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An electrical stimulator device like Geko T-1 provides a 27 mA output current. On the specification for the load impedance range they mention a 200 Ω to 3 kΩ range for 27 mA output. How did they conclude to that for the skin resistance? When I search online I get way higher values, even 100 kΩ. Using Ohm’s law, with an output of 27 mA and 90 V an electrical stimulator can only surpass 3333 Ω. Is it something to do with the use of electrodes?

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  • \$\begingroup\$ 90 volts and 27 mA is 3.33 kohm not ohms. \$\endgroup\$
    – Andy aka
    Jul 24, 2023 at 8:04
  • \$\begingroup\$ Perhaps they measure skin resistance under various conditions - something the medical industry would be interested in since electrical sensors are used for ecg, and higher charges applied with defibrillators. Also consideration would be given to skin resistance for an electric chair... \$\endgroup\$
    – Solar Mike
    Jul 24, 2023 at 8:33

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You've raised an interesting question. The impedance between two spaced-apart electrodes on the body is the sum of the two electrode-to-skin impedances and the impedance along the body path (e.g., arm-to-arm for a Lead I EKG), and does usually run in the 100k ohm range as you've researched.

However, the Geko nerve stimulator is a special case. First, the electrodes are fairly large in area and use hydrogel, both of which help lower the impedance. Second, the conduction distance through the body is extremely short - basically half the width of the calf band, maybe 1-2 cm. Finally, the stimulation is not DC but a short, sharp pulse, letting capacitive effects come into play to lower the impedance.

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I recommend

J. Rosell, J. Colominas, P. Riu, R. Pallas-Areny and J. G. Webster, "Skin impedance from 1 Hz to 1 MHz," in IEEE Transactions on Biomedical Engineering, vol. 35, no. 8, pp. 649-651, Aug. 1988, doi: 10.1109/10.4599.

Skin impedance is fairly complex (no pun intended). It depends on many things. Obviously, electrode area will have a big impact. Skin prep can also have a big impact (electrode prep pads often have some abrasive in them to reduce impedance).

It's also incredibly frequency dependent. At 1 Hz, Webster's group reports (for the electrode size they used) tens of kiloohms to a megaohm. At 100 kHz, they report hundreds of ohms.

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The lumped-element model approximation of skin surface impedance generally looks like this:

schematic

simulate this circuit – Schematic created using CircuitLab

The value of R1+R2+R3 can range from the 100 ohm range to the 100kohm range depending on electrode parameters like surface area and material (dry electrode vs hydrogel, etc). Generally speaking, impedance decreases with both increasing surface area and electrode 'wetness'.

For any electrode, there is a minimum amount of current required before a sensation can be felt. The thresholds for perceptible sensations are function of current density, and therefore change with electrode size; the larger the electrode, the more current is required before a sensation can be felt. Low current density is also generally more comfortable, i.e. a large electrode with more current feels less painful than a small electrode with less current. Very small electrode surface area can damage skin and cause pain at the minimum perceptible sensation level.

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