Bioimpedance measurement of the skin - contact impedance

Discussion in 'The Projects Forum' started by Col, May 15, 2015.

  1. Col

    Thread Starter Member

    Apr 19, 2012
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    Hi,

    I am trying to develop a system for bioimpedance measurement of the skin. I will sweep from 1kHz to 1Mhz, but use 100kHz for most of my tests. I drive quite a high voltage 3-4V differentially (6-8V) at my electrodes and measure the resulting current. I realize two terminal measurements have no chance so I am no focused on 4 terminal measurements for measuring voltage.

    I am finding the the electrode interface impedance has an overwhelming effect on the measurement and can be many orders of magnitude higher than the actual akin impedance (10s-100s KOhm vs 100s Ohm). Even worse is the inconsistency of the contact impedance depending on surface smoothness, pressure, sweat etc. At the moment I am using simple planar electrodes on PCB (no soldermask so the metal protrudes slightly). For example the thumb, with good pressure gives me a good contact and decent signal but even the forearm gives almost nothing.

    In simulation I have developed a number of 4 terminal solutions, which appear to have a good shot at compensating the contact impedance. I use two guard electrodes where a central electrode drives the current and a concentric circle is used in one case to measure voltage with an instrumentation amplifier, and in another measures current between the drive electrode and itself to determine electrode contact impedance. I have now fabricated them and playing around with a them but I am having doubts if a stable and reliable measurement can be made with any of them. Even I cannot seem to compare them since there are so many variable factors around electrode contacts.

    Any electrode or circuit suggestions for a good bioimpedance measurement of the skin in the face of huge and variable electrode contact impedance? Is there any skin bioimpedance model that I could use in the lab to test the circuits under equal conditions? I have been checking the literature, but not found.

    I realize there are already threads on bioimpedance but they seem to elude to this contact problem, that seems to be the biggest challenge.

    Thanks in advance!
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Interesting. I'm curious as to how you managed to distinguish between the two impedances? I would have thought the interface impedance was inextricably in series with the skin impedance?
     
  3. pwdixon

    Member

    Oct 11, 2012
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    Is this where electrode gel comes into the equation?
     
  4. Col

    Thread Starter Member

    Apr 19, 2012
    44
    2
    I separate them only in orders of magnitude, from the literature and from a simple experiment where I use needle electrodes and push them into my skin to the point of mild pain. I see the resulting impedance is 100s Ohm, conversely using planar electrodes the pressure causes a variation of the measurement between about 1-5kOhm for heavy pressure and 100kOhm + for light pressure to of course no contact at all

    Gel electrodes pose two problems. If there is a gel conduction path between the injection electrode and measurement electrode, well forget about the measurement. Also gel connections are still effected by sweat etc and they will deteriorate over time
     
  5. alfacliff

    Well-Known Member

    Dec 13, 2013
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    I am curious how you get impedance readings, not just resistance readings? where is the iductance? where is the capacitance? sounds like you are just getting resistance readings dependant on pressure, contact area, piercing the skin, and such. why not try one of your electrodes being a very large area? that should cut down on some of the variation.
     
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    If you ever manage to get a reliable skin resistance/impedance measurement, what use does the information have?
     
  7. alfacliff

    Well-Known Member

    Dec 13, 2013
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    according to wikki, it's a measurement of body fat. (maybe).
     
  8. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Considering the problems the OP is having in getting a skin resistance measurement, I don't think I'd trust the average medical practitioner to obtain an accurate body fat measurement by this method :).
     
  9. alfacliff

    Well-Known Member

    Dec 13, 2013
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    they usually dont, what you eat, how well your hydrated, exercize and a lot of ther things affect the readings. some so much, they are included in lie detectors.
     
  10. wayneh

    Expert

    Sep 9, 2010
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    Commercial devices use this method - it works fine. Conductive gel is not used and the extent of contact with the "pads" has little impact. The fancier ones use separate signals to differentiat fat and moisture, so they can be reported separately and the impact of moisture on the fat reading can be taken into account.
     
  11. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    The EKG people worked through all of this. Peel-and-stick pads that started out the size of a business card are now down to the size of a dime. But they won't work consistently on fingers because fingers have fingerprints.

    Also, 8 V at 1 MHz smells like an RF burn.

    ak
     
  12. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Don't the commercial devices pass a known small (=safe) current through the skin sample using one pair of probes and measure the voltage developed across the sample using a second pair? The known current implies a variable voltage source in a current-controlled servo loop.
     
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