Electrode impedance is a primary factor in the performance of any electrophysiological recording system.  Electrode impedance describes the electrical characteristics of the complex interface between the metal wire microelectrode and the extracellular recording medium.

Modeling Electrode Impedance

The equivalent electrical circuit of a metal microelectrode immersed in an electrolyte solution is shown in Figure 1. It consists of both resistive and capacitive elements. The resistive portion represents the mobility of charge carriers on each side of the solution/metal interface plus the weak exchange of ions across the so-called "double-layer".

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Figure 1

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The double-layer is created by polarized water molecules at the metal/brain interface. These water molecules form a thin dielectric (Figure 2). Thus, the double-layer is modelled by a capacitor. The exchange of ions across the double layer is weak and the interface is considered polarizable. That is, ions do not physically pass into and out of the electrode. Instead, changes in ionic concentration in the extracellular space attract (or repel) electrons to (or from) the interface.

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Figure 2

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Both the capacitance and the resistance of this interface (Figure 2) are dependent on the size of the surface area of the metal in contact with the solution. In general, more surface area results in lower contact resistance and higher double-layer capacitance.  Recall that higher capacitance values have smaller capacitive reactance at any given frequency.  Thus, the overall effect of increasing surface area is to reduce electrode impedance.

Electrode impedance is related to Johnson (or thermal) noise.  Johnson (or thermal) noise is generated by random movement of electrons in all resistive impedance elements. In general, at any given temperature, Johnson noise is proportional to resistive impedance. If the resistive impedance of the electrode is too high, these random fluctuations will interfere with the electrophysiological recording.   Because electrode impedance is largely determined by surface area of the electrode tip, increasing tip diameter is one way to reduce the Johnson noise inherent to the electrode. Unfortunately, many applications require small tip diameters to obtain sufficient single-unit isolation.  An alternative approach to reducing Johnson noise is to increase the surface area at the electrode tip without increasing tip diameter.  This is the strategy employed by both "bubbling" and electroplating, two methods that reduce electrode impedance without sacrificing the selectivity of the microelectrode.

Measuring Electrode Impedance

The impedance of an electrode is measured by passing a small AC current through the electrode, measuring the voltage drop across a known resistance placed in series with the electrode and using this information to calculate the impedance. In our lab we use a battery-powered sine-wave generator, an oscilloscope and a beaker of saline arranged as shown in Figure 3.  The sine wave generator is set to 100 mV and 100 Hz. The oscilloscope sweep rate and sensitivity is adjusted to display several cycles of the test waveform.  The reference electrode should have a large surface area.

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Figure 3

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Electrode impedance may be calculated directly from the oscilloscope reading using the equation in Figure 4 below.  In our lab, we use a table to make quick determinations of electrode impedance from measured voltages.  In this analysis, R_m is the internal impedance of the oscilloscope (1 megohm in our example), V₀ is the output of the sine-wave generator (100 millivolts in our example) and V_m is the value displayed on the oscilloscope.  The electrode impedance measured will be dependent on the frequency of the signal generator. Electrode impedance will decrease as frequency is increased consistent with the model shown in Figure 1.

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Figure 4

See Als  Table for Computation of Impedance

Practical Considerations

Immerse the electrode to a consistent depth. When a metal microelectrode is immersed in a conductive electrolyte, the wire and the solution form the plates of a capacitor (shown as C_i, above). The insulation coating the electrode wire is the dielectric and the capacitance depends on the depth of immersion. Thus, it is important to immerse the electrode to a consistent depth for each test so as to keep C_i invariant between tests.  I typically immerse the tips to a depth of 2 mm.

Check the voltage output of the signal generator.  Make sure the sine wave is devoid of DC offset (zero mean) by checking it with an oscilloscope or DC voltmeter.

Check the frequency of the sine wave.   Impedance is dependent on the frequency of the signal used to measure it. For impedance measurements to be comparable and consistent, they must be made using the same frequency signal each time.

Procedures to Reduce Electrode Impedance

A small diameter tip is necessary to detect the local ionic changes that are generated extracellularly by an action potential. Wire diameter is chosen based on the cellular properties of the particular brain area being recorded.  The surface area of the tip of the electrode is a strong determinant of the impedance of the electrode.  Both capacitive reactance and contact resistance are reduced by increasing the surface area of the exposed electrode tip. Thus, procedures that increase surface area will result in electrodes with lower impedance.  The impedance reduction procedures described below change the profile of the exposed tip without changing the diameter of the electrode tip. Thus, reducing impedance without sacrificing unit selectivity.

Method 1: the "bubbling" technique. One simple and inelegant method that has worked for us empirically is the byproduct of a procedure known as "bubbling" used to test the continuity of each wire.  This procedure involves passing anodal current through each wire from a DC source, such as a 9 V or 12 V battery, and back through a saline solution.  The current causes bubbles to form in the saline solution at the tip of the wire.  This "bubbling" process confirms the integrity of the connections inside the electrode assembly and also results in a lowering of the impedance measured at the electrode tip.  Impedance is reduced because the strong current that produces the bubbles in solution also causes etching of the electrode tip.  This etching process increases the exposed surface area of the tip without changing the diameter, which determines the recording characteristics of the electrode.  Damage to the insulation may also occur however, so the goal of this method would be to minimize insulation damage, which can be viewed under a stereomicroscope, while etching the tips sufficiently to achieve a useable impedance on each wire.  Too much damage to the insulation will result in a non-selective electrode that is unable to isolate single cells from their neighbors.

Method 2: plating. Another more conventional method of reducing electrode impedance is through electrolytic deposition of an inert metal onto the electrode tip (electroplating).  Electroplating also reduces impedance by increasing the effective surface area without increasing the tip diameter.  Platinum electroplating is accomplished by placing the electrode tips into a solution of platinum chloride and applying a small current such that the platinum in solution is reduced, causing platinum deposition at the tip of the metal electrode. I plate our electrodes using a solution of hydrogen hexachloroplatinate (8% PtCl4 by weight; Sigma Chemicals) with a multi-channel, constant-current plating device available from Eclectic Engineering Studio.

A simple constant-current plating circuit is formed using a large resistance (R) and a DC voltage source (V) as in Figure 5, below. The resistor used in this circuit must be at least ten times larger than the largest anticipated DC resistance of the electrode. The plating current is computed using Ohm's law.

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Figure 5

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In constructing such a circuit, the current (I) should be kept relatively small (1 to 10 microamps), and the return electrode should be either graphite or platinum to prevent contamination of the platinum chloride solution.

Immersion procedure for platinum electrodeposition. Both the plating current and the concentration of the plating solution will affect the immersion time required for sufficient plating. Typically we plate our electrodes at 5 microamps for 4-6 seconds in two separate immersions.  Prior to each immersion, the electrodes are dipped into a 90% EtOH solution to reduce small air bubbles and remove dirt particles that can have a negative impact on electrolytic deposition. Wetting the tips with EtOH and repeating the process several times serves to both reduce and distribute these factors and results in a more uniform impedance across the different wires.  It is important to visually examine the tips of the wires when determining the optimal parameters for time, current, and platinum concentration.  Prepare the electrodes for plating by cutting the tips using fine surgical scissors. Prior to plating, the exposed electrode tip should be “shiny” and the insulation should be intact.  After plating to the desired impedance, the tips will have a rough “matte” appearance and impedance is typically reduced by 50-75%. If the insulation appears ragged or degraded, then the wires have been damaged, indicating that the current, duration or concentration of the plating solution should be reduced.

Variability of results. Finally it is worth noting that the exact parameters change slightly between days and on different electrodes, so we typically practice plating each electrode several times before making a final determination of the correct values.  This approach is possible because extra wire is fed through the guide cannula to be cut off prior to surgery, thus several plating attempts are made before the wires are cut to their final length.  Some variation in the final impedance between wires is normal and likely results from local variables in the plating environment or the mating of each wire through the electrode assembly.

Web Content from:  Schoenbaum, G.  Olfactory Learning and the Neurophysiological Study of Rat Prefrontal Function.  In: CRC Series: Methods and Frontiers in Neuroscience.  Edited by S.A. Simon and M.A.L. Nicolelis, CRC Press, NY, 2000.

This web page coauthored by Kevin B. Austin, Ph.D., Eclectic Engineering Studio, www.EclecticStudio.com