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Page 6 of 30 Kim et al. Soft Sci 2023;3:16 https://dx.doi.org/10.20517/ss.2023.07

programming possibilities (as shown in the bottom row of Figure 2B). Consequently, the mechanical
behavior of the structure is determined by the balance between the flexibility and rigidity of the pattern.
Origami and kirigami are excellent candidates for forming a unique 3D structure by designing the energy
landscape through the process of folding and cutting.

3D printing
3D printing encompasses a wide range of light- and ink-based printing technologies that enable the digital
design and production of 3D objects, and mainly focuses on rapid prototyping. 3D printing typically moves
the laser optics or ink-based pattern generation head to create an object layer by layer, and the pattern area
consisting of resin, powder, or ink is solidified to generate a desired 3D shape during the printing process.
Various 3D printing technologies have been reported [122-124] , and Figure 2C shows representative examples.
Light-based 3D printing technology uses light to sculpt objects through stereolithography (SLA) of
photocurable resin or selective laser sintering (SLS) of polymer powder. Photocurable resin-based 3D
printing technology combines ink-based printing and light-based printing into one platform, creating
structures through the polymerization of photocurable resin via exposure to ultraviolet light sources.
Finally, direct ink writing is a method of immediately drawing a desired shape through the direct injection
of viscoelastic materials under ambient conditions. 3D printing technology is gaining attention as a suitable
methodology to produce 3D structures according to user orders. Continuous development of 3D printing
methods capable of high speeds, high precision, mass production, and high degrees of freedom in the
printing materials are expected to produce high-performance devices for a wide range of applications,
including sensors, actuators, and electronics.

Microelectromechanical systems
With the demand for the miniaturization of devices, Microelectromechanical systems (MEMS), a
multifunctional component that can simultaneously perform different roles such as sensors, electronic
circuits, and actuators, have made significant progress [125-127] . In particular, the integration of 2D MEMS with
flexible/stretchable substrates has shown the possibility of implementing deformable soft electronic devices
[128]
[130]
[129]
in various fields, including biomedical technology , optical devices , and the Internet of Things . In
addition to this design diversity, high-functionalities such as customizable telecommunication and
frequency reconfiguration are required to expand the application area, and additional advantages can be
provided by embedding geometrically complex 3D microstructures in MEMS. Thus far, several
methodologies for installing electronic components in 3D complex morphology, particularly on a
millimeter to micrometer scale, on flexible/stretchable boards have been proposed. Representative examples
include buckling methods that transform 2D precursors into 3D structures through physical
deformation [131-135] and origami/kirigami that obtains a desired shape by cutting or folding materials [136-142] .
This chapter introduces various studies on the fabrication of 3D MEMS devices that can operate in multiple
states/modes through mechanical reconfigurations [Figure 3].

Fu et al. demonstrated a reconfigurable 3D radiofrequency electromagnetic device that can be deformed in
different time sequences via nonlinear mechanical buckling [Figure 3A] . The antennas are elevated and
[143]
exposed in the working mode, and then, the reconfiguration of the system allows the coil to be hidden by
the metallic support and electromagnetically shielded. Electronic components that can be switched to
different states through reversible shape changes highlight their potential for use in advanced
communication devices. Luan et al. demonstrated a double-floor helical-shaped 3D microfluidic
mesostructure integrated with electronic components such as micro LEDs, heaters, thermistors, and
electrodes [Figure 3B] . The 3D hybrid system could withstand various types of elastic deformation,
[51]
including bending and twisting, through the selective bonding technique. 3D structures capable of physical
deformation and endowed with electronic sensing and regulating functions can achieve systematic