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BEAUTY may be only skin deep, but for humanoid robots a fleshy covering is about more than mere aesthetics, it could be essential to making them socially acceptable. A touch-sensitive coating could prevent such machines from accidentally injuring anybody within their reach.

In May, a team at the (IIT) in Genoa will dispatch to labs across Europe the first pieces of touch-sensing skin designed for their nascent humanoid robot, the . The skin IIT and its partners have developed contains flexible pressure sensors that aim to put robots in touch with the world.

“Skin has been one of the big missing technologies for humanoid robots,” says roboticist Giorgio Metta at IIT. One goal of making robots in a humanoid form is to let them interact closely with people. But that will only be possible if a robot is fully aware of what its powerful motorised limbs are in contact with.

“Skin has been one of the big missing technologies in the designing of humanoid robots”

Roboticists are trying a great variety of ways to make a sensing skin. Early examples, such as the CB2 robot, built at Osaka University in Japan, placed a few hundred sensors in silicone skin. But now “many, many sensing methods are emerging”, says Richard Walker of , London. Until a lot of robots are using them, it is going to be hard to say which are best suited for particular applications.

What’s more, there are many criteria the skin has to meet, says Metta: it must be resilient, able to cover a large surface area and be able to detect even light touches anywhere on that surface. “Many of these factors conflict with each other,” he says.

The iCub is a humanoid robot the size of a child of three-and-a-half years old. Funded by the European Commission, it was designed to investigate cognition and how awareness of our limbs, muscles, tendons and tactile environment fuels the development of intelligence. The iCub’s technical specifications are open-source and some 15 labs across Europe have already “cloned” their own, so IIT’s skin design could find plenty of robots to enwrap.

The skin is made up of triangular, flexible printed circuit boards which act as sensors, and it covers much of iCub‘s body. Each bendy triangle is 3 centimetres to a side and contains 12 capacitive copper contacts (pictured). A layer of silicone rubber acts as a spacer between those boards and an outer layer of Lycra that carries a metal contact above each copper contact. The Lycra layer and flexible circuits constitute the two sides of the skin’s pressure-sensing capacitors. This arrangement allows 12 “tactile pixels” – or taxels – to be sensed per triangle. This taxel resolution is enough to recognise patterns such as a hand grasping the robot’s arm. The skin can detect a touch as light as 1 gram across each taxel, says Metta. It is also peppered with semiconductor-based temperature sensors. This version of the skin will be released in May.

Later, IIT plans to add a layer of a piezoelectric polymer called PVDF to the skin. While the capacitance sensors measure absolute pressure, the voltage produced by PVDF as a result of its deformation when touched can be used to measure the rate of change of pressure. So if the robot runs its fingertip along a surface, the vibrations generated by friction give it clues about what that surface is made of. Such sensitivity might help it establish the level of grip needed to pick up, say, a slippery porcelain plate.

Philip Taysom, CEO of British company of Richmond, North Yorkshire, is not a fan of sensing skins based on capacitors, which he says can lose sensitivity with repeated use. Peratech’s answer is a stretchy, elastic material it calls quantum tunnelling composite (QTC). This comprises a polymer such as silicone rubber that is heavily loaded with spiky nickel nanoparticles. A voltage is applied across the skin, and when it is pressed, the distance between the nanoparticles within the polymer diminishes, which results in electrons flowing, or “tunnelling”, from one nanoparticle spike to the next in the area being touched. Crucially, the material’s electrical resistance drops dramatically and in proportion to the force applied, so the touch can be interpreted.

At the Massachusetts Institute of Technology’s Media Lab, is developing a QTC-based sensing skin for a commercial robot-maker which he declines to name. Instead of a tight, conforming skin, Whiton uses a looser covering, more akin to clothing. “We cover ourselves with textiles when we interact with people, so clothing may be a better metaphor as a humanoid’s pressure-sensitive surface covering,” he says.

Natural gestures, like tapping a humanoid on the back to get its attention, or leading it by the arm, can be easily interpreted because QTC boasts high sensitivity, he says. But novel skin capabilities could be on the way, too. For example, QTC can also act as an electronic nose. Careful choice of the material’s base polymer, says Taysom, means telltale resistance changes can be induced by reactions between volatile chemicals in the air – so it can become an e-nose as well as a touch sensor, able to detect, for example, a gas leak in your home. “This shows we can probably build into robots a lot of things that our skin can’t do. It’s another reason not to stick rigidly to the human skin metaphor,” says Whiton.

That’s not to say our skin isn’t a great influence. Shadow Robot will soon start testing a novel human-like touch-sensing fingertip from , a start-up based in California. Its fingertip comprises a rubbery fluid-filled sac that squishes just like a real fingertip, and is equipped with internal sensors that measure vibration, temperature and pressure.

Whichever of the emerging technologies prevail, sensing robot skins should help us get along with our future humanoid assistants, says Whiton. “Right now, robots are about as friendly as photocopiers. The interactions skins encourage will make them much friendlier.”