EMG needle electrodes: electrical impedance.

The electrical impedance of samples of stainless-steel monopolar needle electrodes and concentric electrodes from 3 manufacturers were measured in 154mM saline solution using sine-wave excitation at 4 frequencies from 10Hz to 10kHz. Current densities of less than 2.5x10(-5) amp/cm2 were used. Measurements on both needle and fine-wire electrodes were also made in live rats. Linear dimensions of the exposed tips were measured from which the areas of exposure were calculated. Surface areas of the monopolar needles varied between 0.06 and 0.15mm2. Presoaking of the electrodes for 20 minutes in saline solution containing a small concentration of a wetting agent produced a sixfold to twentyfold reduction in impedance. The average impedance magnitudes of the monopolar electrodes ranged from 1.4 megohms (Momega) at 10Hz to 6.6 kolohms (komega at 10kHz. The phase angle of the impedance due to the capacitive component of the needle-electrolyte impedance ranged between -45 and -71 degrees. The reduction in impedance resulting from presoaking and the concomitant reduction in electronic noise would be an advantage in observing lowamplitude potentials.