Has anyone ever tried this, for example using a high permeability metal such as permalloy on top of stacked coils? Would this increase penetration depth?
Short answer: yes of course. To an electrical engineer, this is an obvious first thing to do. Spoiler: turns out, it doesn’t really seem to make PEMF more biologically effective.
Somewhat longer answer: Over the years I have done a lot of experiments involving high-permeability flux conduits to optimize what is essentially the primary coil magnetic circuit (half), and it is pretty easy to show that you can vary the flux vector concentration and direction using this method, which is pretty much what you would expect. The crucial unknown is: exactly what shape do you want the magnetic field to be for the greatest biological effect?
The other crucial thing no one seems to consider is the fact that we are not building a sinusoidal transformer. We are dealing with very short, shaped single magnetic pulses. The addition of a metal into the magnetic flux circuit may amplify and reshape the flux lines, but it also adds a lot of inductance and so the second-order effect is to reduce dB/dt.
Soooooo, among many other experiments like this I worked with a group interested in veterinary uses of PEMF, specifically they had an interest in injuries to the hoof (of a horse, of course, of course). Their design objective was to achieve a PEMF with flux lines that would look sort of like a water fountain shooting out of the center of a coil to achieve the depth necessary to treat the types of hoof injuries they were interested in.
So, I designed it for them. Basically, it is just a flat, larger diameter coil that has a small inner diameter, is one wire thick (a single spiral), and a much larger outer diameter, placed onto a high permeability metal disk.
I measured the magnetic flux lines, and it generated flux vectors as expected.
But the biological effect was not as expected. In the end, it did not work as well as plain-old vanilla-flavored standard stacked coils, just like the ones we offer now.
This is one of those common traps where we know how to make magnetic fields, but we have no clue how they are actually causing their biological effects. So, when we think we are doing something clever with a magnetic field, the biological outcome often is not at all what we expected.
This happens to me all the time. The answer just seems to be that you need to try a lot of different things, pay close attention, and see what causes what to eventually stumble upon things that really work biologically.
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Excellent, @bob very well articulated response. You nailed it at the top, I’ve tapped into my inner Chartered EE, apprenticeship not Baccalaureate.
The end goal is to help heal wounds from a partial foot amputation. I failed in my quest to cure my diabetic foot ulcer but I kept it at bay for 5 years.
I honestly did give this excellent concept a series of heroic attempts for many years, but it never seemed to be a noticeable improvement over just our plain coils (which act as the primary of an air-core transformer, as far as I can tell). I figure I did enough that if it was going to work that by now I would have stumbled onto something promising. But, alas, nothing.
I would think (hope) that a standard ICES-PEMF system should help you a lot with your recovery from partial amputation. Every time I try a different electro-magnetic configuration, I always end up going back to that basic arrangement because it seems to be most biologically effective.
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Yup my trusty A9 is on the job. The more I use it, the more I realize how robust it is. Kudos to the engineering that went into it. So I ditched my 18650’s and 7809 and just went back to rechargeable PP-3’s. Ran like a beast but too bulky. I’ll have to try another experiment with the new Li-ion caps I just got once I’m done charging, load testing and destroying some.
Thanks again for all you do in support of this community.
load testing electronics can be good fun, but the dang things apparently run on smoke, so if you accidentally let the smoke out, they just quit working for some reason 
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