In search of implantable devices appropriate for medical treatment applications such as the cardiac defibrillator and the deep brain stimulator, researchers are requiring a power source that is rechargeable in vivo, flexible, and biocompatible.
Learning from how an electric eel can generate its own electricity – 600 volts and 1 ampere of electric current – as a defense against its enemies, an energy source that is self-charging, biocompatible, and environment-friendly has been developed by a group of researchers from Switzerland and the United States.
“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 performances similar to what the eel is capable of,” explains co-author Anirvan Guha on what motivated them in using electric eels as a model.
How does an electric eel generate electricity?
Electric eels can generate electricity through their specialized cells called “electrocytes” that essentially work as their biological batteries, explains Guha in an interview with Research Gate. These electrocytes are stacked in series throughout the length of an eel.
Each electrocyte has two membranes – one at the front end and one at the back end – which contain channels for ions to pass through and simultaneously prohibit the entrance of other ions. A biochemical process allows pumping of positive sodium and positive potassium ions out of each electrocyte, storing an electric potential of 150 mV.
When an eel is at rest, the positive potassium ions in each electrocyte flow out of its both frontend and backend membranes creating an electric potential at each end of equal amount but opposing direction, canceling each other and thus, resulting to a zero net potential across the whole electrocyte.
When it needs to produce electricity, it sends signals to all electrocytes to open their sodium channels and close their potassium channels in the backend membrane. The inflow of sodium ions creates a 65-mV electric potential across the backend membrane. This voltage has a direction the same with the 85-mV simultaneously produced by the frontend membrane, resulting in a net potential of 150 mV across the electrocyte.
How the new device copied the electric eel?
“We use the same basic principle of stacking ion gradients across selective membranes to generate electricity,” says Guha.
In order to mimic the eel’s energy generation, the research team recreated the eel’s layered structure of electrocytes by fabricating a sheet containing an array of four different hydrogel droplets, as shown in the above image.
The sheet containing the array of droplets was laser-scored so that it can be folded in a manner that allows the four different droplets to come together causing the sodium and chlorine ions to transfer between droplets. The sheet can generate 110 volts and a power of 27 mW per square meter.
“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,” Guha highlights on their device’s advantages over other small power devices.