Soft, stretchy ‘jelly batteries’ inspired by electric eels

Researchers at the University of Cambridge took inspiration from electric eels, which stun their prey with modified muscle cells called electrocytes.

Like electrocytes, the gelatinous materials developed by the Cambridge researchers have a layered structure, like sticky Lego, that makes them capable of delivering an electric current.

Self-healing gel batteries can stretch to more than ten times their original length without affecting their conductivity – the first time such stretchability and conductivity have been combined in a single material. The results are published in the journal Scientific progress.

Jelly batteries are made from hydrogels: 3D networks of polymers containing more than 60% water. The polymers are held together by reversible on/off interactions that control the mechanical properties of the jelly.

The ability to precisely control mechanical properties and mimic human tissue characteristics makes hydrogels ideal candidates for soft robotics and bioelectronics; however, they must be both conductive and stretchable for such applications.

“It’s difficult to design a material that is both highly stretchable and highly conductive, because these two properties are usually incompatible,” said Stephen O’Neill, first author of the study and a member of Cambridge’s Yusuf Hamied Department of Chemistry. “In general, conductivity decreases when a material is stretched.”

“Normally, hydrogels are made of charge-neutral polymers, but if we charge them, they can become conductive,” said co-author Dr. Jade McCune, also of the Department of Chemistry. “And by changing the salt component of each gel, we can make them sticky and squish them together into multiple layers, which allows us to create a larger energy potential.”

Conventional electronics use rigid metallic materials with electrons as charge carriers, while gelatinous batteries use ions to carry the charge, like electric eels.

Hydrogels adhere strongly to each other thanks to reversible bonds that can form between the different layers, thanks to barrel-shaped molecules called cucurbiturils that are like molecular handcuffs. The strong adhesion between the layers provided by the molecular handcuffs allows the gelatin batteries to be stretched, without the layers separating and, most importantly, without any loss of conductivity.

The properties of gel batteries make them promising for future use in biomedical implants because they are flexible and conform to human tissue. “We can customize the mechanical properties of hydrogels to match human tissue,” said Professor Oren Scherman, director of the Melville Polymer Synthesis Laboratory, who led the research in collaboration with Professor George Malliaras of the Department of Engineering. “Because they do not contain any rigid components such as metal, a hydrogel implant would be much less likely to be rejected by the body or cause scar tissue to build up.”

In addition to their flexibility, hydrogels are also surprisingly strong. They can withstand crushing without permanently losing their original shape and can self-repair when damaged.

The researchers plan future experiments to test the hydrogels in living organisms to assess their suitability for a range of medical applications.

The study was funded by the European Research Council and the Engineering and Physical Sciences Research Council (EPSRC), part of UK Research and Innovation (UKRI). Oren Scherman is a Fellow of Jesus College, Cambridge.

Reference:
Stephen JK O’Neill et al. “Highly stretchable dynamic hydrogels for flexible multilayer electronics”. Science Advances (2024). DOI: 10.1126/sciadv.adn5142