It is not 3d printing but it is a sort of DIY additive manufacturing. Arduino Pro Mini board was made by using a pen plotter and the Electroninks Circuit Scribe roller-ball pen with highly conductive ink to draw a circuit on a sheet of paper. Electronic components are then mounted on it with glue or sticky tape. And it works!
This method for conductive ink printed circuits could be easily adapted for 3d printers. There are also several methods to produce DIY conductive ink with various material mixtures.
Also you shoudl keep in mind that this is made from paper, so if you put too much voltage in, it could catch fire. Stay safe.
Detailed instructions and all the files can be found here:
Conductive ink on paper sheet electronic circuit production is technology that is finding more and more practical applications, you can see interesting sensor here:
Here is one of the many methods to produce homemade conductive ink. This one is easy and cheap, maybe not the most suitable for this type of project. It is made from copper particles suspended in gum arabic water solution.