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. Possible applications would be a self-charging power source for implantable devices such as heart pacemakers, sensors, drug delivery pumps, or prosthetics. Generating electricity inside the body would eliminate the need for replacement surgery and may also provide sustained power for wearable devices such as electrically active contact lenses with an integrated display.
The research team led by AMI Professor of BioPhysics Michael Mayer focused on one animal capable of creating its own electricity: Electrophorus electricus, a knifefish commonly called the electric eel. It is capable of generating up to 600 volts and 100 watts to stun prey or defend itself, but it can also modulate its electrical output, usually to help it navigate through murky waters, and locate its next meal.
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 eel'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.
The team designed an eel-inspired power source that generates 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. A similar effect can be exploited with a second membrane that is more permeable to negatively charged ions. Arranging these compartments and membranes in a repeat sequence thousands of times, somewhat like batteries in a flashlight, makes it possible to generate 110 volts just from salt and water.
Each component of this so-called reverse electrodialysis power source - the compartments and the selective membranes - 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.
Results are still far from matching the capacities of the eel. While the fish can fuel its electrical organs by eating, the prototype system requires the application of an external current to recharge. To reach a useful power level for implants, better and thinner hydrogel membranes are required, along with strategies allowing 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, 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.
Source and top image: Adolphe Merkle Institute
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