The sensor is sensitive enough to easily detect this Peruvian butterfly
(Chorinea faunus) with transparent wings and red-tipped tails,
positioned on a sheet of the sensors.
Credit: Linda Cicero, Stanford University News Service

There’s nothing quite like human touch. Or is there? In September, two groups of California researchers introduced artificial skins that use electricity to sense even the slightest stroke. It’s not the first time electricity has been used in artificial skins, but the new ones are much quicker to conduct current than older efforts. At less than a millimeter thick, the flexible, stretchable sheets could someday be useful in giving prosthetic devices the sensation of touch.

The thin sheets, reported in Nature Materials, can sense pressure changes up to twenty times subtler than previous faux skins. They can even register the tiny touch of a fly’s feet, all in a tenth of a second. This is because “they are basically able to build up more electricity” than older models, says Benjamin Tee, who worked on the Stanford University skins.

To make the skins, Stanford researchers molded mini pyramid shapes onto a thin rubber sheet and attached electrodes—metal strips that conduct charge—to either side. When the pyramid shapes squish under pressure, electricity flow grows, making pressure to the skin register as a difference in touch.

Researchers at the University of California, Berkeley, took a different approach, arranging layers of nanowires—tiny conductors made of silicon and germanium—into a grid, then coating them with rubber. When pressure is applied to the criss-crossed wires, more electrical current flows through them. The Berkeley group’s skin is less sensitive than Stanford’s, but its added flexibility means it would bend better around a prosthesis.

Down the road, the researchers hope to connect these artificial skins to working nerve cells in the body. “We need some kind of circuit to create a sense of feeling,” says Tee. When a prosthetic palm clutches an apple or serves as a butterfly’s landing ground, the touch signal could then pass to the brain, letting someone know the properties of what they’re holding.

The biggest sheet of skin created so far measures seven centimeters on each side, roughly the size of a playing card. With plans underway to make bigger, hand-sized sheets, real possibilities for prosthetics could soon be close enough to touch.