Velcro saline sensors

   
http://lightfield.com/sites/ss%20headS.jpg

Velcro EEG headset using saline solution (instead of paste or gel)

[Photo shows the headset configured for 5 channel alpha synchrony protocol with OpenBCI]

There is a long history of EEG passive sensor systems that use a simple non-sticky & no-residue saline solution between the EEG electrode and skin. The solution is usually soaked into a cloth pad or thin sponge material between the sensor and the scalp. Such saline sensors have been attractive to EEG / BCI / neurofeedback users who do frequent sessions. And who wish to avoid the mess of paste or gel cleanup. Saline systems are not ZERO maintenance, but they do reduce many of the unpleasant per session costs.

On this subpage is a list of some of the commercial saline EEG systems I am aware of.  In Europe these are very well known, based on an elastic head net and posts positioned under the bands. Here in the U.S., the EGI Geodesic Sensor Net saline systems are used in hundreds of research and clinical settings.

Using these saline systems with modern EEG amplifiers such as the ADS1299 in OpenBCI, obviates the need for abrasive skin prep or very low skin impedances. This is because modern amps have an input impedance of hundreds of megohms, even to a gigohm. (The ADS1299 is 1 GOhm). For an excellent paper on this subject see this link. The paper is from the company that produces the Geodesic Sensor nets on the previous commercial system page link. 

You have a choice with these systems, as to which passive electrode sensor type (metal) that you want to use. Some of the commercial systems referred to on that previous link can utilize normal cup type sensors such as gold or silver. Which are then enclosed in a cloth saline soaked pad, and placed into a holder. Farther below on this page we show how to use the OpenBCI gold cup sensors with velcro in this way.

The EGI geodesic sensor net is using Ag-AgCl (silver chloride) sensor posts inside their plastic holders. Silver chloride has one of the most stable signals of all the electrode metals.

With the two-sided (One-Wrap) velcro based system explained below, I first show the sensors based on silver chloride sensor buttons. Silver chloride is needed in my neurofeedback work because I use Infra-Slow Frequency ranges, which go down well below .1 hz to even .001 hz. 

Velcro + saline and regular cup sensors: In a following section [red-colored heading: Cup Sensors] below the next few sections -- I show how gold cup sensors as supplied by OpenBCI, can be used instead. The cup sensors are likely lower maintenance than the buttons. Because of the fewer parts and easier washing procedure.

We all are hoping for the appearance of low cost, tiny, and no maintenance active dry sensor modules. Until that day, saline offers an alternative to the stickiness and cleanup needed with paste or gel electrodes. The One-Wrap velcro band approach could still be used to position active sensors, once these become more widespread.

And headsets, such as the OpenBCI spider claw or the custom tensioned bands of the concept used in the Neuropro / Imec, could also be retrofitted to allow a passive saline approach.


Velcro One-Wrap vs. elastic based headsets

Because no paste is involved, all the saline systems I am aware of, need some type of tensioning system to hold the sensors down on the scalp.  This usually takes the form of elastic bands or a cap.  A downside of a full elastic cap (such as the spandex Electro-Cap) is that after approximately 30 minutes to an hour, if you are doing a long session -- your head begins to feel uncomfortable because of the sustained pressure.  There are actually plates that make up the skull, connected at 'suture' lines.  The plates 'breathe' and move subtly over time.  Elastic will constrict the movement of these plates, causing discomfort.

The velcro system below does not use elastic.  Instead it is just tensioned 'manually' so that the bands are gripping the skull, but not applying a rubber cap type of continuous force.  The velcro material used is 3/4" Velcro-brand One Wrap strap / band material, retail stores carry this in 12 foot rolls. The strap has hooks on one side and loops on the other, so it adheres to itself.

I've interspersed some simple photos below.  Google Sites has shrunk the preview. To see a higher resolution photo, click on the photo to open it in a new tab or window. For simplicity, the next couple photos just show a single channel attached to the velcro headset bands.

[overview]

This shows an overview of the velcro bands on an invisible 'head' (front of head towards top of photo). This is a simple as possible setup for alpha-theta neurofeedback, referential montage at O2. You can see the velcro sensor hanging off the band at the right rear occiput O2 site.  On the left and right of the 'head' are the ear clips.  One acting as reference, and the other as ground/bias.  

In placing the velcro, the circumferential 'hat band' is attached first.  This passes all the 10-20 points on that band such as O1, O2, T3, T4, Fp1 and Fp2.  The band is overlapped on the forehead and tensioned until it is just comfortably snug, not 'tight'-fitting.  Next a front to back band is placed over this, going from Fpz to Oz.  When using multiple channels, I will put the ground/bias sensor on this strap so it is on the midline.

Finally the left/right band(s) are placed, for example over the F3, F4 line.  Or P3 P4 line, or C3, C4 line.  Sensors are attached at the appropriate sites.

In the upper left of the photo is a small amp, the Pocket-Neurobics Pendant unit, two channel, 5 leads.  In the upper right are shown the sensor pieces. Which a following photo will zoom into.  The Craftsman small clamp with the red jaws is used after all sensors have been placed, to fasten the cable bundle securely onto a convenient piece of shirt, such as the collar.  This avoids having the weight of the entire cable set pulling downward on ears or head sensors, and adds to user comfort. Sensor cables here are approx. 40 inches in length each.

[right side view]

This shows a closer view of the bands.  A sensor consists of a standard 'snap-type' female ECG connector plugged into a silver chloride button.  Around the button is a sandwich of highly absorbent micro fiber cloth.  Or 'chamois' cloth.  Saline solution is sprayed onto this pad, which then conducts between the skin and the button.  The ear clips are done similarly.  Both the buttons and ear spring clips are commercial products from MultiBioSensors.  These are used by a number of manufacturers, such as the amps from Nexus and J&J.  But they typically use the buttons with paste, not saline.  Next photo shows a close up of the sensor.

The female snap connectors can be purchased in sets, such as those used for ECG (5 lead) or more.  They are color coded as well.  A small square of the velcro one-wrap is attached to the back of each female snap connector.  Hooks facing outward.  The velcro bands that go over and around the head are oriented so that the soft "loop" material is facing towards the skin.  Thus the bands mate with each other, and with the female sensor holders.

[sensors]

Upper left shows the silver chloride buttons.  These are little plastic buttons with the standard dimension snap posts.  Same dimensions that are used on clothing.  The plastic has been silver plated, then the silver chlorided.  They cost about 50 cents each and last about 30 to 40 sessions until the chloride coating starts to wear off.

Below the buttons are assembled sensors.  In the center is shown a disassembled sensor.  The cloth wrapper is just a strip of chamois-cloth that has a hole punched in the center of one side, then the two sides fabric glued (using Fabri-Tac), leaving an opening at the top and corner.  This is so a button can be slid inside and pushed thru the center hole.

You can see an 'opened' cloth pad at the corner of the yellow Post-It note stack.  One corner of the cloth has been pulled back so you can see how it hinges open.  After placing the button inside, the flap is closed.  Then the black rubber square is placed on top of the button post.  This acts as a moisture and saline barrier to prevent the female snap connector from being in contact with saline for an extended period. This prevents corrosion of the interior of the connector, which is made of nickel plated steel. There is a steel spring inside the female connector that grabs the button post.  So saline cannot be allowed to enter there. 

Finally after the rubber square is placed, then a rubber o-ring is added to the post, so as to seal the gap between the female connector and back of the rubber sheet.  The completed sensor is then snapped into the female connector and at the opposite end, the touchproof connector is attached to your amp.


Application / use of the velcro sensors

In daily operation, the sensor pad is sprayed with saline solution before each session. When spraying the saline onto the pad, apply gradually so that you do not get large amounts of saline flowing behind the sensor and toward the snap. (It takes a couple seconds for the saline to fully soak in.) The snap side is protected with the rubber square and O-ring, but you still do not want to drench that area with saline. Since there is some exposed metal around the front of the female snap.

As with regular paste sensors, you need to move the hair away from the site before (or as you are) placing the sensor down on the skin. This can be done either before you place the sensor, or once the sensor is down on some hair, moving it gently back and forth pushing the surrounding hair out of the way as the sensor seats down on the scalp. You should be able to feel the coolness of the sensor once it finally contacts skin. The saline can penetrate through a bit of hair, but cannot go through to a site where the skin is not exposed at all.

As mentioned previously, it's easiest to apply the circumferential "hat band" strip first. Then a front to back strip. And finally your left to right strip(s) as desired. If you are using the same layout repeatedly with yourself or a specific client, then you also have the option of just pulling the entire headset / sensor net off vertically at the end of the session, maintaining it's shape and sensor positions. (What I generally do with clients however is to disassemble the strips in the reverse order in which they were added.)

The ear clips work similarly to the scalp sensors, except a hole in the ear clip sits between the button and the female snap. So inside the ear clip jaws, one side is the button and the other side the opposite jaw of the clip. Thus there is no need for o-ring, rubber, or cloth sandwich.  Instead just a little strip of saline moistened chamois is placed between the ear and the button as the clip is attached to the ear.  When removing the ear clip, you just pull off the cloth strip at the same time.  The ear clip with a strip of cloth inside the jaws is shown in the photos.


Washing the sensors

After the session is complete you can just let the sensors dry out, no need for cleanup of skin or sensors.  You can do about 20 sessions on a sensor before the salt residue from the saline needs to be washed out.  To do this, the still assembled sensors w/buttons are detached, and placed in a container of water to dilute / wash out the saline. I usually put them in a small container of lukewarm water, stirring around. And then pour that water out and rinse again.

As a simple aid in unplugging the sensor from the snap connector, I just use the handle of a plastic measuring spoon, which happens to have a thin profile. (A plastic knife could be used as well.) Wedging the blade between the sensor and connector, a quick twist of the blade unsnaps it.

After the sensors have been washed, press the excess water out of the sensors with your fingertips and give them a couple shakes to throw off excess water.  Place them on a clean surface to dry, I use the lid of a yogurt container.  After drying out (a few hours or overnight) the cleaned sensors can then be snapped back in.  After 30 to 40 sessions (total)  the buttons will need to be replaced.  (You can usually see their coating has begun to wear off at that point.)  I keep a little 'score card' with tick marks to keep track of how many sessions I've done.  So can see when it is time to wash or replace buttons.

As a precaution to prevent any corrosion inside the female snap connector, every few washouts I will use a q-tip (moistened with synthetic oil) -- to coat the inside of the connector. The synthetic based lubricant will not react with rubber or plastic.


Cup sensors with velcro pockets

The previous paragraphs discussed the silver chloride button sensors. A similar approach can be used with a velcro holder for cup electrodes. The photo below shows how this is done. The sensor holder is a square of the One-Wrap to which a chamois cloth pocket is attached using the Fabri-Tac adhesive. The chamois is a strip which is folded in half. In the very center of the strip is a hole for the sensor lead.

I suggest using an awl or sharp pointed object to make and enlarge a hole in the center of the pad. Just make it large enough to force through your plug. (In the photo shown I used a 1/8" hole punch, which is the same as for the holes on the button sensor strips.) However the awl approach is superior, because it will tend to close up somewhat after you push your wire through -- ensuring that the cup sensor does not widen the hole over time.

[velcro used with cup electrodes]

The Fabri-Tac is used to apply a line of glue to the left and right edges of the chamois strip, leaving the top open for the insertion / removal of the sensor once everything is dry.

After making the pocket, apply two strips of Fabri-Tac to the left and right edges of the velcro soft / loop side.

Be sure to work the glue stripes into the loops a bit so it will hold on to the pocket when dry. Now press the pocket down on top of the velcro square, pressing along the left and right edges to attach the pocket to the square. Don't use so much glue that it goes all over the place and makes a mess. You just need thin stripes, but do want enough to grab onto the cloth. If your stripes are TOO wide they will reduce the clearance needed for your cup sensor.

After you have given the Fabri-Tac enough time to dry (I usually give Fabri-Tac'ed items overnight), the cup sensor can be inserted into the pocket and pulled down to the bottom edge.

In making the chamois pocket you may want to experiment with putting an extra thin line of glue along either side of the hole at the bottom of the pocket, so that the hole does not stretch out of shape over time. Or the thin glue lines could be placed about 1/3 up from the bottom, if you wanted the cup position to rest in the center. 


Sensor components

[components]

This photo shows some of the component packages, bottles.  The synthetic chamois-cloth, called ShamWow on the left.  next to the right is the Fabric glue Fabri-Tac.  Then a spray bottle for the saline solution. Saline solution is made from Potassium Chloride, vs. Sodium Chloride.  The Geodesic Sensor net folks use the KCl, some literature says it may have slightly better characteristics for use with silver chloride sensors than NaCl. I use the Geodesic Sensor recommended solution portions. Worst saline product for corrosion of metals would be sea salt, which contains a synergistic mix of alkali metal salts, and seems to hasten metallic corrosion.

A note on the chamois link above. I picked up the small package shown at a local hardware store some years ago for less than $10. However looking around online now for "synthetic chamois", I find a whole raft of different choices available. The properties of the cloth used are crucial for the success of this saline approach. The original ShamWow was claimed to have been produced in Germany. Some of the newer cloths I see online appear to be made of something called "PVA cloth", poly vinyl alcohol. This is not the same thing as the ShamWow, as the PVA is said to become quite hard like cardboard when dry. All I can say is, be sure the cloth you get is actually made of dense fibers, is highly absorbent, and is not PVA.

Row on bottom shows the One-Wrap package.  Little container at lower right is the same mild disinfectant used in contact lens cleaners. This can be added to the saline solution when mixing.  Polyaminopropyl Biguanide, from Lotioncrafters.com. (Users of Emotiv EPOC sensor pads are suggested to use contact lens cleaner as their saline solution. Contact lens solution uses this same disinfectant.)

Here's the saline solution mixing steps I use to make a 250 ml spray bottle:
  • use warm water from the tap, it dissolves the KCl faster
  • 1/2 teaspoon of KCl in 1/4 liter water (250 ml); stir, let sit until dissolved
  • add three small drops of Dawn dishwashing liquid
  • using a measuring syringe, add 3 ml of Biguanide, stir until dissolved
  • place solution into your spray bottle
When initially spraying your chamois pads, start with some light sprays until the chamois starts absorbing more rapidly. Then as you apply each spray, wait for it to soak in. Continue spraying until 'almost saturated'. In other words there may be a drip or so coming off the pad when held with a corner downward. Let any excess drip off before placement of your sensors. You can also lightly press your thumb / forefinger over the pad after spraying; this will evoke any drips if you've put on too much. With this 'right' amount of saline, the sensors do not drip or run on the head, as the chamois is holding all the saline. Sensor pad should feel cool / wet when it touches down on your scalp or ear. Push the hair aside from the scalp point, before finally touching down. The saline and pads can bridge a small thickness of hair, but if the hair is dense, it needs to be pushed aside first over the sites.


Five channel headset in place

  

Les Fehmi's Open Focus Alpha Synchrony uses five channels: Fpz, Cz, Oz, T3, T4. I have another page explaining this in more detail. Brainbay is used to control the neurofeedback session.

Additional velcro small strips clip the sensor wires to the main left/right or front/back bands, so that the cable bundle routes off the back of the headset. (See photo at the very top of this web page.) In the first photo above you can then see this cable bundle secured at the shirt collar with the Craftsman utility clamp. Wires dangle down a foot or two from the OpenBCI, which is clipped near the top of the shirt.

If working with a client, I will clip the OpenBCI just to the rear of their left shoulder. And then clamp any extra cable lengths to the back of their shirt; to keep cables out of the way of the chair back. Individual touchproof cables used here are about 40 inches in length. Which is a convenient size for routing like this.

It's certainly possible that you could clip the OpenBCI board somehow to the velcro headset itself. But leaving it at a distance has a couple advantages: (1) keeps it away from any saline you are spraying around the sensors. Saline, paste, or any conductive materials on your circuit board are bad news. (2) there is increasing evidence that EMI / RFI from mobile devices can influence cell, brain, organ activity if in close proximity for long time periods. So keeping a moderate distance between the brain being measured and the Bluetooth transmitter, may be prudent.

The Lithium battery / booster / charger circuit is in a pants pocket and power cable goes up to the OpenBCI. See this page on the forum.




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