A smartphone prototype for touch interaction on the whole device surface

Previous research proposed a wide range of interaction methods and use cases based on the previously unused back side and edge of a smartphone. Common approaches to implementing Back-of-Device (BoD) interaction include attaching two smartphones back to back and building a prototype completely from scratch. Changes in the device's form factor can influence hand grip and input performance as shown in previous work. Further, the lack of an established operating system and SDK requires more effort to implement novel interaction methods. In this work, we present a smartphone prototype that runs Android and has a form factor nearly identical to an off-the-shelf smartphone. It further provides capacitive images of the hand holding the device for use cases such as grip-pattern recognition. We describe technical details and share source files so that others can re-build our prototype. We evaluated the prototype with 8 participants to demonstrate the data that can be retrieved for an exemplary grip classification.