Two research groups have demonstrated progress toward fabricating artificial sensors that can emulate the abilities of human skin, an advancement that could lend robots a far more sophisticated sense of touch.
Although the teams used different technologies for their devices, each produced a flexible grid of sensors capable of registering pressures of just a few kilopascals, corresponding to a light touch on human skin. The two groups announced their experimental results in papers published online September 12 in Nature Materials. (Scientific American is part of Nature Publishing Group.)
"The goal was to really develop a material system that can function very similar to our human skin," says Ali Javey, an electrical engineer at the University of California, Berkeley, and an author of one of the new studies. Javey's group built a relatively large pad, seven centimeters on a side, comprising 342 individual sensor pixels with nanowire-based transistors. The entire grid is covered with a flexible layer whose electrical attributes change under even slight compression. "As you apply pressure the conductive property of the rubber changes," Javey says—a change that can be picked up by the underlying sensor pixels. The group demonstrated the sensor array's ability to sense pressures of zero to 15 kilopascals, which Javey says roughly matches the range humans encounter in handling everyday objects.
The group tested the flexibility of its device by bending it more than 2,000 times, after which the researchers report the sensor's performance remained basically unchanged. "They're very flexible," Javey says. "The main reason is the active materials that we're using consist of very thin, nanoscale-type materials."
A separate group at Stanford University based their sensor technology on transistors incorporating a rubber layer patterned with micron-scale raised structures such as pyramids or pillars. The rubber layer's local thickness changes under pressure, along with its local composition as air trapped between the microstructures is temporarily displaced. Both effects serve to change the layer's electrical capacitance, and when incorporated into transistors, the changing capacitance brings about a detectable change in current.
The microstructures also improve the rubber layer's resilience, allowing faster reset times on the order of milliseconds, once the source of pressure is removed from the sensor. "If we open up some space in between the little structures then there is empty space for the rubber to expand into, and they can behave more like springs and bounce back," says Zhenan Bao, a Stanford materials scientist and an author of the study.
The Stanford group demonstrated a measurable change in capacitance with a lone bluebottle fly placed atop the rubber layer, adding just 0.003 kilopascal of pressure. When incorporated into a transistor, the group's pressure sensor registered pressures ranging from zero to roughly 18 kilopascals. That transistor was assembled on a rigid silicon wafer, however, and a more pliable substrate would be needed for a truly skinlike implementation. She says her group is now working on just such an approach.
Bao and her colleagues did, however, demonstrate the flexible potential of their microstructured elastic layer in a simpler device. They constructed a flexible panel with eight crisscrossing electrodes in each direction, whose intersections formed a 64-pixel sensor capable of registering just a few grams of mass on its surface, equivalent to pressures of about 16 kilopascals.
Javey acknowledges that scaling up the size of sensor arrays and scaling down the size of the pixels will be a challenge. But the applications would be many. "For robotics, there is a lot of interest to develop such artificial skin to give the sense of touch to the robots," he says. Producing more lifelike prosthetics is another possibility, although in all likelihood that is a longer-term goal. "In that regard what is really challenging is how the skin is interfaced with our brain," Javey says.
In addition to robotic and prosthetic applications, Bao envisions implantable sensors to monitor blood pressure or a car's steering wheel that can sense when the driver has dozed off and loosened his or her grip. "There are a lot of directions, so we are very excited about this," she says.