Sensitive Smart Skin : New Technology

An array of piezotronic transistors capable of converting
mechanical motion directly into electronic controlling signals

Knitting  zinc oxide nano-wires vertically, researchers of Georgia Institute of Technology have fabricated arrays of piezoelectric transistors which are capable of converting mechanical motion directly into electronic controlling signals.It can sense touch with the same level of sensitivity as the human fingertip, which could result in better bots and prosthetic.

"Any mechanical motion, such as the movement of arms or the fingers of a robot, could be translated to control signals," lead author Zhong Lin Wang of Georgia Tech's School of Materials Science and Engineering said in a news release. "This could make artificial skin smarter and more like the human skin. It would allow the skin to feel activity on the surface."
The transparent and flexible arrays use about 8,000 taxels. A taxel is a touch-sensitive transistor that can generate piezoelectric signals independently, i.e., emit electricity when mechanically agitated. Each of those two-terminal transistors are constructed with 1,500 zinc oxide nano-wires(500-600 nanometers in diameter). In the array the vertical piezotronic transistors are placed between top and bottom electrodes which are made of indium tin oxide aligned in orthogonal cross-bar configurations. A thin layer of gold is deposited between the top and bottom surfaces of the zinc oxide nano-wires and the top and bottom electrodes, forming Schottky contacts. A thin layer of the polymer Parylene is then coated onto the device as a moisture and corrosion barrier.The array density is 234 pixels per inch, the resolution is better than 100 microns, and the sensors are capable of detecting pressure changes as low as 10 kilo-pascals (resolution comparable to that of the human skin), Wang said. The Georgia Tech researchers fabricated several hundred of the arrays during a research project that lasted nearly three years.

Figure shows a scanning electron microscopy image (A) and topological profile image of fabricated strain-gated piezotronic transistor array. An optical image shows (B) the transparent and flexible SGPT array on flexible substrate

The arrays are fabricated on flexible substrates

In the laboratory, the research group has fabricated arrays of 92 X 92 transistors. The researchers used a chemical growth technique at approximately 85 to 90 degrees Celsius, which allowed them to fabricate arrays of strain-gated vertical piezotronic transistors on substrates that are suitable for microelectronics applications.
 The research group measured the tiny polarization changes when piezoelectric materials such as zinc oxide are placed under mechanical stress. Zinc oxide is used because it can accumulate current. In those transistors, then piezoelectric charges control the flow of current through the nano-wires.Passing the control is known as  “strain-gating.” The technique only works in materials that have both piezoelectric and semiconducting properties. These properties are seen in nano-wires and thin films created from the wurtzite and zinc blend families of materials, which includes zinc oxide, gallium nitride and cadmium sulfide.
The arrays could help give robots a more adaptive sense of touch, provide better security in handwritten signatures and offer new ways for humans to interact with electronic devices. "This is a fundamentally new technology that allows us to control electronic devices directly using mechanical agitation," Prof Wang said. "This could be used in a broad range of areas, including robotics, MEMS, human-computer interfaces, and other areas that involve mechanical deformation."

 Potential Applications:

•     Multidimensional signature recording, in which not only the graphics of the signature would be included, but also the pressure exerted at each location during the creation of the signature, and the speed at which the signature is created.
•     Shape-adaptive sensing in which a change in the shape of the device is measured. This would 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.
•     Active tactile sensing in which the physiological operations of mechanoreceptors of biological entities such as hair follicles or the hairs in the cochlea are emulated.

Future work will include producing the taxel arrays from single nano-wires instead of bundles, and integrating the arrays onto CMOS silicon devices. Using single wires could improve the sensitivity of the arrays by at least three orders of magnitude, Wang said.
The research was reported April 25, 2013 in the Journal Science online and will be published in a later version of the print journal. The research has been sponsored by the Defense Advanced Research Projects Agency (DARPA), the National Science Foundation (NSF), the U.S. Air Force (USAF), the U.S. Department of Energy (DOE) and the Knowledge Innovation Program of the Chinese Academy of Sciences.

Source : Georgia Institute of Technology

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