14 September 2010—Today’s advanced robots and prosthetic arms can grab an egg or a plastic cup without crushing it, thanks to tactile sensors on the fingertips. But you wouldn’t say they’re sensitive enough to pat a baby to sleep. For that you’d need to cover the robot arm with pressure-sensitive synthetic skin that could sense a featherlight touch.
Two research groups, one at the University of California, Berkeley, and the other at Stanford, have independently made advances toward such a sensitive system. Their prototypes are as good as human skin at quickly detecting small amounts of pressure: Within 100 milliseconds, they can feel pressures ranging from 15 kilopascals to less than 1 kPa. (The gentlest touch you can feel is 1 kPa.) In the Revolutionizing Prosthetics program, funded by the Defense Advanced Research Projects Agency, a bionic hand needs to feel 0.1 newtons of force over a fingertip, which, if you assume it has an area of about 1 square centimeter, translates to a pressure sensitivity of 1 kPa.
The research teams reported their results on the Web site of the journal Nature Materials on 12 September. The two prototypes are built on the same idea: They contain a pressure-sensitive rubber layer whose electrical properties change in response to pressure, and they have an underlying matrix of transistors that detect this change. But the teams used different materials and mechanisms.
The Berkeley researchers, led by electrical engineering and computer science professor Ali Javey, use an array of germanium-silicon nanowire transistors that they print on a rubbery polymer. A separate pressure-sensitive rubber layer laminated on top of the transistors becomes conductive under pressure, changing the transistor’s voltage and current outputs. The researchers made an 18-by-19 array of pressure-sensing pixels on a square sheet with 7-cm sides, and they can roughly map different amounts of pressure applied to the pixels. One day, says Javey, ”we’d like to be able to cover the body of a robot with this skin.”
Stanford chemical engineering professor Zhenan Bao and her colleagues took a slightly different approach: They molded an array of tiny square pyramids into a thin film of a polymer called polydimethylsiloxane, or PDMS. The flexible film acts as the dielectric material at the gate electrodes of a transistor made from a single crystal of the organic semiconductor rubrene. The capacitance of the pyramidal film changes as the pyramids are squeezed, changing the transistor’s current.
The two designs have trade-offs in flexibility and pressure sensitivity. Stanford’s rubrene transistor is fabricated on a rigid silicon substrate. Berkeley’s nanowire-based skin, meanwhile, can be bent to a radius of 2.5 millimeters as many as 2000 times. But Bao’s team also used just the flexible pyramidal film minus the underlying transistors as a pressure sensor, measuring capacitance directly using external circuits, and were able to sense the weight of a fly placed on the sheet (a mere 3 pascals).
Japanese researchers at the University of Tokyo had previously built the most promising flexible electronic skin using pressure-sensitive rubber and organic transistors embedded on a plastic film. But it was at best half as sensitive as the new prototypes. Besides, says Javey, organic transistors require tens of volts to operate and are unstable, with electrical properties that change in a matter of days. The nanowire-based electronic skin, on the other hand, operates at a low 2 to 3 volts with good chemical and mechanical stability, he says.
The synthetic skins still don’t emulate one important feature of human skin: spatial resolution. On some parts of the body, human skin is able to differentiate between pinpricks 2 mm apart, says Cheol Park, who is working on nanotube-infused polymers for artificial skin at the National Institute of Aerospace. Each pixel in the Stanford and Berkeley prototypes is, respectively, 8 mm and about 3.5 mm to a side. Park hasn’t succeeded in achieving a small enough resolution using carbon-nanotube-infused polymers and is now working with boron nitride nanotubes, which he says show more promise.
Park finds the sensitivity and quick response time of the new skins impressive and the Stanford researchers’ capacitance-measuring approach with the pyramidal film especially innovative. However, the researchers will need to find a way to make large patches of these pressure sensors work reliably and at a low cost, he says.
For now, small patches of the flexible pressure sensors could be used as a coating for surgical tools used in laparoscopy, says Stefan Mannsfeld, a Stanford researcher who led the electronic skin development. Doctors will then be able to feel their way inside a body cavity without jabbing and damaging body tissue.