“Often,” he continues, “these things are built, but then they are left behind or they are destroyed.”
“It’s a great bridge between the mechanics of origami – the geometry of it – and getting a large-scale structure. It’s quite rare ”, says Ann sychterz, an assistant professor of civil engineering at the University of Illinois-Urbana Champaign who was not involved in the study. Sychterz specializes in the design of deployable shelters. “To make this work happen in real life, these are the types of steps needed,” she says.
Bertoldi points out that we already have a well-known deployable shelter: camping tents. Lightweight and compact tents make it easy to backpack in nature. But assembling one in an enclosed space takes time. You need to tie up some metal bars, thread them through narrow holes in the fabric, and lock everything in place. Installation of bar-based structures a lot takes even more time and hands. An ideal emergency shelter sets up quickly when needed and sets up quickly when needed elsewhere.
On their own, origami deployables suffer from a similar problem. Going from 2D to 3D requires taking care of each fold. “The tricky part of forward origami is that usually you have to actuate each hinge, so the actuation becomes really cumbersome,” says Bertoldi.
The team used plastic sheeting or cardboard for the faces of the shelter, but the magic of origami occurs at the hinges. Faces won’t bend, so something has to give way. The hinges were either double-sided tape connecting the laser-cut cardboard or mechanically marked lines in plastic sheets. This allows the structure to fold around itself for inflation and deflation. And to have all the hinges snap into place automatically, his team decided that maybe they could just inflate the pleats at the same time using air pressure.
But blowing air into an inflatable object is more like compressing a spring than assembling a building. It is not bistable. “You compress it and it twists,” says Bertoldi. “But as soon as you take your load off, it bounces.” In other words, you can use the force of the air pressure to warp a folded cardboard package and turn it into an inflatable tent, but then you get stuck making sure that the air stays inside, which, of course, excludes the possibility of having a door.
Stability is about minimizing excess energy: a ball parked in a valley is more stable than a ball halfway up a steep hill. Bistability means designing a structure so that its energy barrier, or the amount of energy needed to lock it in its inflated or deflated states, is right. The barrier cannot be too high, otherwise it cannot be inflated. But the barrier cannot be too low either, because then a gust of wind could collapse it: “It will come back and deflate,” says Bertoldi.
“You have to carefully design your energy barrier,” she continues. “And that’s the essence of the engineering game.”
Bertoldi’s team designed their structures using triangular faces; the energy barrier of each structure depended on how they formed these triangles, the geometry of their connection, and their construction materials. First, they did calculations, then hand-sized physical prototypes shaped like arcs and stars, tinkering with different building materials and researching that sweet spot of energy barrier. “It took us three years to really get to the bottom of it to understand the geometric analysis and the experimental part – how to build it,” says Bertoldi. Every decision from bend angles to face material to hinge construction added a variable that required trial and error. “There have been a lot of failures. Tonnes and tonnes.”
Finally, something clicked. Literally. Pulling on the folded structures to enlarge them, Bertoldi remembers, “at a certain point, you hear a Click on. She compares this feeling to the one you get from those snap bracelets from the 1990s: “It’s something you can really feel with your hands.”