Graphene-based bimorphs for micron-sized, autonomous origami machines

Abstract

Origami-inspired fabrication presents an attractive platform for miniaturizing machines: thinner layers of folding material lead to smaller devices, provided that key functional aspects, such as conductivity, stiffness, and flexibility, are persevered. Here, we show origami fabrication at its ultimate limit by using 2D atomic membranes as a folding material. As a prototype, we bond graphene sheets to nanometer-thick layers of glass to make ultrathin bimorph actuators that bend to micrometer radii of curvature in response to small strain differentials. These strains are two orders of magnitude lower than the fracture threshold for the device, thus maintaining conductivity across the structure. By patterning 2-[Formula: see text]m-thick rigid panels on top of bimorphs, we localize bending to the unpatterned regions to produce folds. Although the graphene bimorphs are only nanometers thick, they can lift these panels, the weight equivalent of a 500-nm-thick silicon chip. Using panels and bimorphs, we can scale down existing origami patterns to produce a wide range of machines. These machines change shape in fractions of a second when crossing a tunable pH threshold, showing that they sense their environments, respond, and perform useful functions on time and length scales comparable with microscale biological organisms. With the incorporation of electronic, photonic, and chemical payloads, these basic elements will become a powerful platform for robotics at the micrometer scale.

Keywords: atomic membranes; bimorph; graphene; origami; self-folding.

Fig. 1.

The basic structure of graphene–glass bimorphs is a sheet of graphene bound to a 2-nm-thick layer of glass (A). The bimorph bends when the glass is strained relative to the graphene. To restrict actuation to take place in specific regions, we pattern in thick pads of photoresist that prevent bending beneath them. The device can then fold and unfold in response to environmental changes. A key parameter in bimorph design is layer thickness: each layer must be of comparable rigidity for the device to bend efficiently. Our glass layers are fabricated to 2-nm thicknesses using atomic-layer deposition. EELS (B) reveals that the glass meets this target size (details are in Materials and Methods). During fabrication, the device is attached to the substrate by an aluminum release layer and consequently does not bend. When the release layer is etched away, the bimorphs fold to a specific angle set by the length of the bimorph between the two pads (C). After release, bimorph hinges behave elastically and will spring back to their rest position if loaded and released (Movies S1 and S2). (Magnification: 20$\times$.)

Fig. 2.

The strain in a graphene–glass bimorph is a function of both temperature and electrolyte content. (A) A laser heats a probe holding a graphene–glass bimorph. (Magnification: 40$\times$.) In response, the glass swells relative to the graphene, and the bimorph coils into a helix with a curvature proportional to the temperature (B). In addition, graphene–glass bimorphs are responsive to pH. In this case, interlayer strain depends on the pH of the surrounding solution and transitions from a finite value to a flat state when a critical pH is exceeded (C). This transition is fully reversible and can be cycled numerous times with the same critical pH setting the unfolding transition. The thermal and chemical mechanisms can be controlled independently and sum together to determine the total strain state in the bimorph. Indeed, the interlayer strain at room temperature in B corresponds to the strain induced by electrolyte effects in C.

Fig. 3.

Graphene–glass bimorphs can be used to fabricate numerous 3D structures at the micrometer scale. These include, but are not limited to, tetrahedron (A), helices of controllable pitch (B and C), high-angle folds and clasps (D), basic origami motifs with bidirectional folding (E), and boxes (F). In Left, we show the device flattened and still attached to the release layer. After they are etched, the bimorphs self-assemble to their targeted 3D geometries (Center). Images of the folded devices were obtained by focal plane stacking. All of the figures in Center are at the same scale. For comparison, we present paper models of the target geometry in Right.

Fig. 4.

Devices made from graphene–glass bimorphs can fold and unfold in fractions of a second in response to local pH changes. When the local pH surrounding a folded tetrahedron exceeds the critical unfolding threshold, the device springs open and into the flat state within 0.5 s (A). Changing the surrounding environment to acidic can refold the tetrahedron within 4 s (B).