One bright April day on a Harvard University lawn, David Melancon stepped out of a white plastic tent carrying a table. Then another. Then he made a few trips to produce 14 chairs. Then a bike, followed by a yellow bike pump. Finally, he carried out a large orange Shop-Vac. Melancon, a PhD candidate in applied mathematics, then closed the tent’s makeshift door behind him. This was what his team dubbed their “clown car” demonstration—proof that a huge number of objects could fit inside a tent which, only a few moments before, had been a flat stack of plastic about the size of a twin mattress, then inflated into an origami-inspired shelter.
Scientists muse over origami. The ancient practice of folding flat paper into art scratches a fundamental itch to make something out of almost nothing. For inventive builders today, origami is less about paper swans, and more about fitting useful structures in tiny spaces. And according to Maslow’s Pyramid ranking the hierarchy of needs, few things are more useful than shelter. (You probably shouldn’t eat or drink an engineer’s origami creation.)
“There are a number of situations—emergency situations, for example—when you need a structure,” says Katia Bertoldi, Melancon’s advisor and a professor of applied mechanics at Harvard. For example, people displaced by natural disasters need immediate shelters. “I can build a shed, and then it's there. But then if I have to move, either I take it apart or I move this huge volume. It is very impractical,” she continues. Cutting down that volume with “deployable” origami structures—which unfold from small, movable volumes into larger useful ones—solves that problem.
How would you deploy an emergency shelter easily? Imagine inflating a folded balloon to deploy a hidden 3D shape. It’s an elegant trick, but a balloon doesn’t hold it’s shape when you take away the air pressure. A standalone origami needs to be bistable. The word is often used in electronics and computer science to describe a circuit with two stable states, but in mechanical design, it basically means the structure has to be sturdy both when it's flat-packed and when it's expanded. It would have to hold its shape while folded, and stay that way while unfolded without sealing in air. Inflatable origami and bistable origami exist, Bertoldi says, “but they’ve never been combined into a single concept.”
Over the last three years, Bertoldi’s team has deconstructed the basic geometry, physics, and structural engineering obstacles to realize that concept. And last Wednesday in the journal Nature, they presented an unprecedented collection of bistable inflatable origami. Folded from either cardboard or corrugated plastic sheets, the pieces snap into place with pressure from an air pump, and hold their own without it. Some of the examples are trinket-sized and look like starbursts or triangular fortune cookies. Others are much larger, like human-sized arches. One stands out: an 8-foot-tall shelter with an 8-foot-wide octagonal floor and a door, unfolded from one single material.
Experts say this step from theory to structure is a promising idea for housing people at natural disaster sites. “It's exciting work,” says Joseph Choma, an associate professor of architecture and founder of the Design Topology Lab at Clemson University. Choma, an expert in foldable structures and materials who was not involved in Bertoldi’s project, says the world needs smarter disaster relief architecture, “especially ones that can be flat-packed, deployed, and then flat-packed again.”
“A lot of times,” he continues, “these things get built, but then they get left behind or they get destroyed.”
“It's a great bridging between the mechanics of origami—the geometry of it—and actually getting all the way to a large-scale structure. That'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 deployable shelter designs. “To actually get this work out into real life, these are the necessary types of steps,” she says.
Bertoldi points out that we already have a well-known deployable shelter: camping tents. Light, tightly-packed tents make it easier to backpack through the wilderness. But assembling one into an enclosed space takes time. You have to link metal bars, thread them through narrow holes in fabric, and lock it all in place. Setting up bar-based structures en masse takes even more time and hands. An ideal emergency shelter gets set up quick when it’s needed, and comes down quick when it’s needed elsewhere.
On their own, origami deployables suffer a similar problem. Going from 2D to 3D requires tending to each fold. “The tricky part of origami before is that typically you need to actuate every hinge, so the actuation becomes really cumbersome,” Bertoldi says.
The team used plastic sheets or cardboard for the shelter’s faces, but the origami magic happens at the hinges. The faces won’t bend, so something’s got to give. The hinges were either two-sided tape connecting laser-cut cardboard, or lines mechanically scored into plastic sheets. That allows the structure to bend around itself for inflation and deflation. And in order to make all the hinges swing into place automatically, her team decided, maybe they could just inflate the folds all at once using air pressure.
But blowing air into an inflatable object is more like compressing a spring then assembling a building. It's not bistable. “You compress it and it deforms," Bertoldi says. "But as soon as you remove your load, it springs back.” In other words, you can use force from air pressure to deform a folded bundle of cardboard and turn it into an inflatable tent, but then you’re stuck making sure the air stays in—which, of course, rules out having a door.
Stability is all about minimizing excess energy: a ball parked in a valley is more stable than one halfway up a steep hill. Bistability means designing a structure so that its energy barrier, or the amount of energy needed to lock it into its inflated or deflated states, is just right. The barrier can't be too high, or else it’s impossible to inflate. But the barrier also can’t be too low, because then a gust of wind could collapse it: “It's gonna flip back and deflate,” Bertoldi says.
“You need to carefully design its energy barrier,” she continues. “And that's most of the engineering game.”
Bertoldi’s team designed their structures using triangular faces; the energy barrier for each structure depended on how they shaped those triangles, the geometry of how they connected, and their construction materials. First they made calculations, then hand-sized physical prototypes shaped like arches and starbursts, tinkering with different building materials and looking for that energy barrier sweet spot. “It took us three years to really get to the bottom of it to figure out the geometric analysis and the experimental part—how to build it,” Bertoldi says. Each decision from crease angles to face material to hinge construction added a variable that required trial and error. “There was a lot of failure. Lots and lots.”
Eventually, something clicked. Literally. When tugging on the folded structures to expand them, Bertoldi recalls, “at a certain point, you hear a click.” She likens that feeling to the one you get from those 1990s snap bracelets: “It's something you can really feel with your hands.”
Years of toiling over design and fabrication details accelerated when the team built a scaled-up arch. Folded up, their largest arch is only 20 centimeters tall and 30 wide—the size of a few coffee table books stacked on each other—and expands to be three times as tall and five times as long. “That was sort of a turning point,” Bertoldi says. “It means we have a simple strategy, it’s clean, and it works.”
Within three months, two of her students were ready to demonstrate their emergency shelter. Standing on a Harvard indoor basketball court with an accordion of large white plastic sheets—the flattened prototype—Melancon and labmate Benjamin Gorissen stood the tightly packed sheets upright near the three-point-line, and switched on a pump. It inflated quickly in every direction. A triangular roof perked up on top; and the outline of a door appeared out of nowhere—true to form for origami. They disconnected the pump and the origami shelter stood unperturbed on the court. This structure spanned about 2.5 meters across and upwards—wide enough for a California king bed and taller than Shaq. Later on, they filmed their “clown car” reveal of how much a shelter like this can contain.
Bringing origami to large structures is quite a new trend, Sychterz says, but a promising one. “If it's an emergency situation, you can bring a lot of them onto the site, and then inflate them,” Sychterz adds. “So this is a great thing to show because, obviously, the initial shape is very, very compact.”
“This is still a prototype,” she continues. “The next questions after that are: How do we actually make it not just mass-producible, but more robust and resilient for natural disasters?” Reusing a shelter will be key—the structures should hold up to many cycles of inflation, deflation, and flat-packing to transport many at a time. Having structures ten or 100 times larger than this proof of concept would also be valuable for disaster relief. But the scaled-up mechanical strain would be far beyond what was tested here.
Choma agrees that solving how to inflate origami into large, rigid structures could also inform the construction of permanent buildings. “Forty percent of the carbon in the world, more or less, is from the built environment,” Choma says, referring to recent global estimates that operation and construction of buildings accounts for an enormous share of greenhouse gas emissions. Choma is working on a project in Kenya that will use his foldable structures as reusable molds for over 700 concrete columns in a building this year, which would decrease emissions by reducing the need for materials, such as timber, that are less sustainable.
Choma notes that origami arches are relatively easy to deploy, so an inflatable one isn't a particularly important leap, but a shelter concept with a functional door is exciting. “The shelter is the most radical and innovative aspect of the project,” says Choma. “It's really quite elegant, also.”
Bertoldi says her team is speaking to companies about potential uses for the prototype designs, but it’s still very early. The next step for her lab's inflatable origami deployables is software: a mechanics simulation tool to predict the correct shapes and materials. “We demonstrated a few shapes that are very interesting,” she says. “But how far can we go?”