Electrode gels

Flies are placed in contact with the foil electrodes using a conductive electrode gel. Two rows of gel are applied using a 3 ml syringe (without a needle) taking care to ensure that all gel residue in the central gap is removed. It is necessary to replace the gel after 2-4 pacing sessions because bubbling and gel dry out affect the conductivity and fly contact, respectively. 

Tronstad C, Johnsen GK, Grinmes S, Martinsen 0J. 2010. A study on electrode gels for skin conductance measurements. Physiol Meas 31,1395-1410. 

Yamamoto and Yamamoto (1976) measured skin impedance on the ventral side of the forearm with a two-electrode system and an AC bridge. They used Beckman silver/silver chloride electrodes filled with gel and measured 30 min after the electrodes had been applied. The skin was stripped with cellulose tape 15 times, after which the entire SC was believed to have been removed. Impedance measurements were also performed between each stripping so that the impedance of the removed layers could be calculated. The thickness of the SC was found to be 40 pm, which is more than the common average values found elsewhere in the literature. For example, Therkildsen et al. (1998) found a mean thickness of 13.3 pm (minimum 8 pm/maximum 22 pm) when analyzing 57 samples from nonfriction skin sites on Caucasian volunteers. However, the moisture increase caused by electrode occlusion and electrode gel has most certainly significantly increased the SC thickness. 

For DC measurement, it is important that the two electrodes are reference electrodes. If the electrodes are made of different metals or surfaces, then a large exogenic DC voltage (possibly > 1 V) may be generated. Often the best choice is two silver chloride reference electrodes. They can be coupled to the skin or tissue via electrode gel, as shown in Figure 7.1, or via a salt bridge to reduce the DC offset from liquid junction potentials. An invasive electrode as a neutral electrode is the most stable DC reference with a unipolar skin potential recording system. 

Commercial electrode gels usually use relatively high concentrations of potassium or sodium chlorides at a neutral pH. Since these concentration levels can irritate the skin, there are different types of gels are on the market offering trade-offs of low resistance versus gentleness to the skin. 

Although electrode gels are needed for good skin contact, hydrating electrolytes produce another source of electric potential, known as a skin diffusion potential, in addition to the skin battery. This diffusion potential arises because skin membranes are semipermeable to ions. The magnitude of this skin potential varies as a function of the salt composition in the electrolyte. 

For fluctuating and high pressures, Ihe principle concern is the separation and contamination of the reference electrode. Gel filled electrodes can develop voids. Liquid filled electrodes wUl suck in process fluids when the pressure in the process is greater than in the reference, solid-state or pressurized reference electrodes with an internal or external salt bridge should be used. Ideally, the differential pressure between the process and the salt bridge should be constant to maintain a constant flow of electrolyte. 

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