"It is a staggeringly small world that is below," said physicist Richard P. Feynman in his famous 1959 talk to the American Physical Society, when he envisioned that physical laws allowed for the fabrication of micro- and nanomachines and that one day we would be able to write the entire Encyclopaedia Britannica on the head of a pin.

Feynman's vision has finally begun to materialize, thanks to ever more sophisticated microelectronics. Micro- and nanoscale machines are poised to become a multibillion-dollar market as they are incorporated in all kinds of electronic devices. Among the revolutionary applications in development are ultradense memories capable of storing hundreds of gigabytes in a fingernail-size device, micromirrors for enhanced displays and optical communications equipment, and highly selective RF filters to reduce cellphone size and improve the quality of calls.

But, again, at very small scales, chemical batteries can't provide enough juice to power these micromachines. As you reduce the size of such a battery, the amount of stored energy goes down exponentially. Reduce each side of a cubic battery by a factor of 10 and you reduce the volume—and therefore the energy you can store—by a factor of 1000. In fact, researchers developing sensors the size of a grain of sand had to attach them to batteries they couldn't make smaller than a shirt button.

In the quest to boost microscale power generation, several groups have turned their efforts to well-known energy sources, namely hydrogen and hydrocarbon fuels such as propane, methane, gasoline, and diesel. Some groups are developing microfuel cells that, like their macroscale counterparts, consume hydrogen to produce electricity. Others are developing on-chip combustion engines, which actually burn a fuel like gasoline to drive a minuscule electric generator.

There are three major challenges for these approaches. One is that these fuels have relatively low energy densities, only about five to 10 times that of the best lithium-ion batteries. Another is the need to keep replenishing the fuel and eliminating byproducts. Finally, the packaging to contain the liquid fuel makes it difficult to significantly scale down these tiny fuel cells and generators.