Machines can see and hear better than humans, but when it comes to the sense of touch, human skin has the advantage.
Now a team of materials scientists from Georgia Institute of Technology has built a flexible, pressure-sensing array of transistors that can be molded to different shapes and is sensitive enough to pick up slight pressures equal to that felt by human fingers.
The sensor could improve robotic prosthetics in a way that would let amputees feel what they hold, and allow robots to sense texture and manipulate delicate objects. The technology could even be embedded in a variety of devices for security measures.
"You could have smart fingerprinting and cover a cellphone with it," said Zhong Lin Wang, co-author of the research study.
Such a cellphone would only respond to owner's fingerprints and no one else's.
The team published their results in the April 25 issue of the journal Science.
To make their sensor, the team used piezoelectric material, a substance that generates a current when it's bent, squeezed or twisted. Do the reverse and send a current into a piezoelectric material, and the material will deform. Although unusual, these materials are commonly used in buzzers and doorbells, and scientists have been exploring them as an alternative energy source.
Wang's team took a different tack: instead of looking at piezoelectric material as a power source, they asked what would happen if the material was used in a transistor, the basic component of every electronic device from radios to computers. Transistors serve as on and off switches inside computer chips, with the two states representing the 1s and 0s that make up the information in a computer.
To make the transistor, they used zinc oxide, which has piezoelectric properties but also works as a semiconductor.
In a lab, they "grew" bundles of microscopic posts of zinc oxide wires between two electrodes of indium tin oxide. Each transistor consisted of about 1,500 of the vertical zinc oxide nanowires and was so small that thousands of these transistors could fit in an area the size of a dime. A polymer layer supported the whole apparatus and a sheet of transistors looks like a square of clear, flexible plastic.
Because the transistors were made from a piezoelectric material, they sent an electric signal whenever the sheet was pressed. What's more, they were able to sense different amounts of pressure coming from different areas. Overall, the transistors were able to create a pattern of the compression, indicating where more tension was located compared to less tension -- just like how a person's skin works.
Zhenan Bao, a Professor of Chemical Engineering at Stanford University, said the impressive thing about the work was integrating the transistors into large sheets where they were able to signal each other. "This work is a nice demonstration of the potential of nanowires for tactile sensing," she wrote in an email.
Bao's own lab has worked on touch-sensitive electronics, with an eye towards building better prosthetics. That work was based on a different principle: measuring the capacitance of rubber membranes. Bao noted that while her design is more sensitive to pressure changes, Wang's is a simpler design because each transistor is also working as the sensing element. In other designs, the sensor and the transistor are two different components.
Another advantage to Wang's design is that it's durable. The polymer base makes the sheet flexible and hard to break or tear and it also acts as a waterproofing layer. The team even dipped the sensor in water for a whole day and it still worked.
The combination of durability and sensitivity opens up lots of possibilities, said Albert Titus, a professor of biomedical engineering at the University of Buffalo. "We can look at what we could build with them," he said, "and how we could play with these." Titus has also been working on systems that mimic living things, and agreed that the method Wang's group invented is a big step towards artificial skins.