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Ren et al. Soft Sci. 2026, 6, 6 Page 7 of 33

nonlinear absorption in photosensitive resins, thereby confining polymerization reactions to a focal volume
(the voxel) . Typical processing setup employs a piezoelectric stage and galvanometer scanner to guide the
[70]
laser focus along a preprogrammed 3D path through the resin [Figure 2E]. In negative photoresists, exposed
regions undergo crosslinking and curing, while exposed areas in positive photoresists can be dissolved and
removed . Since excitation is confined to the voxel, printing resolutions below the optical diffraction limit
[71]
(sub-100 nm) are achievable, enabling seamless free-form 3D geometries . These characteristics make TPP
[72]
particularly attractive for constructing micro- and nano-scale structures, such as microrobots, micro-optical
devices, mechanical metamaterials, and 4D architectures, especially when the resin incorporates stimuli-
responsive chemistries.

Hu et al. employed TPP to fabricate pH-responsive hydrogel microstructures at the microscopic scale,
achieving sub-second actuation and multi-degree-of-freedom deformation for selective micro-object capture
and release . Building on microscale actuation toward richer programmable behavior, Guo et al. assembled
[73]
independent, light-responsive LCE voxels with predefined 3D director fields into lines, grids, and skeletal
forms, enabling optically or thermally triggered anisotropic morphing in complex geometries . Extending
[74]
from mechanics to functional optics and information encoding, Zhang et al. formulated a two-photon-
printable shape-memory photoresist that produces submicron structural-color patterns whose colors and
embedded information vanish upon flattening and rapidly recover when heated above the glass-transition
temperature .
[75]
TPP is one of the most important 4D-printing technologies for microrobotics, but it also has drawbacks. Its
primary limitations include point-by-point exposure restricting throughput and build volume, sensitivity to
focus stability and optical aberrations, constraints on polymerizable resin and photoinitiator efficiency, and
dimensional changes arising from polymerization shrinkage and development . Practical implementation
[76]
further faces challenges including high equipment costs, meticulous alignment and process control, and the
typical mechanical brittleness of highly crosslinked photoresists, though advances in scan strategies,
multibeam parallelization, and tailored resins continue to mitigate these issues.

Scalability for 4D-printed soft microrobots
Scalability remains a key bottleneck for translating 4D-printed soft microrobots from proof-of-concept
demonstrations to reproducible, high-performance devices for mass production. Current platforms face
coupled constraints in throughput, resolution, multi-material integration, and reproducibility. Serial
microfabrication (TPP) enables the finest features but exhibits poor scalability in build time and volume,
while projection-based photopolymerization (SLA/DLP) improves areal throughput but is constrained by
pixel limits, curable materials, and curing-induced shrinkage or distortion. Material extrusion routes
(DIW/FDM) offer relative scalability and cost-effectiveness, yet their minimum feature size is limited by
nozzle and filament dimensions and rheology. Maintaining robust interlayer bonding and defect-free
structures at small scales becomes increasingly challenging. Multi-material 4D designs further narrow the
process window due to registration errors, cross-contamination, and interfacial delamination. At micro-
/sub-mm scales, even minor variability in curing dose, solvent content, or filler dispersion can cause
significant performance differences through changes in stiffness, transition thresholds, fatigue life, and
actuation trajectories. Therefore, scalable manufacturing requires not only faster printing but also integrated
quality control and standardized benchmarking to ensure cross-batch repeatability and reliable actuation in
relevant environments.

INTELLIGENT MATERIALS
Intelligent materials are central to 4D printing, enabling the integration of structure and function to
empower microrobots with on-demand shape transformation and the ability to perform specific tasks.
Currently, researchers have developed various smart materials. In this section, we aim to outline the primary