A new gadget pen created by researchers at the University of Illinois at Urbana-Champaign creates functional electronic circuits on surfaces such as paper and wood . Using liquid metal ink made of silver nanoparticles that only dries when applied to a surface, the pen is meant to be a fast, easy and portable way to create simple circuits in any shape the user can draw, circuits that could be placed on surfaces of virtually any shape.
http://www.popsci.com/technology/article/2011-06/silver-ballpoint-pen-draws-functional-electronic-circuits-paper