By Anwen Bowers
Take a stroll through almost any art gallery and the cultural value of pearls as a status symbol through time is inescapable. From the intricately laced clothes in Elizabethan portraits to the long strings worn by chic youth in the early 20th century photographs, pearls have been a symbol of wealth and glamour. However, shifts in technological capabilities mean that pearls could soon have a much broader range of uses than being merely decorative.
Starting life as a humble piece of grit, pearls gain their ethereal shimmer from nacre, a biological substance secreted by oysters, which eases the discomfort once grit enters their shells. Nacre is produced by a number of different molluscs, and can also be found inside snail shells and coating mother-of-pearl. This material has long been of interest to materials scientists due to its incredible toughness.
Scanning electron microscopy has been used to reveal that the structure of nacre is similar to that of a brick wall, with “bricks” in the scale of micrometres being glued together by an organic adhesive. The bricks themselves are made of aragonite, a form of calcium carbonate with a similar structure to sea shells. These bricks overlay each other, and when pressure is applied they are able to slide against each other which prevents the material from snapping. If any cracks form in nacre then the adhesive acts as a barrier, dissipating the energy along the channels between the bricks and preventing the crack from propagating through the material.
This structure gives nacre the very desirable properties of strength and toughness combined, and a number of strategies have been proposed to create a synthetic material that mimics this brick and mortar structure. Most of the proposed methods have involved a “ground up” strategy of assembling component parts, but this has only ever produced materials that have too high a proportion of adhesive and not enough solid bricks. Nacre itself is 95% aragonite, with only a tiny amount of adhesive holding everything together.
A more recently proposed “top down” method involves a laser which carves into glass and fills the channels with polyurethane glue. This technique has created a material that strongly mimics the properties of nacre. It can be applied to glass or ceramic, both strong materials that are normally limited by their brittleness. By treating them with the polyurethane, scientists have created composite materials that have 700 times the toughness of the original glass!
Biology has been evolving materials for millions of years, to make structures for protection, support, buoyancy, cutting and grinding food, even building organic homes. Whereas slow evolution in the natural world means that each material has tailored properties making it the best choice for different functions, as humans we apply a relatively narrow range of materials to a very broad range of uses. As some materials become depleted, and others face environmental issues, it is vital that we explore other options, and tapping into the wisdom of nature seems like a good place to start.