Next, they put a thin layer of the hydrogel, just 100 microns, on both sides of a piece of elastic adhesive mounting tape.
Essentially what they created was a three-layer "sandwich," with the tape serving as an insulating material sandwiched between the two conducting layers of hydrogel. Next, the researchers attached a copper electrode to each end of the sandwich.
When they ran a current through the electrodes, the sheet expanded and contracted, depending on how much voltage was applied.
This is just the way muscles work; an electrical signal from the nervous system goes to a muscle and causes it to contract or expand. And it's also how speakers make sound. In both cases, current is causing the material to change shape.
In one experiment the scientists attached the other end of the electrodes to a music player and added a current. The rubber sheet vibrated, just like a speaker diaphragm.
If the polymer sheet was squeezed or stretched, either by pinching it or when it vibrated in response to sound, it also generated a small current -- just like some types of condenser or ribbon microphones do.
Suo suggested that the transparent sheet could work as an active noise-cancelling layer on windows. The vibrations from loud noises would make the hydrogel generate an electric current, which could be be used to produce another signal to cancel out the sound.
"We're all, evidently, excited about the possibilities," Rogers said.