Smart Fibres
Nanotechnology
But of all the new technologies being applied to textiles, nanotechnology is generating the most excitement. Nanotechnology is science and technology done at the nth degree of minuteness - the scale of atoms and molecules. It's the use of materials and devices so small that nothing can be built any smaller. Nanomaterials are between 0.1nm (nanometre) and 100nm long. A nanometre is one billionth of a metre (10-9m). Which, to most people means nothing at all. A nanometre is to us what a glass marble is to the Earth. By way of comparison - most atoms are 0.1 to 0.2nm wide, strands of DNA are around 2nm wide, red blood cells are around 7000nm in diameter, and human hairs are typically 80,000 nm across. It is the scale which the basic functions of the biological world operate at.
At scales below 100nm the weird quantum effects of the atomic world start to take hold, changing the optical, electronic or magnetic qualities of materials in unpredictable ways. Nano-scale materials can show physical and chemical properties different to those shown by larger particles of the same material. The relatively larger surface area of the smaller particles, compared to their volume, makes them much more chemically reactive than larger pieces. Silver, popular in jewellery because it is unreactive and tarnishes slowly, shows unique catalytic properties at the nanoscale. Nanoparticles of silver are being incorporated directly into wound dressings because they show strong antibacterial properties. The same approach has been taken to produce socks and shoe liners, which combat smelly feet.
In 1951, the Ealing Studios in Britain made "The Man in the White Suit". The film is a satirical comedy starring Alec Guinness (later Sir Alec Guinness) who plays a brilliant young researcher working in a textile mill, who invents a yarn which repels dirt and never wears out. He has a suit made out of the fabric, which is brilliant white because it cannot be dyed. He is lauded as a genius until both management and the trade unions realise the consequence of his invention - once their customers have purchased enough cloth, demand will crash and put the textile industry out of business. In a weird "life-imitates-art" turn of events, one of the first application of nanotechnology to textiles was the development of spill-proof, self-cleaning trousers.
A self-cleaning cotton fabric known as Nano-Care was developed and is marketed by American company Nanotex, and stain-resistant jeans and khakis made from the material have been available from American firms Gap, Eddie Bauer and Lee Jeans since 2001. Nano-Care fabrics are created by modifying the structure of the cylindrical cotton fibres making up the fabric. At the nano scale, cotton fibres look like tree trunks. Using nanotechniques, these tree trunks are covered in a fuzz of minute whiskers, which creates a cushion of air around the fibers. When water hits the fabric, it beads on the points of the whiskers; the beads compress the air in the cavities between the whiskers creating extra buoyancy. In technical terms, the fabric has been rendered super-non wettable or superhydrophobic. The whiskers also create fewer points of contact for dirt. When water is applied to soiled fabric, the dirt adheres to the water far better than it adheres to the textile surface and is carried off with the water as it beads up and rolls off the surface of the fabric.
A lotus leaf and water droplet through a microscope
(©2006 William Thielicke)
(click to enlarge)
The concept of "self-cleaning" is based on the leaves of the lotus plant. While the self-cleaning abilities of the lotus plant were recognised thousands of years ago – Buddhists venerate the plant because it can grow out of a muddy pond in pristine beauty – the reason why water and dirt skitter off a lotus leaf like drops of mercury, the so-called Lotus-Effect, only became apparent with the advent of the scanning electron microscope. These instruments revealed that the surface of a lotus leaf is rough at the micro-and nanolevels – just like nano-care fabrics, and, just like nano-care fabrics, this roughness means water doesn't spread, but rather forms highly spherical globules. Dirt, which has a greater affinity for water than the surface of the leaf, is simply washed off when it rains. The development of nano-care fabric is a good example of biomimicry: taking a leaf from natures book and applying its properties, perfected over millions of years, to the man-made world.
Unlike conventional water-repellent coatings, the new coating, is permanently bonded onto the fibers of the fabric and will not wash off. And unlike the material in Man in the White Suit, today's self-cleaning fabrics can be made in any color, since the treatment is applied after the fabric has been dyed.
While the range of possible applications of the Lotus-Effect is in inverse proportion to its elemental simplicity, the technology does have its weaknesses. If someone comes along and puts their fingerprint on the material it doesn't function properly on that spot again until the smudge is removed. A lotus leaf repairs itself because it has tiny wax crystals that grow back. Man-made textiles don't have this capacity for self-repair...yet.
Recently, some success has been had in creating fibres directly from nanomaterials themselves. One of nanotechnology's building blocks are tiny, super-strong rolls of carbon atoms known as carbon nanotubes or CNTs. CNTs have a range of unique properties – including the ability to conduct electricity and heat. Although they were first synthesised in 1991, technologists have only recently worked out how to produce yarns made solely from CNTs, opening the door to the production of strong, light, flexible and smart clothing materials. Seventeen times tougher than the Kevlar used for bulletproof vests and six times lighter and twice as strong as steel wire of the same weight and length, CNT fabrics have obvious military applications. In the US, researchers are looking to use the fibres to create body armour. One avenue being explored is using material woven from hollow CNT fibres which have been filled with nanometre-scale magnetic particles. In the presence of a magnetic field generated by, say, a hand-held device, the beads line up, stiffening the fabric to 50 times its normal state.
Creating new fibres and modifying existing ones, both natural and man-made, through the use of nanotechnology offers huge potential to precisely engineer fibres for specific roles. It's likely that in the future combinations of technologies will be used; nanotechnology will be used to refine other approaches, such as micro-encapsulation or electronics. Technology will enable manufacturers to mix and match the capabilities of a variety of fibres and fabrics. Conventionally, there has been a divide between everyday textiles and their cousins – the technical textiles. Technical textiles are materials and products whose performances are more important than their decorative value – such as firefighter and military uniforms, filters and the strong fabrics used by the construction, aerospace, marine industries. Today a convergence between the two is occurring.
