Conductive Ink

http://en.wikipedia.org/wiki/Conductive_ink

http://spectrum.ieee.org/geek-life/hands-on/how-to-brew-your-own-conductive-ink

http://makezine.com/2012/02/23/conductive-ink/

http://www.bareconductive.com/

http://www.psfk.com/2013/11/ballpoint-pen-draws-circuts.html

http://www.tekra.com/content/products/conductive-inks

http://jordanbunker.com/archives/41