Taking a vigorous walk tones the muscles; soon it could recharge your personal electronics too. Researchers at SRI International say they've built a boot that can convert 2.5 hours of walking into enough electricity to provide 20 minutes of cell-phone use. The key ingredient is a polymer that generates electricity when flexed. It's one of the so-called dielectric elastomers, which are more rubbery, flex farther and produce much more power than the piezoelectric materials used in earlier attempts at power-generating shoes, says Ronald Pelrine, who heads the SRI project. The U.S. Defense Advanced Research Projects Agency, which is funding SRI's research, is seeking boots that can power electronics like navigation aids. The polymer could also ease the strain of marching by expanding quickly after each step, giving the foot a small boost, Pelrine says. But any shoe-mounted generator has its power limits, he says, beyond which the shoe would become uncomfortable-"like walking in mud."