Empirical study of unipolar and bipolar configurations using high resolution singlemulti-walled carbon nanotube electrodes for electrophysiological probing of electricallyexcitable cells

Abstract

Identifying the neurophysiological basis underlying learning and memory in the mammaliancentral nervous system requires the development of biocompatible, high resolution, lowelectrode impedance electrophysiological probes; however, physically, electrode impedancewill always be finite and, at times, large. Herein, we demonstrate through experimentsperformed on frog sartorius muscle that single multi-walled carbon nanotubeelectrode (sMWNT electrode) geometry and placement are two degrees of freedomthat can improve biocompatibility of the probe and counteract the detrimentaleffects of MWNT/electrolyte interface impedance on the stimulation efficiency andsignal-to-noise ratio (SNR). We show that high aspect ratio dependent electricfield enhancement at the MWNT tip can boost stimulation efficiency. Derivationof the sMWNT electrode’s electrical equivalent indicates that, at low stimulusvoltage regimes below 1 V, current conduction is mediated by charge fluctuationin the double layer obviating electrolysis of water, which is potentially toxic topH sensitive biological tissue. Despite the accompanying increase in electrodeimpedance, a pair of closely spaced sMWNT electrodes in a two probe (bipolar)configuration maintains biocompatibility and enhances stimulation efficiency and SNRcompared to the single probe (unipolar) configuration. For stimulus voltages below1 V, the electrical equivalent verifies that current conduction in the two probeconfiguration still proceeds via charge fluctuation in the double layer. As an extracellularstimulation electrode, the two sMWNT electrodes comprise a current dipole thatconcentrates the electric field and the current density in a smaller region of sartorius;consequently, the bipolar configuration can elicit muscle fiber twitching at low voltagesthat preclude electrolysis of water. When recording field potentials, the bipolarconfiguration subtracts the potential between two points allowing for the detection ofhigher signal amplitudes. As a result, SNR is improved. These results indicatethat use of the high aspect ratio MWNT in a bipolar configuration can achieve abiocompatible electrode that offers enhanced stimulation efficiency and higher SNR.