Nature abounds with perfect helices. They show up in animal horns and seashells, in DNA and the young tendrils of plants. But helix formation can get complicated: In some cases, the direction of rotation can reverse as a helix grows. The resulting structure has been dubbed the hemihelix, and you may have made one yourself by untwisting part of a telephone cord so much that it flips and spirals in the other direction.
Katia Bertoldi, a professor of applied mechanics at Harvard University, and her colleagues wanted to see how hemihelices form on their own. So they stretched a strip of silicone rubber, glued it to a second, unstretched strip and let the pair go. The researchers reported April 23 in PLOS ONE that they could get a range of shapes to form by tuning the dimensions of the glued rubber pieces. (