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Page 8 of 33 Ren et al. Soft Sci. 2026, 6, 6

Table 2. Representative quantitative metrics of major stimulus-responsive soft material classes
Representative Representative
Material class Representative actuation strain Ref.
modulus response time
LCEs ~0.1-10 MPa Typically, ~10%-50% Seconds-minutes Ge et al. [20]
SMPs ~1-1,000 MPa Large recoverable strain is possible Seconds-minutes Ge et al. [22]
Hydrogels ~1-200 kPa Up to several hundred percent % ms-min Liu et al. [77]
Magnetic
nanocomposite kPa-MPa Field-driven deformation; strain depends ms-s Chung et
[23]
polymers on design and filler loading al.
LCEs: Liquid crystal elastomers; SMPs: Shape memory polymers.

materials used for 4D-printed soft microrobots, including LCEs, SMPs, stimuli-responsive hydrogels, and
magnetic nanocomposite polymers. To complement the qualitative discussion, representative quantitative
properties of these material classes (e.g., modulus range, achievable actuation strain, and typical response
time) are compiled in Table 2 to guide material selection. We hope this provides insights for material
selection in future 4D-printed soft microrobots.

LCEs
LCEs are lightly crosslinked polymer networks that combine long, flexible polymer chains with short, rigid
rod-like mesomorphic units. These mesogens or liquid crystal molecules are integrated into the backbone
(main chain) or attached as pendants to the backbone (side chain) . Their distinct spatial alignments enable
[78]
LCEs to exhibit different mesophases, including nematic, smectic, and cholesteric. Most LCEs are
thermotropic, meaning that when the temperature exceeds the phase-transition temperature, the mesogens
become disordered . This disorder correlates with the macroscopic reversible anisotropic shape change of
[79]
the LCEs, specifically manifested as contraction along the mesogenic orientation and expansion along the
orthogonal direction [Figure 3A] . When the external stimulus is removed, the LCEs can revert to their
[80]
original shape. Beyond direct heating, LCEs can be triggered by light, electro-, or magneto-joule heating, and
solvent, exhibiting rapid, fully reversible deformation. This unique coupling of softness, reversibility, and
programmable anisotropy positions LCEs as a cornerstone material for fabricating soft robotics, sensors, and
adaptive devices.

Jiang et al. developed a 4D-printed LCE composite reinforced with continuous fibers . By adjusting the
[81]
printing pathway and selecting conductive or structural fibers, they programmed reversible folding, boosted
actuation force and energy absorption, and achieved electrically induced shape deformation [Figure 3B].
Based on the same material, Zhou et al. fabricated a monolithic LCE robot trained to snap for self-sustained
motion in a thermal gradient . By modulating substrate adhesion or applying light, it switched between
[82]
rolling and jumping and achieved real-time steering [Figure 3C]. Extending from devices to architected
materials, Kotikian et al. printed multi-material LCE lattices with spatially programmed nematic director
fields . By setting local composition and using an inverse-design scheme, they realized predictable,
[83]
reversible shape morphing across lattice topologies [Figure 3D].

As one of the most commonly used materials for 4D-printed microrobots, LCEs combine large reversible
anisotropic strain, programmable orientation fields, and multi-stimulus actuation (heat, light,
electromagnetic induction, solvents, etc.), and they exhibit excellent compatibility with printing processes
such as DIW, TPP, and DLP. Their drawbacks include relatively low intrinsic modulus and force output,
constrained response frequency due to thermal diffusion, hysteresis under cyclic loading, and phase-
transition temperatures that are difficult to match to physiological conditions.