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Artists and material scientists alike bend, melt and mold materials into useful and aesthetically pleasing forms. But nothing human hands have made can match the intricacy of convoluted corals or the delicate and unique geometry of a snowflake. In a study published yesterday in Science researchers exploited nature’s sculpting methods to create visually stunning 3-D structures that may change the way nano- and micro-materials are made.
Organisms alter their growth patterns in response to changes in their environments. For example, a seashell may switch from a spotted to a striped pattern if there is a change in the temperature, acidity or carbon dioxide level of the water. Wim L. Noorduin, a physical chemist at the Harvard School of Engineering and Applied Sciences (SEAS), used the same concept to coax self-assembling materials to ripple, spiral and bend into structures that resemble leaves, stems, flowers, vases and corals.
The fantastic micro-bouquets showcased in this slide show are not sculpted, but rather grown by design. Noorduin and his colleagues built these crystal structures in a stepwise fashion: first grow the vase, then the stems and finally the petals. The original images are black and white but the researchers false-colored each structure according to the sequence in which it was formed.
Many nanostructures such as silicon memory cells are etched using lithography, a precise but expensive and labor-intensive technique that can only be used on flat surfaces. “There is nothing now to create 3-D structures,” says professor of material science at SEAS, Joanna Aizenberg, who is principal investigator of the study and a pioneer in biomimetics (the use of biological systems as templates for creating materials or designing machines). The new technique is the first that can design and build 3-D structures. It is simple, cheap and efficient, as a whole forest of micro-flowers can assemble themselves simultaneously.
Although the structures created in this study are just for show, the technique has potential for future applications. The folds of these 3-D microstructures pack a large amount of surface area into a tiny space—an important consideration for the production of chemicals that depend on catalysts, substances that speed up chemical reactions. The more surface area available, the more catalysts you can add—and the more efficient the reaction.
The process can also be used to make nonsymmetrical (chiral) structures that may be useful for microcircuits, because chirality plays a role in conductivity.
The technique still needs to be refined before it can be used in these types of applications. The team has developed a mathematical model that maps how the structures evolve, which is important for designing new shapes. Noorduin says they are now working on devices that will allow them to very precisely control the environmental conditions in order to standardize shapes and sizes. They will also need to figure out how to maintain the same level of control for other materials such as carbon, which is used for nanotubes.
Three years after having initiated the project, Noorduin says he still goes back to admire some of his favorite samples. He sits in front of the scanning electron microscope and peers through the lens: “It feels like diving into a strange coral reef,” Noorduin says. “You can spend hours looking at them.”