Scientists studied how electric eels generate power and used knowledge to build soft batteries

By Brian Santo, contributing writer

There are some people who might jump at the chance to have their DNA spliced with some eel DNA for the thrill of being able to literally shock people, whereas others might not want to risk some unintended and possibly socially disadvantageous side effect, such as exuding a mucoprotein slime to keep their scales supple and healthy. And that is why someone invented sheets dotted with hydrogels that mimic the mechanism that eels use to generate electricity — no mad scientist genetic experiments necessary, thank you.

With refinement, the technique could lead to the development of artificial organs that could provide a generous jolt of power to medical devices (medication dispensers or pacemakers, for example), artificial organs, and prosthetics (e.g., augmented reality contact lenses).

Most animals generate some modest amount of electricity. The process typically involves transferring sodium and potassium ions back and forth.

Eels have evolved a way to amplify the process of ionic transference with a vengeance. The mechanism is based on a special type of cell called an electrocyte, each of which can produce over 150 millivolts. In eels, thousands of electrocytes are lined up in series. These rows are also stacked. The biology is such that rows and stacks are effectively in series so that their potential is combined.

The ultimate result is that eels are able to generate pulses of hundreds of volts at roughly 1 amp. In humans, meanwhile, voltage and current are typically measured on the nanoscale.

A multidisciplinary team of researchers based at the University of Michigan, along with colleagues at the University of Fribourg (Switzerland) and the University of California at San Diego, were looking for a means of providing larger amounts of electricity than humans can generate on their own and in a format that was compatible with biological systems — “biocompatible.” Mimicking the mechanism that eels use seemed a promising possibility.

What they devised are sheets (membranes) covered with dots (compartments) of polyacrylamide hydrogel that are reminiscent of paper strips of Candy Buttons . The sheets are patterned with a repeating sequence of cation- and anion-selective hydrogels interspersed with a low-salinity gel and a high-salinity gel. Each repeated unit is a tetramer — a polymer comprised of four monomers. Where biological systems often swap Na and K ions, this system uses Na and chloride.

Charge-selective (yellow and green) and freshwater and saline (blue and red) hydrogels printed on a sheet that has been laser-cut in a Miura fold pattern. Image: Biophysics group, Adolple Merkle Institute.

The compartments need to line up in a precise order, however. The system uses what the researchers called a scalable stacking or folding geometry, which means that the membranes can be manipulated to bring the hydrogel cells in physical contact with each other. Specifically, the system uses the Miura fold, a technique also used to pack solar panels on satellites for launch.

Upon initiation of contact, each tetramer in sequence forms ionically conductive pathways. The researchers said that this establishes “electrolyte gradients across tens to thousands of selectively permeable compartments,” according to their paper .

Through reverse electrodialysis, each of these tetrameric gel cells generated 130 to 185 mV, roughly similar to the voltage generated by a single electrocyte of an eel.

The researchers said that they have used these sheets to create an artificial electric organ that can produce a steady but low current. It generates 110 volts “at open circuit,” or 27 milliwatts per square meter per gel cell.

In several ways, the artificial organ’s overall performance still does not measure up to eels’ systems of electrocytes. For example, the researchers devised a means of resetting their artificial electric organ and were able to cycle it through multiple pulses, though with diminishing effectiveness.

But this version was meant to simply prove the concepts. The team is confident that with further research, including bringing advanced techniques to bear on the system (microfluidics, for example), their artificial organ can be improved.

They said that if they can continue to improve the technology, “then these artificial electric organs may open the door to metabolically sustained electrical energy for powering implants, wearables, and other mobile devices.”