances are very small and, therefore, no attenuation of the high frequency parts of the biosignal exists. At low frequencies, the contact impedances can be very large and, hence, significant attenuation of low frequency components of the biosignal can exist. The overall signal is not only attenuated, it is also distorted with its low frequency components selectively reduced. The measurement system in effect acts as a high pass filter and the signal is differentiated. In the case of the ECG, the P, S, and T waves are deformed, leading in particular to a modification of the S-T segment. The S-T segment is of vital importance to the electrocardiologist, hence the importance of avoiding such biosignal distortions.

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