Liquid metal in hydrogel generates electricity with a twist

A soft, stretchable device that converts movement into electricity in dry and wet environments could have applications in marine environments, claim researchers at North Carolina State University. 

“Mechanical energy – such as the kinetic energy of wind, waves, body movement and vibrations from motors – is abundant,” said Michael Dickey, corresponding author of a paper on the work and Camille & Henry Dreyfus Professor of Chemical and Biomolecular Engineering at NC State. “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 heart of the energy harvester is a liquid metal alloy of gallium and indium that is encased in a hydrogel.

Stretchy but tough fibres have potential for robotics and textiles

The water in the hydrogel contains ions that assemble at the surface of the metal, which can induce charge in the metal. Increasing the area of the metal provides more surface to attract charge. This generates electricity, which is captured by a wire attached to the device.

“Since the device is soft, any mechanical motion can cause it to deform, including squishing, stretching and twisting,” Dickey said in a statement. “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 experiments, researchers found that deforming the device by a few millimetres generates a power density of approximately 0.5mW m-2. This amount of electricity is said to be comparable to several popular classes of energy harvesting technologies.

hydrogel
Researchers at NC State University have created a soft and stretchable device that converts movement into electricity. The device works in wet or dry environments and has a host of potential applications (Image: Veenasri Vallem)

“However, other technologies don’t work well, if at all, in wet environments,” Dickey said. “This unique feature may enable applications from biomedical settings to athletic wear to marine environments. 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 researchers have two related projects under way. One is aimed at using the technology to power wearable devices by increasing the harvester’s power output whilst the second evaluates how this technology could be used to harvest wave power from the ocean.

A paper detailing the work – Soft Variable Area Electrical Double Layer Energy Harvester – is published in Advanced Materials.