Researchers have prototyped sensor-packed robot bugs that mimic biological digestive systems to meet energy needs, employ a Janus interface for a steady supply of nutrients and move on the water's surface like a water strider.
Back in 2017, DARPA proposed a program to develop and deploy thousands of floating sensors aimed at gathering environmental data like "ocean temperature, sea state, and location as well as activity data about commercial vessels, aircraft, and even marine mammals moving across the ocean."
Called the Ocean of Things – and similar in essence to the multitude of sensor-packed smart devices that collect info across the Internet of Things – the project page states that sensor data would be uploaded to government-owned cloud storage for analysis, and that the OoT would support military missions while also being open to research bodies and commercial concerns.
Professor Seokheum Choi from Binghamton University has been working on just such a device for the last 10 years or so, funded by the Office of Naval Research. Now Choi and team have developed a tiny aquatic robot that can skim across the surface, and is powered by onboard bacteria instead of common energy systems like solar, kinetics or thermal.
"Researchers are actively pursuing a variety of innovative strategies to enable self-sustaining robots that harvest energy directly from their marine surroundings," the team notes in its paper. "These strategies include leveraging solar power, kinetic energy from waves or currents, the osmotic potential of saline waters, thermal gradients, and moisture-driven energy sources.
"Despite the innovative nature of these approaches, the variable availability of light and mechanical energy in marine settings, combined with the relatively low energy yields from salinity gradients, thermal differentials, and moisture levels, presents significant challenges. Those limitations hinder the ability to guarantee the reliable and continuous operation of aquatic robots solely based on current energy harvesting technologies."
The powerplant of the new system is built around a microbial fuel cell employing spore-forming bacteria known as Bacillus subtilis for a mini generator inspired by biological digestive processes that converts organic matter into electricity via catalytic reduction-oxidation reactions.
"When the environment is favorable for the bacteria, they become vegetative cells and generate power, but when the conditions are not favorable – for example, it’s really cold or the nutrients are not available – they go back to spores" said Choi. "In that way, we can extend the operational life."
The anode in the fuel cell is fashioned from polypyrrole-coated carbon cloth – selected for its excellent conductivity and ability to support bacterial colonization. The electron-accepting cathode is also carbon cloth, but is decorated with polypyrrole-coated platinum, and chosen for its "catalytic properties to accelerate oxygen reduction." The final part of the puzzle is a Nafion 117 membrane for selective proton transfer.
The integrated powerplant also features adjoining hydrophobic and hydrophilic surfaces to allow "the unidirectional flow of organic substrates" from ocean water to supply the bacterial spores with nutrients.
A single fuel cell setup managed "a maximum power density of 135 µW cm-2 and an open-circuit voltage of 0.54 V" but scaling up to a six-unit array resulted in observed power generation of almost a milliwatt. That output might be relatively small in the grand scheme of things, but it's enough for the small DC motor that sits atop the platform as well as onboard sensors.
"To achieve smooth aquatic locomotion, the robot employs the rotational force of the motor, which exerts a reaction force on the platform, propelling it forward across the water surface without direct force on the water itself," explained the researchers, while the "hydrophobic characteristic contributes to the main buoyancy force." The teeny bot's legs have also been treated with a hydrophobic coating so it can glide across the water's surface like a water strider.
As such, the idea here is to be able to deploy fleets of tiny data gatherers wherever they are needed at any given time, rather than being tethered to one location throughout their operational lifespan.
"While this work successfully demonstrates self-sustainable mobility on water surfaces powered by an integrated MFC array, the exploration into practical applications such as localization, sensing, and signal processing and transmission in aquatic robotic platforms remains an area ripe for development," noted the team. More work on long-term performance and suitability for varying environmental conditions also need to be undertaken. But the current system serves as a proof of concept for the novel design.
A paper of the research has been published in the journal Advanced Materials Technologies.
Source: Binghamton University