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When the four miniribs were bonded together in the printing process to form one larger rib, the rib as a whole could curve due to the difference in temperature response between the materials of the smaller ribs: If one material is more responsive to temperature, it may prefer to elongate. But because it is bonded to a less responsive rib, which resists the elongation, the whole rib will curve instead.
The researchers can play with the arrangement of the four ribs to “preprogram” whether the rib as a whole curves up to form part of a nose, or dips down as part of an eye socket.
Shapes Unlocked
To fabricate a lattice that changes into the shape of a human face, the researchers started with a 3-D image of a face — to be specific, the face of Gauss, whose principles of geometry underly much of the team’s approach. From this image, they created a map of the distances a flat surface would require to rise up or dip down to conform to the shape of the face. Van Rees then devised an algorithm to translate these distances into a lattice with a specific pattern of ribs, and ratios of miniribs within each rib.
The team printed the lattice from PDMS, a common rubbery material which naturally expands when exposed to an increase in temperature. They adjusted the material’s temperature responsiveness by infusing one solution of it with glass fibers, making it physically stiffer and more resistant to a change in temperature. After printing lattice patterns of the material, they cured the lattice in a 250-degree-Celsius oven, then took it out and placed it in a saltwater bath, where it cooled to room temperature and morphed into the shape of a human face.
The team also printed a latticed disc made from ribs embedded with a liquid metal ink — an antenna of sorts, that changed its resonant frequency as the lattice transformed into a dome.
Van Rees and his colleagues are currently investigating ways to apply the design of complex shape-shifting to stiffer materials, for sturdier applications, such as temperature-responsive tents and self-propelling fins and wings.
This research was supported, in part, by the National Science Foundation, and Draper Laboratory.
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