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Page 10 of 30 Kim et al. Soft Sci 2023;3:16 https://dx.doi.org/10.20517/ss.2023.07

including origami/kirigami and deployable devices. Park et al. fabricated a wirelessly powered lighting
[161]
system with a built-in capacitor [Figure 4F] . A freestanding LED was developed using transient materials
capable of triggering 3D transformation and mechanical interlocking elements composed of female-type
lugs and male-type hooks. Mechanical interlocking systems provide straightforward access to transformable
or freestanding optoelectronics without material limitations. Yong et al. reported an atomically thin
graphene-based deformable and strain-insensitive photodetection device with controllable stretchability via
a kirigami-inspired architecture [Figure 4G] . The kirigami architecture enabled strategic redistribution of
[156]
stress concentrations through kirigami notches and island bridge motifs. Through this strategy, a
photodetection device that can be driven under high tensile and torsional deformation was demonstrated.

Energy devices
As microsystems become wireless and perform power-consuming tasks, the importance of microenergy
devices capable of energy generation and storage in micro-autonomous systems is increasing. Although
many energy-harvesting platforms such as solar cells [13,162,163] , batteries [164-166] , photovoltaic cells [167,168] and
triboelectric devices [169,170] have made impressive progress in performance, further improvements in shape
deformability and mechanical/electrochemical stability against deformation are required for a wide
application. In this regard, the implementation of an energy device with a complex and hierarchical 3D
geometry can achieve excellent mechanical properties and stable operations even under extreme
deformation. It can also increase the areal capacity, surface area accessibility, and charge/discharge speed
rate . Considerable efforts have been made to achieve these improvements, and various manufacturing
[171]
concepts of energy harvesters applicable to 3D structures with various forms are introduced in this chapter
[Figure 5].

Han et al. demonstrated 3D piezoelectric mesostructures by a controlled, nonlinear buckling process,
converting thin films of piezoelectric polymers into sophisticated 3D piezoelectric microsystems
[172]
[Figure 5A] . These ultralow-stiffness 3D mesostructures, composed of functional piezoelectric materials,
created interesting opportunities for energy harvesting from complex modes of motion induced by
vibrations. Kim et al. developed a highly stretchable piezoelectric energy harvester by grafting a kirigami
structure on a flexible polyvinylidene fluoride (PVDF) film [Figure 5B] . To overcome the low
[173]
piezoelectric coefficient of PVDF, the generation of opposite charges was eliminated using phase
depolarization, and the neutral axis was optimized using a backing layer. Furthermore, the kirigami pattern
has been designed to generate the highest average stress within the constraint by performing finite element
analysis, resulting in high performance. Yang et al. also fabricated a high-areal MXene aerogel capacitor
with microlattices assisted by templates developed using 3D printing techniques [Figure 5C] . Transition
[174]
metal carbides (MXene), a promising material for high-performance energy storage, were injected into a
3D-printed hollow template to produce an MXene aerogel with effective electrolyte penetration and rapid
ion diffusion. The high design freedom of structures and diversity of material choices could be obtained
using various 3D printing-based direct fabrication methods. Xia et al. introduced a new mechanism for
dynamically reconfiguring materials by exploiting electrochemically driven alloying/dealloying reactions
[Figure 5D] . 3D silicon-coated microlattices were transformed into sinusoidal patterns via cooperative
[175]
buckling in response to an electrochemical silicon-lithium alloying reaction. Various architecture designs
could be achieved by programming the domain boundaries to form particular patterns and exhibit the
capabilities of implantable energy storage systems. Guo et al. demonstrated a kirigami-based stretchable,
[176]
deformable, inorganic thermoelectric generator (TEG) for body heat energy harvesting [Figure 5E] . The
3D adjustable architecture enables vertical heat conduction to utilize the temperature difference between the
nonplanar heat source and the environment. The kirigami-based TEG exhibited high-generation
performance with deformation reliability during body movement. Miao et al. fabricated magnetic
material-based energy-harvesting systems, including 3D piezoelectric devices, for noncontact conversion of