Soft Robotic Structures Fold Themselves Up in Hot Water

Roboticists built self-folding 3D models of a rabbit, tuna fish, and starfish to demonstrate a new approach for making compliant, controllable robotic structures
Image: MIT/University of York/KIST
To demonstrate a new approach for making compliant, controllable robotic structures, researchers built self-folding 3D models of a rabbit, tuna fish, and starfish. The structures fold themselves up from flat sheets after immersion in hot water.

Over the last few years there’s been an increased focus on robots that can build themselves. This is especially pertinent when you’re dealing with robots that are fiddly to make, which includes (at the moment) most robots that are soft and compliant. It seems like soft robots would be quite happy to be 3D printed, but in practice, they need to be made out of highly deformable materials that only behave themselves if you take the trouble to mold them instead, which is tedious any annoying.

At ICRA on Tuesday, Cynthia Sung, who was previously with Daniela Rus’s group at MIT and is now a professor at the University of Pennsylvania, presented a new approach for making compliant, controllable robotic structures. Called additive self-folding, this origami-inspired technique involves creating 3D shapes made out of a long strip of self-folding 2D material, and all you have to do is add some hot water.

The video does a good job of showing exactly how this works, but essentially, you start with a 3D model of a thing, and then use software to chop that model up into lots of different layers. Each of those layers is attached to the layer above and below it with a little folding flap. Once you’ve got the design all ready to go, you print it out (using a vinyl cutter) on a sandwich of mylar and PVC.

Pre-folding, you end up with a long and skinny strip consisting of connected cross sections of the 3D object you’re looking to create, but as soon as you drop the strip into some nearly boiling water, the PVC heats up and then shrinks wherever the mylar has been cut out, and the strip almost instantly accordionizes and zigzagifies (two technical terms that I just made up) into its final shape in a matter of just a few seconds.

The software also includes an option to add in some actuation in the form of antagonistic tendons (made of fishing line) that squish down one side of the structure while allowing the other to spring up, resulting in a controlled curve.

The researchers were able to use these techniques to design and fabricate a fairly complex robotic starfish, with five legs (or arms) each of which uses four tendons to be actuated in two dimensions. Fabricating the legs took just 50 minutes a piece, which is really not all that much considering their range of motion. By grouping the tendons together, six servos control clockwise and counterclockwise bending of all the legs, as well as upward and downward bending of groups of two and three legs. If, at this point, you decide to call it a five fingered manipulator instead of a starfish, suddenly it’s much more useful.

In addition to making it faster and easier to fabricate relatively complex robots like this, there’s also potential for fully autonomous fabrication, particularly in scenarios where weight and volume are constrained, like in the context of space travel or planetary exploration. You could bring along a big roll of material and a cutter, make the robots you need on the fly, and then squish them back down flat whenever you’re not using them.

The most time consuming part of this whole business is dealing with the tendons, which requires a human and accounts for half of the total fabrication time. The researchers are exploring ways of embedding the tendons during the fabrication process, which may make things slightly more complicated, but should also make it significantly faster.

“Self-folded Soft Robotic Structures With Controllable Joints,” by Cynthia Sung, Rhea Lin, Shuhei Miyashita, Sehyuk Yim, Sangbae Kim, and Daniela Rus from MIT, the University of York, and KIST, was presented this week at ICRA 2017 in Singapore.

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