Researchers at the Georgia Institute of Technology (Georgia Tech), Atlanta, have fabricated arrays made of bundles of vertical zinc oxide nanowires, which are capable of converting mechanical motion into electronic controlling signals. The arrays could help give robots and prosthetic devices a more adaptive sense of touch, provide better security in handwritten signatures, and offer new ways for humans to interact with electronic devices. The research was reported online April 25 in the journal Science, at the Science Express website, and will be published in a later version of the print journal.
The arrays, piezotronic transistors, rely on tiny polarization charges formed when piezoelectric materials such as zinc oxide are moved or placed under strain. The transistors are made up of bundles of about 1,500 individual nanowires, which are each between 500 and 600 nanometers in diameter. The arrays include more than 8,000 functioning piezotronic transistors. Each transistor can independently produce an electronic controlling signal when placed under mechanical strain. These touch-sensitive transistors, based on arrays of tactile pixels or “taxels,” could provide improvements in resolution, sensitivity, and active/adaptive operations compared to existing techniques for tactile sensing, the researchers said. Their sensitivity is comparable to that of a human fingertip.
Zhong Lin Wang, PhD, a Regents’ professor and Hightower Chair in the Georgia Tech School of Materials Science and Engineering, and his co-authors fabricated several hundred arrays of transistors during the three-year project. The array density is 234 pixels per inch, the resolution is better than 100 microns, and the sensors are capable of detecting pressure changes comparable to that of the human skin, Wang said. Because the arrays would be used in real-world applications, the researchers evaluated their durability; the devices still operated after 24 hours immersed in both saline and distilled water.
The arrays are transparent, which could allow them to be used on touchpads or other devices for fingerprinting. They are also flexible and foldable. According to the study, “the real-time detection of shape changes caused by stretching or twisting is a desirable feature for sensors embedded in an artificial tissue or a prosthetic device.”
“Any mechanical motion, such as the movement of arms or the fingers of a robot, could be translated to control signals,” said Wang. “This could be used in a broad range of areas, including robotics, MEMS [microelectromechanical systems], human-computer interfaces, and other areas that involve mechanical deformation. This could make artificial skin smarter and more like the human skin. It would allow the skin to feel activity on the surface.”
Future work will include producing the taxel arrays from single nanowires instead of bundles, and integrating the arrays onto complementary metal-oxide-semiconductor (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 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.