Ever since the advent of smartwatches, technologists have been looking to expand interactions beyond the confines of the small watch face, now there is new technology developed at Carnegie Mellon University that suggests turning the entire lower arm into a touchpad.

Developed by the Human-Computer Interaction Institute’s (HCII's) Future Interfaces Group, the new SkinTrack system allows for continuous touch tracking on the hands and arms. It also can detect touches at discrete locations on the skin, enabling functionality similar to buttons or slider controls.

SkinTrack allows for continuous touch tracking on the hands and arms. Image credit: CMU.“A major problem with smartwatches and other digital jewelry is that their screens are so tiny,” says Gierad Laput, PhD student in HCII and part of the research team. “Not only is the interaction area small, but your finger actually blocks much of the screen when you’re using it. Input tends to be pretty basic, confined to a few buttons or some directional swipes.”

Previous “skin-to-screen” approaches have employed flexible overlays, interactive textiles and projector/camera combinations that can be cumbersome. With SkinTrack, the user wears a ring that produces a low-energy, high-frequency electrical signal. When the finger gets near to the skin or touches the skin, that signal propagates through the skin.

By using electrodes integrated into the watch’s strap, it is possible to pinpoint the source of those electromagnetic waves because the phases of the waves vary. For example, electrodes corresponding to the 12 o’clock and 6 o’clock positions on the watch can detect phase differences that can determine the position of the finger along the width of the arm; electrodes at the 3 o’clock and 9 o’clock positions can determine the finger’s position along the length of the arm.

The researchers found that they could determine when the finger was touching the skin with 99% accuracy and could resolve the location of the touches with a mean error of 7.6 millimeters. That compares well with other on-body finger-tracking systems and approaches touchscreen-like accuracy.

SkinTrack has been demonstrated to be useful as a game controller, for scrolling through lists on a smartwatch, for zooming in and out of onscreen maps and for drawing. A number-pad application enables users to employ the back of the hand as a dial pad for the onscreen number pad; hovering a finger over the hand acts as a cursor, highlighting numbers on the screen to aid in targeting touch points.

The system has some limitations. Keeping the ring powered up is one challenge. Another is that signals tend to change as the device is worn for long periods, owing to factors such as sweat and hydration, as well as the fact that the body is in constant motion.