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Page 2 of 33 Ren et al. Soft Sci. 2026, 6, 6

INTRODUCTION
Three-dimensional (3D) printing, also known as additive manufacturing (AM), is a versatile, digitally
guided, layer-by-layer process for directly fabricating complex 3D geometries, a paradigm first introduced in
1986 by Chuck Hull . It has become a crucial method from model to device for microrobots, enabling rapid
[1]
iteration, integration of different materials, and sub-millimeter-scale feature control . However, traditional
[2-6]
3D-printed structures are typically geometrically fixed after curing, meaning in-situ reconfiguration,
autonomous deformation, and closed-loop behavior often require additional components or external
mechanisms. These limitations have spurred a shift toward temporally programmable materials [7-12] . Four-
dimensional (4D) printing extends 3D printing by encoding materials with anisotropic responses and
functional gradients to stimuli, enabling printed structures to predictably alter their shape, properties, or
functionality. As a cutting-edge technology, 4D printing accelerates the construction of microstructures and
smart functional devices for diverse applications, including targeted drug delivery , tissue engineering ,
[14]
[13]
stents , and sensing .
[15]
[16]
As an emerging branch of AM, 4D printing reduces design and manufacturing complexity by encoding self-
transformation into simplified printable architectures that deform on demand. This significantly shortens
building times and minimizes support usage, even for complex geometries . With the widespread rise of
[17]
AM in industrial applications, 4D printing has sparked strong interdisciplinary interest . In the field of soft
[18]
microrobots, this technology achieves on-demand shape transformation and multi-degree-of-freedom
actuation through the unique integration of stimulus-responsive polymers with composite materials and a
high-resolution fabrication process. The material library encompasses hydrogels, elastomers, and hybrid
systems, which require a balance between printability, responsiveness, and robustness . Specifically, liquid
[19]
crystal elastomers (LCEs) can convert mesophase orientation into macroscopic motion upon thermal and
optical signals [20,21] . Shape memory polymers (SMPs) provide programmable recovery capabilities for multi-
material structures . Magnetically controllable elastomers or hydrogel matrices enable remote torque and
[22]
force transmission . The selection of fabrication routes should consider the physicochemical properties of
[23]
materials and their intended applications. Reported techniques for constructing soft microrobots include
direct ink writing (DIW) , fused deposition modeling (FDM) , stereolithography (SLA) , digital light
[24]
[25]
[26]
processing (DLP) , and two-photon polymerization (TPP) . Furthermore, to achieve programmable
[27]
[28]
deformation or reliable motion of microrobots, precise control through appropriate actuation schemes is
essential. Energy inputs for driving soft microrobots encompass light , heat , magnetic field ,
[29]
[31]
[30]
electricity , ultrasound , and chemical stimuli . In summary, materials, manufacturing, and actuation
[33]
[32]
[34]
form the fundamental trinity of 4D-printed soft microrobots. Materials and manufacturing enable the
physical realization of devices, while actuation endows them with mobility. Here, we adopt a broad definition
of “microrobots” to include micrometer- to millimeter-scale miniaturized mechanical devices capable of
controlled and programmed deformation under external stimuli; offboard actuation and control are
considered sufficient, and onboard power or computation is not required. In practice, decisions across these
three domains are tightly coupled, and the trade-offs between them ultimately constrain performance and
determine the scope of feasible applications.
Currently, research on 4D-printed soft microrobots has been conducted in multiple areas. Some researchers
focus on selecting and synthesizing suitable materials [35-37] , some emphasize developing appropriate
manufacturing methods [38-40] , and others explore broader application scenarios [41-43] . In this review, we
summarize recent advances in 4D-printed soft microrobots, covering how materials, fabrication strategies,
and stimuli combine to achieve diverse functionalities, as illustrated in Figure 1. Following an overview of
printing technologies and various responsive materials in Section “FABRICATION STRATEGIES” and
Section “INTELLIGENT MATERIALS”, we analyze different actuation responses of 4D-printed soft
microrobots in Section “STIMULI”. Then, in Section “APPLICATIONS”, we survey the applications of 4D-
printed soft microrobots with outstanding behaviors in real-world scenarios. Finally, we discuss perspectives