Our sense of touch is made possible thanks to thousands of "mechanoreceptors," which are distributed throughout our skin. The more pressure that's applied to one of these sensors, the more electrical pulses it sends to the brain, thus increasing the tactile sensation that we experience. Led by Prof. Zhenan Bao, scientists at Stanford University have now created synthetic skin that contains electronic mechanoreceptors, which could give prosthetic limbs or robots a sense of touch.

The flexible plastic skin is made up of multiple layers – one of those serves as a pressure sensor, while another houses an ink jet-printed organic electrical circuit. Embedded on the sensory layer is an array of tiny rubber pyramids, each one filled with electrically-conductive carbon nanotubes. When pressure is applied to the skin, those pyramids compress, causing the nanotubes within them to move closer together.

This increases their collective conductivity, allowing more of the natural electrical field emitted by the object being touched to pass through to the circuit layer. It responds by sending out electrical pulses – the more pressure that's applied, the greater the conductivity and the more frequent the pulses.

If the skin were to be used on a robot, those pulses could then be analyzed by a computer.

For amputees wishing to feel through the fingertips of a prosthetic hand, however, the pulses would go to the brain. The scientists have already made strides towards this goal, by stimulating in vitro mouse brain cells using signals generated by the skin. They did so via optogenetics, a process in which neurons are activated by pulses of light. The light-sensitive proteins traditionally used in optogenetics didn't stimulate the brain cells for long enough to allow the signals to register, however, so new proteins had to be engineered.

Bao and her team are now working on making improvements to the skin, including making it more elastic and giving it the ability to "feel" heat and cold.

MIT, Georgia Tech and the University of Southern California are among some of the other institutions that are also developing touch-sensitive skin, using a variety of approaches.

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