Moist steel studs on a shielded flex strip: where do motion artifacts come from?

PavelKamenikPavelKamenik czech republic
in Electrodes

I'm building an open-source EMG suit: 16 boards with one ADS1299 each, one bias drive for
the whole body. The electrodes are 316L steel press studs, 6-8 mm, each in a ring that keeps
the skin's own moisture under it (no gel). The studs are pressed through gold (ENIG) pads of
a two-layer flex strip (polyimide on the trunk, stretchable copper-on-TPU on the limbs); the
second layer is ground as a shield, P and N run side by side under it, and the strip runs at
most a few tens of centimetres to its board. The garment is cotton with a static-dissipative
layer.

My worry is motion artifacts: changing capacitance, changing loop area, and triboelectric
charge as the strip and the fabric move.

  1. In your experience, did motion artifacts come mostly from the leads or from the skin
    under the electrode?
  2. Shield grounded vs driven at the body's common-mode voltage: did driving it help?
  3. Passive leads vs an op-amp follower at each electrode: worth it for EMG?
  4. Steel electrodes without gel: what offsets and drift have you seen?

The design: https://github.com/Project-NIC/NIC-Crazy_Ivan/blob/main/rubaska/CONSTRUCTION.md
Thanks for any experience.

Comments

  • bcidecodedbcidecoded UK

    Your construction notes already plan a resistor-dummy test. I'd run that with both matched and deliberately unequal P/N source impedances, keeping the electrode ends fixed while only the strip moves. Then repeat the movement with the strip held still and only the contact disturbed. That could help separate lead-motion sensitivity from contact-motion sensitivity before comparing the complete suit. I haven't measured this hardware.

  • wjcroftwjcroft Mount Shasta, CA
    edited November 5

    See this related post and pdf paper link. And the graph therein.

    https://openbci.com/forum/index.php?p=/discussion/2889/best-types-of-metal-for-electrodes-resolved

    Caption
    Fig. 2. Current-induced polarization of electrodes, shown as apparent resistance against time. Typical responses of single tin, gold, silver, sintered AgjAgCl and disposable AgjAgCl (marked with an asterisk) electrodes to constant current pulses of G5 nA. Subsequent current pulses were of alternating polarity, and six voltage responses were divided by the injected current and averaged. The responses of platinum and stainless steel electrodes went almost immediately out of scale because of rapid polarization (shown schematically with the dashed line). The lower panel shows recordings with higher amplification made with the sintered AgjAgCl and three disposable AgjAgCl electrode models, as indicated by the arrow.

    The paper focuses on an unusual type of EEG used in slow cortical potentials and infra-low frequency work. However electrode metals and electrolytes have been researched for decades and silver chloride is the 'standard'.

    More related Forum posts here:

    https://www.google.com/search?q=electrode+metals+site:openbci.com

    William

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