Tiny generators embedding thousands of nanowires produce electricity whenever the wires are subjected to mechanical strain, and can be used to power microscopic sensors without the need for batteries.

 The team has been working on nanoscale generators that harness the piezoelectric effect — which allows the transformation of mechanical waves into an electrical signal, and is used among other things in certain types of microphones, for the last five years, finding once more that reducing the size of components means a much improved efficiency, especially compared to its size.
The generators are large arrays of hundreds and even thousands of zinc oxide nanowires. The output voltage is proportionate to the mechanical strains being applied to the wires, and the team confirmed they can produce a peak voltage of 1.26 volts and peak power density of 2.7 milliwatts per cubic centimeter when the material on which they are deposited is subject a straining of a mere 0.19 percent.
The research shows how the wires can then generate current as they are compressed in a flexible enclosure, eliminating the contact with a metallic electrode that was required in earlier devices.
[Xu et al., nature nanotech., (2010), doi:10.1038/nnano.2010.46]
Because the generators are completely enclosed, they can now be used in a variety of environments, harvesting the mechanical forces in seawaves, sonic waves, or even running shoes to power devices without the need for a battery.
The team has already produced two nanosensors working in conjunction with the generators, one to measure the pH of liquids and a second one that can detect the presence of ultraviolet light, both of which work by measuring the amplitude of voltage changes across the device.
The generators were manufactured via a chemical process designed to facilitate low-cost manufacture on flexible substrates. Moreover, extensive tests carried out on nearly one thousand of these nanogenerators showed that, thanks to the absence of moving parts, they can be operated over time without loss of generating capacity.
Future research efforts will go into scaling these nanogenerators up for more and more practical applications.