The Digital Tattoo Interface is a 2" x 4" conceptual device created by engineer Jim Mielke for the Greener Gadgets Design Competition in 2008 . The device is a subdermal implant that is inserted under the skin and acts as a display for a tatto and also as a bluetooth interface device .
How It works:
The implant is inserted into the arm through a small incision, at this point the silicon and silicone device is tightly rolled into a tube. From here it unfolds to lie beneath the skin just above the muscle . After it is inserted, tubes from the device are attached to a vein and an artery to allow blood to flow into and out of a small battery on the back of the device, the battery itself is powered by converting the components found in blood into electricity, namely glucose (sugar) and oxygen, the same way that the rest of your body is powered .
The Tattoo is not just a display but the Bluetooth aspect of it would allow for it to interect with your other devices such as your cell phone. In the picture above you can see a woman using it to interact with her celphone. A touch display appears on her arm allowing her to enter a phone number. As well a display appears beside it showing a visual display of the person she is calling. After he call is done the display returns to the tattoo state.
While technology like this invariably raises concerns about the safety of mixing technology with the human body. One aspect though can be placed directly into the pro column, the device would actually be able to monitor the wearers blood to warn them about disorders and possible problems such as blood born pathogens.
As of the writing of this (November 2011) there is no new news regarding the Digital tattoo interface. The original concept design was unveiled at the Greener Gadgets Design Competition and since then there has been no new information. The original design was created of silicone and silicon and utilized a black ink display, with the current advances in Organic LED technology it would be interesting to see what sort of display capabilities a modern iteration of this technology could do.
 Lisa Zyga, "Electronic Tattoo Display runs on Blood", Physorg.com. [Online] Available: http://www.physorg.com/news122819670.html [Accessed Nov 27th 2011].
 Geekologie, "Digital Tattoo is Conceptual, Questionable - Geekologie", Geekologie.com. [Online] Available http://www.geekologie.com/2008/02/digital_tattoo_is_conceptual_q.php [Accessed Nov 27th 2011]
 Rosa Golijan, "Digital Tattoo Interface Turns your Skin Into A Display", Gizmodo.com. [Online] Available: http://gizmodo.com/digitaltattoointerface/ [Accessed Nov 27th 2011].
Rosa Golijan, "Digital Tattoo Interface Turns your Skin Into A Display", Gizmodo.com. [Online] Available: http://gizmodo.com/digitaltattoointerface/ [Accessed Nov 27th 2011].