New tech turns your arm into a smartwatch trackpad

Researchers at North Carolina State University have developed technology that turns a smartwatch user’s forearm into a trackpad, allowing the user to scroll, zoom, click on icons, and so on. The system relies on a radar system that tracks the movement of a user’s fingers on the forearm and monitors pressure on the skin of the forearm, providing the same level of force-touch detail available on laptop trackpads.

“Smartwatch screens are too small for the touch interactions people use every day on phones and laptops, such as two-finger scrolling, pinch-to-zoom, or pressing harder to trigger a different command,” says Chenhan Xu, corresponding author of a paper on the work and an assistant professor of computer science at NC State. “Our goal was to develop a small, wearable technology that would provide that level of interactive control for users.”

To that end, the researchers developed technology called EpiPad, which turns the skin of the forearm next to the watch into a trackpad.

“With the EpiPad, a compact millimeter-wave radar worn on a wristband reads the tiny dents fingertips make as they press the skin, so the system tracks multiple fingers and how hard each one presses,” says Yuliang Fu, first author of the paper and a Ph.D. student at NC State. “We want users to interact with their smartwatch in a way that feels familiar, and EpiPad gives people the input expressiveness they would have when using a MacBook trackpad.”

When a user first puts on the EpiPad wristband, they conduct a short series of standard actions that allows the technology to adapt to the new user. Altogether, this calibration process takes less than four minutes.

The existing prototype resembles a wristband with a circuit board sticking out of it. However, the technology could be incorporated into conventional smartwatch devices, with a less obtrusive profile. The actual sensor array that allows EpiPad to monitor motion and force-touch is approximately 5 millimeters by 4 millimeters.

For proof-of-concept testing, the researchers recruited 20 study participants and conducted a range of experiments aimed at evaluating EpiPad’s accuracy and ease of use.

In terms of accuracy, the researchers assessed on-screen tracking accuracy, input gesture recognition, and force-touch. EpiPad performed well across all three categories.

“Perhaps the more intuitive evaluation was when we asked study participants to perform a set of real-time smartwatch input tasks using three methods: a traditional touch screen, a tilt cursor, and the EpiPad,” says Fu. “EpiPad outperformed the other two tasks with regard to both error rate and efficiency.”

“This approach could let future smartwatches, and screenless wearables such as AI glasses, use the wearer’s own skin as a full touch surface,” says Xu. “The functional components of the EpiPad, such as the radar sensor array, are relatively inexpensive – and even that cost would go down if these components were incorporated into wearable devices at scale. We’re talking about less than $10.

“We are open to working with industry partners to explore how this technology could be incorporated into next-generation devices,” Xu says. “Given the trend of creating smaller and smaller wearable electronics, having an easy and reliable way for users to interact with those devices may be a particularly attractive feature.”

The paper, “EpiPad: Enabling Multi-Finger and Force-Sensitive Skin Interface for Smartwatches,” was presented at the UbiComp/ISWC conference, which was held October 13-15 in Shanghai. The paper was co-authored by Rakshita Ranganath, a Ph.D. student at NC State; and Huining Li, an assistant professor of computer science at NC State.