How would you like a speaker system in your living room that was completely clear? Or, better yet, an active noise-canceling window that could mute the outside? The technology is already here, and a team of materials scientists at Harvard is showing us what’s now possible with ionic conduction, or using ions to carry the current that powers this new generation of electronics. The principle is embodied in the team's clear, disc-shaped speaker, details of which are published in the August 30th issue of Science.
The speaker consists of two layers of saltwater gel sandwiched around a thin rubber sheet. When a high enough voltage is passed through the gel it causes the rubber to contract and vibrate, playing the music in the video. It's actually capable of producing every frequency in the audible spectrum, from 20Hz to 20KHz.
The conductor can be stretched to several times its size
In typical conductors like copper, the atoms occupy fixed positions, passing electrons from one to another down the chain. But in ionic conductors the ions actually move from one location to another inside the structure. That means the conductor can be stretched to several times its size without an increase in resistivity, so it's still easy for current to pass through the system. That property ideally makes ionics more practical for the flexible electronics of the future. Ionic conductors aren't a new idea, but their propensity for breaking down under high voltage led many to think they were impractical. However, the Harvard team found a way to prevent unwanted chemical reactions from taking place in the system by using rubber as an insulator, controlling the voltage where the gel contacts the electrodes.
"We're really approaching the type of soft machine that biology has to offer."The gels needed to make these kinds of ionic conductors work also get along better with living organisms, making them more suited to implants like artificial muscles. Co-lead author Christoph Keplinger explains in an interview that the team’s vision for the future is "soft machines." Not only could these kinds of ionic systems be used wherever you need something transparent to move, say in haptic smartphone displays, they can already replicate a lot of our bodies’ functions, like moving and sensing, says Keplinger. "We’re really approaching the type of soft machine that biology has to offer."