Biomimicry: Electric eels and Batteries


What is Biomimicry?


Biomimicry or Biomimetics is the study of models, systems, and elements from the natural world in order to find solutions to complex human problems. All living organisms that exist today have evolved and adapted their structure over time based on natural selection. This field of science has been studying these evolutionary structures in an effort to create newer technology inspired by biological functions at the macroscopic and nanoscale level. In addition, nature has already solved numerous engineering problems: self-assembly, self-healing, harnessing solar power to name a few.


Some classical examples often cited for this subject are studying birds for human flight, which was one of the earliest cases. Leonardo da Vinci, although unable to recreate it, was vastly interested in anatomy of birds and their ability to fly. The Wright Brothers, who were successful in flying the first flying machine in 1903, admitted to be inspired by pigeons in flight. American biophysicist Otto Schmitt developed the concept of “biomimicry” in the 1950s.


Other examples of biomimicry are the streamlined design of bullet train (Shinkansen 500 Series) mimics the beak of kingfisher to improve aerodynamics, the tiny hooks in the bur fruits inspired Velcro tape, LED lights can be designed to mimic the pattern of scales on fireflies’s abdomens, improving their efficacy and many more. The mechanism by which electric eels generate electricity is currently being examined in order to develop an unconventional soft battery that could be useful in the next generation of soft-bodied robots, pacemakers and prostheses.


How do electric eels produce electricity?


Fig. 1: How an electric eel’s electrocytes work (Schroeder et al. / Nature)


Electric eels (Electrophorus electricus) are ray-finned knifefish. The fish has three organs (Sachs’ organ, Hunter’s organ and Main organ) in its body that produce electric discharges. The organs contain specialized cells called electrocytes. There are hundreds of these electrocytes packed together, separated by insulating tissue, which allows the electric current to flow forward in one direction, towards its prey, in water.

The nervous system of the fish sends neurotransmitters to the electrocytes. The electrocyte, when in rest, pumps out Na+ and K+ ions out of the cells to maintain a positive charge outside and negative inside. When the signal arrives, it opens up the pumps and positively charged ions flow back in. In doing so, it creates an electric charge gradient in a way that one side of the cell is positively charged inside and negatively charged outside and vice versa for the other side of the cell.

This results in an alternating current that can carry a current forward, turning the electrocyte into a biological battery. The coordinated arrival of the signal from the brain occurs such that all the electrocytes get charged at the same time and act like several cells in series forming a battery. This battery can generate up to 600 volts which can instantly kill any prey or predator present in the water.

Cells called “electroreceptors” are buried in their skin that helps them sense this electric field and the changes caused to it by other surrounding fishes. For example, the Peter’s elephantnose fish, a weakly electric fish, has a special organ called Schnauzenorgan that has these electroreceptor cells which allows it to intercept signals from other fishes, sense its surroundings and detect shape and size of nearby objects. Apart from electric eel, other strongly electric fishes include electric catfish that can generate up to 350 volts with an organ taking up its entire torso and electric ray, with kidney shaped electric organs on either side of its head that produces around 220 volts of electricity.


In an attempted attack, an electric eel uses two phases to approximate the target’s location and then incapacitate it. Initially, starting by emitting two or three strong pulses, causing the prey’s muscles to spasm, sending waves to reveal their location. Lastly, it produces a series of high voltage discharges which generates more intense muscle contractions. The eel can also curl in a way that causes the electric fields produced at both ends of the organ to overlap. The continuous valley of electric charges finally immobilise the prey.


Biomimicry inspired from the electric eel



The Italian scientist Alessandro Volta in the year 1799, designed the first synthetic battery called the “voltaic pile” using zinc and copper discs separated by salt-soaked cardboard. This design was inspired by the body of an electric eel. (Fig. 2: Google images)

At the University of Fribourg, Michael Mayer and colleagues have designed a soft battery that has potential to charge pacemakers, prosthetics, and medical implants. This model generates electric discharge by using “the gradients of ions between miniature polyacrylamide hydrogel compartments bounded by a repeating sequence of cation- and anion-selective hydrogel membranes”



The red gel contains saltwater whereas blue ones contain freshwater, the ions will flow from red to blue gel, but will not be able to as the gels are separated. This changes when the green and yellow gel sheet is put on top of it as shown in Fig. 2, providing the ions a channel to travel.

While the green gels allow only positive ions to pass through them, the yellow gels are selective to negative ions. This selectivity leads to positive ions to flow into the blue gel from only one side and negative ions flow

in from the other side, resulting in formation of an ion Fig. 3: Schroeder et al./Nature

gradient across the blue gels as seen in the electrolyte cells. Thousands of these arrangements in a row can produce up to 110 volts. It takes only pressing the gels together to trigger this succession of ion exchange.


Problems in Design and Future developments


Despite the clever design, having large sheets of the battery will be difficult to manage and not be able to fit into small spaces like pacemakers. For this, Max Shtein, an engineer at University of Michigan suggested using origami to fold the gel sheet in such a way that the hydrogel clumps still stay in contact in the right order. Another problem is the short few hours for which the gel remains active and produces electricity till the ion concentration equalizes and the battery goes flat. The battery is reactivated by applying a current to reset the gel concentrations to the high salt and low salt hydrogel clumps. Although the soft battery model has enormous potential, there is great scope for rectifying the design further to make it more compact and have a smaller recharge period.


Jayshree Agrawal

Department of Biochemistry and Biotechnology

St. Xavier's College, Ahmedabad


Reference:


  1. Schroeder, T., Guha, A., Lamoureux, A. et al. An electric-eel-inspired soft power source from stacked hydrogels. Nature 552, 214–218 (2017). https://doi.org/10.1038/nature24670

  2. Young, Ed. “A New Kind of Soft Battery, Inspired by the Electric Eel.” The Atlantic, 13 Dec. 2017, https://www.theatlantic.com/science/archive/2017/12/a-new-kind-of-soft-battery-inspired-by-the-electric-eel/548261

  3. Matchar, Emily. “Electric Eels Inspire a New Type of Battery.” Smithsonian Magazine, 17 Jan. 2018, https://www.smithsonianmag.com/innovation/electric-eels-inspire-new-type-battery-180967801

  4. Wikipedia contributors. (2021, May 7). Biomimetics. In Wikipedia, The Free Encyclopedia. Retrieved 09:26, May 14, 2021, from https://en.wikipedia.org/w/index.php?title=Biomimetics&oldid=1021919729

  5. Nelson, Elenor. (2017, November) How do fish make electricity?[Video Lesson] https://ed.ted.com/lessons/how-do-fish-make-electricity-eleanor-nelsen







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