Bendy Liquid Metal Coils Could Make Stretchable Loudspeakers

An illustration of the fabricated stretchable acoustic device. Credit: Korea University View full size image

Coils of liquid metal could be used to make stretchable loudspeakers and microphones, potentially leading to new kinds of hearing aids, heart monitors, and wearable and implantable devices, researchers say.

Increasingly, researchers worldwide are developing flexible electronics, such as batteries,video displays and solar panels, that could one day make their way into clothing and even human bodies. However, developing flexible devices that can record and play back sound has proved challenging, because scientists have had trouble developing audio components that can remain mechanically stable after they are stretched.

Acoustic devices often rely on rigid metal coils that can both emit and detect sound. Now, scientists in Korea have created a stretchable acoustic device by replacing this rigid coil with a deformable, liquid metal coil.
The new coil was made of Galinstan, a highly conductive liquid metal alloy of gallium, indium and tin. The researchers used a syringe to inject Galinstan into a spiral channel in a thin film of flexible silicone rubber. They then attached copper wires to the end of the coil and a neodymium magnet (made from an alloy of neodymium, iron and boron) to the center of the coil.

The liquid metal coils could be used to make stretchable acoustic devices, such as loudspeakers and microphones.
Credit: Korea University

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The scientists operated the device by electrically charging the liquid metal coil, turning it into an electromagnet that could push back and forth off the neodymium magnet to either detect or emit sound. They were able to record sounds, such as the human voice and a beeping alarm clock, and play them back while the device was attached to the wrist or was being stretched by hand.
The researchers found the device can be stretched up to 50 percent its length 2,000 times without any noticeable loss of acoustic performance. It could also play back sounds across the frequency range of human hearing.
"Our stretchable loudspeaker can give off the sound level of typical earphones,"study co-author Jeong Sook Ha, a chemical engineerat Korea University in Seoul, told Live Science.
Potential uses for this device include "body-attachable and wearable acoustic applications such as sensing biological signals, hearing aids and notification of information via sound," Ha said. For example, "our stretchable acoustic devices may be used as a wearable health-monitoring device for measuring heart rate in the future."
Further research is needed to improve the device so that it can more efficiently convert electricity into sound, the researchers said. Potential ways to improve the gadget include using a stronger magnet, reducing the thickness of the silicone rubber encasing the coil, and increasing the number of turns in the coil. The researchers would also like to replace the solid neodymium magnet with a deformable one to make the device even more stretchable.
"If a stretchable magnet with sufficiently strong magnetic fields is developed, a completely stretchable loudspeaker could be realized," Ha said.
The scientists detailed their findings online today (July 16) in the journal Scientific Reports.