NYC Resistor’s Nick Vermeer is working on conductive ink:

I’ve recently had success in making a conductive ink using a fine copper powder suspended in an acrylic airbrush medium. This paper on conductive epoxies was really the key to getting this ink working.

The paper shows that etching the metal filler slightly before mixing it with the binder improves the conductivity of the ink. In this test I first used ammonium persulfate as the initial etching solution. After decanting off the resulting copper sulfate solution, the powder was then washed with deionized water. The wet powder was then mixed with an acrylic airbrush medium to make the resulting ink.

There is still quite a bit of experimentation to be done, but this is a very encouraging result!