US researchers have developed a power source inspired by the electric eel, allowing the generation of 110V just from salt and water.
The research team involved personnel from the University of Fribourg’s Adolphe Merkle Institute (AMI), the University of Michigan, and the University of California San Diego, led by AMI Professor of BioPhysics Michael Mayer. They focused on the electrophorus electricus, a knifefish which is commonly called the electric eel.
The electric eel can generate up to 600V and 100W to stun prey or defend itself. Interestingly, it can modulate its electrical output, using the electricity to help it navigate through waters with low visibility.
The researchers are interested in using the power generation capabilities of the electrophorus electricus and applying it to the integration of technology into living organisms. These systems, like heart pacemakers, sensors, drug delivery pumps or prosthetics, require a form of power source that is biocompatible and flexible. By engineering a system capable of generating electricity inside the body in a sustained manner, it is possible to avoid replacement surgery, while still providing bio-compatible device with the energy required to keep running.
The researchers reverse-engineered the animal’s electric organ. This organ is made up of long and thin cells called electrocytes that span 80% of the eel’s body in parallel stacks.
Signals from the brain trigger these cells to generate a small voltage by allowing sodium ions to rush into one side of the cell, and potassium ions out on the other side. The resulting voltages along the stacks of the cells add up.
The team then designed a power source inspired by this system, generating electricity based on the salinity difference between fresh water and salt water. Sea salt is made of a positive ion (sodium) and a negative ion (chloride). When a permeable compartment of salt water is put in contact with a similar compartment of fresh water, the salt has a natural tendency to migrate into the fresh compartment until all the water has the same salt concentration.
If, however, a membrane that is more permeable to positive ions than to negative ones is placed between these two compartments, then the positive ions rush into the low salt compartment, leaving behind a negatively charged high salt compartment.
The researchers then implemented a second membrane that is more permeable to negatively charged ions. Arranging these compartments and membranes in a repeat sequence thousands of times makes it possible to generate 110 volts just from salt and water.
The power source, dubbed the reverse electrodialysis power source consists of compartments made of hydrogel, which contains water and can conduct salt ions. These components can be assembled on clear plastic sheets using a commercial 3D printer.
Because the power source consists of individual compartments with small capacities, the voltages must be triggered at the same time. The researchers did this 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 researchers are still looking ways to improve the system. While the electric eel can fuel its electrical organs by eating, the system currently requires the application of an external current to recharge. The hydrogel membranes can also be improved – with thinner membranes, the solution will be able to reach a useful power level of implants. There also needs to be a strategy to allow their reactivation inside a living organism.
“The power characteristics of our artificial electric organ are at least a factor of 1,000 lower than those of the eel, and the fish’s ‘packaging’ is also very efficient,” says Mayer.
“Currently we might be able to power the very lowest energy devices, but I do think it is realistic to improve the performance by a factor of ten with better membranes, and then possibly by another factor of ten by efficient engineering.”
According to Mayer, another major challenge will be to tap into the body’s metabolic energy, for example by using 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.
[Image: Printed sheets - This photo depicts the printed, high voltage implementation of the artificial electric organ. A 3D bioprinter was used to deposit arrays of gel precursor droplets onto plastic substrates, and the droplets were cured with a UV light to convert them into gels. Alternating high-salinity and low-salinity gels (red and blue gels, respectively) were printed onto one substrate, and alternating cation-selective and anion-selective gels (green and yellow gels, respectively) onto a second substrate. When overlaid, these connect to form a conductive pathway generating up to 110 V. Credit: Thomas Schroeder ([email protected]) and Anirvan Guha ([email protected]).]