MIT engineers have built a wireless, battery-free underwater camera, capable of harvesting energy by itself while consuming very little power, according to a new paper published in the journal Nature Communications. The system can take color photos of remote submerged objects—even in dark settings— and convey the data wirelessly for real-time monitoring of underwater environments, aiding the discovery of new rare species or monitoring ocean currents, pollution, or commercial and military operations.

We already have various methods of taking underwater images, but according to the authors, “Most of the ocean and marine organisms have not been observed yet.” That’s partly because most existing methods require being tethered to ships, underwater drones, or power plants for both power and communication. Those methods that don’t use tethering must incorporate battery power, which limits their lifetime. While it’s possible in principle to harvest energy from ocean waves, underwater currents, or even sunlight, adding the necessary equipment to do so would result in a much bulkier and more expensive underwater camera.

So the MIT team set about developing a solution for a battery-free, wireless imaging method. The design goal was to minimize the hardware required as much as possible. Since they wanted to keep power consumption to a minimum,  for instance, the MIT team used cheap off-the-shelf imaging sensors. The trade-off is that such sensors only produce grayscale images. The team also needed to develop a low-power flash as well, since most underwater environments don’t get much natural light.

The solution to both challenges turned out to incorporate red, green, and blue LEDs. The camera uses the red LED for in situ illumination and captures that image with its sensors, then repeats the process with the green and blue LEDs. The image might look black-and-white, per the authors, but the three colors of light from the LEDs are reflected in the white part of each image. So a full-color image can be reconstructed during post-processing.

“When we were kids in art class, we were taught that we could make all colors using three basic colors,” said co-author Fadel Adib. “The same rules follow for color images we see on our computers. We just need red, green, and blue—these three channels—to construct color images.”

Instead of a battery, the sensor relies on piezo-acoustic backscatter for ultra-low-power communication after the image data has been encoded as bits. This method doesn’t need to generate its own acoustic signal (as with sonar, for instance), relying instead on modulating reflections of incident underwater sounds to transmit data one bit at a time. That data is picked up by a remote receiver capable of recovering the modulated patterns, and the binary information is then used to reconstruct the image. The authors estimate that their underwater camera is about 100,000 times more energy-efficient than its counterparts, and could run for weeks on end.

Naturally, the team built a proof-of-concept prototype and did some testing to demonstrate that their method worked. For instance, they imaged pollution (in the form of plastic bottles) in Keyser Pond in southeastern New Hampshire, as well as imaging an African starfish (Protoreaster lincklii) in “a controlled environment with external illumination.” The resolution of the latter image was good enough to capture the various tubercles along the starfish’s five arms.

The team was also able to use their underwater wireless camera to monitor the growth of an aquatic plant (Aponogeton ulvaceus) over several days, and to detect and locate visual tags often used for underwater tracking and robotic manipulation. The camera achieved high detection rates and high localization accuracy up to a distance of about 3.5 meters (about 11 and a half feet); the authors suggest longer detection ranges could be achieved with higher-resolution sensors. Distance is also a factor in the camera’s energy harvesting and communication capabilities, per tests conducted in the Charles River in eastern Massachusetts. As expected, both those critical capabilities decrease with distance, although the camera successfully transmitted data 40 meters (131 feet) away from the receiver.

In short, “The tetherless, inexpensive, and fully-integrated nature of our method makes it a desirable approach for massive ocean deployments,” the authors wrote. Scaling up their approach requires more sophisticated and efficient transducers, as well as higher-power underwater acoustic transmissions. It’s possible that one could also make use of existing mesh networks of buoys on the ocean surface, or networks of underwater robots like Argo floats, to remotely power the energy-harvesting cameras.

“One of the most exciting applications of this camera for me personally is in the context of climate monitoring,” said Adib. “We are building climate models, but we are missing data from over 95 percent of the ocean. This technology could help us build more accurate climate models and better understand how climate change impacts the underwater world.”

DOI: Nature Communications, 2022. 10.1038/s41467-022-33223-x  (About DOIs).