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Stanford researchers patent pressure-sensitive e-Skin

Soon, robots may be able to feel — not emotion, but touch-related sensory information they capture through an “e-Skin” Stanford researchers have developed using an electronic sensor that rivals the sensitivity of human skin.

The e-Skin uses sensors to detect pressure changes mimicking human skin. Each of the pyramids in the sensors, shown here under a microsope, is 6 microns, about a tenth of the human hair diameter.

The e-Skin could be used in a vast array of future applications, including prosthetic limbs, burn victims’ treatment, touch-screen displays such as iPhones and iPads, and medical instruments that need to make controlled incisions. The skin is sensitive enough to detect the landing of a butterfly or small insect, and researchers say it could give robots “feeling.”

Furthermore, bandages could be equipped with the sensors to ensure they are applied with the proper tightness. Sensors could also be incorporated into steering wheels to detect when a drunk or fatigued driver loses his or her grip on the wheel, triggering an alarm to alert the driver or automatically slow down the vehicle, said Benjamin Tee, co-author and graduate student in electrical engineering.

Zhenan Bao, associate professor of chemical engineering, has led the research since the summer of 2005. She has worked on the development of flexible electronics for many years, and the e-Skin project came out of the intention to apply her group’s electronic expertise to robotics.

Her team has patented the technology and their work was published on Sept. 12 in “Nature Materials.”

The e-Skin is composed of a thin, highly flexible layer of rubber sandwiched between two electrically conducting layers. The researchers call their rubber “micro-structured” because it is molded into a grid of tiny pyramids that number between several hundred thousand and 25 million. The grid’s pores fill with polymer when the rubber is compressed, fluctuating the rubber’s ability to hold electrical charge.

The rubber stores electrical charge, and a change in pressure on the rubber changes the capacitance of the rubber, or the amount of electric charge the rubber can hold. When the rubber is compressed, the capacitance increases, and when the rubber expands, the capacitance decreases. The change in capacitance is detected by the electrodes, measuring the pressure on the surface. The total thickness of the e-Skin can be 100 microns or fewer.

Stefan Mannsfeld, co-author and former postdoctoral researcher in chemical engineering, said that by using micro-structured rubber films, the team was able to make the rubber layer behave more like an ideal spring.

The largest sheet of skin produced so far is about seven square centimeters, and the sheet has the ability to wrap “like a sticker” around sharp angles and curves, Mannsfeld said.

Other research teams, including one at the UC-Berkeley, also have been working on the advancement of electronic sensors. However, while the team at Berkeley essentially laminates a transistor with a pressure-sensitive surface, the Stanford team’s skin makes the transistor itself pressure-sensitive. The Stanford team’s e-Skin is more sensitive because of the micro-structured rubber, which can quickly rebound to its original shape.

“We started with transistor device and said, ‘Can we put something in them to turn them into a pressure-sensor?’” Mannsfeld said.

Although the main components of the e-Skin are biocompatible — the rubber is used in breast implants — Bao said the team will need to use more biocompatible polymers and surface molecules before the skin can reach its full biomedical potential.

“In the future, we would like to make these devices more skin-like,” Bao said.

“Currently we make these devices on plastic substrates, but the substrates we are using right now aren’t elastic enough to enable them to behave like what real skin is like… We want to demonstrate some of the biomedical applications that these sensors will enable in the future,” she said.

The research was funded in part by the National Science Foundation and the Office of Naval Research.