Stretchy, see-through material conducts electricity

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A thin skin of Jell-O—like material made of salty gel and rubbery tape can work as a completely transparent loudspeaker. The new device can carry current — and it’s stretchier and more see-through than today’s best transparent electronic materials, Christoph Keplinger of Harvard University and colleagues report in the Aug. 30 Science.

The squishy sandwich could one day form the flesh of soft robots or merge with biological tissues to make artificial muscles or skin. People could even hook it up to their iPods to pump up the volume, sticking the clear membrane right on the screen.

The sandwich’s design is simple and elegant, says Muhammad Alam of Purdue University in West Lafayette, Ind. “The materials they use are familiar to everyone,” he says. The Harvard researchers’ “real innovation is how they put them together.”

Previously devised transparent electronics rely on materials such as metallic wires, carbon nanotubes and graphene sheets to conduct electricity. In recent years, researchers have used creative tricks to make electronics more flexible (SN: 11/17/12, p.18) but with limited success. And even the most transparent designs let only about 95 percent of light pass through. For devices like TV and computer screens, this just isn’t transparent enough, says Stanford engineer Zhenan Bao.

So Keplinger came up with a different idea for conducting electricity: His team used rubber tape sandwiched between gels swollen with salt water. The salt’s positive and negative ions carry current. “Instead of electrons traveling though metal wires, now you have ions traveling through solution,” Keplinger explains.

When the team applies a voltage to the sandwich, positive charges line up on one side of the rubber sheet and negative charges line up on the other. The opposite charges attract, squeezing the rubber layer like a balloon in a vise grip. By switching the voltage on and off, the researchers could force the rubber to rapidly shrink and expand. And like a traditional loudspeaker, these vibrations can generate sound.

“It’s a completely new way of doing things,” Bao says. Already, she and other scientists are thinking up ways to use the material.

The sandwich could help soft robots flex in different directions, like an octopus arm does, says physicist Herbert Shea of the Swiss Federal Institute of Technology in Lausanne. “If someone has an injured hand, you could put this fake octopus on top, and it could do all sorts of complicated motions,” he says.

And if the material is placed in the heel of a shoe, it may even be able to generate electricity as a person walks, squishing it with each step, says materials scientist Yang Yang of the University of California, Los Angeles. “That might be fun.”

Now Keplinger’s team is trying to make the moist gels less prone to evaporation. Replacing the water with molten salt could keep them from drying out.

Clear gel and rubber form a see-through loudspeaker that audibly vibrates, translating an amplified electrical signal into sound.
Credit: Courtesy of Christoph Keplinger and Jeong-Yun Sun, Whitesides and Suo Research Groups, Harvard University

A transparent loudspeaker plays music from a YouTube video, when fed audio output from a laptop,
Credit: Courtesy of Christoph Keplinger and Jeong-Yun Sun, Whitesides and Suo Research Groups, Harvard University