Call it one small step for material science, one giant leap for origami. Researchers have created the first heat-reactive polymer material that can not only remember its current shape but also memorize new ones. The material—which currently requires high temperatures to change shape and reset its memory—could lead to a new generation of reusable self-folding materials that could be useful for everything from medical implants to shape-shifting electronics.
Self-folding materials aren’t new. The first generation of shape memory polymers folded into a single predetermined shape whenever they were heated. Later generations could be triggered by other stimuli, such as light, electrical charges, or a magnetic field. But they all relied on a property known as elasticity. When cool, their stringy polymers coil up. They straighten out into a new shape when heated, and then they bend right back to the default shape once they cool off again. In this way, they keep a “memory” of their original shape.
But elastic shape memory materials can only memorize two or three shapes. A 2005 Science paper offered a possible route to hundreds or even thousands: Rather than elasticity—the tendency for a material to come back to the same shape—the paper demonstrated a way to trigger a change in a material's plasticity, that is its ability to be reshaped. "The question was ... can we incorporate these two shape-shifting behaviors in one polymer?" says Tao Xie, a chemical engineer at the State Key Laboratory of Chemical Engineering in Hangzhou, China.
To make a material that is both plastic and elastic, Xie and colleagues started with a known elastic material: crosslinked poly(caprolactone), or PCL. To give the material plasticity, they added a chemical called 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD). If it works, then above and below PCL's elastic temperature point the material should flip between a default shape and one other shape. But if the temperature is raised above the plasticity threshold, then the TBD kicks in by creating chemical bonds between the polymer chains. If you physically manipulate the material into a new shape before this plastic "annealing" process starts, then the default shape gets replaced.
But the trick for Xie was to combine PCL and TBD in such a way that the elastic and plastic temperatures were far enough away from each other that the material can switch cleanly between its different shapes. Otherwise, it could become a chaotic shape-shifting mess, like the death scene of the liquid metal T-1000 in the film Terminator 2. (You're welcome, sci-fi geeks.)
After months of fine-tuning the mixture of these chemicals, the team nailed the critical temperature gap. The new substance has transition temperatures of 70°C and 130°C for elasticity and plasticity, respectively. To demonstrate its multishape capabilities, Xie's team turned a 30-millimeter square of the material into an origami masterpiece that could fold between two shapes using elasticity and change into other shapes using plasticity.
Not only did the material fold into multiple different shapes, but it could also snap between them hundreds of times with little sign of fatigue—a critical feature if the material is to be used in real-world applications, they report today in Science Advances.
The team is already working on a version of the material that works at lower temperatures. "The biggest challenge for us is not necessarily technical, but rather our imagination of what the possibilities are with this type of shape-shifting behavior," Xie says. He considers flexible electronics to be one possible "killer application." Imagine an electronic newspaper that becomes plastic in the heat of your hands but always folds back down when you're done reading it.
The new material is a “step forward” in shape-programmable systems, says Timothy White, a chemical engineer at the Air Force Research Laboratory at the Wright Patterson Air Force Base in Dayton, Ohio, who was not involved in the research. Among the possible applications on his mind is a "reconfigurable antenna." Not only could it be bent into many different shapes, but it would still always be able to retract.