An interdisciplinary team of researchers at Arizona State University (ASU) has discovered a new nanotechnology effect, the ability of moving water molecules by light. This is a far better way than current methods such as damaging electric fields and opens the way to a new class of microfluidic devices used in analytical chemistry and for pharmaceutical research. For example, this makes possible to design a device that can move drugs dissolved in water, or droplets of water and samples that need to be tested for environmental or biochemical analyses.
The discovery could have an important effect on the fledgling field of microfluidics, said Tony Garcia, an associate professor in the Harrington Department of Bioengineering. The use of an ordinary beam of light to move water around without the need for potentially damaging electric fields, air bubbles (which can denature proteins), or moving microscopic mechanical pump parts (which are expensive to make and difficult to repair) could significantly aid development of microfluidic devices, which are themselves tiny, sophisticated devices that can analyze samples.
What exactly found the researchers?
The ASU team theorized and then proved that a change in water wettability -- the ability of the water molecules to easily move across a surface -- when induced by light can be significantly amplified through a combination of very high nanoscale roughness and chemically coating the surface with molecules.
"We have been working on the problem of using light to move microscopic amounts of water around for drug delivery and microanalysis applications," said Tom Picraux, , professor of chemical and materials engineering. "However, we were stymied by the vexing problem of the combined small effect created and the high degree of attraction that water retains on even a very waxy, or hydrophobic, flat surface.
"Our advance came when we realized that if the surface was roughened at the nanoscale, not only would we obtain the 'lotus leaf effect,' but we could also magnify the small change in water repelling controlled by light to a level that can overcome the hysteresis, or the attraction, that causes water to stick even when a drop is pushed along, " Picraux said. "Rohit Rosario, [a postdoctoral researcher,] mathematically derived the theory for surface change amplification and proved it in the laboratory."
The image above shows two water drops illuminated with a fluorescent dye. The drop on the left (the one that looks round) is sitting on a nanowire surface with a very hydrophobic coating. The drop on the right, which is spread out, is sitting on a flat surface with the same coating. This picture shows that the nanowires create a lotus leaf-like surface. ASU researchers have made that surface photoresponsive and can make the drop on the left move in response to light. (Credit: ASU)
The lotus leaf effect is a fairly well known phenomenon that combines the microscopically rough surface of the plant's leaves with a waxy chemical coating and leads to high water repellency and self-cleaning of the surface. It is already employed commercially in stain repelling pants
So what's next?
The ASU team now is working to design a device that can move drugs dissolved in water, or droplets of water and samples that need to be tested for environmental or biochemical analyses.
Another potential application is reducing the amount of proteins or enzymes needed for testing during drug development. Usually, making and purifying these candidate drugs is time-consuming and small amounts are made at a time.
In a microfluidic device, the cells, DNA, or proteins that are used to test the candidate drug efficacy also are reduced so that a small amount of candidate drug can be mixed with its target and the result recorded. This reduces the time needed to screen all of the drug candidates and allows as many tests as possible to be run simultaneously.
The research work has been published by Journal of Physical Chemistry:B on July 29, 2004. The paper is called "Lotus Effect Amplifies Light-Induced Contact Angle Switching" and is available here if you're an American Chemical Society (ACS) subscriber or if you decide to purchase it for $25.
Sources: Arizona State University news release, via Medi-lexicon.com, July 30, 2004; Journal of Physical Chemistry:B, July 29, 2004
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