Researchers discover new type of spring inspired by nature

Researchers discover new type of spring inspired by nature
Researchers at Harvard University say they have discovered a biological mechanism for coiling that may enable an unusual type of spring. The work, based on the tendrils of the cucumber plant, has revealed a type of spring that is soft when pulled gently and stiff when pulled strongly.

What they found was that, instead of unwinding to a flat ribbon under stress – as an untwisted coil normally would – the cucumber's tendrils actually coil further.

The researchers, led by Professor Lakshminarayanan Mahadevan, recently published their work in Science. Sharon Gerbode, lead author of the paper, said: "What we found was this new kind of spring that no one had characterised before. What's strange about the cucumber tendril is that if you pull on the ends, it actually overwinds, adding more turns to both helices."

Gerbode and coauthor Joshua Puzey started exploring the phenomenon with a silicone model. They stretched a sheet of elastic silicone, secured the ends, and then spread a thin layer of silicone across its surface. When it cured, they cut a thin strip off the model, held both ends and watched it coil into a pair of perfect helices. When they pulled on both ends, however, it simply unravelled and lay flat, adding no extra coils as they had hoped.

Refining their approach, Gerbode and Puzey glued a fabric ribbon to one side of their silicone model and a copper wire to the other side. This saw the silicone strip form a pair of helices that overwound, just like the cucumber tendril.

The structure is a spring made of two joined, opposite handed helices whose bending stiffness is higher than their twisting stiffness. In other words, the materials involved have to make it easier for the ribbon to twist axially than to change its curvature.

Mathematical models have allowed the team to understand the parameters and synthesise a simple principle for the design of these springs.

Prof Mahadevan suggests the benefits of understanding cucumber coils might be useful in technology, but noted the work was driven by pure curiosity, not with an end product in mind. "This is likely to be useful anywhere a spring is needed with a tunable mechanical response," he said.

Author
Graham Pitcher

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