Soft Squishy Hydrogel with a Liquid Metal Heart Proves Its Worth as a Mechanical Energy Harvester

Scientists at North Carolina State University and Sungkyunkwan University in South Korea have developed a soft and stretchable device for turning motion into electricity — and, unlike rival designs, it works fine underwater.

"Mechanical energy — such as the kinetic energy of wind, waves, body movement, and vibrations from motors — is abundant," says Michael Dickey, NC State professor and corresponding author of the paper on the work. "We have created a device that can turn this type of mechanical motion into electricity. And one of its remarkable attributes is that it works perfectly well underwater."

The prototype gadget the team has developed doesn't look much like a generator: It's a soft and bendy piece of hydrogel swollen with water containing dissolved salts, inside of which is a liquid metal alloy made of gallium and indium. As the device twists and deforms, the contact between the metal and the hydrogel shifts — and generates electricity.

"Since the device is soft, any mechanical motion can cause it to deform, including squishing, stretching and twisting," says Dickey. "This makes it versatile for harvesting mechanical energy. For example, the hydrogel is elastic enough to be stretched to five times its original length."

In testing, the team found that even slight deformations of the device could generate power at a density of 0.5 mW m⁻² — comparable to rival energy-harvesting technologies which, unlike the hydrogel device, can't work underwater.

"This unique feature may enable applications from biomedical settings to athletic wear to marine environments," Dickey claims. "Plus, the device is simple to make. There is a path to increase the power, so we consider the work we described here a proof-of-concept demonstration."

The team's paper has been published under open-access terms in the journal Advanced Materials, while the researchers are now working on increasing the generator's power output and designing it into a system for harvesting wave power from the ocean.