Electrical, electromagnetic, and energy devices

Complex, 3D micro-/nanostructures are ubiquitous in biology, where they provide versatile functionality in mechanical actuation (cytoskeletons), fluid transport (vascular networks), electrical communication (neural circuits), and others. Analogs do not exist even in the most advanced electronics technologies due to an absence of capabilities for 3D micro-/nanofabrication with high-performance electronic materials such as inorganic semiconductors. For example, techniques such as 3D printing or two-photon polymerization do not permit structuring of device-grade silicon. In contrast, capillary or residual stresses can be used to induce bending in thin, released layers from semiconductor wafers in a way that is naturally compatible with modern planar technologies. The result provides access to 3D geometries through either rotations of rigid plates to yield structures such as tilted panels, rectangular cuboids, pyramids, or other hollow polyhedra, or rolling motions of flexible films to form tubes, scrolls, or other cylindrical shapes.

Self-folded structures with device-grade silicon provide new capabilities, such as higher efficiency and small form factor of capillary force-assisted 3D photovoltaic devices (Figure 3a).^{Reference Guo, Li, Ahn, Duoss, Hsia, Lewis and Nuzzo27} Here, the forces mediated by an evaporating water drop fold very thin (1 μm), single-crystalline Si foils into deterministic shapes. The light-trapping that follows from these shapes improves the device performance by more than 300% compared to planar counterparts, and by more than 600% when combined with a 3× optical concentrator.^{Reference Guo, Li, Ahn, Duoss, Hsia, Lewis and Nuzzo27} Elsewhere, capillary forces have also been utilized to assemble a variety of off-planar and polyhedral geometries, including high Q microwave inductors,^{Reference Dahlmann and Yeatman43} optomechanical structures,^{Reference Syms44} and polyhedra with device-grade silicon,^{Reference Gracias, Kavthekar, Love, Paul and Whitesides25} 3D plasmonic resonators,^{Reference Cho, Keung, Verellen, Lagae, Moshchalkov, Van Dorpe and Gracias45} Faraday cages,^{Reference Leong, Zarafshar and Gracias3} microcube antennas, and three-axis devices (Figure 3b–c).^{Reference Gracias, Cho, Hu, Bonafos, Fujisaki, Dimitrakis and Tokumitsu46}

Figure 3. Electronic, electromagnetic, and energy devices. (a) Images of capillary-assisted folding of 3D, mm-scale photovoltaic devices. Reprinted with permission from Reference Reference Guo, Li, Ahn, Duoss, Hsia, Lewis and Nuzzo27. Photograph courtesy of Xiaoying Guo, Zachery Johnson, and Alex Jerez (University of Illinois at Urbana–Champaign). (b) Optical images of capillary force-assembled and self-sealed magnetic field-sensitive three-axis sensors.^{Reference Gracias, Cho, Hu, Bonafos, Fujisaki, Dimitrakis and Tokumitsu46} (c) Scanning electron microscope image of a capillary-assisted self-folded cube with a 50-nm-thick, twin-loop, gold split-ring resonator defined by e-beam patterning on 50-nm-thick Al₂O₃ panels. Reprinted with permission from Reference Reference Cho, Keung, Verellen, Lagae, Moshchalkov, Van Dorpe and Gracias45. © 2011 Wiley. (d) Schematic and experimental images of pop-up origami using device-grade silicon on an elastomeric substrate (color scale shows maximum strain, ε_max). Reprinted with permission from Reference Reference Xu, Yan, Jang, Huang, Fu, Kim, Wei, Flavin, McCracken, Wang, Badea, Liu, Xiao, Zhou, Lee, Chung, Cheng, Ren, Banks, Li, Paik, Nuzzo, Huang, Zhang and Rogers47. © 2015 American Association for the Advancement of Science. Reference Reference Zhang, Yan, Nan, Xiao, Liu, Luan, Fu, Wang, Yang, Wang, Ren, Si, Liu, Yang, Li, Wang, Guo, Luo, Wang, Huang and Rogers48. © 2015 National Academy of Sciences. (e) Schematic of self-rolled-up microtubes (S-RUMs) of C/Si/C and their use as anodes for lithium-ion batteries. Reprinted with permission from Reference Reference Deng, Ji, Yan, Zhang, Si, Baunack, Oswald, Mei and Schmidt69. © 2013 Wiley. (f) Cross-sectional view of a S-RUM ultracompact capacitor comprising ∼13 windings and rolled from a 600-μm-long planar capacitor. Reprinted with permission from Reference Reference Bufon, Gonzalez, Thurmer, Grimm, Bauer and Schmidt54. © 2010 American Chemical Society. (g) Metallic S-RUM helical microantennas on a polymer bilayer with internal diameters (ø) indicated. Reprinted with permission from Reference Reference Karnaushenko, Karnaushenko, Makarov and Schmidt58. © 2015 Nature Publishing Group.

Newer approaches exploit the release of stress in prestrained elastomer substrates to cause 2D sheets of electronic materials or devices to spontaneously transform, without directed intervention, into 3D architectures with complex, targeted geometries (see Figure 3d).^{Reference Xu, Yan, Jang, Huang, Fu, Kim, Wei, Flavin, McCracken, Wang, Badea, Liu, Xiao, Zhou, Lee, Chung, Cheng, Ren, Banks, Li, Paik, Nuzzo, Huang, Zhang and Rogers47,Reference Zhang, Yan, Nan, Xiao, Liu, Luan, Fu, Wang, Yang, Wang, Ren, Si, Liu, Yang, Li, Wang, Guo, Luo, Wang, Huang and Rogers48} When patterned into appropriate geometries, functional membranes composed of advanced semiconductor materials can be delivered to prestretched elastomer substrates, where they chemically bond at a selected set of points, lithographically defined as spatial modulations in surface chemistry. Releasing the tension in the substrate induces compressive stresses that cause the unbound regions to lift out of the plane, with a trajectory in bending, twisting, and translating dictated by (1) the 2D layout of the original structure, (2) the configuration of bonding points, and (3) the magnitude and orientation of the prestretch. This simple process, which has some similarities to mechanisms in children’s pop-up books, offers remarkable levels of versatility in 3D design, in a way that maintains compatibility with state-of-the-art 2D technologies, including those in electronics, photonics, and MEMS.

Demonstration experiments illustrate the ability to achieve more than 100 different 3D geometries using this technique, ranging from open-filamentary spirals, baskets, and networks, to chiral membranes, fractal plates, and closed-form architectures.^{Reference Xu, Yan, Jang, Huang, Fu, Kim, Wei, Flavin, McCracken, Wang, Badea, Liu, Xiao, Zhou, Lee, Chung, Cheng, Ren, Banks, Li, Paik, Nuzzo, Huang, Zhang and Rogers47,Reference Zhang, Yan, Nan, Xiao, Liu, Luan, Fu, Wang, Yang, Wang, Ren, Si, Liu, Yang, Li, Wang, Guo, Luo, Wang, Huang and Rogers48} In all cases, finite element analysis algorithms quantitatively capture the shapes, thereby providing indispensable design capabilities for rapidly exploring layout ideas. A unique feature of this mechanically directed approach to 3D assembly is that the structures can remain tethered to an underlying elastomer substrate, such that their 3D geometries can be reversibly adjusted through mechanical strain. Example devices include tunable spiral inductors, in which the 3D geometry, and therefore, the quality factor and resonance frequency, can be tuned, and rotating micromirrors whose tilt angles can be adjusted for beam steering and modulating the transmission.

Another 3D micro- and nanoscale construct of interest to materials used in electrical, electromagnetic, and energy devices is the self-rolled-up microtube (S-RUM). Here, strain induced by lattice, stoichiometric, or thermal mismatch deformation of ultrathin membranes made of semiconductors, dielectrics, metals, and polymers, or hybrid hierarchical composites,^{Reference Li2,Reference Prinz, Seleznev, Gutakovsky, Chehovskiy, Preobrazhenskii, Putyato and Gavrilova49–Reference Mi and Bianucci52} yield structures with cylindrical symmetry from planar precursors.^{Reference Huang, Koric, Yu, Hsia and Li53} Demonstrated devices include passive electronic components (capacitors, inductors, antennas, etc.) for radio frequency integrated circuit miniaturization,^{Reference Bufon, Gonzalez, Thurmer, Grimm, Bauer and Schmidt54–Reference Huang, Li, Gong and Li59} fully integrated field-effect transistors with ultrasmall bending radii,^{Reference Grimm, Bufon, Deneke, Atkinson, Thurmer, Schaffel, Gorantla, Bachmatiuk and Schmidt60} lasers,^{Reference Bianucci, Mukherjee, Dastjerdi, Poole and Mi61,Reference Dastjerdi, Djavid and Mi62} vertical ring resonators for 3D photonic integration,^{Reference Tian, Veerasubramanian, Bianucci, Mukherjee, Mi, Kirk and Plant63–Reference Madani, Kleinert, Stolarek, Zimmermann, Ma and Schmidt66} and batteries^{Reference Yan, Xi, Si, Deng and Schmidt67–Reference Deng, Ji, Yan, Zhang, Si, Baunack, Oswald, Mei and Schmidt69} (Figure 3e–g). In terms of functionality, S-RUM inductors are 1–2 orders of magnitude smaller and >5× higher in resonant frequency compared to their 2D counterparts (i.e., planar spiral inductors, with operation that is almost independent of substrate conductivity).^{Reference Huang, Yu, Froeter, Xu, Ferreira and Li55,Reference Froeter, Yu, Huang, Du, Li, Chun, Kim, Hsia, Rogers and Li56} Large-area S-RUM capacitor arrays and helical antennas can also be achieved.^{Reference Sharma, Bufon, Grimm, Sommer, Wollatz, Schadewald, Thurmer, Siles, Bauer and Schmidt57,Reference Karnaushenko, Karnaushenko, Makarov and Schmidt58} Pairs of inductors have been proposed as integrated microtransformers with near-perfect coupling coefficients.^{Reference Huang, Li, Gong and Li59,Reference Schmidt, Deneke, Kiravittaya, Songmuang, Heidemeyer, Nakamura, Zapf-Gottwick, Muller and Jin-Phillipp70} In photonics, S-RUMs naturally form ring-resonator cavities. When coupled with III–V-based quantum dots or quantum wells as the gain materials, lasing is possible at ultralow thresholds, under both optical and electrical pumping.^{Reference Bianucci, Mukherjee, Dastjerdi, Poole and Mi61,Reference Dastjerdi, Djavid and Mi62} S-RUM resonators provide convenient out-of-plane optical coupling schemes for 3D photonic integration, outside of the scope of possibilities with conventional planar microcavities.^{Reference Tian, Veerasubramanian, Bianucci, Mukherjee, Mi, Kirk and Plant63–Reference Madani, Kleinert, Stolarek, Zimmermann, Ma and Schmidt66} Evanescent fields along with the hollow core geometry make S-RUM resonators inherently suitable for optofluidic sensing and potentially for lab-on-a-chip applications.^{Reference Bernardi, Kiravittaya, Rastelli, Songmuang, Thurmer, Benyoucef and Schmidt71,Reference Harazim, Quiñones, Kiravittaya, Sanchez and Schmidt72}

Lithographically patterned metamaterials

Metamaterials are synthetic and often micro- and nanostructured periodic or patterned structures that provide novel mechanical, electromagnetic, or chemical properties. Currently, a significant challenge is the parallel fabrication of true 3D metamaterials with material versatility and micro-/nanoscale patterns. Origami approaches can be used to create building blocks such as micro- or nanopatterned polyhedra that can be tiled or aggregated to form 3D periodic metamaterials.^{Reference Randhawa, Gurbani, Keung, Demers, Leahy-Hoppa and Gracias73} Alternatively, micropatterned thin films can be folded up directly into complex materials with periodic 3D patterns, as demonstrated with metals, polymers, and hydrogels (Figure 4a–d).^{Reference Shenoy and Gracias6,Reference Gracias7,Reference Hawkes, An, Benbernou, Tanaka, Kim, Demaine, Rus and Wood19,Reference Bassik, Stern and Gracias23,Reference Nagpal38,Reference Mahadevan and Rica74,Reference Silverberg, Evans, McLeod, Hayward, Hull, Santangelo and Cohen75} Previously assembled metamaterials include egg-crate-like, lithographically patterned self-folded sheets capable of supporting 128,000× their weight, negative Poisson ratio structures, and origami materials that facilitate mechanically stable lattice defects.^{Reference Bassik, Stern and Gracias23,Reference Silverberg, Evans, McLeod, Hayward, Hull, Santangelo and Cohen75–Reference Silverberg, Na, Evans, Liu, Hull, Santangelo, Lang, Hayward and Cohen77} These demonstrations illustrate how complex curved and folded materials with hundreds to potentially millions of folds and patterns limited only by the resolution of planar lithography could be mass-produced.

Figure 4. Metamaterials and tissue/cell engineering devices. (a) Zigzag Miura-ori patterns (a well-known rigid origami fold pattern) in a thin film atop a thick elastic substrate that is compressed biaxially, manifested here in a drying slab of gelatin with a thin skin that forms naturally and showing the physically driven self-organization of Miura-ori. Scale bar 35 μm. Reprinted with permission from Reference Reference Mahadevan and Rica74. © 2005 American Association for the Advancement of Science. (b) Metallic metamaterials with egg-crate-like patterns similar to that in Figure 1c, but self-folded and with significantly smaller dimensions; the materials have a 5:1 rigid segment to flexible hinge (45 μm) ratio. Schematic shows the flat fold pattern with red lines representing −90° valley folds, dark lines representing +90° mountain folds, and gray areas representing rigid panels. Reprinted with permission from Reference Reference Bassik, Stern and Gracias23. © 2009 American Institute of Physics. (c) Bidirectionally self-folded polymeric metamaterial with SU-8 (a photocrosslinkable epoxy) rigid faces and differentially cross-linked hinges. Scale bar 250 μm. Reprinted with permission from Reference Reference Jamal, Zarafshar and Gracias95. Image credits: Mustapha Jamal. © 2011 Nature Publishing Group. (d) Photograph of a paper sheet folded into a Miura-ori metamaterial with a complex defect structure, showing how columns of edge dislocations (highlighted yellow) generate a grain boundary. Reprinted with permission from Reference Reference Silverberg, Evans, McLeod, Hayward, Hull, Santangelo and Cohen75. © 2014 American Association for the Advancement of Science. (e) Fluorescent and phase-contrast time-lapse images of an entrapped, dividing HeLa cancer cell visualized with a green cytoskeleton imaged using GFP-tubulin and red nucleus imaged using the histone H2B-mCherry in a self-rolled-up microtube device. Scale bar 15 μm. Reprinted with permission from Reference Reference Xi, Schmidt, Sanchez, Gracias, Carazo-Salas, Jackson and Schmidt86. © 2014 American Chemical Society. (f) Rolled-up polymeric device with dual fluidic channels through which flows fluorescein (green) or rhodamine B (red). Scale bar 500 μm. Reprinted with permission from Reference Reference Jamal, Zarafshar and Gracias95. Image credits: Mustapha Jamal. © 2011 Nature Publishing Group.

Biomedical devices, robotics, and surgery

A number of curved and folded structures have been assembled that could mimic anatomical structures or confined geometries of relevance to tissue engineering and lab-on-a-chip devices (Figure 4e–f). For example, S-RUMs can guide and accelerate the outgrowth of neurons with exceptional directionality and speed,^{Reference Schulze, Huang, Krause, Aubyn, Quinones, Schmidt, Mei and Schmidt78,Reference Froeter, Huang, Cangellaris, Huang, Dent, Gillette, Williams and Li79} and control the mechanism of neuronal stem cell migration.^{Reference Koch, Meyer, Helbig, Harazim, Storch, Sanchez and Schmidt80} Strain engineering has also been used to generate lab-in-a-tube devices,^{Reference Smith, Xi, Makarov, Mönch, Harazim, Quiñones, Schmidt, Mei, Sanchez and Schmidt81} and curved and folded structures have been used to study the division of single cancer cells, microvascular mimics, drug delivery constructs, bioartificial organs and enable 3D microfluidic tissue scaffolds.^{Reference Randall, Gultepe and Gracias82–Reference Arayanarakool, Meyer, Helbig, Sanchez and Schmidt87}

Out-of-plane bending, folding, twisting, and shape change can also be utilized to enable mechanical actuation of small 3D structures. Techniques such as microfabrication, printed circuit board (PCB) MEMS and lamination have been used to create structures that mimic insects or self-morphing robots (Figure 5a–d).^{Reference Suzuki, Shimoyama, Miura and Ezura88–Reference Paik, An, Rus and Wood90} At smaller size scales, untethered self-folding microgrippers have been used to biopsy gastrointestinal tissue in live pigs, deliver drugs, and capture single red blood cells (Figure 6a–c).^{Reference Gultepe, Yamanaka, Laflin, Kadam, Shim, Olaru, Limketkai, Khashab, Kalloo, Gracias and Selaru91–Reference Malachowski, Jamal, Jin, Polat, Morris and Gracias93} S-RUM approaches have been used to create self-propelled structures such as catalytic micromotors that move by bubble propulsion (Figure 6d) and spermbots consisting of a S-RUM is attached to a sperm cell (Figure 6e). These devices have been used to perform a variety of tasks, such as movement of cargo, or even enable mechanized functions, such as drilling holes into cells and tissues.^{Reference Magdanz, Guix and Schmidt94} Challenges in robotic origami micro- and nanodevices include miniaturization, energy harvesting, speed, reversibility, material versatility, biocompatibility, and programmability.

Figure 5. Robots. (a) Spherical five-bar mechanical linkages fabricated from printed circuit board (PCB)-microelectromechanical (MEMS), in front of a larger panel used to mass-produce them. A US dime provides scale. (b) A monolithic bee (Mobee) fabricated using PCB-MEMS (a US penny provides scale). (a–b) Reprinted with permission from Reference Reference Sreetharan, Whitney, Strauss and Wood89. © 2012 IOP Publishing. (c–d) Self-assembly steps of a cm-scale robot by folding. Reprinted with permission from Reference Reference Felton, Tolley, Demaine, Rus and Wood96. © 2014 American Association for the Advancement of Science.

Figure 6. Micro- and nanosurgical tools. (a) Endoscopic and zoomed image of dust-sized, self-folding microgrippers inside a porcine colon. Scale bar 2 mm. Inset shows zoom of the microgrippers (black spots in the larger image). The size of the gripper shown in the inset is 1 mm when fully open. Reprinted with permission from Reference Reference Gultepe, Yamanaka, Laflin, Kadam, Shim, Olaru, Limketkai, Khashab, Kalloo, Gracias and Selaru91. © 2013 Elsevier. (b) Optical and fluorescent images of all-polymeric, self-folding theragrippers tightly closed around and gripping a clump of cells. Scale bar is 1 mm. Reprinted with permission from Reference Reference Malachowski, Breger, Kwag, Wang, Fisher, Selaru and Gracias92. © 2014 Wiley. (c) Optical image and schematic of self-folding single-cell grippers holding onto a red blood cell. Scale bar 10 μm. Reprinted with permission from Reference Reference Malachowski, Jamal, Jin, Polat, Morris and Gracias93. © 2014 American Chemical Society. (d) Tapered catalytic self-rolled-up microtube (S-RUM) catalytic self-propelled motors moving in a screw-like motion (top image) and drilling into a single fixed HeLa cancer cell (bottom images). Reprinted with permission from Reference Reference Solovev, Xi, Gracias, Harazim, Deneke, Sanchez and Schmidt97. © 2011 American Chemical Society. (e) The vision of spermbots: capture and delivery of single spermatozoa to the oocyte (egg cell) by a magnetic microtube that is controlled by an external magnetic field. The inset shows a microscopic image of a bovine spermatozoon entering a 20-μm-long S-RUM. Scale bar 20 μm. Reprinted with permission from Reference Reference Magdanz, Guix and Schmidt94. © 2014 Informa UK, Ltd.