I have some contributions to make on Zeo sensors (it was I who wrote all the Wikipedia Zeo,_Inc.#Effect_on_users section (as it stands today) mentioned in message 1, BTW).
I'm not an expert writing authoritatively; in particular the suggestions I make are probably extremely inappropriate if you use electrodes in a life-critical medical environment. I'll try to distinguish what I've found out, what I've read, and what I think is reasonable! All this may depend upon each person's skin, hairline, etc.
While Zeo was still a going concern, I saw some postings about using disposable adhesive gel ECG electrodes with offset press-stud connections, which was reported to give better results than the Zeo headband.
Electrodes used were Ambu Blue Sensor Monitoring Electrode.
P-00-S/<number in pack, e.g. 50>.
From my reading: ECG electrodes come with several different connectors; only the press studs are suitable. In the Ambu range the final "S" means stud; "A" and "F" are different connectors (but more on this later). Most electrodes have connectors in the middle of the pad, which is not suitable for Zeo; it is essential to use offset connectors. The electrode is basically a pad covered with an electrically conductive gel, to give good skin contact over a largish area. It can be fixed to the body with, say, sticky tape, or an electrically conductive glue can be used (the glue is water-soluble; you can wash it off with water, don't use alcohol). There is nothing magical or high-tech about what an electrode is; there is even a project to make electrodes with the plastic inserts in beer-bottle crown tops, using a flour, water, and salt conductive (but not adhesive) gel for use in countries that get given ECG machines but can't afford disposable electrodes.
I think I also read somewhere of someone taping the Zeo transmitter directly to their forehead, getting a signal.
The problem with gel electrodes is that they dry out. The Ambu electrodes come in packs of 50; the pack has an expiration date, and an opened pack must be used within 30 days. For non-critical applications like Zeo I would simply use them until they became unreliable. I tend to refrigerate things to make them last longer, but doubt that it would help with an open pack of electrodes. Hermetic sealing of the pack and squeezing out air should help (this is recommended by sources, not just my idea). The blue center of Ambu electrodes should never be pressed, but only the white ring; always avoid any pressure on the center, especially when pressing an electrode onto the skin to get it to adhere firmly and removing tape.
General advice for nurses is that electrodes should be placed on dry skin. For ECG, contact is improved by shaving the skin and removing dead skin cells by rubbing with a rough paper or cloth.
I read somewhere (Amazon review?) that you could get two nights' use out of the Ambu electrodes, but that results on the third night were not reliable.
It's basically a matter of money; you can buy electrodes and use 3 every 2 days, discarding opened packs after 30 days and unopened ones on expiry. This adds up over a year; you can try to extend electrode life to reduce the outlay. I estimate that using electrodes every day, once only (as specified) and buying one pack at a time from Ambu, costs £442 per year. Using electrodes several times, buying larger quantities to reduce carriage costs, and buying from sources such as eBay reduces the cost progressively, until a single order of a years' supply of short-dated P-00-A electrodes (wrong type, but easily modified, see elsewhere in this post) to be used 5 times (much more is possible), and beyond their expiry date, costs £26.
What I have tried: first let me say that I haven't done the extensive comparative testing to determine absolute maximum mileage - it would take lots of packs and many months. But, by Feb 2014, I know I (YMMV) can get at least 10 days out of each set of 3 electrodes, ignore the expiry date on the pack, and ignore the "use within 30 days of opening" warning if I follow some simple procedures.
I've used gel electrodes, both fresh with their own glue and older with tape; they stay in place very firmly and never come off during sleep or when up, and I never have the feeling that they are precariously held, though I worry about the irreplaceable transmitter falling down the toilet or into the wash-basin! I haven't had any issues with the electrodes or Hypafix, Transpore and Micropore tape damaging or marking my skin. I wash my forehead and dry it by rubbing roughly with a towel (to help remove dead skin cells), then place the electrodes and rub the white edge (but not the blue middle where the gel is) to help them stick, adding surgical tape to tired electrodes.
If electrodes dry out, let's see if we get improvement by keeping them moist. A single experiment with adding a drop of water to electrodes that were fading didn't work (maybe it could be made to work? But the following way seemed better). So I got a small plastic container with lid, put some cotton wool in and saturated it with water, with a bit of free water at the bottom, and a few drops of bleach added to inhibit mould. Every morning I put the protective plastic layer back on the electrodes and lay them on top of the cotton wool in the humidor; the idea is that they are in a humid atmosphere, not wet (which would wash the adhesive and the conductive gel off). I used a set of electrodes for 5 days this way. For the first two days the adhesive glue held strongly. By the third day the glue was weakening, but the electrodes still gave a reliable signal if held in place, so I put 2.5cm-wide tape on where the electrodes weren't sticking (but I didn't need to tape all 3 electrodes even on the 5th day; two lower electrodes in a line tend to be OK, the one sticking out higher up is less flat and prone to unsticking). I can get about 6 days this way, and more with could Tensive adhesive conductive gel (see below).
What I think is reasonable: it would need months of comparison tests to confirm this, but I am storing my opened packs of electrodes in a large plastic lidded container with absorbent cloth or paper soaked in water with a drop of bleach - my overnight humidor writ large. I think both the 30-day limit and the printed expiry date are ultra-conservative, for medical use (you don't want the tiniest chance that your defibrillator electrodes are dead, or your patient will be), so being able to exceed the limits doesn't actually prove that the humidor works - maybe the electrodes would work OK without? Maybe someone will try comparative testing?
A way to revive fading electrodes is with Parker Tensive adhesive conductive gel, designed I think for dry TENS electrodes. Amazon reviews complain that it leaves a terrible sticky mess, and recommend applying the gel and letting it dry for 24 hours; I disagree. The manufacturer says to apply and use immediately. I put the smallest possible amount on the middle and spread it with a finger, immediately before use. There is a bit more sticky mess the next day than with the original electrode, but it comes off easily with water. One application of Tensive (I've never tried more) gives the electrode another 4-6 days life, though with 6 days electrode life anyway it may not be worth bothering with it.
There seem to be Chinese producers of electrodes which may be suitable, but the minimum order has lots of zeroes after it. I doubt there's enough of a market to import these things for home use in small packs; I'm certainly not going to do it. Maybe a Chinese seller might find it worthwhile to offer small quantities on eBay, if they are suitable?
I accidentally bought a pack of (cheap, short-dated) type P-00-A instead of P-00-S; I discovered that they were just the press-stud electrodes with a small plastic housing added to plug 4mm connectors into, and I was able to cut off the extra bit with diagonal cutters, leaving effectively P-00-S. The first one took 3 cuts and about a minute, subsequent ones one cut (you cut the base of the housing, being careful not to press or twist the center of the electrode) and 5 seconds. So if P-00-A or P-00-K are available at bargain price and you don't mind spending a few seconds, they can be made to work.
Tapes for fixing electrodes that don't stick well: 3M Micropore; 3M Transpore (cheaper, holds more firmly with slight risk of skin damage, though not like ouch-traditional-sticky plasters); Hypafix Self Adhesive Dressing Retention Tape. I have a roll of 5cm wide Hypafix; I cut it to half-width for Ambu electrodes. I find Hypafix best as it is more conformable and doesn't lift at the edges. I use the same tape for 2-3 days and store the electrodes with it in place, for convenience not economy. Old tape can be peeled off the electrodes, with a little difficulty, trying to hold the electrode edges, not center; or it can be trimmed off with scissors (I usually peel). I haven't had any skin damage from tape or electrodes.
I might experiment with fixing the Zeo transmitter directly to my forehead with the full width of the Hypafix tape, or maybe some type of headband. Maybe push metal sew-on press studs into the transmitter to increase the area? Maybe use press studs with something like the African beer-bottle plastic circle with conductive gel, adhesive or not?
While everyone will have seen the article on making a Zeo-type dry headband, gel electrodes also work fine.
I hope this is helpful.
10 Feb 14: In case anyone comes here via a search, I've added some further comments in the second part of post #282. I've also slightly updated this post (mainly re Hypafix, Tensive and electrode life - later experience has confirmed the rest), valid as of 19 Feb 14. I don't think there were any comments to the original post affected by the edit.