Nature provides a wonderful source of inspiration for scientists. From the development of airplanes and flying machines (influenced heavily by the study of birds) to the development of advanced parallel processing based on the herd behavior of animals, scientists have always looked to the natural world to provide ideas for new technology.
One recurring need is the development of lightweight materials that are also tough. These materials are in constant demand for their use in vehicles; they save fuel costs, help engines to run more efficiently, and also have a high material strength to protect the overall integrity of the structure. One commonly used class of materials are “composites” – mixtures of two or more materials which, by themselves, aren’t very interesting but become incredibly strong when used in combination. One new development in this area was recently disclosed in the American Chemical Society journal Nano Letters. It involves using the material that lines abalone shells, a compound called nacre (also called mother-of-pearl), for the production of airplane components. Nacre is extremely interesting as it’s shatter-resistant and strong, but also very lightweight. Nacre is made out of calcium carbonate (the same material found in chalk and in oyster shells) and a protein. By combining plates made out of a very hard material with a filler of a very soft, but ductile, material results in a composite that is both shatter-proof and lightweight. However, trying to reproduce the microscopic structures of the nacre that are responsible for it’s strength has been difficult, and as a result manufacturing materials on a large scale using nacre has been impossible so far.
Now, however, researchers in Finland (in collaboration with the Finnish paper company UPM) have developed a method for making nacre-like coatings that can be painted directly onto building walls or the skins of aircraft to produce a very strong and lightweight reinforcement.
This is an important breakthrough. There is a problem with using single-component materials for structural components. If you choose something tough, like steel, you run into problems with weight. Lighter materials such as ceramics can hold up under a great deal of stress, but they can easily be shattered. It’s the combination of multiple components – such as nature performs with nacre – that allows the sum to be greater than the parts. The researchers at Helsinkis University of Science and Technology have now disclosed a process whereby strong, disc-shaped particles of clay are combined with a soft, plastic material called polyvinyl alcohol and a binder agent. When these two materials are mixed together in a water solution, the polymeric alcohol coats the surface of the clay and produces a slurry. This slurry can either be pressed into a paper, or directly painted over a surface like a wall.
When dried, the resulting paper or coating consists of row after row of the hard clay disks, all interconnected by the springy columns of the polymer, which act almost like shock absorbers. The coatings are very strong and lightweight, with a great deal of stiffness. This so called “nanoclay” has a similar structure to nacre, which combines tiny particles of hard carbonate with the soft and springy protein. Because of the thermal properties of the clay, the coatings can also act as a heat / fire shield.
Perhaps the most exciting part of this idea is that the polymer can be easily modified before use, giving rise to a wide range of possible final structures. Softer polymers and / or sticker binding agents may result in coatings that are even more resilient than this initial, proof-of-concept material. Further work is being done to push the boundaries of toughness and strength. The clay and polymer are both nontoxic, and are applied in a water solution, meaning that there are benefits for the environment and for the health of workers, as well. All of this from the inspiration of a natural material – imitation is definitely sincere flattery.