PerspectiveMaterials Design

Folding structures out of flat materials

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Science  08 Aug 2014:
Vol. 345, Issue 6197, pp. 623-624
DOI: 10.1126/science.1257841
Exploiting origami design principles.

Intricate 3D structures, such as this core structure of a lightweight sandwich plate, can be formed by folding sheet materials. Curved creases are adopted in this example.

PHOTO: ZHONG YOU

Origami is the art of intricately folding a sheet of paper into elaborate three-dimensional (3D) sculptures and objects. In this issue, two reports focus on different aspects of the thriving field of origami engineering. On page 644, Felton et al. (1) report on origami morphing structures, and on page 647, Silverberg et al. (2) report on origami-based metamaterials.

The most intuitive and direct engineering application of folded-sheet geometries is to produce foldable structures or machines that use the well-understood folding motions of origami. The work of Felton et al. is a good example of the design principles used in this field. They developed self-folding machines (crawling robots) in which a type of self-folding composite, activated by embedded electronics, actuate a plate assembly set along a prearranged origami crease pattern. For such applications, a subset known as rigid origami is ideal for immediate engineering adoption. Rigid origami permits continuous motion between folded states along the predetermined folding creases without the need for twisting or stretching of folded facets. It belongs to a family of mechanisms where bodies are connected by revolute hinges (a common example is a door hinge).

The creation of an origami machine starts with the synthesis of a crease pattern, then an analysis of its mobility and motion sequences, and finally the synchronization of motions and identification of the actuation locations. To date, noticeable research progress has been made for single-vertex crease patterns or multivertex crease patterns with decoupled motions. Yet, a major challenge still lies in determining the mobility of a crease pattern that includes multiple vertices, where motion around each vertex is coupled with those of its neighbors. For example, when only four folds meet at each vertex, the crease pattern becomes overconstrained. An overconstrained system has mobility only when motions around each vertex are compatible with those around its adjacent vertices, attained under specific pattern geometries. It is not a trivial issue to find these geometries.

To complete a self-folding machine, suitable actuation materials and devices must be identified and then integrated into the folding patterns. Felton et al. used a type of shape-memory polymer (one that, after a suitable stimulus, returns to a previous imprinted shape). Numerous other systems have also been proposed, including that of Kuribayashi-Shigetomi et al. (3), in which a 3D microstructure was created purely by a cell's traction force, and that of Pickett (4), who explored microscale origami with the intention of manufacturing microelectro-mechanical systems from self-folding origami membranes. Such fundamental work opens the way to the creation of origami structures and machines that are modular, self-assembled, self-reconfigurable, and capable of adapting to their environments (5, 6). These structures and machines are most suitable for applications in which shape transformations from 2D surfaces to 3D objects, and vice versa, are essential, such as large-space antennas and morphing aircrafts.

Another major engineering adoption of origami has been to use folded geometries to create metamaterials (ones with artificial internal structure) with a target mechanical property, such as strength, stiffness, or failure behavior, rather than a target folding motion. Current origami metamaterial applications range from single tubes, to composite folded core (foldcore) sandwich structures (similar to “egg-crate” structures, but the surface is achieved by folding a sheet), to bistable spatial structures.

Because most sheet materials used in engineering applications are relatively rigid in comparison with paper, rigid origami can again be used to parameterize the structures. This approach allows the patterns to be readily manufactured from modern materials such as plastics, metals, or composite fiber-reinforced sheets that have distinctive material properties. For example, Nojima (7) explored a number of patterns that gave cylindrical tube shapes. These tubes could neatly collapse by rotating one end of the tube against the other, making them suitable for applications as space masts. Another example is seen in Ma and You (8), where a highly efficient automobile crash box with a prefolded rigid origami pattern was created. The box exhibits a failure mode that consumes much more energy when subjected to axial crushing in a collision.

By far the most popular engineering adaption, at least as measured in terms of research interest, is seen with foldcore sandwich panels, which gained recognition because of the potential use of their open-channel design, continuous manufacturing process, and abundant number of design parameters in comparison with honeycomb and foams (9). With suitably chosen geometrical parameters and selective alteration of some of the geometrical designs, foldcores can potentially exhibit superior structural properties, e.g., better energy absorption, over traditional structures made from honeycomb and foam.

Most of the current research on foldcores has focused on cores generated with a zigzag rigid origami pattern known as the Miura pattern. Silverberg et al. observed an interesting bistable feature in the Miura pattern, and they proposed that this property could be used to create functional materials whose compressive modulus can be actively altered. In fact, the bistable feature also exists among many other origami patterns that are overconstrained mechanisms. A slight error in dimensions can lead to bistability. Bistability is also common in origami structures consisting of curved creases (see the figure); such structures can “pop” between 3D shapes (10).

Research exploration of bistability is still in its infancy, with many interesting problems still to be solved. Silverberg et al. cleverly made use of this feature in the design of metamaterials. The combination of structural and morphing capabilities that enhances or alters a particular material characteristic is likely where the most innovative applications of origami engineering lie. Such structures would be self-folding and single or multifunctional, with applications for such capabilities as yet scarcely imagined.

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