New mesh structures can transform into configurations that are more complex than what other shapeshifting materials and structures can achieve. The new design method can be used to determine the specific pattern of flat mesh structures to print, given the material's properties, in order to make the structure transform into a desired shape. The technique could be used to design deployable structures such as tents or coverings that automatically unfurl and inflate in response to changes in temperature or other ambient conditions.
As a demonstration, the researchers printed a flat mesh that, when exposed to a certain temperature difference, deforms into the shape of a human face. They also designed a mesh embedded with conductive liquid metal that curves into a dome to form an active antenna — the resonance frequency of which changes as it deforms.
To impart double curvature to a shapeshifting sheet, the researchers switched the basis of the structure from a continuous sheet to a lattice, or mesh. First, a temperature-induced bending of the lattice's ribs would result in much larger expansions and contractions of the mesh nodes than could be achieved in a continuous sheet. Second, the voids in the lattice can easily accommodate large changes in surface area when the ribs are designed to grow at different rates across the sheet. Each individual rib of the lattice bends by a predetermined degree in order to create the shape of, for example, a nose rather than an eye socket.
For each rib, four skinnier ribs are incorporated, arranging two to line up atop the other two. All four mini-ribs were made from carefully selected variations of the same base material to calibrate the required different responses to temperature. When the four mini-ribs 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 arrangement of the four ribs can be adjusted 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.
The lattice was printed from PDMS, a common rubbery material that naturally expands when exposed to an increase in temperature. The material's temperature responsiveness was adjusted 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, the lattice was cured in a 250 °C oven. It was then placed in a saltwater bath where it cooled to room temperature and morphed into the shape of a human face.