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Letting Nature Be the Architect

Single-celled algae called diatoms could provide the structure for mass-produced nanodevices.
October 4, 2005

Inspired by the intricate beauty of the shapes formed by microorganisms – and by those organisms’ ability to reproduce rapidly – a group of researchers centered in Georgia may have found an efficient way to create nanoscale parts for next-generation electronics.

Chemical engineer Kenneth Sandhage of the Georgia Institute of Technology and a team of biologists, geneticists, and electronic engineers have published details of a new process for converting the finely-detailed silica skeletons of diatoms, a type of single-celled algae, into synthetic replicas made of materials such as titanium dioxide, which conducts electricity and could be used in electronic devices.

The new techniques exploit the diatom’s own ability to reproduce, and can be used to mass-produce intricate three-dimensional structures.

“Excellent work” is the description applied by Karl Berggren, head of the Quantum Nanostructures and Nanofabrication Group at MIT, who was not involved in the research. “It’s a new concept for certain big problems in nanofabrication.”
Sandhage says he got the idea after sitting next to a marine biologist on a bus trip. She showed him the elaborate, Christmas ornament-like structures made by diatoms. Sandhage decided to try growing the organisms as templates for potential nanodevices.

That part is easy, since diatoms reproduce through cell fission, creating two exact copies of their silica shells. After 40 generations, a single diatom will have multiplied itself into a trillion copies.

Sandhage then uses a handful of methods to either coat the diatom shells with metallic substances or completely replace them. He uses materials such as titanium dioxide (also known as titania) that are better conductors and can withstand thermal stress, two important features of materials to be used in electronics.

The resulting structures have features measured in tens of nanometers, comparable to the smallest features of chips produced today using conventional photolithographic techniques. The difference: complex three-dimensional shapes can be produced much more quickly using Sandhage’s approach.

That’s important because three-dimensional chip designs could help chipmakers keep delivering more powerful microprocessors at the pace set by Moore’s Law, which says that the number of transistors that can fit on a chip doubles roughly every two years.

Conventional photolithography can be used to build three-dimensional structures by adding and etching one layer of silicon at a time, but it’s a frustratingly slow process, says Berggren.

Pointing to an image published in Sandhage’s article – which appeared in the International Journal of Applied Ceramic Technology – Berggren says, “There’s no way I know of that we could make this structure without the technologies that they’re developing.”

Sandhage’s project isn’t the first time researchers have used organic templates to produce nanoscale devices and materials. Angela Belcher, a professor in MIT’s Department of Materials Science and Engineering, has used viral proteins to assemble a variety of materials, and a startup called Cambrios is pursuing commercial applications of her work.

Daniel Solis, a graduate student in Belcher’s lab, is working on viruses that can attach themselves to gold electrodes and coat themselves with semiconductor material; eventually he hopes to use the viruses to make working transistors.

Diatoms could provide templates for many other types of structures – but exactly what types is not yet clear. Sandhage hopes that the hundreds of thousands of examples of uniquely-shaped diatoms in nature will inspire engineers to consider new design possibilities for processors and memory chips.

Sandhage’s colleagues are already learning about how diatoms’ genes determine their shape, with the hope of allowing engineers to design diatoms to their own specifications.

The genome of one diatom species has been completely sequenced, and another is on the way. Mark Hildebrand, a molecular biologist at the Scripps Institution of Oceanography and partners with Sandhage, believes that the diversity of natural diatom shapes suggests, though counter-intuitively, that there are just a few core genes that control these shapes.

If there are only a few key genes, he says, then relatively few mutations would be required to cause the huge variety of existing shapes. Hildebrand hopes that identifying these genes and manipulating both the genes and the environments in which the diatoms grow up will allow researchers to create novel structures.

That’s a hope seconded by Lucent Technologies’ Joanna Aizenberg, who has produced tiny lenses inspired by the structure of sponges.

“Being able to understand the genetics – how diatoms produce the variety of their forms – may give us the way to produce non-natural forms using their genetic codes,” says Aizenberg.

Sandhage cautions that engineering the diatoms and arranging them into useful structures for electronic devices is “not a trivial challenge.” Aizenberg and Berggren say they agree – but both are reservedly optimistic. 

“There may be limits to how arbitrarily they can engineer them,” says Berggren. “[But] I think they’re going to be able to engineer these diatoms to make different kinds of structures.”

Meanwhile, Sandhage has already developed a couple of uses for his new structures, including using materials that catalyze chemical reactions as the coating for diatoms. The large ratio of surface area-to-volume in structures based on diatoms makes them into ideal catalysts when floating free in a solution, Sandhage says.

He has used catalyst-coated diatoms to destroy pesticides, a technique that might eventually be used to prevent the runoff of dangerous chemicals into streams and groundwater. He has also made photo-luminescent structures by coating diatoms with materials that glow under certain wavelengths of light. The structures could one day be used in computer displays.

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