What is a Transparent Ionic Conduction Speaker?


A transparent ionic conduction speaker is a relatively new form of technology. In 2013, a research team from Harvard University introduced transparent ionic conduction speakers to the public. Jeong-Yun Sun and Christoph Keplinger have pioneered a never before seen application of ionic conductivity by creating a see-through artificial muscle that can produce sounds spanning the entire audible spectrum.




It consists of a thin sheet of rubber sandwiched between two layers of a saltwater gel. It’s as clear as a window. A high-voltage signal that runs across the surfaces and through the layers forces the rubber to rapidly contract and vibrate. It then produces sounds that span the entire audible spectrum, 20 hertz to 20 kilohertz. But this is not an electronic device. Published in the August 30 issue of Science, it represents the first demonstration that electrical charges carried by ions, rather than electrons, can be put to meaningful use in fast-moving, high-voltage devices.


Ionic conductors


Ionic conductors could replace certain electronic systems; they even offer several advantages. For example, you can stretch ionic conductors many times their normal area without an increase in resistivity—a problem common in stretchable electronic devices. Secondly, they can be transparent, making them well suited for optical applications. Thirdly, the gels used as electrolytes are biocompatible. So, it would be relatively easy to incorporate ionic devices—such as artificial muscles or skin—into biological systems. After all, signals carried by charged ions are the electricity of the human body, allowing neurons to share knowledge and spurring the heart to beat. Bioengineers would dearly love to mesh artificial organs and limbs with that system.

The transparent ionic conduction speaker is suitable for robotics, mobile computing, and adaptive optics fields. Engineered ionic systems can achieve a lot of functions that our body has. They can sense, they can conduct a signal, and they can actuate movement. It can be said that with this technology science is approaching the type of soft machine that biology has to offer. The audio speaker represents a robust proof of concept for ionic conductors. This is 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). The two criteria are important for applications but which would have ruled out the use of ionic conductors in the past.


The downsides


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.


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