Researchers at the University of Fribourg’s Adolphe Merkle Institute, the University of Michigan, and the University of California San Diego, have developed a power source inspired by the electric eel. In a study published by the scientific journal Nature, they have demonstrated a method that suggests that self-powered batteries for biological applications such as pacemakers may one day become a reality.

Could a knifefish, the electric eel, inspire a self-charging power source for implantable devices such as heart pacemakers, sensors, drug delivery pumps, or prosthetics? Researchers at the University of Fribourg’s Adolphe Merkle Institute (AMI), the University of Michigan, and the University of California San Diego took up the challenge. The ongoing integration of technology into living organisms requires some form of power source that is biocompatible, flexible, and able to draw energy from inside a biological system. Generating electricity inside the body would eliminate the need for replacement surgery in some cases and may also provide sustained power for wearable devices such as electrically active contact lenses with an integrated display.

The researchers began by reverse-engineering the animal’s electric organ. It is made up of long and thin cells known as electrocytes that span 80% of the eel’s body in parallel stacks. Triggered by the animal’s brain, these cells each generate a small voltage almost simultaneously by allowing sodium ions to rush into one side of the cell and potassium ions out on the other side of the cell. The resulting voltages along stacks of these cells add up to a much larger potential, up to 600 volts.

The team led by the AMI Professor of BioPhysics, Michael Mayer, designed an eel-inspired power source that generates electricity based on the salinity difference between compartments of fresh water and salt water separated by ion-selective membranes. Arranging these compartments and membranes in a repeat sequence hundreds of times, somewhat like batteries in a flashlight, makes it possible to generate 110 volts just from salt and water.

Each component of this power source is made of a hydrogel, a solid-seeming polymer cage that contains water and can conduct salt ions. These components can be assembled on clear plastic sheets using a commercial 3D printer. Like the eel, the power source has individual compartments with small capacities, so the voltages must be triggered at the same time. The eel does this with its nervous system; the researchers achieve this task most efficiently by bringing all the cells into contact simultaneously, using a folding strategy of the printed sheet that was originally developed to unfold solar panels in space.

The results are still far from matching the capacities of the eel. According to Mayer, the major challenge will be to tap into the body’s metabolic energy, for example by mobilizing ion differences in zones such as the stomach fluids, or by converting  mechanical muscle energy to electrical energy, which could then be stored and released from an artificial electric organ.

Article: Schroeder, TBH et al., An electric-eel-inspired soft power source from stacked hydrogels, Nature; 14th December 2017