Zhenan Bao and colleagues made their transparent elastic films by airbrushing a solution of carbon nanotubes onto the top and bottom surface of a flat silicone sheet. After coating, the researchers stretched the sheet. When the sheet then relaxed, the nanotubes naturally formed wavy, spring-like structures. These structures act as electrodes that can accurately measure the amount of force applied to the material.

In fact, the set-up behaves like a capacitor, with the silicone layer storing electrical charge, as a battery does. When pressure is applied to the sensor, the silicone layer compresses, which alters the amount of electrical charge that it can store. This charge is measured by the carbon nanotubes on top and below the silicone.

When the composite film is stretched again, the nanotubes straighten out in the direction they are stretched. The electrical conductivity of the thin film does not change as long as the material is not stretched beyond the initial stretch amount. Indeed, the film can be stretched up to two and half times its initial length, and in any direction without damage, always reverting back to its original dimensions, even after many stretches. When fully stretched, the film has a conductivity of 2200 S/cm and can detect a pressure of around 50 kPa, which roughly corresponds to that of a "firm finger pinch", according to the researchers.

"Ours is probably the first stretchable, transparent, skin-like sensor – with or without carbon nanotubes," team member Darren Lipomi told nanotechweb.org.

The film might find applications in a host of areas, including screens for mobile devices that can sense a range of pressures (not just touch); sensors for touch screens that are collapsible, stretchable and virtually indestructible; transparent electrodes for solar cells that could be wrapped around the curved surfaces of vehicles and buildings without wrinkling; and sensors for robots and artificial intelligence systems.

Other applications
"Other systems could also benefit – such as those requiring 'biofeedback' – for example, 'smart' steering wheels that could sense if the driver was falling asleep," added Lipomi. "Artificial skin made from the material might also be used to restore the sense of touch to amputees (if they were fitted with prosthetic limbs covered with the skin, for example), injured soldiers and burn victims."

Spurred on by its preliminary results, the team now hopes to improve the sensitivity to pressure of its device. For the films made so far, the researchers mainly focused on making the structures stretchable and transparent and not on the sensitivity as such. They would also like to integrate the skin-like pressure sensors with neurons, as well as try out the electrodes in solar cells.

"In the future, it should also be possible to use such films to design organic, skin-like devices with other human – and "superhuman" – characteristics, such as the ability to sense moisture, temperature, light and even chemical and biological species," stated Lipomi.

More details about work, which was funded by the US Intelligence Community, can be found in Nature Nanotechnology journal.