Printed Circuit Boards (PCB's) are usually fabricated on FR4 (a glass reinforced epoxy). You can now print your prototype PCB layout out on paper. Conductive Ink printing isn't a new technology, but introducing the concept using a low cost ink jet printer is relatively new. A team from The University of Tokyo, Microsoft Research Cambridge, and Georgia Tech published a paper in 2013 outlining this process. The process is particularly useful in large area sensors and high frequency applications such as RFID and antennas. These types of printable inks may include materials like silver and carbon in powder or flake form. The inks are usually classified as either Fired High Solid (FHS) systems or the more common Polymer Thick Film (PTF) systems and can be deposited on different types of mediums. Circuits printed on paper usually have components that are placed and held with conductive tape or glue. The ink can be soldered to, but not very reliably at this time. The printed circuits have a good amount of flexibility and can be bent or folded to fit different design constraints. Several companies exist today that sell the ink including Methode and AgiC. If you don't have a printer that supports these types of cartridges, AgiC also sells conductive ink pens and even erasers. For you do-it-yourselfers, there is an article from IEEE Spectrum that outlines how to make these conductive inks and pens. -NS