If you're releasing a robot into the aquatic environment with no intention of retrieving it, that bot had better be biodegradable. Swiss scientists have gone a step better than that, with li'l robots that can be consumed by fish when their job is done.
We've already seen a number of experimental "microbots" that can be equipped with sensors and other electronics, then turned loose to wander the wilderness while recording and/or transmitting environmental data.
In most cases, the idea is that when their mission is complete, the tiny, inexpensive devices will simply be abandoned. With that fact in mind, their bodies tend to be made largely out of biodegradable materials. That said, non-biodegradable plastics and toxic chemicals often still factor into their construction.
Prof. Dario Floreano, PhD student Shuhang Zhang and colleagues at Switzerland's EPFL university set out to change that, with their new aquatic robots. Each motorboat-shaped bot is about 5 cm long (2 in), weighs an average of 1.43 grams, and can travel at one-half to three body lengths per second.
Oh yes, and they're made out of fish food.
More specifically, their hulls are made out of commercial fish feed pellets that have been ground into a powder, mixed with a biopolymer binder, poured into a boat-shaped mold, then freeze-dried.
In the center of each robot's body is a chamber filled with a nontoxic powdered mixture of citric acid and sodium bicarbonate (aka baking soda). That chamber is sealed with a gel plug on the bottom of the hull, and connected to a propylene-glycol-filled microfluidic reservoir that forms the top layer of the robot's body.
Once the bot has been placed on the water's surface, water gradually starts making its way through the semi-permeable plug. When that water mixes with the powder in the chamber, a chemical reaction occurs, producing CO2 gas. That gas expands into the reservoir, pushing the glycol out of a hole in the back end of the robot.
In a phenomenon known as the Marangoni effect, the expelled glycol reduces the surface tension of the surrounding water, pushing the robot forward as it does so – aquatic insects such as water striders utilize this same effect. And importantly, the glycol isn't toxic.
So how might these robots actually be utilized?
Well, initially a batch of them would be placed on the surface of a pond, lake or other body of water. As they proceeded to randomly squiggle their way across the surface, onboard sensors would gather data such as water temperature, pH, and pollutant levels. That data could be wirelessly transmitted, or obtained from some of the bots that were able to be retrieved.
Eventually, their hulls would become waterlogged enough that they would become soft, and start to sink. At that point, fish or other animals could eat them. In fact, an alternate possible use for the robots is the distribution of medicated feed in fish farms.
Even if not eaten, all of the robot-body components would still biodegrade. Needless to say, one challenge now lies in producing sensors and other electronics that are likewise biodegradable – or even edible.
"The replacement of electronic waste with biodegradable materials is the subject of intensive study, but edible materials with targeted nutritional profiles and function have barely been considered, and open up a world of opportunities for human and animal health," says Floreano.
A paper on the study was recently published in the journal Nature Communications.
Source: EPFL
