If my plan is to ALWAYS use them stacked can coils be glued together rather than the athletic tape method?
yes
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I have been using them stacked almost all the time, but my mother will use them a couple times a day for about a half hour to help with digestion. On those occasions, she has to have the coils placed side to side. What I have done is create socks for the coils: I take stretchy cloth and sew a sock that I pull over the coils and this will keep them clean (I can launder the socks when they get dirty) and hold stacked coils together which I can remove the sock and use separately as the need arises. So, I would not glue them together, as this method I’ve adopted allows for much more flexibility.
Another approach you could consider is having different sets of coils for different purposes. I have found as I have experimented that other configurations are sometimes more helpful. I use stacked the most by far, but since the coils are not that expensive, I keep at least one set with side by side and one single coils in addition to the stacked. I use coban wrap on each of these sets on the coils so that i can tape them to the body area and then remove the tape and apply new tape each day/session and the coban does not need to be replaced often.
Think of the coils as consumables, as opposed to durable goods. As the copper in the wires is bent back and forth during use, it work hardens, becomes more and more brittle and eventually fails. I seem to get 6-8 months out of the ones that get daily use. So I have several. Also I have some for specific uses, some glued together, etc. I also keep extras on hand.