Biology has inspired many human inventions and we now have an emerging field termed bio-inspired material science, especially with novel materials taking a cue from burr, tooth and seashells.
As George de Mestrel was walking his dog one day in the woods of Switzerland, some burrs (prickly sacs of the plant that hold seeds) stuck to the dog’s body and Mestrels’s pants. Shaking them off was not easy. He then looked at them carefully and found hook-like attachments that held the burr on to the dog and to his trousers. Even as he shook them off, he thought if only I can add a loop on the other side of the hook, I can make a device that can replace a zipper. He worked on the idea for 8 years and the fastener Velcro was born.
Velcro is the first well-known man-made device or material that was inspired by what is seen in biology. Plants and animals contain various curious structures and devices in their body which help them in their daily life. For the cockleburr and similar plants (we have them in India, called Banokra, Chota Dhatura, Marulam Athangi), the burr is a transportable seed bag which is dispersed across the land by the animal it sticks to. And for man it became an inspiration to invent a fastening device.
Biology has inspired many such human inventions and we now have an emerging field termed bio-inspired material science. Universities and R &D centers across the world have set up laboratories where scientists study the unusual (and clever) materials and devices that are built into or made by plant and animal bodies, try understand and mimic or model them and create new materials of unusual properties.
A great example is the material out of which the spider weaves its web. The web is a trap into which the prey, even heavy ones such as beetles and lizards, fall and cannot get out. The spider silk has a density and tensile strength comparable to some metallic alloys, and can be stretched up to five times its length without breaking. This has inspired an emerging industry that attempts to make spider silk- like materials for building structures such as motor car and aircraft bodies!
The footpad of an ordinary household lizard is a marvel in itself. We all wonder how in the world it climbs a wall so fast and runs upside down on the ceiling. Electron microscopic study of the footpad shows millions of elaborate tiny hair like protuberances called setae which help the foot cling on to any surface – wet or dry, smooth or rough. Each seta by itself is nothing to speak of in strength- but arrange thousands or millions together, they offer strength, the ability to hold on to any surface, yet release and reattach as the gecko or lizard moves -it works marvels.
Shivaji is reported to have used one such giant gecko (called udumbu in Tamil) to climb a fort wall. While he used the actual animal, today’s materials scientists are inspired by the gecko to generate artificial materials with the same remarkable properties.
An important point that emerges when one studies these unusual biological structures is the set of microstructures that are assembled to make the macro-object. While each strand of the spider silk or the seta of the lizard is by itself too fragile, weak and brittle, when we put them together (as a silk fiber or footpad), we gain strength, stability, toughness and macroscopic properties. A single broomstick is brittle and easily broken, but tie them up together as broom, it becomes strong and unbreakable. It does not have to be only organic materials like silk or setae. Minerals and inorganic materials too are organized in nature to produce strong and tough objects).
A group of mechanical engineers at the McGill University in Montreal, Canada have studied the structures of the tooth (made largely of inorganic material) and sea shells called nacre.
The tooth enamel is an organization of long rods perpendicular to the surface of the tooth, held together by a small fraction of proteins. It is this that offers the tooth enormous strength. The nacre is made up of microscopic tables of chalk, with some proteins and polysaccharides holding them together. It is the interfaces between the layers that “give” when the material is stressed, distributing the energy easily and thus making the entire assembly strong and resistant to breakage.
See how counterintuitive this idea is! Shock is absorbed by the microstructures and interfaces, thus dissipating the energy smoothly, offering the tooth or the nacre enormous strength. The McGill group is attempting to use this principle of putting in interfaces and microstructures in glass, in an attempt to make glass less brittle, unbreakable and even bendable. What was otherwise hard and brittle can be made more pliable for use.
The maverick painter and artist Salvador Dali once painted a “melting” clock dripping like syrup out of a table. Well, he was not crazy but a foreteller of the future (as many artists are). Bio-inspired materials may throw in many surprises and applications in days to come.