A team of engineers in the US has fabricated flexible, skin-like arrays of nanowire transistors that convert mechanical motion into electronic signals and are as sensitive as a human fingertip, according to the researchers. This means that the arrays could help robots to adjust intuitively the force they use to grasp things, be used in human prosthetics, as well as offer new ways for us to interface with a variety of electronic devices.
Reproducing a human sense of proprioceptive touch with electronics has proved difficult for roboticists. Existing tactile sensors tend to be made of materials with a resistivity that changes characteristically when touched, but the devices have frustratingly low resolutions – pixels of around 1 mm. The Georgia Institute of Technology team, led by Zhong Lin Wang, reduced the pixel size drastically, to 20–50 µm, and improved on resistive sensitivities by a factor of at least 30 by exploiting a unique physical phenomenon – the piezoelectric effect.
When a piezoelectric semiconductor is subject to mechanical strain, the symmetry of its component crystals gets distorted, creating a polarization charge along the length of the material. Wang used this principle a few years ago to create a new electrical component from bundles of zinc-oxide nanowires held vertically between electrodes: the piezoelectric transistor. Unlike conventional field-effect transistors – that have a current source, a drain and a gate electrode that controls the flow between them – the piezoelectric transistor comprises only source and drain electrodes. The internal piezoelectric polarization of the material acts as the gate, thus modulating the current by dominating how electrons flow at each end of the wire.