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Skin electrode impedance?

J

Joerg

Jan 1, 1970
0
Joe said:
What is the purpose?

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.

One can buy very good electrodes and conductive jelly for such things,
typically intended for use in Electro CardioGram (ECG) machines.

Those we have, and they are definitely too small.

<http://www.diytrade.com/china/pd/7681402/Multifunction_Suction_ECG_elec
trodes.html>

But for 100 ohms you will need a sheet of stainless steel and a puddle
of electrode gel, or a metal bucket full of salt water.

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.
 
J

Joerg

Jan 1, 1970
0
Joe said:
Joerg said:
Joe Gwinn wrote:
[...]
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.

OK. The bucket of salt water is actually the original method, so that
wasn't completely a joke.

<http://en.wikipedia.org/wiki/Willem_Einthoven>

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.
 
R

Robert Macy

Jan 1, 1970
0
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-Reusable-Electrode-Pigtail-Connector/...

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.

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.
 
J

Joerg

Jan 1, 1970
0
Robert said:
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


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.

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.


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.
 
R

Robert Macy

Jan 1, 1970
0
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.

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.
 
J

Joerg

Jan 1, 1970
0
Robert said:
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?

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

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

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.

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.


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.
 
J

John Walliker

Jan 1, 1970
0
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.

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
 
J

Joerg

Jan 1, 1970
0
John said:
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.

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.

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.

I believe the pad from 3M is coated.

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

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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