3D printed electrode holders for g.tec g.GAMMA cap
I've been requested several time to publish those, here is a repo containing the 3D models I used to combine the gold cup electrodes with "my" g.tec EEG cap. As you'll see, there's room for improvements: https://github.com/jfrey-xx/OpenBCI_3DPrint_misc
Comments
A couple questions:
* I didnt realize the holder cylinder was the same diameter all the way from outside to inside. This seems like a large stretch for the elastic holes on the g.GAMMAcap. Will such stretching have any "fabric fatigue" effects on the holes over time?
* Have you ever seen the inside of the Electro-caps? With those, the metal contact does not have to be right up against the scalp. As long as the gel can bridge the distance, that is all that is required. This implies that the electrode contact could be 'outside' the cap, in the 'top' of the holder. Then a smaller diameter cylinder to the scalp. The holder would be a two piece affair with a narrower cylinder going thru the hole.
* You mentioned that g.tec had some holders used with the g.GAMMAcap. Can you post a couple photos of those? Are they the same dimensions? Ones I've seen on their site are smaller. ("Ladybird"?)
http://www.gtec.at/Products/Electrodes-and-Sensors/g.Electrodes-Specs-Features
Thanks again, William
I agree with the snap connector idea, it's what I use with the Ag-AgCl buttons in the velcro saline system.
https://sites.google.com/site/biofeedbackpages/velcro-sensors
See the "commercial systems" subpage there on the variety of commercial saline systems already on the market.
Snap pellets used in velcro headset. Actually smaller ones exist than these as well:
http://stens-biofeedback.com/collections/parts-and-supplies/products/mb-pellets
Now I think an interesting combination would be to use the saline sensor approach with the g.GAMMAcap2. In other words, NOT using the sticky and hard to clean up congealing gel. But instead use a freely flowing saline solution, soaked into a material such as the Sham-wow chamois cloth shown on the velcro saline headset page.
In this case the plastic holder would be some kind of two piece idea. The top piece would be the cylinder, just wide enough in diameter to adapt the snap button. With a hole in the top center for the snap to go thru.
The rear of this top section would have 4 pins at the 4 quadrants of the cylinder edges. These would friction mate with 2nd piece, a flat circle (or square) plate with 4 holes aligning with the top portion. (A hole in the center of this bottom plate would also help the saline absorption.)
To assemble, a rectangular piece of the chamois cloth is wrapped over the bottom plate, then snapped into the top cylinder. The cloth strip overlaps a bit underneath the button and makes good contact with it, as it abuts directly. Width of cloth strip is slightly less than the pin spacing, so cloth strip fits between the pins.
Before use all the chamois cloths are saturated with saline using a syringe.
As with all saline systems, you must ensure the cloth pad is in contact with the skin of the scalp by parting the hair in that area. With an elastic cap covering the whole head, access to these touch points is trickier than say with the EGI geodesic sensor net or the velcro system. But could probably be finessed with some back & forth + rotational circular movement of each outer sensor assembly + elastic cap, until skin is reached. Some intervening hair is allowed, it just cannot be many layers of it.
You can see the same kind of scalp contact motions used in the video demos of the g.SAHARA comb sensors. They too need a bit of jiggling and back and forth until the combs contact the scalp.
William
Although a plastic printed holder adapted for saline would be easier to manufacture than manually creating the components of chamois pockets, rubber backing sheet, etc. An O-ring between the snap post and female snap would still be a good idea to prevent any saline solution from getting up into the female snap.
William
This all depends on the liquid absorption and square centimeters or volume of the pad used in the sensor.
Emotiv used rather hard felt pellets (link to their 'hydration pack' on the 'commercial' subfolder of the saline site.).
Not a lot of 'space' in that hardness for holding onto the water. The Sham-wow chamois material used above is HUGELY absorptive. I've done 2 hour sessions with these sensors, and they are still going strong.
Another factor is that with these flat sensors, the back is covered by rubber sheet, and the front is down on the scalp. So most of the surface area of the sensor is not available for evaporation. The Emotiv pads on the other hand are just hard little felt cylinders that evaporate all along their length between holder and skin. So the Emotiv comparison (although a useful data point) does not predict the performance of this particular saline design.
https://www.google.com/search?q=electro-cap+electro-gel
The Electro-gel also has some kind of stiffening agent as part of the formula. So after it is injected and connects between scalp and electrode -- it solidifies slightly so as to hold the connection over a long period of time. I'm sure the DIY gel with salt water and aloe also works well, but may be somewhat more "runny".
One consideration with gels is that you don't want the gel to run down any farther than the current 10-20 site it is being injected at. If the gel runs down by force of gravity, it can make a conductive bridge to other 10-20 sites, thus averaging / distorting your signal over multiple sites.
---
I'd definitely consider the dry comb sensors with the g.GAMMAcap. Would eliminate the time and mess factor. But the dry combs do reduce the signal strength slightly. So far folks using the combs are happy. If you use the combs, go for the smaller holes.