1 IV. Electrodes In order to measure biopotentials, we must convert the ionic activity of the excitable cells of interest to electrical signals. In order to measure biopotentials, we must convert the ionic activity of the excitable cells of interest to electrical signals. This conversion process, a transduction of ions to electrons, is accomplished by the use of specially designed electrodes. This conversion process, a transduction of ions to electrons, is accomplished by the use of specially designed electrodes.

2 A. Fundamentals 1. Electrochemistry 1. Electrochemistry A- = anion in electrolyte (in this case, the ion with the greatest affinity for the metal electrode) A- = anion in electrolyte (in this case, the ion with the greatest affinity for the metal electrode) C = metallic atom C = metallic atom e- = electron e- = electron Electrode/Electrolyte Interface

3 2. Half-Cell Potentials 2. Half-Cell Potentials Half-cell potentials (i.e. the difference in the potential of the metal relative to that of the electrolyte) arise because one ion in the electrolyte has a greater affinity for the metal used in the electrode. Half-cell potentials (i.e. the difference in the potential of the metal relative to that of the electrolyte) arise because one ion in the electrolyte has a greater affinity for the metal used in the electrode. The magnitude and polarity of this potential if no current (ionic or electronic) is flowing, is a function of: The magnitude and polarity of this potential if no current (ionic or electronic) is flowing, is a function of: type of metal type of metal surface cleanliness surface cleanliness electrolyte type and concentration electrolyte type and concentration These potentials could be over 1-V in amplitude These potentials could be over 1-V in amplitude

4 3. Polarizable vs. Nonpolarizable 3. Polarizable vs. Nonpolarizable When a dc current passes through the electrode, its half-cell potential is altered Vhc = Vhc|i=0 + Vp where Vp = Vr + Vc + Va Vr = ohmic overvoltage Vc = concentration overvoltage Va = activation energy overvoltage Vp = polarization overvoltage If Vp is roughly 0, then the electrode is called a nonpolarizable electrode. Polarizable electrodes will have a detectable Vp

5 B. Surface Electrodes

6 1. Equivalent Circuit Vhc = half-cell potential Vhc = half-cell potential RB = bulk resistance of "electrolyte RB = bulk resistance of "electrolyte R = leakage resistance across double-layer R = leakage resistance across double-layer C = capacitance across double-layer C = capacitance across double-layer

7 2. Applications Electrocardiography (ECG) measurements Electrocardiography (ECG) measurements Noninvasive electrogastrography (EGG) measurements Noninvasive electrogastrography (EGG) measurements Electroencephalography (EEG) measurements Electroencephalography (EEG) measurements

8 C. Microelectrodes Designed to penetrate the membrane of a cell, or be placed in close proximity to individual neurons Designed to penetrate the membrane of a cell, or be placed in close proximity to individual neurons Tip sizes range from 0.05 to 10 mm Tip sizes range from 0.05 to 10 mm Typically have very high impedances Typically have very high impedances

9 1. Glass Micropipette

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11 2. Metal

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13 3. Microelectronic

14 4. Applications Electromyography (EMG) measurements (metal needle) Electromyography (EMG) measurements (metal needle) Electrocorticography (ECoG) measurements Electrocorticography (ECoG) measurements Intracellular potential measurements Intracellular potential measurements