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Skin-like sensor flexible enough for prosthetic limbs

The new sensor's key element is a transparent film of carbon nano-springs, created by spraying nanotubes onto a thin layer of silicone, enabling the sensor to stretch and bounce back sans wrinkles.

Researchers at Stanford are developing new sensors so flexible and pressure-sensitive that they could be used to make touch-sensitive prosthetic limbs, pressure-sensitive badges, and more.

By incorporating a transparent film of carbon nano-springs, the sensor "can register pressure ranging from a firm pinch between your thumb and forefinger to twice the pressure exerted by an elephant standing on one foot," says postdoctoral researcher Darren Lipomi, co-author of a paper published October 23 in the journal Nature Nanotechnology. "None of it causes any permanent deformation."

The sensor is stretchy in all directions and rebounds to its original shape. Steve Fyffe

The team built those nano-springs by airbrushing nanotubes (which are in liquid suspension) onto a thin layer of, you guessed it, silicone. They then stretched the silicone to pull some of those little bundles into alignment in the direction of the stretching. Upon release, those bundles rebounded back to their original, randomly oriented dimensions, while the nanotubes actually buckled and formed nanostructures that look like springs.

By stretching the silicone a second time in a perpendicular direction to the first stretch, some of the other nanotube bundles aligned in this second direction, thus rendering the sensor completely stretchable in any direction with full rebound.

"After we have done this kind of pre-stretching to the nanotubes, they behave like springs and can be stretched again and again, without any permanent change in shape," Zhenan Bao, associate professor of chemical engineering, said in a school news release.

She adds that repeated stretching below the original stretch length doesn't significantly alter electrical conductivity; maintaining conductivity in stretched and compressed forms is key because the sensors measure the force being applied to them through the spring-like nanostructures, which are serving as electrodes.

One layer of silicone actually stores electrical charge much in the way a battery does, and when pressure is exerted on the sensor (think of an elephant stomp), that layer compresses, thus altering the electrical charge it can store. It is this change that is sensed by carbon nanotubes, prompting the sensor to transmit information about pressure levels.

The team thinks that even types of pressure-induced deformation (stretching versus compression) can be sorted out due to the very patterns of deformation. With compression, for instance, a bull's-eye pattern is more likely, where the greatest deformation is concentrated at the center.

This sensor is not as sensitive to pressure as Bao's team's previous efforts; they created one sensor so sensitive it can detect pressures "well below the pressure exerted by a 20 milligram bluebottle fly carcass." For this sensor, the focus was less on sensitivity and more on elasticity, but Bao insists that with some modifications this sensor could boast the best of both worlds.