Conductive ink

Conductive ink is an ink that results in a printed object which conducts electricity. It is typically created by infusing graphite or other conductive materials into ink. 
Conductive inks can be a more economical way to lay down a conductive trace when compared to traditional industrial standards such as etching copper from copper plated substrates to form the same conductive traces on relevant substrates, as printing is a purely additive process producing little to no waste stream which then has to be recovered or treated. 
Silver inks have multiple uses today including printing RFID tags as used in modern transit tickets, they can be used to improvise or repair circuits on printed circuit boards. Computer keyboards contain membranes with printed circuits that sense when a key is pressed. Windshield defrosters consisting of resistive traces applied to the glass are also printed. Many newer cars have conductive traces printed on a rear window, serving as the radio antenna.
Printed paper and plastic sheets have problematic characteristics, primarily high resistance and lack of rigidity. The resistances are too high for the majority of circuit board work, and the non-rigid nature of the materials permits undesirable forces to be exerted on component connections, causing reliability problems. Consequently such materials are only used in a restricted range of applications, usually where the flexibility is important and no parts are mounted on the sheet. Conductive ink was once thought to be used in creative t-shirt marketing design. In this instance, sound controllers attached to the clothing could be activated depending on where a user pressed the shirt. Ultimately, this use never reached mainstream popularity.