CLUMPS of wet hair could be the latest thing in nanotechnology, creating microscopic spiral structures that may form drug delivery systems and surfaces whose colour can be tuned.
While nature makes use of spiral structures of all sizes, engineers have found it hard to create small spirals. 鈥淲e can make helical structures on the molecular scale and on macroscales, but at the level of nanometres and micrometres it has never been achieved,鈥� says .
Now Aizenberg and her team have found a way to make nanospirals self-assemble. They start with an array of epoxy-resin bristles, each 300 nanometres thick and 4 to 9 micrometres long. These are immersed in a mixture of ethanol and water and then taken out and left to dry.
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As the liquid evaporates, its surface tension draws the bristles together in an effect related to capillary action. At first, the tension pulls groups of four bristles together to form what look like miniature Eiffel Towers. Then, as the liquid evaporates further, the four bristles slide past one another and intertwine, making the towers twist. Finally, groups of twisted bristles are pulled together into larger swirls (see a gallery of nanospiral images at www.newscientist.com/article/dn16372).
For this to work, a range of parameters must be optimised. Bristle stiffness and the surface tension of the liquid have to be chosen carefully, and adhesion between the bristles has to be just right. 鈥淚f it is too weak they will spring back when they dry out, if it is too strong they will never slide and turn around each other,鈥� says Aizenberg. This makes the choice of materials crucial.
In early experiments, the structures spiralled clockwise or anticlockwise at random. To control the direction of twist, the assembly can be tilted or the bristles given an elliptical cross section, says Aizenberg (Science, ).
A uniform direction of twist would be vital for some of the applications Aizenberg has in mind. As the size of these spirals is comparable to the wavelength of visible light, an array of them would affect light in interesting ways. This has already been seen, says Aizenberg: 鈥淭he colour of our substrate actually changes during assembly.鈥�
The hope is to develop tunable optical devices that change colour as the spirals curl and uncurl, but Aizenberg is reluctant to say more. 鈥淚 don鈥檛 want to go into details before our studies are complete.鈥�
鈥淥ne idea is for surfaces that change colour as the spirals curl and uncurl鈥�
Further experiments have shown that as the bristles twist, they can grasp tiny spheres. This could lead to a new kind of adhesive which, in principle, could be switched off by re-wetting the bristles to make them untwist.
This grasping property could also be used to target drug delivery to a specific location inside the body, says Aizenberg. A set of bristles twisted around a particular drug might be made to untwist and release their cargo in the right spot by the presence of a particular chemical.
