The technology behind Shrinky Dinks can make better robots

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Imagine a robotic arm that does not require any movement mechanisms, or surgical tools that can expand or reshape in the body. While they may sound strange and wondrous, in fact they already exist.

These crazy instruments are all based on shape memory polymers, a category of materials that can change themselves. Now researchers at Stanford University have created a shape memory polymer that is stronger and better performing than any of its predecessors. They published their work in the magazine last week ACS Central Science.

“[Shape-memory polymers] have been commercialized for some time in our lives, ”says Shayla Nikzad, a PhD student at Stanford University and a lead author on the article.

A polymer is a very long molecule made up of many smaller molecules linked together in a giant chain. It is no exaggeration to believe that polymers rule our world. The DNA behind the scenes in your cells is a polymer; Silk and gelatin are also made of polymers. In your everyday life, you are likely to come across man-made polymers that are made up of plastic and synthetic rubber.

Shape memory polymers are a very special type of polymer. When bending, the molecules in the polymer form special bonds. And when you expose them to a stimulus – light, electricity, or most often a change in temperature – those bonds break and snap the polymer back to its original state.

In other words, shape memory polymers “remember” their original shape and can transform themselves back into this true shape. Many engineers find this skill irresistible.

“So we can use shape-memory polymers to create a device … that changes shape without actually pulling or stretching,” said Kai James, a professor of aerospace engineering at the University of Illinois Urbana-Champaign, who is not an author was on paper.

If you’ve ever played with a shrinky dink, you’ve seen a shape memory polymer in action. If you put one in the oven, it will shrink and harden. Of course, you can’t easily undo a Shrinky Dink afterwards; In fact, many shape memory polymers only “snap” back in one direction.

Shape memory polymers are much more than just toys, however. In many ways, they are already shaping the world around us. Some wires have a shape memory coating that can shrink to give the wire better insulation. Shape memory polymers in the fabric can make it more breathable in the heat and waterproof in the cold.

[Related: We finally have a working supersolid. Here’s why that matters.]

Researchers are also working on using shape memory polymers in hospitals, such as self-suturing sutures and stents that can fit into arteries and then expand to increase blood flow.

Nikzad and her colleagues are particularly interested in using shape memory polymers in robots. By making artificial muscles from it, they could make robotic arms that don’t need heavy mechanisms like machines or actuators to move.

“But one of the biggest obstacles,” says Nikzad, “is simply the fact that the materials are essentially not very strong.”

Engineers could Use shape memory polymers to build a robotic arm that is lighter and more flexible than its mechanical counterparts, but it wouldn’t carry as much without breaking and falling apart. And shape memory polymers that go back to their original state don’t provide much force that can move or lift objects.

It is possible to make stronger shape memory polymers, but there is a tradeoff: they can only act over lengths too short for most robots. Imagine a high-tech version of the. before Tyrannosaurus rex‘s little arms.

The Stanford Group wanted to see if they could do something about this mystery. Working with chemists like Stanford graduate student Christopher Cooper, the article’s other lead author, they took an existing polymer called PPG – which is now used to refine leather and make paintballs – and modified its atomic structure to help the atoms to form better bonds, creating a brand new molecule.

“That’s your usual organic chemistry,” says Nikazad.

When they tested it, they found that they had developed a new material that could store six times as much energy as any previous shape memory polymer. A plastic muscle made from this material can lift objects up to 5,000 times its own weight – and that is not unaffordable: the raw materials cost around 11 US dollars per pound.

“These researchers were able to overcome the tradeoff,” says James.

The researchers want to continue working on their materials. The shape memory effect of their polymer, for example, only works in one direction. That’s something they think they can improve on. “For example, you could make something that heats up and stretches, then cools and shrinks,” says Nikzad. This process repeats itself no matter how many times you heat it up or cool it down.

That could certainly create stronger, lighter robots. In addition, James said, in the multitude of applications that shape memory polymers already have, the Stanford Group’s new materials could be used in much more than just robots.


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