Cucumber Inspires New Bio-Inspired Twistless Spring
August 31, 2012

Cucumber Plant’s Coiling Ability Studied By Researchers

April Flowers for - Your Universe Online

Researchers at Harvard University have developed a new spring that is soft when pulled gently and stiff when pulled strongly — all from studying a cucumber.

Captivated by the strange coiling behavior in the grasping tendrils of the cucumber plant, the research team found that unlike a normal coil, which would unwind to a flat ribbon under stress when untwisted, a cucumber tendril would actually coil further. Understanding this counterintuitive behavior required a combination of head scratching, physical modeling, mathematical modeling and cell biology — and a large quantity of silicone.

The study, published in the August 31 issue of Science, describes the mechanism by which coiling occurs in the cucumber plant and suggests a new type of bio-inspired twistless spring.

Simple curiosity about the natural world directed this inquiry into the properties of cucumber springs.

"Nature has solved all kinds of energetic and mechanical problems, doing it very slowly and really getting it right," says lead author Sharon Gerbode, a former postdoctoral fellow at SEAS who has now advanced to a faculty position in the physics department at Harvey Mudd College. "But few people have studied biological mechanisms from the point of view of a physicist or an engineer. We barely had to scratch the surface with this question about the cucumber–how does it coil? What could be a simpler question? And what we actually found was this new kind of spring that no one had characterized before."

The coiling tendrils of climbing plants like cucumbers, sweet peas, and grape vines allow the plants to pull themselves to the sunlight and secure themselves to trees or trellises. Yet, previously, no one has studied the biological or physical mechanism of this coiling at the level of the plant's cells and tissues.

A cucumber's tendril starts out as a straight stem until it latches onto something. Then, secured at both ends, it forms both a left-handed helix and a right-handed helix joined at the center by a "perversion" — a strikingly Victorian term coined by Charles Darwin for the point at which the coiling changes direction.

"It's easy to create one of these twistless springs with a telephone cord, 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," says Gerbode.

A fibrous ribbon, made of thread-like cells called gelatinous fiber (g-fiber) cells, runs the length of each tendril, Gerbode and colleagues found when they took a closer look inside. This ribbon appears to provide the force required for the tendril to form a helix without the benefit of muscles while only being two cells thick.

The team thought that if the cells on one side of such a ribbon were to contract, it would force the ribbon to curve and coil, so they tried to reconstruct this fiber ribbon with a silicone model. A sheet of elastic silicone was stretched, secured at each end, and then a thin layer of silicone caulk was applied across the surface. When the caulk cured, they cut a thin strip off the model, held both ends, and watched it coil into a pair of perfect helices. Unlike the cucumber tendril, though, when they pulled on both ends, it unraveled and lay flat.

"This is when I spent a lot of time pulling on telephone cords," Gerbode admits.

The clue, as it turns out, was inside the g-fiber cells. These cells have been studied extensively in trees; they have the ability to shrink or elongate, thanks to a special type of architecture in the cell wall.

"What we think may be happening is that the inner cell layer of the tendril has more lignin in it, which is a sort of glue that gives cell walls stiffness and holds together the cellulose microfibrils, which are like rebar in the cells," explains Joshua Puzey, graduate student of organismic and evolutionary biology. "We thought this stiffness must be related to the coiling somehow."

Puzey and Gerbode glued a fabric ribbon to one side of their model and a copper wire to the other, and at last, the silicone strip formed a pair of helices that overwound like the cucumber tendril.

This spring is made of two joined, opposite-handed helices whose bending stiffness is higher than their twisting stiffness. To form this specific structure, the materials involved have to make it easier for the ribbon to twist axially than to change its curvature.

Through mathematical models developed by team members L. Mahadevan and Andrew McCormick, the team was able to fully understand the parameters and synthesize a simple principle for the design of these springs.

By extracting the fiber ribbon from a cucumber tendril, the research team had already noticed that moisture was playing a role in the spring's behavior. As the extracted ribbon dried out, its stiffness increased and it coiled more tightly. Lignin is also known to be hydrophobic, repelling water. They measured the mechanical response of young tendrils and older ones, finding that the older tendrils put up much more resistance to pulling, a fact that they explained using a combination of theory and computer simulations.

The team has not yet explored these findings from an evolutionary perspective, but they hypothesize that the mature coil structure allows the climbing plants just the right amount of structural flexibility.

"You want the plant to make a nice strong, secure connection, but you also don't want it to be too stiff or to snap," explains Gerbode. "You want it to have a little bit of flexibility so that if the wind blows or an animal brushes past it, it doesn't break. So one possibility is that this overwinding allows the plant to easily accommodate small motions, but then if something really serious happens it can get very stiff and protect itself."

The team needs to study the coils in numerous species and attempt to reconstruct the evolutionary history of the tendril's morphology to gain a better understanding of the evolutionary significance, which could provide important ecological insights.

"The advantage of using a tendril is that the plant saves on complex machinery to build structural supports such as trunks and branches," Mahadevan says. "The disadvantage is that it must depend on other species to build these supports. Thus, tendrils are an adaptation that is likely to develop only in regions replete with vegetation that can provide supports and where competition for resources is intense."

"The real question remains this: how difficult is it to evolve such tendril-like solutions?"

Now that nature has done the hard work, though, Mahadevan suggests that the benefits of understanding cucumber coils might be useful in technology–but hastens to add that this work was driven by pure curiosity, not with an end product in mind.

"This is likely to be useful anywhere we need a spring with a tunable mechanical response," he says.