The team, led by Ali Javey, made the new e-skin by first spin coating a polymer sheet just 25 microns 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 in it.

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 told "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 added. 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 more functionalities, such as thermal and light sensing, as well as just pressure sensing 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, revealed Wang.

The present work is detailed in Nature Materials doi:10.1038/nmat3711

Further reading

Spray process applies silver nanowire network to plastic, glass and fabric (Jun 2011)
Hydrogel electronics makes its debut (Jun 2012)
Hairy solution to making sensitive artificial skin (August 2012)
Semiconducting CNT networks make bendy circuits (Mar 2012)