So there's a reason we aren't doing this... Right?
TLDR: If we can vapor-deposit parlyene, gold, and photoresist we should be able to make extremely thin flexible implantable electronic devices.
So rather than creating PCBs for all our implantable electronics needs, why not start off with the biocompatible coating as the substrate? In the paper I posted below the authors created an electrode array by using a 2-micron sheet of parlyene as the substrate (which I assume was vacuum deposited), onto which the gold circuit was additively photolithographically patterned (photoresist deposition -> gold deposition -> exposure). The main thrust of the paper was the use of organic transistors to record brain activity, so I'm not sure whether it would be difficult to integrate traditional CMOS components (microcontrollers, LEDs, other SMD components) onto a gold circuit on parlyene, seeing as how I've got no clue how to deal with making PEDOT::PSS semiconductor components much less the board itself. But at least making the circuit board doesn't sound impossible. I'm guessing the hardest part involves the vacuum deposition of each of the materials. @chironex on a scale of 1 to difficult how difficult would it be?
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Possibly even using something like conductive thread could work, if only as a prototype.
I know I've looked quite a bit into flexible/diffuse conductors for tattoos and it's quite the puzzle...
What comes to mind for me is Pyralux. It's 9 bucks for a 6in square of it on Adafruit. That's a brand name of course, but apparently you can etch it with ferric chloride just like a normal pcb.
Do you think that it would be a issue even if the whole thing is encased in silicon? So me it seems like it would have no ware to go. I will look into Pyralux some more maybe see about a cheaper (non branded) substitute.
You can get really really tiny SMD components, like you could inhale them and not even realize it. If you could secure those to a flexible pcb using some sort of flexible and conductive glue, then there would be less mechanical stress on the circuit.
That way we wouldn't have to redevelop the components themselves.
There's not much reason to get insanely tiny unless you have a really complex circuit; the effort isn't worth it when you have a large battery and other irreducible constraints like coating thickness. It depends on how small you're trying to get, but I would say that in nearly all cases going smaller would be better achieved by using a thinner PCB fab, optimizing placement and design before shrinking components.