A new technique to print out, fold up and deploy self-building structures could one day make it much easier for surgeons to place artery stents, or astronauts to install new, lightweight space habitats.
The designs build upon an architectural concept called "tensegrity," a term coined by Buckminster Fuller in the 1960s (who also patented the first tensegrity shapes in 1962). Tensegrity, or "tensional integrity," structures hold themselves in shape via rigid struts held in place with interconnected high-tension cables. The Kurilpa Bridge in Brisbane, Australia, and a new radio antenna tower being constructed atop Santiago, Chile’s Metropolitan Park hill are two typical examples of tensegrity structures.
Though they’re very strong, they’re heavy, as they are constructed with metal struts and cables. Georgia Tech engineers Glaucio Paulino and Jerry Qi wanted to apply those same tensional advantages to objects that could be used for more than just bridges and antennae, such as space habitats or heart stents.
Paulino and Qi devised a method to create 3D printable, lightweight, foldable versions of these designs, with tubes made of a plastic-like material called a shape memory polymer connected with printed elastic tendons.
By heating up the tubes, the strut material becomes programmed to “remember” the open configuration. It can then be flattened and folded up, and once the whole design is re-exposed to heat, the whole package slowly unfolds into its final, open configuration—no motors involved.
Paulino and Qi also found that by programming different parts of their designs to unfold at varying temperatures, their designs could unpack themselves in stages to prevent the cables from getting tangled.
Because the entire design can be squashed down into a package that is essentially fully assembled, it takes up much less space than conventional tensegrity designs.
“If you compare tensegrity designs with any other type of structure, they are extremely light and very strong,” Paulino says. “The beauty of this system is that there is an extra degree of freedom that allows the tensegrity to deform, to change shape, have dramatic shape change, and support any type of loading in any direction.”
Paulino and Qi’s lab models are the size of a child’s tabletop toy, four to five inches across on a side, and look like nothing so much as a highly organized stack of sticks being held in place by taut fishing line. When fully unfolded, the struts are hard and rigid, while the elastic cables are softer and more flexible. The designs, when fully assembled, do have some give—if you squeeze them, the shape will deform. But they snap right back into shape when released.
The team used hot water baths to demonstrate how the high-temperature unpacking process works, but even a tool like a heat gun or hairdryer would do the trick. It just has to be consistent—which, at the current stage of development, can be problematic, Paulino says. Controlling vibration has also been a challenge in other types of tensegrity designs, as well.
Paulino and Qi chose to use simple designs for ease of lab testing, but Paulino says there’s no limit to what could be done on the design front.
Their idea is that polymer tensegrity structures can be scaled up and made much more complex, as for space structures, or down, to the size of something that could fit in the human body. Imagine a stent that could be inserted into an artery, Paulino says, which self-deploys once in position. Or if space-bound structures were to be made of similar shape memory polymers, they’d also weigh much less than a similar structure made of metal, allowing for cheaper launches of pre-assembled frames that could be used for lab or living quarters in space.
Those are still just concepts at this point, though he added he has had some interest from medical colleagues, and that NASA has already been exploring tensegrity as an approach for future space missions.
Robert Skelton, who has researched tensegrity for ocean and space applications for decades at Texas A&M University, says Paulino and Qi’s work has an efficiency perk over other types of tensegrity designs.
“A nice advantage of Paulino and Qi's work is the small amount of energy required to stiffen the [struts],” Skelton wrote via e-mail. Skelton added that a similar principle is in action when you pull out a metal tape measure: it’s pre-stressed to be slightly curved when it’s pulled out, but flat while rolled up. Pre-stressed structural elements have been an important approach for space construction, such as on the Hubble Space Telescope, whose solar arrays were deployed with such pre-stressed metal strips that are rigid once fully opened.
“The impact [of shape-memory tensegrity structures] will be just as broad, with a large variety of applications, on earth and in space,” Skelton added.
So the next thing Paulino says he and Qi will be tackling is taking their concept to scale—up and down. And because all that’s required is a 3-D printer and the right material, it could be done from anywhere once the technique is perfected.
“It took a while to reach this level, but we feel that we have a good starting point for the next steps,” Paulino says. “We’re very excited about it. Certainly we don’t know everything that still needs to be done, but we have confidence that we have the capability to make good progress on the idea.”