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Skin electrode impedance?

Discussion in 'Electronic Design' started by Joerg, May 12, 2013.

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  1. Joerg

    Joerg Guest

    Folks, does anyone know what the impedance (resistance) is for large
    skin electrodes? Either per square inch or for typical apps like these
    6" by 4" for lumbosacral stimulation:

    http://www.amazon.com/Uni-Patch-Reu...d=1368376409&sr=8-62&keywords=patch+electrode

    I assume each secion is 3" by 4". On my own skin using aluminum foil I
    see about 400ohms at a few kilohertz, so if paralleled I should get
    200ohms. I need to get into the <100ohms range.
     
  2. Joerg

    Joerg Guest

    It will be for an animal trial, among other parameters we will be
    measuring the resistive path from electrodes inside cardiac vessel to
    the skin because the technology we are developing needs this path to be
    low-Z.

    Those we have, and they are definitely too small.

    It's not that bad, but the largest commercial electrodes I can find that
    would be available for overnight shipment are 6" by 4". Worst case we'll
    have to parallel.

    All AC >2kHz, we do not need DC conductivity.
     
  3. Joerg

    Joerg Guest

    No kidding, last week I did use a bucket of saline (9mg/liter of salt)
    here in the lab for tests.

    Well, I just ordered a stack of 6" by 4" lumbosacral stimulation
    electrodes for overnight delivery. Knocking on wood they'll get here in
    time.
     
  4. Robert Macy

    Robert Macy Guest

    Although the model does NOT include any piezo effects which can play
    haovc, I got a lot of information from this site:
    <http://www.biomedical-engineering-online.com/content/3/1/25>
    in Figure 7, contact impedance drops as a function of
    Z < 200 near 180Hz and Z < 100 near 400Hz keeps dropping as you go on
    up in freq
    somewhere I have a VERY accurate PSpice model for EKG contacts which
    does include the piezo generator, but may have lost it in the multiple
    head crashes. If you need more than this site's info, let me know.
     
  5. Joerg

    Joerg Guest


    I believe they meant kOhm. This application is in a way different league
    where we need <100ohms if possible. Also, the electrodes will have gel.
    I have ordered some of the ones that are marketed for lumbosacral
    stimulation. Biggest ones I could find for overnight shipping.
    Unfortunately they come UPS and they list one day later as guaranteed
    arrival than the shipper. That would be too late :-(

    So I might need a McGyver option here.


    We can't use EKG electrodes at all, they are only good for amplifier
    pickup. What we need is stimulator electrodes. I have seen some listend
    as low as 8 ohms but they didn't have a fast enough shipment option.
     
  6. Robert Macy

    Robert Macy Guest

    I only transferred what I was given. BUT I know my epithelial is over
    200k, which means if you can break through that, you've got really
    pretty low impedance. As I remember, something like a bag of salt
    water, what is that? 100-200 S/m>? depending on how drunk you are?

    Aren't strong magnetic fields a form of stimulus without the need for
    contact?

    If I were you and needed conductive contacts NOW, I'd use copper tape
    - large areas. And if that isn't good enough, salt water soak and an
    ACE bandage over the whole thing.
     
  7. Joerg

    Joerg Guest

    I've always looked for a reason to have a couple of cold ones on the job :)

    We need conductive contact that can function from roughly 1.5kHz to
    10kHz. The changes in the AC current are demodulated, it's about sensing
    inside the body.


    We've got a solution now, this one:

    http://solutions.3m.com.sg/wps/portal/3M/en_SG/SG-IP/Main/solutions/pa/pads/

    If I use both sides in parallel I should be there. On my own body this
    size (mimicked with aluminum foil) has worked quite well and got me way
    under 100ohms.
     
  8. If you are using a two-terminal system then I can understand why you
    need such a low impedance - you want to swamp impedance variations of
    the external electrode so that you only see things close to the
    internal electrode.

    However, you are likely to get much better results with two external
    electrodes, one delivering the stimulus current and the other as a
    reference point for the voltage measurement.

    Al is not a good candidate for stable measurements at low frequencies
    as the surface oxide layer properties will vary according to past
    usage. Traditionally, Ag coated with AgCl provides a chemically
    reversible electrode with stable properties.

    As I'm sure you know, electrode impedances are strongly dependent on
    current density, as is the conductivity of the skin.

    John
     
  9. Joerg

    Joerg Guest

    Like half of a Kelvin-method, yes, that would improve things. For now
    it's ok though, all we want to know is changes with the heart beat.
    Since I am measuring current a slight variation in electrode impedance
    would be ok. The system can also compensate for that (it does not simply
    measure amplitude, it watches resonance effects) but there comes a limit
    where we can't get enough current in there any longer.

    I believe the pad from 3M is coated.

    Yes, and some of that doesn't work in our favor. But we can go to
    several kHz so the capacitance of the epidermis helps.
     
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