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.

ABSTRACTWe report a simple method to strain, and thereby program, shape memory polymers by compressing planar thermoplastic sheets. This work is motivated by the limited number of commercially available prestrained polymer sheets; current examples include: Shrinky Dinks, Eastman's Embrace, and polyurethane shrink films. However, these commercial specimens limit the sample thickness, polymer composition, and amount of stored strain. We show here that melt pressing can strain thermoplastic sheets over a range of thicknesses and polymer chemical compositions. After pressing (and thus, straining), the polymer sheets can self‐fold out‐of‐plane into complex geometries using two different actuation mechanisms, both of which locally release strain stored in the polymer. Three‐dimensional geometries are attained experimentally with both thick (~12 mm) and thin (~1 mm) strained polymer samples with a range of polymer compositions. Digital image correlation maps the strain profile within the melt pressed samples while a Mooney–Rivlin and geometric model predicts the average strain and folding response of the samples, respectively. The model predictions agree well with experimental results. These findings enable self‐folding with a broader design space such as polymer chemical composition, sample thickness, strain within the sample, and external stimulus. Techniques presented here should translate to other thermoplastic polymers, thus making this technique a viable tool to increase the available pool of materials available for self‐folding devices. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46889.

Self-folding converts two-dimensional (2D) sheets into three-dimensional (3D) objects in a hands-free manner. This paper demonstrates a simple approach to self-fold commercially available, millimeter-thick thermoplastic polymer sheets. The process begins by first stretching poly(methyl methacrylate) (PMMA), polystyrene (PS), or polycarbonate (PC) sheets using an extensometer at elevated temperatures close to the glass transition temperature (Tg) of each sheet. Localizing the strain to a small strip creates a “hinge,” which folds in response to asymmetric heating of the sheet. Although there are a number of ways to supply heat, here a heat gun delivers heat to one side of the hinge to create the necessary temperature gradient through the polymer sheet. When the local temperature exceeds the Tg of the polymer, the strain in the hinged region relaxes. Because strain relaxation occurs gradually across the sheet thickness, the polymer sheet folds in the direction toward the heating source. A simple geometric model predicts the dihedral angle of the sheet based on the thickness of the sheet and width of the hinge. This paper reports for the first time that this approach to folding works for a variety of thermoplastics using sheets that are significantly thicker (∼10 times) than those reported previously.

Self-folding represents an attractive way to convert two-dimensional (2D) material sheets into three-dimensional (3D) objects in a hands-free manner. This paper describes a simple approach to self-fold pre-strained polystyrene (PS) sheets using microwaves.

Self-folding represents an attractive way to convert two-dimensional (2D) material sheets into threedimensional (3D) objects in a hands-free manner. This paper demonstrates a simple approach to selffold thin pre-strained polystyrene (PS) sheets using microwaves. While the PS sheets are transparent to microwaves, patterns of screen-printed ink containing graphene absorb microwaves and cause the underlying printed sections of the sheet to warm up. When the local temperature in the inked region exceeds the glass transition temperature of PS ( 103 C), the strain in the inked regions of the film relaxes gradually across the sheet thickness, which causes the PS sheet to fold. The resulting dihedral angle is proportional to the width of the hinge printed by graphene ink. The geometry and azimuthal orientation of the sample inside the microwave reactor affect the quality of the folding due to the nonuniformity of the microwave energy inside the reactor. While self-folding has previously utilized heat, light, photo-chemistry, and solvent swelling, this paper reports the use of microwaves, which can deliver large amounts of energy remotely.