PerspectiveMaterials Assembly

Designing two-dimensional materials that spring rapidly into three-dimensional shapes

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Science  09 Jan 2015:
Vol. 347, Issue 6218, pp. 130-131
DOI: 10.1126/science.aaa2643

One of the hallmarks of living organisms, including humans, is the ability to actively respond and adapt to stress. Even plants, which lack a nervous system, react to different stimuli by changing their shape. Some well-known examples are the wrinkled edges of leaves as a response to compression during growth, wrinkles of skin to accommodate flexing and bending movements or age-drying stresses, or the mimosa's rapid folding of its leaves via propagating waves of osmotic pressure when touched. Similar phenomena have been used in engineered artificial two-dimensional (2D) materials that can respond to external stimuli, but these applications typically are simple, slow, and work only along one dimension. On page 154 of this issue, Xu et al. (1) demonstrate a new paradigm of designing functional materials that can quickly snap into complex 3D architectures via localized buckling.

Getting into shape.

(A) An atomic force microscopy image shows a silk self-rolling helical microtube. (B) Confocal microscopy image of pH-stimulated self-rolling microtubes created from silk bilayer sheets. (C) Coarse-grain computer models of Janus flowers shown in relaxed and stress-responsive conditions.

Buckling phenomena have been widely explored as a means for measuring elastic properties of ultrathin polymer films (2), as periodic templates for organized nanoparticle assemblies (3), as a complex patterning approach for metal-polymeric sandwiched films (4), for bistable patterning of nanoparticle-polymer multilayers (5), and for chiral patterning of periodic porous materials (6). The underlying physics of wrinkle formation is well understood and arises from the mechanical instability that develops in compressed thin films (7). However, all of these phenomena have an essentially 2D nature, with “penetration” into the third dimension being extremely limited. Furthermore, attempts to overstress the materials result in irregular large-scale folding and crumpling.

Thus, efforts to break through into the realm of 3D shapes now exploit controlled interfacial stresses. Among them are multistep folding of polymer bilayer films into 3D pyramids and “flowers” (8), chemomechanical mediation of a gel-embedded high–aspect ratio nanorod array (9), self-rolling of helical microtubes from laterally patterned hydrogel sheets (10), and self-rolling of bilayer gel sheets with different shapes into helical and smooth tubules (11) (see the figure, panels A and B). Complex shapes can also be induced by thermal control of out-of-plane transformation of gels with halftone-patterned cross-linking (12). These transformations and the resulting 3D shapes are predicted by either analytical considerations for simple shapes (7, 8) or finite-element analysis for complex shapes (9).

In the case of nanoscale materials, more sophisticated computer modeling is required to predict corresponding transformations (see the figure, panel C). Moreover, tuning composition, thickness, and mechanical properties with fine resolution can be a very challenging feat of materials engineering. Even if some astonishing examples of shape transformations have been demonstrated, these materials are usually slow to respond (on a time scale of minutes), frequently require controlled environment (e.g., swollen hydrogels need liquid water or humidity), and are often limited to fairly simple shapes and transformation paths.

New design principles for materials with fast-snapping microstructures suggested by Xu et al. cleverly combine conventional uniform compressive stresses with multiple preprogrammed “weak” points that allow buckling to occur in a hierarchical fashion. Compressive stresses initiate a cascade of buckling events that spring the initial surface-confined planar architectures into 3D complex structures. In this way, various metal and silicon serpentines and ribbons are converted into a collection of microscopic tents, tables, baskets, flowers, boxes, stars, and many other shapes. An amazing variety of conveniently separated “first floor” and “second floor” buckled microstructures are demonstrated as well.

Although the prospective applications of this technique are astounding in breadth and impact, there are more intriguing questions to address. How can the extremely tedious planar processing of large-scale microfabrication be improved or replaced by more facile approaches, such as directed assembly, with similar outcomes? Can these architectures be “unbuckled”—for example, can an open box be closed? Can these structures eventually be sustainable in their 3D shape as stand-alone structures after release from supporting substrates? How does snapping into a 3D architecture affect the global functions of nanostructured constructs, such as electronic, optical, or magnetic properties? Can structures that mimic the mimosa, in which a highly localized stimulus with a pinpoint compression results in a rapid cascade of 3D shape transformations, be made on much larger scale? And finally, what are the limits of scale? Perhaps one day, if you order a house complete with furnishings and a white picket fence, you may just receive a box with a label that reads, “Compress to unfold.”

References and Notes

  1. Acknowledgments: We thank A. Alexeev for computer simulation images and R. Geryak for assistance. Supported by Air Force Office of Scientific Research grant FA9550-14-1-0269 and NSF grants DMR-1002810 and CBET-1402712.
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