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CIENTISTS AT CORNELL UNIVERSITY have
created a “synthetic tree” microfluidic device that mimics transpiration, the process by which a plant pulls water through its roots and up to its leaves. In transpiration, evaporation of water through the leaves creates a large negative pressure (tension in the liquid) that drives the process. The analogous large negative pressures generated in the microfluidic device suggest that other processes requiring large pressure differences, such as high-performance liquid chromatography, could be performed in microfluidic devices without external pumps. Assistant professor of chemical and biomolecular engineering Abraham D. Stroock and graduate student Tobias D. Wheeler defined the minimum requirements for transpiration as a water-filled conduit coupled to its surroundings by two membranes. The challenge, Stroock says, was finding the right material for the membranes that separate the bulk water inside the plant from the water in the atmosphere or soil. “The breakthrough was finding the material that could place the liquid in equilibrium with subsaturated phases of water,” he says. “That’s the step that generates these negative pressures and drives the liquid through the so-called plant.” The breakthrough material turned out to be a poly(hydroxyethyl methacrylate) hydrogel. From this hydrogel the team built synthetic trees consisting of “leaf” and “root” networks—each with 80 microchan-
NUCLEAR MEDICINE Temporary reactor shutdowns bring shortage of isotopes A shortage of medical isotopes has developed in Europe, according to the European Association of Nuclear Medicine. The association says that nuclear medical clinics can no longer carry out a large part of their diagnostic and therapeutic activities and that many patients face danger to their health. The isotopes—primarily molybdenum-99, from which technetium-99m is generated—are used in more than 80% of routine nuclear-imaging tests for procedures such as cardiac blood-flow imaging, bone scanning for secondary tumors, and renal function monitoring. The shortage stems from temporary
shutdowns of the major nuclear reactors that make the isotopes. They are down for several reasons, including routine maintenance, repairs following a lightning strike, and the correction of operating errors, pointed out Andrew Hilson, a nuclear medicine specialist at the Royal Free Hospital, in London, in a recent BBC Radio interview. The situation became serious after the late-August idling of a reactor in Petten, the Netherlands, which normally supplies about 25% of world demand. At the time, its operator, NRG, said it observed gas bubbles escaping from the pipe leading into a key cooling system. As a result,
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NRG won’t run its normal October irradiation cycle. This is not the first time the medical world has faced tight isotope supplies. Last December, NRG upped production to nearly 40% of global demand after a reactor in Chalk River, Ontario, was closed because of problems encountered during routine maintenance. Hilson was one of a trio of nuclear medicine specialists to write to British medical journal BMJ recently to warn about looming shortages (2008, 337, a1577). Their letter said that most commercial reactors are 40 years old and that new capacity is urgently needed.—PATRICIA SHORT
TO BIAS WHEELER
MICROFLUIDICS: ‘Synthetic tree’ pumps water with negative pressure
nels of various lengths arranged in a circle—connected by a single “trunk” microchannel (Nature 2008, 455, 208). “For simplicity, we built this entire network from the root region through the trunk to the leaf region all in a single material,” Stroock says. “The architecture allows us to separate the root from the environment of the leaf.” Because they are separated, the leaf and root networks can experience different water environments. Plant roots are typically in a wetter environment than plant leaves. The water then spontaneously follows the chemical potential gradient from the roots to the leaves. Stroock and Wheeler achieved presStroock holds the transpiration sures in the microdevice of about −70 atm. microfluidic device. Under these conditions the water is metastable, meaning that it is prone to vaporization. “Our method opens up a new route to study the metastable state of liquids under tension,” Stroock says. “Even for water, the world’s most studied substance, the most basic thermodynamic and kinetic properties CLOSE-UP have never been mapped in this regime.” An optical micrograph of The work is a “significant development,” the “trunk” connecting says Pablo G. Debenedetti, professor of chemito a “leaf” network. cal engineering at Princeton University. “It shows how, taking inspiration from nature, it is possible to engineer the reliable, safe, and efficient handling of metastable liquids. The work is remarkable because of the very satisfying way in which it combines a solid grounding in thermodynamics, appreciation for the mechanism of water transport in vascular plants, and clever selection and microfabrication of a synthetic material so as to mimic the action of a tree’s root, xylem, and leaves.”—CELIA ARNAUD ROBERT BA R KER /COR NE LL
TRANSPIRATION MIMIC
