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Exactly what it says on the tin: how would one go about making a subdermal (i.e. under the skin) Faraday cage?
Specifically, are there any physical or biological principles that could be applied to do such a thing, while doing so in a fashion that is consistent with known biology and physics?
Preferably, this could be something that an infant could be genetically modified with and that they could "grow into", but I'm willing to take all comers.
Inspiration here.
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:
[Apology for a minor jest: the proper term is supradermal, but "Superdermis" sounded more heroic]
Tattoos made with conductive ink have been proposed. All the examples I've seen so far have been surface ink. I'm not sure if that's for reasons of toxicity or because people don't seriously want to have permanent circuitboards inscribed in the skin. But the surface ones are conductive enough to attach lights and power sources.
See Silver Nanowire Inks for Direct-Write Electronic Tattoo Applications
It would be difficult to get a biological process that'd create a metalic mesh, becuase the thing DNA is good at making, protein, and similar biology-ish chemicals is the thing metal isn't.
However, all is not lost! It doesn't necessarily have to be metal! There are conductive proteins, and it might be feasible that a correctly designed mesh would have similar properties to a faraday cage.
Heres at least one suggestion;- https://pubmed.ncbi.nlm.nih.gov/27409066/
I'm not too sure the physics of this, and I'm not sure an internal faraday cage is going to be particularly useful, since the goal is to keep that EM energy away from the body, rather than ground it inside of the body. That might require a physicist to chime in on this. I might add, that grounding is important, the EM captured and diverted by the cage has to go somewhere. Newton does not negotiate with terrorists and sci-fi authors, conservation laws are a constant in any concievable universe worth concocting.. So you'd have to work that problem out too.
If you put it on the surface though, you could have your character displaying a nice metalic sheen, since one property of conductivity is reflectivity.
Blood vessels are a wonderful criss-crossing madness on their own. I'm not entirely certain how feasible or survivable this is, but since there is iron in our blood and copper in the blood of molluscs I'd imagine it wouldn't be too big of a stretch, even if nothing in nature currently has this, for the body to use iron/copper atoms in the construction of a person's veins at a relatively high amount/concentration.
This may lead to something like the veins themselves being slightly metallic with all the good and the bad that comes from that. Who/whatever has these veins would be rather conductive throughout their whole body and will have a bit of a bad time at airports and all sorts of situations that involve metal detectors.
Bad news: to protect against high voltages, you need a very low resistance, probably only to be found in metals.
So unless you go the way of some semi-plausible body-temperature organic superconductor, which you could, the only alternative is a metallic mesh, probably gold, or copper or silver sheathed with something biocompatible and non reactive.
The advantage with gold is that beside having very low resistance, you can have really tiny wires, that do not interfere with the organism and need little sheathing.
To thread the wires throughout the skin, you'll need either specialized mites (genetically engineered Demodex for example, except they'd need to burrow - so, something more like D. Medinensis) or nanomachines, and a gold-based skin lotion to supply them with the raw materials. The lotion could contain millions of really tiny carabiners, and the mites would assemble them in chains and thread them throughout the organism.
Or the mites could assemble the carabiners themselves out of ordinary materials, building exotic compounds such as bond-strained thiafulvalene-catenane (fulvalene was one of the first groups used in high temperature superconduction back in the '80s, and catenanes are a novel class of pseudopolymers that possess literally infinite flexibility. Best of all, they could be built from ordinary organic CHNOPS if one had the appropriate molecular machinery. The only hand-waving here is the hypothesis that bond-straining certain groups could "gift" them with superconductivity, as it seems to happen with thiocarbo-pseudohydride).
If your idea is protect the whole body from microwaves you are out of luck. No way to metallize a human body skin is scientifically plausible. But human body skin and muscles are rich in salt and salt is conductive. Maybe if you connect such tissues to earth you cold make a reasonable Faraday Cage. But attention!!! It can work to block low energy electromagnetic waves. It will not protect from strong microwave action. As Adrian Colomitchi has commented it is not a good idea, and, connect skin and muscles to earth cam make you much more in risk of ordinary electric shock.
But if your idea is people were born with a natural "Tin foil hat" you could be more lucky.
You need to metallize only the skull. I think, maybe, because human skull is mostly made of mineral (calcium) matter some none could mess up with DNA to make the human skull bone metallized with iron, maybe cooper.
