een up close, hydrogen looks like a recipe for success. Small and simple-one proton and one electron in its most common atomic form-hydrogen was the first element to assemble as the universe cooled off after the big bang, and it is still the most widespread. It accounts for 90% of the atoms in the universe, two-thirds of the atoms in water, and a fair proportion of the atoms in living organisms and their geologic legacy, fossil fuels. To scientists and engineers, those atoms offer both promise and frustration. Highly electronegative, they are eager to bond, and they release energy generously when they do. That makes them potentially useful, if you can find them. On Earth, however, unattached hydrogen is vanishingly rare. It must be liberated by breaking chemical bonds, which requires energy. Once released, the atoms pair up into two-atom molecules, whose dumbbell-shaped electron clouds are so well balanced that fleeting charge differences can pull them into a liquid only at a frigid-252.89°Celsius, 20 kelvin above absolute zero. The result, at normal human-scale temperatures, is an invisible gas: light, jittery, and slippery; hard to store, transport, liquefy, and handle safely; and capable of releasing only as much energy as human beings first pump into it. All of which indicates that using hydrogen as a common currency for an energy economy will be far from simple. The papers and News stories in this special section explore some of its many facets. Consider hydrogen's green image. As a manufactured product, hydrogen is only as clean or dirty as the processes that produce it in the first place. Turner (p. 972) describes various options for large-scale hydrogen production in his Viewpoint. Furthermore, as News writer Service points out (p. 958), production is just one of many technologies that must mature and mesh for hydrogen power to become a reality, a fact that leads many experts to urge policymakers to cast as wide a net as possible. In some places, the transition to hydrogen may be relatively straightforward. For her News story (p. 966), Vogel visited Iceland, whose abundant natural energy resources have given it a clear head start. Elsewhere, though, various technological detours and bridges may lie ahead. The Viewpoint by Demirdöven and Deutch (p. 974) and Cho's News story (p. 964) describe different intermediate technologies that may shape the next generation of automobiles. Meanwhile, the f ires of the fossil fuel-based "carbon economy" seem sure to burn intensely for at least another half-century or so [see the Editorial by Kennedy (p. 917)]. Service's News story on carbon sequestration (p. 962) and Pacala and Socolow's Review (p. 968) explore strategies-including using hydrogen-for mitigating their effects. Two generations down the line, the world may end up with a hydrogen economy completely different from the one it expected to develop. Perhaps the intermediate steps on the road to hydrogen will turn out to be the destination. The title we chose for this issue-Toward a Hydrogen Economyreflects that ...
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W hen a child sees a bird flying past or the fluttering wings of a butterfly, does it inspire thoughts about how to build airplanes? Or does it simply convey the idea that flight is possible? We are immersed in the natural world, so it is not surprising that it inspires the design of engineered structures, or that we would like to probe this world further to learn all its secrets. The four Reviews in this issue highlight this two-sided relationship as it applies to the development of new materials. From nature we have learned how soft brittle materials like chalk are made tougher through composite structures. Some of these same design principles have been applied to the much wider range of building blocks available to the engineer (Mayer, p. 1144). Advances in materials processing, particularly in the area of polymers, are also making it possible to fabricate systems with advanced optical capabilities (Lee and Szema, p. 1149), inspired by living eyes and by creatures that we have recently learned can see even though they appear to lack eyes. On the flip side, new material systems have been designed to probe and manipulate biological interactions. Two Reviews examine highly complementary questions. The first summarizes what has been learned about the interactions of cells with surfaces that possess features at different size scales (Stevens and George, p. 1135). The second explores how cells sense the stiffness and strength of underlying substrates (Discher et al., p. 1139). From both we see how new materials are being used to probe cell cycles and interactions and to coax cells to grow in desired ways. The drive toward medical applications is helping push research at the biology/materials interface. On the small scale, nanotechnology is being explored to find new methods for cancer detection and treatment (Service, p. 1132). A Science of Aging Knowledge Environment (SAGE KE) feature story by Davenport explores the progress and problems of engineering tissues in the lab. This endeavor could revolutionize medicine, potentially reversing illnesses such as diabetes, heart disease, and liver failure. As research at the interface between materials and biology increasingly overlaps, we look forward to seeing how each continues to inspire developments in the other.
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