Electric eel inspires new, battery-free power source

The concept could be a biocompatible power source for implants, prosthetics, and wearable devices.

To stun its prey, an electric eel can generate a powerful electrical discharge of 100 watts using thousands of specialized cells called electrocytes. In a new study, University of Fribourg’s Michael Mayer and colleagues have developed a power source that mimics the electric eel. We spoke with one of the authors, Anirvan Guha, about the work.

ResearchGate: What motivated this study?

Anirvan Guha: The main motivation behind this study was our fascination with the unique physiology of the electric eel. This is an organism which, within the constraints of biology, is able to generate 600 volts and 1 ampere of current, which is enough electric power to seriously harm a human being. Unlike today’s traditional batteries, which use chemical reactions of toxic metals to produce electricity, the eel is able to generate these outstanding voltages and currents simply by using naturally occurring salt gradients within its body. We wanted to see if it was possible to mimic the eel’s strategy of generating power, and whether or not we could achieve performance similar to what the eel is capable of.

RG: Can you give us a brief insight into how an electric eel generates electricity?

Guha: The electric eel has specialized cells in its body called “electrocytes” which function as biological batteries. Each electrocyte has an anterior and a posterior membrane, and both of these membranes contain ion channels which allow certain ions to pass through while preventing the transport of others. In the eel’s resting state, potassium ion channels in both membranes are open, allowing potassium to flow out of the cell in both directions. This movement of positively-charged potassium ions creates a separation of charges across each membrane, resulting in a voltage of 85 mV across each membrane, in opposing directions. These voltages cancel, resulting in no voltage generation across the cell in the resting state.

When the eel wants to produce a pulse of electricity, it sends a coordinated series of nerve signals to all the electrocytes in its body simultaneously. These signals lead to the opening of sodium channels and the closing of potassium channels in the posterior membrane of the cells. The subsequent influx of positively-charged sodium ions establishes a potential of 65 mV across the posterior membrane, which, crucially, is in the same direction as the 85 mV across the anterior membrane. This allows these voltages to add, creating a modest 150 mV across each electrocyte in the firing state. While this voltage on its own is not especially powerful, the eel’s electric organ contains thousands of these cells stacked back-to-back, allowing the voltages of many cells to add to a value in the range of hundreds of volts from head to tail of the eel.

RG:  How did you mirror this?

Guha: We use the same basic principle of stacking ion gradients across selective membranes to generate electricity. In our system, which we call the artificial electric organ, we employ a method called reverse electrodialysis, in which alternating reservoirs of high and low salinity solutions are separated by alternating cation-selective and anion-selective membranes to create electricity. The major innovation of our system is that we use hydrogels as the material for the reservoirs and the charge-selective membranes. This allows us to manipulate all the components of our system as liquids, which aids in automation of the assembly process, before solidifying the gels and putting them into contact.

Printed array. Image courtesy of Anirvan Guha.

RG: What could artificial electric organs be used for?

Guha: Hydrogels are moldable, flexible, and transparent – characteristics not typically associated with traditional energy storage devices. They can also be biocompatible, and are commonly used to create contact lenses. We think that they could, with further development, be used as biocompatible power sources for implants, prosthetics, and wearable devices. One potential application is the creation of a layered contact lens which is able to generate energy, which may be used to power a sensor or augmented reality display within the lens.

RG: What was the most challenging aspect of the design process?

Guha: From the beginning, we felt that generating voltages on the order of what the eel can produce would be a good demonstration of the capabilities of the artificial electric organ. However, it was not trivial to find a way to assemble the sequence of thousands of hydrogels necessary to create such a voltage. Ultimately, our breakthrough came when we considered inkjet printing as a means of precisely positioning large arrays of hydrogels.  We were able to print thousands of hydrogel droplets onto two separate substrates, which we brought together to create a consecutive sequence of almost 2,500 hydrogels, generating 110 volts.

RG: What are the next steps for your research?

Guha: While the voltages we were able to produce are comparable to those of the eel, the power that we have been able to generate is very low, limiting the application space for our system as it stands right now. Thinner hydrogel films and more selective membranes could likely increase the performance of this artificial electric organ by at least an order of magnitude.

Featured image courtesy of Anirvan Guha.