Theoretical limiting transfer function for electrodes in biological tissue

Electrophysiological recordings are limited by the filtering at the electrode-tissue (or electrode-solution tissue) interface. From the diffusion equation a series impedance inversely dependent on the square root of frequency, will govern the transfer function, implying a high-pass filter with 5 dB per decade rolloff. In perfused living rat heart, gold (Au), platinum (Pt) and Ag/AgCl electrodes were studied potentiostatically with 10 mV RMS sinusoidal excitation. All electrode types showed a high pass filter transfer function. In the physiologic frequency range, attenuation was characterized by rolloffs of 8.82±0.09 dB/dec (Au), 8.12±0.53 dB/dec (Pt) and 6.25±0.88 dB/dec (Ag/AgCl), all higher than theoretical. Transfer function behavior is consistent with diffusionally controlled charge carriers and is material independent. This insight identifies avenues for altering bioelectrode transfer function only by using Debye capacitive materials or semiconductor heterostructures