Starfish embryos swim in formations such as 'living crystals', which can inform the design of swarms of self-assembling robots
MIT scientists have observed that when several starfish embryos spin to the surface, they are attracted to one another and spontaneously assemble into organized crystal-like structures. Credit: Courtesy of the researchers, colored by MIT News
In the early stages, long before growing their signature appendages, starfish embryos resemble tiny beads, spinning in the water like miniature ball bearings.
Now, MIT scientists have observed that when several starfish embryos spin onto the surface of the water, they are attracted to one another and spontaneously assemble into surprisingly organized crystal-like structures.
Even more curiously, these collective “living crystals” can exhibit a peculiar elasticity, an exotic property in which the spinning of individual units—in this case, the embryo—creates much larger ripples throughout the structure.
The researchers found this rippling crystal configuration can persist for a relatively long period of time before dissolving as individual embryos mature.
“This is absolutely incredible—these embryos look like beautiful glass beads, and they come to the surface to form this perfect crystal structure,” said Nikta Fakhri, Thomas D. and Virginia W. Cabot Professor of Career Development Physics at MIT. “Like a flock of birds being able to evade predators, or to fly more smoothly because they can organize in this large structure, perhaps this crystal structure has some advantages that we haven’t realized yet.”
Beyond starfish, he said, this self-assembling collection of rippling crystals could be applied as a design principle, for example in making robots that move and function collectively.
“Imagine building a swarm of soft, rotating robots that can interact with each other like these embryos,” said Fakhri. “They could be designed to self-regulate to ripple and crawl through the ocean to do useful work. This interaction opens up a whole host of exciting new physics to explore.”
Fakhri and his colleagues have published their results in a study appearing today in Natural.
Spin together
Fakhri said the team’s observation of starfish crystals was a “coincidental discovery.” His group has studied how starfish embryos develop, and in particular how embryonic cells divide at the earliest stages.
“Sea stars are one of the oldest model systems for studying developmental biology because they have large and optically transparent cells,” said Fakhri.
The researchers observed how the embryos swam as they matured. Once fertilized, the embryo grows and divides, forming a shell which then grows tiny hairs, or cilia, that propel the embryo through the water. At a certain point, the cilia coordinate to rotate the embryo in a certain direction of rotation, or “chirality”. Tzer Han Tan, one of the group members, noticed that as the embryos swam to the surface, they kept turning, towards each other.
“Once in a while, a small group gathers and dances,” said Fakhri. “And it turns out that there are other marine organisms that do the same thing, like algae. So, we thought, this is interesting. What happens when you put a lot of them together?”
In their new study, he and his colleagues fertilized thousands of starfish embryos, then watched them swim to the surface of a shallow dish.
“There are thousands of embryos in the cup, and they start to form these crystal structures that can grow very large,” said Fakhri. “We call them crystals because each embryo is surrounded by six neighboring embryos in a hexagon that is repeated throughout the structure, very much like the crystal structure in graphene.”
Crystal swaying
To understand what might trigger embryos to clump together like crystals, the team first studied a single embryonic flow field, or the way water flows around an embryo. To do this, they placed a single starfish embryo in the water, then added much smaller beads to the mixture, and took pictures of the beads as they flowed around the embryos on the surface of the water.
Based on the direction and flow of the beads, the researchers were able to map the flow field around the embryo. They found that the cilia on the surface of the embryo pulsed in such a way that they twisted the embryo in a certain direction and created whirlpools on either side of the embryo which then attracted the smaller beads.
Mietke, a postdoc in Dunkel’s applied mathematics group at MIT, worked these flow fields from one embryo into a multi-embryo simulation, and ran the simulation forward to see how they behaved. The model yielded a crystal structure similar to that observed by the team in its experiments, confirming that the crystallization behavior of the embryos was most likely the result of their hydrodynamic and chirality interactions.
In their experiments, the team also observed that once a crystal structure was formed, it persisted for days, and during this time spontaneous ripples began to spread throughout the crystal.
“We could see these crystals spinning and swaying for a very long time, which was really unexpected,” he said. “You would expect these ripples to die off quickly, because water is thick and would dampen these oscillations. This tells us that the system has some kind of strange elastic behavior.”
Long-lasting spontaneous ripples may be the result of interactions between individual embryos, which rotate with each other like gears that lock into each other. With thousands of gears rotating in a crystal formation, many individual spins can trigger greater collective movement throughout the structure.
The researchers are now investigating whether other organisms such as sea urchins exhibit similar crystalline behavior. They are also exploring how these self-assembly structures can be replicated in robotic systems.
“You can play with these interaction design principles and build something like a swarm of robots that can actually work on the environment,” he said.
Researchers build embryo-like structures from human stem cells
Nikta Fakhri, The strange dynamics of living chiral crystals, Natural (2022). DOI: 10.1038/s41586-022-04889-6. www.nature.com/articles/s41586-022-04889-6
Provided by the Massachusetts Institute of Technology
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