Professor Zhong Lin Wang holds one of the piezotronic transistor arrays

Device mimics touch with sensitivity of fingertip

A device able to mimic the sense of touch with sensitivity comparable to a human fingertip could revolutionise robotics.

A team of researchers from the Georgia Institute of Technology have fabricated arrays of piezotronic transistors, which rely on the piezoelectric effect to convert mechanical motion directly into electronic controlling signals, using bundles of vertical zinc oxide nanowires.

The arrays could help give robots a more adaptive sense of touch, pave the way for “smart” artificial skin, provide better security in handwritten signatures and offer new ways for humans to interact with electronic devices.

"Any mechanical motion, such as the movement of arms or the fingers of a robot, could be translated to control signals," explains Zhong Lin Wang, a Regents' professor and Hightower Chair at the university’s School of Materials Science and Engineering.

"This could make artificial skin smarter and more like the human skin. It would allow the skin to feel activity on the surface."

The arrays include more than 8,000 functioning piezotronic transistors, each of which can independently produce an electronic controlling signal when placed under mechanical strain.

These touch-sensitive transistors – dubbed "taxels" – have an array density of 234 pixels per inch, the resolution is better than 100 microns, and the sensors are capable of detecting pressure changes as low as 10 kilopascals – resolution comparable to that of the human skin, Wang said.

The research project lasted nearly three years and scientists fabricated several hundred of the arrays, which are transparent, flexible, foldable and durable – the devices still operated after 24 hours immersed in both saline and distilled water.

Among the potential applications are multidimensional signature recording, in which not only the graphics of the signature would be logged but also the pressure and speed used to write it.

The device could also be useful in applications such as artificial prosthetic skin, smart biomedical treatments and intelligent robotics in which the arrays would sense what was in contact with them.

"This is a fundamentally new technology that allows us to control electronic devices directly using mechanical agitation," Wang adds.

"This could be used in a broad range of areas, including robotics, microelectromechanical systems, human-computer interfaces and other areas that involve mechanical deformation."

Mimicking the sense of touch electronically has previously been done by measuring changes in resistance prompted by mechanical touch.

The devices developed by the Georgia Tech researchers rely on a different physical phenomenon – tiny polarization charges formed when piezoelectric materials such as zinc oxide are moved or placed under strain.

The vertically-aligned taxels operate with two-terminal transistors. Instead of a third gate terminal used by conventional transistors to control the flow of current passing through them, taxels control the current with a technique called "strain-gating" based on the piezotronic effect.

In the piezotronic transistors, the piezoelectric charges control the flow of current through the wires just as gate voltages do in conventional three-terminal transistors.

The technique only works in materials that have both piezoelectric and semiconducting properties, properties seen in nanowires and thin films created from the wurtzite and zinc blend families of materials, which includes zinc oxide, gallium nitride and cadmium sulfide.

The research was reported on April 25 in the journal Science online, at the Science Express website, and will be published in a later version of the print journal Science.

The research has been sponsored by the Defense Advanced Research Projects Agency, the National Science Foundation, the US Air Force, the US Department of Energy and the Knowledge Innovation Program of the Chinese Academy of Sciences.

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