WASHINGTON, Aug. 29 (Xinhua) -- U.S. researchers announced Thursday they had designed gel-based ionic conductors that are highly stretchable, fully transparent to light of all colors, offering new opportunities for designers of soft machines.
Stretchable electronics is a field of growing interest, particularly because softer, more stretchy devices would be well- suited for biomedical applications, like implants.
But existing stretchable conductors are mostly electronic conductors. Although they conduct electricity well, they perform poorly when they are significantly stretched, and they lack transparency and are not as biocompatible as ionic conductors.
Researchers from the Harvard University reported in the journal Science that they combined saltwater, which is packed with dissolved ions, with a polymer to make ionic hydrogels capable of operation at frequencies beyond 10 kilohertz and voltages above 10 kilovolts.
To demonstrate their performance, they used hydrogels to build a transparent actuator that can generate large strains and a transparent loudspeaker that produces sound over the entire audible range.
"The audio speaker represents a robust proof of concept for ionic conductors because producing sounds across the entire audible spectrum requires both high voltage and high-speed actuation (to vibrate quickly) -- two criteria which are important for applications but which would have ruled out the use of ionic conductors in the past," the researchers said in a statement.
"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," they said.
The researchers said their conductors solved both of these problems by sandwiching a thin sheet of insulating rubber between two layers of a saltwater gel, thus opening up a vast number of potential applications including not just biomedical devices, but also fast-moving robotics and adaptive optics.
"It is a surprise that we can make a high-frequency device like a loudspeaker using the ionic conductor," Professor Zhigang Suo, who led the Harvard research group, told Xinhua.
"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," said Suo.
"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."
Suo and his colleagues plan to work with companies to apply this technology 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," said Sam Liss, director of business development in Harvard's Office of Technology Development.