An electric-eel-inspired soft power source from stacked hydrogels

Miniature hydrogel compartments in scalable stacked and folded geometries were used to prepare a contact-activated artificial electric organ. The electric eel can generate electrical discharges of 100 watts to stun prey, but should you X-ray an eel, you wouldn't find a battery pack inside. Instead, thousands of cells called electrocytes are arranged along its body, each producing a small ion gradient and therefore a potential difference across them. Now, Michael Mayer and colleagues have developed a hydrogel-based system that mimics the electrocyte mechanism and could be used as a soft power source for robotics. They arrange sets of ion-selective hydrogels in series to generate ion gradients across a group of four hydrogel droplets. These droplets can either be arranged in series in a microfluidic set-up, or be stacked in parallel by folding up an array of hydrogels using origami principles. The net result is a power source that is able to generate voltages similar to those generated by the electric eel. Progress towards the integration of technology into living organisms requires electrical power sources that are biocompatible, mechanically flexible, and able to harness the chemical energy available inside biological systems. Conventional batteries were not designed with these criteria in mind. The electric organ of the knifefish Electrophorus electricus (commonly known as the electric eel) is, however, an example of an electrical power source that operates within biological constraints while featuring power characteristics that include peak potential differences of 600 volts and currents of 1 ampere1,2. Here we introduce an electric-eel-inspired power concept that uses gradients of ions between miniature polyacrylamide hydrogel compartments bounded by a repeating sequence of cation- and anion-selective hydrogel membranes. The system uses a scalable stacking or folding geometry that generates 110 volts at open circuit or 27 milliwatts per square metre per gel cell upon simultaneous, self-registered mechanical contact activation of thousands of gel compartments in series while circumventing power dissipation before contact. Unlike typical batteries, these systems are soft, flexible, transparent, and potentially biocompatible. These characteristics suggest that artificial electric organs could be used to power next-generation implant materials such as pacemakers, implantable sensors, or prosthetic devices in hybrids of living and non-living systems3,4,5,6.