Inspired by Jellyfish, Johns Hopkins APL Researchers Float a Versatile Sensor Platform
Schools of Velella velella, as shown above, are pushed across the ocean surface by wind and currents.
Thu, 04/27/2023 - 16:32
Using a design inspired by one of the ocean’s best sailors, a team of scientists at the Johns Hopkins Applied Physics Laboratory (APL) in Laurel, Maryland, is developing a versatile, low-cost sensor for ocean observations. The APL-designed sensors are modeled after Velella velella, which are floating jellyfish-like organisms that sail across the surface of the ocean.
The new sensor platforms, which were developed with support from the Office of Naval Research and in collaboration with the Naval Postgraduate School and a small local aerospace electrical engineering company, incorporate advanced low-power electronics that can measure key oceanic factors, such as salinity, temperature and position. The collected data is then transmitted to researchers using satellite communication for real-time global monitoring.
“We’ve started deploying these [platforms] with salinity sensors because salinity is one of the more difficult oceanographic measurements to measure precisely,” said Daniel Ayoub, an electrical engineer in the Research and Exploratory Development Department (REDD).
Historically, ships and buoys were used to capture salinity data, but recent space missions like NASA’s Aquarius satellite collected more data in its first few months than had been collected by ships and buoys throughout the previous century. Even though satellites are capable of capturing data from large portions of the ocean quickly, data from buoys and in-water sensors, such as the Velella sensors, provides localized, high-resolution readings. Each set is useful for varying research needs, and when combined, create a more complete understanding of the ocean.
Salt makes water denser, and while surface water circulation is driven primarily by surface winds, changes in seawater density and temperature drive ocean currents deep below the surface. Global ocean circulation models suggest that these density-driven deep currents play a significant role in mediating our planet’s climate, as well as ocean nutrient and carbon dioxide cycles.
“If we can better monitor ocean health and how environments around the world are changing, then we can make better decisions on how to mitigate or adapt to those changes,” said Leslie Hamilton, a materials science engineer and Science of Extreme and Multifunctional Materials assistant program manager.
A prototype of the sensor is tested in the Pacific Ocean.
Credit: Johns Hopkins APL
Biomimetic and Biodegradable
The funky-looking sensors are biomimetic, meaning they’re inspired by nature, and in this instance specifically inspired by the shape of the Velella velella. The organisms are from the same family as jellyfish and other stinging animals such as corals and sea anemones. They feed on plankton as they float across the sea and are nicknamed “by-the-wind sailors.” Velella velella typically form large schools and are propelled by winds pushing on their sails.
“We are inspired by the Velella velella because of its sail,” said Jamie MacMahan, an oceanography professor at the Naval Postgraduate School. “The majority of observational buoys sit beneath the surface and their movements are dictated by near-surface ocean currents. With the Velella velella’s shape, we have a naturally-propelled sailing buoy providing a different spatial coverage complementing traditional buoys.”
To pack such a technologically heavy punch in such a small package, many components of the sensor are multifunctional.
“Not only does the sail capture wind, it’s also a feature to house electrical components, like antennae, that need to sit above the waterline” said Kyle Lowery, a mechanical design engineer at APL and lead designer of the sensor exterior. “And the bottom of our Velella velella resembles a ship’s keel, which simultaneously houses the bulk of the electronics, provides a convenient location for the salinity sensor and lowers the sensor’s center of gravity for stability.”
The sensor’s bodies are currently made out of silicone, but APL researchers are working in tandem to develop a silicone-like structural material that’s biodegradable.
“When we put these sensors out into the ocean, we want them to persist as long as we need them, but then we want them to go away,” explained Joel Sarapas, a chemist in REDD. “Our goal is that once we perfect this degradable elastomer, the sensor will operate for a desired life span, maybe six months, and then begin to degrade.”
The team is also working on anti-fouling coatings to slow the growth of subaquatic organisms on the sensor platform, another major problem faced by measurement tools deployed in the ocean.
In areas of the ocean that are nutrient and plankton scarce, real Velella velella can capture the power of the Sun using symbiotic algal cells, and turn that power into energy in a process similar to photosynthesis. Each of APL’s Velella-inspired sensors harvests the energy of the Sun too, using two solar arrays above its fin to power the tiny electronics it holds.
“As it floats in the ocean, the solar energy will recharge the battery, which is really quite small,” said Ayoub, holding up a battery the size of his thumb. The salinity sensor, smaller than Ayoub’s pinkie, will be constantly submerged in the ocean and continuously measuring salinity levels. Researchers just have to set their desired cadence for which data is reported to them from the sensor.
An early prototype of the Velella sensor (left) compared to a real Velella.
Adaptable Aquatic Communicators
“These platforms and sensors are incredibly adaptable,” said Jeff Maranchi, manager of REDD’s Research Program Area. “We can vary the sensor suite for other ocean measures, supporting a large range of research interests.”
The electronics are designed to periodically report temperature, position and salinity via satellite communication. An external small business partner designed the controlling and communication electronics. To test these capabilities, an early electronics prototype was deployed via weather balloon from the Delaware coast. The prototype is currently in the middle of the Atlantic, following the Gulf Stream, and has been reporting position and temperature multiple times a day for over 20 weeks.
To properly protect the electronics and optimize the design of the sensor, researchers used computational modeling to design the exterior casing, with buoyancy and self-righting capabilities. They worked closely with APL fabricators to 3D-print the device and manufacture a mold that looked as realistic as possible.
“Designing the interior cavity, where the electronics package was held, was as important as the exterior,” said Craig Leese, an engineered materials fabricator. “We incorporated watertight air pockets that served as floats for the Velella sensor. Once we 3D-printed the mold, we used a glossy surface finish on both the interior and exterior to create a translucent finished, molded part.”
To perform initial buoyancy tests, the team set the Velella velella afloat in the APL pond. This was followed by a tethered released in the Pacific Ocean. Recently, researchers at the Naval Postgraduate School deployed the latest iteration of their design in Monterey, California. The sensor is currently tethered to a pier to allow for continued testing and evaluation in a real ocean environment.
To date, the team has focused on designing, fabricating and testing the initial prototypes. As they move forward into future testing phases, they’ll expand to deploy a collection of Velella velella that resemble their natural school structure and are capable of providing larger sets of critical oceanic data.
The Applied Physics Laboratory, a not-for-profit division of The Johns Hopkins University, meets critical national challenges through the innovative application of science and technology. For more information, visit www.jhuapl.edu.