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Harvard Engineers Propose 'Molecular Car'
Theorists suggest new approach to shuttling target molecules like passengers
Cambridge, Mass. - May 14, 2004 - Facing researchers' desire to manipulate materials on an ever-smaller scale, engineers in Harvard University's Division of Engineering and Applied Science suggest that a "molecular car" could shuttle target molecules just as conventional vehicles transport passengers. Their efforts, which could satisfy growing demand to control the behavior of individual atoms for electronic, biomedical and nanotechnology applications, will be described in the May 25 issue of the Proceedings of the National Academy of Sciences, appearing online this week.
The molecular car now exists only in computer simulations, but Zhigang Suo and his colleagues are looking for ways to overcome the hurdles between the drawing board and the showroom.
"Imagine if you could drive a tiny vehicle that shuttles individual molecules from one place to another on a chip," says Suo, Gordon McKay Professor of Mechanics and Materials at Harvard. "This level of control could open the door to countless applications."
The molecular car proposed by Suo and graduate student Wei Hong could help scientists in their quest to create increasingly small patterns for myriad applications, such as lithography to create smaller electronics, or delivery of self-assembling molecules to a place where they react. The vehicle would be propelled by an array of electrodes, taking advantage of the slight dipole, or electrical polarity, present in materials because of uneven distribution of electrons.
A molecular car has an "engine," "wheels," and a "passenger seat." At the heart of the car, its engine is an electric dipole. Energy is supplied by an array of electrodes beneath the substrate surface. When the electrodes are charged sequentially, the resulting electric field makes the car move. Because the electric field pattern is programmable, with individual electrodes able to be turned on and off, the car can move forward or backward, make a sharp turn or park.
"We propose harnessing the power of dipoles in a novel way, so that these cars move very fast and yet remain on the substrate," Suo says.
To pick up passenger molecules, the car would draw molecules to a receptor using a favorable pH or charge. The car would then move to its destination and reverse the process to cause the molecules to disembark.
Inter-molecular attractions lead to a degree of clumping of the cars when the electrodes aren't activated, which Suo says presents the possibility of conducting microreactions within these aggregations. The cars could be pulled together to foster reactions, and then split to distribute reagents as needed.
"The concept of the molecular car raises as many questions as potential solutions," Suo says. "How can the car be controlled under difficult conditions such as thermal fluctuation? What molecules will bind well to the substrate yet move fast? What will the on-chip 'highway' look like? We're now seeking answers to these questions."
Suo and Hong's work is supported by the National Science Foundation, the U.S. Department of Energy, and the Division of Engineering and Applied Science at Harvard.
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