Shapeshifting Faces

The holiday season is already long gone, but the trauma of wrapping oddly shaped presents is still fresh in mind. Wrapping a soccer ball perfectly for Christmas might just be the hardest task the holiday experience has to offer. And similarly to wrapping a soccer ball perfectly, and on a level of difficulty much higher, MIT’s soft robotics scientists have figured out a way to warp a new “shape-shifting” material into the form of a human face!

This concept of “shape-shifting” becomes more complicated when involving curvature and 4D materials, which are materials that shift from their original shape to a new one when exposed to a certain external environment like temperature or humidity. These 4D materials are usually made of hydro-reactive polymers (fibers that react to water) and thermo-reactive polymers (fibers that react to temperature).

All the way back in September (when the world wasn’t as much of a mess), the researchers managed to create a 3D mesh material that distorted into the shape of a human face once exposed to a temperature of 250ºC. The mesh allowed for a property known as double curvature, a form that describes the property of a substance that can curve in two perpendicular directions at once, to apply to a flat sheet-like material instead of only applying to double-curved structures like a human face.

For example, a piece of paper has no curvature at all, while a soccer ball has double curvature. This makes it hard to perfectly wrap the paper around the soccer ball because the paper would have to contort and expand in the right places for the wrapping to fit uniformly and relatively well.

There are already shape-shifting materials, however, they have only been shifted into relatively simple shapes such as spherical caps or dome-like shapes but nothing as complex as a human face before. These simple shapes have been the easiest to reconstruct, not particularly because they are common shapes (think of the soccer ball), but because most materials are very limited in the amount they grow because of the way the property of double curvature works.

When Wim Van Rees, assistant professor of mechanical engineering at MIT and overseer of soft robotics (a type of robotics that deals with constructing robots from highly malleable materials that often mimics the inner body of a human), first imagined this abstract design for contorting mesh two years ago, he imagined a material without any physical limits that could be transformed from a thin flat sheet into a complex shape. He wanted to create a 4D shape, a shape designed to deform over time, that could eventually shift into shapes that resemble something as complex as a jellyfish’s external body. Van Rees started with a face and first asked how he and his team could get a material that could shift into a shape that complicated.

Yet, when he and his team first tried to implement the formulas calculated to transform the sheet into a human face, they ran into a problem: they found that when they tried to deform a continuous sheet of material, the curvature of the material did not allow for the idealized calculations that Van Rees derived for the optimum expansion and contraction of the material.

The next step for Van Rees was to switch from a continuous sheet of substance to a mesh-like structure. Their idea of a mesh design had two parts; one, the lattice’s ribs that bend because of an increase in temperature would expand and contract much more than just a continuous sheet. These ribs are small inter-tangled rods that are designed to expand as temperature increases. Second, the spaces in between the lattice structure of the mesh easily accommodate sizable changes in the surface area as a result of the ribs’ design to grow.

The ribs were made of Polydimethylsiloxane (PDMS), a common silicone-based, rubbery material that expands when exposed to a specifically high heat. The researchers also infused the ribs with glass tubes so that they would be less responsive to the increase in temperature. The glass tubes were more resistant to temperature changes, and thus, when fused into the ribs, the ribs themselves became more temperature resistant since that part of the rib was more resistant. The researchers also designed the lattice sheet so that it would contort at different rates depending on where in the “face” the sheet was being contorted to. That means that the researchers specified where they wanted an eye socket to be, for example, vs. where the nose would be.

For every lattice rib incorporated, four smaller ribs were arranged so that they lined up with two ribs on top of two other ribs. Each smaller rib was made of a variation of the same material as the original mesh ribs, so there would be different responses to temperature when heat was being inflicted on the structure as a whole.

When the smaller ribs were put together to make one bigger rib, the rib as a whole curved due to the change in temperature. This was because if all the ribs were responsive to the temperature difference, they would all elongate due to the nature of their structure, which would then not only cause the mesh to fail but would also cause the mesh to elongate as a whole. Instead, when bonded to a less responsive rib, the mesh curved, creating the structure of a fully formed human face.

Although a shape-shifting 4D material sounds intriguing, the practical use for a shape-shifting face based on temperature does not seem like it would have much practical use. However, Van Rees hopes that it could someday be used to make a jellyfish-like structure that could expand and contract based on the temperature in the water. Furthermore, it could be used to make tents that expand based on a given temperature as well.

Currently, this new way mesh structure is being investigated in a way that some of its properties may be incorporated into stiffer materials so that eventually the mesh can be used for everyday use. It would be possible to make self-propelling fins or even wings that could be used in environmental protection units, for moving through the water, and even unfolding tents that respond to an increase in temperature.