Self-assembly principle offers useful devices, deep insight
05:12 PM CDT on Sunday, September 5, 2004
Heiko Jacobs' novel manufacturing methods can be compared to baking a magical cookie – one with chocolate chips that spontaneously form smiley faces before the batch comes out of the oven.
Dr. Jacobs' ingredients aren't found in any kitchens, though. Instead of flour, sugar and eggs, the engineering professor combines tiny bits of electronic gadgetry and common chemicals.
After a little mixing, and no more than five minutes, this gadgetry comes together to form hundreds of LEDs, the tiny indicator lights in everything from car dashboards to digital cameras. And nearly all these self-assembled LEDs work reliably, successfully turning on and off when connected to a power supply.
"From a layman's point of view, it's true that what we do is just suspend components in a liquid, agitate the liquid, and sit back and observe the process of spontaneous self-assembly," says Dr. Jacobs, who, along with two colleagues from the University of Minnesota, described the results last month in the Proceedings of the National Academy of Sciences.
Attracting scientists from physics, chemistry, engineering and biology, self-assembly research is blossoming in several universities and a handful of companies. Scientists are mixing together this diverse expertise to explore deep questions in biology. And they're applying what they learn to the manufacture of simple microelectronics on a massive scale.
In 2001, George Whitesides built one of these simple devices using a not-so-simple technique. The Harvard chemist strung together some crumb-sized electronic components along a length of flexible wire. When he dipped the string into the right mixture of chemicals, it spontaneously folded up into a working, albeit very basic, memory device. Proteins, basic biological structures, put themselves together in much the same way.
"Biology provides the inspiration," says Dr. Whitesides, who has reported his results in Science and in the Proceedings of the National Academy of Sciences. "Cell membranes, proteins, nucleic acids – no machines put these together."
It's not just the living world that's full of self-assembly examples. Anyone who has ever "grown" crystals in a basement science experiment, or looked at a salt crystal under a microscope or magnifying glass, has seen a self-ordered system.
Arguably the world's most famous self-assembled molecule, DNA, was described more than 50 years ago. And given the tremendous progress in chemistry and biochemistry ever since, "sometimes we're asked, 'What's new here?' " says Dr. Whitesides.
What's new is applying these biological principles to build useful, miniature machines. The University of Washington's Karl Böhringer, for instance, is piecing together millimeter-sized parts to create self-assembling micropumps.
A puff of air pushes Dr. Böhringer's tiny pump-parts across a notched surface. When a part hits one of the notches, it's pulled snugly into the site. Those that miss are recycled and sent across the surface a second time.
Oil and water
Researchers experiment with a variety of chemical coatings and dips. But the basic physical process that nudges the parts properly into place is the same one displayed anytime you vigorously stir a few tablespoons of oil into a glass of water.
In the glass, oil and water repel each other. So the oil adjusts its shape to minimize its contact with the water. Dr. Böhringer's chemical bath is more complicated, but his dipped pump parts lock together for essentially the same reason that the oil droplets clump together when your stirring stops.
"It's just a matter of calculating the forces," says Dr. Böhringer, who described his process in June at a workshop in Hilton Head, S.C., on the science of sensors.
The science of self-assembly intrigues more than just academics. Companies that manufacture tiny sensors are getting into the act, too, inspired by a new requirement from one of the world's largest retailers.
By June 2005, all of the shipping crates and pallets sent through Wal-Mart's massive supply chain must be labeled with tiny radio frequency identification tags. Because of Wal-Mart's size, this means a market for an estimated 1 billion tags. RFID tags are already used to track everything from library books to apparel.
The small devices, roughly the dimensions of a few sticks of gum, are basically "more complex bar codes," says Jay Tu, director of manufacturing for one RFID supplier, Alien Technology.
Alien, based in Morgan Hill, Calif., uses self-assembly to build massive quantities of RFID tags. The company relies on a manufacturing process that's roughly similar to printing newspapers. But instead of newsprint, it's thin sheets of material stamped with circuitry and wiring that are passed, roll-to-roll, through various machines.
The brain of each RFID tag is a tiny silicon chip, less than a millimeter on a side. To build self-assembled tags, Alien mixes these chips into a chemical bath. When the circuitry-stamped sheet is fed through the bath, the chips spontaneously find the right places to attach themselves onto the sheet's surface.
Dr. Tu says the technology is capable of producing 1 million tags per hour, a significantly higher volume than would be possible using traditional assembly-line technology.
Self-assembly's ability to scale up to gargantuan quantities is one of the technology's most beautiful features, says the University of Minnesota's Dr. Jacobs. "Whether there's 100, 1,000 or 10,000 components to be put together, once the self-assembly reaction starts, it should take roughly the same time to finish," he says.
Robotic assembly lines, by contrast, handle each part one at a time. "If you want to make 10 times the number of components on one of these lines, it takes 10 times as long," says Dr. Jacobs.
Despite the recent industrial and academic successes, several challenges stand in the way of more widespread use of self-assembly technology. One of the biggest practical problems is that the tiny gadget-ingredients aren't available on a large scale. So university researchers wind up doing twice the work – building the tiny components in the first place and then figuring out how to best piece them together.
Another challenge is maintaining reliability even as self-assembly recipes get more complicated and involve additional steps. In manufacturing, the main measure of reliability is yield – the percentage of products coming off the assembly line that work properly.
"Modern computer chip factories go through hundreds of steps and still have yields well over 90 percent," says Dr. Böhringer. "We go through a couple of steps and we're lucky to be in the high 90s. And if we add steps to the process, our yields quickly go down from there."
Confident that these problems will be solved, self-assembly's boosters say the technology may eventually be used to build billboard-sized display screens, billion-transistor computer chips and a host of other devices.
The biggest prize, though, has nothing to do with faster, cheaper electronics and everything to do with a deeper understanding of all life – from single-celled protozoa to the family pet.
"I'm sitting in my kitchen looking at this large collection of cells put together into this amazing thing called a cat," says Dr. Whitesides, fielding questions by phone. "What is the difference between a cat and crystal of salt, which also is the product of self-assembly?"
The difference, he continues, is that the crystal of salt assembles on its own with no outside influence. It's just one of hundreds of examples of so-called static self-assembly, a process well understood by chemistry for decades.
A cat, though, puts itself together only when there is a flux of energy through the system. And understanding the mystery of how living systems build themselves up from basic molecular parts, says Dr. Whitesides, "is one of the really deep, conceptual areas in science right now."
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