Reprogrammable Self-Folding Thermoplastic Sheets Using Elastic Filaments

Abstract

Materials that fold into new shapes have diverse applications in packages, remote deployment of objects, robotic actuators, and transformative toys. There are a variety of mechanisms to induce folding in materials. In this paper, we take inspiration from how humans fold their limbs at joints using tensile stress from filaments (i.e., muscle fibers) that actuate rigid bodies (i.e., bones surrounded by tissue). We mimic this mechanism using strained elastic filaments to fold plastic sheets subjected to uniform heat. The sheets are typically several centimeters in length and width. The hinge regions are hundreds of microns thick, and the rest of the sheet is millimeters thick. Three factors affect the final shape of the folded object: the location of the elastic filaments, the initial strain in the filaments, and the location of engraved hinge lines on the polymer surface. A geometric model predicts the folding angle as a function of the initial strain in the elastic filament. This technique can form pyramids, boxes, cranes, and modular tessellated shapes. After the elastic filaments are removed, the folded objects revert to their initial state at elevated temperatures in a repeatable manner. It is possible to reprogram the materials to unfold into a different shape when reheated by adding an extra heating step to the process. This approach enables the folding of thermally unresponsive thermoplastics, retention of the folded shape after cooling, restoration of the original shape upon additional heating (i.e., shape memory), and the ability to create various shapes from the same initial sheet by strategically employing elastic filaments.