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Ren et al. Soft Sci. 2026, 6, 6 Page 13 of 33

of actuation strength with volume and with the available field or gradient, which constrains the operational
range to very small spaces. Hysteresis and magnetothermal losses can cause heating and drift, and careful
surface functionalization and encapsulation are required to ensure long-term biostability and minimal
cytotoxicity.

Material challenges
Despite rapid progress, material selection for 4D-printed soft microrobots remains constrained by several
coupled technical factors. First, printability, responsiveness, and robustness form an inherent trade-off:
formulations that enable large stimulus-induced strain often exhibit lower modulus, weaker interlayer
bonding, and higher susceptibility to creep, hysteresis, and fatigue, whereas mechanically robust networks
frequently respond more slowly or with smaller deformation amplitudes. Second, the fidelity of anisotropy
programming (e.g., director alignment, gradient formation, or multi-material interfaces) is often limited by
process-induced defects, voxel-level heterogeneity, and relaxation after printing, leading to variability in
shape-morphing trajectories. Third, long-term stability in aqueous or ionic environments can be
compromised by hydrolytic or enzymatic degradation for biodegradable networks, stress relaxation, and
solvent or ion exchange, which shift transition thresholds and reduce repeatability . In addition, repeated
[103]
swelling and deswelling cycles can progressively alter network morphology and mechanical properties,
accelerating damage accumulation and performance drift [104] . Notably, hydrogels that appear tough under
monotonic loading may still fail under cyclic actuation due to fatigue fracture, highlighting the importance of
reporting fatigue-relevant metrics such as fatigue thresholds or crack propagation rate in addition to fracture
energy [105] . Finally, translation requires attention to biocompatibility and safety, including potential particle
leaching, residual monomers, sterilization tolerance, and predictable degradation products [106] . Addressing
these issues typically demands synergistic optimization of polymer chemistry, filler-matrix interactions, and
printing parameters, together with standardized mechanical and actuation characterization protocols .
[2]

STIMULI
Stimuli play a crucial role in inducing deformation of 4D-printed soft microrobots. They can trigger changes
in composition, arrangement, phase, molecular structure, conformation, molecular/atomic packing, and
other factors within smart materials, releasing stored stress/strain and converting them into deformation and
motion. Currently, common actuation methods include heat, light, electric field, magnetic field, ultrasound,
and chemical stimuli. Each approach presents distinct trade-offs in penetration depth, spatiotemporal
addressability, energy density, environmental compatibility, and safety. Beyond qualitative pros and cons,
actuation performance is ultimately governed by a set of technical limits: (i) energy coupling efficiency,
namely how effectively the stimulus generates stress/strain; (ii) response time set by transport processes
(diffusion/thermal conduction) or dynamic balance (torque vs. viscous drag); (iii) control bandwidth and
stability (step-out, overshoot, or crosstalk in multi-field operation); and (iv) safety and compatibility
constraints (thermal dose, electrochemical reactions, ultrasound intensity, and imaging/actuation
interference). Therefore, the selection of an actuation scheme should align with the microrobot’s material
composition and target application scenario. Detailed comparisons and recent advancements for each
method are provided in the subsequent sections.

Light
Light serves as a non-contact, rapidly switchable stimulus capable of delivery with high spatiotemporal
precision. By adjusting its wavelength, intensity, polarization direction, and exposure pattern, it can drive
4D-printed soft microrobots. Activation generally follows two pathways. In photothermal approaches,
absorbers (e.g., dyes [107] , carbon nanomaterials [108] , plasmonic particles [109] ) convert optical energy into heat,
inducing local phase transitions, modulus changes, or differential thermal strains that bend or twist printed
structures. In photochemical schemes, photoswitches (e.g., azobenzene, spiropyran) or photo-labile bonds
alter molecular conformation, crosslinking density, or mesogen order [Figure 5A] .
[110]