Mimicking nature

when it comes to building new structures, human ingenuity is no match for nature, especially living cells. Even at the simplest level, molecules assemble into amino acids, then into proteins, which in turn form a variety of different structures.

But now, with the work of Samuel Stupp of the University of Illinois at Urbana-Champaign, usa , there might be a possibility of mimicking the natural building process. Stupp and his colleagues have created molecules which assemble themselves hierarchically over several sizes, from clusters to thick films ( Science , Vol 276, No 5315).

The properties of the thick films are essentially like those of the molecules themselves because the basic units, the molecules, are all aligned in the same direction The basic building-block of the structure is a molecule which was synthesised by Stupp and his team about two years ago. The molecule is called rodcoil by the scientists and has some peculiar properties.

One half of the molecule is made up of biphenyl esters which impart rigidity to the molecule. The other end of the molecule is made up of isoprenes connected with styrenes, causing this end to be flexible. On the rigid end sit sticky water-loving phenolic groups, while on the flexible end sit slippery water-fearing methyl groups.

The process of forming the thick films is hierarchical with three-steps. The rodcoils themselves assemble into small, about 100-molecule sized, mushroom-shaped clusters. The rigid ends form the stems of the mushrooms while the flexible ends form the caps. Once the mushrooms are about 5 nanometres thick in diameter (a thousand-millionth of a metre), different clusters pack side by side into sheets.

Surprisingly, the sheets have all the mushroom shaped clusters with the same side up. The sheets then stack up in layers to form a thick film. As to why the hierarchical building takes place in this fashion giving the film an orientation because all the rigid ends are on one side and the flexible ones on the other is still not known.

But the possibilities of such oriented stacked films are immense. Among the applications being explored are anti-icing coatings for aircraft wings. Since one of the ends of the film is water-fearing, or hydrophobic, the wings could be coated with the film to prevent ice build-up for a long time. They could also be used to fabricate anti-blood clot linings in artificial blood tubes.

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