Folding and assembly methods for forming three-dimensional mesostructures

Abstract

In recent years, there has been an increasing interest in three-dimensional (3D) mesostructures with feature sizes between tens of nanometres and hundreds of micrometres. By forming 3D mesoscale architectures in advanced materials, the resulting materials systems are capable of offering novel acoustic, optical, thermal, mechanical and electronic properties that are not available in natural world, and are, thereby, also known as metamaterials. Owing to the tremendous application prospects of metamaterials in various advanced areas (e.g., electronics, photonics and energy storage), the design and the fabrication of 3D mesostructures has attracted growing attentions. In addition to the various techniques of 3D printing, another two classes of strategies have been developed, including the stress-controlled folding method and the mechanically guided assembly method. In the context of stress-controlled folding method, advanced techniques such as 4D printing and micro-/nano-scale origami were proposed. As for the 4D printing, the planar structures formed by 3D printing techniques have a bilayer or multilayer heterogeneous architecture, in which mismatched strains resulted from external stimulation (e.g., heating) lead to 2D-to-3D transformation through self-folding or self-rolling. As for the micro-/nano-scale origami, the folding deformations are typically resulted from the forces induced by capillarity, thin-film residual stresses or mechanical stimuli response of active materials (for example, hydrogels, shape-memory polymers and shape-memory alloys). In the mechanically guided assembly method, a strategically designed thin 2D precursor is fabricated by modern planar technologies (e.g., photolithography) and then transfer-printed onto a prestretched elastomer substrate. Then strong sites of adhesion are created between the 2D precursor and the substrate by selective bonding. Release of the prestretched substrate gives rise to compressive forces at the bonding areas, thereby transforming the 2D precursor into a 3D configuration by compressive buckling. The key design parameters of this method include: the geometric pattern, thicknesses and mechanical properties of the 2D precursor; the position of selective bonding; and the magnitude of the prestrain in the elastomeric substrate. These two methods provide additional, unique options for the manufacturing of 3D mesostructures. The stress-controlled folding method usually applies to a limited class of 3D geometries, such as simple curved shells (e.g., tubes and scrolls), polyhedra and cylindrical structures. In comparison, the mechanically guided assembly method provides a route to more complex 3D topologies, because of the coupled translational and rotational deformations that can be controlled during the compressive buckling. In this review, we summarize the latest progress of these two methods, and introduce the basic design principles and fabrication techniques. The resulting mesostructures with representative topologies are illustrated, along with the relevant design method and applications. Opportunities exist in the development of an integrated approach to combine effectively these existing methods, which might facilitate progress towards the goal of establishing methods that allow for rapid formation of arbitrary 3D architectures in any constituent materials.