This post is an interesting combination between two fields of technology, digital fabrication and DIY computing/electronics.
Appropriate Audiences is an interesting group of hacker/designers who have modified a 3D printer to be able to tattoo onto both two and three dimensional surfaces (illustrated in the video above).
Working with Appropriate Audiences, Alexandre Echasseriau hopes to make a digitally fabricated version of INKO, a hand tattooed iPad keyboard/case. But the tattoo ink used for INKO isn't just any regular ink, it is a electrically conductive ink, enabling for a circuit to be impregnated into the leather in a durable and flexible manner.
The conductive ink used is a product sold by BARE Conductive, who in addition to selling the ink, offer a variety of interactive peripherals including circuit boards, LEDs and pin batteries. A video about their products can be seen below.
For further reading about this project see the original Design Milk article here.