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Page 4 of 30 Kim et al. Soft Sci 2023;3:16 https://dx.doi.org/10.20517/ss.2023.07

device performances or unique functionalities that cannot be achieved with existing 2D devices [58-69] . There is
a method for converting a 2D structure into a 3D structure by mechanically guiding the assembly
(buckling) [70-75] , folding (origami) [76-78] , or cutting (kirigami) [79-84] a 2D structure along a predesigned pattern,
as well as directly fabricating the desired 3D structure (3D printing) [Figure 2] [85-87] . The assembly processes
for the aforementioned 3D structure manufacturing methods are detailed below, along with material
availability, unique structural features, and design and manufacturing advantages of each method. In terms
of material availability, methods such as mechanically-guided assembly, origami, and kirigami, which
produce 3D structures by transforming and assembling 2D structures, can be applied to nearly all types of
advanced materials, including metals, semiconductors and polymers. The 3D printing method has generally
focused on conductive ink with optimized rheology and incorporates various types of materials.
Furthermore, it is necessary to discuss the unique structural features of each method, which are closely
related to the device’s function. The mechanically-guided assembly is characterized by a deformable
substrate. Origami and kirigami have foldable creases, while 3D printing has a relatively high degree of
structural freedom. Next, the design and manufacturing advantages of each method are discussed. A
mechanically-guided assembly could be introduced to multilayer 2D precursors to obtain a dense and
complex 3D architecture with overlaid layouts and entanglement points. Both origami and kirigami
methods, though primarily through kirigami’s reduction in stress concentration, facilitate control of curves
and provide high design flexibility. 3D printing has the advantages of programmability, scalability, and low
entry barriers in the fabrication of soft electronics. By carefully applying these 3D structure manufacturing
methods with unique characteristics in the right places, 3D structures have been incorporated into various
application fields, such as energy-harvesting [88-95] , biomedical [96-106] , sensors [107-116] , and metamaterials [117-121] .

Mechanically-guided assembly
The mechanically-guided assembly has been studied as a precise and well-controlled 2D-3D conversion
method that can extend geometry to 3D while maintaining compatibility with 2D microsystem technology.
Figure 2A shows the mechanical assembly process of a 3D structure consisting of three key steps: precursor
production, transfer printing, and mechanical buckling. This process begins by manufacturing 2D
precursors through general lithography techniques on source wafers or other planar substrates. Selective
bonding sites and sacrificial layers are formed through sputtering or deposition technologies. The 2D
precursor is then transferred to a pre-stretched elastomer substrate through a transfer printing process using
a polydimethylsiloxane (PDMS) stamp or water-soluble tape to induce strong covalent bonding of the
contact sites. Finally, by releasing the compression applied to the elastomer substrate, which serves as a
platform to provide the mechanical force needed to drive the 3D assembly, the 2D precursor is converted to
a precisely-designed 3D geometry through spatial deformation, in-plane/out-of-plane conversion, and
rotational motion. This assembly process is expected to produce various 3D structures with unique
architecture and functions by controlling key factors that affect the assembly, such as changing the 2D
layout of the precursor, adjusting the pre-strain characteristics of the assembly substrate, and patterning for
selective bonding.

Origami and kirigami
Origami and kirigami, which translate to “paper folding” and “paper cutting,” are methods of converting 2D
flat objects into 3D structures using “folding” and “cutting” as basic processes, respectively. Although many
studies do not establish a clear boundary between origami and kirigami, “folding” is a common feature of
the two methodologies. These methods can modify the shapes of various materials, such as metals,
polymers, hydrogels, graphene, and paper, from the macroscale to the nanoscale. As shown in Figure 2B,
the origami structure is typically folded to a compressed volume compared to the initial state, while the
kirigami structure has an expanded configuration from the initial state. In addition, a hybrid
origami-kirigami design that combines the two concepts is emerging to realize more complex