April 25, 2006
Biology and Electronics May Soon Meet Inside Your MP3 Player
UA's nanotechnology research group is using proteins from living cells to "grow" wires on microchips.
Their work promises to revolutionize the way microchips are made by combining biology and electronics "” leading to smaller, faster and more efficient circuits for cell phones, computers, MP3 players and a thousand other microelectronic devices.But that's only one of the benefits of this research.
The work holds promise in several areas, such as improving testing methods for anticancer drugs, connecting molecule-sized transistors to the outside world, and extracting electricity from highly efficient photosynthesis proteins that could be used to replace today's far less efficient solar cells.
Proteins called microtubules (MT) are the key to all of this.
MTs form long, thin strands that can be turned into tiny wires, said Materials Science and Engineering (MSE) Professor Pierre Deymier, one of the co-founders of UA's Nanotechnology Interdisciplinary Research Team (NIRT).
Connecting Nano to Micro
"Microtubules are about 25 nanometers in diameter," Deymier said. "They're hollow, with an inside diameter of about 15 nanometers. But they can grow to be 100 microns long. So they're prefect for making nano-sized interconnects. They have nano-scales in their cross section, but they have micron-scale lengths."
This makes them ideal for connecting nano-sized components to standard microchip-sized circuit elements. The difference between nano-sized components and micro-sized ones is about 1 to 1,000 "” about the size difference between a foot-long ruler and three football fields lined up end-to-end.
15 nanometers is incredibly small "” much smaller than the connections in today's commercially available microchips. To get an idea of just how small they are, consider that you could line up more than 66,000 of these 15-nanometer wires in a slot just one millimeter wide.
The NIRT team works at the interface of biology, chemistry, materials science, and electrical engineering. It includes co-founders Pierre Deymier in MSE and James B. Hoying from Biomedical Engineering, as well as Srini Raghavan and Brian Zelinski from MSE; Olgierd Palusinski from Electrical and Computer Engineering; Ian N. Jongewaard from Pediatrics; Roberto Guzman from Chemical and Environmental Engineering; and Ludwik Adamowicz from Chemistry.
MTs also are ideal for making circuit connections because they already "know" a lot about connecting components. Nature uses them to segregate DNA and chromosomes in a dividing cell. During mitosis (cell division) MTs grow and shrink, appear and disappear, as they're needed.
Exploiting Natural Cellular Processes
"Our strategy is to look at what's happening in the cell, extract these protein elements from the cell, modify them genetically so they can be attached to metal surfaces, and then set up processes that exploit the biology for circuit assembly," Deymier explained.
MTs form the proper connections in cells through a complex process that involves several proteins. They grow from nucleation sites, searching and probing for the correct connection. A capping protein identifies the target site.
NIRT researchers have been able to attach the MTs to circuit sites and then have caused them to grow to capping proteins, which are located at the proper connection sites. After an MT attaches and caps, it becomes stable and the process can be reversed to disassemble the MTs that did not attach.
"We can write different sequences," Deymier explained. "With proteins, there are 20 amino acids that you can put in various sequences. So we can connect to the nucleating protein and capping target proteins at different sites and at different times as we need them."
Proteins don't conduct electricity. So after they are attached, they serve as guides for copper molecules that are deposited on them to form wires.
A Significant Breatkthrough
This is where the NIRT team has achieved a significant breakthrough. If the MTs are coated on the outside, the resulting wires are about 45 nanometers in diameter. But Raghavan and his students have discovered a way to control the coating process so that only the inside of the tube is coated. This results in wires that are 15 nanometers in diameter "” less than half the size of those formed on the outside.
Since the tubes are coated on the inside, they're insulated by the protein coating. "So if you think about two tubules crossing each other, the metals are not going to touch," Deymier said. "This cuts down on the number of circuit layers needed in the chip and reduces processing costs."
Other Promising Applications
In their natural state, MTs are involved in mitosis. Some cancer treatments depend on blocking mitosis to stop cancer cell growth.
Anti-cancer drugs are tested now by putting them in a test tube with MTs to see if MT growth slows. But the MTs are not in a configuration that mimics their natural placement in a cell.
"Since we can make the MTs grow in specific patterns, we also could grow them in a configuration that is an analog of how they are ordered in a cell," Deymier said. "So we would have a geometry that is similar to the cell's geometry and we could test the drugs in a more realistic environment."
In another area, researchers in university and industry labs have been working on transistors that are only as large as a single molecule. One of the big issues is how to connect them to the outside world. " How do you bridge the scale of these nano devices to the scale of chips that you put into your computer?" Deymier asked. One way might be to connect them using MTs.
"In electronics, you have micro scale and even macro scale," Deymier said. "You need to bring in electricity and bridge the scales. You have to go from macro to micro to nano and back again. These microtubules may be a good way of bridging that."
They also may be the link between the outside world and proteins that act like solar cells.
"Currently, photovoltaic cells are not too efficient," Deymier said. "Plants are much more efficient, and we're working with researchers at the University of Tennessee who are using plant proteins to efficiently convert sunlight to electricity. But their main problem is getting those electrons out into micro-sized circuits where they can be used. We believe microtubles can be used to bridge that gap."
On the Net: