Inspired by origami, Prof. Yan Chen’s group in Tianjin University designed helical structures with tunable chirality, and achieved chirality switching through mechanism bifurcation and hierarchical chirality by introducing a helical tessellation between the chiral units. The results were published in Applied Physics Letters with the title " Helical structures with switchable and hierarchical chirality " in May, 2020 ( https://doi.org/10.1063/5.0005336).
       Chirality refers to the asymmetric configurational property of an object, indicating that the object is unable to be superposed onto its mirror image by pure translation or rotation. Since chirality has a significant effect on the mechanical, optical, or electromagnetic properties of materials, it presents as a new trend of research in biological, chemical and materials communities. However, the realization of many properties such as switchable and hierarchical chirality in man-made structures has remained a challenge due to the constraints in manufacturing and related techniques. Here, the authors proposed chiral structures with tunable chirality based on origami technique and achieved switchable and hierarchical chirality through mechanism theory.
       Firstly, the authors proposed eggbox-based chiral units for constructing homogeneous and heterogeneous chiral structures (Fig. 1), and demonstrated theoretically and validated experimentally for tuning the chirality of these structures by modulating their geometrical parameters.

Fig. 1 Origami-based chiral units and structures

       In general, the chirality of a designed structure is fixed. It requires reconfiguration to achieve chirality switching in man-made chiral structures owing to the different construction of structures with different chirality. Therefore, the authors replaced the paper chiral structure with the linkage based on the kinematic equivalence between rigid origami and linkages. The chirality switching was achieved by taking advantage of mechanism bifurcation.
       In addition, by introducing a helical tessellation between the chiral units, the authors designed hierarchical structures with chirality transferring from construction elements to the morphological level (Fig. 3) and discussed the effect of geometrical parameters on their helical properties and energy. The hierarchically chiral structure has two helices, defined as the major helix and the minor helix, which have identical chirality and greatly-differed helical properties, leading to two zero-height configurations of the structure during the unwinding process.

Fig. 2 Chirality switching

Fig. 3 Helical structures with hierarchical chirality

       This design and analysis approach could facilitate the development of man-made metamaterials with new chiral features, which may serve in engineering applications, including switchable electromagnetic metamaterials, morphing structures, and bionic robots. It can be also used as theoretical models to understand the mechanism of chirality in nature.