Harvard University scientists have built a clear, artificial “muscle” in the laboratory and controlled its rapid vibrations with such precision that it can be turned into a sophisticated speaker, playing the delicate flute melody of Edvard Grieg’s “Peer Gynt.”
Although the demonstration of a clear gel projecting classical music piped from YouTube is striking, the researchers were initially motivated by a broader engineering problem: the fundamental difference between our gadgets and our bodies.
“The whole project started with this mismatch in mechanics between living organisms, which are soft, and our electronics world, which is hard and stiff,” said Christoph Keplinger, a post-doctoral researcher at Harvard and one of the scientists who built the muscle, described in a paper published in the journal Science on Thursday.
As the field of robotics has advanced, the rigid, opaque metals used to build electronic circuits has become more of a limitation. To build lifelike devices or sensors that can be implanted in the body, scientists will depend on a new generation of stretchable electronics, with far different properties than the chips found in today’s cellphones or laptops.
Keplinger and colleagues set out to build a conductor that, unlike a stiff metal wire that carries a current of electrons, could conduct electricity the way it happens in the body, using ions—charged atoms or molecules.
That meant they would have to overcome deep-seated biases held by scientists. Already used in batteries, ionic conductors come with challenges. They tend to be slow, thus limiting their applications. And as chemistry students who have stuck electrodes into a beaker of salt water know, applying a high voltage can cause hydrogen and chlorine gas form at the electrodes—the kind of chemical reaction that would be unwelcome in a device.
The researchers overcame those hurdles by inserting a sliver of clear rubber between the electrodes, building a circuit in such a way that they could stop problematic chemical reactions from occurring. They attached the electrodes to a clear gel soaked in salt water. When voltage is applied to the gels, they squeeze the rubber. By controlling the voltage, scientists could thus control how much and how rapidly the rubber was squeezed. And they could control it at frequencies high enough to create audible vibrations—sound.
In an accompanying commentary, John A. Rogers, a professor of materials science and engineering at the University of Illinois at Urbana-Champaign, called the work “promising and remarkably simple,” with potential applications for surgical tools, sensors that can wrap around curvy, soft structures in the body, and implants.
Keplinger also sees potential use in speakers that could be integrated into the screens of smartphones, or, used to control the optical properties of lenses.
Asked why his group chose a familiar piece of classical music to demonstrate the ability of its invention, Keplinger said the team thought long and hard about what piece of music to use.
“My intention behind this was to choose something that’s kind of timeless,” avoiding songs whose popularity might be more evanescent, Keplinger said. That’s because he hopes the work will be the start of a long-lasting shift in how people think about flexible electronics.
“This first paper is a proof of principle that could change the attitude of people toward ionic conductors,” Keplinger said, helping them “realize they have really unique properties that could be exploited.”