Researchers have developed prototypes of sensor-equipped robot bugs that can mimic biological digestive systems for energy needs and move across the water like water striders. Back in 2017, DARPA proposed a plan to develop and deploy thousands of floating sensors aimed at collecting "environmental data such as ocean temperature, sea state and location, as well as data on the activity of commercial ships, aircraft and even marine mammals moving across the ocean."
The project is called Ocean of Things, and its essence is similar to a large number of sensor smart devices that collect information through the Internet of Things. The project page states that sensor data will be uploaded to government-owned cloud storage for analysis. The "Ocean of Things" will support military missions and is also open to research institutions and commercial organizations.
Professor Seokheum Choi of Binghamton University has been working on such a device for more than a decade with funding from the Office of Naval Research. Now, Choi and his team have developed a tiny aquatic robot that can glide on water, powered by onboard bacteria rather than common energy systems such as solar energy, electricity or heat.
"We are actively exploring various innovative strategies to enable self-sustaining robots to harvest energy directly from the marine environment. These strategies include harnessing solar energy, kinetic energy generated by waves or currents, the osmotic potential of salt water, thermal gradients, and moisture-driven energy," the research team noted in the paper.
"Despite the innovative nature of these approaches, the varying availability of light and mechanical energy in marine environments, coupled with the relatively low energy production from salinity gradients, thermal differences and humidity, present significant challenges. These limitations hinder the ability to ensure reliable and sustained operation of aquatic robots relying solely on current energy harvesting technologies."
The power plant of the new system is built around a microbial fuel cell that uses a spore-forming bacterium called Bacillus subtilis, a small generator inspired by the biological digestion process that converts organic matter into electricity through a catalytic reduction-oxidation reaction.
"When conditions are favorable for the bacteria, they turn into asexual cells and produce energy, but when conditions are unfavorable - for example, it's very cold or there are no nutrients - they turn back into spores. This way, we can extend the operational life," Choi said.
The fuel cell's anode is made from polypyrrole-coated carbon cloth -- a material chosen for its excellent conductivity and ability to support bacterial growth. The electron-accepting cathode is also carbon cloth but decorated with polypyrrole-coated platinum, chosen for its "catalytic properties that accelerate oxygen reduction." The final piece of the puzzle is the Nafion117 membrane for selective proton transfer.
The integrated power unit also features adjacent hydrophobic and hydrophilic surfaces, enabling "one-way flow" of the "organic matrix" in seawater to provide nutrients for bacterial spores.
A single fuel cell device had a "maximum power density of 135µWcm-2 and an open circuit voltage of 0.54V," but upon scaling up to an array of six cells, the observed power generation reached almost a milliwatt. This output may be relatively small overall, but it is sufficient for the small DC motors and onboard sensors on top of the platform.
The researchers explain: "In order to achieve smooth water motion, the robot uses the rotational force of the motor to exert a reaction force on the platform, pushing it forward on the water surface without directly acting on the water itself. At the same time, "the hydrophobic properties help to generate the main buoyancy force." The legs of the small robot are also treated with a hydrophobic coating so that it can glide on the water like a water strider."
The aim, therefore, is to be able to deploy the microdata collector where it is needed at any given time, rather than tethering it to one place for its entire operational life.
The research team noted: "While this work successfully demonstrated self-sustained movement capabilities on the water surface driven by an integrated MFC array, the exploration of practical applications such as positioning, sensing, signal processing and transmission of aquatic robotic platforms is still an unexplored area. More work is needed on long-term performance and applicability under different environmental conditions. But the current system can serve as a proof-of-concept for novel designs."
The research paper has been published in the journal Advanced Materials Technology.