Summary: water + copper particles + room-temperature liquid metal (consisting of indium and gallium) = highly conductive gel with interesting properties.

Drying it slowly to evaporate the water allows simply getting conductive traces. Drying it fast allows printing objects that transform their shape when heated.

Commentary from me: indium and gallium are expensive metals. This is promising stuff, but not promising enough to go replicating at once. For most use cases, cables, soldering and PCBs are still the better option.