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Researchers at the Georgia Institute of Technology’s School of Materials Science and Engineering have developed an array of piezotronic transistors that convert mechanical motion directly into electronic signals. “This could make artificial skin smarter and more like the human skin. It would allow the skin to feel activity on the surface,” said Zhong Lin Wang, a Regents’ professor and Hightower Chair at the school.
The devices track tiny polarization charges formed when piezoelectric materials such as zinc oxide are moved or strained. The piezoelectric charge controls the piezotronic transistor’s gate voltage. This approach requires materials that have both piezoelectric and semiconducting properties such as nanowires and thin films based on the wurtzite and zinc blend families of materials, which include zinc oxide, gallium nitride, and cadmium sulfide.
The research arrays have 92 by 92 elements with a density of 234 pixels or “taxels” per inch (see the figure). The transistor elements consist of bundles comprising approximately 1500 individual nanowires. The nanowires are about 500 and 600 nm in diameter. The active strain-gated vertical piezotronic transistors have top and bottom electrodes made of indium tin oxide aligned in orthogonal cross-bar configurations. The array is waterproof due to a thin layer of Parylene.
The transistors can detect pressures as low as 10 kilopascals, which is similar to what human skin can detect. The current arrays are transparent, so they can be used on top of displays. The technology has many potential uses, from mechanical skin to shape detection. It can also provide pressure-sensitive signature recording without an active stylus.