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

required for practical applications such as strain, speed, energy efficiency, and repeatability [215,216] . In this
chapter, we introduce actuators with 3D architectures that can perform very delicate and complex tasks in
several fields, including electronics and robotics [Figure 8].

Ning et al. reported a micro-sized complex 3D mechanical framework that integrated multiple,
independently addressable piezoelectric thin-film actuators for vibration excitation and precise control
[217]
through the guided assembly of heterogeneous materials [Figure 8A] . This approach combines transfer
printing for integrating heterogeneous materials and structural buckling for 2D to 3D geometric
transformation to realize complex and hierarchical architectures. In addition, the location of the
piezoelectric material and metal layer was optimized to ensure integrity during compression buckling based
on quantitative finite element analysis. This study demonstrated the possibility of measuring fluid properties
with separated sensitivity to viscosity and density based on multiple 3D vibration modes of piezoelectric
microactuators. Xiang et al. reported photo-driven hydrogel actuators that can be used in underwater
[218]
photo-mechanical applications [Figure 8B] . Hydrogels are made of transparent polyurethane, consisting
of dynamically covalent crosslinked hexaarylbiimidazole and permanently crosslinked pentaerythritol, and
exhibit anisotropic deformation in response to visible light with mechanical stability and repeated utility. As
one of the applications of biomimetic actuators, underwater flowers with rapid photo-driven locomotion
have been demonstrated. Han et al. reported submillimeter-scale terrestrial robots with complex 3D
geometries and multi-material construction, capable of unusual actuation modes on a solid surface
[219]
[Figure 8C] . The balance between the force associated with the shape memory alloy under heating
conditions and the elastic resilience of the encapsulating shell provided the basis for reversible deformation.
It has been shown that various modes of locomotion are possible, from bending, twisting, and expansion at
global heating to linear/curve crawling, walking, turning, and jumping at laser-induced local heating. By
integrating a photonic component and a colorimetric sensor into the proposed actuator body, the possibility
of wireless communication and localization was suggested. Deng et al. reported a laser-rewritable magnetic
composite film that could be reprogrammed digitally using a direct laser writing method [Figure 8D] .
[220]
The magnetic anisotropy of the composite film was encoded through digital laser writing into the local area,
resulting in multimodal 3D shaping under the same actuation magnetic field. They fabricated a multistate
electrical switch as an application example of a multistate 3D structure driven by magnetically induced
buckling. Skylar-Scott et al. introduced a method for designing and fabricating voxelized soft matter using
multi-material multi-nozzle 3D printing, where the composition, function, and structure of materials can be
spatially programmed [Figure 8E] . Using this method, they fabricated a pneumatic actuator-embedded
[212]
soft robot with millipede-like features utilizing a combination of rigid and soft elastomers. By arranging the
two actuator sets asymmetrically, the power stroke for each actuator group can achieve forward movement.
Zhu et al. created electrothermal actuator arrays capable of rapid, large-angle, and reversible elastic folding
[221]
through the controlled and localized Joule heating of micro-origami creases [Figure 8F] . The proposed
system is capable of reprogramming the folds of these creases by overheating, providing a method for
fabricating a permanent and plastically folded 3D shape. A 3D functional microsystem that flexibly folds
and unfolds flap wings according to the current was demonstrated. Yi et al. proposed a manufacturing
strategy that can rapidly fabricate magneto-active machines on a large scale by integrating the roll-to-roll
processing of 2D patterns and 3D origami folding using magnetic composite sheets, where hard magnetic
particles are dispersed in a polymer matrix [Figure 8G] . The introduction of a roll-to-roll platform
[222]
enables the automatic production of 2D patterns of various geometries with high throughput. The magnetic
sheet has magnetic responsiveness while maintaining paper-like foldability so that 3D magneto-origami
machines with various shapes and movements could be manufactured on demand.