Ionic conductors could replace certain electronic systems and offer several advantages.
1. ionic conductors can be stretched to many times their normal area without an increase in resistivity — a problem common in stretchable electronic devices.
2. They can be transparent, making them well-suited for optical applications.
3. 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.
Science – Stretchable, Transparent, Ionic Conductors
“The big vision is soft machines,” said co-lead author Christoph Keplinger, who worked on the project as a postdoctoral fellow at SEAS and in the Department of Chemistry and Chemical Biology. “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. We’re really approaching the type of soft machine that biology has to offer.”
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.
“It must seem counterintuitive to many people, that ionic conductors could be used in a system that requires very fast actuation, like our speaker,” said Sun. “Yet by exploiting the rubber layer as an insulator, we’re able to control the voltage at the interfaces where the gel connects to the electrodes, so we don’t have to worry about unwanted chemical reactions. The input signal is an alternating current, and we use the rubber sheet as a capacitor, which blocks the flow of charge carriers through the circuit. As a result, we don’t have to continuously move the ions in one direction, which would be slow; we simply redistribute them, which we can do thousands of times per second.”
The researchers plan to work with companies in a range of product categories, including tablet computing, smartphones, wearable electronics, consumer audio devices, and adaptive optics.
“With wearable computing devices becoming a reality, you could imagine eventually having a pair of glasses that toggles between wide-angle, telephoto, or reading modes based on voice commands or gestures,” suggested Liss.
For now, there is much more engineering and chemistry work to be done. The Harvard team chose to make its audio speaker out of very simple materials — the electrolyte is a polyacrylamide gel swollen with salt water — but they emphasize that an entire class of ionically conductive materials is available for experimentation. Future work will focus on identifying the best combinations of materials for compatibility, long life, and adhesion between the layers.
ABSTRACT
Existing stretchable, transparent conductors are mostly electronic conductors. They limit the performance of interconnects, sensors, and actuators as components of stretchable electronics and soft machines. We describe a class of devices enabled by ionic conductors that are highly stretchable, fully transparent to light of all colors, and capable of operation at frequencies beyond 10 kilohertz and voltages above 10 kilovolts. We demonstrate a transparent actuator that can generate large strains and a transparent loudspeaker that produces sound over the entire audible range. The electromechanical transduction is achieved without electrochemical reaction. The ionic conductors have higher resistivity than many electronic conductors; however, when large stretchability and high transmittance are required, the ionic conductors have lower sheet resistance than all existing electronic conductors.
Science – Development of actuator technologies with capabilities that can match or exceed those found in biology represents a topic of long-standing interest within the advanced robotics community. One promising and remarkably simple class of such an “artificial muscle” exploits a dielectric elastomer (an electrical insulator) sandwiched between a pair of mechanically compliant electrodes
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