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cover tory simple that almost anyone could solve it in a few minutes using pencil and paper. Last year’s experiment, however, showed that DNA computing could solve a mathematical problem with more than a million possible solutions, far too complex for anyone to calculate without a computer. Despite his success, Adleman believes existing DNA computing PROFESSOR ANUPAM MADHUKAR technology is too error-prone to compete against conventional computers. Nevertheless, the fact that something as pervasive in biological systems as DNA can work as a computer raises intriguing possibilities. “It’s possible that we could use DNA computers to control chemical and biological systems in a way that’s analogous to the way we use electronic computers to control electrical and mechanical systems,” Adleman says. In fact, he is not the only computer researcher at USC who has found inspiration in the human body.
BIOLOGICAL INSPIRATION Ted Berger, professor of biomedical engineering, has been working to build a brain prosthetic device for many years that could take over brain functions lost through injury or disease. By bombarding live brain slices from rats with a huge range of electrical impulses and studying the signals that emerge, Berger has been able to model larger and larger groups of neurons. John Granacki, director of the Advanced Systems Division at the Information Sciences Institute, has fabricated computer circuits that mimic Berger’s neuron models. Armand Tanguay, professor of electrical engineering, has designed a precisely shaped photonics array to connect to the hippocampus of the brain. Berger’s team is preparing to test the neural prosthetic device on live rat brain slices. But their single biggest remaining challenge is to attach the man-made hardware to the wetware of the brain. The challenge fits neatly into the USC nanotechnology niche of “smart regulated bio-implant systems,” and Berger has begun exploring a possible nanotech solution to the remaining hurdle. Meanwhile, Berger’s research has spun off a neuron-based speech recognition system that is better than the human ear; it is under commercial development. He and Granacki have received grants from the National Science Foundation and others to develop a next-generation computer chip based on how neurons compute.
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QUANTUM DOTS Some of the School’s longest-running research in nanotechnology involves photonics, an area of strength for almost two decades. In the late 1980s, Anupam Madhukar’s work with semiconductors began to suggest a novel approach to making smaller devices, such as lasers and transistors, than are possible with standard silicon technology. Madhukar, the Kenneth T. Norris Professor of Engineering and professor of materials science and physics, deposited a layer only a few molecules thick of the semiconductor indium arsenide onto a thick gallium arsenide layer. A seven- percent difference in the spacing between the atoms of the two materials resulted in the formation of tiny crystalline pyramids called quantum dots. The pyramids are about seven nanometers high and 20 nanometers in diameter at the base. “Because of the strain induced by the difference in spacing of the atoms, the indium arsenide buckles and creates the pyramidal bumps,” Madhukar explains. “Nature’s own elastic forces form them spontaneously.” Quantum dots contain 10,000 to a million atoms arranged “epitaxially,” i.e., in a defect-free relationship to the underlying semiconductor surface atoms. They offer new electronic properties not present in either much larger or much smaller collections of atoms. Devices such as lasers, detectors and transistors based upon them require a fraction of the power of standard silicon devices. Since publication of dot breakthroughs in Science and Physics Today, introducing and utilizing the concept of “surface stress engineering,” Madhukar has continued to develop this class of quantum dot technology under a USC-led $5 million multi-institution federal grant. Madhukar’s group has found dots that detect the presence of certain wavelengths of infrared radiation. Properly configured, quantum dots could work as the senses and the brains of new machines. He has been trying to attach his quantum dot detectors to biological molecules and cells to create what might be termed smart, biochemical sensors. More recently, his interests have expanded to another class of quantum dots, nearly spherical semiconductor, metal or ceramic nanoparticles with diameters from two to 50 nanometers suspended in a solution. As the lead lab in another concurrent SOME OF ANUPAM MADHUKAR’S POST DOCTORAL RESEARCHERS.
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