Luxo, Jr., Pixar's trademark animated Luxo balanced-arm lamp, is based on a classic design known as the anglepoise lamp, invented by British designer George Carwardine in 1932. Almost ninety years later, the anglepoise lamp has helped inspire a novel approach to building multifunctional shapeshifting materials for robotics, biotechnology, and architectural applications, according to a new paper published in the Proceedings of the National Academy of Sciences.
Meanwhile, physicists at Case Western Reserve University and Tufts University have stumbled on another promising approach to creating novel shapeshifting materials. The researchers remotely manipulated the ordinarily flat surface of a liquid crystal without any kind of external stimulus (such as pressure or heat), changing its physical appearance merely with the nearby presence of a bumpy surface. It's early days, but the researchers suggest their approach could someday enable materials that can shapeshift with the ease of The X-Men's Mystique. They described their work in a new paper published in the journal Physical Review Letters.
Developing novel shapeshifting materials is a very active area of research because there are so many promising applications, such as building artificial muscles—manmade materials, actuators, or similar devices that mimic the contraction, expansion, and rotation (torque) characteristics of the movement of natural muscle. For instance, in 2019, a team of Japanese researchers spiked a crystalline organic material with a polymer to make it more flexible, demonstrating their proof of concept by using their material to make an aluminum foil paper doll do sit-ups. Most artificial muscles are designed to respond to electric fields (such as electroactive polymers), changes in temperature (such as shape-memory alloys and fishing line), and changes in air pressure via pneumatics.
Later that same year, MIT scientists created a class of so-called "4D materials" that employ the same manufacturing technique as 3D printing but which are designed to deform over time in response to changes in the environment, like humidity and temperature. They're also sometimes known as active origami or shape-morphing systems.
The MIT structures can transform into much more complicated structures than had previously been achieved, including a human face. These kinds of shapeshifting materials might one day be used to make tents that can unfold and inflate on their own, just by changing the temperature (or other ambient conditions). Other potential uses include deformable telescope lenses, stents, scaffolding for artificial tissue, and soft robotics.
T is for Totimorphic
What's unique about the latest research from the Harvard team is that their assemblies of interlocking blocks, or cells, can take on and maintain any number of configurations; most shapeshifting materials are limited to just a handful. That's why they are called "totimorphic" structural materials.
“Today’s shapeshifting materials and structures can only transition between a few stable configurations, but we have shown how to create structural materials that have an arbitrary range of shape-morphing capabilities,” said co-author L Mahadevan of Harvard's John A. Paulson School of Engineering and Applied Sciences (SEAS). “These structures allow for independent control of the geometry and mechanics, laying the foundation for engineering functional shapes using a new type of morphable unit cell.”
The trick to any shapeshifting material is to find the sweet spot where both rigidity and elasticity (or conformability) are optimized. If a material has too much conformability, it can't maintain the different shapes it adopts because the configuration won't be stable. If a material is too rigid, it won't be able to take on new configurations at all. That's where the anglepoise lamp comes in. The lamp head "is infinitely morphable by virtue of its having a set of opposing springs in tension that change their lengths while the total energy remains constant," the authors wrote.
In other words, Luxo Jr.'s head will remain stable in any position because its springs will stretch and compress however they need to in order to counteract the force of gravity. The technical term is a "neutrally stable structure": a structure in which the rigid and elastic elements are ideally balanced, enabling them to transition between an infinite number of positions or orientations while still remaining stable in all of them. Mahadevan and his colleagues essentially built an assembly of unit cells as building blocks, connected by individual switchable hinges, to get the same balance between rigidity and conformability.
“By having a neutrally stable unit cell, we can separate the geometry of the material from its mechanical response at both the individual and collective level,” said co-author Gaurav Chaudhary, a postdoctoral fellow at SEAS. “The geometry of the unit cell can be varied by changing both its overall size as well as the length of the single movable strut, while its elastic response can be changed by varying either the stiffness of the springs within the structure or the length of the struts and links.”
As a proof of concept, the team demonstrated that a single sheet of their totimorphic cells could curve up, twist into a helix, bear weight, and even morph into face-like shapes. “We show that we can assemble these elements into structures that can take on any shape with heterogeneous mechanical responses,” said co-author S. Ganga Prasath, another SEAS postdoctoral fellow. “Since these materials are grounded in geometry, they could be scaled down to be used as sensors in robotics or biotechnology or could be scaled up to be used at the architectural scale.
A touch of Mystique
The collaborative research between CWRU and Tufts scientists is a bit more fundamental in nature. Liquid crystals hold great interest because they can shift in response to a localized stimulus, like heat or light or the pressure of touch. The CWRU/Tufts team experimented with so-called nematic liquid crystals: a phase where cigar-shaped molecules are arranged in parallel, yet the material can still flow like water. It's an example of an "orientable Newtonian fluid," a class that also includes certain polymers and tobacco mosaic virus.
The point where the water in a glass meets the air is basically flat unless there is an external disturbance. Most liquid crystal surfaces work the same way. But in these experiments, the scientists placed a patterned substrate on the opposite side of a thin film of nematic liquid crystal just a few hundred nanometers thick. The defects in the substrate enabled the researchers to control how the molecules aligned throughout the material, resulting in a bumpy surface at the water/air interface, with no need for any external stimulus. And the change was significant: between a 30 to 70 percent increase in height compared to the flat surface.
The next step is to achieve even better control over the surface shape by applying external electric fields or by varying the temperature. The team is also interested in expanding their experiments to "smectic" liquid crystals, in which the oriented molecules also form layers. "This is a groundbreaking approach and could prove to be the starting point for future applications—many which we cannot yet imagine," said co-author Charles Rosenblatt, another CWRU physicist.
The authors suggest that these kinds of malleable liquid crystals could help improve microchips or enable the construction of fluid microscopic tools, assuming different shapes to perform repairs and then flowing back into their original shape. There could even be more exotic long-term applications. "Think Mystique from X-Men—you know, shapeshifting," said co-author Timothy Atherton, a physicist at Tufts. "By doing what we've done, we've taken the first step toward altering the surface of something—maybe not skin, but other materials—without touching them or heating them."
DOI: PNAS, 2021. 10.1073/pnas.2107003118 (About DOIs).
DOI: Physical Review Letters, 2021. 10.1103/PhysRevLett.126.057803 (About DOIs).
