I appreciate all the replies but they all seem to involve making circuit connections by applying heat. If you have to reflow the whole assembly, then you've lost the big advantage they pitch from being able to drop traces on delicate materials without significantly heating them. e.g. If you want to print a circuit on a living bone, you can't reflow that whole creature who the bone belongs to, without killing it.
I'm not clear if their technique works for the junction of one metal trace onto something like a metal pin of an IC? Would heat conduct through to the rest of the pin, potentially damaging the surrounding tissue or whatever it's mounted atop? Did they study heating up highly conductive junctions and how well they can control how far the heat travels while still obtaining a good "weld"? I guess the idea might be they sort of "spot weld" the trace to the pin very quickly with high heat? I don't know, maybe instead of one "weld" they could do clusters of several very small ones at staggered time intervals to limit the maximum heat absorbed by surrounding material?
That's without even getting into adhesives, etc. to hold the components in place against those delicate materials before (and potentially after, depending on trace strength) they are soldered.
They say they built a circuit, but from what I can tell from my brief skim all they really did was lay down traces on exotic substrates, with the real circuitry connected to it but mounted somewhere else. I'm genuinely eager for someone to point out a counterexample.
Thanks!