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ANALYTICAL CURRENTS Swell hydrogel devices Some polymer hydrogels display dramatic, reversible volume changes in response to stimuli, such as varying temperature, pH, or electric fields. The phenomenon has led researchers to create hydrogel actuators, which are sometimes described as artificial muscles. Unfortunately, the time required for a hydrogel to expand or contract is often slow, ranging from tens of seconds to minutes. In general, the response time increases in proportion to the system size. So why not go small? David Beebe and co-workers at the University of Illinois at Urbana– Champaign and the University of Wisconsin–Madison used microlithographic techniques to pattern miniature hydrogel actuators for controlling the flow of liquids in submillimeter channels. In one configuration, the pH of the working fluid determined the hydrogel valve settings, sending acids in one direction and bases in
Polymer hydrogel pillars swell in response to pH changes, regulating liquid flow. (Adapted with permission. Copyright 2000 Nature Publishing Group.)
another, and shutting off completely at pH 6.7. In another setup, a control fluid operated a valve that was separated from the working fluid by a thin membrane. Actuation times were only
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DNA transcription close-up
seconds due to the minuscule scales of the hydrogel structures, which may lead to novel biosensors and drug delivery systems. (Nature 2000, 404, 588–590)
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During DNA transcription, RNA polymerase (RNAP) molecules tend to move discontinuously, pausing at various sites before important transcription events. Until recently, most experiments into the dynamics of DNA tranPipette scription have relied on studies of bulk complexes halted by nucleotide starvation to provide a snapshot of RNAP activity. Because these types of studies involve large numbers of DNA molecules, some details are averaged out. Carlos Bustamante and co-workers at the University of California–Berkeley, the University of Wisconsin–Madison, and Lawrence Berkeley National Laboratory now report that they have directly observed transcription pausing and arrest activity by E. coli RNAP. The researchers began by tethering a single DNA molecule between two streptavidincoated, 2.2 µm polystyrene beads. One bead was mounted on a glass pipette tip, and the other was held in a laser optical trap. The researchers controlled the tension on the DNA strand by varying the flow of fluids past the beads and manipulating the trapping laser. Because DNA contracts when RNAP is actively transcribing, the researchers can record RNAP dynamics by monitoring the distance between the beads with a video microscope. Bustamante’s group studied transcription as a function of the surrounding solution conditions and the tension applied to the DNA strand. (Science 2000, 287, 2497–2500)
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Schematic of RNAP transcription of DNA tethered between polystyrene spheres. (Adapted with permission. Copyright 2000 American Association for the Advancement of Science.)
