Empirical study of unipolar and bipolar configurations using high resolution single multi-walled carbon nanotube electrodes for electrophysiological probing of electrically excitable cells

Abstract

Identifying the neurophysiological basis underlying learning and memory in the mammalian central nervous system requires the development of biocompatible, high resolution, low electrode impedance electrophysiological probes; however, physically, electrode impedance will always be finite and, at times, large. Herein, we demonstrate through experiments performed on frog sartorius muscle that single multi-walled carbon nanotube electrode (sMWNT electrode) geometry and placement are two degrees of freedom that can improve biocompatibility of the probe and counteract the detrimental effects of MWNT/electrolyte interface impedance on the stimulation efficiency and signal-to-noise ratio (SNR). We show that high aspect ratio dependent electric field enhancement at the MWNT tip can boost stimulation efficiency. Derivation of the sMWNT electrode's electrical equivalent indicates that, at low stimulus voltage regimes below 1 V, current conduction is mediated by charge fluctuation in the double layer obviating electrolysis of water, which is potentially toxic to pH sensitive biological tissue. Despite the accompanying increase in electrode impedance, a pair of closely spaced sMWNT electrodes in a two probe (bipolar) configuration maintains biocompatibility and enhances stimulation efficiency and SNR compared to the single probe (unipolar) configuration. For stimulus voltages below 1 V, the electrical equivalent verifies that current conduction in the two probe configuration still proceeds via charge fluctuation in the double layer. As an extracellular stimulation electrode, the two sMWNT electrodes comprise a current dipole that concentrates the electric field and the current density in a smaller region of sartorius; consequently, the bipolar configuration can elicit muscle fiber twitching at low voltages that preclude electrolysis of water. When recording field potentials, the bipolar configuration subtracts the potential between two points allowing for the detection of higher signal amplitudes. As a result, SNR is improved. These results indicate that use of the high aspect ratio MWNT in a bipolar configuration can achieve a biocompatible electrode that offers enhanced stimulation efficiency and higher SNR.

Sub-10-nm intracellular bioelectronic probes from nanowire–nanotube heterostructures

“…Several approaches have been taken to achieve these goals (2,11). Sharp microelectrodes, which access the cell interior by direct insertion of metal (1,12) or carbon (13,14) microelectrodes, and more recently chipbased arrays of vertical metal electrodes (11,(15)(16)(17) have been used to record intracellular and intracellularlike signals. Further development of these probes with respect to achieving the above miniaturization goals does, however, face intrinsic limits related to increasing electrode impedance with decreasing size.…”

High spatial resolution single multiwalled carbon nanotube electrode for stimulation, recording, and whole cell voltage clamping of electrically active cells

“…2͑a͒ and 2͑b͔͒. We attribute this advantage to the lower access resistance of the sMWNT electrode ͑R access = 500 k⍀͒ 16 compared to the glass micropipette electrode ͑R access =5Ϯ 2 M⍀, average for five tips͒. Thus, during the 1 ms pulse, the sMWNT electrode requires a lower minimum power level of approximately 86 W to elicit muscle twitching compared to approximately 480 W needed by the glass micropipettes.…”

“…Nanoscale electrodes capable of penetrating the membrane can potentially enable the high resolution probing of spatially confined regions such as the dendritic spine while achieving higher SNR and enhanced stimulation efficiency. Recently, Kawano et al, 13 Kouklin et al, 14 Schrlau et al, 15 and de Asis et al, 16 fabricated sub-100 nm tip diameter probes based on carbon nanostructures. Schrlau et al 15 demonstrated intracellular recording with amorphous carbon nanopipettes; however, extracellular recording, stimulation and voltage clamping of electrically active cells using nanoscale tip electrodes have not yet been reported.…”

“…In a prior investigation, we presented a process flow for fabricating sub-50 nm diameter sMWNT electrodes based on a single MWNT mechanically attached to a nickel coated support structure. 16 Passing dc current through the MWNT/Ni interface creates a mechanically robust, Joule's heat induced weld of the MWNT to the nickel adhesion layer and changes the junction from a Schottky diode to an Ohmic contact with a resistance of 40 k⍀. 17 In addition, we showed that parasitic shunt impedances in the sMWNT electrode's support structure are negligible, that the sMWNT electrode's access resistance is lower compared to glass micropipette electrodes, and that the sMWNT electrode can detect the cell resting membrane potential.…”

“…Recording of a stable Ϫ80 to Ϫ90 mV membrane rest potential ͑V rest ͒ verified penetration of the cell membrane by the MWNT tip. 16,[18][19][20][21][22] Next, the sMWNT electrode was reconnected to the Grass Stimulator delivering variable amplitude, 1 ms voltage pulses at 1 pulse per second. The sMWNT electrode required V min = 1.2Ϯ 0.3 V ͑four electrodes, 20 trials͒ whereas glass micropipettes necessitated V min = 8.8Ϯ 2.3 V ͑four tips, 21 trials, R access =5+2 M⍀͒.…”