The tactile sensitivity of human skin is hard to re-create, especially over large, flexible surfaces. But two California research groups have made pressure-sensing devices that significantly advance the state of the art.
One, made by researchers at Stanford University, is based on organic electronics and is 1,000 times more sensitive than human skin. The second, made by researchers at the University of California, Berkeley, uses integrated arrays of nanowire transistors and requires very little power. Both devices are flexible and can be printed over large areas; they are described this week in separate papers in the journal Nature Materials.
Highly sensitive surfaces could help robots pick up delicate objects without breaking them, give prosthetics a sense of touch, and give surgeons finer control over tools used for minimally invasive surgery. “Our goal is to mimic the human skin,” says Zhenan Bao, professor of chemical engineering at Stanford. Human skin responds quickly to pressure and can detect objects as small as a grain of sand and light as an insect.
The core of Bao’s device consists of a clear silicon-containing polymer called PDMS. This material’s ability to store charge is directly related to its thickness. A few years ago, researchers led by Takao Someya at the University of Tokyo took advantage of this property, using PDMS as the insulating layer in flexible organic transistors that acted as pressure sensors. But these sensors were limited: when compressed, PDMS molecules change conformation, and it takes time for them to return to their original state.
Bao addressed this problem by patterning the polymer material with arrays of micropillars that stand up from the touchable surface. This design allows the material to flex and quickly return to its original shape, which means it’s possible to take pressure measurements in quick succession. The microstructuring also improves the sensitivity of the device. The gentlest pressure that human skin can detect is about one kilopascal; Bao’s devices can detect pressures that are 1,000 times more gentle.
This approach can be used to make flexible materials with inexpensive printing techniques, but the resulting device requires high voltages to operate. Ali Javey, professor of electrical engineering and computer science at the University of California, Berkeley, has built low-power tactile sensors based on arrays of inorganic nanowire transistors. The transistors are arranged beneath, and connected to, a layer of a commercially available conductive rubber that contains carbon nanoparticles. When the rubber is compressed, its electrical resistance changes, and this can be detected by the transistors. “The nanowires are being used as active electronics to run the tactile sensor on top,” he explains.
Nanowire transistors offer low-voltage operation and fast switching speeds in a flexible surface. Whereas Bao’s devices require about 20 volts to operate, Javey’s need less than five volts.
Javey has made sensor arrays that are about 50 centimeters squared. Bao has built circular arrays that are just over 10 centimeters in diameter. Both researchers say the size of their devices is limited only by the tools in the lab–in Javey’s case, the size of the contact printer, and in Bao’s case, the size of the mold used to shape the PDMS.
Artificial skin could offer major advantages for robotic manipulation, says Matei Ciocarlie, research scientist at Willow Garage, a personal-robotics company based in Menlo Park, California. When a robot is manipulating an object, that object may often be hidden from cameras and other sensors, so tactile sensing can provide useful feedback. Touch sensing can also help robots avoid obstacles and locate objects in difficult environments. “Artificial skin must be able to cover large, irregular surfaces on the robot, have adequate sensitivity and dynamic range–all highly significant challenges that these new technologies promise to address,” says Ciocarlie.
The new electronic-skin devices “are a considerable advance in the state of the art in terms of power consumption and sensitivity,” says John Boland, professor of chemistry at Trinity College at the University of Dublin. “The real advance, though, is moving away from a flat geometry to a flexible device that could be used to make something in the shape of a human finger,” he says.
Surgical tools tipped with very sensitive tactile sensors could give doctors better control over how much force they use during minimally invasive surgeries. And large-area, flexible electronic skin could conform to the curves of future prosthetic devices. “Today’s prosthetics are crude–they can grasp but provide no tactile feedback,” notes Boland.
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