Cambridge Researchers Build Stretchable 'Jelly Batteries' Inspired by Electric Eels for Wearables and Implants

On Wednesday, July 17, Cambridge University announced that its scientists had built a soft, jelly-like battery modeled on the electric eel, a material meant for wearable devices and soft robotics and, potentially, for implantation in the brain.

The design borrows from the eel's electrocytes, the modified muscle cells the animal uses to stun its prey. The researchers imitated them with layered, stretchy, sticky materials that the university's news release likened to Lego, and that can deliver an electric current. The work was published in Science Advances, a peer-reviewed U.S. journal.

The headline result is a pairing of two properties that usually work against each other. According to the research, the self-healing batteries can stretch tenfold without any loss of conductivity, the first time a single material has combined the two. First author Stephen O'Neill, of Cambridge's Yusuf Hamied Department of Chemistry, explained the difficulty: a material that is highly stretchable and highly conductive is hard to design, because conductivity typically falls as a material is stretched.

The batteries are built from three-dimensional polymer networks known as hydrogels, which are more than 60% water. On-off interactions hold them together and precisely control their mechanical properties, so that they mimic the characteristics of human tissue. That makes hydrogels a natural fit for soft robotics and bioelectronics, as long as they can also conduct electricity and flex lengthways.

Co-author Dr. Jade McCune, who works in the same lab as O'Neill, said the team got past the problem by giving a charge to polymers that are normally neutral, which makes them conductive. Changing the salt component of each gel makes it sticky, she said, so the gels can be pressed together in multiple layers to build a larger energy potential. Unlike conventional electronics, which use rigid metallic materials with electrons as the charge carriers, the jelly batteries carry charge with ions, as eels do. The layers cling to one another through reversible bonds formed by barrel-shaped molecules called cucurbiturils, which McCune compared to molecular handcuffs. Those bonds let the batteries stretch without the layers separating and, critically, without losing conductivity.

The team believes the softness of the material and its ability to mold to human tissue make it promising for biomedical implants. Research lead Professor Oren Scherman, director of Cambridge's Melville Laboratory for Polymer Synthesis, said the mechanical properties of the hydrogels can be customized to match human tissue. Because they contain no rigid parts such as metal, he said, a hydrogel implant would be much less likely to be rejected by the body or to cause a build-up of scar tissue. He added that the polymers are durable: they can be squashed without permanently losing their shape, and they can repair themselves after damage.

The implant uses described so far are possibilities rather than demonstrations. The source mentions delivering drugs in the brain and controlling epilepsy and other neurological conditions as potential applications.

The next step is to test the hydrogels' suitability for a range of medical uses in experiments with live organisms. The project was funded by the European Research Council and the Engineering and Physical Sciences Research Council, part of UK Research and Innovation.