A humanoid robot that fights in front of ordinary soldiers, takes bullets for them, and then downs a shot of diesel fuel to continue the fight is the wish-list item that Ray Baughman of the University of Texas at Dallas, Richardson, recalls from a 2004 discussion with John Main, a program manager with the Defense Advanced Research Projects Agency. Baughman hasn't delivered the battery-free robowarrior that Main had in mind, but he and 10 colleagues have devised two types of artificial muscles whose flexing components double as part of the fuel cell that powers them (Science 2006, 311, 1580). In one design, a strip made of platinum-coated carbon nanotubes remains straight until the fuel cell it is part of is switched off. That's when charge quickly builds on the nanotubes, which respond by flexing. The second prototype is based on shape-memory nickel-titanium wires coated with catalytic platinum particles. When the fuel-for example, a vapor of methanol-bathes the wire, a catalytic reaction generates heat, which triggers a reversible shape change in the alloy. In effect, these artificial robotic muscles work by breathing fuel rather than by drawing electricity from batteries.