We envision the future of wearable devices to be implantable, where devices are no longer attached to the body, but rather, they are embedded as part of the body and provide more ambient experiences. The revolution will not only change the wearable devices in functionality, but also dramatically alter the way we perceive and interact with the world. From wearable to implantable, screen based interface is on the verge of disappearing. Future interactions will be weighted towards human language of symbols, languages and gestures, which are ultra-subtle and intuitive. However, this revolutionary technology will not be maturely developed or widely applied anytime soon. It will take a long period of refinement to bring the implantable devices into the main digital markets. This thesis contributes an interface design space that outlines the possible transitions in the input and output metaphors from wearable to implantable, as well as technical solutions and design innovations that efficiently overcome the challenges raised from the interface paradigms. Specifically, touchscreen on wearable devices faces input efficiency challenge. Designing touch input on the screen requires improving expressiveness of simple touch gestures. An alternative solution is designing intuitive hand gestures that are performed around the device. Approaches include not only adding new sensing capabilities to the devices, but also leveraging the devices themselves as on-body sensors. In spite of this, devices that are worn or embedded can be used as on-body actuators, providing haptic feedback as an alternative output channel. Finally, skin becomes more important as interface that can be turned into visual or haptic display. Upon summarizing the key feature, scenario and challenges of the interface paradigms, the thesis undertakes a broad exploration of design, implementation and evaluation for the interfaces.