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Number 387, August 28, 1998 by Phillip F. Schewe and Ben Stein
ELECTRIC NOISE CAN INCREASE HUMAN TACTILE SENSATION of mechanical forces, new experiments have shown, opening possibilities for electric devices that can enhance sensitivity to touch in the elderly, stroke patients, and people with diabetes. Researchers in Massachusetts (contact Jim Collins or Kris Richardson, Boston University, 617-353-0390) applied a small mechanical force to the finger pads of 11 young, healthy subjects. The force was ordinarily too weak for the subjects to detect, with a magnitude of approximately 0.01 Newtons, roughly equivalent to pressing a pencil tip onto the finger very lightly. However, when the researchers applied this force along with 2 milliamps' worth of randomly fluctuating electrical current through the fingerpad, 9 of the 11 subjects then reported detecting the mechanical stimulus---without feeling the electric current. The researchers speculate that the electrical noise helped nerve cells in the finger pad to reach their threshold for firing a signal through the nervous system. Moreover, they observed that the 2 milliamps of electrical noise appeared to maximize the detectability of the stimulus--suggesting evidence for the phenomenon of "stochastic resonance" (Updates 121 and 293). Although a previous experiment provided evidence for stochastic resonance in tactile sensation of mechanical forces--by using random mechanical vibrations as the noise--this is the first human experiment in which the signal and noise were from different kinds of sources. (Richardson et al., Chaos, September 1998; this paper is available in PDF format--free PDF reader available at Adobe Acrobat web site.)
A TORQUE WRENCH FOR DNA, an experimental tool for twisting the double-helix molecule at one of its ends, can yield once-elusive information on its mechanical properties. DNA is much more than a database of genetic information; it is a versatile machine that can duplicate itself, build and repair cells, and regulate cell functions. By attaching one end of a DNA molecule to a surface and affixing a tiny magnetic bead to the free end, experimentalists in France devised a clever way to twist a single molecule and stretch it with external magnets (T.R. Strick et al, Science, 29 March 1996). But with heat constantly jiggling the molecule and introducing complicated motions, physicists once feared these experiments would not provide useful data. Now, two theory groups have independently filtered out the contributions of this random motion. Considering a DNA molecule going from an unstretched state to a stretched state, they factor in the possible combinations of random twists and bends that the DNA molecule can exhibit in going towards its final outstretched state. With this theoretical contribution, the experiments can now provide a new determination of an elusive mechanical parameter of DNA, its twisting stiffness (C. Bouchiat and M. Mezard, Physical Review Letters, 16 February 1998; J.D. Moroz and P. Nelson, Macromolecules, September 1998, see Lay Language Paper by Moroz and Nelson.)
CHEMISTRY MICROCHIPS might be to chemistry what integrated circuits have been to electronics---a way of rapidly carrying out a myriad of transactions in a compact, controlled environment. Drawing on transistor design principles, early versions of the chemistry chips use voltage (biology has long been able to sort charged molecules by size by pulling them through filters) and gates (based on surface tension) to steer fluid nanoblobs through circuits of capillaries. Applications of the chips include the human genome project and combinatorial chemistry for drug design. (Science News, 15 August 1998.)
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