A BIZARRE effect in which a light beam switches the capillary action of a narrow tube on and off could help make 鈥渓abs-on-a-chip鈥� more practical. These devices could revolutionise tasks such as diagnosing disease, but to make them work engineers need to devise an efficient way to pump fluids around them.
Moving minute quantities of fluids around a microchip poses big problems, as high pressures are needed to force fluid through the ultra-narrow pipework. These high pressures make miniaturised versions of conventional motor-based pumps impractical. Some microfluidics engineers have tried using electricity to drive fluid through the tubes, but this requires high voltages that can lead to overheating and only works with liquids that conduct electricity.
But in a paper published online last week in the surface chemistry journal Langmuir (DOI: 10.1021/la0259631), Tony Garcia and his colleagues at Arizona State University report a new way to move microlitre quantities of fluid around a chip. Their technique uses light to modify a coating on the inner surface of a capillary tube, increasing its affinity for water and drawing fluid into the tube.
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At the heart of Garcia鈥檚 technique are the shape-shifting molecules of the organic chemical spiropyran. In their normal, unpolarised state the spiropyran molecules repel water. But when the molecules are exposed to ultraviolet light they rearrange themselves to create positively and negatively charged regions, and in this form they attract water molecules. Shining visible light causes the molecules to become unpolarised again.
The team deposited a layer of spiropyran onto the inside of a glass capillary tube 0.5 millimetres in diameter. When they shone weak ultraviolet light on the tube the water level inside it rose by 2.8 millimetres.
The Arizona team couldn鈥檛 use the tube as a microsyringe to draw in fluids and expel them again, because shining visible light onto the tube did not cause the water level to fall. That鈥檚 because wet surfaces such as the walls of the filled tube have a stronger affinity for water than dry glass, holding the water inside the tube.
Other experts say it may be possible to work around this problem by modifying the tip of the tube to help draw the fluid out. 鈥淭his is something that can be solved in principle,鈥� says Itamar Willner, a chemist at the Hebrew University in Jerusalem. He envisions Garcia鈥檚 device being used as a tiny pen that could lay down microcircuits or dot a surface with minuscule amounts of chemicals.
In the meantime, though, the current system could be used as a tiny, one-way valve in which UV light turns on fluid flow by causing the spiropyrans to draw water across a short, water-repelling gap. 鈥淭he light would act like a bridge,鈥� says Garcia. 鈥淥nce you get across that bridge, everything would happen by capillary action.鈥� Such a valve could help control the flow of reagents in a microscale 鈥渓ab-on-a-chip鈥� device.
