Synthetic Skin Gets A Soft Touch

Electronic skin that can mimic the human sense of touch is a step closer to reality. Such artificial skin could be used to give robots or prosthetic limbs a sense of touch. Two groups now independently report sensitive, flexible pressure sensors that can be incorporated into large-scale skinlike materials.

Zhenan Bao and coworkers at Stanford University make highly sensitive, flexible pressure sensors by sandwiching microstructured polydimethylsiloxane films between arrays of flexible electrodes (Nat. Mater., DOI: 10.1038/nmat2834). Air gaps between the microstructures lead to changes in device capacitance when applied pressure pushes the air out. These capacitance changes correspond to the applied pressure.

The sensors are sensitive enough to detect even a tiny insect on the surface, which exerts a pressure of only a few pascals, but they also respond to pressures as high as 15 kPa. The gentlest human touch exerts about 1 kPa.

The devices use pyramidal microstructures, but other shapes are also possible. "Using a variety of structures, we can tune the sensitivity range and the load that the pressure sensors can experience," Bao says. "By incorporating all of them in a sensor array, we can make sensors that function over a broad range, just like our skin."

In other work, Ali Javey and coworkers at the University of California, Berkeley, demonstrate that nanowire arrays can work as the circuitry needed for low-voltage macroscale artificial skin (Nat. Mater., DOI: 10.1038/nmat2835). The nanowire circuits can be operated at lower voltages than are needed for the organic transistors usually found in flexible electronics, Javey says.

The Berkeley team makes the arrays by transferring nanowires grown on a silicon substrate to a flexible "receiver" substrate. The simplest way to achieve the transfer is by contact printing—growing the nanowires on a cylindrical drum and then rolling the drum across the receiver substrate. "The nanowires get aligned and transferred to the receiver substrate by this simple process," Javey says. "You get parallel arrays of printed nanowires."

So far, they've made devices that are 7 × 7 cm² with an 18 × 19 pixel array. "Ultimately, we want to make it as big as possible, so we can cover the entire body of a robot," Javey says. "Right now, we’re limited by the size of the processing tools we have in an academic lab."

"These groups are at the forefront of a new and exciting field of research into biointegrated electronics," says John A. Rogers, a materials scientist at the University of Illinois, Urbana-Champaign, who works with flexible electronics. "The work is exceptional for its focus, not only on materials or individual devices, but on full system-level demonstrations with impressive levels of function."