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Ren et al. Soft Sci. 2026, 6, 6 Page 9 of 33

Stimuli-responsive hydrogels
Hydrogels are water-rich polymer networks (typically containing over 70% water by weight) formed through
covalent chemical crosslinking or physical crosslinking via noncovalent interactions such as hydrogen
bonding, hydrophobic interactions, ionic interactions, and crystallite formation . The interplay of
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interactions fixes a 3D network that swells in water as osmotic pressure is balanced by network elasticity.
Owing to their high porosity and permeability, hydrogels facilitate rapid diffusion of oxygen and nutrients
and their moduli can be tuned across multiple orders of magnitude . External stimuli such as humidity,
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temperature, light, pH, electrical, and magnetic signals can all modulate hydrogels, simultaneously inducing
changes in their volume, stiffness, and shape [Figure 3E] .
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Using a photocurable ceramic elastomer slurry paired with a hydrogel precursor, Wang et al. printed
hydrogel-ceramic laminates by DLP . By programming dehydration of the hydrogel layer, flat patterns
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morphed into 3D shapes that converted to pure ceramics after sintering, guided by a curvature model
accounting for both dehydration and sintering [Figure 3F]. In related work, Zhang et al. employed linearly
responsive transparent hydrogel as artificial muscles to drive cooperative buckling in printed micro-
metastructures . By tailoring printing power, layer thickness, and unit-cell geometry, locally isotropic or
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anisotropic deformations were programmed, enabling thermally reconfigurable metalattices for pixel-level
information display and concealment [Figure 3G]. Moving beyond planar self-folding, Huang et al. direct-
laser-wrote two-photon-polymerizable, stimuli-responsive hydrogel building blocks and assembled them
using Denavit-Hartenberg (DH) parameters to prescribe 3D kinematics, thereby achieving true 3D-to-3D
transformations exemplified by a microscale “transformer” switching between a race car and a humanoid
robot [Figure 3H] .
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For soft microrobots and 4D printing, hydrogels offer multiple advantages including biocompatibility, ease
of processing, low cost, and seamless integration with other materials. However, their high-water content
typically results in low tensile strength, creep, susceptibility to dehydration, and solvent-limited actuation
speeds. Additional challenges include long-term stability, fatigue under cyclic loading, and maintaining
fidelity in complex 3D constructions. Ongoing solutions combine graded crosslinking density architectures
and multi-material design to deliver robust, biomimetic hydrogel actuators and sensors .
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SMPs
SMPs are polymer networks that can be temporarily fixed in a programmed shape and then recover their
original geometry upon exposure to a stimulus, most commonly heat . According to their network
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structure, SMPs can be primarily classified into two types: chemically crosslinked (glassy thermosets or
semicrystalline rubbers) and physically crosslinked (amorphous thermoplastics or semicrystalline block
copolymers) . This memory effect arises from two structural elements: stable netpoints that define the
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permanent shape and switching segments that undergo reversible phase transitions to store and release
strain. The shape-memory programming cycle typically comprises three stages: first, deforming the material
to the desired shape under heating; second, fixing the temporary shape by cooling under constraint; and
finally, recovering the original shape upon reheating [Figure 4A] . Although debates exist about classifying
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them as 4D printing due to the need for post-print programming, they remain a crucial component of 4D
printing materials.

Kong et al. developed a dynamic covalent SMP network that transitions from rigid to pliable with heating
and functions as an artificial muscle, combining reversible actuation, solvent resistance, self-healing,
recyclability, and biodegradability . By setting the recovery temperature close to physiological levels, the
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material achieves body-relevant shape recovery [Figure 4B]. Building on reconfigurability, Li et al.
formulated mechanically robust covalent adaptable network SMPs for high-resolution DLP printing . By
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leveraging extreme deformability at reconfiguration temperatures alongside a glass transition near 75 °C, one