Ionogels Could Conduct And Stretch In Soft Machines

Some engineers want to ditch hard metals and plastics and instead build machines with soft, shape-shifting materials to make wearable sensors or robots that can deform to squeeze through tight spaces. But to power these machines they need conducting materials that can bend and stretch along with moving parts. Hydrogels stretch and conduct electricity, but they eventually dry out and shrivel in air. Now researchers have created a gel that contains an ionic liquid instead of water—a so-called ionogel—that works as a transparent, stretchable conductor in open air (ACS Appl. Mat. Interfaces 2014, DOI: 10.1021/am501130t).

Zhigang Suo of Harvard University, Yong Mei Chen of Xi’an Jiaotong University, in China, and their colleagues made this ionogel by first mixing acrylic acid, poly(ethylene glycol) diacrylate, and α-ketoglutaric acid in the ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate. They then irradiated the mixture with ultraviolet light for two hours. This caused the polymer chains to crosslink, forming a substance that Suo says has the consistency of Jell-O.

The team tested the ionogel by making a simple actuator out of an elastic polymer that changes shape in response to an applied voltage. They stretched the polymer over a 66.5-mm-diameter plastic frame and then attached circular sheets of ionogel to the top and bottom of the polymer. Finally, the researchers attached the sheets to aluminum electrodes connected to a power source. When they applied a voltage, the ionogel stretched with the polymer. The ionogel could stretch and relax for at least 1 million cycles without failing. And it could expand to 4.6 times its original length without breaking.

Suo says he would like to test the electrical and mechanical properties of other ionic liquids in the gel. For anything worn on the body, researchers would have to demonstrate that the liquid is biocompatible. But there are thousands of ionic liquids, he says, paving the way for a wide variety of applications.