Cucumber tendrils inspire artificial muscle

Cucumbers and some other plants employ mechanical means to maximise the amount of sunlight they receive. Tightly coiling tendrils seek out support and pull the plant stems upwards. This phenomenon – which goes by the charming name of tendril perversion – is the inspiration for a new form of artificial muscle fibre developed by engineers at MIT.

Other approaches to artificial muscle have included shape-memory alloys, servo motors, polymers which respond to external stimuli, and hydraulics. However, drawbacks have limited their application, such as high weight and bulk of equipment and slow response times. The MIT team, working under the direction of Prof Polina Anikeeva, claims that the new fibre-based system is both lightweight and fast to respond.

In a paper in Science, Anikeeva and colleagues explain how they used a fibre-drawing technique to combine two dissimilar polymers – a stretchy cyclic copolymer elastomer and a semi-crystalline thermoplastic polyethylene – into a single strand of fibre. The two materials have very different thermal expansion coefficients, and when warmed the material which tends to expand faster is held back by the other material. In the same way that the bimetallic strip in the thermostat bends away from in electrical contact, the fibre starts to bend towards the slower-expanding material.