Biological Mechanism for Coiling and a Strange New Type of Spring Discovered
Researchers have finally unraveled the biological mechanism behind the creeping of a plant’s tendrils, leading to the discovery of an unusual type of spring.
The new type of spring is quite soft when pulled gently upon, but becomes stiff when pulled upon strongly. Rather than unwinding into a flat ribbon form under stress, “the cucumber’s tendrils actually coil further. Understanding this counterintuitive behavior required a combination of head scratching, physical modeling, mathematical modeling, and cell biology — not to mention a large quantity of silicone.”
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A cucumber plant’s tendril starts as a straight stem that grows until it finds something to support its weight. Once it has secured itself at both ends, it differentiates into a left-handed helix and a right-handed helix that are connected at the center.
“It’s easy to create one of these twistless springs with a telephone cord,” says Gerbode, “and they’re annoying. But with the phone cord, you can pull on both ends and it will straighten out into a flat ribbon. What’s strange about the cucumber tendril is that if you pull on the ends, it actually overwinds, adding more turns to both helices.”
The previously unknown key to the tendril’s behavior is a spring made of “two joined, opposite-handed helices whose bending stiffness is higher than their twisting stiffness. In other words, to form this specific structure, the materials involved have to make it easier for the ribbon to twist axially than to change its curvature.” When the structure is combined with other factors such as the structure of the g-fiber cells and moisture variation, it allows for the interesting behavior of the tendril.
The researchers think that this discovery will lead to new types of bio-inspired twist-less springs. “This is likely to be useful anywhere we need a spring with a tunable mechanical response.”
Source: Harvard School of Engineering and Applied Sciences
Image Credits: Joshua Puzey and Sharon Gerbode
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