Besides having tentacles, squid and octopi are also both known for their color-changing skin. Well, soft-bodied robots may soon also share that attribute, thanks to research being carried out at Cornell University. Led by assistant professor Rob Shepherd, a team of grad students there has developed an electroluminescent rubber "skin" that not only emits light in different colors, but that can also do so while being stretched to more than six times its original length.

Known as a hyper-elastic light-emitting capacitor (HLEC), the skin consists of three layers – two transparent ionic (salt-based) hydrogel electrodes, between which is a matrix of silicone.

That matrix contains pixels composed of zinc sulfide, which are in turn doped with various transition metals. As an externally-delivered electrical current passes between the electrodes and through that matrix, different metals emit different wavelengths of light – copper produces blue light, and magnesium makes yellow, just as a couple of examples.

Additionally, because the capacitance of the matrix changes under deformation, it's possible for the HLEC to detect when it's being touched, then change color accordingly.

While it all might simply seem like a neat party trick, the technology does potentially have some very practical applications. It has been suggested, for instance, that health care robots could use the material to display patients' temperature and pulse – it would also be possible to create displays that could be rolled up. More than that, however, the ability for robots to "express" themselves in one more fashion could make us like them better.

"When robots become more and more a part of our lives, the ability for them to have emotional connection with us will be important," say Shepherd. "So to be able to change their color in response to mood or the tone of the room we believe is going to be important for human-robot interactions."

The technology has already been used to develop a simple soft-bodied robot, that changes colors while using pneumatic actuators to crawl like an inchworm.

A paper on the research was recently published in the journal Science.

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