Summary: Current distribution under carbon impregnated rubber electrodes

[Guest guest]

Ref: Journal of Medical Enmgineering & Technology; Vol. 30,

#6, Nov/Dec 06, P. 368 - 381 http://www.tandf.co.uk/journals

Current flow for electrical stimulation [TENS] is uneven;

causes high current density at the electrode connection and

up to 1/2 less density at the edges. Shape is not

important in selection [round has same troubles as square

or rectangular], Skin blood flow alters current movement on

the electrode surfaces. Lists 13 common sites / purposes of

use, primarily chiropractic; most important is site location

vs shape or area. 5 brands tested have varying

construction details, notes skin temperature of 10 deg C change

can result in a current change of up to x 50 which causes

skin conductivity changes [increases].

Direct application of Ohm's law is impossible due to bulk

tissue resistivity variations; is only approximate, no guide

given for the error to be expected.

All were tested at from D.C. up to 50 KHz. Carbonized

rubber with salt gel had 82 +/- 19 ohms [total area];

hydrogel [as a group] had over 2 megohms; some with weaved

conductors had over 10 megohms. When using A.C., impedance

went lower, varied directly with A.C. current, still x 10

higher with hydrogel, still uneven distribution, highest

currents at the connection.

Use of carbon impregnated electrodes had a higher applied

voltage to get the same total current and muscle

contraction force. P. 379 has: " ... when stimulation current

is low, such as for TENS or microcurrents, these new

electrodes may be totally ineffective. " ... " When local

heating is applied to skin, the situation is worse. With

non-uniform current distribution, changes in local blood flow

due to heat shift the current away from the non-heated

areas of the skin. "

P. 380 - " If electrode impedance is low, then a small

change in blood flow has little effect on the movement of

current. But if the impedance is high, as in hydrogels,

there is a substantial shift in current away from the skin

by the blood having a lower resistance than surrounding

tissue and increases total current. " " ... blood resistivity

is less than 2 ohms / cm^2. "

END

Conclusions:

Either stay away from them or get the instrumentation

needed to evaluate the brand, type, size on hand.

For beginners - eliminate a trouble source by following

directions in the files.

Joe.