WHY don鈥檛 your eyelids get stuck to your cornea? And what makes cartilage so slippery that your knees can bend smoothly? Some clues may lie in what happens when a pair of 鈥渕olecular hairbrushes鈥� get soaking wet.
Water-based solutions lubricate our joints and prevent them wearing out, but how they do this is a mystery, says Jacob Klein, a physicist specialising in the properties of soft matter at the University of Oxford and the Weizmann Institute of Science in Israel. Water alone is not viscous enough to be a good lubricant.
A polysaccharide called hyaluronan has long been thought to play a role. Its long, chain-like molecules are strongly attracted to water molecules, and it can bind up to 1000 times its own weight of water. Hyaluronan is present in the eye and many other body fluids. 鈥淭here is a lot of it in our joints,鈥� Klein says.
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But how exactly does hyaluronan act as a lubricant? Klein and his team suspected that one end of these long molecules might attach itself to the surface of cartilage, leaving the rest of the molecule sticking out like the bristles on a hairbrush. Their guess was that the bristly coating might provide the lubrication.
To test the idea they set about mimicking these bristles. First, they created a type of long-chain molecule called a diblock copolymer, which they made water-attracting at one end and water-repelling at the other. They then anchored the water-repelling ends to thin sheets of mica ceramic, forming brush-like structures. When they dipped a pair of these brushes in a range of liquids, they found that water-lubricated brushes passed over each other much more easily than brushes immersed in any other substance (Nature, vol 425, p 163).
Klein thinks the artificial bristles release positively charged sodium ions into the water trapped in the space between the bristles. When another water-soaked brush approaches, the clouds of positive ions repel one another. 鈥淭hey just don鈥檛 like being pushed together,鈥� he says.
In addition, the bristles become negatively charged. These attract water molecules, which cluster around them in a kind of shell. 鈥淭he water shells are held quite tenaciously to the charge, but at the same time are very fluid,鈥� Klein says. 鈥淭hey act like molecular ball bearings.鈥� That鈥檚 why charged brushes reduce the friction far more than uncharged ones, he suggests.
Klein says the work could form the basis of therapies for people with a painful condition called dry eye syndrome, in which the cornea is poorly lubricated. It could also help reduce friction in artificial joints.
