Inspired by Electric Eels, Scientists Create Wearable Underwater Generators

Its been over three years since my first triathlon, but I still cringe thinking about that initial dive into the water. See, I can’t really swim. If you were to watch a race between me and a housecat, I’d strongly suggest putting your money on the cat. In spite of my poor technique, I came out on the other side, as excited as one can be when they’re facing miles of biking and running ahead of them. When the race was finally over, my internal science-nerd monologue resumed, and I thought, “Wouldn’t it be awesome if all that kinetic energy I just used could be converted into electricity?”

Unbeknownst to me, researchers at the Beijing Institute of Nanoenergy and Nanosystems were working on just that, and they’ve invented flexible underwater nanogenerators (Bionic Stretchable Nanogenerator, BSNG) that can harness electricity, as you swim.

An electric eel.

Wearable electronics are everywhere, from Apple watches, to Fitbits, to the ill-fated Google Glass. The market just keeps growing, and its expected to be worth $52 billion USD by 2022. These technologies aren’t just useful for tracking your fitness journey, or texting on the go; scientists have already developed wearable technologies that can accurately monitor your vital signs. New products are thin, stretchable, and even waterproof. Some companies go so far as to say they could save your life.

While the wearable technology industry is quickly developing, we’re still using the same batteries we’ve always used.

“Rapid developments of wearable electronics have an urgent demand for a sustainable power supply which could match them.” said Professor Zhou Li, a group leader at the Bejing Institute of Nanoenergy and Nanosystems, and one of the authors on the study.

This led the researchers to a question: What if your wearable tech could make its own electricity? For the past few years, the research team of Dr. Zhong Lin Wang, a professor at the Georgia Institute of Technology, have been working on how to harvest energy from the human body and convert to electricity for portable and implantable electronic devices.

Eels: the real deal

One way machines can generate their own electricity through triboelectric charging. In this scenario, two dissimilar objects come in contact, electrons flow between them to bring the system back to equilibrium, and an electric current is generated. It’s just like rubbing your hair against a balloon and watching it stand up (See this article on triboluminescence for more info on the exchange of energy.). Their simplicity makes triboelectric nanogenerators a low cost and lightweight energy source, but until now, no one has been able to make them work reliably underwater.

To solve an engineering problem, sometimes the best source of inspiration is nature. Over the course of life’s 3.5 billion years on earth, evolution has yielded some pretty nifty organic machines. So when they wanted to make a waterproof and flexible generator, researchers turned their focus to an animal that does this everyday: the electric eel.

Diagram of an electric eel (left) and the BSNG (right). Credit: Zou et al. 2019

“Electric eel” is kind of a misnomer for the Electrophorus electricus, because they’re not actually eels, they’re a type knifefish. Nonetheless, these sparky water noodles thrive in their ecosystem by utilizing their voltage abilities to defend themselves against predators (surprisingly, it can use electricity as a sensory system to locate prey, much like echolocation).

Eels generate electricity through stacks of electrocytes, ionized cells that act similar to a battery. The inside of these cells contains a high concentration of charged potassium ions, and low concentration of charged sodium ions. When the eel aims to shock, a neurotransmitter signals the membrane to let the ions pass through the cells, letting potassium out and sodium in, generating an electric current. The transport of these ions within the eel can generate up to 600 volts! That’s like the push of energy (or voltage) needed for 5 waffle irons.

Diagram of a diver using the BSNG. Credit: Zou et al. 2019

How it works

With the eels on the brain, the researchers sought to create a device that mimics the structure of the electrocyte, creating an ion channel that can open and close with mechanical force. The team spent months trying to find the exact material, eventually settling on silicone interlayered with sections of a polymer called polydimethylsiloxane (referred to as PDMS).

The mock cells are filled with deionized water and saltwater (which acts as an electrode), which mimics the behavior of potassium and sodium in the electrocyte. When the generator stretches back and forth through the regular motion of a swimmer, the biomimetic channels open and close, creating a continuous alternating current. Underwater, their structure can be stretched to over twice its length, and can generate up to 170 volts of electricity on land, and 10 volts of electricity underwater (which could sufficiently power a signaling devie).

The device works in two ways: primarily as a generator, but also as a sensor. Four generators are attached to the swimmer near their elbows and knees. As the swimmer moves, the electric signal is transmitted via Bluetooth to a monitoring device on land. Based on the signal, you can see whether the swimmer is moving at a constant pace using a constant stroke, or if they’re struggling. In an emergency situation, the swimmers can tap a wireless trigger charged by the generator to send an emergency signal, letting responders know that they’re in need of assistance.

Currently, bionic stretchable nanogenerators (BSNGs) promise to provide better monitoring for divers in emergency situations, and provide swimming trainers with a data-driven method to analyze technique and performance. In the future though, these generators could become more sensitive and miniaturized, leading to some surprising applications.

“We hope that BSNG could have more opportunity for potential applications in electrical skins, soft robots, wearable electronics, and implantable medical devices,” said Professor Li.

Recording of a swimmer using the BSNG. Credit: Zou et al. 2019

If I were to wear the BSNG in my next triathlon, not only could I generate a bit of electricity while I swim, but the suit could also monitor my vital signs, helping safety officials differentiate between my nontraditional (see also: ineffective) swimming style, and a person having a legitimate medical emergency. While I’m not looking forward to my next swim, I do like the idea of becoming a bionic electric eel, so that’s fun.

–Lissie Connors

Lissie Connors is not an electric eel. She’s a human person that writes about science for Physics Central and the American Physical Society. For more science shenanigans, you can follow her on twitter (@LissieOfficial). 

Further Reading

A bionic stretchable nanogenerator for underwater sensing and energy harvesting, (2019),  Nature Communications, volume 10, article 2695

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