Pop-up cookie molds: self-folding elastomer sheets using thermal expansion of embedded air chambers

Self-folding is a promising concept in the fields of smart materials and robotics, which allows self-assembly and multi-functionality. Although there have been approaches to realize self-folding mechanisms using different techniques, and their practical applications are not sufficiently investigated. Here, we propose untethered, self-folding polymer structures with embedded air chambers, which easily transform two-dimensional flat shapes into preprogrammed three-dimensional (3D) shapes when heat energy is transferred into the structures resulting in thermal expansion of the air chambers. This mechanism is particularly useful for casting materials that require shape and phase changes simultaneously if the proposed structures are used as molds. As a practical application, we present pop-up cookie molds that can bake 3D-shaped cookies from flat cookie dough. Analytical modelings based on the ideal gas law, the principle of minimum potential energy, and the lumped capacitance method for heat transfer are established and experimentally validated using prototypes. These provide a prediction of the structure's bending curvature during the baking process. Furthermore, other target shapes, including bending, twisting, and their combinations, are also achieved by configuring various chamber designs. Finally, practical prototypes with more complex shapes are proposed, and their performances are demonstrated.