Biology â€“ Electric Eel
Editor | On 25, Feb 2018
From Nature last month, an article describing scientistsâ€™ search for safer, more natural ways to power devices that go into our bodies. Sticking toxic battery elements inside the body, and having to periodically replace them with invasive surgery is rarely viewed by patients or clinicians alike as any kind of ideal solution?
So, in classic TRIZ, â€˜someone, somewhere already solved your problemâ€™ mode, the paper recognizes that a possible â€˜someoneâ€™ is nature. One organism that is pretty good at generating biocompatible power (for itself, at least) is the electric eel, and scientists have now used the high-voltage species as a blueprint for a promising new self-charging device that could one day power things like pacemakers, prosthetics and even augmented reality contact lenses.
Electric eels generate voltage using three pairs of abdominal organs that produce electricity: the main organ, the Hunter’s organ, and the Sach’s organ. These organs make up four-fifths of its body, and give the electric eel the ability to generate two types of electric organ discharges: low voltage and high voltage. These organs are made of electrocytes, lined up so a current of ions can flow through them and stacked so each one adds to a potential difference.
When the eel finds its prey, the brain sends a signal through the nervous system to the electrocytes. This opens the ion channels, allowing sodium to flow through, reversing the polarity momentarily. By causing a sudden difference in electric potential, it generates an electric current in a manner similar to a battery, in which stacked plates each produce an electric potential difference.
In the electric eel, some 5,000 to 6,000 stacked electroplaques can make a shock up to 860 volts and 1 ampere of current (860 watts) for two milliseconds. Such a shock is extremely unlikely to be deadly for an adult human, due to the very short duration of the discharge. (Atrial fibrillation, for example, requires that roughly 700 mA be delivered across the heart muscle for 30 ms or more, far longer than the eel can produce. Still, this level of current is reportedly enough to produce a brief and painful numbing shock likened to a stun gun discharge, which due to the voltage can be felt for some distance from the fish. When it comes to humans, the eel is essentially discharging a high-voltage shock as a defence mechanism. This is certainly a use of the capability, but its primary function is to help stun and catch prey. Either way, the central skill required is to be able to generate the electrical charge in the first place.
To recreate the electric eel effect, researchers from the University of Fribourg, the University of Michigan and the University of California San Diego turned to the difference in salinity between fresh and saltwater. They deposited hydrogel, ion-conducting blobs onto clear plastic sheets and separated them with ion-selective membranes. Hundreds of blobs containing salt and freshwater were arranged in an alternating pattern. When the team had all these gel compartments make contact with one another, they were able to generate 100 V through what is known as reverse electrodialysis, where energy is generated through differing salt concentrations in the water.
While the eel triggers the simultaneous contact of its electrocytes using a neurotransmitter called acetylcholine as the command signal, the team achieved this by carefully working a special origami pattern â€“ called a Miura-ori fold â€“ into the plastic sheet. This meant that when pressure was applied to the sheet, it quickly snapped together and the cells shifted into exactly the right positions to create the electricity.
The device, which the team calls an artificial electric organ, isn’t in the same ball park as an eel in terms of output, but the researchers do have some ideas around how to boost its efficiency. It points to the metabolic energy created by ion differences in the eel’s stomach, or the mechanical muscle energy, as some of the possibilities, but does note that recreating these would be a major challenge.
“The electric organs in eels are incredibly sophisticated, they’re far better at generating power than we are,” Mayer said. “But the important thing for us was to replicate the basics of what’s happening.”
Translating solutions from the natural world into useful products and services always make for great stories, and hereâ€™s hoping that this one makes it all the way to that rare family of genuine biomimetic innovation success stories.
In the meantime, perhaps the most useful thing we can do here is try and understand the electric eelâ€™s amazing killer charge solution from a contradiction solving perspective.
One way to think about what the eel (actually, itâ€™s not really an eel, but it looks like one, and so thatâ€™s what the biologists called it) has achieved is â€˜merelyâ€™ a jump along the Object Segmentation trend. Or several jumps. We know that the â€˜fieldâ€™ always wins over mechanical solid, fluidic or gaseous solutions eventually, and for a whole host of reasons, not least of which is that fields are inevitably more efficient from a resource-usage perspective. So, one way of modelling the electric eel is to say it has successfully evolved a solution to the Productivity versus Power (killer charge is about volts x amps) conflict. Something like this:
Sure enough, Principle 28, Mechanics Substitution is one of the most frequently used strategies to solve this kind of productivity-power conflict.
Mapping the step-change at this system level is of interest, but it doesnâ€™t contribute anything to the understanding of the scientists and thus how they might engineer their own equivalent solution. Taken at their level, the conflict becomes something much more like the desire to generate power when the only resources within the system able to assist are chemical in nature. Hereâ€™s how we might map that problem onto the Matrix:
So how do these suggestions correspond to the electric eel solution? Pretty well it seems:
- Principle 1 (Segmentation) â€“ long chain of individual electrocytes
- Principle 19 (Periodic Action) â€“ momentaryâ€¦
- Principle 13 (The Other Way Around) – â€¦reversal of polarity
- Principle 24 (Intermediary) â€“ sodium
- Principle 3 (Local Quality) â€“ local differences in salt concentration
Now, perhaps, the scientists need to look at some of the other Principles? I know what Iâ€™d doâ€¦