Chemist Zhenan Bao at Stanford University, California, and her colleagues used the elastic polymer polydimethylsiloxane (PDMS)1. Bao took a piece of PDMS measuring six centimetres square with pyramid-shaped chunks cut out of it at regular intervals. When the PDMS is squashed, the pyramid-shaped holes that were previously filled with air become filled with PDMS, changing the device's capacitance, or its ability to hold an electric charge.

To make it easier to detect the changes in capacitance, Bao stuck the PDMS capacitor onto an organic transistor, which can read out the differences as a change in current. The team used a grid of transistors to track pressure changes at different points across the material.

The PDMS-based skin is sensitive to the lightest of touches: Bao tested her device by placing a bluebottle fly and a butterfly on it, both of which were clearly 'felt'.

( The use of pressure-sensitive rubber makes this artifical skin flexible. )

Ali Javey at the University of California, Berkeley used semiconductor nanowires pulled into the shape of a grid using a technique called contact printing. The grid was then laid out on a flexible pressure-sensitive rubber — a material that changes its electrical resistance under pressure2.

In the 7-centimetre-square grid, the criss-crossing nanowires act as transistors. Each transistor is like a pixel, and the pressure-induced current change at each individual position can be read out. And because it's made mainly of rubber, the device is bendy. "Because we're using very small inorganic semiconductors, the devices are very flexible," explains Javey. He has bent the sensor into a U-shape with each arm of the 'U' separated by a gap of just 5 millimetres and it still works.

Update 8-Dec-2011: In Roger Zelazny's Hugo award-winning 1966 novel This Immortal gives sf fans a glimpse of this idea. A wrestling robot uses special skin described as a radar mesentery to sense its opponent. End update.

For more exciting robot skin stories, gently caress these links:

• Robot Skin Soft Like Yours
  The new artificial skin gives you that real human skin feeling...
• Robots Need Realistic Skin!
  The authors of the paper identified three specific qualities that must be replicated to give that genuine human skin feeling...
• Underwater Robot With Touch Sensitive Skin
  Comes in handy when lights serve little practical purpose in murky, sediment-laden water....
• E-Skin For Robots Stretches Like Yours
  Stretchable e-skin for robots is being developed by Japanese researchers who apparently hate being touched by traditional rigid, metallicky robots....