According to a team of researchers from KAIST and MIT's Media Lab, "On-body electronics promises a seamless integration of computers and sensors with our bodies.” Normally such implementations involve constructing a sort of electronic sticker that is then applied to the body; however, the engineers have come up with a machine actually capable of printing conductive ink on an insulating surface that's already applied to a person's skin.
The aptly named BodyPrinter takes the form of a CNC ink deposition system, made small enough to strap onto someone's fingers, arms, back, belly, forehead, neck, laps, and shoulders. The device is calibrated by moving the dispensing head to different body positions, and can then travel along the x, y, and z axes to follow body curvature to extrude conductive ink via a syringe.
Everything is controlled by an Arduino Uno with a CNC shield. Circuits can be designed in a CAD package like Autodesk Fusion 360, and the BodyPrinter's user interface also features some drawing functionality.
Once applied, printed materials can be used for a wide variety of tasks, and experiments were undertaken involving flex sensors, LED outputs, and more. This kind of application technique alleviates some of the calibration and design issues inherent in “traditional” 2D sticker application. A very quick explanation of how the system works is seen in the video above.