Researchers have developed a prototype of sensor-packed robotic insects that mimic biological digestive systems to meet their energy needs, use a Janus interface to regularly provide nutrients, and move on the water surface like a walking insect.
In 2017, DARPA proposed a program to develop and deploy thousands of floating sensors aimed at collecting environmental data such as “ocean temperature, sea state and location, and activity data on commercial vessels, aircraft, and even marine mammals moving through the ocean.”
Dubbed the Ocean of Things, which is essentially a plethora of smart devices packed with sensors that gather information across the Internet of Things, the project page states that sensor data will be uploaded to government cloud storage for analysis, and that OoT will support military missions while also being available to research organizations and commercial companies.
Professor Seokheum Choi of Binghamton University has been working on just such a device for the past 10 years, funded by the Office of Naval Research. Now Choi and his team have developed a tiny aquatic robot that can glide across the surface and is powered by bacteria on board, rather than common energy systems like solar, kinetic or thermal.
“Researchers are actively pursuing several innovative strategies to enable self-sufficient robots that harvest energy directly from marine environments,” the team writes in their paper. “These strategies include solar energy, kinetic energy from waves or currents, osmotic potential of saltwater, thermal gradients, and moisture-driven energy sources.
“Despite the innovative nature of these approaches, the variable availability of light and mechanical energy in marine environments, combined with relatively low energy yields from salinity gradients, thermal differences and humidity levels, pose significant challenges. These limitations hinder the ability to guarantee reliable and sustained operation of aquatic robots based solely on current energy harvesting technologies.”
Binghamton University
The new system's power plant is built around a microbial fuel cell that uses spore-forming bacteria. Bacillus subtilis Inspired by biological digestion processes, for a mini generator that converts organic matter into electricity through catalytic reduction-oxidation reactions.
“When the environment is suitable for bacteria, they develop into vegetative cells and produce power, but when conditions are not suitable—for example, when the weather is too cold or nutrients are not available—they revert to spores,” Choi said. “In this way, we can extend the operational lifespan.”
The anode in the fuel cell is made of polypyrrole-coated carbon cloth, chosen for its excellent conductivity and ability to support bacterial colonization. The electron-accepting cathode is also carbon cloth, but decorated with polypyrrole-coated platinum, chosen for its “catalytic properties to accelerate oxygen reduction.” The final piece of the puzzle is a Nafion 117 membrane for selective proton transfer.
The integrated power plant also has adjacent hydrophobic and hydrophilic surfaces to allow “one-way flow of organic substrates” from ocean water and provide nutrients to bacterial spores.
A single fuel cell setup achieved “maximum power density of 135 µW cm-2 and open circuit voltage of 0.54 V,” but scaling up to an array of six units resulted in an observed power output of almost a milliwatt. This output may be relatively small in the big picture, but it is sufficient for the small DC motor and onboard sensors on top of the platform.
“To achieve smooth movement in the water, the robot uses the rotational force of the motor, which applies a reaction force to the platform and propels it forward on the water surface without directly applying force to the water itself,” the researchers explain. “The hydrophobic property contributes to the main lift force.” The small robot's legs were also treated with a hydrophobic coating, so it can glide over the water surface like a water track.
The idea is to be able to deploy small fleets of data collectors wherever and whenever they are needed, rather than having them be tied to a single location for their entire operational life.
“While this work successfully demonstrates self-sustained locomotion on water surfaces powered by an integrated MFC array, exploration of practical applications such as localization, sensing, and signal processing and transmission on aquatic robotic platforms remains an area ripe for development,” the team noted. Further work on long-term performance and suitability to changing environmental conditions is needed. However, the current system serves as a proof of concept for the new design.
An article of the research was published in the journal Advanced Materials Technologies.
Source: Binghamton University