Natural and artificial helical structures

Helical or coiled structures are very common in several biological materials, such as proteins and nucleic acids. They appear also at a macroscopic level in certain human organs, as in the case of the spiral anatomy of the heart muscle bands or the helical twisting of the umbilical cord. Further examples can be found in the rest of the natural world, such as in the structure of certain trees or even in the agglomeration of galactic nebulae and in plasma jets of quasars. Beyond the biological and natural domains, artificial helical structures from the nano- to the macro-scale have been developed by science and technology. Nanosprings made of zinc oxide, helical microtubules of graphitic carbon, helical screws and gears, and the helical flying machine dreamed about by Leonardo da Vinci are just a few outputs of the human interest for this shape. This paper intends to provide a brief overview on natural and artificial examples of helical structures, showing how their geometrical properties have been exploited to achieve different purposes.