Velcro was designed by studying the structure and properties of cocklebur flowers

A new branch of science and technology has emerged in recent years. Called bio-inspired technology, it learns from the properties and behaviour of plants and animals, particularly their modes of defence and offence, and attempts to produce new technological products, inspired by these properties. It was about 10 years ago that scientists understood how the household lizard can run effortlessly on the ceiling, defying gravity and without dropping off to the floor. The trick lies in the millions of tiny hairs that the palm of the lizard’s fingers, or the toe-pads. These attach to the surface through very weak attractive forces called the van der Waals interaction. The force of each hair on the toe-pad is negligible, but put together thousand and millions of them, and that adds up to considerable strength (shave of these hairlets- and the lizard becomes essentially immobile). Based on an understanding of this phenomenon, scientists have been able to make tape-based adhesives (like ‘Post It’).

Likewise are the flowers of the plant cocklebur inspired the Swiss engineer George de Mestral to invent Velcro. This plant, which occurs in India, particularly in the Madurai region of Tamilnadu (where it is called Maroolimatthai in Tamil), has a number of small ball-like flowers, each decorated with lots of short pin-like hairs all around, which stick to your socks and clothes. Studying the structure and properties of these protruberances, Dr Mestral designed Velcro, the hook-and-loop fastener.

The latest in the series is inspired by the porcupine, a large rodent which carries as many as over 30,000 quills on the back surface of its body. It derives its name from the French ‘pirc spin’ or spined pig. We know it in India as mullam panri in Tamil, Yedu Pandi in Telugu and Sayal in Hindi. Its style of attacking its predator or enemy is deadly. It launches itself backwards with great speed and clashes its hind quarters against the target. As it does so, it drives its quills deep inside the enemy’s body. The quill has an unique structure, which allows for easy penetration into the skin and beneath, but extremely painful during its removal or extraction.

It was this aspect of the porcupine quill that attracted biomedical engineers from Harvard and MIT, led by Drs. Jeffrey Karp of Harvard and Robert Langer of MIT, who have published a paper in the December 26, 2012 issue of PNAS (US). First, they studied the micro-structural details of the quill using field emission scanning electron microscopy. They found the quill tom have a sharp, thin, tapering and razor-sharp tip. This tip contains a layer of backward facing barbs on its surface. These barbs overlap slightly, and the size of the barbs becomes larger, as we move away from the tip to the body of the quill.

The sharp tip enables easy penetration. But when it has to be removed by pulling it out, each of the barbs resists by opening up (a bit like an umbrella), adhering to the skin, damaging it and causing laceration. This makes for easy penetration and painful extraction, making the enemy suffer and learn for life not to go near the porcupine. The researchers measured the force of penetration by using a quill on a muscle tissue, as well as the force of removal or extraction. The force needed to penetrate was about 0.3 Newton units (or N), while that for removal was over 0.44N. In comparison, when the quill of the barbless African porcupine was tried, the force of penetration was twice more, 0.71 N, suggesting that the sharpness of the tip of the barbed quill made entry easier. And the force of pulling out the barbless quill was easier, 0.11 N versus 0.44 N. And when they compared the tissue damage in both cases, they found that the barbed quill needs less energy, going deeper and causing less damage when entering the tissue than the barbless quill. It is when we pull the quill out that the barbed quill causes greater havoc and more pain than the African one.

Of what use is all this? Why not make synthetic quills using polymers like polyurethane (PU) and compare its effects with the natural ones? To this end, they made both barbed and barbless PU quills, as well as a prototypic hypodermic needle with microscopic barbs. As with the natural, ones, the PU-barbed needle adhered to the tissue far better than the PU-barbless needle. And the former adhered to the tissue far better than the latter. The authors thus conclude that such a biomimetic or bio-inspired polymer patch (a small sheet decorated uniformly with an array of micro- or nano-scale PU-barbed quills) would be useful for the development of mechanically interlocking tissue adhesives or needles, trocars and surgical staples.

Last, there is this often-asked question of how these porcupines mate and make babies, with all these quills all over. One acceptable answer goes like this. Both the male and the female approach each other walking on their hind legs. Once ready, they stand belly to belly while the female turns her back on him and arches her tail over her back. The actual mating takes but a minute and, after a long gestation, a porcupette is born.