Researchers at the University of California at Berkeley have integrated three distinct electronic components to create touch-sensitive "electronic skin" or e-skin. The new technology combines semiconducting carbon-nanotube transistors, pressure-sensitive polymer sensors and organic light-emitting diodes (OLEDs) – which are integrated over large areas on a single plastic substrate. The result is a mechanically flexible sensor network that responds to a finger touch by immediately lighting up. And the harder it is touched, the brighter the light.

The researchers believe that the technology could help enhance the sense of touch in robots of the future and even find use in applications such as touchscreen wallpapers. Medical applications, such as "e-bandages" that monitor a patient's health in real time, might also be possible.

Led by Ali Javey, the team made the new e-skin by first spin coating a polymer sheet just 25 µm thick on top of a silicon-wafer substrate and subsequently hardening the plastic by baking it in an oven at 300 °C. The electronic components were then vertically built on top of the plastic surface using conventional microfabrication processes. Once the electronics were stacked, the plastic backing layer was peeled away leaving a free-standing film with the sensor network embedded within it.

Active matrix

Each pixel in the active matrix of the device contains a nanotube transistor with its drain electrode connected to the anode of an OLED. A pressure-sensitive polymer is laminated on top of the OLED and it is in electrical contact with the cathode of the OLED at each pixel. The top surface of the polymer is made conducting by coating it with silver ink and acts as the ground contact. When the device is touched, current flows through the polymer layer and switches the OLED on.

"Our e-skin is the first flexible system that responds to pressure stimuli of varying intensities and provides a real-time response by emitting light through the integrated OLED display," team member Chuan Wang says. "In the system, OLEDs are turned on only where the surface is touched and the intensity of the emitted light depends on the amount of pressure applied. This basically allows us to visualize the applied pressure."

The e-skin can be laminated on a variety of surfaces, curved or otherwise, he adds. Potential applications include robot skin, interactive wallpaper and interactive in-vehicle dashboards. "I can also imagine things like e-bandages applied to a person's arm that would continuously monitor blood pressure and pulse rates, for example, while providing real-time feedback."

The Berkeley team is now busy integrating additional sensing capabilities – such as those that respond to thermal and light stimuli – into its e-skin system. "We are also experimenting with the possibility of having the whole system built using roll-to-roll printing processes for large-scale, low-cost fabrication of the sensor networks, reveals Wang.

The e-skin is described in Nature Materials 10.1038/nmat3711.