We examine implanted user interfaces of very small mobile devices implanted at fixed locations underneath the human skin. Such devices stay always with the user, making user interfaces available at all times. We discuss four core challenges that implanted user interfaces face: providing input, producing output, communicating, and remaining powered. We investigate these four areas in a technical evaluation where we surgically implant study devices into a cadaveric specimen arm and find that traditional interfaces do work through skin. We then deploy a prototype device on participants and cover it using artificial skin in order to simulate implantation. We close with a discussion of medical consideration of implanted user interfaces, risks and limitations, and project into the future.This was a joint research project done in collaboration with a PhD student from Hasso Plattner Institute, University of Potsdam, Germany, and an Anatomy Professor at the University of Toronto. The evaluation and its associated procedures were carried out at the Department of Anatomy at the University of Toronto. The procedure of the study underwent ethics review and received approval from the Office of Research Ethics, University of Toronto.