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Stretchable, glowing capacitors used to make a touch-sensitive robot

A hydrogel sandwich provides some interesting electrical properties.

Stretchable, glowing capacitors used to make a touch-sensitive robot
Larson, et. al., Science

For many of us, the term "robot" still evokes an image of R2D2 or a terminator-style collection of metal parts. But there's no reason to limit our construction materials to hard parts. A number of labs are working on soft-bodied robots and have shown they can do some rather interesting things, like squeeze through narrow spaces.

A team of researchers from Cornell and the Istituto Italiano di Tecnologia has taken a soft-bodied robot and made it glow. Their method of producing the light, however, has some interesting side effects: it allows the robot to determine how much it has flexed, and it makes the robot responsive to touch.

These days, "glow" is usually synonymous with "LED." But the authors used a very different technique, relying on what's called an electroluminescent phosphor—basically, something that glows when it's place in an alternating electric field. The phosphors (zinc sulfide, in this case) can be embedded in a silicone gel, making them stretchable and bendable. Different dopants in the phosphor will cause it to glow in different colors.

But to get any light, you still need to deliver an alternating electric field to them. Here, they relied on a hydrogel, specifically a lithium chloride solution inside a gel made of a polymer called polyacrylamide. The conductive solution transmits currents; the authors sandwiched the silicone-phosphor layer in between two polyacrylamide gel layers. The whole thing could then be sealed in a stretchable polymer, creating a durable material.

The authors show that the resulting material is extremely stretchable. They could pull it out to 480 percent of its initial length, and it glowed all the way. The only reason it failed at five times its initial length is that the metal electrodes that supply the current to the polyacrylamide layer detached; presumably, a more robust electrical connection would allow it to extend even farther.

Glowing, stretchy stuff.

The neat thing about this structure is that it inherently creates a capacitor, as there are two conducting sheets sandwiching an insulating layer (the one that contains the phosphor). And the actual capacitance of the material changes when it's stretched, allowing the authors to track the degree of stretching.

This has some interesting consequences. For one, it's possible to determine the configuration of the material by reading its capacitance. That gives the material the equivalent of proprioception, the ability to tell where it's located in space. The other thing is that it makes the material touch sensitive—if you give it a poke, it can tell because there's a 25 percent increase in capacitance.

To show that all of this works in practice, the authors made a soft-bodied robot with the material. It moves a bit like an inchworm, so it's not exactly exciting to watch, but it does keep glowing all through its movements.

The one sticking point here is the horrible efficiency. A good LED bulb puts out about 80 lumens per Watt; this thing got only 40 millilumens. The authors think they can improve on this substantially, but even commercial electroluminescent devices only achieve about four lumens per Watt.

Still, nobody was suggesting to use something like this as a light bulb, and there are a number of cases where it could be useful. And, even if the ability to glow is never used on robots, the capacitance sensing-abilities could find a variety of applications.

Science, 2015. DOI: 10.1126/science.aac5082  (About DOIs).

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