Twistless spring with tunable mechanical response

As a consequence of research into the functioning of climbing tendrils in the cucumber plant, Cucumis sativus, a team of researchers at Harvard University have applied for a patent for a new type of spring. They discovered that when tension is increased on the coiled tendrils the plant responds by counterintuitively overwinding the coils in the tendril. The researchers – Dr Sharon Gerbode, Dr Joshua Puzey, Andrew McCormick and Professor L. Mahadevan – hypothesized “that the mature coil structure allows the climbing plants just the right amount of structural flexibility” (from the 30th August 2012 Press Release of School of Engineering and Applied Sciences, Harvard University, ‘Uncoiling the cucumber’s enigma’, http://www.seas.harvard.edu/news-events/press-releases/uncoiling-the-cucumbers-enigma). Gerbode explains, “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. 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 researchers were able to replicate the effect artificially, making an artificial tendril spring from a silicon rubber strip core with fabric ribbon glued to one side and a copper wire glued to the other. Mahadevan suggests that this technology “…is likely to be useful anywhere we need a spring with a tunable mechanical response.” Puzey says in the video that the researchers made, that “The discovery of tendril overwinding and its mechanical consequences hints at the possibility of biomimetic springs that can be fine tuned to achieve designer auto-adaptive behaviour with applications that arise everywhere that springs arise” (http://www.guardian.co.uk/science/video/2012/aug/30/plant-tendrils-overwind-pulled-video).

Key Differentiators

Existing helical/coiled springs are, typically, made of steel and have a constant ratio between strength of tensioning force and spring extension. When the force becomes too great the spring becomes over-extended, causing the structure of the steel to permanently change. The spring becomes deformed and has to be replaced. Because the new type of spring proportionately extends less with increasing load the risk of the spring becoming over-extended becomes less, so there would be less replacement of springs.

Biomimicry Story

As described above, the researchers investigated the observed behaviour of the cucumber plant climbing tendrils by making artificial tendrils that mimicked the real tendrils’ behaviour, in particular the coil overwinding response to increased tendril tension. The proposed new type of spring that the patent has been applied for is fundamentally an artificial tendril, and entirely inspired by the natural tendril.

Challenges Solved

Existing coil springs both untwist as the tension on them increases and also fail to respond adaptively to increased load. This new design is twistless and adapts to an increased load by increasing the number of coils in the the spring–a tunable auto-adaptive response.