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strong non-reciprocal behaviors . In addition, through mechanical deformation or reconfiguration of the
3D assembly constituting these metamaterials, their characteristics can be dramatically improved or tuned
according to the purpose. With the development of 3D metamaterials, various devices in imagination can be
realized, and metamaterials have high potential applications in various fields. Thus, in this chapter, we
review studies on fabricated 3D metamaterials through periodic arrangements of mesostructures or
elaborate designs [Figure 9].

Cheng et al. demonstrated a dome-shaped hierarchical metamaterial with a gradient helical structure and
ultrahigh strength and plasticity, inspired by the Pantheon in Rome, which can withstand high loads with
low density [Figure 9A] . The introduction of the helical arch structure induced a property transition
[226]
from brittle to ductile and could have high compliance and energy absorption capacity. They demonstrated
that their proposed strategy is a promising method to mitigate the instability of ductile materials with the
torsional effect of spiral arch domes and to suppress the decay of brittle composite materials through energy
dispersion. Pan et al. fabricated a bifunctional chiral metasurface with giant asymmetric chirality in the
mid-infrared range by bending a 3D bent surface through origami fabrication using a focused ion beam
[227]
(FIB) [Figure 9B] . The tensile stress generated from the FIB induced an asymmetrically curved array of
split ring resonators, resulting in the formation of a metasurface with abundant spatial degrees of freedom.
The giant circular dichroism and asymmetric chirality properties are expected to promote the design of
multifunctional chiral optical devices. Fan et al. presented a 3D flexible frequency-selective surface (FSS)
that can be conformally attached to complex surfaces while maintaining stable transmission performance
[57]
[Figure 9C] . Through mechanical buckling, the 3D metal structure is precisely controlled to induce the
elasticity of the FSS, increasing inductance inside the metal cell and decreasing capacitance between unit
cells. The proposed technology proved to be an efficient method for applying technologies such as
electromagnetic wave shielding and spatial filtering to non-developable and flexible surfaces. Farzaneh et al.
fabricated mechanical metamaterials with user-specified alternating Poisson’s ratios, sequentially deformed
by leverage principles of differential stiffness and self-contact [Figure 9D] . The degree of freedom in the
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unit cell design was guaranteed through an analytical approach using complementary software tools capable
of designing 2D and 3D metamaterials. Finally, it is demonstrated that an alternating Poisson’s ratio can be
achieved even when a 3D lattice consisting of microscale unit cells is extended to a desired 3D volume.
Chen et al. demonstrated an on-chip and electromechanically reconfigurable nano-kirigami system with
optical functionality driven by the electrostatic force between a top-suspended gold nanostructure and a
[56]
bottom silicon substrate [Figure 9E] . Through the design of the nano-kirigami pattern, they could achieve
broadband non-resonant optical reconstruction at visible wavelengths and narrowband resonant optical
reconstruction at near-infrared wavelengths. The proposed optical nano-kirigami design can be
reconstructed with high contrast at the submicron level, proving its applicability to fields such as physics,
nanophotonics, photonics, and MEMS.

Micro-fluidic system
Microfluidic engineering is a technology that can handle a small amount of fluid using microchannels
(typically tens to hundreds of microns) and has been applied to various fields, including lap-on chips, drug
delivery systems, and biopharmaceuticals [229-231] . For further advances in microfluidic technology, extending
microfluidic networks from 2D to 3D is considered a promising method for improving fluid manipulation
performance such as high-efficiency mixing, separation, and detection. Moreover, complex microfluidic
structures in 3D can achieve complexity that is difficult to obtain on the 2D plane; therefore, they are
expected to be actively used in biological vascular network simulations, especially for disease model
investigation, tissue development, and drug screening [232,233] . Thus far, many methodologies, including
mechanical buckling and 3D printing, have been developed to fabricate 3D microfluidics, and this chapter
introduces various applications of 3D microfluidic channels with complex structures [Figure 10].