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14 HEALTH AND LABORATORY MAGAZINE

STARFISH
EMBRYOS SWIM

LIKE A “LIVING CRYSTAL”
IN ORGANIZED FORMATION

Their swirling, clustering behavior may someday
inform the design of self-assembling
robotic swarms.

A starfish embryo, in its pling crystal assemblage shell that then sprouts tiny or the way in which water a very long time, which
earliest stages, before could be applied as a de- hairs, or cilia, that propel flows around the embryo. was absolutely unexpect-
it sprouts its distinctive sign principle, for example an embryo through the To do this, they placed a ed,” she says. “You would
tentacles, looks like a small in building robot swarms water. At a certain point, single starfish embryo in expect these ripples to
bead and spins in the water that move and function the cilia coordinate to spin water, then added much die out quickly, because
like a miniature ball bear- collectively. an embryo in a particular smaller beads to the mix, water is viscous and would
ing. Now, MIT researchers rotational direction, or and took images of the dampen these oscillations.
have discovered that when “Imagine building a swarm “chirality.” Tzer Han Tan, beads as they flowed This told us the system has
multiple starfish embry- of soft, spinning robots one of the group members, around the embryo at the some sort of odd elastic
os spin up to the water’s that can interact with each noticed that as embryos water’s surface. behavior.”
surface, they naturally other like these embryos,” swam to the surface, they
gravitate together and Fakhri says. “They could be continued spinning, toward Based on the direction The spontaneous,
spontaneously assemble designed to self-organize each other. and flow of the beads, long-lasting ripples may be
into a surprisingly organ- to ripple and crawl through the researchers were the result of interactions
ized, crystal-like structure. the sea to do useful work. “Once in a while, a small able to map the flow field between the individual em-
These interactions open up group would come to- around the embryo. They bryos, which spin against
Even more curious still, this a new range of interesting gether and sort of dance found that the cilia on the each other like interlocking
collective “living crystal” physics to explore.” around,” Fakhri says. “And embryo’s surface beat in gears. With thousands of
can exhibit odd elasticity, it turns out there are other such a way that they spun gears spinning in crystal
an exotic property whereby Fakhri and her colleagues marine organisms that do the embryo in a particu- formation, the many indi-
the spinning of individual published their results on the same thing, like some lar direction and created vidual spins could set off
units — in this case, embry- July 13, 2022, in a study in algae. So, we thought, this whirlpools on either side of a larger, collective motion
os — triggers significantly the journal Nature. Co-au- is intriguing. What happens the embryo that then drew across the entire structure.
larger ripples across the thors include Tzer Han Tan, if you put a lot of them in the smaller beads.
entire structure. Alexander Mietke, Junang together?”
Li, Yuchao Chen, Hugh Mietke, a postdoctoral The team is now inves-
This rippling crystal config- Higinbotham, Peter Foster, In their new study, she and researcher in Dunkel’s ap- tigating whether other
uration can persist over rel- Shreyas Gokhale, and Jörn her colleagues fertilized plied mathematics group organisms such as sea
atively long periods of time Dunkel. thousands of starfish at MIT, worked this flow urchins exhibit similar
before dissolving away as embryos, then watched as field from a single embryo crystalline behavior. They
individual embryos mature, SPINNING TOGETHER they swam to the surface into a simulation of many are also exploring how this
according to the scientists. of shallow dishes. embryos, and ran the sim- self-assembling structure
Fakhri says the group’s ulation forward to see how could be replicated in ro-
“It’s absolutely remarkable observations of starfish “There are thousands of they would behave. The botic systems.
— these embryos look like crystals was a “serendipi- embryos in a dish, and they model produced the same
beautiful glass beads, and tous discovery.” Her team start forming this crystal crystal structures that the “You can play with this
they come to the surface has been studying how structure that can grow team observed in its exper- design principle of interac-
to form this perfect crystal starfish embryos develop, very large,” Fakhri says. iments, confirming that tions and build something
structure,” says Nikta and specifically how em- “We call it a crystal be- the embryos’ crystallizing like a robotic swarm that
Fakhri, the Thomas D. and bryonic cells divide in the cause each embryo is sur- behavior was most likely a can actually do work on the
Virginia W. Cabot Career very earliest stages. rounded by six neighboring result of their hydrodynam- environment,” she says.
Development Associate embryos in a hexagon that ic interactions and chirality.
Professor of Physics at MIT. “Starfish are one of the is repeated across the Article: “Odd dynamics of
“Like a flock of birds that oldest model systems for entire structure, very similar In their experiments, the living chiral crystals” by
can avoid predators, or fly studying developmental to the crystal structure in scientists also observed Tzer Han Tan, Alexander
more smoothly because biology because they have graphene.” that once a crystal struc- Mietke, Junang Li, Yuchao
they can organize in these large cells and are optically ture had formed, it per- Chen, Hugh Higinbotham,
large structures, perhaps transparent,” Fakhri says. JIGGLING CRYSTALS sisted for days, and during Peter J. Foster, Shreyas
this crystal structure could this time spontaneous rip- Gokhale, Jörn Dunkel and
have some advantages The scientists were To understand what might ples began to propagate Nikta Fakhri, 13 July 2022,
we’re not aware of yet.” observing how embryos be triggering embryos to across the crystal. Nature.
swim as they mature. Once assemble like crystals, the
Beyond starfish, she says, fertilized, the embryos researchers first studied a “We could see this crystal DOI: 10.1038/s41586-022-
this self-assembling, rip- grow and divide, forming a single embryo’s flow field, rotating and jiggling over 04889-6