An almost-invisible film of jelly can now be hooked up to a laptop to blast out your favourite tunes. The stretchy speaker, which can produce sounds that span the entire audible range, is the first to use ions in place of electronics. The technology could one day be used to build both noise-cancelling windows and music-playing smartphone screens.
The team that created the device, led by Zhigang Suo of Harvard University, took inspiration from the way electric signals are transmitted in the human body. There, the flow of charged atoms called ions – rather than the electrons that carry charge in electrical devices – is what allows neurons to share information or trigger the heart to beat.
Suo's team combined saltwater – which is packed with dissolved ions that are free to move around – with a polymer. The result was a flexible but solid substance known as a hydrogel.
The speaker was created by sandwiching a thin sheet of transparent, insulating rubber between two layers of gel, which were both connected to copper electrodes.
Ionic music
Electrical audio signals from a computer, fed to the hydrogel via the electrodes, caused the ions in both layers to flow. That physical movement then caused the insulating rubber in the middle to vibrate at specific frequencies, producing different sounds.
The team foresees many potential applications. One is as a coating for TV, laptop or smartphone screens that doubles up as a speaker. It might also be used to create soundproof windows, using the film to generate noise-cancelling vibrations.
In contrast with other bendy electronics, the material can be stretched more than five times its length without an increase in resistivity, a property that might allow other applications to be dreamed up.
Hydrogels are inexpensive and easy to make but the problem is they don't last long. They dry out as their water evaporates. In order to develop longer-lasting devices, Suo is seeking alternative, flexible ionic conductors that do not readily evaporate. "The current work is really a proof of concept," he says.
Journal reference: Science 10.1126/science.1240228
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