By determining how a desert plant’s seed pods unfurl when they get wet, biologists discovered new principles for designing materials that respond to their environments. Eventually, the insights may help engineers improve satellites or develop artificial muscles.
Biologist Matthew Harrington and his colleagues at Germany's Max Planck Institute set out to explain the mysteries of the “ice plant,” Delosperma nakurense, a succulent that grows in arid and semi-arid regions of eastern Africa. The plant has adapted to dry conditions by producing special seed capsules that protect their cargo with a boxlike lid of five petal-like sections. When it rains, the capsules unfold. When the capsules dry up, they return to their original shape.
The cycle can happen over and over, though by the time ice plant seed capsules perform their origami feats, they’ve long since been separated from the living plants that created them.
“Generally speaking, dead things don’t move,” said Harrington, whose examination of ice plant origami is published June 7 in Nature Communications. “So when they do, it’s of obvious interest.”
Living plants like the Venus' flytrap can move as well, but Harrington says the lessons embedded in nonliving systems are generally easier to transfer to technological applications, because they don't involve complicated, dynamic processes.
When the researchers examined the seed capsules using a variety of microscopy techniques and computer simulations, they discovered that the ice plant’s relatively complex movements come from a simple mechanism.
Specialized cells in the capsules are filled with a cellulose layer that can absorb large amounts of water. As the cellulose swells, the cell’s unique honeycomb structure translates the increasing pressure into movement along a single axis — like blowing into a noise-maker party favor. The curved shape of the valve also contributes to the reversible movement.
“The organism has encoded in its structure a way to achieve movement while dead. That is interesting by its own right,” says Eleni Katifori, a Rockfeller University physicist who studies the principles behind the behavior of living things and was not involved in this new research. “I think it should be investigated more. What does it mean for the plant? How can we modify it? I would like to see the mechanism examined more quantitatively.”
Harrington and his colleagues will continue studying how water interacts with the swellable cellulose filler, but in the meantime they're trying to create synthetic materials that can mimic the seed capsule’s behavior. That ice plants produce their self-folding materials in an environmentally friendly manner adds to the appeal of understanding them. “Biological organisms have had billions of years to experiment with different ways of making materials better,” Harrington says.
Image: Seed capsule from the ice plant Delosperma nakurense in the hydrated, unfolded state. (M. J. Harrington).
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Citation: "Origami-like unfolding of hydro-actuated ice plant seed capsules." By Matthew J. Harrington, Khashayar Razghandi, Friedrich Ditsch, Lorenzo Guiducci, Markus Rueggeberg, John W.C. Dunlop, Peter Fratzl, Christoph Neinhuis & Ingo Burgert. Nature Communications, June 7, 2011.
