Sea urchin sperm could inform miniature robot design

Sperm have a unique sense of direction. The reproductive cells of many species are tuned to seek out eggs, no matter how far or how difficult the journey. Take sea urchin sex. First, males and females will puff out clouds of sperm and egg cells into the ocean. To find and fertilize an egg in the open waters, the sperm of these spiny seabed critters follow a chemical bread crumb trail. And engineers are tapping this clever attraction method for smarter, destination-seeking robots of their own. 

A study published December 9 in the journal Physical Review E details the similarities between the trajectory of sea urchin sperm and computer systems that use a type of real-time search approach called extremum seeking. Engineers from the University of California, Irvine and University of Michigan made a mathematical model of the sperm’s pathway to better understand its behavior. According to the authors, assessing the sea urchin’s biological nature could help design miniature robots that follow cues from sources in the same way.

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Since the 1920s, engineers have used extremum seeking as an adaptive control technique to program technologies that help steer or direct systems for maximum function. It’s been used to control and optimize fuel flow in flight-propulsion systems, combustion for engines and gas furnaces, and anti-lock braking systems in cars. At its basics, a system’s extremum seeking algorithm tracks a signal beacon emitted by a source, says Mahmoud Abdelgalil, who studies dynamics and control at UC Irvine and was the lead author of the paper.

When you think of robotic designs, sea urchin sex isn’t quite what comes to mind. But Abdelgalil says their reproductive cells are a useful and well-studied biological model. To find an egg, sea urchin sperm use chemotaxis, where the cells move in response to a chemical stimulus. Sea urchin eggs specifically secrete a compound called a sperm-activating peptide, which interacts with the sperm’s flagellum, controlling how it beats. This curves and bends the sperm’s direction on a path toward the egg. 

“Sperm don’t have a GPS,” Abdelgalil says. “They don’t know ahead of time where the egg is. So they measure the local concentration [of the peptide] at the current position, then they use that information and move in the direction of increasing concentration levels—which we like to call the direction of the concentration gradient.”

It’s the same for an extremum seeking robot: It doesn’t have coordinates or other information about the target’s location—all it knows is that it can measure and follow the dynamic signal from the current position. Abdelgalil got the idea to look at sea urchin sperm when he saw a previously published paper detailing their behavior under a microscope. The trajectory of the sperm looked nearly identical to a proposed model of an extremum-seeking unicycle robot, a simple machine that can only control its orientation and move in a forward direction. 

“As soon as I saw the two pictures, I realized that this is more or less the same,” he says. So, in the new study, Abdelgalil and his colleagues illustrated how key components of the sea urchin sperm’s navigation strategy resemble hallmark features of extremum seeking. 

This extremely effective searching strategy, which evolved over time in nature, could be useful in fine-tuning future system designs and technologies. Extremum seeking algorithms with minimal sensors could help steer miniature robots, like those being tested for targeted drug delivery. Research groups have already explored drug delivery microrobot designs that utilize external signals, Abdelgalil says. For instance, Abdelgalil mentions that researchers at ETH Zurich in Switzerland developed a tiny starfish larva-inspired robot that is guided by sound waves and might one day be useful in delivering drugs directly to specific diseased cells in the body. “I hope my work will eventually be applied in studying or designing microrobots that employ extremum seeking to autonomously navigate environments and find the exact locations of infected cells that need drugs,” he says.

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Abdelgalil also notes that other organisms seem to have some form of extremum seeking, including bacteria searching for food or algae moving in the direction of light. “We can learn from the behavior of these microorganisms to design our robots that behave in a well-defined way when there is no one commanding them,” he says. “This can enhance the autonomy of our more traditionally operated robots.”