Biomolecular computer is next breakthrough

Biomolecular computer is next breakthrough – research underway for new types of computer data storage

Christopher T. Freeburn

Halobacterium salinarium seems far removed from the world of computers. The microscopic plant has little in common with silicon, the invaluable element that makes possible today’s speedy personal computers. But a protein produced by this salt-marsh bacterium soon may share in silicon’s glory by allowing computers to store gigabytes of data in a space no bigger than a sugar cube.

Since the late seventies, Robert Birge, director of the WM. Keck Center for Molecular Electronics at Syracuse University, has been exploring the potential of the protein bacteriorhodopsin to serve as a vehicle for data storage. Originally studying rhodopsin, a light-activated protein found in the human eye, he switched to bacteriorhodopsin – whose similarity to rhodopsin prompted its name – as a low-cost alternative. Birge’s team hopes to use the protein to produce memory cards that can store three-and-a-half gigabytes of information.

In 1980, while he was at the University of California at Riverside, Birge was joined by Richard Lawrence of Hughes Aircraft, who was also interested in developing organic computational or data-storage devices. By 1982, they had made one of the first prototypes of a biomolecular computer component.

The device featured a thin layer of bacteriorhodopsin on a small, disk-shaped mirror. Specific sites on the disk were exposed to green and red lasers, causing photochemical changes to the protein that in turn allowed for the transference of data to and from the disk. The process was not very efficient, and maintaining the protein disk required temperatures too low for practical applications.

Eventually, Birge succeeded in solving the temperature problem. “Our memories now work at 30[degrees]C 86[degrees]F, which is slightly above room temperature,” says Birge. “In fact, they work best at 35[degrees]C [95[degrees]F], but we cut it back to 30[degrees] because that’s a good trade-off between temperature, control and data longevity.”

To produce a memory device with a high storage capacity, the Syracuse team added the innovation of “volumetric” (three-dimensional) memory. The protein is embedded in polymer cubes, allowing data to be written to or read from the memory simultaneously. In theory, an optical volumetric memory could store a thousand times as much data as an optical two-dimensional memory. However, technical difficulties mitigate that potential. “It is closer to a factor of 300,” says Birge.

The drive for smaller silicon components requires the construction of ever more expensive and elaborate fabrication plants. As Birge notes, silicon computing is approaching the “wall” of practical limits. “It is not a wall imposed by physics [but] by economics,” he says. “Every time you drop the size of a semiconductor chip by a factor of two, the cost of producing it goes up by a factor of five…. It is not unusual for a fabrication plant to cost over $1 billion.”

Biomolecular devices, on the other hand, are much less expensive to produce and to change. In the case of bacteriorhodopsin, it can be prepared in relatively large quantities in temperature- and oxygen-controlled vessels, a process resembling the fermentation of beer. Birge’s team uses a strain of the bacterium genetically engineered to produce an excess of the protein.”We get roughly 500 milligrams [of protein] from a 10-liter batch,” Birge says.

Furthermore, mutated versions of bacteriorhodopsin can be made without much difficulty, so that computer components can be optimized at a rate that the semiconductor industry cannot duplicate. “For them to make a change on a single component in their computer chip requires many weeks of lithography,” says Birge. “We can do it in a matter of four or five days in the laboratory” – and at far less expense.

Another appeal factor: Bacteriorhodopsin is environmentally friendly. “Semiconductor electronics is a highly polluting industry,” notes Birge. By contrast, the only toxic ingredients used to produce biomolecular components are the polymers used to make the protein cubes. This has prompted his group to look for nontoxic substitutes.

Several barriers remain to be surmounted before volumetric memories reach the commercial market.! “We still have reliability problems,” Birge explains. As data are written, variations in the refraction of light in the cube can make it difficult to accurately locate and read data. Another problem, according to Birge, is the lack of lasers with good coherence properties for writing data at the highest density.

A third issue is homogeneity. The distribution of protein molecules within the cube must be uniform for the memory to function effectively. But during the polymerization process that creates the cubes, the pull of gravity causes the protein to separate slightly from the polymer, producing a minute gradation within the cube. While the effect is not a problem for most applications, some require ultrahomogeneity.

As Birge points out, a fully biomolecular computer – one using only organic molecules without semi-conductors – will remain science fiction for some time. Such a machine would have to mimic the human brain, the world’s only true biomolecular computer, and science is nowhere near reproducing that.

“I have never made the assumption that biomolecular electronics will replace semiconductors,” says Birge. “What I envision in the future is a hybrid system that takes advantage of the best characteristics of both and combines them into a system with enhanced capabilities.” He believes that the biomolecular components for such a hybrid system could appear commercially in five to 10 years.

Other bioelectronics researchers echo Birge’s optimism. “In my mind, there is no question about it: Organic molecules are here to stay,” says Peter Rentzepis, presidential chair and professor of chemistry at the University of California at Irvine, who is working on his own volumetric memory using other organic molecules.

Indeed, a commercially available volumetric memory seems a virtual inevitability. “Somebody will do it, no question about it,” says Birge. “And when they do, it will have a significant impact on technology and on the way people view the use of biological molecules.”

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