A flexible, transparent electrical device that could give us new a type of implantable power source for health monitors, medication dispensers, and even augmented-reality contact lenses takes inspiration from the electrical eel. The technology uses soft cells made from a hydrogel and sodium chloride to form what could be considered a biocompatible artificial electric organ. It can generate 100 volts but at low current, sufficient and safe for a cardiac pacemaker, for instance.

Michael Mayer of the University of Fribourg, Switzerland, and his colleagues believe their approach might be developed for bother wearable devices and implants, offering no toxicity problems and none of the bulk of conventional batteries. Ultimately, it might be adapted to build bioelectric systems that generate electricity from metabolic or other processes in the body. So, where does the eel slip into the picture?

"The eel polarizes and depolarizes thousands of cells instantaneously to put out these high voltages," explains team member Max Shtein. "It's a fascinating system to look at from an engineering perspective - its performance metrics, its fundamental building blocks and how to use them." The researchers knew that one secret to the eel's success is a phenomenon known as transmembrane transport. Specialized electrical organs contain thousands of alternating compartments, each with an excess of either potassium or sodium ions. These compartments are kept apart by selective membranes. In the resting state, the ions are kept separate, but when it needs to generate a voltage, the membranes allow the ions to flow together creating a burst of power.

The team used sodium and chloride ions instead of sodium and potassium in their 3D printed aqueous hydrogel droplets and produced thousands of tiny cells on a polymer substrate. The alternating droplets mimic the eel's electrical compartments and a charge selective hydrogel acts as the separating membranes. To generate power, the two sheets are pressed together, connecting saline and freshwater droplets across the charge-selective droplets in series. As the salty and fresh solutions mix, the charge-selective droplets move the sodium and chloride ions in opposite directions, producing an electric current.  [Schroeder et al., Nature, (2017) 552(7684), 214; DOI: 10.1038/nature24670]

An additional trick inspired by the electric eel involved finding a way for ion shuffling to occur almost instantaneously as it does in the eel. This was instigated using an origami technique called a Miura fold, which has been used to fold solar panels for satellite launch into space. The team turned this on its head alternating droplet types in a precise pattern on a flat sheet that had been laser-scored in a Miura pattern. When pressure was applied, the sheet quickly folded together, stacking the cells in exactly the right positions.

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase.