Wearable electronics have come along in leaps and bounds in the past few years, and much work is underway to develop electronic "tattoos" and skin electronics, in which ultrathin silicon devices and other electronic components just a few microns thick can be directly laminated on to the surface of skin. Researchers are now able to manufacture light sources, such as organic and polymer LEDs, onto plastic substrates to make sensing devices, but these technologies are still too bulky (at millimetres thick) and not yet flexible enough for wearable, non-invasive systems capable of detecting vital signs and other clinically important parameters. And systems that are thinner have not been stable enough in air to last for more than a few hours.

Now, a team led by Takao Someya has developed a film less than two microns thick made from alternating layers of inorganic silicon oxynitrite and organic parylene. The film, which contains a digital display lit up by red, green and blue polymer LEDS (PLEDs) on a substrate just a few microns thick, is stable in air for up to a few days. The researchers have also succeeded in attaching transparent indium tin oxide (ITO) electrodes to it without damaging it to make the e-skin.

Someya and colleagues' composite film is thin enough to be applied to the skin like a temporary tattoo and flexible enough to follow body movement. The PLEDs themselves were just three microns thick in all and six times more efficient than previously reported ultrathin PLEDs.

The devices can be used for medical applications, like measuring blood oxygen concentration and pulse rates. Such information is measured by an ultraflexible optical sensor in the device and the data obtained can be presented either by an analogue display or a digital display laminated onto the body (the hand or face, for example). The colour and the intensity of PLED can also be changed to show, in a simpler way, if your heart is beating faster or slower – something that will be important not only for doctors monitoring their patients, but which might even come in useful on a romantic date, quips Someya.

Like a second skin

"Think about how smart phones have transformed our lives," he says. "While they are just an extension of your desktop computer or laptop, they are much smaller and so much more personal.

In the same way, ultrathin e-skin has the potential to transform our lives. You will no longer have to carry an external device, but will actually be able to wear it without being hampered because it is neither bulky nor rigid. You'll wear it like your second skin."

The applications are many, he tells nanotechweb.org. "An electrician, for instance, will be able to carry a building's plans or an electrical diagram of equipment on the back of their hand, without having to lumber heavy documents around. Perhaps more importantly, wearing electronics closer to the skin allows for more accurate health monitoring and for more precise information from the sensors embedded in the film. The device is much more comfortable since patients will not have to carry monitoring equipment with them or put it on and take it off all the time. This could allow for continuous, round the clock monitoring."

Other possible applications are in sport, he adds. "Athletes, be they professional or amateur, could have access to statistics such as their heart rate, sugar levels and or distance covered at their fingertips to help them optimize their training sessions. Personalized, skin-level electronics will also improve privacy – for instance if the information is displayed inside your own palm."

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