Frame shift: subepithelial (superdermal) Faraday cage
This is an answer to a slightly different question: Putting the conductive layer under the dermis is hard, because the bottom edge of the dermis is irregular and intertwined with the hypodermis. It could be placed more deeply (superficial fascia, etc), but I'd rather look to the boundary of the two major layers of the skin: the basement membrane. Putting the cage above the dermal layer means that it defeats miniature spying devices with which the victim might have been tattooed via weapons shot at protests (I've seen a paper proposing these about 20 years ago, ought to look for it...) or injected/vaccinated etc., which could hide in the dermis itself.
The basement membrane provides a fairly continuous barrier around the body. We just need to make it electric. For this I'll suggest beta-carotene as raw material. This is pretty much what biology uses for wire - in plants, it collects light for photosynthesis, and in humans, related compounds (vitamin A and more specifically 11-cis-retinal) are used to detect light. If we eat it in large excess, the skin will take on an orange color. We're going to change that color so it absorbs a wider range of (lower) frequencies.
The trick of this molecule is in the double bonds (=) which can be shifted to either side and still connect all the atoms correctly, if charge is introduced to one end and the other end. All we need to do to make it more graphite-like (conductive) is to extend this network by an enzymatic reaction that links these structures together with another bond. We need to evolve a new enzyme from scratch, most likely, which is a task of some difficulty but definitely doable, especially since there are many known domains that bind to and react with vitamin A derivatives. I only took the briefest look at PubMed to scan for something already in existence, and there's a chance if you dig into KEGG or other compilations of biochemical pathways you can find something useful already from nature.
When making such an enzyme, we should tweak it during development so that:
- It has a relatively poor binding affinity to beta-carotene. We don't want it stealing retinoic acid needed during development or vitamin A the baby needs for night vision - only when beta-carotene is consumed in excess will an appreciable amount bind and react.
- The reaction has some reversibility. We're not going to spend a fortune in energy to make this happen, and we don't mind if the network can spontaneously be broken down when beta-carotene concentrations are low. This will cause the network to double as a vitamin storage for the infant and ensure it is possible to give up the cage with a prolonged period of sticking to the Recommended Daily Allowance, if there is a medical imaging issue. But vitamins with megadoses of beta carotene have been very easily available; and if that fails, there are carrots (yikes!), so this is not a large bother.
- The enzymes for the reaction need to include binding sites that attach somewhere on proteins of the basement membrane, for obvious reasons. Otherwise we're laying these down in random places. They should be secreted by basal epithelial cells, I suppose. Their production should be very strongly repressed in any tissue of the eye, and their activity should be allosterically inhibited by ceramide or other compounds common in the retina. More safety considerations would be needed ... we don't want any foul ups!
[Apology for a minor jest: the proper term is supradermal, but "Superdermis" sounded more heroic]