Material: Heat-sealed TPU films
The sheets are made of PLA, a bio-compatible polymer. They consist of two layers that were printed with a 3D printer. During printing, the PLA polymer is extruded through the hot nozzle of the printer. As a result, it is stretched along the printing direction. It is frozen in the stretched configuration, but remembers its original, relaxed, length. Once put in hot water, the PLA relaxes and shrinks along the printing direction.
The sample that you are holding consists of two layers. Each of them was printed along one direction, perpendicularly to the other. Therefore, when heated, the top and bottom surfaces shrink in perpendicular direction, imposing a saddle-like curvature on the sheet, causing it to bend into nontrivial 3D shapes. The exact configurations are not arbitrary. The can be computed by the theory of incompatible elastic sheets.
The Racah Institute of Physics (The Hebrew University)
The sheet that you hold seems to be a regular flat plastic foil. However, a specific, nontrivial internal structure is encoded within it. It was imprinted during is fabrication and causes the sheet to bend and fold into three-dimensional shapes, once inserted into hot water.