Buying Dry / Active Electrodes
Could someone recommend some off-the-shelf dry active electrodes for use with OpenBCI? Chip speaks ill of some cheap ones in another thread. Are better ones readily available, even if they cost more?
Comments
This is a nice find however. Cognionics uses a Ag-AgCl "comb" like plastic sensor tip that flexes with pressure. And g.tec also uses a comb, gold in their case. Other dry sensor systems I've run across also use this comb idea. Perhaps this Biopac comb could be made to connect with a tiny PC board we come up with, using ideas from some of the published active sensor circuit papers that are out there. I bet there is some tiny highly integrated sensor chip already out there.
This comb from Biopac is much more comfortable looking than the "bed of nails" photos on the OpenEEG active sensor site. :-)
My hunch is that using this as a passive sensor, with no saline solution or conductive surface liquid / gel / paste interfacing to the skin -- that the impedance would tend to vary somewhat. Based on how much pressure is on the points. ECube is getting a decent signal, but then we'd also ask, how stable is that over the course of a half hour or hour long session? Saline sensors stay moist and conductive for such time periods.
William
That link you provide is to their online shop area. Curious about the company I found this,
http://www.floridaresearchinstruments.com/about/staff/
So they look knowledgeable. I'm going to write them and ask about their experience using this Ag-AgCl comb in a dry fashion. Originally (previous post) I thought these came from Biopac. But since FRI's pricing is more reasonable (esp. the wires), perhaps it is the other way around.
My guess is that the skin impedance would vary quite a bit using this in a dry fashion. Usually Ag-AgCl is always used with a paste, gel, or saline interface. But Cognionics has a similar approach, using active amplification and a flexible sensor. So perhaps with the really high impedance amp we have (1 GOhm), that might help compensate. Still the CMRR with such widely varying impedances might suffer.
William
Once you have a cap with holes in it, it is an easy process to thread a comb sensor through that from the inside, then clip on the electrode wire on the outside surface. Now the comb will self align to your head when you pull on the cap.
Karl also points out these BraiNet 10-20 placement bands (mesh cap)
would be another way to get the equivalent of a cap with hole positions.
http://bio-medical.com/products/brainet-placement-caps.html
[Note the holes on that BraiNet are possibly 9mm or so in diameter, so the comb sensors might squeeze through that hole. Solution would be to place a thin rubber sheet washer, say .75in diameter -- between back side of comb and the cap. Then attach lead.]
While the comb electrodes will work well for hair areas, they would be uncomfortable directly on the forehead. In those positions you would use the other non-comb version.
I have been speaking with a number of people about
Dry Electrodes. I tried to explain some
of the options. Go to our web site www.FRI-FL.com and click on the Manuals
tab. Download EEG Manual. This is a work in progress and it will be
continually updated.
Karl Zurn, Florida Research Instruments
http://www.mdpi.com/journal/sensors
2014 Review paper (covering many current dry sensors)
http://www.mdpi.com/1424-8220/14/7/12847/pdf
Searching that journal for papers on EEG,
http://www.mdpi.com/search?q=eeg&journal=sensors&search=Search
2014 Soft, Comfortable Polymer Dry Electrodes for High Quality ECG and EEG Recording
http://www.mdpi.com/1424-8220/14/12/23758
Abstract: Conventional gel electrodes are widely used for biopotential measurements, despite important drawbacks such as skin irritation, long set-up time and uncomfortable removal. Recently introduced dry electrodes with rigid metal pins overcome most of these problems; however, their rigidity causes discomfort and pain. This paper presents dry electrodes offering high user comfort, since they are fabricated from EPDM rubber containing various additives for optimum conductivity, flexibility and ease of fabrication. The electrode impedance is measured on phantoms and human skin. After optimization of the polymer composition, the skin-electrode impedance is only ~10 times larger than that of gel electrodes. Therefore, these electrodes are directly capable of recording strong biopotential signals such as ECG while for low-amplitude signals such as EEG, the electrodes need to be coupled with an active circuit. EEG recordings using active polymer electrodes connected to a clinical EEG system show very promising results: alpha waves can be clearly observed when subjects close their eyes, and correlation and coherence analyses reveal high similarity between dry and gel electrode signals. Moreover, all subjects reported that our polymer electrodes did not cause discomfort. Hence, the polymer-based dry electrodes are promising alternatives to either rigid dry electrodes or conventional gel electrodes.
Image of the EPDM rubber combs from that paper:
See section 3.3.1 of that paper for how they put their tiny active board on the back of the EPDM combs; including schematic and more photos. Active board is a modified OpenEEG circuit!
All the authors on that paper are from imec.be, here's an article on their site. Wonder if we could buy some? Image below shows the EPDM comb mounted on top of the active board.
http://www2.imec.be/be_en/press/imec-news/imec-polymer-dry-electrodes.html
Here's a high rez image of their circuit board you can zoom into. Open in another tab in your browser, then zoom in.
http://www.mdpi.com/sensors/sensors-14-23758/article_deploy/html/images/sensors-14-23758f4.png
The TI chip on that tiny board is, http://www.ti.com/product/tlc2272
However... I also realized there are conductive filaments made for 3D printers. These are ABS or PETG based. Some using various forms of graphite or carbon black, another using carbon nanotubes.
http://www.makergeeks.com/coabs3dfi10.html
About 10K ohms/cm
http://3dprint.com/3701/3dxtech-carbon-nanotube-3d-printer-filament/
http://www.3dxtech.com/carbon-nanotube-filament/
http://www.3dxtech.com/content/3DXNano_ESD_PETG_Data_Sheet_v1.pdf
http://www.3dxtech.com/content/3DXNano_ESD_ABS_Data Sheet_v1.pdf
From 10M to 1G ohms per ?
These are of course plastic, not rubber. But with the 3D printing process you could make the comb teeth "fan out" in a way radially that would distribute the forces on the skin more gently than comb teeth that are strictly perpendicular, like the IMEC combs. In other words, the fanned out teeth would be more similar to the Cognionics combs. Since Joel @biomurph and Conor @conor_obci have the 3D printers right there, could be a way to experiment with comb sensors of varying form factors, possibly mated to active boards.
While looking around the Cognionics site, I found a couple new pages I had not seen before,
http://www.cognionics.com/index.php/products/hd-eeg-systems/quick-20-dry-headset
and
http://www.cognionics.com/index.php/products/mini-systems/cap
William
If some OpenBCI people want to try out EEG Dry Electrodes I
can offer a special price for Package of
25 Disposable / Reusable Dry Electrodes & 3 Lead Wires (FRI-2140-1) $29.95
and this including shipping. It is a
savings of about $10. They can be seen
on our web site www.fri-fl.com If anyone is interested send via PayPal $29.95
to [email protected]
with a note that it is for the OpenBCI Dry Electrode Offer.
Regards,
Karl Zurn
Hi Joel,
The electrode package is in the mail. I have some other packages (www.fri-fl.com) I
would be happy to give OpenBCI people a discount on. We have quite a few items in development and will
keep you updated on our progress. [email protected]
Karl Zurn
http://conorrussomanno.com/2015/02/16/3d-printed-eeg-electrodes/
Conor, I was looking around for the resistance in ohms of your BuMat filament, but did not easily find it. How does it compare to the 10K ohms figure mentioned on the Jan 29 post above? I guess one way to measure it is with an ohmmeter just between the back contact and say one of the front tips.
http://fri-fl-shop.com/product-category/electrode/electrodes/eeg-electrodes/
And manual for use of the straps and electrodes. What they have done with the velcro straps is punch regularly spaced holes, that can mate with the comb or cone electrodes. This seems like a reasonable compromise with these particular electrodes which attach with a side-gripping spring jaw clamp. One potential downside is that the fixed hole spacing may not line up completely with the 10-20 site you are aiming for. But would likely be 'close'. Karl may want to add a comment here regarding their choice of hole spacing on the straps. (I'm sure which was chosen carefully.) The chin strap is elastic, but the Velcro One-Wrap straps are not.
Karl, I wonder if have you also tried out your comb / cone sensors with the elastic band BraiNet straps. That would be another option that may provide closer 10-20 positioning. But I don't know if the holes are large enough on the BraiNet.
On Jeremy's @jfrey thread on the g.GAMMA cap printed electrode holders this also seems like a good system with pre-positioned 10-20 holes on an elastic / adaptive cap. So it would likely work with these dry combs as well.
If Chip @chipaudette or Joel @biomurph have played with the FRI dry comb sensors, I wonder what they have found out relative to impedance and performance with and without gel injected.
Finally, it might be possible to 3D print some kind of adapter with a post that would mate with the holes of the comb / cone sensors. Perpendicular to the post would be a flat plate that is glued to a small piece of One-Wrap. This could then provide arbitrary positioning of the sensors, as is done with the velcro saline system headset. But the geometry of this may be awkward given that there must be room for the spring clamp to attach. And it also precludes gel injection.
William
In
regard to the BraiNet cap the holes are too big for our dry electrodes but you
can put one of our TD-22 Electrode Collars “washers” on the inside and they
will work. If anyone would like I to try
these collars I could send a few at no charge. Email me at [email protected] Also I could send a few of our dry
electrodes. These need one of our cables
but you could uses an alligator to try them out.
Karl
Zurn, FRI www.fri-fl.com
Most Ag-AgCl buttons (and I think this comb) are plastic with a very thin silver plate coating, then chloride-ed. The 3D printed combs that Karl is experimenting with are also this plating process.
The buttons I use with the velcro saline system are this way and snap into standard ECG female snap connectors. Generally since the buttons just stay in their sockets over many sessions (20 or so), they only get removed / reinserted a couple times. I do a rinsing / cleaning cycle about every 20 sessions. Then at about the 40 session mark, the chloride on the front of the button has started to wear off. So time to replace the button. But that's cool, because they only cost 50 cents each.
The button posts do get slightly scratched by an insert / remove
operation. But because of the rare remove operations, this is not an
issue.
Sintered Ag-AgCl disk electrodes that I've seen are very expensive. The Othmer Method ILF training uses these, they are supposed to be more stable than even the plated variety. Another down side to sintered is that they can only be cleaned in distilled water with USP grade salt. Any mineral impurities are said to degrade performance.
----
An interesting development in the last couple years, has been the creation of a new type of dry passive electrode comb. The base material is a dense form of PU polyurethane, plated with Titanium, so the comb teeth have some flexibility. The outer surface is then processed into a nano-ized TiN, Titanium Nitride. The big neuro company ANT in NL is developing this. No products available yet that I'm aware. I have several papers they have published in this area if anyone is interested PM me. Here's a comparison between several dry passive types: link here
http://www.andreaproject.eu/
http://www.ant-neuro.com/
We have introduced an EEG Headband. I can offer the OpenBCI community a
discounted price of $59.95 Including
shipping to the US. Export add $5. This is a $15 savings. Send Payment via PayPal to
FloridaResearchInstruments@gmail and mention this offer.
FRI-2140-3 DRY EEG
Dry/Wet Electrodes, EEG Headband and 5
Cables (15 Disposable /
Reusable Dry Electrodes and 10 EEG Cup Disposable/Reusable Electrodes and 5 Cables– Red, White, Green, Black and Brown) see our web site www.fri-fl.com under Products
Karl Zurn, Florida Research Instruments