harvard's ionic gel speakers are stretchable and almost invisible

harvard’s ionic gel speakers are stretchable and almost invisible
jeong-yun sun (left) and christoph keplinger (right)
image © eliza grinnell

in a materials science laboratory at harvard’s school of engineering and applied sciences, a transparent disk connected to a laptop fills the room with music — the transparent ionic speaker consists of a thin sheet of rubber sandwiched between two layers of a saltwater gel, and it’s as clear as glass. the device isn’t electronic though, it represents the first demonstration that electrical charges carried by ions, rather than electrons, can be used in fast-moving, high-voltage products. ‘ionic conductors could replace certain electronic systems; they even offer several advantages,’ says co-engineer jeong-yun sun. the completely transparent and stretchable speaker can be expanded to many times its normal size without an increase in resistivity — a problem common in traditional stretchable electronic devices. in addition, the gels used as electrolytes are bio-compatible, which means it would be relatively easy to incorporate ionic devices — such as artificial muscles or skin — into biological systems.

‘we’d like to change people’s attitudes about where ionics can be used,’ says christoph keplinger, co-engineer. ‘our system doesn’t need a lot of power, and you can integrate it anywhere you would need a soft, transparent layer that deforms in response to electrical stimuli—for example, on the screen of a tv, laptop, or smartphone to generate sound or provide localized haptic feedback—and people are even thinking about smart windows. you could potentially place this speaker on a window and achieve active noise cancellation, with complete silence inside.’

transparent loudspeaker, capable of producing sound across the entire audible range
image courtesy christoph keplinger, jeong-yun sun, whitesides and suo research groups, harvard university

the audio speaker represents proof of concept for ionic conductors because producing sounds across the entire audible spectrum requires both high voltage (to squeeze hard on the rubber layer) and high-speed actuation (to vibrate quickly) — two criteria that are important for applications but that would have ruled out the use of ionic conductors in the past. the traditional constraints are well known: high voltages can set off electrochemical reactions in ionic materials, producing gases and burning up the materials. Ions are also much larger and heavier than electrons, so physically moving them through a circuit is typically slow. the system invented at harvard overcomes both of these problems, opening up a vast number of potential applications including not just biomedical devices, but also fast-moving robotics and adaptive optics.

transparent loudspeaker, capable of producing sound across the entire audible range
image courtesy christoph keplinger, jeong-yun sun, whitesides and suo research groups, harvard university

a membrane of transparent, insulating rubber is sandwiched between two layers of transparent, conductive gel
image courtesy christoph keplinger, jeong-yun sun