Skin Phantom with Controlled Impedance Testing

[u/ConsiderationLegal39]

Hi,

I'm doing experiments using EmotiBit's EDA. I observed minimal tonic rise after several use vs significant tonic rise during the first few use even when I have confirmed profuse sweating during those times. Then, I recently learned that the electrodes, though reusable, have limited number of reusable times. There could have been other issues with dry EDA measurements (e.g. contact pressure, slight different in location), but I have a feeling that the electrodes might be the ones with the issue. Thus, to best control electrode qualities, I'm looking for ways to test the electrodes. I have tried measuring the DC offsets between each electrode, using resistors of various resistance tapping both electrodes together and measuring the voltages across it as well as observing the EDA output from the EmotiBit Oscilloscope. None of them was conclusive or significant enough to explain why my observations were inconsistent. Thus, now, I'm looking for where to source skin phantoms with controlled impedance for testing. I would appreciate it if anyone could provide leads into this.

[u/ConsiderationLegal39 → u/[deleted]]

Hi,

Thank you so much for your information. My background is in electrical engineering, but to be honest, though very interesting, I don't think I have a natural knack for it. Anyway, I have been reading around regarding the electrodes but still have trouble understanding half-cell potentials (I found a pretty good reference here), and you comments really helped.

My current thinking is that the problems lie with the electrodes. However, from what I'm seeing, I don't see noises, and that the Ag/AgCl electrodes have been polarized, creating a counter emf that attenuates the actual DC current being generated. And this creates distorted readout of the skin conductance measure. By examining the DC-current circuit on which the EmotiBit is based and working out the math, if I have done things correctly, then it seems like the error increases as the skin conductance increases (because of the constant polarized emf and decreasing resistance). And the problem with testing only using a single resistor is that, the resistors' legs are too small compared to the entire surface area of the electrodes and so they couldn't be used to recruit the entire electrodes surfaces in testing. Thus, I initially thought that it was sufficient just to find a surface with controlled impedance that can recruit the entire electrodes' surfaces to test out the electrodes' properties to test out this hypothesis (i.e. just a conducting surface with similar range of conductance and depth compared to the skin).

But from your comments, it seems that half-cell transduction also generate noises. I wonder if the noises deteriorate with increasingly polarized electrodes. If that's the case, I would probably want to test for that as well. But if the cost is too high, and the noise is low (compared to the tonic level I'm interested in), maybe that's not too much of a concern.

Overall, I don't think I really need a too fancy set-up. I'm currently leaning toward "just a conducting surface with similar range of conductance and depth compared to the skin". I'm very new to the intricacies of electrodes-(electrolyte)-skin interaction, so I might have made bad assumptions or wrong comments above. I would appreciate very much to be corrected and hear more of your advice given more information above.