The December 2022 issue of IEEE Spectrum is here!

Close bar

Stretchable Touch Pad Could Become Wearable Touch Screen

The biocompatible touch pad could one day be implanted into skin

2 min read
Playing a virtual piano using a stretchable touch pad on a person's forearm.
Photo: Kim et al. Science (2016)

Video: Kim et al. Science (2106)

A new, stretchable transparent touch pad can be used to write words and play electronic games, and it may even one day be implanted inside the body, its inventors say.

Touch pads and touch screens are on nearly every smart device these days. But they can’t go on anything flexible, such as the human body. Scientists have explored stretchable touch panels based on carbon nanotubes, metal nanowires, and other advanced materials, but the performance of these stretchable touch panels fell off sharply when they were stretched. Just as bad, they also fell apart over time when repeatedly stretched.

To overcome these problems, scientists at Seoul National University created a touch pad made of the same kind of soft and very stretchable hydrogel used to make soft contact lenses. The hydrogel involved contains lithium chloride salts, which are electrically conductive and help the hydrogel hold onto the water it needs to stay soft.

In operation, the same small AC voltage is applied to all four corners of the touch pad. Because the signals are in phase and the hydrogel acts as a capacitor, no current flows until the device is touched. When a grounded object such as a human finger makes contact with the touch pad, it closes the circuit, and current flows from each corner in proportion to how close the finger is to that corner. Sensors then measure the difference in current that each corner of the pad receives to determine the finger’s position.

Using the touch pad, the researchers were able to draw a stick figure, write “Hello world!” and play the piano and chess. Moreover, the touch pad still worked even when stretched to more than 10 times its normal area. It also worked when bent around the skin of a person’s arm. 

The touch pad is transparent, letting through 98 percent of visible light. This suggests that it could be combined with a stretchable video display to create a stretchy touch screen.

The scientists noted that the screen is biocompatible. 
“If you want to interface electronics with biology, you ideally want soft, stretchable electronics with the same kinds of properties seen in biology,” says John Rogers, a materials scientist at the University of Illinois at Urbana-Champaign, who did not take part in this research.”The researchers have demonstrated a nice advance that adds to the growing tool kit of stretchable electronics for interactions with the body. Skin would be a great starting point for such an electronic interface, since implanting there would probably pose minimal health impacts.”

The research team did note that after 100 cycles of stretching and relaxation, touch pad performance did decrease slightly. They suggest this is because water evaporated from the hydrogel. “Addressing that drying-out process would be an area for future progress,” Rogers says.

The scientists detailed their findings this week in the journal Science.

Video: Kim et al. Science (2106)
The Conversation (0)

Deep Learning Could Bring the Concert Experience Home

The century-old quest for truly realistic sound production is finally paying off

12 min read
Image containing multiple aspects such as instruments and left and right open hands.
Stuart Bradford

Now that recorded sound has become ubiquitous, we hardly think about it. From our smartphones, smart speakers, TVs, radios, disc players, and car sound systems, it’s an enduring and enjoyable presence in our lives. In 2017, a survey by the polling firm Nielsen suggested that some 90 percent of the U.S. population listens to music regularly and that, on average, they do so 32 hours per week.

Behind this free-flowing pleasure are enormous industries applying technology to the long-standing goal of reproducing sound with the greatest possible realism. From Edison’s phonograph and the horn speakers of the 1880s, successive generations of engineers in pursuit of this ideal invented and exploited countless technologies: triode vacuum tubes, dynamic loudspeakers, magnetic phonograph cartridges, solid-state amplifier circuits in scores of different topologies, electrostatic speakers, optical discs, stereo, and surround sound. And over the past five decades, digital technologies, like audio compression and streaming, have transformed the music industry.

Keep Reading ↓Show less