> > On the self-assembly line

Funny how you can get a group of materials scientists into a room, and a joke about baby-making turns into an earnest discussion. That's what happened at the recent workshop "Fundamentals and Applications of Mesoscopic Self-Assembly," sponsored by the U of C Materials Research Science and Engineering Center, one of 25 such centers funded by the National Science Foundation. The workshop was organized by associate professor of physics David G. Grier, who cracked the joke while explaining self-assembly.

"Nature has an incredible ability to organize itself into beautiful and complex hierarchical structures-all without a factory and all done at the nanoscale. This," Grier flashed a slide of a happily drooling nine-month-old on the screen behind him, "is my daughter Zöe. She has a visual processor, holographic capabilities, biomechanical actuators, and all of it was constructed by her own cells. She is an amazing self-assembled thing. Inspiring, isn't it?"

Several chuckles went up from the crowd of physicists, chemists, biologists, computer scientists, graduate and undergraduate students, venture capitalists, intellec- tual-property experts, and corporate scouts gathered at the Kersten Physics Teaching Center. They came to hear research on how itsy-bitsy particles organize themselves and how scientists are learning to "guide" such self-assembly, getting nature, in essence, to manufacture computer circuitry and medical devices. On the agenda were scientists from Chicago, Stanford, Princeton, Harvard, and Lucent Technologies, all of whom work at the nanoscale, or the level of one-billionth of a meter.

Heinrich M. Jaeger began the day by slapping a petri dish of BBs onto the overhead projector. After rolling around for a while, the BBs settled into a neat arrangement, albeit with a hole or two. "Beautiful. But you can see the defects." He applied a vibrating device to the petri dish, and most of the gaps worked themselves out.

Jaeger, professor in physics, the James Franck Institute, and the College, and his research group use a similar technique to get metallic particles to self-assemble into nanoscopic "wires" that may eventually conduct heat. They begin by heating chains of a chemical compound called diblock copolymers and allowing the particles to cool on a surface. As they cool, the copolymer chains randomly arrange themselves into snaking grooves. Next, Jaeger sends volts of electricity through the snaking copolymer chains, causing the chains to iron themselves out into straight lines-just as the BBs rearranged themselves when the petri dish vibrated. The straight lines provide a neat template on which he then applies gold or silver. The metallic particles, he's discovered, assemble themselves along the copolymer grooves as if "they know where they want to go." It's not Zöe, a student laughs, but it's a start.

Later in the day, Zöe's father returned to the dais to talk about his patent-pending nanoscopic tool, HOT, or holographic optical tweezers. Grier, who also holds appointments in the James Franck Institute, the Institute of Biophysical Dynamics, and the College, began his project with the same problem raised by Jaeger's BBs: the inevitable defects in self-assembly.

What if we could prevent defects, Grier has asked, by using light to guide self-assembly? Researchers have long known that nanoscopic particles react to lasers like moths to a porch light: as a particle scoots around and comes into contact with a tightly focused light beam, it gets trapped. Move the laser, and the particle moves with it. With HOT, Grier and postdoc Eric R. Dufresne, SM'98, PhD'00, have figured out how to bounce the beam off of a liquid-crystal hologram, splitting a single laser into lots of tweezers-he's produced as many as 400. Grier can upload a pattern onto the hologram, and the particles attracted to the beams are guided into assembling themselves along the pattern too.

Grier believes the tweezers will work in three dimensions, causing particles to assemble vertically as well as horizontally-something his team hopes to demonstrate in the not-so-distant future.

During Grier's talk and at other times throughout the day, a buzz emanated from distinct areas of the room as corporate scouts and researchers discussed possible applications. With the assistance of venture-capital firm ARCH Development Corporation, Grier has launched the Arryx Company to market the invention. Grier's technology could be useful in biomedicine and telecommunications. HOT, for example, could one day reach inside a single living cell to rearrange abnormal genetic material or make a diagnosis, eliminating the need for some biopsies and invasive procedures. The technology also could be used to power optical switches that direct high-speed Internet traffic without electronics.

Quipped Grier during a break, "We scientists are stupid. We have to hold meetings like this to push us to get concrete." Baby-making, apparently, is only the start.-S.A.S.

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