SENSITIVITY TRAINING: Bao's lab creates flexible, transparent sensors using carbon nanotubes. (Photos: Linda A. Cicero (top); Steve Fyffe)
Stretch It Out of Shape and This Superskin Springs Back
For Zhenan Bao, 2011 was a busy year.
As prolific as she is inventive, the associate professor of chemical engineering made significant advances in organic and carbon nanotube semiconductor technology, applicable to flexible electronics for displays, sensors and solar cells. Among the latest innovations from her lab is a transparent pressure sensor that can be repeatedly stretched to more than double its initial length in any direction with no permanent deformation or loss of sensitivity. One eventual application could be a touch-sensitive artificial skin for prosthetic limbs or robots.
The key was developing a method for creating a stretchable and transparent conducting film of carbon "nano-springs." Single-walled carbon nanotubes in liquid suspension are sprayed onto a thin layer of silicone, which is then stretched, causing the nanotubes to organize themselves into structures that look and behave like tiny springs. The sensor is made from two layers of this nanotube-coated silicone that sandwich a layer of another type of silicone. Pressure on the sensor compresses the middle layer, changing the amount of electrical charge it can store. The change is detected by the nanotube-coated layers, which act as a capacitor.
"The sensor can register pressure ranging from a firm pinch between your thumb and forefinger to twice the pressure exerted by an elephant standing on one foot," says Darren Lipomi, a postdoc in Bao's lab. That pales in comparison to the sensitivity of a similar device Bao and her colleagues developed in 2010, which can detect a 20-milligram fly alighting. But because in this instance the focus was on making the sensor transparent and stretchable, "we did not spend very much time trying to optimize the sensitivity aspect," Bao says.
She's confident that with a few modifications they can create sheets that are both super elastic and ultra responsive.
SMART APP: Lew and Dharmaraja (top) and Duran devised touch screen keys that find users' fingertips. (Photos: Steve Gladfelter (upper left); Leo Hochberg (upper right); Steve Fyffe) Summer Program Yields a Cheaper, More Versatile Braille Writer
Under the guidance of Stanford mentors, a student in a summer research program helped develop an ingenious application that turns a touch screen tablet into a Braille writer—an innovation that may replace dedicated devices costing up to 10 times as much.
Adam Duran, a senior at New Mexico State University, was one of 15 participants in the Army High-Performance Computing Research Center summer institute. Held annually at Stanford since 2009, the eight-week program brings students from around the country to work with faculty and graduate-student mentors on open-ended research challenges.
This year's challenge was to develop applications for the Android touch screen tablet. Duran's mentors, Adrian Lew, an assistant professor of mechanical engineering, and Sohan Dharmaraja, PhD '11, had planned to create a character-recognition app that would use the device's camera to transform pages of Braille, the system of raised dots used by the visually impaired, into standard text. But after consulting with the Office of Accessible Education they soon realized that "The killer app was not a reader, but a writer," Dharmaraja says.
Lew explains: "Imagine being blind in a classroom, how would you take notes? What if you were on the street and needed to copy down a phone number? These are real challenges the blind grapple with every day."
Duplicating the eight-key Braille keyboard on a tablet was trivial. The real question was: How would someone who cannot see find the keys on a smooth screen? The solution was elegant in its simplicity. They wouldn't. The keys, instead, would find them. When users touch their fingertips to the glass, the keys orient themselves under them.
This app offers a level of customization not possible with dedicated Braille machines, which can cost up to $6,000. On a tablet with spoken menus enabled, a user could switch among different modes—from the alphabet to mathematical or chemical notation, say—by dragging a finger across the screen.
"No standard Braille writer can do this," says Charbel Farhat, chair of the aeronautics and astronautics department and director of the summer program. "I'm always amazed by what the students can accomplish in these courses, but this was something special."
