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Flexible sensor can feel butterfly’s touch

STANFORD (US)—By sandwiching a precisely molded, highly elastic rubber layer between two parallel electrodes, researchers have created an electronic sensor that can detect the slightest touch.

“It detects pressures well below the pressure exerted by a 20-milligram bluebottle fly carcass we experimented with, and does so with unprecedented speed,” says lead researcher Zhenan Bao, an associate professor of chemical engineering at Stanford University.

The key innovation in the new sensor is the use of a thin film of rubber molded into a grid of tiny pyramids, says Bao. The work is described this week in the journal Nature Materials.

Previous attempts at building a sensor of this type using a smooth film encountered problems.

“We found that with a very thin continuous film, when you press on it, the material does not have room to expand,” says Stefan Mannsfeld, a former postdoctoral researcher in chemical engineering and a study coauthor.

“So the molecules in the continuous rubber film are forced closer together and become entangled. When pressure is released, they cannot go back to the original arrangement, so the sensor doesn’t work as well.”

“The microstructuring we developed makes the rubber behave more like an ideal spring,” Mannsfeld says. The total thickness of the artificial skin, including the rubber layer and both electrodes, is less than one millimeter.

The speed of compression and rebound of the rubber is critical for the sensor to be able to detect—and distinguish between—separate touches in quick succession.

The thin rubber film between the two electrodes stores electrical charges, much like a battery. When pressure is exerted on the sensor, the rubber film compresses, which changes the amount of electrical charges the film can store. That change is detected by the electrodes and is what enables the sensor to transmit what it is “feeling.”

The largest sheet of sensors that Bao’s group has produced to date measures about seven centimeters on a side. The sheet exhibited a great deal of flexibility, indicating it should perform well when wrapped around a surface mimicking the curvature of something such as a human hand or the sharp angles of a robotic arm.

Bao says that molding the rubber in different shapes yields sensors that are responsive to different ranges of pressure. “It’s the same as for human skin, which has a whole range of sensitivities,” she notes. “Fingertips are the most sensitive, while the elbow is quite insensitive.”

The sensors have from several hundred thousand up to 25 million pyramids per square centimeter. Under magnification, the array of tiny structures looks like the product of an ancient Egyptian micro-civilization obsessed with order and gone mad with productivity.

But that density allows the sensors to perceive pressures “in the range of a very, very gentle touch,” Bao says. By altering the configuration of the microstructure or the density of the sensors, she thinks the sensor can be refined to detect subtleties in the shape of an object.

“If we can make this in higher resolution, then potentially we should be able to have the image on a coin read by the sensor,” she says. A robotic hand covered with the electronic skin could feel a surface and know rough from smooth.

That degree of sensitivity could make the sensors useful in a broad range of medical applications, including robotic surgery, Bao adds.

In addition, using bandages equipped with the sensors could aid in healing of wounds and incisions. Doctors could use data from the sensors to be sure the bandages were not too tight.

Automobile safety could also be enhanced. “If a driver is tired, or drunk, or falls asleep at the wheel, their hands might loosen or fall off the wheel,” says Benjamin Tee, graduate student in electrical engineering and a study coauthor.

“If there are pressure sensors that can sense that no hands are holding the steering wheel, the car could be equipped with some automatic safety device that could sound an alarm or kick in to slow the car down. This could be simpler and cost less than other methods of detecting driver fatigue.”

The team also invented a new type of transistor in which they used the structured, flexible rubber film to replace a component that is normally rigid in a typical transistor. When pressure is applied to their new transistor, the pressure causes a change in the amount of current that the transistor puts out.

The new, flexible transistors could also be used in making artificial skin, Bao says.

The project was partially funded by the National Science Foundation and the Office of Naval Research.

More news from Stanford: http://news.stanford.edu/

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