Interesting and insidious. They would need to be self replicating to be viable I figure or produced in such quantity that cost wouldn't perhaps be an issue if they weren't self replicating. Perhaps made from something abundant. They would need to be built in a way where molecular structure was from readily viable or easily sourced materials in high abundance. Oxidization would be a constant threat, requiring not only replication, but also rejuvenation, or perhaps a parasitic nature. Only atomic structure low on the periodic table meets this demand for stability, leading me to belive carbon is the go to. Blood and biological tissue are usually highly oxygenated. Biological structures could provide both necessities, but also require the living organism to not recognize the nanoparticle as a threat immunologically. Either precious metals or perhaps carbon would be necessary in the process to create and sustain such a creation, both of which process differently in an organic body, but conduct electrical frequency at sustainable and replicatible rates, but both conduct electrical impulses differently. Highly toxic metals store within organic bodies readily, stay long term, but are often fatal over time or are in such rare quantity upon the planet they would likely not be a viable source for mass production, leading me to believe it would need to be carbon, silicate or aluminum based to be viable. The body readily recycles cells and adds new carbon as well as other trace elements, but would likely ditch all unnecessary processes quickly. It would definitely need to be at a RNA or DNA level to sustain itself. Maybe this is why the push for MRNA technology? Far beyond what I have studied, but again you always have great rabbit holes to study anon. 😁